Fundamentals and applications of phosphorus nanomaterials 9780841236585, 0841236585

Table of contents :
Fundamentals and Applications of Phosphorus Nanomaterials......Page 2
Fundamentals and Applications of Phosphorus Nanomaterials......Page 4
Library of Congress Cataloging-in-Publication Data......Page 5
Foreword......Page 6
Subject Index......Page 8
Figure 1. The structures of phosphorus allotropes.......Page 10
Synthesis of Red and Black Phosphorus Nanomaterials......Page 12
Mechanical Methods......Page 13
Thermal Growth Strategies......Page 14
Figure 3. (a) Schematic figure of one example of the vaporization-condensation synthesis method: red phosphorus nanodots on reduced graphene oxide 13. Reproduced with permission from reference 13. Copyright 2017 American Chemical Society. Nanomaterials synthesized by the vaporization-condensation method: (b) SEM image of the red phosphorus-carbon nanotube composite 15. Reproduced with permission from reference 15. Copyright 2014 American Chemical Society. (c) SEM image of the red phosphorus-porous carbon nanofiber composite 16. Reprinted from Li, W.; Yang, Z.; Jiang, Y.; Yu, Z.; Gu, L.; Yu, Y. Crystalline red phosphorus incorporated with porous carbon nanofibers as flexible electrode for high performance lithium-ion batteries. Carbon 2014, 78, 455-462, Copyright 2014, with permission from Elsevier. (d) TEM image of the phosphorus-metal organic framework 18. Reproduced with permission from reference 18. Copyright 2017 John Wiley and Sons.......Page 16
Figure 4. (a) High-resolution TEM image and selected area diffraction pattern of the fibrous red phosphorus synthesized with iodine as a catalyst 19. Reproduced with permission from reference 19. Copyright 2005 John Wiley and Sons. (b) Crystalline red phosphorus synthesized by heating amorphous phosphorus under vacuum 21. Reprinted from Wang, F.; Ng, W. K. H.; Jimmy, C. Y.; Zhu, H.; Li, C.; Zhang, L.; Liu, Z.; Li, Q. Red phosphorus: an elemental photocatalyst for hydrogen formation from water. Appl. Catal., B 2012, 111, 409-414, Copyright 2012, with permission from Elsevier. (c) SEM image of the crystalline red phosphorus nanorods synthesized on the bismuth-doped silicon wafer 23. Reproduced with permission from reference 23. Copyright 2009 John Wiley and Sons. (d) SEM image of the Hittorf’s phosphorus microbelt synthesized with the assistance of bismuth nanodroplet 24. Reprinted from Liu, Y.; Hu, Z.; Jimmy, C. Y. Liquid bismuth initiated growth of phosphorus microbelts with efficient charge polarization for photocatalysis. Appl. Catal., B 2019, 247, 100-106, Copyright 2019, with permission from Elsevier.......Page 17
Figure 5. (a) SEM image of the iodine doped red phosphorus nanosphere 25. Reproduced with permission from reference 25. Copyright 2017 American Chemical Society. (b) TEM image of the porous red phosphorus nanoparticles on graphene 26. Reproduced with permission from reference 26. Copyright 2018 American Chemical Society. (c) TEM image of the red phosphorus hollow sphere synthesized through the solution route 27. Reproduced with permission from reference 27. Reproduced with permission from reference 27. Copyright 2017 John Wiley and Sons.......Page 18
Figure 6. Optical image of the low-pressure transport route synthesized large black phosphorus single crystals in a silica ampoule, where areas marked with 1, 2 and 3 represent the bulk precursor residue, violet phosphorus and the black phosphorus as the main product, respectively 38. Reprinted from Nilges, T.; Kersting, M.; Pfeifer, T. A fast low-pressure transport route to large black phosphorus single crystals. J. Solid State Chem. 2008, 181 (8), 1707-1711, Copyright 2008, with permission from Elsevier.......Page 19
Figure 7. (a) AFM images of a piece of black phosphorus flake obtained immediately after cleaving (left) and after exposure to air for 3 days (right) 40. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nature Nanotechnology, Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Ou, X.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black phosphorus field-effect transistors. Nat. Nanotechnol. 2014, 9 (5), 372. Copyright 2014.. (b) Optical image of multilayered phosphorene after Ar+ plasma thinning. Scale bar is 5 μm 43. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nano Research, Lu, W.; Nan, H.; Hong, J.; Chen, Y.; Zhu, C.; Liang, Z.; Ma, X.; Ni, Z.; Jin, C.; Zhang, Z. Plasma-assisted fabrication of monolayer phosphorene and its Raman characterization. Nano Res. 2014, 7 (6), 853-859. Copyright 2014.......Page 20
Figure 8. (a) Schematic diagram of the solvent exfoliation of black phosphorus in various solvents via tip ultrasonication 47. Reproduced with permission from reference 47. Copyright 2015 American Chemical Society. (b) Optical photograph of solvent-exfoliated black phosphorus dispersions in NMP solvent with various centrifugation conditions (1: as-prepared, 2:500 rpm, 3:5000 rpm, 4:10000 rpm, and 5:15000 rpm) 47. Reproduced with permission from reference 47. Copyright 2015 American Chemical Society. (c) Concentration of black phosphorus dispersions from part (b) 47. Reproduced with permission from reference 47. Copyright 2015 American Chemical Society. (d) AFM height profile of black phosphorus nanosheets in (b). Black phosphorus solution was deposited onto a 300 Si substrate with 300nm SiO2 for measurement. The heights are 1:16, 2:40, 3:29, and 4:128 nm 47. Reproduced with permission from reference 47. Copyright 2015 American Chemical Society. (e) and (f) Preparation diagram and TEM characterization of liquid exfoliated black phosphorus quantum dots 50. Reproduced with permission from reference 50. Copyright 2015 John Wiley and Sons.......Page 22
Figure 9. (a) Schematic diagram of the electrochemical exfoliation process of phosphorene using tetrabutylammonium hexafluorophosphate (TBAP) and DMF as the electrolyte 54. Reproduced with permission from reference 54. Copyright 2017 John Wiley and Sons (b) Schematic of the in-situ Raman measurement setup to monitor electrochemical intercalation process of black phosphorus, (left: before intercalation, right: after intercalation) where the cetyl-trimethylammonium bromide (CTAB) and NMP were used as the electrolyte 56. (c) Electrochemical gate current as a function of the applied electrochemical voltage potential, where the inset is the false-color scanning electron microscope (SEM) image of the intercalated black phosphorus transistors with the scale bar of 5 μm 56. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nature,Wang, C.; He, Q.; Halim, U.; Liu, Y.; Zhu, E.; Lin, Z.; Xiao, H.; Duan, X.; Feng, Z.; Cheng, R.; Weiss, N.; Ye, G.; Huang, Y. C.; Wu, H.; Cheng, H. C.; Shakir, I.; Liao, L.; Chen, X.; Goddard, W.; Huang, Y.; Duan, X. Monolayer atomic crystal molecular superlattices. Nature 2018, 555, 231-236. Copyright 2018. (d) Optical images of a piece of MoS2 crystal before (left) and after (right) THAB intercalation. The scale bars of 5 mm 57. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nature, Lin, Z.; Liu, Y.; Halim, U.; Ding, M.; Liu, Y.; Wang, Y.; Jia, C.; Chen, P.; Duan, X.; Wang, C. Solution-processable 2D semiconductors for high-performance large-area electronics. Nature 2018, 562 (7726), 254. Copyright 2018. (e) AFM image of the exfoliated black phosphorus with the quaternary ammonium intercalation 57. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nature, Lin, Z.; Liu, Y.; Halim, U.; Ding, M.; Liu, Y.; Wang, Y.; Jia, C.; Chen, P.; Duan, X.; Wang, C. Solution-processable 2D semiconductors for high-performance large-area electronics. Nature 2018, 562 (7726), 254. Copyright 2018.......Page 23
Bottom-Up Growth Strategies......Page 24
Figure 10. (a) Schematic for CVD growth of black phosphorus 59. Republished with permission of IOP Publishing, from Smith, J. B.; Hagaman, D.; Ji, H.-F. Growth of 2D black phosphorus film from chemical vapor deposition. Nanotechnology 2016, 27 (21), 215602. (b) Cross-sectional TEM images of amorphous black phosphorus thin films synthesized by PLD method 60. Reproduced with permission from reference 60. Copyright 2015 John Wiley and Sons. (c) AFM morphology of the black phosphorus quantum dots on silicon substrate synthesized by MBE method 61. Reproduced with permission from reference 61. Copyright 2018 John Wiley and Sons. (d) Height profiles along line 1 and 2, respectively 61. Reproduced with permission from reference 61. Copyright 2018 John Wiley and Sons.......Page 26
High-Pressure Conversion......Page 27
Figure 12. (a) TEM image of the ball-milling synthesized black phosphorus-carbon black nanocomposite as a lithium-ion battery electrode material 6. Reproduced with permission from reference 6. Copyright 2007 John Wiley and Sons. (b) Schematic apparatus for the pressurization synthesis of black phosphorus thin film 68. Republished with permission of IOP Publishing, from Li, X.; Deng, B.; Wang, X.; Chen, S.; Vaisman, M.; Karato, S.-i.; Pan, G.; Lee, M. L.; Cha, J.; Wang, H. Synthesis of thin-film black phosphorus on a flexible substrate. 2D Mater. 2015, 2 (3), 031002.. (c) Optical photo of the synthesized black phosphorus thin film on a PET substrate 68. Republished with permission of IOP Publishing, from Li, X.; Deng, B.; Wang, X.; Chen, S.; Vaisman, M.; Karato, S.-i.; Pan, G.; Lee, M. L.; Cha, J.; Wang, H. Synthesis of thin-film black phosphorus on a flexible substrate. 2D Mater. 2015, 2 (3), 031002. (d) Schematic diagram of the synthesis process of the crystalline black phosphorus thin film on a sapphire substrate 69. Reproduced with permission from reference 69. Copyright 2018 John Wiley and Sons.......Page 28
Other Methods......Page 29
Figure 13. (a) Schematic diagram of the wet-chemical synthesis of phosphorus nanosheets 71. (b) TEM image of the wet-chemically synthesized polycrystalline holey black phosphorus nanosheets 71. Reproduced with permission from reference 71. Copyright 2016 John Wiley and Sons. Copyright 2016 John Wiley and Sons.......Page 30
Figure 14. (a) Schematic diagram of the production process of the black phosphorus nanoribbon 72. (b) TEM micrograph of black phosphorus nanoribbon drop-cast from the liquid dispersion shown in the inset, where the scale bar is 10 µm 72. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nature, Watts, Mitchell C., Picco, L.; Russell-Pavier, F. S.; Cullen, P. L.; Miller, T. S.; Bartuś, S. P.; Payton, O. D.; Skipper, N. T.; Tileli, V.; Howard, C. A. Production of phosphorene nanoribbons. Nature 2019, 568, 216-220, Copyright 2019.......Page 31
References......Page 32
Introduction......Page 38
Figure 1. Morphologies of different types of red phosphorus. a) and b) crystals believed to belong to red phosphorus type III and type II, respectively, synthesized through heating red phosphorus. Adapted with permission from reference 2. Copyright 1947, American Chemical Society. c) and d) type II phosphorus synthesized from white phosphorus vapor deposition on bismuth-doped silicon wafer, scale bars are 100 μm and 1 μm for b) and c), respectively. Reproduced with permission from reference 11. Copyright 2009, Wiley-VCH. e) wire structures of fibrous red phosphorus synthesized through deposition under vacuum of amorphous red phosphorus. Reproduced with permission from reference 12. Copyright 2014, Royal Society of Chemistry. f) platelets of violet phosphorus. Reproduced with permission from reference 13. Copyright 2014, Royal Society of Chemistry. g) mixture of fibrous phosphorus and violet phosphorus made through reaction between amorphous red phosphorus, tin, tin(IV) iodide, and red phosphorus thin film using an evacuated ampoule, scale bar is 500 μm. Reproduced with permission from reference 14. Copyright 2016, Wiley-VCH. h) fibrous phosphorus synthesized using amorphous red phosphorus and CuCl2 in an evacuated silica ampoule. Reproduced with permission from reference 15. Copyright 2013, Wiley-VCH.......Page 39
Figure 2. Structural differences between fibrous phosphorus and violet phosphorus. a) crystal structure of fibrous phosphorus with enhanced image depicting the P9, P2 and P8 groups; b) crystal structure of violet phosphorus with enhanced images showing the P9, P2 and P8 groups and how the bonds between groups of cages are made through the P9 group forming and angle of almost 90°. The crystal structure for fibrous phosphorus was obtained from reference 6. and the crystal structure for violet phosphorus was obtained from reference 7. Structures were plotted using VESTA 16.......Page 40
Characterization of Red Phosphorus Type II......Page 41
Characterization of Red Phosphorus Type III......Page 42
Characterization of Fibrous Phosphorus and Violet Phosphorus......Page 43
Introduction of Black Phosphorus and Phosphorene......Page 44
Figure 6. Structure of phosphorene showing a) the armchair direction on a sideview; b) zigzag direction on a sideview; c) top view of a single layer with preferred thermal and electrical conductance; d) P-P atomic configuration, with bond angles and bond lengths. Adapted with permission from reference 47. Copyright 2015, American Chemical Society.......Page 46
Characterization of BP and Phosphorene......Page 47
Figure 7. Raman, AFM, photoluminescence, and powder XRD characterization of black phosphorus and phosphorene. a) Raman spectra of bulk black phosphorus (green), one-layer phosphorene (blue), and two-layers phosphorene (red). Reproduced with permission from reference 28. Copyright 2014, American Chemical Society. b) Visual representation of the Raman vibrational modes of orthorhombic black phosphorus. Reproduced with permission from reference 60. Copyright 2015, Wiley-VCH. c) Polarization resolved Raman spectra for one-layer phosphorene by laser excitation for different polarization angles. Reproduced with permission from reference 61. Copyright 2015, Springer Nature. d) AFM image of a one-layer phosphorene sheet showing a height of 0.85 nm. Reproduced with permission from reference 28. Copyright 2014, American Chemical Society. e) Photoluminescence spectra for bulk black phosphorus (red) and one-layer phosphorene (blue), the high intensity peak for phosphorene corresponds to its electronic band gap. Reproduced with permission from reference 28. Copyright 2014, American Chemical Society. f) Powder XRD diffractogram of bulk black phosphorus showing the (020), (040), and (060) planes. Reproduced with permission from reference 29. Copyright 2014, Springer Nature. The inset shows the morphology of bulk black phosphorus crystals synthesized through a mineralizer agent process in a sealed ampoule. Reproduced with permission from reference 64 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0).......Page 48
Phosphorus Vapor......Page 49
Phosphorus Clusters......Page 50
References......Page 51
Introduction......Page 58
Figure 1. a)-c) Selected AFM scans of three BP flakes in air taken at different times after exfoliation. a) Adapted with permission from 22. Copyright 2015 IOP Publishing Ltd. b) Adapted with permission from 26 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). c) Gamage, S.; Li, Z.; Yakovlev, V. S.; Lewis, C.; Wang, H.; Cronin, S. B.; Abate, Y. Nanoscopy of Black Phosphorus Degradation. Adv. Mater. Interfaces 2016, 3 (12), 1600121. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.......Page 59
Figure 2. a) Integrated intensity of the Ag2 Raman mode as a function of time under different exposure conditions: air, vacuum, O2/H2O mixture and 300 nm thick parylene layer-capped flake in air. The faster decay rate observed in air (5.5 min-1) compared to under the O2/H2O mixture exposure (36 min-1) is due to the photon flux (1.7 × 104 W cm-2 and 1.8 × 103 W cm-2, respectively). Reproduced with permission from 28. Copyright 2015 Springer Nature. b) Transfer characteristics (IVg) of a BP FET device at selected times over the first hour of exposure (curves are offset by 100 nA for clarity). Adapted with permission from 22. Copyright 2015 IOP Publishing Ltd. c) Local sheet resistance maps of a 24 nm thick flake capped by ~3 nm Al2O3 layer. Adapted with permission from 26 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). d) F(δ) curves for a 5.3 nm thick BP drumhead in high vacuum and after 3, 7, 11 and 26 h of exposure to ambient conditions. Adapted with permission from 30 under a Creative Commons License (http://creativecommons.org/licenses/by/3.0/).......Page 60
Passivation Methods: Encapsulation and Functionalization......Page 61
Figure 3. a) Hole mobility for encapsulated and unencapsulated BP FETs versus ambient exposure time. Reproduced with permission from 25. Copyright 2014 American Chemical Society. b) Sheet resistance maps of a 16 nm-thick flake capped by 25 nm Al2O3. Adapted with permission from 26 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). c) (Left) Schematic of the BN–BP–BN heterostructure device fabrication process. (Right) Optical image of the BN–BP–BN heterostructure after O2-plasma etching (the etched area is enclosed within the white line) and of the BN–BP–BN Hall-bar device. Adapted with permission from 47 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).......Page 62
Figure 4. a) Time dependence of the normalized PL intensity of the monolayer phosphorene samples treated by different methods: exfoliated 1L phosphorene (red), 1L phosphorene with PxOy (~11 nm) capping layer produced by O2 plasma etching (green) and 1L phosphorene with dual passivation layers of PxOy and 5 nm of ALD Al2O3 (black). All PL was measured in ambient condition under the same laser excitation. Inset: zoom in plot for the exfoliated 1L phosphorene sample. Adapted with permission from 53 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). b) Optical images of flakes covered by a thin PxOy layer and a Al2O3 + PxOy double layer after fabrication and respectively, after 3 days and 30 days. The images show the higher passivation effect of the double capping layer. Adapted with permission from 53 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). c) HRTEM image of the hybrid structure, where the C60 molecules can be seen passivating the edges of the BP layer. Inset: low magnification TEM image (top). STEM image (middle) and EDX elemental mapping (bottom) of the C60-BP hybrid structure to show the successful presence of the fullerenes along the borders. Adapted with permission from 60 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).......Page 63
Figure 5. a) Preparation process to obtain few-layer phosphorene enriched aqueous dispersions. The workflow is represented by the test tubes, going the order of the different steps from left to right. Deoxygenated water with 2% (wt/vol) SDS was prepared by ultrahigh-purity Ar purging. The exfoliation of the BP crystal was performed in a sealed container using tip ultrasonication. Then, this solution was centrifuged to remove unexfoliated BP crystals. The remaining FL-BP dispersion was ultracentrifuged to precipitate large flakes. The supernatant was finally redispersed in deoxygenated water. Reproduced with permission from 67. Copyright 2016 National Academy of Sciences. b) Relative absorbance as a function of time, measured at 465 nm for: the standard few-layer BP dispersion exfoliated in CHP, in NMP, in IPA and the BP exfoliated in CHP in a glovebox (CHP GB). Adapted with permission from 66 under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/).......Page 64
References......Page 65
Introduction......Page 72
Structure and Properties of White Phosphorus......Page 73
Figure 1. Comparison between the violet and fibrous red phosphorus. a) Building units of the violet and fibrous phosphorus 22. b) Interconnecting tubes of fibrous phosphorus (left) versus violet phosphorus (right). Reproduced with permission from 22. Copyright 2005 Wiley.......Page 74
Figure 2. High-resolution electron micrograph for a) Violet phosphorus b) fibrous phosphorus. Reproduced with permission from 22. Copyright 2005 Wiley.......Page 75
Figure 3. Atomic structure of (a) multi-layer and (b) monolayer black-P 34. Reproduced with permission from 34. Copyright 2015 Royal Society of Chemistry.......Page 76
Figure 5. Band Structure of Phosphorene 42. Reprinted by permission from Springer Nature: Springer Nature, Nature Reviews Materials (ref 42), Copyright 2016.......Page 77
Figure 6. Functionalization of monolayer black-P using a nitrene moiety 47. Reproduced with permission from 47. Copyright 2018 American Chemical Society.......Page 79
Figure 7. (a) Top and side view of Black-P and (b) Blue-P structures (c) dislocations that part of converting from Black-P to Blue-P (d) A-B stacks of Blue-P structure at equilibrium 33. Reproduced with permission from 33. Copyright 2014 American Physical Society.......Page 80
References......Page 82
Introduction......Page 90
Phosphorus Cluster Cations......Page 91
Figure 2. CID mass spectra of selected ions obtained at relatively low collisional energies: (a) , (b) , (c) , (d) , (e) (f) (g) and (h) . Reproduced with permission from 19. Copyright 2015 John Wiley & Sons.......Page 92
Figure 3. Lowest-energy configurations of odd-numbered cluster cations of (m= 1-12). Reproduced with permission from 28. Copyright 2010 Elsevier.......Page 93
Figure 4. CID mass spectra of selected ions obtained at relatively high collisional energies: (a) , (b) , (c) , (d) , (e) and (f) . Reproduced with permission from 19. Copyright 2015 John Wiley & Sons.......Page 95
Figure 6. Mass spectrum of phosphorus cluster anions obtained by laser ablation of RP in the m/z range of 3000–16000, A portion of the spectrum is shown in the inset. The experiment was performed on the FT ICR MS. Reproduced with permission from 18. Copyright 2019 Acta Physico-Chimica Sinica.......Page 96
Figure 7. Photoelectron spectra of -clusters (n=2-9) recorded at photo energy = 3.49 eV (left) and = 4.66 eV (right). Reproduced with permission from 34. Copyright 1931 American Institute of Physics.......Page 98
Figure 9. CID mass spectra of selected ions obtained at relatively high collisional energies: (a) , (b) , (c) , (d) , (e) , (f) (g) , (h) and (i) . Reproduced with permission from 33. Copyright 2016 Elsevier.......Page 99
Figure 10. Lowest-energy configurations of odd-numbered cluster anions of (m= 1-9). Reproduced with permission from 33. Copyright 2016 Elsevier.......Page 100
Figure 11. CID mass spectra of some selected anions, which were obtained at different values of ’s: (a) = 0.15 V; (b) = 0.4 V; (c) = 0.3 V; (d) = 0.5 V; (e) = 0.225 V; and (f) = 0.5 V; The frequency offset is set to be 100 Hz relative to cyclotron frequencies of corresponding precursor ions. Reproduced with permission from 33. Copyright 2016 Elsevier.......Page 101
Figure 12. Lowest energy structures and their corresponding isomers for (n = 1–14) clusters. For each size, the lowest-energy isomers are reported in bold character. Reproduced with permission from 41. Copyright 2017 Elsevier.......Page 102
Metal Phosphide Clusters......Page 103
Other Relative Clusters......Page 104
A Brief Overview......Page 105
Figure 13. Morphology characterization of BPQDs. a) TEM image of BPQDs. b) Enlarged TEM image of BPQDs. c,d) HRTEM images of BPQDs with different lattice fringes. Scale bar=5 nm. e) Statistical analysis of the sizes of 200 BPQDs measured from TEM images. f) AFM image of BPQDs. g,h) Height profiles along the white lines in (f). i) Statistical analysis of the heights of 200 BPQDs measured by AFM. A morphology sketch of BPQD is shown as an inset in (a). Reproduced with permission from 11. Copyright 2015 John Wiley & Sons.......Page 106
Applications of Laser Ablation in PQDs......Page 107
Summary and Outlook......Page 108
References......Page 109
Introduction......Page 114
General Methods......Page 115
Alkali and Alkaline-Earth Metal Phosphides......Page 117
Transition Metal Phosphides......Page 119
Lanthanide and Actinide Phosphides......Page 120
Main-Group (Post-Transition) Metal Phosphides......Page 122
Conclusion......Page 123
References......Page 137
Black Phosphorus Based Photodetectors......Page 146
Overview of Surface Illuminated BP Based Photodetectors......Page 147
Figure 1. (a) Typical surface-illuminated BP based photodetector with bottom gate. (b) Fast response to visible light, and (c) Slow response to ultraviolet in a BP based photodetector. (d) Schematics of a MIR BP based photodetector with interdigitated electrodes and its power-dependent response. ((a) Reproduced with permission from reference 5. Copyright 2014 American Chemical Society; (b)&(c) Reproduced with permission from reference 7. Copyright 2015 American Chemical Society; (d) Reproduced with permission from reference 10. Copyright 2016 American Chemical Society.)......Page 148
Photodetectors Based on BP Homojunctions......Page 149
Photodetectors Based on BP-TMDCs Heterojunctions......Page 150
Figure 4. (a) Cross section schematics of a BP based dual-gate field-effect transistor for bandgap tuning to enable longer wavelength detection. (b) Photocurrent tuned by top and bottom gate at 7.7 µm wavelength at a cryogenic temperature of 77K. (c) Absorption spectra of black arsenic-phosphorus, indicating smaller bandgap and wider spectral detection range up to 10 µm. ((a)&(b) Reprinted by permission from Springer Nature: Springer Nature, Nature Communication 12, Widely tunable black phosphorus mid-infrared photodetector, X. Chen et al., Copyright (2017); (c) Reprinted with permission of AAAS from Science Advances 30 Jun 2017: Vol. 3, no. 6, e1700589. © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC) http://creativecommons.org/licenses/by-nc/4.0/).......Page 151
Figure 5. Illustration of BP based photodetector with plasmonics from of bowtie antenna and bowtie apertures. Bowtie antennas were utilized to enahance responsivity while bowtie apertures were designed to increase polarization sensistivity. (Reproduced with permission from reference 25. Copyright 2018 American Chemical Society.)......Page 152
Figure 6. (a) Structure of a BP/MoS2/BP/Au photodetector for polarization resolving. (b) Polarization-resolved spectral photoresponse of the two diodes. (Reprinted by permission from Springer Nature: Springer Nature, Nature Photonics 26, Polarization-resolved black phosphorus/molybdenum disulfide mid-wave infrared photodiodes with high detectivity at room temperature, J. Bullock et al., Copyright (2018).)......Page 153
Waveguide-Integrated Photodetectors in the Near-Infrared......Page 154
Figure 7. Common types of waveguide-integrated photodetectors. (a) Schematic of the waveguide- integrated Ge based photodetector. (b) Schematic of the waveguide-integrated III-V based photodetector/laser. (c) Schematic of the waveguide-integrated graphene based photodetector working at 1.55 µm. (d) Schematic of the waveguide-integrated TMDC based photodetector working at 1.16 µm. ((a) Reprinted by permission from Springer Nature: Springer Nature, Optical and Quantum Electronics 30, Heterostructure modeling consideration for Ge-on-Si waveguide photodetectors, A. Palmieri et al., Copyright (2018); (b) Reproduced with permission from reference 36. Copyright 2015 MDPI; (c) Reprinted by permission from Springer Nature: Springer Nature, Nature Photonics 40, Chip-integrated ultrafast graphene photodetector with high responsivity, X. Gan et al., Copyright (2013); (d) Reprinted by permission from Springer Nature: Springer Nature, Nature Nanotechnology 42, A MoTe2-based light-emitting diode and photodetector for silicon integrated circuits, Y. Bie et al., Copyright (2017).)......Page 155
Figure 8. Reported works of waveguide-integrated BP based photodetectors working in the NIR. (a) Schematic of the waveguide-integrated BP based photodetector with high responsivity and low dark current. (b) Mode profile showing light-BP interaction. (c) Eye diagram of the BP based photodetector working at 3 GHz. (d) Schematic of the 3D integration of BP based Photodetector with Si photonics and nanoplasmonics. (e) Mode profile showing Si photonics-BP-plasmonic interaction. ((a-c) Reproduced with permission from reference 45. Copyright 2015 Springer Nature; (d&e) Reproduced with permission from reference 46. Copyright 2017 American Chemical Society.)......Page 156
Waveguide-Integrated BP Based Photodetectors in the MIR......Page 157
Figure 9. Waveguide-integrated BP based photodetector working in the MIR. (a) Schematic of the device. (b) Zoom-in view of the BP based photodetector. (c) Optical image of the device. (d) Simulation of light propagation and distribution in the whole integrated system at 3.78 µm. (e) Light distribution in a cross section of the grating structure. (f) Contour plot of gate and drain dependent photocurrent under 237 µW illumination at 3.78 µm. (g) Energy band diagrams for the four quadrants labeled I, II, III, and IV in (f). (h) Power dependent photocurrent and responsivity of the phototransistor. (i) Spectral responsivity of three photoconductors with distinct BP thickness and orientation. (j) Percentage of light propagating upward and downward at the output grating coupler with and without metal gate. (Reproduced with permission from reference 11. Copyright 2019 Americal Chemical Society.)......Page 158
Conclusion......Page 159
References......Page 161
Introduction......Page 166
Figure 1. The molecular structures of (a) Hittorf phosphorus and (b) fibrous phosphorus. Reproduced with permission from reference 16. Copyright 2019, Elsevier.......Page 167
Crystal Structure and Morphology Control......Page 168
Figure 3. (a–c) Representative SEM images, (d) XRD patterns, the blue curve was the simulated pattern based on the crystallographic data, (e) atomic structure and (f–h) TEM, HRTEM and SAED analysis of fibrous phosphorus submicron fibers obtained at -0.06 MPa, 100 mg RP and 550 °C. Reproduced with permission from reference 13. Copyright 2014, The Royal Society of Chemistry.......Page 170
Figure 4. SEM images of (a) micro-fibrous P/SiO2, (b) smashed-fibrous P, and XRD patterns of (c) micro-fibrous P/SiO2 and (d) smashed-fibrous P, bulk-fibrous P. The standard XRD patterns of fibrous P are placed in (c) and (d) as the references. (e) Time course of the hydrogen evolution on micro-fibrous P/SiO2 and smashed-fibrous P. (f) Comparison of the activity of photocatalytic hydrogen evolution on different elemental photocatalysts. The light source used or referred here are all visible light, except the mesoporous crystalline Si (full spectrum). Reproduced with permission from reference 18. Copyright 2016, Wiley-VCH.......Page 171
Cocatalyst Loading......Page 172
Figure 6. (a, b) SEM and (c, d) TEM images of resulted hierarchical YPO4/P hollow microspheres with different magnifications. Inset is the HRTEM image of the marked frame region in (d). (e) The possible formation process of hierarchical P/YPO4 hollow spheres. Reproduced with permission from reference 29. Copyright 2012, Elsevier.......Page 173
Figure 8. (a) Schematic representation of the proposed charge trapping model in g-C3N4 (left) and RP/g-C3N4 (right). Normalized fs-TA decay kinetics (dotted lines) with exponential fitting curves (solid lines) of the sample dispersions (0.1 mg mL-1) in H2O probed at 560 nm under 400 nm excitation: (b) short time scale, (c) long time scale; (d) ns-TA decay kinetics of the sample dispersions (0.1 mg mL-1) in H2O probed at 460 nm under irradiation of 400 nm laser. Reproduced with permission from reference 34. Copyright 2017, Wiley-VCH.......Page 174
Photocatalytic Mechanism......Page 175
Figure 9. (a) Photocatalytic inactivation efficiencies, (b) Level of 1O2, (c) Level of •O2–, (d) Level of •OH, (e) Level of H2O2, were measured with red phosphorus in the presence of various scavengers (l-histid

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PRIMER ON PHOSPHORUS NANOMATERIALS This work is a comprehensive introduction to the fundamental physical and chemical properties of phosphorus nanomaterials. These emerging nanomaterials have band gaps that facilitate applications in energy, environmental, industrial, and clinical fields. Both experienced and new researchers will find useful information in chapters describing the properties, synthesis, and applications of phosphorus nanomaterials.

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VOLUME 1333

FUNDAMENTALS AND APPLICATIONS OF PHOSPHORUS NANOMATERIALS

ACS SYMPOSIUM SERIES

ACS SYMPOSIUM SERIES

FUNDAMENTALS AND APPLICATIONS OF PHOSPHORUS NANOMATERIALS

JI

JI

Fundamentals and Applications of Phosphorus Nanomaterials

ACS SYMPOSIUM SERIES 1333

Fundamentals and Applications of Phosphorus Nanomaterials Hai-Feng (Frank) Ji, Editor Department of Chemistry, Drexel University Philadelphia, Pennsylvania, United States

Sponsored by the ACS Division of Inorganic Chemistry, Inc.

American Chemical Society, Washington, DC

Library of Congress Cataloging-in-Publication Data Library of Congress Cataloging in Publication Control Number: 2019048654

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Contents Preface .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

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1. Synthesis of Red and Black Phosphorus Nanomaterials .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  Yihang Liu, Dingzhou Cui, Mingrui Chen, Zhen Li, and Chongwu Zhou

1

2. Introduction and Characterization of Phosphorus Nanomaterials .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  27 Pedro E. M. Amaral and Hai-Feng Ji 3. Degradation of Black Phosphorus upon Environmental Exposure and Encapsulation Strategies To Prevent It .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  47 Yu Kyoung Ryu, Andres Castellanos-Gomez, and Riccardo Frisenda 4. Physical and Chemical Properties of Phosphorus .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  61 Ishaq Alalq, Jie Gao, and Bin Wang 5. Phosphorus Clusters and Quantum Dots .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  79 Xianglei Kong, Lei Mu, Ming Zhou, and Shumei Yang 6. Synthesis of Phosphides .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  103 Michael Shatruk 7. Black Phosphorus Based Photodetectors .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  135 Bowei Dong, Li Huang, Chengkuo Lee, and Kah-Wee Ang 8. Photocatalytic Property of Phosphorus .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  155 Yang Liu, Jie Li, Zhuofeng Hu, and Jimmy C. Yu 9. Electronic Applications of Black Phosphorus Thin Films.  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  179 Arnob Islam and Philip X.-L. Feng Editor’s Biography .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  195 Indexes Author Index .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  199 Subject Index .  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  201

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Preface The discovery of carbon nanotubes and the isolation of graphene from bulk graphite were individually responsible for launching entire scientific fields of inquiry into 1D and 2D nanomaterials, respectively. Researchers are taking inspiration and insights from carbon nanotubes and graphene and applying it to new or recently rediscovered 2D materials that do possess a band gap, such as black phosphorus (BP) and red phosphorus (RP). These materials may be suitable for optoelectronic applications from the near infrared region through to the visible. Over 1000 papers were published in the last two years on this topic, compared to 1 GPa) (72).

Phosphorus Clusters P4 is the standard state for pure phosphorus, apart from that, other cage-like phosphorus structures have been detected, either in an ionic form, small clusters, or as rings as part of the structure for other molecules and the pentagonal arrangements in red phosphorus. Although P=P bonds in elemental phosphorus Pn (n > 2) have not been observed, it is possible that Pn cluster-type or closed ring (as in benzene) molecules or ions could exist. This could be based on cages where three edges meet at the corner (3). It has been theorized that a P8 molecule with Oh symmetry would be less stable energetically than 2 P4 molecules linked together (73), suggesting that phosphorus cages would not be possible for molecules in which the angles between the P atoms are 90° or larger. However, other works have shown that a P8 cage is indeed more stable than 2P4 molecules (74, 75). Nevertheless, a P8 species has still not been detected. Numerous publications from calculations have suggested the possibility of small phosphorus ionic species such as (n = 1,…,15) (76, 77, 78), (n = 1,….,15) (77, 79) or neutral Pn (n up to 28) (80, 81) cluster. Other publications showed evidence of the detection of 39

small cationic species (82). Also, a recent publication suggests three new different covalently linked structures of phosphorus that are more stable than P4 at room temperature (83). Apart from the small phosphorus clusters, there is also great interest in finding large phosphorus analogous to the carbon fullerenes/nanotubes due to their outstanding mechanical, electrical, optical and structural properties (84). Early work (85) shows that these fullerene-like phosphorus Pn (14 ≤ n ≤ 60) clusters exhibit instability with respect to separation into P4 molecules. However, Karttunen et al. showed that large phosphorus cages can be stable if slight modifications are made in their structure (86). In their work, they showed that pointing the phosphorus lone pairs to the inner side of the cage will greatly reduce strain. These structures would not be perfect spheres like the carbon ones (Figure 9a) and their exterior would have the appearance of black phosphorus sheets with some P atoms pointing inwards. Clusters of Pn (n up to 720) were predicted to be thermodynamically stable and the stability would increase with n, however, the cages would eventually collapse to monolayer sheets of black phosphorus if n becomes too large (86). Karttunen et al. also suggested in the same work the possibility of the existence of ring-shaped allotropes (Figure 9b). These ring-shaped allotropes of phosphorus are predicted to be as stable as fibrous red and violet phosphorus, suggesting the ring-shaped chains can be stable in air (86). Recent work have shown the evidence to the existence of large spectrometry (87), stable nonionic species have not been detected yet.

(n = 65) clusters by mass

Figure 9. a) Proposed structures for the phosphorus cage-like structures; b) proposed structures for the phosphorus ring-like structures. Reproduced with permission from reference (86). Copyright 2007, WileyVCH.

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Chapter 3

Degradation of Black Phosphorus upon Environmental Exposure and Encapsulation Strategies To Prevent It Yu Kyoung Ryu,* Andres Castellanos-Gomez, and Riccardo Frisenda Materials Science Factory, Instituto de Ciencia de Materiales de Madrid(ICMM-CSIC), Madrid E-28049, Spain *E-mail: [email protected].

Along this chapter we discuss about one of the main issues in black phosphorus research: its environmental instability. We will discuss the works were the role of the environmental exposure has been studied and the reported works describing strategies to effectively encapsulate black phosphorus to preserve its pristine properties.

Introduction In 2013, Yuanbo Zhang and co-workers demonstrated that graphene was not the only elemental two-dimensional material that could be isolated by exfoliation of a bulk layered crystal (1). Atomically thin layers, and even one-atom thick layers, of phosphorus can be isolated by mechanical and chemical exfoliation of bulk black phosphorus, a stable layered allotrope of phosphorus, crystals. Bridgman grew black phosphorus single crystals for the first time in 1914. He was investigating phase transformations of white phosphorus under high pressure and discover the black phosphorus allotrope by serendipity. In his original manuscript Bridgman said: “…Black phosphorus was discovered during an attempt to force ordinary white phosphorus to change into red phosphorus by the application of high hydrostatic pressure…” (2). Regarding the optical and electronic properties of black phosphorus, in bulk it is a semiconductor with a direct bandgap of 0.35 eV and charge carrier mobilities in the order of 10000 cm2/V·s (3). Thin flakes of black phosphorus have been recently used in field-effect transistors (FETs) showing mobility values up to 1000 cm2/V·s (4–6) at room temperature and up to ~4000 cm2/V·s at low temperatures (7–9). Moreover, due to its small and direct bandgap black phosphorus has a bright perspective for broadband photodetection where transition metal dichalcogenides are limited because of their large bandgap (10–16). Unlike other 2D materials isolated to date, however, black phosphorus is rather environmentally instable. Therefore, the attainment of either stable thin layers or effective passivation methods is required to produce reliable BP-based optoelectronic devices (17–19). In the following we will © 2019 American Chemical Society

discuss about this issue and about the strategies developed to overcome this environmental instability.

Origin of the Instability under Ambient Conditions Most of the two-dimensional materials isolated prior to 2014 (graphene, h-BN, MoS2, etc…) are rather stable at atmospheric conditions which facilitated the fabrication of devices based on these materials. Black phosphorus, however, shows a relatively large reactivity (20, 21). This reactivity is originated by its particular puckered honeycomb crystal structure. In black phosphorus, the phosphorus atoms have 5 valence shell electrons available for bonding with a valence shell configuration 3s23p3. Therefore, each phosphorus atom bonds to three neighboring phosphorus atoms through sp3 hybridized orbitals, making the phosphorus atoms to form a puckered honeycomb lattice (orthorhombic, with space group Cmca) and each phosphorus atom also has a lone pair (a pair of valence electrons that are not shared with another neighbouring atom), which makes phosphorus very reactive to air. In fact, exfoliated flakes of black phosphorus are highly hygroscopic and tend to uptake moisture from air (22). Several initial studies report on quick water accumulation at the surface of exfoliated flakes (23–29). Figure 1 shows some examples from the literature of atomic force microscopy images of exfoliated black phosphorus flakes left in ambient conditions that progressively accumulates water with time (22, 26, 27).

Figure 1. a)-c) Selected AFM scans of three BP flakes in air taken at different times after exfoliation. a) Adapted with permission from (22). Copyright 2015 IOP Publishing Ltd. b) Adapted with permission from (26) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). c) Gamage, S.; Li, Z.; Yakovlev, V. S.; Lewis, C.; Wang, H.; Cronin, S. B.; Abate, Y. Nanoscopy of Black Phosphorus Degradation. Adv. Mater. Interfaces 2016, 3 (12), 1600121. Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. Apart from the topographical changes induced by the moisture uptake of the black phosphorus flakes exposed to air, the environmental exposure also induces degradation of the optical, electrical and mechanical properties of black phosphorus. In fact, Favron et al. measured the evolution of the Raman intensity upon exposure to different atmospheres showing a clear degradation of black phosphorus in air (Figure 2a) (28). 48

Figure 2. a) Integrated intensity of the Ag2 Raman mode as a function of time under different exposure conditions: air, vacuum, O2/H2O mixture and 300 nm thick parylene layer-capped flake in air. The faster decay rate observed in air (5.5 min-1) compared to under the O2/H2O mixture exposure (36 min-1) is due to the photon flux (1.7 × 104 W cm-2 and 1.8 × 103 W cm-2, respectively). Reproduced with permission from (28). Copyright 2015 Springer Nature. b) Transfer characteristics (IVg) of a BP FET device at selected times over the first hour of exposure (curves are offset by 100 nA for clarity). Adapted with permission from (22). Copyright 2015 IOP Publishing Ltd. c) Local sheet resistance maps of a 24 nm thick flake capped by ~3 nm Al2O3 layer. Adapted with permission from (26) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). d) F(δ) curves for a 5.3 nm thick BP drumhead in high vacuum and after 3, 7, 11 and 26 h of exposure to ambient conditions. Adapted with permission from (30) under a Creative Commons License (http://creativecommons.org/licenses/by/3.0/). The electronic properties of black phosphorus are also strongly modified by the environmental exposure as illustrated by Island et al. and Wood et al. through direct electrical transport measurements as a function of the exposure in air (22, 25) (see Figure 2b). Interestingly, from these transport measurements one can conclude that there are two different regimes: one at shorttimescale where the modifications are mainly due to physisorption of O2 and N2 species and another one at longer timescales were the water absorption starts. In the short-timescale (less than 1 hour) there is an instant shift of the threshold voltage to the left (shown in Figure S6 (e) from the supplementary information file in ref. (22)) and an overall decrease in conductance and these changes are reversible by putting the device in vacuum. A vacuum to air zero bias resistance cycling experiment showed that during the first cycles up to the 7th, the initial value was recover (figure S4, from the supplementary information file in ref. (22)). At long-timescale the conductance of the devices steadily decreases until the device breakdown. Complementary to the macroscopic electrical transport measurements microscopic measurements with microwave impedance microscopy are a non-destructive, good asset to probe the effect of the environmental exposure on black phosphorus (26, 29). As an example to illustrate the importance to study locally the mechanism of interaction between the BP surface and its degradation agents, figure 2c shows an example of a black phosphorus flake capped with a thin layer of Al2O3 that cannot effectively encapsulate completely the black 49

phosphorus flake (26). From the microwave impedance microscopy images one can conclude that the degradation process occurs first at the edges of the flake and then it is extended towards the center of the flake. This study allowed the authors to optimize the stability in air of the BP by capping it with a double layer of Al2O3/hydrophobic fluoropolymer film, instead of a single Al2O3 layer. Moreno-Moreno et al. demonstrated that exposing thin flakes of black phosphorus to air also degrades its mechanical properties: after 24 h or exposure to ambient the Young’s modulus of thin black phosphorus flakes decreases by a factor of two (see Figure 2d) (30). Regarding the physical and chemical mechanisms behind this environmental degradation of black phosphorus, Favron et al. and Zhou et al. have deduced that the degradation most likely occurs as a result of photo-induced oxidation from oxygen absorbed in accumulated water at the surface of exfoliated flakes exposed to ambient conditions. (28, 31, 32). Recently, Kim et al. have studied by scanning Kelvin probe microscopy (SKPM) the degradation process of black phosphorus in dark. They have observed that the degradation rate is layer-dependent, since the electronic structure of a 2D material changes with its thickness (33). Zhang et al. have combined experimental results and first-principle calculations to study the reaction kinetics of black phosphorus with oxygen contained in the water. They have concluded that the main products are the PO23-, PO33- and PO43- species. They also showed the possibility to store BP in deoxygenated water under ambient light without degradation for at least 15 days (34). The exact role of water in the oxidation mechanism of the black phosphorus is not fully understood yet. As a consequence, further experimental results and theoretical models are being developed. A recent work has proposed that OH- ions are responsible for the degradation of BP, through experimental observation and density functional theory calculations (DFT). They showed that the degradation of BP has therefore a dependence with the pH, with linear increasing degradation rate from pH 4 to 10 (35). Finally, Luo et al. studied the kinetics of black phosphorus oxidation under 5% O2/Ar, 2.3% H2O/Ar and 5% O2/2.3% H2O/ Ar mixture. The highest rate of oxidation was found in the mixture of oxygen and water. The authors concluded that water continued to interact chemically with the oxidized BP surface promoting the oxygen dissociation further (36). The stepwise reactivity of H2O with a phosphorene surface once it was oxidized was also studied theoretically (37).

Passivation Methods: Encapsulation and Functionalization These works focused on the air-induced degradation of black phosphorus have motivated works to find effective methods to encapsulate black phosphorus at the early stages of isolation/fabrication to increase its longevity (25, 26, 38). Several encapsulation techniques have been reported to date including polymer dielectrics (10, 39), atomic layer deposited (ALD) oxides (25, 26, 40–42), hybrid metal organic chemical vapor deposition (MOCVD)/ALD coating of BN/Al2O3 (43), electronbeam evaporated metallic thin films oxidized under ambient air (44), van der Waals heterostructures using boron nitride, MoS2 and graphene flakes (7–9, 38, 45–48), and chemical functionalization (49–52). In two different works, a thin native oxide layer on top of the black phosphorus surface has been formed by oxygen plasma (53) or thermal annealing etching (54). In both works, the presence of the oxide proved to stop the degradation of BP. Tian et al have produced a thin native Pox layer around 2 nm thick on the bottom side of black phosphorus films of typically 20 nm thick. This bottom oxide layer was formed under exposure in air for 30 min during the mechanical exfoliation 50

and metallic contacts deposition. The authors used the charge transfer induced at the bottom Pox/BP interface to simulate synaptic behavior along the 45° angle spaced electrodes of a field-effect transistor, exploiting the in-plane anisotropy of the material. After fabrication and electrical measurements at vacuum, from room to high temperature, the devices were stored in a N2 box under dark and the authors checked that the bottom oxide layer was not modified for a few months (55). Another work demonstrated the use of ozone generated by ultraviolet light under atmospheric pressure, to form phosphorus oxide on top of thick black phosphorus films. This oxide is etched away by rinsing with deionized water, leaving a thinner BP surface, which presents a retarded degradation effect in air. They found an etching rate of 12 ± 2 nm/h and could achieve a single-layer phosphorene film under this process (56).

Figure 3. a) Hole mobility for encapsulated and unencapsulated BP FETs versus ambient exposure time. Reproduced with permission from (25). Copyright 2014 American Chemical Society. b) Sheet resistance maps of a 16 nm-thick flake capped by 25 nm Al2O3. Adapted with permission from (26) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). c) (Left) Schematic of the BN–BP–BN heterostructure device fabrication process. (Right) Optical image of the BN–BP–BN heterostructure after O2-plasma etching (the etched area is enclosed within the white line) and of the BN–BP–BN Hall-bar device. Adapted with permission from (47) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). Figure 3a-b shows the results of encapsulating black phosphorus using a thick AlOx capping layer (deposited through atomic layer deposition) (25, 26). While uncapped field-effect transistors break down in a few days, field-effect transistors with encapsulation show performances (e.g. mobility and conductance) that do not change over several days (25). Microwave impedance microscopy measurement technique confirms that black phosphorus devices encapsulated with a thick AlOx layer shows little to no degradation (26).

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Figure 4. a) Time dependence of the normalized PL intensity of the monolayer phosphorene samples treated by different methods: exfoliated 1L phosphorene (red), 1L phosphorene with PxOy (~11 nm) capping layer produced by O2 plasma etching (green) and 1L phosphorene with dual passivation layers of PxOy and 5 nm of ALD Al2O3 (black). All PL was measured in ambient condition under the same laser excitation. Inset: zoom in plot for the exfoliated 1L phosphorene sample. Adapted with permission from (53) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). b) Optical images of flakes covered by a thin PxOy layer and a Al2O3 + PxOy double layer after fabrication and respectively, after 3 days and 30 days. The images show the higher passivation effect of the double capping layer. Adapted with permission from (53) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). c) HRTEM image of the hybrid structure, where the C60 molecules can be seen passivating the edges of the BP layer. Inset: low magnification TEM image (top). STEM image (middle) and EDX elemental mapping (bottom) of the C60-BP hybrid structure to show the successful presence of the fullerenes along the borders. Adapted with permission from (60) under a Creative Commons License (http://creativecommons.org/ licenses/by/4.0/). Sandwiching black phosphorus flakes between boron nitride flakes (bottom and top encapsulation, see Figure 3c) is a more advanced route toward passivation which has proven to be a method to obtain exquisite electronic properties for graphene and MoS2 devices. This enhancement of the device performance is due to the atomically flat, dangling-bond-free and defect free surface of hexagonal boron nitride (47, 57, 58). Thanks to this improved performance in these fully 52

encapsulated devices it has been possible to observe interesting physical phenomena not observable in low-mobility electronic devices such as the Shubnikov-de Haas oscillations and the integer quantum Hall effect (7–9, 46, 47). Figure 4a) shows the passivation effects of a PxOy layer formed by an oxygen plasma to thin down the initial exfoliated thick phosphorene layer (orange curve). This oxide layer was found to have a dual passivation role: its presence itself slowed down the degradation rate under ambient exposure. Then, for a dual passivation process depositing further a Al2O3 layer by ALD to increase the lifetime of the device, the oxide served as a protection layer to prevent the reaction of the phosphorene with the ALD precursors (pink curve) The optical images reinforce the longer time stability of double capped flakes compared to one that was also covered by the oxide layer (53).

Figure 5. a) Preparation process to obtain few-layer phosphorene enriched aqueous dispersions. The workflow is represented by the test tubes, going the order of the different steps from left to right. Deoxygenated water with 2% (wt/vol) SDS was prepared by ultrahigh-purity Ar purging. The exfoliation of the BP crystal was performed in a sealed container using tip ultrasonication. Then, this solution was centrifuged to remove unexfoliated BP crystals. The remaining FL-BP dispersion was ultracentrifuged to precipitate large flakes. The supernatant was finally redispersed in deoxygenated water. Reproduced with permission from (67). Copyright 2016 National Academy of Sciences. b) Relative absorbance as a function of time, measured at 465 nm for: the standard few-layer BP dispersion exfoliated in CHP, in NMP, in IPA and the BP exfoliated in CHP in a glovebox (CHP GB). Adapted with permission from (66) under a Creative Commons License (http://creativecommons.org/licenses/by/4.0/). Another strategy pursued to improve the stability of the black phosphorus under ambient conditions and consequently, its performance as device is the functionalization of its surface with different elements and molecules. The adsorption of Ag+ ions on the black phosphorus surface via cation-π interactions constitutes an example of such approach. To produce such metal ion modified surface, a mechanically exfoliated BP flake transferred to a 300 nm SiO2/Si substrate was immersed in a N-methyl-2-pyrrolidine (NMP) solution with silver nitrate (1x10-6 M). They observed an improvement on the hole mobility from 796 to 1593 cm2/V s and the ON/OFF ratio from 5.9x104 to 2.5x106 after 1 h immersion, compared to the original device before passivation (59). In another work, the edges of BP nanosheets were functionalized by covalent bonding with C60 molecules (see Figure 4c). The fullerenes were chosen due to its high hydrophobicity. These C60 functionalized black phosphorus films showed higher stability against oxidation and good photocurrent characteristics (60). The functionalization of a black phosphorus surface with antioxidant molecules following processes such as fluorination (61) or treatment in imidazolium-based ionic liquids (62) 53

were pursued. A sulfur doped thin layer BP field effect transistor showed around 77% of the mobility and an ION/IOFF ratio of 103 still after 21 days under ambient exposure (63). Aluminum doped black phosphorus flakes were used to produce FETs that presented an n-doping effect and unnoticeable degradation after 10 days under ambient conditions (64). Finally, density functional theory calculations were performed to estimate the ratio of BP doping with scandium atoms at which the flake was protected from the oxygen and water molecules, but also retained the semiconductor behavior (65). An alternative approach to address the environmental instability issue consists on using liquid phase exfoliation to obtain suspensions of black phosphorus nanosheets. It has been demonstrated that liquid phase exfoliation can produce large quantities of high-quality few-layer black phosphorus nanosheets. These black phosphorus nanosheets are surprisingly stable in suspension (even when deoxygenated water is used as solvent for the liquid phase exfoliation), probably due to the solvation shell protecting the nanosheets from reacting with oxygen (Figure 5) (66–71). In summary, in this chapter we have discussed the issue of the environmental instability in black phosphorus and the works devoted to develop strategies to effectively encapsulate black phosphorus to preserve its pristine properties.

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Chapter 4

Physical and Chemical Properties of Phosphorus Ishaq Alalq, Jie Gao, and Bin Wang* School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States *E-mail: [email protected].

Phosphorus has unique structures and properties, which lead to a variety of different applications. Particularly, motivated by the study of graphene and other two-dimensional (2D) materials, many forms of phosphorus have been revisited from the perspective of 2D materials, different from its early studies mostly as a bulk material. This new perspective motivates exploration of phosphorus to be used in optoelectronics, catalysis, and batteries. In this chapter, we overview different phases of phosphorus ranging from the most active white phosphorus, to amorphous and crystalline red phosphorus, and to the most stable black phosphorus with an emphasis on the correlation between these atomic structures and the physical and chemical properties. We also discuss the challenge for largescale synthesis of phosphorus with controllable atomic structures.

Introduction Phosphorus is of immense importance in living systems. It is an essential element to construct the phosphodiester bonds that link mononucleotide units forming DNA and RNA and to power synthesis of complex molecules of life through reversibly removal and addition of the phosphate bond between adenosine diphosphate (ADP) and adenosine triphosphate (ATP). Moreover, as a crucial element for producing fertilizers, its demand escalates as the global population continues to grow. This increased consumption together with very limited, localized reserves and its slow cycling of natural mobilization raise serious concerns about its substainibility (1, 2). The predominant source of phosphorus is phosphate rock. Phosphate rock deposits can be sedimentary or igneous. Currently, sedimentary phosphate deposits are exploited to produce more than 80% of the world’s production of phosphate rock (3). Recently the United States Geological Survey has estimated that the economically extractable phosphate reserves amounted to approximately 70 billion tons around the world, which was predicted to be exhausted in 50–100 years, with peak production in the next 15–20 years (4). After this peak, manufacture of phosphorus fertilizers will require extraction from increasingly dilute minerals using greater energy inputs at higher costs. © 2019 American Chemical Society

Phosphorus has a variety of different allotropes, ranging from the least stable white phosphorus (white-P) to the most stable black phosphorus (black-P), with strikingly different properties. The most abundant types are white and red phosphorus (5), while the black-P is the most stable form (6). The different colors show that they have distinct physical properties, where the color itself characterizes the electronic and optical band gap of the allotropes. These different physical and chemical properties further lead to a broad range of applications. In addition to these common allotropes, motivated by the intriguing electronic and optical properties of 2D black-P, other forms of low-dimensional phosphorus have been explored as well. In this chapter, we highlight the different atomic structures of the most common forms of phosphorus, including the white, red and black phosphorus. Their chemical and physical properties will be separately discussed, with an emphasis on their intrinsic electronic properties and the structure-property relationship of the different allotropes of phosphorus.

Structure and Properties of White Phosphorus White-P is a transparent, colorless, and highly flammable solid (7). When exposed to air, the color quickly becomes yellow. It is the least stable allotrope among all of the phosphorus and is highly sensitive to the temperature change (8, 9). Particularly, self-ignition occurs in the air at 34 °C. This high instability is caused by its atomics structure. In white-P, four P atoms form a tetrahedral P4 molecule and each P atom forms three chemical bonds with its neighbours (10). This configuration results in a distorted 3p atomic orbitals leading to its thermodynamic instability. The configuration of P valence electrons is 3s2 3p3; formation of three chemical bonds with the neighboring P atoms leaves a lone pair of electrons in each P. It looks like that the lone pairs avoid interacting with each other, favoring van der Waals interaction between the P4 molecules in white-P. The van der Waals arrangement of the P4 clusters leads to the relatively low density in this allotrope. White-P is slightly soluble in water; it has been a convenient way for white-P to be transported or stored to avoid possible self-ignition when exposed to air. Though white-P is intrinsically instable in the air and only slightly soluble in water, by constriction of individual P4 molecule in self-assembled tetrahedral structures, P4 molecules could be air-stable and water-soluble (11). Along the same line, the P4 units can be good ligands for coordination with transition metals to build up complexes (12). There are three different forms of the tetrahedral packaging; α, β, and γ refittings (8, 13). The common type is the α phase of white-P, a type that is very soluble in carbon disulfide and somewhat soluble in benzene, alcohol, and acetic acid. In this phase at room temperature, the P4 molecules can rotate around their center. The structure of white-P in the α phase was proposed to be similar to α-Mn with the P4 replacing the metal atom (10); in this way, the white-P in the α phase has four crystallographically different P4 molecules. The β white-P is mainly prepared from α at atmospheric pressure with a temperature of 197 K or at a higher pressure than 1.0 GPa at ambient temperature (14, 15). The β-phase has three different molecules in the asymmetric unit, and the symmetry is reduced to triclinic, different from cubic in the α-phase. The P4 molecules are arranged in the same manner as the metal atoms in γ-Pu (15). The P-P distance is between 219 and 221 pm (15). Rapid quenching followed by slow warming can convert the α-phase into the γ phase, which remains stable up to about 160 K (16). If the quenching is not fast enough, the α-phase may be converted to the β phase directly. At higher temperature, the γ phase irreversibly transforms to the β phase, which finally reversibly transforms to the α-phase at a temperature of about 197 K (16). The γ phase is very similar to the β 62

phase in terms of the arrangement of the P4 molecules; both of them form distorted hexagon nets, and the subtle difference is the exact position of the P4 molecules. Okudera et al. showed that the γ to β phase transition can be achieved by sliding the hexagon stacks halfway along the quasi-triangle edge oriented parallel the [101] direction in the γ monoclinic cell corresponding to the [211] direction in the β triclinic phase (16). Low dimensional molecular structures (0D, white-P) and polymer structures (1D, phosphorus chains) are considered the building blocks for other allotropes as discussed later in this chapter.

Structure and Properties of Red Phosphorus By controllably heating white-P, up to six different phases of red phosphorus (red-P) can be produced. Many of these phases are either amorphous or poorly cystallized (17–19). The commercially available red-P is amorphous, and its structure is rather challenged to be determined. Recently, by combining several different experimental characterization tools and quantum mechanical calculations, Zhang et al. showed that the amorphous red-P is very likely a linear inorganic polymer with a broad molecular weight distribution; the average length of the zig-zag ladder-type chain is about 100 nm (20). Hittorf’s (or violet) phosphorus (violet-P) and the fibrous red-P are two crystallized forms of red-P. The violet-P structurally consist of layered pentagonal tubes (21). The building units are closely related to the fibrous red-P although these two are linked together in different configruations (8). Both crystal structures consist of tube-like structures created by subunits of P8 and P9 phosphorus cages and dumbbells of P2 (two P atoms) as shown in Figure 1. The tubes can be viewed as zigzag double chains forming a corrugated ladder, which is similar to the aforementioned building block for amorphous red-P. In the violet-P monolithic space, each atom formed by layers of crosssectional pentagon tubes (19). A structural analysis obtained by Thurn and Krebs showed a density of 2.36 g cm-3 for an unit cell containing 84 atoms (19).

Figure 1. Comparison between the violet and fibrous red phosphorus. a) Building units of the violet and fibrous phosphorus (22). b) Interconnecting tubes of fibrous phosphorus (left) versus violet phosphorus (right). Reproduced with permission from (22). Copyright 2005 Wiley. Red-P structure consists of a polymeric network that contains various arranged units. The edge of fibrous red-P platelets shares crystals with violet-P that barely can set apart by either color saturation or plate-like shape. That aside, fibrous red-P crystals split into fine filaments following 63

mechanical stress while smaller fragments of crystal platelets are resultant of the split in the case of violet-P (22). Thermodynamically, these two red-P crystal structures have very similar energies, less than 3 meV per atom in calculations (22). The difference between the violet-P and the fibrous red-P is the orientation between the interconnected tubes. That is, in the violet-P, the tubes are perpendicular to each other forming interpenetrating tube systems while in fibrous red-P, the tubes are parallel with each other (22). Note the twisted structure in the violet-P lowers the energy of the lone-pair electrons located at the bridge atoms that connect the perpendicular tubes (between P(21) and P(21’) in Figure 1b), resulting in a downshifted molecular frontier orbitals and a shortened P-P bond length in the violet-P, which is about 217 pm in violet-P, shorter than the 222 pm in fibrous red-P (22).

Figure 2. High-resolution electron micrograph for a) Violet phosphorus b) fibrous phosphorus. Reproduced with permission from (22). Copyright 2005 Wiley. The two structures can be distinguished from each other under a high-resolution electron microscope, as presented in Figure 2. The strands in violet-P are perpendicular to one another, whereas parallel to each other in those of fibrous phosphorus as displayed in Figure 2b. The violet and fibrous red phosphorus exhibit comparable packing density of tubes. Both allotropes have equally estimated densities of 2.37 g/cm3. In comparison, the lowest van der Waals gap in violet-P holds a value of 306 pm, whereas in fibrous red-P, the distance is slightly higher, at 315 pm, implying the response to the different stacking arrangements and the dispersion of phosphorus filaments. The red-P is less reactive than the white-P. This allotrope is commercially prepared by heating white-P up to 400°C or by the ultraviolet radiation of white-P (23). The violet-P crystals appeared in the chemical vapor transport reactions to grow black-P from commercially available amorphous redP, indicating the violet-P may play a role in the phase change between red-P and black-P (24). This work also showed that the transition from violet-P to black-P at low temperatures (475°C) could be mostly driven by the entropy contribution (24). Calculation of the energy that is needed to exfoliate violet-P into single layers showed a value around 0.35 J/m2, which is smaller than the value of black-P and similar to the value of separating graphite into graphene (25). Calculation showed that monolayer violet-P has a direct band gap of about 2.5 eV using a hybrid functional and corrected van der Waals interaction; this band gap value is larger than the value of 1.5 eV for the monolayer black-P in the same type of calculation. In addition, the band gap is not very sensitive to the thickness for violet-P, which is also different from the thickness-dependent black-P as discussed later in this chapter (25). Due to its large theoretical capacity and commercial availability, red-P has been considered a promising material for application in lithium and sodium batteries. Because of its low electronic 64

conductivity and a significant volume change during Li+ and Na+ insertion and extraction, active research of combing amorphous red-P with porous carbon and carbon nanotube has been conducted (26–28). Note the chemistry behind is probably very similar to another active area of using sulfur as the electrode material where similar problems (poor conductivity and volume expansion) and approaches have been applied (29–31). In addition, due to its biocompatibility and efficient photothermal ability, amorphous red-P has also been used for treatment of bone-implant-associated infection (32).

Structure and Properties of Black Phosphorus Black-P was discovered last in the phosphorus family though it is thermodynamically more stable than the white and red phosphorus. It is a member of layered materials or 2D crystalline atoms, and its non-planar layered structure distinguishes itself from other popular 2D materials such as graphene, hexagonal boron nitride, and transition metal dichalcogenides (TMDCs) (33). Because of the weak van der Waals interaction, monolayer or few layers of black-P may be exfoliated from the bulk phosphorus; the single layer may be referred as phosphorene, as an analogy to graphene.

Figure 3. Atomic structure of (a) multi-layer and (b) monolayer black-P (34). Reproduced with permission from (34). Copyright 2015 Royal Society of Chemistry. Motivated by the study of graphene and TMDCs, black-P is revisited from the perspective of 2D material, different from its early studies as a bulk material. This monolayer allotrope is characterized by its high carrier mobility and thickness-dependent band gap, and has potential applications in batteries, optoelectronics, and transistors (33, 35–41). Meanwhile, the structure of monolayer blackP is highly anisotropic; the x and y directions of the black-P unit cell correspond to the armchair and zigzag direction, respectively, as shown in Figure 3. This in-plane structural anisotropy is quite unique in the family of 2D materials and leads to unique electronic and optical properties (34, 42, 43). Black-P can exist in different crystalline structures, including simple cubic, orthorhombic, and rhombohedral structures (45). The orthorhombic structure is the most stable one of black-P at room temperature. As a result of its thermostability, the orthorhombic phase is incombustible and insoluble in most solvents. The rhombohedral, arsenic allotropes structure can form when high pressure is applied to the orthorhombic phase. The rhombohedral structures, which consists of sixmembered layers, is packed similarly to the cyclohexane chair arrangement, resulting in a structure that mimics puckered graphite structures (46). Trigonal structure is arsenic, a gray type of black-P that consists of crystal structured rings with chair conformation of P6 rings (8). 65

Figure 4. Lattice and electronic structures of bulk black-P. (a) Crystal structure of bulk black-P, (b) Brillouin zone path of the primitive cell, (c) Electronic band structure with the HSE06 hybrid functional (red solid) and the mBJ potential (blue dashed). The effect masses along different directions are indicated (44). Reprinted by permission from Springer Nature: Springer Nature, Nature Communications (ref (44)), Copyright 2014. The orthorhombic black-P comprises of few layers constrained by van der Waals interaction, where each atom within a single layer is covalently bonded together with three adjacent P atoms constructing “puckered honeycomb” structure (Figure 4). Note in this buckled structure there are two different kinds of P-P bonds: the shorter one is 222 pm bonding the nearest P atoms in the same plane while the longer one, the length of which is 224 pm, connects P atoms between different planes (43). Note the configuration of P valence electrons is 3s2 3p3; three of these valence electrons of each P are shared to construct the three covalent bonds and the remaining lone pairs of electrons point out of the crystal plane. This particular configuration leads to its structural degradation and possibilities for chemical functionalization based on Lewis acid-base pairs as will be discussed later in the text (47).

Figure 5. Band Structure of Phosphorene (42). Reprinted by permission from Springer Nature: Springer Nature, Nature Reviews Materials (ref (42)), Copyright 2016. The electronic property of monolayer black-P is sensitive to atomic defects (6, 48, 49), as in other 2D materials (50). Here we focus the discussion on the pristine black-P. Black-P presents in the electronics and energy markets as an auspicious raw material because of its high charge mobility and a direct band gap of 0.3 to 2.0 eV dependent on its thickness (7). Monolayer black-P has an direct band gap of ~ 2 eV (51). Though the band gap reduces to 0.3 eV in the bulk (Figure 4), the feature of the direct band gap persists when adding additional layers in few-layer black-P and in the bulk (Figure 5), which is quite different from TMDCs such as MoS2. The latter ones show a transition from direct to indirect band gap when adding adjacent layers to the monolayer (52). The band gap of black-P can be tuned by an external electric field due to the total potential difference across the think film of black-P when the thickness is large enough (53). For example, the band gap of a 10-nm-thick black-P can be 66

continuously tuned from ~ 300 to below 50 meV using a displacement field up to 1.1 V/nm (53). In addition, mechanical strain can also be adopted to tune the band gap of black-P (54). Similar to other low-dimensional materials, the exciton in monolayer and few-layer black-P possesses large exciton binding energies, the calculated value of which is about 800 meV in monolayer balck-P (51), similar to the computational and experimental value in monolayer MoS2 (52, 55). Based on the polarization-resolved photoluminescence measurements at room temperature, the exciton binding energy has been estimated to be around 0.9 eV, which is similar to the aforementioned calculations (56). The direct band gap in monolayer and few-layer black-P is valuable for optoelectronic applications because optical excitation and radiative recombination can occur without involvement of phonons. Black-P’s properties can be compared with the two primary layered materials potentially used in electronics - graphene and TMDCs. The presence of zero bandgap in graphene leads to low current switch ratio, different from the high current on/off ratio in TMDCs, the value of which could reach ~108 in single crystal MoS2 samples (57). The carrier mobility in graphene is much higher than it in TMDCs. Black-P thin film features a narrower band gap than MoS2, which sets a spot between more significant band gaps in most TMDCs and zero bandgap in graphene. The reported on/off ratio of black-P varies from 104 to 107 at room temperature; this large variation may be caused by different thickness and orientation of the black-P samples (58). Furthermore, the encapsulated black-P transistors displays hole mobility over 5,000 cm2 V-1s1 in vacuum at room temperature, which is limited by phonon scattering, and FET mobility up to 45,000 cm2 V-1s-1 at cryogenic temperatures (59, 60). Recent reports also showed improved electron mobility over 1,000 cm2 V-1s-1 in electron-doped black-P transistors by hBN encapsulation (61, 62). These numbers are comparable with supported graphene samples (42). As discussed above, the anisotropic structure of black-P results in anisotropic in-plane electronic, optical and thermal properties. For example, calculations suggested that the carrier effective masses along the G-X direction in the Brillouin zone are 0.15 m0 (hole) and 0.17 m0 (electron), which are slightly larger than those in bulk black-P. In contrast, the hole effective mass along the G-Y direction shows a strongly layer-dependence, decreasing from 6.35 m0 in the monolayer to 1.81 m0 in the bilayer and to 0.71 m0 in the bulk (44). In a phonon-limited scattering model, based on the calculated effective mass along different directions and in the deformation potentials, a large anisotropic carrier mobility can be predicted; Across all doping levels, the conductance along the armchair direction is about one order of magnitude higher than it along the zigzag direction (63). Note this anisotropic carrier mobility can in principle be controlled by using mechanical strains as predicted by first-principles calculations (64). The thermal conductivity is also anisotropic, and the ratio between zigzag and armchair direction is about 2 for thick black-P films (thicker than 15 nm) and decreases to ~1.5 for thinner films (65). Thermal measurement by micro-Raman spectroscopy showed that the thermal conductivity along the zigzag and armchair in very thin-film black-P could be about 18 and 12 Wm1K-1, respectively (42). In other words, thermal conductivity is higher along the zigzag direction (65, 66). Note the exact number of the thermal conductivities is sensitive to the thickness of the samples, and as thickness of the film reduces, the thermal conductance decreases (65). Because of this difference in preferred directions for thermal and charge conduction, which are orthogonal in monolayer black-P, it is predicted to be an excellent candidate as a thermoelectric material (67). 67

As another consequence of the anisotropic structure, calculations suggest that, for photon energy close to the bandgap energy, only light with a component of the polarization along one of the crystal direction – the armchair direction – can be absorbed, while monolayer and few-layer black-P are almost transparent to zigzag-polarized light in the same energy range (44, 51, 68). This anisotropic optical property can thus be used to determine the direction of the crystallographic directions and find applications in liquid crystal displays and optical quantum computers. The optical absorption spectra of multilayer black-P is very sensitive to the thickness (69). The main challenge in using black-P for broader applications is the fast degradation as a result of the lone pair atoms on the surface and their reactions with oxygen (70). Monolayer black-P exhibits graphene-like properties; however, chemical species used in graphene may not be sufficient for functionalization of phosphorene due to the different electronic configurations of carbon and phosphorus (71), which have valence electrons in configurations of 2s22p2 in carbon and 3s23p3 in phosphorus, respectively.

Figure 6. Functionalization of monolayer black-P using a nitrene moiety (47). Reproduced with permission from (47). Copyright 2018 American Chemical Society. Chemical modification of phosphorene naturally takes place whenever exposed to O2 at any concentration level (71, 72). Imide-functionalization requires a significant concentration of N2H2 to arise (71). Covalent functionalization may increase the chemical stability of the black-P (73), but also normally results in defect levels in the samples, which may hinder its intrinsic transport properties (74). Studies have shown that chemical degradation of exfoliated black-P is slowed down because of the covalent functionalization and remain stable for weeks and thereupon boost the potential semiconducting properties. Functionalization using van der Waals interaction through polymers has also been used, which meanwhile gains extra features such as dispersibility, higher thermal stability, and increment chemical activity without altering much the carrier mobility (74). We recently reported stable functionalization of the surface of black-P based on the Lewis acidbase chemistry (47). We showed that chemcial modification using nitrene-derived species introduces a strong P-N dative bond at the black-P interface without changing its intrinsic electronic structure. That is, the direct band gap persists. The Lewis basic P atom attacks the Lewis acidic nitrene, through a free pair of electrons, as shown in Figure 6. By modifying the nitrene using different electron

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donating and withdrawing groups, we showed that the work function of black-P could be tuned by more than 2 eV (47). As mentioned above, black-P under high pressures can be converted to an arsenic type A7 phase, so-called blue-P, and a simple cubic phase (75). This A7 phase was recently studied by first-principles simulations (33). Figure 7 shows the atomic structure of blue-P and compares it with the black-P. Though blue and black phosphorus share in-plane hexagonal structure, which is similar to graphene from the top view, both phosphorus structures distinguish themselves from graphene in the course of the bulk of layer stacking and semiconducting properties. Both black-P and blue-P are different from graphene because of their buckled atomic structures. The side view of black-P and blue-P clearly shows their difference, that is, the cross section of black-P shows the armchair structure while blue-P has a puckered zigzag structure.

Figure 7. (a) Top and side view of Black-P and (b) Blue-P structures (c) dislocations that part of converting from Black-P to Blue-P (d) A-B stacks of Blue-P structure at equilibrium (33). Reproduced with permission from (33). Copyright 2014 American Physical Society. Zhu et al. found the armchair-ridge structure in black-P is disparate from the puckered zigzag shape found the blue type (33). The covalent bonds between blue-P atoms is 2.27 Å, and this bond length is very close to the ones in black-P as discussed above. The P-P bond in blue-P has a large binding energy of 5.19 eV/atom; this value is slightly different from black-P by ~2 meV/ atom indicating both of them are equally thermal stable. The interlayer interaction in blue-P is 6 meV/atom with a distance of 5.63 Å. Note these values are obtained from density functional theory (DFT) calculations, and it is a known challenge for DFT to calculate accurately the van der Waals interactions (76). In the process of structural transformation from black-P to blue-P, a dislocation occurs without adjusting the bond angles during the flipping P atoms from down to up spot along the monolayer of black-P with an activation barrier of 0.47 eV/atom. Note this value is very likely overestimated due to the constrain introduced in the calculations (33). The indirect band gap of 2 eV calculated for blue-P exhibits the semiconductor applicability. The value of the band gap (between 1.2 to 2.0 eV) is inversely proportional to the number of layers. Altering the thickness of the slab varies the band gap value by nearly double the amount situated at an independent number of stacking layers. Note DFT calculation predicted that the indirect band gap could be switched to a direct band gap under a transverse electric field above 2 V/nm (77).

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Similar to black-P and many other 2D materials, mechanical strain can be used to tune the band gap. In-plane strain range −10% < σ < +10% leads to a significant change of the band gap. A similar conclusion has been reached in other reports (78, 79). Altering this band gap by up to 50% within this non-planar structure shows alluring value in nanoelectronics and offers flexibility in adjusting the material thickness in the industry of electronics (33, 80, 81). As mentioned, the blue-P can be achieved by converting black-P at high pressures and may not be easily obtained in experiments. Recently, molecular beam epitaxial growth of single layer blueP has been achieved on an Au (111) substrate using black-P as the precursor (82). The scanning tunneling spectroscopy (STS) measurement shows a band gap of 1.1 eV of epitaxial blue-P on Au (111). This difference between the STS measurement and the aforementioned calculations may result from the lattice strain induced in the epitaxial growth and the interfacial interaction between blue-P and the metal substrate. Similar interfacial interaction has been shown to cause significant modulation of the electronic properties of other 2D materials such as graphene and hexagonal boron nitride (83–88). To reduce such an interfacial reaction, Zhang et al. prepared blue-P on a tellurium monolayer functionalized Au(111) surface to achieve quasi-free-standing monolayer blue-P (89), though the detailed electronic properties have not yet been reported. Other applications of blue-P, such as application in lithium ion batteries and construction of a pn heterostructure using blue- and black-P (90, 91), have been proposed but experimental validation has not yet been reported. Though it is not the focus of this chapter, we would like to note that large-scale synthesis of black-P with controllable thickness is still a challenge. Black-P may be prepared from red-P through sequential heating and cooling (7, 35). In addition, black-P single crystals can be prepared from a solution of white-P in liquid bismuth, but it is unlikely that the thickness can be controlled (92). In fact, more than hundred years ago, Bridgman reported preparation of black-P from white-P at a moderate temperature under a high pressure (14). Liquid exfoliation of black-P to produce fewlayer and monolayer samples with high quality of crystal structures have also been reported both in simulations and experiments (93–95). It is very valuable to develop methods that can prepare singlecrystal thin films of black-P at the wafer scale with controllable thickness, which is important to explore its unique electronic, optical and thermal properties. When talking about the preparation and application of phosphorus, we also need to pay attention to its original mineral resources. Developing sustainable phosphorus supply to its application in industry is critical. Here we briefly overview the cycle of phosphorus in nature and human’s role in this process. Phosphorus is mainly cycling through water, soil and sediments. Different from many other biogeochemical cycles, the atmosphere does not play a major role in cycling phosphorus. Phosphine (PH3) is the only known gaseous form of the phosphorus compound that can be produced with low concentration under certain conditions (2). Phosphorus in the atmosphere usually exists with dust particles; its global cycle is restricted to the solid and liquid phases. In the natural phosphorus cycle, most phosphates come from phosphate rock, such as calcium phosphate, or in ocean sediments. Phosphate ions released from rocks by erosion and rain make the way into soil, from which organic matter, such as plants and fungi, can absorb phosphorus and use it in cellular processes. The phosphorus in the organic matter are then consumed by humans and animals. After animals and plants die, phosphates return to the environment driven by bacterial decomposition. Eventually, phosphorus accumulates in sediments or rock formations again to close the cycle. This massive natural biogeochemical cycle of phosphorus is slow, probably taking millions of years to complete; the particularly slow process in the phosphorus cycle is moving it though the 70

soil and ocean. The slow cycle of phosphorus thus limits the natural supply of phosphorus. Most phosphorus products are currently for agriculture application, and human activities have significantly impacted phosphorus cycles by directly applying phosphate fertilizer on farmland extensively to support the rapidly growing world population. Plants then absorb phosphorus from the soil and convert the inorganic phosphorus to organic phosphorus in cellular processes. Decomposition of organic phosphorus plays an important role in the natural cycle of phosphorus. If these organic phosphorus can be converted back to the inorganic forms directly before accumulation in sediments, this process could in principle allow a long-term, controllable release of phosphorus (96). Indeed, recycling of organic wastes, such as animal wastes, in many traditional agricultural systems remains a valuable approach. For example, incineration is one of the methods adopted to treat the oversupplied livestock and poultry in areas with intensive farming. Other chemical technologies can be developed for removal of phosphorus from these animal wastes to recover phosphorus-containing salts as discussed below. Microorganisms may also be leveraged to facilitate the cycle of environmental phosphorus for its sustainable use. For example, some types of bacteria can be used to increase the solubility of the insoluble forms of phosphorus (97). Note the low solubility of some phosphorus salts that slowly accumulate in sediments in oceans are normally inaccessible to plans. Photodegradation of organic phosphorus could be an alternate sustainable approach for converting organic phosphorus into its inorganic forms. Previous works showed that phosphate could be generated when the organic phosphorus is continuously exposed to light mediated by a photosensitizer (98). The amount and rate of photodecomposition of organic phosphorus depend on the light sources and solution composition (96). It is not clear if this photodegradation process is cost competitive. Pyrolysis could also be implemented to promote the cycling of organic phosphorus in animal manure into biochar that can be further used for fertilizers. It is also desirable to catalytically convert organic phosphorus from waste water into inorganic forms, for effective recapture of phosphorus. This approach may allow remotely extracting and converting phosphorus in distributed infrastructure, thus reducing the cost for transportation of water, which is costly, to centralized facilities. Furthermore, carbothermal reduction of low-grade phosphate to phosphorus is another approach with promising prospect for the sustainable use of phosphorus (99).

Conclusion Here we overview different phases of phosphorus ranging from the most active white phosphorus, to amorphous and crystalline red phosphorus, and to the most stable black phosphorus with an emphasis on the relationship between these atomic structures and the physical properties. Many optoelectronic properties of various phases of phosphorus have been explored in the literature, and many of these properties are sensitive to the precise atomic arrangement and thickness; controllable synthesis of phosphorus with targeted structural characteristics will be crucial and remains a challenge. Developing sustainable phosphorus supply is valuable not only to its application in industry but also to the farms.

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Chapter 5

Phosphorus Clusters and Quantum Dots Xianglei Kong,1,2,* Lei Mu,1 Ming Zhou,1,3 and Shumei Yang1 1The State Key Laboratory of Elemento-organic Chemistry, College of Chemistry,

Nankai University, Tianjin 300071, China 2Collaborative Innovation Center of Chemical Science and Engineering,

Nankai University, Tianjin 300071, China 3Department of Physics, Anhui Normal University, Wuhu 241000, China *E-mail: [email protected].

Cluster is characterized by its unique role in chemistry and physics due to the fact that it bridges isolated atoms and bulk counterparts. It is also found that the structures and properties of clusters change dramatically with their sizes. Characterized by its diverse bonding patterns, phosphorus clusters have attracted many interests in past years. With the help of laser ablation mass spectrometry and other relative technologies, many kinds of phosphorus clusters have been generated, identified and studied. Their structures and properties are also widely studied by theoretical calculations. This chapter reviews both experimental and theoretical progresses about phosphorus clusters and relative species. On the other hand, the phosphorus quantum dots have attracted a great deal of attention due to its unique structures, properties, and applications in recent years. A quick overview on this topic is also included in the chapter, in order to attract more interests from broad research fields and make a bridge across the cluster study and nanomaterials to benefit scientists in both fields.

Introduction As a very interesting and important form of matter that bridges isolated atoms and bulk counterparts in the condensed phase, clusters are characterized by the fact that their structures, stabilities, and other properties change nonmonotonically with their sizes (1–5). Since it is well known that phosphorus is characterized by its diverse bonding patterns (6–10), a better understanding about phosphorus clusters will greatly deepen our knowledge about its relative materials. Fortunately, these clusters can be generated by a process of laser evaporation or ablation and detected and further studied by mass spectrometry, photoelectron spectroscopy, photoelectron detachment experiments and other experimental technologies. Advances in laser technology have opened up new possibilities in laser–matter interaction and nanomaterials processing. The © 2019 American Chemical Society

interaction between the laser and solid materials might be highly nonequilibrium in some cases, and lots of new species, including neutral clusters, cluster anions and cations can be generated and detected by a mass spectrometer. A typical experimental set up for laser ablation mass spectrometry can be found in the wonderful paper about fullerenes by Smalley et al in 1985 (4). Based on these technologies and the rapid development of computational chemistry, the variety of structural phases of phosphorus has also stimulated both experimental and theoretical researches in phosphorusrelated clusters in the past 40 years. On the other hand, the zero-dimensional phosphorus quantum dots, which can be thought as clusters with very large sizes, have attracted a great deal of attention due to its unique structures, properties, and applications (11, 12). The design of these novel materials has now emerged as a promising way to deal with some basic problems. The advantages of these new materials, such as high surface areas, variable diameters and functionalities, endowed them great performance in energy, catalysis, and environmental applications (12). And the aforementioned study on the relative clusters can provide us more fundamental aspects about their structures, compositions and properties at the molecular level. Thus, in the second half of this chapter, we will have a quick review on the exciting nanomaterials, and some progresses about the laser ablation method in this field will be summarized and discussed.

Phosphorus Cluster

Figure 1. Mass spectra of phosphorus cluster cations obtained by laser ablation of RP in the m/z ranges of (a) 0-3000 and (b) 3000-10,000. Parts of the spectra are further shown in the respective insets. Reproduced with permission from (18). Copyright 2019 Acta Physico-Chimica Sinica. Phosphorus Cluster Cations Cationic phosphorus clusters of were firstly studied by Martin et al. Phosphorus clusters of (n 145, the superiority of n = 8k+1 ions becomes insignificant. Although odd-numbered cluster ions are much abundant than even-numbered ions, the difference of intensity between odd- and even-numbered cluster ions decreases with increasing cluster sizes (18).

Figure 2. CID mass spectra of selected ions obtained at relatively low collisional energies: (a) , (b) , (c) , (d) , (e) (f) (g) and (h) . Reproduced with permission from (19). Copyright 2015 John Wiley & Sons. Collision-induced dissociation (CID) mass spectra of some phosphorus cluster cations (4 ≤n ≤ 25) were also reported (16). Based on their experimental results, Huang et al. suggested that dissociation processes of cations were characterized by two main fragmentation pathways. One is to loss stable daughter ions including and , the other is the ejection of P4 fragment. Using a high-resolution Fourier transform ion cyclotron resonance (FT ICR) mass spectrometer, Mu et al. had systematical studied the CID behaviors of cationic clusters 81

(m=6-13) (19). Figure

2 shows some of the results. For and , only fragment ions of were detected under relatively low activation energies. At higher collision energies, corresponding fragment ions formed by sequential loss can be detected. All fragment ions observed clearly indicate that the main dissociation channels of phosphorus cluster ions of (m = 6-11) are derived from the loss of P4 units. Some even-numbered cluster ions were also isolated and performed CID experiments. Figure 2g and Figure 2h shows the results of and , respectively. Similarly, both dissociation pathways are characterized by loss of P4 unit, and no product ions from other dissociation pathways were observed. Theoretical calculations were also preformed for by several groups (20–28). Recently, Xue et al. suggested the lowest-energy structures of the odd-numbered cationic phosphorus clusters up to 25 atoms by the combination of simulated annealing and density functional theory (DFT) methods (28). These structures are shown in Figure 3.

Figure 3. Lowest-energy configurations of odd-numbered cluster cations of (m= 1-12). Reproduced with permission from (28). Copyright 2010 Elsevier. For the magic clusters in which the number of phosphorus atoms can be described by 8k+1 (k=3-8), CID experiments were also applied, and the results are shown in Figure 4. As the first ion in this list (k=3), shows a very similar spectrum to those of even-numbered cluster ions with smaller sizes shown in Figure 3. Its primary dissociation channel is also characterized by the loss of P4 unit. By increasing the collisional energy applied, fragments ions by the sequential loss of P4 unit, including and , were clearly identified (Figure 4a). However, for , the second ion in the 8k+1 list, its CID mass spectrum is much more complex (Figure 4b). Besides the fragment ions formed by the sequential loss of P4, other fragments, such as

and

character in its CID mass spectrum is that the signal of fragment ion

82

, were also identified. Another is much stronger than others,

,

suggesting a possible new dissociation pathway characterized by the direct loss of P8 unit. For

, which was formed via a loss of P2 unit (Figure 4c).

the largest fragment ions were found to be

, which was most likely formed by the direct

The most dominate fragment ion observed here is

dissociation via a loss of P8 unit. The CID mass spectrum of

in the lower mass region is very close

, indicating similar mechanism for the generation of these fragment ions. Similarly, the

to that of

primary dissociation pathway of is the loss of P8 unit. However, a careful comparison between Figure 4c and Figure 4d in the same m/z region of 200-1300 shows some difference. One is that the ion of

in Figure 4d is much more abundant than that in Figure 4c; the other is that some fragment , did not emerge in the CID mass spectrum of

ions identified in Figure 4d, such as

and

These facts suggest that most of the

ions observed in Figure 4d should be products of

direct loss of P10 unit, instead of products of and observed for

should be product ions of (Figure 4e). The ions of

. by a

by the loss of P2 unit. And the fragment ions of

by the sequential loss of P4 unit. Similar results were also and

were generated from the precursor ions of

by the loss of P4 and P8 unit, respectively. The strong signal of should be mainly product of by loss of P10 unit. The three pathways characterized by the loss of P4, P8 and P10 units can also be used to explain the CID results of

(Figure 4f).

83

Figure 4. CID mass spectra of selected ions obtained at relatively high collisional energies: (a) , (b) , (c) , (d) , (e) and (f) . Reproduced with permission from (19). Copyright 2015 John Wiley & Sons.

84

Phosphorus Cluster Anions

Figure 5. Mass spectra of phosphorus cluster anions obtained by laser ablation of RP in the m/z ranges of (a) 0-3000 and (b) 3000-10,000. Parts of the spectra are further shown in the respective insets. Reproduced with permission from (18). Copyright 2019 Acta Physico-Chimica Sinica.

Figure 6. Mass spectrum of phosphorus cluster anions obtained by laser ablation of RP in the m/z range of 3000–16000, A portion of the spectrum is shown in the inset. The experiment was performed on the FT ICR MS. Reproduced with permission from (18). Copyright 2019 Acta Physico-Chimica Sinica.

85

In their laser vaporization experiments, Huang et al. also reported mass spectra of anionic phosphorus clusters, as shown in Figure 5 (25). It is found that the difference between oddnumbered and even-numbered cluster ions was significant for anions up to and the signal from even-numbered cluster ions with n>25 was too weak to be detected in the experiments (29). For large cluster ions of (n ≥ 25), those ions with n = 8k+1 (k ≥ 3) also showed greater intensity than their neighbors. Bulgakov et al. also studied different phosphorus clusters across a wide size range using mass spectra and found similar results (30–32). Kong studied these cluster anions in a wider m/z region. It is found that the previously reported 8k+1 rule was found to be significant and insignificant for cluster ions with k18, respectively (Figure 6) (33). And very large phosphorus cluster anions up to n~500 were observed by using a FT ICR mass spectrometer. It is also suggested that the stability difference between odd- and even-numbered cluster ions decreased with increasing cluster size. For both cations and anions, the observed intensity alternation of even/ odd-numbered clusters may vanish for cluster ions with n > 1000. Jones et al. used the pulsed arc cluster ion source to generate the clusters, and photoelectron detachment measurements had been performed on phosphorus cluster anions (34). Figure 7 shows some of these results. These photoelectron spectra were used to determine the energies required for electron detachment in occupied orbitals for the clusters, providing experimental basis for understanding their molecular structures and orbitals. The CID mass spectra of phosphorus cluster anions (3 ≤ m ≤ 20) have been investigated by Yang et al. with their FT ICR mass spectrometer (33). Results showed that the fragmentation pathways of the phosphorus cluster anions are very different from those of cation ions. The CID mass spectra of these anions under low and high collisional energies are shown in Figure 8 and Figure 9, respectively. As shown in Figure 8a, was the only fragment ions from the precursor ions of detected at low-collisional energy. When the collisional energy increased (Figure 9a), was also observed, which could be thought of as fragment ions from through the sequential loss of the P2 unit. For anions of , the results were quite similar (Figure 8b,9b). However, for , the CID mass spectra are different (Figure 8c,9c). Only fragment ions of could be detected under low-collisional energy conditions, and no anions of was observed. Despite this, weak signals for could still be observed under a higher collisional energy.

86

Figure 7. Photoelectron spectra of -clusters (n=2-9) recorded at photo energy = 3.49 eV (left) and = 4.66 eV (right). Reproduced with permission from (34). Copyright 1931 American Institute of Physics.

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Figure 8. CID mass spectra of selected ions obtained at relatively low collisional energies: (a) , (b) , (c) , (d) , (e) , (f) (g) , (h) and (i) . Reproduced with permission from (33). Copyright 2016 Elsevier.

Figure 9. CID mass spectra of selected ions obtained at relatively high collisional energies: (a) , (b) , (c) , (d) , (e) , (f) (g) , (h) and (i) . Reproduced with permission from (33). Copyright 2016 Elsevier. 88

Both fragment ions of and were detected in the CID mass spectrum of obtained under low-collisional energy (Figure 8d). With a higher collisional energy, fragment ions of were also observed (Figure 9d), but the signal of was much stronger than those of the others. It may be suggested that the ions of is more stable than other anions. However, careful inspection of the distribution of cluster anions across the whole spectrum shows that the intensity of is, in fact, weaker than that of , and . The results suggest two parallel dissociation channels of , which are characterized by the loss P2 or P6 unit, respectively. For anions of , and , only fragment ions of were observed under low-collisional energy (Figure 8e-8i), indicating that their primary dissociation channels are all characterized by the loss of a P2 unit. And fragments characterized by sequential loss of unit were observed under high-collisional energies for all these anions (Figure 9e-9i). Theoretical calculations were also performed for -clusters (m= 1-9) (33–35). As shown in Figure 10, most of structures of anions are unlike those of their counterparts of cations. The dissociation channels characterized by unit loss calculated based on these structures are more energetically favorable for (3 ≤ m ≤ 9), which is consistent with the experimental results.

Figure 10. Lowest-energy configurations of odd-numbered cluster anions of (m= 1-9). Reproduced with permission from (33). Copyright 2016 Elsevier. However, the dissociation pathways of are very complicated. For , the first ion in the 8k+1 list (with k = 3), its dissociation process is different from those of cluster anions with smaller sizes. As shown in Figure 11a, its primary dissociation was characterized by the loss of unit. Under high-collisional energy, fragments of (n = 6-11), resulting from the sequential loss of unit, were clearly identified (Figure 11b). It was also found the fragmental ions of to be 89

always more abundant than other ion fragments under different collisional energies, indicating the existence of direct loss. However, CID mass spectra of are quite different. With low-collisional energy, only fragment ions of were observed (Figure 11c), indicating the main dissociation channel of is the loss of , not the loss of . Under the high collisional energy, other fragment ions of , and were also observed (Figure 11d). Among them, the ions of and are formed via the dissociation of , and the ions of are more likely formed via the direct dissociation of through the loss of . For , the dissociation channels are more complex (Figure 11e). The largest fragment ion was found to be , which was formed through the dissociation of precursor ions via the loss of . Other fragment ions of , and were also observed under a high-collisional energy, which are likely formed by the sequential losing of or unit.

Figure 11. CID mass spectra of some selected anions, which were obtained at different values of ’s: (a) = 0.15 V; (b) = 0.4 V; (c) = 0.3 V; (d) = 0.5 V; (e) = 0.225 V; and (f) = 0.5 V; The frequency offset is set to be 100 Hz relative to cyclotron frequencies of corresponding precursor ions. Reproduced with permission from (33). Copyright 2016 Elsevier.

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Neutral Phosphorus Clusters

Figure 12. Lowest energy structures and their corresponding isomers for (n = 1–14) clusters. For each size, the lowest-energy isomers are reported in bold character. Reproduced with permission from (41). Copyright 2017 Elsevier. Unlike the cations and anions, neutral clusters cannot be detected by mass spectrometry directly. But theoretical calculations about neutral phosphorus clusters have been investigated by different 91

method for a long time (35–41). For example, small phosphorus clusters of (n = 1–6) have been investigated by Wang et al. . It is found that the two clusters of and had nonplanar structures, which were different from those of their anionic forms (36). For , Chen et al. have found that the nonplanar cage-shape structure is more stable than planar ones. Guo et al. had investigated groundstate structures of neutral phosphorus clusters of (n = 2-15), and compared them with those of their cationic and anionic forms (40). It is found that unlike those cationic and anionic clusters, the most stable neutral clusters are even-numbered. Recently, a systematic theoretical investigation of (n=1-15) were also performed by Mahtout et al.. The most stable structures and some other isomers of these clusters are shown in Figure 12 (41).

Phosphorus Related Binary Clusters Metal Phosphide Clusters Metal phosphide clusters were also widely investigated by several research groups. Using a 1064 nm Nd: YAG laser and the mixture of Co metal and red phosphorus, Greenwood et al. have investigated cluster cations of (M=Ni, Co, y=2,4,6,8) and anions of (42). At least 110 anions have been identified, which are distributed through the range 2≤x≤16, 4≤y≤17. Among them, ions of (x=2-5), (x=5-7), (x=4-8) and (x=2-6,7) are the abundant ones. And the observed preponderance of even numbers of P atoms also indicates that structural units exist in these clusters. This suggestion is also supported by the CID experiments on the selected ions in the FT ICR mass spectrometry. Yi et al. generated binary anionic cobalt phosphide clusters up to by laser ablation of CoP. It is found that composition maps for the two experiments (by Greenwood and Yi, respectively) overlap, but with generally lesser y/x ratios in the case of CoP (43). In Yi’s study, the reactions of selected cluster ions with H2S, NO2 and N2O have also been investigated with a Fourier transform ion cyclotron resonance mass spectrometry. The results show that more reactive species undergo addition at those under-coordinated Co atoms. For cluster ions, Ju et al. have investigated the formation and stability of them using laser ablation (44, 45). Similarly, Han et al. have produced Mn/P, Ti/P binary cluster ions and studied them using tandem mass spectrometry (46). An odd–even variation in the ionic peak strength related to the number of P atoms in the mass spectrum is observed both series. It is also found that the units of P4 and P2 can be readily stripped by a process of laser photo-dissociation. For , both cation and anion clusters were produced by 532 nm laser ablation on a tablet of well-mixed chromium and red phosphorus powder by Han et al. (47, 48) Results show that the odd-even oscillation exists in the intensity of the series, in which mass peaks with even P atoms are higher than those with odd ones. The peaks of and are the most prominent ones in the mass spectra, indicating the existence of sub-structures. And some other intense peaks of were also identified in the mass spectra. In order to better understand these relative cluster ions, DFT calculations have been performed on geometrical structures and dissociation channels of some binary cluster ions. Kuang et al. have studied the ions of (M = Fe, Co or Ni; n = 2, 4, 6 or 8), and found that the lowest energy + structures of MP n cluster ions were constructed by bonding a twofold or fourfold M atom with 92

a or unit (49). Since the M–P bond is weaker than the P–P bond, the most likely dissociation channel of the MP n + cluster ions is the detachment of a or fragment, which is consistent with the experimental results. For (m = 2, 4, 6, 8) clusters, the suggested lowest energy structures have similar bonding characters (50). The same group also studied the geometrical structures and possible dissociation channels of (n = 2–8) cluster ions (51). The lowest energy structures of them can be regarded as the outcome of bonding between Mn atom and one or two units of or and . Similar calculation results for Ti/P binary cluster ions are also reported by the same group (52). Generation of gold phosphides via laser ablation of red phosphorus and nanogold mixtures was performed using laser desorption ionization TOF MS by Havel’s group (53). A variety of gold phosphide cluster ions have been observed, including (n =1, 2–88 (even numbers)), (n = 1-7, 14–16, 21–51 (odd numbers)), (n = 1-6, 8, 9, 14), (n = 19, 14–16), (n = 1-6, 14, 16), (n = 1-6), (n = 1-7), (n = 16, 8), (n = 1-10), (n = 1-8, 15), (n = 1-6), and (n = 1, 2, 4) in positive ion mode and (m = 1–5), (n = 2,3,5–11, 13–19, 21–35, 39, 41, 47, 49, 55 (odd numbers)), (n = 4–6, 8–26, 30–36 (even numbers), 48), (n = 2–5, 8, 11, 13, 15, 17), (n = 6–11, 32), (n = 1, 2, 4, 6, 10), , and in negative ion mode. These generated gold phosphides might inspire the synthesis of new Au-P materials with specific properties. Xu et al. investigated the clusters of (n = 1–8) using DFT methods (54). It is found that the phosphorus-doped gold clusters have different evolutionary paths from pure gold clusters. And those clusters with odd-numbered gold atoms prefer planar structures, with the others prefer 3D structures. The same group also studied the clusters of in different valence states. It has been found that the lowest energy structures for the cationic, neutral and anionic species of are also very different (55). Theoretical study of the electronic structures of , and preformed by Hoffmann’s group shows some very interesting results. It is found that these compounds contain a framework of condensed and rings forming parallel channels, which are filled by lead, thallium, or mercury atoms (56). Other Relative Clusters People have known that arsenic can substitute for phosphorus in various molecular structures for a long time. Thus, Havel’s group recently has generated 479 binary (m =1–6, n =1–200) clusters in the gas phase by the laser ablation method (57). In both positive and negative ion mode, only clusters with odd (m + n) values were detected by mass spectrometry. For oxygen, Martin et al. have studied the binary system of phosphorus and oxygen using the vapor of red phosphorus in the 1980s (13). The mass spectra were obtained by evaporating phosphorus into a 1 mbar He gas containing 1% O2. Interestingly, the mass peaks are all shown in bunches, which are separated by 16 mass units successively. These peaks can be identified as the oxide cluster cations of , and for each bunch, the value of 2n+m is equal to a given integer. It is also found that most of the oxide clusters observed in the experiments contain an odd number of phosphorus atoms. Havel et al. used laser ablation of phosphorus and sulfur mixture to generate a wide range of clusters with various P:S ratio (58). Clusters (m = 1–8, 11), (m = 2–11), (m =1–8), (m =1–6), 93

(m = 1–6), (m = 1–3), (m =1–2), (m = 1–2), (m = 1–2) and (m =1–2) were detected. Later by careful optimization of the laser energy, many new clusters not found in their previous work were further determined (59). By laser ablation of ground mixtures of red phosphorus powder and powders of C, Si, B, and Al, Binary phosphorus cluster ions of (n = 1–16, 18, 20–22, and 24–26 and m = 1–3 and 5), (n = 1–6 and m = 1–5, 7–9, 11, 13, 15, 17, 19, and 21), and (n = 1–3 and m = 2–22) have been observed in mass spectra by Liu et al. (60) Fisher et al. also produced a series of carbon–phosphide anions, (n = 3–9), (n = 3–9), , and (n = 4, 6), and investigated them theoretically (61). Allan et al. systematically studied carbon phosphides experimentally and carried out extensive ab initio studies about them (62–64). The two‐dimensional carbon phosphide compounds were also predicted by Fu et al. (65) Using a modified vaporization – condensation method, a red phosphorus–single‐walled carbon nanotube (RP‐SWCNT) composite was obtained by Zhu et al. (66) Recently, Havel’s group generated a wide range of carbon–phosphide ions using the precursor of graphene–red phosphorus (67). The detected ions include: (m = 3–47), (m = 2–44), (m = 1–42), (m = 1–39), (m = 1–37), (m = 1–34), (m = 1–31), (m = 1–29), (m = 1–26), (m = 1–24), (m = 1–21), and (m = 1–19). And when nanodiamond composites with red/black phosphorus were used in the experiments, cluster ions of (n = 0–28), (n = 0–16), and (n = 0–14) were observed. For other Group 14 (Si, Ge, Sn, and Pb)/P binary clusters, Liu et al. also studied them in both positive and negative ion modes by methods of laser ablation mass spectrometry. The relative intensity of is related to that of , indicating that the formation of can be thought the direct replacement of phosphorus atoms by silicon atoms. For , and , the formed small-sized binary cluster ions consist of two kinds of atoms almost with all possible compositions, while large-sized cluster ions contain metal atoms mainly (68).

Phosphorus Quantum Dots A Brief Overview As examples of special phosphorus clusters, phosphorus fullerenelike structures have been theoretically considered as the analogies to their carbon counterparts, and also as the zerodimensional materials. Haeser et al. investigated theses fullerenelike structures by Hartree–Fock (HF) method in 1992 (69). After that, other fullerenelike Pn clusters were also studied theoretically (70). With the rapid development of low-dimensional materials in the past 20 years, the zerodimensional nanomaterials of phosphorus quantum dots (PQDs), have been recently prepared by chemical methods and attracted lots of interests because of their unique electronic and structural properties. Due to their quantum confinement, edge effects and modifications on the surfaces, their electronic and optical properties of PQDs are quite different from those of bulk phosphorus. Based on these unique properties, some of them have been successfully used in opto-electronics, photovoltaic devices and biological analysis (11, 12). We will give a quick overview of the nanomaterials in this section. Some progresses about them based on laser ablation method will be discussed in the next section. 94

The black phosphorus quantum dots (BPQDs) were first synthesized by Zhang et al. by a facile top-down approach (11). In their experiments, BP powder was added to a mortar containing Nmethyl pyrrolid (NMP). After grounding, the mixture was transferred into a glass vial and then sonicated for three hours. The resultant dispersion was centrifuged to obtain the supernatant containing BPQDs. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) are used to characterize the obtained BPQDs. As shown in Figure 13, the obtained BPQDs have an average lateral size of 4.9 nm and thickness of 1.9 nm, corresponding to about 4 layers.

Figure 13. Morphology characterization of BPQDs. a) TEM image of BPQDs. b) Enlarged TEM image of BPQDs. c,d) HRTEM images of BPQDs with different lattice fringes. Scale bar=5 nm. e) Statistical analysis of the sizes of 200 BPQDs measured from TEM images. f) AFM image of BPQDs. g,h) Height profiles along the white lines in (f). i) Statistical analysis of the heights of 200 BPQDs measured by AFM. A morphology sketch of BPQD is shown as an inset in (a). Reproduced with permission from (11). Copyright 2015 John Wiley & Sons. After the first example, different synthetic methods for BPQDs were explored. These methods can be classified as: exfoliation (including ultrasonic and electrochemical ways), solvothermal treatment, blender breaking and milling, and laser ablation. Cui et al. have just summarized different synthetic methods and conditions for preparing BPQDs in their recent review papers (12). Some points should be emphasized here: 1) among those methods, the ultrasonic liquid-phase exfoliation is still the most widely used ones (12, 71–78); 2) different sonication methods, such as probe sonication or bath sonication, can have a great effect on the products, as well as the solvent used in the processes; (12, 78, 79), 3) comparing with liquid-phase exfoliation techniques, solvothermal treatment is more facile and controllable to produce small-sized BPQDs on a large scale (80); 4) interestingly, a kitchen blender can be very effective in preparing ultra-small sized BPQDs (81); 5) although the laser ablation method is considered as a clean and tunable technique, its application is PQDs is still limited (82, 83); 6) besides the widely used precursors of BP powers, crystals or sheets, red phosphorus (RP) can also be used as the precursors (84, 85). 95

In order to further apply the BPQDs in different fields, the functionalized modification is surely important. Thus, they are usually modified by different substances or combined with other functionalized substances, including hybrid with nanosheets, doping on films, modified by polymers, self-assembly and forming complexes with other molecules, including proteins (12). The BPQDs show many unique and important properties, suggesting their great applications in many fields. These BPQDs show great near infrared photothermal performance, good ultraviolet–visible absorption spectroscopy, nonlinear optical properties and excellent memory performance. Their potential applications are demonstrated in many research fields, such as ultra broadband saturable absorbers, biomedicine applications, photovoltaics, optoelectronics and flexible devices. Applications of Laser Ablation in PQDs As shown in the previous sections in this chapter, laser ablation has played a very important role in the study of phosphorus clusters. Combined with mass spectrometry, many novel species can be observed and identified. In fact, as a versatile method, laser ablation of solid materials in liquids has been widely used in the preparation of many different metal nanoparticles (NPs), semiconductor NPs and carbon-related materials. We would like to focus our discussions in a few cases of its application in PQDs. We do hope the readers can enhance the link between phosphorus cluster study and materials, developing new methods to promote the progress for both research fields. The laser ablation process is complicated. Depending on the applied laser wavelength and the material, the laser pulse can penetrate into the material with a certain depth (86). The material is converted to plasma containing various species including electrons, atoms, molecules, clusters and ions, which are characterized by their high densities and temperatures. Thus the method can be applied in vacuum as pulsed laser deposition (PLD) to fabricate thin films. It also can be used in the synthesis of nanowires and nanotubes. In 1993, Henglein et al. applied the method to metal targets in solvents to form colloidal solutions. After that, the method of laser ablation in liquid has been greatly developed. With a 1064 nm Nd:YAG pulsed laser, Ge et al. prepared PQD solution using BP in diethyl ether (82). The laser energy is 30 mJ pulse-1. The pulse width and repetition rate are 6 ns and 5 Hz, respectively. The sample was kept in a sealed glass vial and irradiated by the laser for 20 minutes. The TEM image of the PQDs is shown in Figure 14 The average size of the PQDs is 7 nm. An intense and stable photoluminescence emission of the PQDs in the blue-violet wavelength region is clearly observed. Unlike other quantum dots, these emission peaks are not red-shifted with progressively longer excitation wavelengths. This excitation wavelength-independence is derived from the saturated passivation on the periphery and surfaces of the PQDs. The large numbers of electrondonating functional groups on the surface cause the electron density to be dramatically increased and the band gap to be insensitive to the quantum size effect. Using a similar way, Ren et al. also successfully synthesized BPQDs with an average diameter of about 6 nm and a height of about 1.1 nm (83). In their experiment, the applied solvent is isopropyl ether (IPE). And the Nd:YAG pulsed laser is operated with a rate of 10 Hz at a power density of 140 mWcm-2. The ablation time was 30 min. The obtained BPQDs exhibit a stable blue–violet light emission with a relatively high photoluminescence quantum yield ( about 20.7%), which is three times that of BPQDs prepared by means of probe ultrasonic exfoliation. All these studies reflect some advantages of the laser ablation method and show great potential of these materials in biomedical application. As a test, HeLa cells were incubated with the laser ablation synthesized BPQDs for 3 hours. Compared with confocal microscopy images of the control group, those incubated show 96

a blue photoluminescence emission under excitation at 405 nm. The results also indicate the low cytotoxicity and excellent biocompatibility of the BPQDs in HeLa cells, which is similar to those of carbon quantum dots.

Figure 14. (a) A TEM image of the PQDs and a photo of PQD solution under visible light in the inset. HRTEM images of circles in (a) with lattice parameters of (b) 0.53 nm, (c) 0.34 nm and (d) 0.26 nm, respectively. Reproduced with permission from (82) under a Creative Commons License (https://creativecommons.org/licenses/by/4.0/).

Summary and Outlook Briefly, in the past 40 years, phosphorus clusters have been widely studied by experimental and theoretical chemists. With the aid of the laser ablation mass spectrometry, many kinds of phosphorus cluster ions have been generated and identified in the gas phase. These species can be further studied by other experimental technologies including photoelectron detachment experiments. And the rapidly developing calculation methods, programs and high performance computers help us much better understand the bonding and structures of these diverse clusters than before. On the other hand, the zero-dimensional PQDs have been firstly synthesized in 2015. Since then, many studies have been devoted to this field. Various synthetic and modification methods have been invented and applied for the nanomaterials. And their important properties in many aspects, including energy states, optical and electronic properties, have been discovered or predicted, inspiring their applications or potential applications in many research fields, such as bioimaging, intelligent electronics and optoelectronics. Although the laser ablation method has been proved as the most powerful method for the generation of many kinds of novel phosphorus-related clusters, it should be noticed that its application in the synthesis and modification of PQDs and relative nanomaterials is still limited. And detailed understanding about these nanomaterials in the molecular level is still lacking. The qualities of the nanomaterials synthesized by different methods or different research groups are quite different. These challenges may become the bottlenecks of PQDs for future development. Considering the great success of laser ablation method in many other nanomaterials, we believe that there is still much room for improvement in this approach. It also can be applied as one of effective methods to characterize these materials. And what is more, the method may offer great potential to produce novel nanomaterials with new chemical structures and properties, just like the diverse species that has been generated in the laser ablation mass spectrometric experiments. 97

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21627801).

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Chapter 6

Synthesis of Phosphides Michael Shatruk* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States *E-mail: [email protected].

Phosphorus combines with nearly all metals in the Periodic Table, affording a wealth of compositions and structural arrangements in the resulting phosphides. This chapter discusses typical approaches to the synthesis of binary phosphides, including direct reactions between elements, chemical vapor transport, flux synthesis, as well as synthesis from solution and a few other, less common approaches. Specific examples are discussed to demonstrate common considerations that go into the synthesis of phosphides. The conditions used for the preparation of almost all known binary phosphides have been compiled in a tabular format.

Introduction Phosphorus is a prolific reactant, forming binary compounds nearly with all elements in the Periodic Table. Nevertheless, the synthesis of such compounds, especially metal phosphides, can be quite challenging, requiring extreme safety precautions, rigorous exclusion of air and moisture, and often high temperatures. In this chapter, we discuss general aspects and some specific examples encountered in the synthesis of phosphides. We focus primarily on binary phosphides, since they offer more than sufficient coverage of the typical preparative methods encountered in the phosphide chemistry. Our main goal is to generalize the approaches to the synthesis of phosphides across the Periodic Table by compiling and discussing the conditions used in the synthesis of almost all known binary phosphides, as such information is not readily available in a single literature source. For the discussion of the rich structural chemistry of these fascinating materials, we refer the reader to an excellent and comprehensive review by von Schnering and Hönle (1). Finally, we should remark that our coverage is by no means comprehensive, as the chemistry of phosphides is a vast subject that is impossible to cover in a brief book chapter. Therefore, the current overview aims to strike a balance between adequate coverage of the existing literature and a tutorial-type approach to the presentation of the topic.

© 2019 American Chemical Society

General Methods The approaches to the synthesis of phosphides can be classified in two large groups: 1. The solution synthesis typically uses white phosphorus (Pwhite), which is built of discrete P4 molecules. The Pwhite polymorph is highly reactive and notorious for its hazardous pyrophoric nature. This reactant has to be stored in the inert atmosphere or under water and must be handled with utmost care. It is generally not readily available for laboratoryscale synthesis, especially when one needs to scale up a reaction, and therefore we do not discuss such preparations to a great extent. The use of Pwhite in the solution chemistry of polyphosphides was explored especially fruitfully by the group of Marianne Baudler, who summarized the corresponding developments in two review articles (2, 3). 2. The solid state synthesis mainly uses the more stable red phosphorus (Pred) polymorph, which has an amorphous oligomeric structure. The reactions typically require elevated temperatures, but the heating rate and the amount of starting materials should be controlled very carefully to avoid excessive pressure buildup due to fast evaporation of Pred. Moreover, the subsequent cooling can lead to condensation of the pyrophoric Pwhite polymorph, and thus extreme care must be exercised when opening the reaction vessel and exposing the products to air. As an additional precaution, the residual Pwhite can be converted to Pred by annealing at 275°C for 3-5 days, prior to opening the vessel to analyze the products. These two branches of phosphide chemistry were notably driven by comprehensive works of Baudler and von Schnering, respectively. Pioneering efforts of Jeitschko and co-workers in developing the synthesis of phosphides in molten metals (flux reactions) should be also mentioned (4). In the case of solution reactions, several general methods can be pointed out: 1a. Cryogenic conditions are used for very exothermic reactions between Pwhite and highly electropositive metals (e.g., alkali metals). Cooling the reaction mixture to –78°C in an acetone-CO2 slush bath or carrying out the reaction in liquid ammonia are common techniques in such cases (3). 1b. Room-temperature reactions with Pwhite are more appropriate when the metal reactants are not as aggressive. The reactions are usually carried out in solvents which readily dissolve Pwhite (toluene, DMF, etc.). 1c. Solvothermal conditions can be also applied to synthesize phosphides via reactions of Pwhite or Pred with metals or metal halides. Such method was also used for the synthesis of nanoscale phosphide materials (5).

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1d. Nucleophilic activation of Pred by alkali metal alkoxides in refluxing solvents is a relatively new method, which might become more popular due to the concerns about the highly hazardous nature and limited availability of Pwhite (6). The solid state methods can be classified in a similar fashion, based on the specific techniques: 2a. High-temperature annealing is usually performed between 350 and 1150°C, depending on the reactivity of the metal used. Here, frequently helpful is Tammann’s rule (7), which states that the reactivity of a solid becomes more pronounced at temperatures above 2/3 of its melting point (in Kelvin) (8). Intermediate grinding and annealing of the reaction mixture is often required to achieve a phase-pure product. Slow cooling vs. rapid quenching can also impact the reaction outcome, depending on the thermal stability of the target phase. Addition of a trace amount of I2 or CuI as mineralizers is sometimes used to improve the reaction rate and the crystallinity of final products. 2b. Reactions in molten metals (so-called fluxes) take advantage of increased mutual solubility of elements while also partially suppressing the tendency of phosphorus to rapid evaporation. Thus, the flux serves as a reaction medium, although sometimes it might be incorporated into the reaction products (4). After the reaction, the flux can be removed, usually by dissolution in dilute acids, as long as the target product is resistant toward acid treatment (and many of phosphides are!). If the product dissolves in acid, then the molten flux can be removed by high-temperature centrifugation using a two-crucible method (9) or a specially designed alumina crucible with a filter insert (so-called Canfield crucibles) (10). Flux synthesis often affords high-quality crystals, the surface of which can be additionally polished to remove traces of the flux. 2c. Chemical vapor transport (CVT) (11) has been used effectively for crystal growth of many phosphides. Addition of transporting agents, such as I2 or Br2, allows simultaneous transport of phosphorus and metal in the form of volatile halides and controlled growth of high-quality crystals in a zone remote from the location of starting materials along the temperature gradient of the reaction vessel. 2d. The Faraday method can be considered as a CVT, during which vapors generated by evaporation of Pred in the cold zone of the reactor are passed over much hotter metal contained in the hot zone. The rate of phosphorus diffusion and its vapor pressure can be varied by controlling the temperature of the cold zone, and this method has been used effectively for phosphidation of some high-melting metals which were melted by induction (RF) heating. The disadvantage of the method is its more elaborate setup. Nevertheless, it has become very popular in the catalysis community due to the possibility of topotactic conversion of metal foams into phosphide foams, which thus preserves the high surface area of the catalyst (12).

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2e. RF heating and arc-melting are rarely used for the synthesis of phosphides by direct reaction between elements, due to the fast (and dangerous) evaporation of phosphorus from the reaction mixture. Nevertheless, some metal-rich phosphides were synthesized by a two-step procedure which involved arc-melting pre-annealed compacted mixtures of red phosphorus and high-melting transition metals. Table 1, provided in the end of this chapter, lists many examples of synthetic protocols used to prepare binary metal phosphides. These examples should give the reader a general idea for conditions involved in the preparation of phosphides. We now discuss selected examples of these synthetic approaches for elements from different regions of the periodic table.

Synthesis of Phosphides Alkali and Alkaline-Earth Metal Phosphides Alkali and alkaline-earth metals react with phosphorus readily and exothermically. As a result, care must be taken to avoid the excessive release of heat, which could lead to rapid evaporation of phosphorus and explosion of a reaction vessel. To prevent such problems, the reaction mixtures should be heated slowly and sometimes allowed to react at lower temperatures before raising the temperature to the desired level. Addition of trace amounts of iodine or sulfur also help to alleviate the problem of the fast exothermic reaction, as described in detail by von Schnering and Menge for the case of MgP4 synthesis (13). Another concern is the reaction between these electropositive metals and silica tube at elevated temperatures. This problem can be averted by carbonizing the inner surface of the silica tube, by using a Nb or Ta tube or a corundum crucible sealed inside the silica tube, or sometimes by performing reactions in metal fluxes. Nb and Ta might react with phosphorus at elevated temperatures, and in such cases products should be carefully analyzed for possible contamination with these metals. We recommend that a reader interested in the synthesis of a particular compound listed in Table 1 consult the original source for the more accurate experimental details. As can be seen from Table 1, the majority of alkali or alkaline-earth metal phosphides were prepared by direct reactions between the metal and red phosphorus. In some cases, however, such direct methods failed. For example, K3P could be only obtained when vapors of white phosphorus reacted with molten potassium metal at 700°C (14). The phosphorus-rich compound KP15 could be synthesized by the direct reaction between elements, but much larger crystals were obtained by using a CVT setup (15). Barium phosphides Ba3P2, Ba5P4, and BaP2 were prepared by the reaction between elements in Pb flux (16), but these compounds can be also obtained by the direct annealing of elements in the solid state (16–18). In contrast, while SrP2 could be synthesized both in Pb flux and by the direct reaction between elements (16), Sr3P2 could be obtained only by the latter method (19). In some cases, higher polyphosphides were obtained by reacting a pre-synthesized lower phosphide with additional amount of phosphorus. For example, BaP10 was obtained by annealing BaP3 with Pred. Attempts to synthesize BaP10 by the direct reaction between elements always led to the formation of Ba3P14 (20). Conversely, some lower phosphides were obtained by thermal decomposition of the higher ones. For example, annealing BaP2 above 750°C led to the partial loss of phosphorus and the formation of Ba3P4 (21): 106

while alkali metal phosphides AP15 (A = Li–Cs) were found to decompose cleanly toward A3P7 upon heating (22):

The formation of different polymorphs should be also mentioned, although it is not detailed in the limited compilation of synthetic data presented in Table 1. As an example, the reaction between Ba metal and Pred in the 1:3 ratio produces two forms of BaP3, depending on the annealing temperature. Both polymorphs exhibit similar melting points, ~750°C, and if the reaction mixture is heated above this temperature, the more thermodynamically stable high-temperature polymorph is obtained. On the other hand, the low-temperature polymorph can be obtained by annealing a mixture of elements at 700°C or re-annealing the high-temperature polymorph at this temperature (23). In further discussion, the possibility of polymorph formation will be mentioned only in cases where it is instructive for contrasting different synthetic methods. Alkali metal phosphides are also the most common among phosphide materials whose synthesis was probed in solution. As mentioned before, more details can be found in the reviews by Baudler (2, 3), and we only provide here a few general remarks, which are also illustrated by representative examples in Table 1: 1. The solution synthesis frequently requires the use of complexing ligands in order to improve the solubility of alkali metal cations. For example, the reaction between Pwhite and LiCH2PPh2·tmeda (tmeda = tetramethylethylenediamine) in a refluxing mixture of tetrahydrofuran (thf) and hexane led to the formation of [Li(tmeda)]3P7 (24). 2. The phosphides obtained in solution frequently yield themselves to further cation exchange. Thus, subjecting a solution of Cs3P11 in liquid ammonia (at –40°C) to a Liexchange resin led to the isolation of [Li(NH3)4]3P11·5NH3 by crystallization (25). A reaction between Cs3P11 and (NEt4)I in liquid NH3 resulted in a mixed-cation salt, (NEt4)Cs2P11 (26). 3. Polyphosphide anions obtained by solid-state methods are typically quite stable, being constrained by the crystalline lattice. In contrast, the polyphosphides anions produced in solution can be highly fluxional, showing facile conversion to other forms. For example, Cs3P11 reacts with Te in liquid NH3 to form CsP7 (27). The latter, we should note, was the first polymeric polyphosphides crystallized via solution chemistry. 4. The structures of polyphosphides fragments obtained in solution are typically not observed among those obtained by solid-state methods. In this vein, it is interesting to compare two syntheses of NaP5: the reaction of Na metal and Pwhite in diglyme resulted in the formation of the planar aromatic P5– anion, which was identified by 31P NMR spectroscopy but could not be crystallized as a pure compound (28), while the solid-state reaction between Na and

107

Pred at 800°C and 3 GPa led to the formation of an insoluble NaP5 with the crystal structure containing an extended polyphosphide motif (29). Transition Metal Phosphides In contrast to the alkali and alkaline-earth metals, transition elements exhibit higher melting points and lower reactivity toward phosphorus, especially as one proceed to the middle of the transition series and from 3d to 4d to 5d metals. Consequently, the preparation of transition metal phosphides was approached via various methods, in some cases rather exotic. For example, some of the preparations proceeded via dropping lumps of Pred into a metal melt produced by RF heating, using a specially designed apparatus (30, 31), while others required homogenizing annealings that lasted as long as 5 months (32, 33). Given the aforementioned properties of transition metals, direct annealing of elements poses two major problems: 1. The reaction might require multiple re-grindings and re-annealings until a pure product is obtained. Many of the synthetic conditions encountered in Table 1 did not necessarily led to a single-phase material, and quite a few original references mention the presence of other phosphide byproducts. (Unfortunately, some of the older papers do not provide sufficient clarity as to the purity of the materials being produced.) 2. The much higher volatility of phosphorus calls for most serious precautions, in order to avoid explosive pressure build-ups. Typically, the reaction mixture should be heated very slowly, so that the desired temperature is reached within 24-48 h. As a result, several approaches have been developed to combat these problems. We discuss them below, providing a few specific examples in each case. An approach which might be considered a “brute-force” method is to compact a mixture of transition metal and Pred powders and pre-anneal them at certain temperature, which does not lead to substantial evaporation of phosphorus. The result is a pre-reacted pellet, which then can be subjected to arc-melting or RF heating in order to complete the reaction. Such approach was frequently used, especially in the early investigations of the transition metal phosphides extensively pursued in Sweden tin 1960s. As an example, a number of titanium phosphides were discovered in such way, where the annealing of pre-compacted Ti/Pred mixtures could take from 2 to 30 days at 800°C, followed by rapid arc-melting of the pellets to minimize the loss of phosphorus (34–36). This approach was also used to prepare lower metal phosphides, which then were arc-melted with additional amount of metal to obtain higher phosphides, e.g., Zr3P (37), Zr2P (38), and Zr14P9 (39) were obtained by arc-melting stoichiometric mixtures of Zr and ZrP. The problem of fast evaporation of phosphorus can be relieved by using the Faraday method, in which the vapors of phosphorus, released at ~400-500°C and essentially composed of P4 molecules, are passed over metal heated to much higher temperature. Such approach was demonstrated for the preparation of TaP (the metal target was held at 1000°C) and Ti2P (800°C), as well as for some lanthanide and actinide phosphides, as will be discussed in the next section. The use of metal fluxes as reaction medium was shown to be an effective way to facilitate the reaction between a high-melting transition metal and phosphorus (4). Most commonly, Sn flux has been used. Although Sn forms several binary phosphides, they do not impede the synthesis 108

but rather serve as intermediates which slow down evaporation of phosphorus and allow it to react with the transition metal at higher temperatures. The Sn flux is afterwards removed by dissolution in dilute HCl (1:1 v/v), which also dissolves the binary Sn phosphides but usually does not affect the transition metal phosphide. For instance, all known technetium phosphides could be prepared by reacting the elements in Sn flux for 7-28 days at 950°C (40, 41). In addition, the flux method often leads to the formation of large, high-quality single crystals appropriate for physical property measurements. The inertness of transition metals can be also circumvented by the use of halogen as a mediator, which essentially catalyzes the reaction between the metal and Pred by the formation of more reactive halides, followed by their combination to form a more thermodynamically stable phosphide and the release of halogen. In the chemistry of phosphides, the use of I2 as a catalyst, also called a transport agent or a mineralizer, has become especially common. The mass of I2 added to the reaction is typically quite small – on the order of 1-2 at.%. This approach was used successfully to prepare Re6P13 (42) and ReP4 (43), which were obtained by annealing mixtures of elements for 7 days at 930°C. FeP4 was obtained in a similar fashion, but by annealing for 21 d at 630°C (44). It is likely that the reaction could proceed much faster if a higher temperature were used. This approach can be extended to a CVT process performed in a temperature gradient. In such way, crystals of VP2 (45) and Cr12P7 (46) were grown using I2 and Cl2, respectively, as transport agents. The addition of I2 as mineralizer was also employed in the synthesis performed in Sn flux. It was found to both lower the reaction temperature and accelerate the reaction, but most importantly, to result in purer products. In this way, MoP2 and WP2 were prepared by 7-14 day reactions in Sn flux at 850°C and 1000°C, respectively (47). In contrast, CrP4 and MoP4 were obtained by direct reactions between elements at 1200°C and 65 kbar, although under such drastic conditions the reaction was completed in 2 h (48). It is interesting whether this phosphide could be produced by a flux method or whether the reaction could be facilitated by addition of I2. It also should be pointed out that the action of the mineralizer can be more dramatic than just accelerating the reaction. In some cases, the addition of mineralizer is critical for obtaining the desired phosphide. For example, Cu2P20 was obtained by annealing a mixture of Cu3P and Pred in the presence of CuI, but the same reaction performed without CuI produced a mixture of Cu2P7 and elemental phosphorus (49). Other important parameters that should be kept in mind are the heating and cooling rates. While we do not mention them explicitly in Table 1, their knowledge might be sometimes crucial for the reaction to succeed. AgP15 was synthesized by reacting Ag and Pred in the presence of I2, first for 5 h at 650°C and then for 15 h at 500°C (50). One of the most important aspects of this synthesis, however, is the cooling rate from 500°C to room temperature. The slow cooling at 1.3°C/h was shown to be important for obtaining large crystals of AgP15. In other cases, rapid quenching of the reaction from the annealing temperature might be critical for obtaining the desired product or isolating a specific polymorph. An example case of EuP3 will be discussed below. Lanthanide and Actinide Phosphides While we singled out the phosphides of f-elements into a separate section, the problems encountered in their synthesis partially mirror those already discussed for the transition metals. 109

While the f-metals are generally more reactive, they also exhibit rather high melting points, typically exceeding 1000°C. In addition, the formation of very stable monophosphides with the NaCl-type structure impedes the synthesis of other phosphide phases, especially metal-rich ones. Indeed, as can be seen from Table 1, the majority of lanthanides and actinides form phosphides with the phosphorus content not exceeding 50 at.%, and the compositional variety is much more limited as compared to the phosphides of transition metals. In this section, we only consider a few selected examples that expand the range of synthesis methods discussed thus far. von Schnering and co-workers successfully prepared a series of polyphosphides, LnP5, for yttrium and all lanthanides, except europium, by reacting Ln and Pred in alkali-halide fluxes just at 500-600°C. The fluxes used included molten KI or its mixture with LiCl, NaCl, or CsCl, and a small amount of I2 as mineralizer (51, 52). After the reaction, the flux could be easily removed by dissolution in water. In the same fashion, LaP7 (53) and EuP7 (54) were obtained. The synthesis of YP5 also demonstrates another advantage offered by the flux method – the possibility to obtain metastable phases not accessible by simple high-temperature annealing of constituent elements. While YP5 was prepared by annealing a mixture of Y metal and Pred in a LiCl-KI flux at 520°C, this phosphides was found to decompose at 475°C in its pure form (55). Thus, the presence of the alkalimetal flux alters the phase equilibria in this system and allows the formation of YP5 above the limit of its thermal stability. Although the aforementioned monophosphides LnP can be prepared by direct reaction between elements at 700-900°C (56), an alternative method was devised by Rowley and Parkin (57), who thermolyzed a mixture of Na3P and anhydrous LnCl3 (Ln = Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Yb) at 700-800°C, producing pure LnP just in 4 h. The NaCl byproduct could be easily removed by trituration of the reaction products with MeOH. Among lanthanides, europium showed a greater variety of phosphide compositions, due to its tendency to act as divalent metal. Thus, similar to Ca, Sr, and Ba, this lanthanide forms the EuP3 phosphide, but the structure of the material showed strong dependence on the preparation method and the thermal treatment of the sample (58). The synthesis in the LiCl-KCl flux at 630°C led to the formation of α-EuP3, while the direct reaction between elements at 850°C resulted in β-EuP3 polymorph. The α-form appears to be metastable as it transforms into the β-form upon heating. The synthesis of actinide phosphides is complicated by radioactivity of the metals. The compositional variety is also rather limited, which might be due to the limited number of reactions performed. Indeed, the more commonly explored actinides, thorium and uranium, form more diverse phosphide compositions as compared to other 5f metals. The lower thorium phosphides were synthesized by the Faraday method (ThP and Th3P4) (59), while the higher ones were obtained by direct reactions between elements in the presence of I2 (Th2P11 (60) and ThP7 (61)). Some actinide monophosphides were synthesized by passing PH3 gas over the corresponding metal (UP at 385°C (62)) or metal hydride (NpP at 350°C (63), PuP at 600°C (64)). The obtained products were subsequently homogenized by annealing at 1000-1500°C.

110

Main-Group (Post-Transition) Metal Phosphides The moderate melting points and chemical activity of the majority of post-transition metals makes their reactivity toward phosphorus intermediate between those observed for transition elements, on the one side, and alkali and alkaline-earth metals, on the other side. In general, the synthesis of post-transition metal phosphides is less challenging as compared to the preparation of transition metal phosphides. Among the corresponding reactions compiled in Table 1, it is worth pointing out the synthesis of monophosphides MP (M = Al, Ga, In) by the reaction between the corresponding metal and Zn3P2. The advantage of this method is that Zn3P2 sublimes without decomposition and, therefore, this reactant can be prepared in very pure form. The metathesis reaction between Zn3P2 and the triel element at 800-900°C produces metallic zinc, which can be sublimed from the desired MP product by increasing the temperature to 1000-1100°C (65). The synthesis of phosphides of high-melting main-group elements, such as boron and silicon, requires unconventional approaches. For example, higher boron phosphides were produced by decomposition of boron monophosphide: B13P12 was obtained by heating BP at 1100-1400°C (66), while B6P was obtained by heating BP in the flow of H2 gas at 1250°C (67). The synthesis of BP itself is quite challenging, since the direction reaction between elements requires high annealing temperatures, at which the vapor pressure of phosphorus can become hazardously high. This problem is similar to the one encountered in the synthesis of phosphides of high-melting transition metals. Nevertheless, it has been shown that BP can be also obtained by reaction between the elements in Sn flux at 800°C or by metathesis between BI3 and Pred at 900°C. The workup procedure for the metathesis route is extensive but affords a pure sample of BP (68). In the same vein, silicon phosphides SiP and SiP2 had to be prepared by high-pressure annealing of a mixture of elements at 1400-1800°C and 30-50 kbar (69), but crystals of SiP2 were also obtained by CVT in 900°C → 500°C temperature gradient, also starting from a mixture of elements (70). Another interesting observation is the lack of binary phosphides for some of the main-group pelements – a situation not observed for the groups of s‑, d‑, or f-elements considered in the preceding sections. Thus, no binary phosphides are known for C, Hg, Sb, or Bi. The lack of phosphides for the latter two elements might not be particularly surprising, as it is not unusual for the elements of the same group to interact poorly with one another. Although C–P bonds are abundant in metalorganic chemistry, binary carbon phosphides with well-defined crystal structures are unknown. The absence of mercury phosphides is also puzzling, especially given the broad range of reported mercury phosphide halides, whose crystal structures rely on extensive Hg–P bonding (71). Interestingly, Pb had also belonged to the list of metals that showed apathy toward phosphorus, but recently Pöttgen et al. reported the remarkable synthesis of PbP7 in Pb flux (72). The polyphosphide was prepared by a careful stepwise annealing procedure, in which Pb and Pred were heated in 1:1 ratio for 2 d at 150°C, then 2 d at 300°C, and finally 2 d at 400°C, before cooling the reaction to room temperature. The Pb flux was removed by treatment with a mixture of H2O2 and glacial CH3COOH, affording pure PbP7. This result is very inspiring, as it suggests that the use of alternative synthetic methods might be promising not only for discovering the first mercury phosphide, but also for synthesizing yet unknown phosphides of other metals.

111

Conclusion We hope that this brief overview has provided the reader with good appreciation of the compositional variety of metal phosphides and the arsenal of methods used for their synthesis. These methods are also applicable to the synthesis of many other inorganic materials, especially pnictides and chalcogenides. Beyond the general approaches discussed in this chapter, new possibilities can be envisioned in the synthesis of phosphides. For example, the use of two other allotropes, violet and black phosphorus, for the preparation of phosphides is essentially unexplored. The structures of these allotropes feature, respectively, infinite one-dimensional tubules and corrugated sheets of phosphorus atoms, which can exhibit different reactivity and lead to different structures of the reaction products. They are also more inert than Pred and could be interesting to use as starting materials for high-pressure syntheses. One of the problems in this regard is the difficulty of the largescale synthesis of Pviolet and Pblack. Over many years, the preparation of phosphides has been a real challenge for synthetic chemists, who often had to demonstrate remarkable creativity and synthetic prowess to discover new phosphide materials and obtain them in phase-pure form. Some of these challenges still remain unresolved. The author would like to encourage new generations of scientists to find such unresolved problems and continue solving them with knowledge, ingenuity, and enthusiasm. The promising uses of phosphides as water splitting electrocatalysts, thermoelectrics, magnetic refrigerants, and precursors to 2D materials should continue to stimulate interest to this fascinating and diverse group of materials.

112

Table 1. Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

113

Alkali and alkali-earth metals Li3P

annealing

Li + Pred, at 680°C

(73)

LiP

annealing

Li + Pred, 3 d at 450°C

(74)

Li3P7

annealing

Li + Pred, 3 d at 600°C

(75)

LiP5

annealing

Li + Pred, 500°C

(76)

LiP7

annealing

Li + Pred, 500°C

(76)

[Li(tmeda)]3P7

solution

LiCH2PPh2·tmeda + Pwhite, reflux in thf/hexane

(24)

[Li(NH3)4]3P11·5NH3

solution

Li ion-exchange resin + Cs3P11 in liq. NH3 at –40°C

(25)

Na3P

annealing

Na + Pred, 3 d at 650°C

(73)

NaP

annealing

Na + Pred, at 450°C

(77)

Na3P7

annealing

Na + Pred, 4 d at 500°C

(78)

Na3P11

annealing

Na + Pred, 4 d at 500°C

(78)

NaP5 (molecular)

solution

Na + Pwhite in diglyme

(28)

NaP5 (extended)

annealing

Na + Pred, 1 h at 800°C and 3 GPa

(29)

NaP7

annealing

Na + Pred + CuI, 7 d at 550°C

(79)

NaP15

annealing

Na + Pred + I2, 5 h at 650°C, 15 h at 500°C

(50)

K at 700°C + Pwhite at 80°C

(14)

K3P

CVT

K4P3

annealing

K + Pred, 400°C

(80)

KP

annealing

K + Pred, at 490°C

(77)

K4P6

annealing

K + Pred, 0.5 d at 400°C; 1 d at 600°C

(81)

K3P7

annealing

K + Pred, 3 d at 900°C

(82)

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

114

K + Pred, 3.6 d at 470°C

(83)

K + Pred, 650°C → 200°C

(15)

annealing

Rb + Pred, 2 d at 600°C

(84)

Rb3P7

annealing

Rb + Pred, at 460°C; grinding; at 460°C

(82)

Rb3P11

annealing

Rb + Pred, 2.5 d at 500°C

(83)

RbP11

annealing

Rb + Pred, 14 d at 460°C

(85)

RbP15

annealing

Rb + Pred, 2 d at 550°C

(22)

Cs4P6

annealing

Cs + Pred, 2 d at 650°C

(84)

Cs3P7

annealing

Cs + Pred, at 460°C; grinding; at 460°C

(82)

Cs3P11

annealing

Cs + Pred, 3 d at 450°C

(83)

CsP7

solution

Cs3P11 + Te in liq. NH3

(27)

CsP15

annealing

Cs + Pred, 2 d at 550°C

(22)

(NMe4)2RbP7·NH3

solution

(NMe4)I + Rb3P7 in liq. NH3 at –40°C

(26)

Cs2P4·2NH3

solution

Cs + P2H4 at –78°C; liq. NH3 at –40°C

(86)

(NEtMe3)Cs2P7·2NH3

solution

Cs3P7 + EtMe3I in liq. NH3

(87)

(NEt4)Cs2P11

solution

(NEt4)I + Cs3P11 in liq. NH3 at –40°C

(26)

Be3P2

annealing

Be + Pred, at 700°C; grinding; at 700°C

(88)

BeP2

annealing

Be + Pred, at 800-1000°C

(89)

Mg3P2

annealing

Mg + Pred, at 750°C

(90)

MgP4

CVT

Mg + Pred + I2, 900°C → 850°C

(13)

Ca5P8

annealing

Ca + Pred, 3 d at 630°C; grinding; 3 d at 700°C

(91)

K3P11

annealing

KP15

CVT

Rb4P6

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

115

CaP3

annealing

Ca + Pred, at 650°C

(92)

Sr3P2

annealing

Sr + Pred, at 1000°C

(19)

Sr3P4

annealing

SrP2, at 735°C

(21)

SrP2

annealing

Sr + Pred, 15 d at 800°C

(16)

SrP3

annealing

Sr + Pred, 2 h at 1150°C

(93)

Sr3P14

annealing

Sr + Pred, 2 h at 850°C

(94)

Ba3P2

annealing

Ba + Pred, at 800°C

(17)

Ba4P3

annealing

Ba + Pred, 2 d at 450-500°C; 2 d at 600-630°C; 3 d at 700°C; grinding; 1 h at 1220°C

(95)

Ba5P4

Pb flux

Ba + Pred, 2 d at 950°C

(16)

Ba3P4

annealing

BaP2, at 750°C

(21)

Ba5P9

annealing

Ba + Pred, 1 h at 650°C

(96)

BaP2

annealing

Ba + Pred, 2 w at 800°C

(16)

BaP3

annealing

Ba + Pred, 6 d at 900°C

(23)

Ba3P14

annealing

Ba + Pred, 2 h at 850°C

(93)

Ba + Pred, 1 h at 1000°C and 30 kbar

(97)

BaP3 + Pred, 4 w at 780°C

(20)

Ti + Pred, 2 d at 800°C; arc-melting

(34)

Ti at 800°C + Pred at 450°C, 30 d; arc-melting

(35)

BaP8

P-annealing

BaP10

annealing

Transition and lanthanide metals Ti3P Ti2P

annealing/ arc-melting CVT/ arc-melting

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

TiP

annealing/ arc-melting annealing/ arc-melting annealing

TiP2

annealing

Zr3P

arc-melting

Zr + ZrP

(37)

Zr2P

arc-melting/ RF heating

Zr + ZrP, arc-melting; RF 6 h at 1700°C

(38)

Zr7P4

annealing/ arc-melting/ RF heating

Zr + Pred, 3 d at 900°C; arc-melting; RF at 1650°C

(100)

Zr14P9

arc-melting

Zr + ZrP

(39)

Zr + Pred + ZrCl4, at 850°C

(101)

annealing/ arc-melting annealing/ arc-melting arc-melting/ RF heating

Hf + Pred, 3 d at 850°C; arc-melting; 6 d at 1000°C

(102)

Hf + Pred, 3 d at 850°C; arc-melting; 40 d at 1150°C

(103)

Hf + Fe + Hf P, arc-melting; RF 6 h at 1400°C

(104)

annealing/ arc-melting annealing

Hf + Pred, 1 d at 850°C; arc-melting

(105)

Hf + Pred, 3 d at 600°C; 2 d at 900°C

(106)

annealing/ arc-melting

V + Pred, 2 d at 800°C; arc-melting

(107)

Ti7P4 Ti5P3

116

ZrP2 Hf3P Hf2P Hf7P4 Hf3P2 Hf P V3P

annealing

Ti + Pred, 3 d at 800°C; arc-melting

(36)

Ti + Pred, 3 d at 800°C; arc-melting

(36)

Ti + Pred, 10 d at 850°C; grinding; 10 d at 850°C

(98)

Ti + Pred, 2-5 h at 550°C; 12-20 h at 650°C

(99)

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

V2P

annealing/ arc-melting/ RF heating

VP

Conditions

Ref.

117

V + Pred, 8 d at 900°C; arc-melting; RF 8 h at 1175°C

(108)

annealing

V + Pred, 8 d at 850°C; grinding; 8 d at 850°C; grinding; 8 d at 850°C

(109)

VP2

annealing/ CVT

V + Pred, 3 d at 800°C; +I2, 5 d at 800°C → 700°C

(45)

VP4

annealing

V + Pred + I2, 7 d at 550°C

(110)

Nb3P

arc-melting

Nb + NbP

(111)

Nb2P

arc-melting

Nb + Pred

(112)

Nb7P4

arc-melting

Nb + NbP

(113)

Nb + Pred, at 750°C; arc-melting

NbP

annealing/ arc-melting annealing/ arc-melting annealing

Ta3P

Nb5P3 Nb8P5

Ta2P Ta5P3

Nb + Pred, 1 d at 800°C; arc-melting

(114)

Nb + Pred, 4 d at 1000°C

(115)

arc-melting

Ta + TaP

(116)

annealing/ arc-melting RF heating

Ta + Pred, 1 d at 750°C; arc-melting

(117)

Ta + TaP, at 2000°C

(118)

Ta at 1040°C + Pred at 470°C

(118)

TaP

CVT

Cr3P

annealing

Cr + Pred, 5 d at 1050°C

(119)

Cr2P

annealing

Cr + Pred, 5 d at 1050°C

(119)

Cr + Pred + Cl2, 1030°C → 970°C

(46)

Cr12P7

CVT

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

(120)

P-annealing

Cr + Pred, 2 h at 1200°C and 65 kbar

(121)

P-annealing

Cr + Pred, 2 h at 1200°C and 65 kbar

(48)

annealing/ arc-melting annealing/ arc-melting/ RF heating

Mo + Pred, 5-7 d at 800°C; arc-melting

(122)

Mo + Pred, 7 d at 900-1100°C; arc-melting; RF at 1580-1680°C

(123)

annealing

CrP2 CrP4

Mo8P5

Ref.

Cr + Pred at 1000°C

CrP

Mo3P

Conditions

118

Mo4P3

annealing

Mo + Pred at 1000°C

(123)

MoP

annealing

Mo + Pred, 2 d at 1000°C; grinding; 2 d at 1000°C

(124)

MoP2

Sn flux

Mo + Pred + I2, 7-14 d at 850°C; HCl (1:1 v/v)

(47)

MoP4

P-annealing

Mo + Pred, 2 h at 1200°C and 65 kbar

(48)

W + Pred at 1000°C

(120)

W + Pred + I2, 7-14 d at 1000°C; HCl (1:1 v/v)

(47)

WP

annealing

WP2

Sn flux

Mn3P

annealing

Mn + Pred, 5 d at 1050°C

(119)

Mn2P

annealing/ RF heating

Mn + Pred, 1 d at 800°C

(125)

MnP

RF heating

Mn + Pred at 900-1150°C

(120)

MnP4

P-annealing

Mn + Pred, 1-2 h at 1230-1430°C and 30-55 kbar

(126)

Tc3P

Sn flux

Tc + Pred, 7-28 d at 950°C; HCl (1:1 v/v)

(40)

Tc2P3

Sn flux

Tc + Pred, 7-28 d at 950°C; HCl (1:1 v/v)

(40)

TcP3

Sn flux

Tc + Pred, 21 d at 940°C; hot HCl (1:1 v/v)

(41)

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

119

TcP4

Sn flux

Tc + Pred, at 950°C; HCl (1:1 v/v)

(40)

Re2P

Sn flux

Re + Pred, 7 d at 930°C; boiling HCl (1:1 v/v)

(127)

Re3P4

Sn flux

Re + Pred, 7 d at 800°C; boiling HCl (1:1 v/v)

(128)

Re6P13

annealing

Re + Pred + I2, 7 d at 930°C

(42)

Re2P5

Sn flux

Re + Pred, 7 d at 950°C; boiling HCl

(129)

ReP3

annealing

Re + Pred + I2 at 800°C

(41)

ReP4

annealing

Re + Pred + I2, 7 d at 930°C

(43)

Fe3P

annealing

Fe + Pred, 5 d at 1050°C

(119)

Fe2P

annealing/ RF heating

Fe + Pred, 14 d at 900°C; RF 30 min at 1450°C

(130)

FeP

RF heating

Fe + Pred, at 900-1150°C

(120)

FeP2

annealing

Fe + Pred, 8 d at 1000°C; grinding; 30 d at 800°C

(131)

FeP4

annealing

Fe + Pred + I2, 21 d at 630°C

(44)

Ru2P

annealing

Ru + Pred, at 1100°C

(132)

RuP

annealing

Ru + Pred, at 1000°C

(120)

RuP2

annealing

Ru + Pred, 8 d at 1000°C; grinding; 25 d at 800°C

(131)

RuP3

Sn flux

Ru + Pred, 6 h at 1000°C; boiling HCl (5N)

(133)

RuP4

Sn flux

Ru + Pred, 1 d at 730°C; boiling HCl (1:1 v/v)

(134)

OsP2

annealing

Os + Pred, 8 d at 1000°C; grinding; 25 d at 800°C

(131)

OsP4

Sn flux

Os + Pred, 1 d at 730°C; boiling HCl (1:1 v/v)

(134)

Co2P

annealing

Co + Pred, 5 d at 1050°C

(119)

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

120

CoP

RF heating

Co + Pred, at 900-1150°C

(120)

CoP2

annealing/ CVT

Co + Pred, 5 d at 600-700°C; 7 d at 750°C → 650°C

(110)

CoP3

Sn flux

Co + Pred, 1 d at 450 °C; 7 d at 675°C; HCl (1:1 v/v)

(135)

Rh2P

annealing

Rh + Pred, 3 d at 950 °C

(33)

Rh3P2

Sn flux

Rh + Pred, 7 d at 1150 °C; dilute HCl

(136)

Rh4P3

annealing

Rh + Pred, at 900-1100°C

(137)

RhP2

annealing

RhP3

Sn flux

Ir2P

P-annealing

IrP2

annealing

IrP3

annealing

Ir + Pred, 8 d at 800°C; grinding; 10 d at 800°C; grinding; 10 d at 800°C; grinding; 10 d at 800°C Ir + Pred, sev. d 1000°C

Ni3P

annealing

Ni + Pred, 8 d at 1050°C

(119)

Ni + Pred, 2 d at 800°C; arc-melting; 10 d at 800°C

(142)

Ni + Pred, 2 d at 800°C; arc-melting; 100 d at 800°C

(143)

Ni + Pred, at 700-900°C

(144)

Ni8P3 Ni5P2 Ni12P5

annealing/ arc-melting annealing/ arc-melting annealing

Rh + Pred, 8 d at 800°C; grinding; 10 d at 800°C; grinding; 10 d at 800°C; grinding; 10 d at 800°C Rh + Pred, 12-15 h at 1150°C; hot dilute HCl Ir + Pred, 30 min at 1800°C and 150 kbar

(138) (139) (140) (138) (141)

Ni2P

annealing

Ni + Pred, 8 d at 1050°C

(119)

NiP

annealing

Ni + Pred, 4 d at 850°C

(145)

Ni5P4

annealing

Ni + Pred, 1 d at 900°C, 5 d at 800°C

(146)

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

NiP2

Sn flux

Ni + Pred, 12-15 h at 1150 °C; hot dilute HCl

(139)

NiP3

Sn flux

Ni + Pred, 1 d at 450°C; 7 d at 675°C; HCl (1:1 v/v)

(135)

NiP3

Sn flux

Ni + Pred, 1 d at 450°C; 7 d at 675°C

(135)

121

Pd15P2

annealing

Pd + Pred, 2 d at 830°C; 150 d at 795°C

(32)

Pd9P2

annealing

Pd + Pred, 2 d at 700-750°C; grinding; 2 d at 700-750°C

(147)

Pd3P

annealing

Pd + Pred, a few h at 450-500°C; 1 d at 600-700°C

(148)

Pd7P3

annealing

Pd + Pred, 1 d at 700°C; 10 min at 1200°C; 1-3 d at 760°C

(149)

PdP2

annealing

PdCl2 + Pred, 2 d at 500°C

(150)

Pt5P2

annealing

Pt + Pred, sev. d at 600-800°C; 21 d at 570°C

(151)

PtP2

Sn flux

Pt + Pred, 24 h at 1200°C; warm HCl (2N)

(152)

Cu3P

annealing

Cu + Pred, at 750°C

(49)

CuP2

Sn flux

Cu + Pred, 12-15 h at 1150°C; hot dilute HCl

(139)

Cu2P7

annealing

Cu + Pred, 7 d at 550-700°C

(153)

Cu2P20

annealing

Cu3P + Pred + CuI, 7 d at 550°C

(49)

AgP2

annealing

Ag + Pred, 7 d at 550-700°C

(154)

Ag3P11

annealing

Ag + Pred, 35 d at 370°C

(154)

AgP15

annealing

Ag + Pred + I2, 5 h at 650°C, 15 h at 500°C

(50)

Au2P3

annealing

Au + Pred, 3-30 d at 825°C

(155)

Sc3P

annealing

Sc + ScP, 7 d at 950°C; grinding; 1 w at 950°C

(156)

Sc7P3

annealing

Sc + ScP, 7 d at 950°C; grinding; 1 w at 950°C

(156)

Lanthanides and Actinidesb

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition Sc3P2

Synthetic Method arc-melting/ annealing

Conditions

Ref.

Sc + ScP, arc-melting; 2 d at 1000°C

(157)

122

ScP

annealing

Sc + Pred, 1.25 d at 1000°C

(158)

YP

annealing

Y + Pred, 20 h at 1100°C; grinding; 10 h at 1100°C

(159)

YP5

LiCl-KI flux

Y + Pred + I2, 14 d at 420°C

(55)

LaP

annealing

La + Pred, 20 h at 1100°C; grinding; 10 h at 1100°C

(159)

LaP2

CVT

La + Pred + I2, 5 d at 1080°C → 650°C

(160)

LaP5

CVT

La at 750°C + Pred at 580°C, 7 d

(161)

LaP7

NaCl-KI flux

La + Pred + I2, at 540°C

(53)

CeP

annealing

Ce + Pred, at 700-900°C

(56)

CeP2

annealing

CeP + Pred, 7 d at 700°C

(162)

CeP5

KI flux

Ce at 750°C + Pred + I2 at 580°C, 7 d

(163)

PrP

annealing

Pr + Pred, at 700-900°C

(56)

PrP2

annealing

Pr + Pred, 7 d at 750°C

(162)

PrP5

KI flux

Pr at 750°C + Pred + I2 at 580°C, 7 d

(51)

NdP

annealing

Nd + Pred, 20 h at 1100°C; grinding; 10 h at 1100°C

(159)

NdP5

CVT

Nd at 750°C + Pred at 580°C, 7 d

(161)

SmP

annealing

Sm + Pred, 20 h at 1100°C; grinding; 10 h at 1100°C

(159)

SmP5

KI flux

Sm at 750°C + Pred + I2 at 580°C, 7 d

(51)

EuP

solution

Eu + PH3 in liq. NH3 at –78°C; → 25°C; 18 h at 730°C

(164)

Eu3P2

annealing

Eu + Pred, at 1300°C (Mo crucible)

(165)

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

123

Eu4P3

annealing

Eu + Pred, 12 h at 1050°C (Nb crucible)

(166)

Eu3P4

annealing

Eu + Pred, 1 d at 450°C; 100°C/d to 900°C

(21)

EuP3

annealing

Eu + Pred, at 850°C

(58)

EuP7

LiCl-KI flux

Eu + Pred + I2, 1 d at 380°C; 14 d at 540°C; dilute HCl (2N)

(54)

GdP

annealing

GdCl3 + Na3P, 4 h at 700-800°C

(57)

GdP5

KI flux

Gd + Pred + I2, 3 d at 800°C; dilute HCl (1:1 v/v)

(167)

TbP

annealing

TbCl3 + Na3P, 4 h at 700-800°C

(57)

DyP

annealing

DyCl3 + Na3P, 4 h at 700-800°C

(57)

DyP5

CsCl-KI flux

Dy + Pred + I2, 14 d at 540°C

(52)

HoP

annealing

HoCl3 + Na3P, 4 h at 700-800°C

(57)

HoP5

CsCl-KI flux

Ho + Pred + I2, 7 d at 530°C

(52)

ErCl3 + Na3P, 4 h at 700-800°C

(57)

Tm + Pred + I2, 14 d at 510°C

(168)

Yb + PH3 in liq. NH3 at –78°C; → 25°C; 3 h at 550°C

(164)

Yb + Pred + I2, 14 d at 520°C

(169)

Lu + Pred, at 700-900°C

(56)

ErP

annealing

TmP5

LiCl-KI flux

YbP

solution

YbP5

LiCl-KI flux

LuP

annealing

LuP5

LiCl-KI flux

Lu + Pred + I2, 14 d at 510°C

(168)

ThP

CVT

Th at 1650°C + Pred at 400°C

(59)

Th3P4

CVT

Th at 800-1000°C + Pred at 400°C

(59)

Th2P11

LiCl-KI flux

Th + flux at 530°C + Pred + I2 at 500°C, 14-21 d

(60)

annealing

Th + Pred + I2, 24 h at 400 °C; 14-21 d at 580°C

(61)

ThP7

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

124

PaP2, 1 min at 1000°C

(170)

annealing

PaH3 + Pred, 1 d at 700°C

(170)

UP

annealing

U + PH3, sev. h at 385°C; 2 h at 1400°C

(62)

UP2

annealing

U + Pred, 1 d at 400°C; 7 d at 800°C

(171)

U3P4

annealing

U + Pred, 1 d at 600°C; 2 d at 900-1000°C

(172)

NpP

annealing

NpH3 + PH3, 5 h at 350°C; 2.5 h at 1000°C

(63)

Np3P4

annealing

Np + Pred, 16 h at 750°C

(173)

PuP

annealing

PuH3 + PH3, sev. h at 600°C; 2 h at 1500°C

(64)

AmP

annealing

AmH3 + Pred, at 580°C

(174)

CmP

annealing

CmH3 + Pred, at 580°C

(174)

BkP

annealing

Bk + Pred, 12 h at 540°C

(175)

Pa3P4

decomposition

PaP2

Post-transition metals Zn3P2

CVT

Zn + Pred, 70-80 h at 780-800°C → 450-460°C

(176)

ZnP2

CVT

Zn3P2, 90-100 h at 870-880°C → 480-490°C

(176)

ZnP4

P-annealing

Zn + Pred, 2 h at 1000°C and 40 kbar

(177)

Cd3P2

CVT

Cd + Pred; 70-80 h at 670-680°C → 450-460°C

(176)

Cd7P10

P-annealing

Cd + Pred, 30 min at 600°C and 25 kbar

(178)

Cd at 850°C + Pred at 500°C

(179)

CdP2

CVT

CdP4

annealing

Cd + Pred, 3-4 d at 570°C

(180)

B13P2

annealing

BP, at 1100-1400°C

(66)

BP + H2, 11h at 1250°C

(67)

B6P

Table 1. (Continued). Details of Synthetic Procedures for Binary Metal Phosphidesa Composition

Synthetic Method

Conditions

Ref.

125

B + Pred, 1 d at 400°C; 0.5 d at 800°C; HCl (1:1 v/v)

(68)

annealing

Al + Zn3P2, 5 h at 800-900°C; 30 min at 1000-1100°C

(65)

GaP

annealing

Ga + Zn3P2, 1 d at 800°C; 30 min at 1000-1100°C; HCl

(65)

InP

annealing

In + Zn3P2, 2 d at 800°C; 30 min at 1000-1100°C; HCl

(65)

InP3

P-annealing

In + Pred, 30 min at 1200°C and 30 kbar; → 600°C for 1.5 h

(181)

TlP3

annealing

Tl + Pred, 28 h at 480-500°C

(182)

TlP5

annealing

Tl + Pred, 4 d at 350°C; grinding; 12 d at 365°C

(183)

Si12P5

annealing

amorphous SiP film; 30 min at 1100°C

(184)

SiP

P-annealing

Si + Pred, 1700-1800°C at 40-50 kbar

(69)

SiP2

CVT

Si + Pred, 900°C → 500°C

(70)

GeP

Bi flux

Ge + Pred, 4 d at 950°C; 5 d → 300°C; 1 d at 300°C; dilute HCl/H2O2

(185)

GeP3

P-annealing

Ge + Pred, 1 h at 800°C and 30 kbar; → 500°C for 3 h

(186)

GeP5

P-annealing

Ge + Pred, 1 h at 900°C and 65 kbar; → 500°C for 3 h

(186)

Sn4P3

solvothermal

Sn + Pred in ethylenediamine, 40 h at 200°C; 0.1 M HCl, 12 h

(187)

BP

Sn flux

AlP

SnP

annealing

Sn + Pred, at 700°C

(188)

Sn3P4

annealing

Sn + Pred, 7 d at 460°C

(189)

SnP3

annealing

Sn + Pred, 7 d at 400°C; grinding; 7 d at 520°C

(190)

PbP7

Pb flux

Pb + Pred, 2 d at 150°C; 2 d at 300°C; 2 d at 400°C; H2O2/CH3COOH (1:1 v/v)

(72)

1. CVT = chemical vapor transport; P-annealing = annealing under pressure. 2. If the synthesis involved multiple steps, the corresponding stages are separated from each other by semicolons (;). The rates of temperature increase or decrease are not provided; they can be found in the original articles. 3. If the duration of an annealing step is not mentioned, then the original source did not provide such information. b Sc and Y have been grouped with lanthanides due to the similarity of chemical properties. a Notes:

Acknowledgments The author gratefully acknowledges support of research endeavors in his laboratories by the National Science Foundation (award DMR-1905499).

References 1. 2. 3. 4. 5.

6.

7. 8. 9.

10. 11.

12. 13. 14. 15. 16. 17.

18. 19. 20.

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Chapter 7

Black Phosphorus Based Photodetectors Bowei Dong, Li Huang, Chengkuo Lee, and Kah-Wee Ang* National University of Singapore, Department of Electrical and Computer Engineering, 4 Engineering Drive 3, Singapore 117583 *E-mail: [email protected].

The layered two-dimensional (2D) black phosphorus (BP) has found significant applications in nanoelectronics and nanophotonics. In particular, it has been proven as a promising material for photodetectors due to its narrow direct bandgap ranging from 0.3 eV to 2 eV, high carrier mobility, modulation capability, and polarization sensitivy. The modulation capability of BP based photodetector is achieved by gate control using field effect transistor structure. Homojunction and heterojunction have been demonstrated to suppress dark current while enhancing carrier collection efficiency. Plasmonic effect and avalanche breakdown are exploited to boost the responsivity further. To extend the detection wavelength of BP, electric field modulation and arsenide doping are investigated, leading to a wide detection wavelength up to 8 µm. Owing to the ease of integrating BP with diverse substrates due to BP’s 2D nature, waveguide-integrated BP based photodetectors are realized in the near-infrared (NIR) for telecommnunication applications and in the mid-infrared (MIR) for on-chip sensing applications. High speed and high responsivity can be achieved by optimizing the device design. Despite that the mechanical robustness of BP on waveguide remains challenging, nevertheless, wafer-level growth of BP is highly desirable to realize scalable integration for mass production.

© 2019 American Chemical Society

Introduction Ever since black phosphorus (BP) was rediscovered as a layered two-dimensional (2D) material in the year of 2014 (1), numerous works on BP-based photodetectors have been reported. The narrow finite bandgap, high carrier mobility, and anisotropic lattice structure of BP promise broadband detection from ultraviolet to mid-infrared (MIR), relatively low dark current, effective photocarrier collection, and polarization sensitivity. In this chapter, we review the development of BP-based photodetectors, highlight the merits of the important works, and share our opinion on the possible improvements in these works. To better explain the development progress of BP based photodetectors, we briefly introduce the evaluation criteria for photodetectors. Responsivity indicates the amount of extra current (or voltage) generated by unit incident power, which is an indicator most commonly used to evaluate the performance of a photodetector. Besides responsivity, there are another few parameters to assess photodetectors. Cut-off wavelength is constrained by the bandgap of the detection material and determines the spectral detection range of the photodetector. The capability to detect longer wavelengths up to MIR is especially important as there is a variety of applications in MIR while the solution to highly integrated MIR systems is still inadequate. Dark current is also of importance because it is closely linked to the power consumption and noise level. Noise level and responsivity together determine the noise equivalent power (NEP), which is the amount of light power needed to generate a signal that is equal to the noise, indicating the weak power detection limit. Apart from how much a photodetector responds to the light signal, we also care about how fast a photodetector responds in occasions such as data communication. The speed of a photodetector could be characterized by the rise and fall time of photocurrent, the 3dB bandwidth in frequency response, and the eye diagram. Furthermore, the ability to detect polarization enables a wide range of applications such ellipsometry, automotive detection in hazy weathers (2), and three-dimensional orientation analysis of bio-molecules (3). During the past few years, research on BP based photodetectors have been dedicated to improving responsivity, extending spectral detection range, supressing dark current, investigating time response, enhancing polarization sensitivity, and realizing waveguide integration.

Surface Illuminated BP Based Photodetectors Overview of Surface Illuminated BP Based Photodetectors In surface-illuminated photodetectors, light travels through free space and is received by the photodetector from its surface. They are suitable for free space communication and imaging applications using photodetector arrays. Due to the lack of optical confinement, the interaction between light and the photodetector is largely limited by the thickness of the detection material. As a result, the absorption could be exceptionally weak in surface-illuminated photodetectors using 2D BP as the absorbing layer. In this case, the efficiency of photocurrent collection is notably important. Effort has been done to facilitate photocarrier separation and collection, including selecting proper contact metal, using p-n junctions, adopting shorter channel length, and utilizing field-effect to modulate doping condition and barrier height. Employing back reflector and plasmonics to enhance light-matter interaction have also been explored to improve responsivity. In addition, avalanche effect has been observed in InSe/BP heterostructure (4), which could be exploited to multiply photocarrier in order to achieve high responsivity. Although not all the demonstrations succeeded in achieving a phenomenal improvement, the ideas behind them are worthy of investigation for further 136

optimization. Thanks to the trap-induced photoconductive gain, high responsivity BP based photodetectors have been achieved with surface-illumination for a broad range from ultraviolet to MIR. Photodetectors Based on Metal-BP-Metal Structures with or without Gate In 2014, Buscema et al. observed the photoresponse of a BP based field-effect transistor (Figure 1a) for 640 to 940 nm wavelength with a responsivity of 4.8 mA/W at 640 nm (5). Meanwhile, Engel et al. demonstrated a metal-BP-metal photodetector on SiO2/Si substrate for multi-spectral imaging (6). Submicron resolution was achieved. These results revealed BP’s potential for broadband photodetection beyond the detection range of large bandgap TMDCs. However, compared to commercial detectors, the responsivity was still too low, especially when considering that the BP flake in the work of Engel et al. was already 120 nm thick. At the early stage of BP research, the low responsivity of BP based photodetectors was possibly due to a lack of design of the device structure. The channel length, thickness, and contact material may be chosen randomly, resulting in the poor performance. With further optimization, high performance BP based photodetectors could be realised for broadband spectrum.

Figure 1. (a) Typical surface-illuminated BP based photodetector with bottom gate. (b) Fast response to visible light, and (c) Slow response to ultraviolet in a BP based photodetector. (d) Schematics of a MIR BP based photodetector with interdigitated electrodes and its power-dependent response. ((a) Reproduced with permission from reference (5). Copyright 2014 American Chemical Society; (b)&(c) Reproduced with permission from reference (7). Copyright 2015 American Chemical Society; (d) Reproduced with permission from reference (10). Copyright 2016 American Chemical Society.) In 2015, ultraviolet BP based photodetector with high responsivity has been reported by Wu et al. (7) Using a simple metal-BP-metal structure with 4.5 nm thick BP channel, the detector achieved a responsivity in the order of 104 A/W at 310 nm, corresponding to a photoconductive gain of 107108. To explain the mechanism for the high gain, time response of the photocurrent was studied for visible-to-near-infrared and ultraviolet wavelength range. While fast response (a few millisecond) was observed for visible-to-near-infrared wavelengths (Figure 1b), a combination of fast and slow 137

response (a few hundred seconds) were presented for ultraviolet response (Figure 1c). The slow response was originated from the delayed carrier recombination through trap states. These traps, on the other hand, contributed to the large photoconductive gain by lengthening the carrier life time in BP. Longer wavelength with lower photon energy is less likely to excite electrons from valence band to the trap states, therefore, the visible to near-infrared light induced a faster response but a lower responsivity than ultraviolet. Their results indicate that there is a trade-off between responsivity and detection speed when leveraging trap-induced gain for photodetection. In 2016, Huang et al. demonstrated high responsivity (~106 A/W @ 633 nm, ~103 A/W @ 900 nm) photodetectors based on field-effect transistors with 8-nm thick BP channel for wavelengths ranging from 400 to 900 nm (8). Unlike previous works using Ti as the metal contact for photocarrier collection, they chose Ni as the contact. The high workfunction Ni largely reduced the contact resistance for holes and enabled more efficient photocarrier collection. A channel length study was performed on the same BP flake with different contact distances. Under the same drain bias, responsivity was observed to be inversely proportional to the square of channel length. This is because in a shorter channel, photocarriers are accelerated by a larger electric field to travel through a shorter transit distance before being collected. In addition, temperature dependence showed that both dark current and photocurrent increased with decreasing temperature, due to less carrier scattering in cryogenic conditions. It is worth noting that their detectors were operated in the on-state of the BP transistor, with a large dark current which leads to high power consumption and may also compromise the detectivity. The authors claim that when gate was biased to off-state, the Schottky barrier was too high for the photocarriers to reach the contact before recombination. It is possible to overcome this problem if the initial doping in BP could be controlled with future development of material growth technique and the appropriate contact material could be selected accordingly. Several works on photodetectors based on BP field effect transistors for short-wavelength infrared and MIR showed that optimized photocurrent could be achieved near the off-state where dark current is supressed (9–12). Guo et al. reported BP based photodetectors for a MIR wavelength of 3.39 µm with a responsivity of 83 A/W under 0.5 V bias (10). Their detector with high gain benefited from the trap states in BP’s bandgap and the interdigitated electrodes for efficient photocarrier collection (Figure 1d). The back-gate voltage was applied to tune the fermi level in the BP channel for the optimization of photocurrent and NEP. The 3dB bandwidth of their photodetector was in the order of kilo-Hertz due to the long carrier life time, as expected with the presence of such high gain. Photodetectors Based on BP Homojunctions Yuan et al. demonstrated a polarization-sensitive BP based photodetector (13). A ring-shaped electrode (Figure 2a) was designed to guarantee that the polarization dependence is solely from the in-plane anisotropy of BP. To efficiently separate photo-generated electron-hole pairs, a vertical homogeneous p-n junction was formed by electrical gating using ionic gel. The gate bias contributed to enhancing the polarization sensitivity (Figure 2b). Utilizing the ambipolar property in BP, lateral homogeneous BP p-n junctions could be formed by electrostatic gating from two adjacent gates on a uniform BP layer (Figure 2c) (14). Unlike the vertical junctions with large active area, the effective area of lateral diodes are limited by the junction width, which may restrain the device from achieving a high efficiency because only those light illuminated on the junction could be efficiently converted to electric signal.

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Figure 2. (a) Microscope image of a BP based photodetector with a ring-shaped electrode and a vertical homo-p-n junction formed by ionic gel gating, and (b) Photocurrent mapping of the device showing gate bias increased the polarization sensitivity. (c) Microscope image of a lateral p-n photodiode formed by electrostatic gating on BP. ((a)&(b) Reprinted by permission from Springer Nature: Springer Nature, Nature Nanotechnology (13), Polarization-sensitive broadband photodetector using a black phosphorus vertical p-n junction, H. Yuan et al., Copyright (2015); (c) Reprinted by permission from Springer Nature: Springer Nature, Nature Communication (14), Photovoltaic effect in few-layer black phosphorus PN junctions defined by local electrostatic gating, M. Buscema et al., Copyright (2014).) Photodetectors Based on BP-TMDCs Heterojunctions Photodetection using narrow bandgap materials is usually accompanied with high dark current due to thermal excitation, compromising the detectivity of the photodetectors. Introducing a larger bandgap material to form a vertical heterojunction is an effective solution to overcome this drawback. While the narrow bandgap material enables long wavelength detection, the large bandgap material supresses dark current. In the meantime, the built-in field in the heterojunction separates the photogenerated electron-hole pairs for efficient photocurrent collection. Deng et al. demonstrated BP/MoS2 based p-n diodes for photodetection and photovoltaic energy conversion at a visible wavelength of 633 nm (Figure 3a) (15). They investigated the modulation of diode performance from back gate and diode bias. When the device is operating under a reverse bias, dark current could be supressed to achieve high photocurrent-dark-current ratio. Similar structure for near-infrared has been studied by Ye et al. (16) 139

Ye et al. then realised a photodetector for visible to near-infrared light using BP-on-WSe2 structure (Figure 3b) (17). The WSe2 layer with the larger bandgap was used as the conducting channel with low dark current, meanwhile BP with a smaller bandgap serves as a photogate and the absorption material for near-infrared. Polarization dependent photocurrent mapping of the structure was also investigated.

Figure 3. (a) Schematics of a BP/MoS2 hetero p-n diode. (b) Photodetector using WSe2 as the conducting material while BP serves as photogate, absorbing material and introduces polarization sensitivity. ((a) Reproduced with permission from reference (15). Copyright 2014 American Chemical Society; (b) Reproduced with permission from reference (17). Copyright 2017 Elsevier.) Enabling Longer Wavelength Detection by Reducing Bandgap The minimum of the layer-dependent bandgap of BP is 0.3 eV when BP is in bulk form, corresponding to a cut-off wavelength of 4.1 µm. Measures should be taken to further reduce the bandgap if it is desired to go beyond this cut-off wavelength to detect photons with energy lower than 0.3 eV.

Figure 4. (a) Cross section schematics of a BP based dual-gate field-effect transistor for bandgap tuning to enable longer wavelength detection. (b) Photocurrent tuned by top and bottom gate at 7.7 µm wavelength at a cryogenic temperature of 77K. (c) Absorption spectra of black arsenic-phosphorus, indicating smaller bandgap and wider spectral detection range up to 10 µm. ((a)&(b) Reprinted by permission from Springer Nature: Springer Nature, Nature Communication (12), Widely tunable black phosphorus mid-infrared photodetector, X. Chen et al., Copyright (2017); (c) Reprinted with permission of AAAS from Science Advances 30 Jun 2017: Vol. 3, no. 6, e1700589. © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC) http://creativecommons.org/licenses/by-nc/4.0/). Both experimental results and theoretical calculations show that the bandgap of BP could be tuned by applying a vertical electric field, as a result of Stark effect (18, 19). Leveraging this mechanism, we could further narrow BP’s bandgap to below 0.3 eV so that more molecule fingerprints located at longer wavelengths could be covered by BP’s detection spectral range. Chen et 140

al. demonstrated tunable BP based photodetectors with dual-gate (Figure 4a) at the temperature of 77 K (12). The bottom gate serves to tune the BP’s absorption edge while the top gate controls the doping in BP. Their results show that the bandgap of BP could be reduced by 0.13 eV with 1.14 V/nm vertical displacement field from the bottom gate to enable photodetection up to 7.7 µm wavelength, while the photocurrent could be optimised when BP channel is modulated to charge-neutral point by the top gate (Figure 4b). Besides electrical control of BP’s bandgap, another approach to enabling MIR detection beyond 4.1 µm is adopting black arsenic-phosphorus (b-AsP) alloy as the detection material. The bandgap of b-AsP shrinks from ~ 0.3 eV to 0.16 eV with arsenic mole fraction increasing from 0% to 83%, extending the detection range to longer wavelengths (Figure 4c) (20). B-AsP based photodetectors have been demonstrated under room temperature for wavelengths up to 8 µm (21–23). As the bandgap gets smaller, more attention needs to be paid to reducing dark current, particularly when room temperature detection is still preferred. Compared to photodetectors with only b-AsP as the conducting channel, heterojunctions of b-AsP/MoS2 have been proven to effectively improve the detectivity by more than one order of magnitude for MIR detection under room temperature (21). Plasmonic Enhanced BP Based Photodetectors Plasmonics are able to localize free-space light within an area of sub-wavelength dimension (24). Introducing plasmonics to enhance light-matter interaction is a concept frequently considered in the design of photodetectors. Venuthurumilli et al. explored using plasmonic resonance in periodic metal nanostructures to enhance the performance of BP based photodetectors for near-infrared (Figure 5) (25). Bowtie antennas were utilized to increase responsivity, while bowtie apertures were claimed to supress absorption of zigzag-polarized light so as to increase polarization sensitivity. However, only a small amount of enhancement is achieved. Even in the simulation, the change of BP absorption due to the plasmonic resonance is far from impressive. To better justify the motivation to introduce metal nanostructures into BP based photodetectors, more theoretical calculations need to be performed in order to analyse how strong the plasmonic effect is at the target wavelength and if there will be any loss brought by the metal structure which partially covers the BP surface.

Figure 5. Illustration of BP based photodetector with plasmonics from of bowtie antenna and bowtie apertures. Bowtie antennas were utilized to enahance responsivity while bowtie apertures were designed to increase polarization sensistivity. (Reproduced with permission from reference (25). Copyright 2018 American Chemical Society.)

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Avalanche BP Photodetectors Leveraging avalanche breakdown, the number of photocarriers are able to multiply through impact ionization and weak signal could be magnified. Recently Gao et al. observed ballistic avalanche phenomena in InSe/BP heterostructures and demonstrated avalanche photodetectors at 4 µm wavelength under the temperature of 10 K (4). However, the avalanche phenomena only existed under cryogenic conditions in their devices, and gradually disappeared as temperature rose. BP Based Photodetectors for Polarization Resolving While many reported works on BP based photodetectors have explored the polarization sensitivity, demonstrating photocurrent varying with the polarization of incident light, most of them fail to decouple the effect of light intensity and polarization. For example, strong light with zigzag polarization and weak light with armchair polarization could induce the same amount of photocurrent. Neither the light power nor the polarization could be identified unless one parameter is already given. Until recently, Bullock et al. designed photodiodes based on BP/MoS2 heterojunctions with polarization resolving capability for MIR (26). They first extracted the complex refractive index of BP from measured reflection spectroscopy of BP on gold, and then used the extracted parameters to optimize MoS2/BP/Au structure for enhancing BP absorption. The Au layer serves as a back reflector and an electrode to collect photocurrent. They further proposed BP(armchair)/MoS2/BP(zigzag)/Au stack for polarization resolving (Figure 6a). Under a certain bias direction, only one of the BP/MoS2 diodes is in its working mode and will only respond to the light component that is polarized along BP’s armchair orientation (Figure 6b). This diode would become inactive once the bias is reversed while the other diode comes into play for detecting the light component polarized along the direction orthogonal to the previous one. By separating the light intensity along two orthogonal orientations, the possible polarization of linearly polarized incident light could be narrowed down to two directions.

Figure 6. (a) Structure of a BP/MoS2/BP/Au photodetector for polarization resolving. (b) Polarizationresolved spectral photoresponse of the two diodes. (Reprinted by permission from Springer Nature: Springer Nature, Nature Photonics (26), Polarization-resolved black phosphorus/molybdenum disulfide mid-wave infrared photodiodes with high detectivity at room temperature, J. Bullock et al., Copyright (2018).)

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Waveguide-Integrated BP Based Photodetectors Overview of Waveguide-Integrated Photodetectors Traditional photodetectors commonly adopt surface illumination scheme. Light shines vertically on the detection materials. The generated photocarriers are transported vertically and arrive at collection electrode. The surface illumination scheme has three major drawbacks. Firstly, the optical absorption path and the carrier transit path are coupled. The absorption layer has to be thick because the bottom and top portion of the deposited absorptive layer have low quality for photo carrier generation and transportation. The thick absorption layer may limit the detection speed. Secondly, the light only passes once through the absorption layer where most of the power transmits instead of gets absorbed. Thus, the light-matter interaction is limited. Thirdly, the outof-plane scheme hinders the miniaturization of photonics system due to its difficulty in on-chip integration. Consequently, research interests migrate to waveguide-integrated photodetectors. In waveguide-integrated photodetectors, light is routed in waveguides with sub-micron dimension. The absorption layer is directly deposited or positioned on top of the waveguide. Light is evanescently coupled to the absorption layer to generate photo carriers. The waveguide-integrated photodetector brings several benefits. The optical absorption path is decoupled from the carrier transit path. The evanescent coupling allows light to propagation parallelly with the absorption layer, and to gradually get fully absorbed to achieve the highest efficiency. Furthermore, due to the high efficiency, the light matter interaction region can be as small as tens of µm2 to hundreds of µm2. Compact chip-scale integrated photonics system is expected. Waveguide-Integrated Photodetectors in the Near-Infrared The development of waveguide-integrated photodetectors focuses on the near-infrared (NIR) wavelength range in its early years staring from the 1990s (27). Silicon (Si) waveguide fabricated from silicon-on-insulator (SOI) wafer is the most common platform for light routing. After the fabrication of Si waveguide with proper doping for ohmic contact, the wafer is passivated by silicon dioxide (SiO2) upper cladding with a selective opening window at the targeted absorption region. Germanium (Ge) is selectively epitaxially grown as the absorption layer thanks to the small lattice mismatch between Ge and Si (28, 29). A particular example is shown in Figure 7a (30). In 2007, Yin et al. reported a Ge-on-Si (GOS) based waveguide-integrated photodetector with a detection speed of 31 GHz at 1.55 µm wavelength for telecommunication applications (31). The waveguideintegrated Ge based photodetector works under 2 V reverse bias. The dark current of 267 nA is reported while a responsivity of 1.16 A/W is achieved. Besides Ge, SiGe alloy (32), Germanium-tin (GeSn) alloy (33) and doped Si (34, 35) are also sought for waveguide-integrated photodetectors. III-V and II-VI materials are the cornerstones in second-generation semiconductors. They find crucial applications in active devices such as light emitting devices and light detection devices (Figure 7b (36)) (37, 38). A broadband waveguide-integrated InGaAs based photodetector was demonstrated in 2010 (39). The InGaAs based photodetector mesa is integrated with the Si photonics chip through divinyldisiloxane benzocyclobutene (DVS-BCB) bonding. Under zero bias, the photodetector achieves a low dark current of 10 pA with a good responsivity of 1.1 A/W at 1.55 µm. Linear responsivity is realized under different reverse biases while 30 GHz detection speed is demonstrated. Most importantly, this device operates from 1.50 µm to 1.64 µm, covering the whole S, C, and L bands. 143

Figure 7. Common types of waveguide-integrated photodetectors. (a) Schematic of the waveguideintegrated Ge based photodetector. (b) Schematic of the waveguide-integrated III-V based photodetector/ laser. (c) Schematic of the waveguide-integrated graphene based photodetector working at 1.55 µm. (d) Schematic of the waveguide-integrated TMDC based photodetector working at 1.16 µm. ((a) Reprinted by permission from Springer Nature: Springer Nature, Optical and Quantum Electronics (30), Heterostructure modeling consideration for Ge-on-Si waveguide photodetectors, A. Palmieri et al., Copyright (2018); (b) Reproduced with permission from reference (36). Copyright 2015 MDPI; (c) Reprinted by permission from Springer Nature: Springer Nature, Nature Photonics (40), Chip-integrated ultrafast graphene photodetector with high responsivity, X. Gan et al., Copyright (2013); (d) Reprinted by permission from Springer Nature: Springer Nature, Nature Nanotechnology (42), A MoTe2-based lightemitting diode and photodetector for silicon integrated circuits, Y. Bie et al., Copyright (2017).) Since the discovery of 2D graphene in 2004, graphene has been proven as a promising material for many applications including photo detection due to its zero bandgap, high electron mobility, and strong mechanical robustness. The waveguide-integrated graphene based photodetector was reported in 2013 (40). The schematic of the device is shown in Figure 7c. The monolayer graphene was transferred on top of the Si waveguide. Then electrodes were deposited for carrier collection. Under zero bias, a responsivity of 0.1 A/W is achieved while detection speed more than 20 GHz is realized. However, the zero bandgap of graphene also results in high dark current. In order to solve this problem, Wang et al. reported a graphene/silicon heterostructure waveguide photodetector (41). At 1.55 µm, the dark current is suppressed to 30 nA under 30 mW illumination. Other types of 2D materials are also utilized for waveguide-integrated photodetectors, especially transition metal dichalcogenide (TMDC). In 2017, an MoTe2-based light emitting diode (LED) and photodetector was reported which achieved full integration with Si photonics for 1.16 µm wavelength application (42). The schematic of the device is shown in Figure 7d. The device fabrication is similar to the waveguide-integrated graphene based photodetector. Although a dark current at pA level was reported, the responsivity is relatively low at 4.8 mA/W. The operating bias is as high as ±15 V; and the detection speed is limited to 0.2 GHz. As the wavelength moves to beyond 2 µm, Ge becomes transparent. Therefore, III-V, II-VI, and graphene become the material for photo detection. Waveguide-integrated GaInAsSb and graphene 144

based photodetector working at 2.3 µm (43) and 2.75 µm (41) were demonstrated with a responsivity of 1.4 A/W and 0.13 A/W respectively. It is noteworthy that doped Si is also a promising material for photo detection beyond 2 µm. In 2015, Jason et al. reported high speed photo detection at 2 µm using boron doped Si. The detection speed is as high as 15 GHz with a responsivity of 0.3 A/W (44). However, the working reverse bias of this photodetector is 30 V. Waveguide-Integrated BP Based Photodetectors in the Near-Infrared

Figure 8. Reported works of waveguide-integrated BP based photodetectors working in the NIR. (a) Schematic of the waveguide-integrated BP based photodetector with high responsivity and low dark current. (b) Mode profile showing light-BP interaction. (c) Eye diagram of the BP based photodetector working at 3 GHz. (d) Schematic of the 3D integration of BP based Photodetector with Si photonics and nanoplasmonics. (e) Mode profile showing Si photonics-BP-plasmonic interaction. ((a-c) Reproduced with permission from reference (45). Copyright 2015 Springer Nature; (d&e) Reproduced with permission from reference (46). Copyright 2017 American Chemical Society.) There are several reports on waveguide-integrated BP based photodetectors working in the NIR. The first work dates to 2015 from Dr Mo Li’s group from the University of Minnesota (45). In this BP based phototransistor, BP was directly transferred on top of Si waveguide. The source and drain electrodes were fabricated followed by Al2O3 deposition as the gate dielectric. Graphene was used as the gate electrode (Figure 8a). BP was placed closely to the Si waveguide in order to achieve a strong evanescent coupling between the light propagating in Si waveguide and the BP layer as shown in Figure 8b. Under the gate bias of -8 V and source/drain bias of 0.4 V, dark current of 220 nA and responsivity of 0.15 A/W were achieved in a device with 11.5 nm thick BP working at 1.55 µm. The thin BP layer also helps realize a detection speed of 3 GHz. The respective eye diagram for high speed is presented in Figure 8c. In another device with 100 nm thick BP, though the detection speed was comprised, 0.7 A/W responsivity was demonstrated under the source/drain bias of 2V and the same gate bias of -8 V. Following the first work, Dr Mo Li’s group further demonstrated the threedimensional (3D) integration of BP based photodetector with Si photonics and nanoplasmonics (46). As shown in Figure 8d, light is directed upward by Si photonics grating coupler. The source and drain electrodes are placed close enough to form a 60 nm air slot that supports collective oscillation of electron clouds, i.e. plasmonics. While the electrodes provide carriers collection function, they 145

also serve as a focusing structure to concentrate light beyond the diffraction limit in the extremely narrow gap. The electric field is significantly enhanced in the nanoslot region to provide strong lightBP interaction as shown in Figure 8e. Under the source/drain bias of 1.5 V, 10 A/W responsivity was achieved in such BP photoconductor. However, the maximum detection speed is limited to 150 MHz in the phototransistor due to the device RC time constant. Waveguide-integrated BP based photodetector also found its application for 2 µm wavelength (47). Dark current between 20 to 30 µA is revealed in devices with around 40 nm thick BP. For 2 µm wavelength, a constant responsivity of 0.3 A/W was reported in the temperature range from 20°C to 65°C. The maximum detection speed of 4 Gbit/s was demonstrated. This is promising for telecommunication application at 2 µm to complement the overcrowded 1.55 µm. Apart from Si waveguide, BP based photodetector was successfully integrated with chalcogenide glass (CHG) waveguide (48). The CHG waveguide and source/drain electrodes are fabricated by deposition and lift-off. Two perpendicular electrode pairs are aligned respectively with the armchair and zig-zag direction of BP for the characterization of specific polarization. Although the BP photoconductor operates in a broad wavelength range from 2.1 µm to 2.5 µm with a small bias of 0.1 V, the responsivity of the device is limited to 10 mA/W. Waveguide-Integrated BP Based Photodetectors in the MIR MIR beyond 3 µm is a critical wavelength range for sensing applications. It contains two atmospheric transmission windows (3-5 µm and 8-12 µm), benefiting thermal imaging and remote sensing (49). It also covers many vibrational molecular fingerprints of chemical and biological agents, including C-H, N-H, O-H, etc. (50) Up to date, methane sensing (51), glucose sensing (52), protein detection (53), exhaled breath monitoring (54), and genome detection (55) have been demonstrated using MIR spectroscopy. MIR waveguide-integrated photodetector plays an important role in miniaturization of photonic systems to realize chip-scale spectroscopic sensing. Nevertheless, the limitation of the selection of absorptive layer material hinders the development of integration. IIIV and II-VI compounds are restricted due to the lattice mismatch between Si and these materials. Graphene experiences high noise level and high power consumption due to high dark current. The BP is promising for MIR waveguide-integrated photodetector. It has a small direct band gap of ~0.3 eV; and possesses good integration capability on varied substrate materials by virtue of its 2D nature. The waveguide-integrated BP based photodetector working in the MIR was demonstrated in 2019 by Huang et al. (11) Theoretically, the direct bandgap of ~ 0.3 eV in bulk form BP corresponds to the absorption cutoff at 4.13 µm. A roll-off in the extinction coefficient of BP is also observed from 3.5 µm afterwards. However, in the work of Huang et al., satisfactory responsivities were achieved in the waveglength range between 3.68 µm to 4.03 µm in both phototransistor and photoconductor. The schematic and optical image of the device are shown in Figure 9a-9c. The device fabrication of Si photonics devices was completed using an 8 in complementary metal-oxide-semiconductor (CMOS) fabrication line. After Si waveguide pattern definition, the whole wafer was covered by depositing SiO2 upper cladding. The upper cladding was thinned down to 200 nm above the Si waveguide top surface to provide passivation, to ensure a smooth and flattened surface for BP integration, and to allow significant evanescent wave-BP interaction for photo detection. The following BP based photodetector fabricaiton is similar to Dr Mo Li’s work in 2015. When operating near the cutoff wavelength of BP where absorption is weak, the light-BP interaction is enhanced by exploiting the optical confinement in the Si waveguide and grating structure to overcome the limitation of absorption length constrained by BP thickness. Figure. 9d 146

and 9e show the simulation of light propagation and distribution in the whole integrated system at 3.78 µm, and light distribution in a cross section of the grating structure respectively. The light was launched in the Si waveguide system by an optical fiber via an input grating coupler. The transverse electric (TE) polarized light was guided to the output grating coupler and directed upward to the BP based photodetector.

Figure 9. Waveguide-integrated BP based photodetector working in the MIR. (a) Schematic of the device. (b) Zoom-in view of the BP based photodetector. (c) Optical image of the device. (d) Simulation of light propagation and distribution in the whole integrated system at 3.78 µm. (e) Light distribution in a cross section of the grating structure. (f) Contour plot of gate and drain dependent photocurrent under 237 µW illumination at 3.78 µm. (g) Energy band diagrams for the four quadrants labeled I, II, III, and IV in (f). (h) Power dependent photocurrent and responsivity of the phototransistor. (i) Spectral responsivity of three photoconductors with distinct BP thickness and orientation. (j) Percentage of light propagating upward and downward at the output grating coupler with and without metal gate. (Reproduced with permission from reference (11). Copyright 2019 Americal Chemical Society.) Figure. 9f presents the contour plot of gate and drain dependent photocurrent in the phototransistor under 237 µW illumination at 3.78 µm. The amount of photogenerated carriers is only dependent on the illumination power. Hence, the photocurrent is determined by the 147

photocarrier collection efficiency, which is modulated by the energy band alignment controled by the gate bias Vg and the source/drain bias Vd. The contour map is divided into four quadrants I, II, III, and IV with distinct energy band alignment (Figure 9g). As shown in Figure 9f, in quadrant IV at Vg-V0 = 1.5 V and Vd = -1 V, the maximum photocurrent of around 30 µA is achieved. When Vg-V0 is positive, the fermi level is closer to the energy band of BP, resulting in a reduced hole barrier. Negative Vd also helps reduce hole barrier. However, the photocurrent finds its maximum at specific Vg-V0 because as Vg further increases, the BP moves into the heavily n-doped regime where electron traps are occupied and could not contribute to photoconductive gain. Figure 9h shows the power dependent photocurrent and responsivity of the phototransistor. A sublinear relationship is observed. At low illumination power, due to the limited amount of photo generated carriers, there are few occasions of radiative recombination that deteriorates the responsivity. Meanwhile, electron traps which are attributed to photoconductive gain are active. Nevertheless at high illumination power, more radiactive recombinations happen and electron traps are fully occupied due to the huge amount of photo generated carriers. The spectral responsivity of three photoconductors with distinct BP thickness and orientation is shown in Figure 9i. Satisfactory responvities are achieved in all three devices. The maximum of 7 A/W and 1 A/W are realized at 3.68 µm and 4.05 µm respectively under 5 µW illumination power. Comparing device A&B in which the armchair direction of BP is aligned with the TE direction of light, it is demonstrated thicker BP grants higher responsivity due to higher interaction volume. However, thicker BP may result in higher dark current and slower detection speed. Comparing device B&C whose BP thickness are the same, it is observed higher responsivity is achieved when the BP armchair direction is aligned with the TE direction of light. And the difference increases drascticly as wavelength rises. It suggests that special care must be taken when transferring the BP flake to ensure proper alignment. Figure 9j compares the percentage of light propagating upward and downward at the output grating coupler with and without metal gate. Without metal gate, almost 50% of light are scattered upward and downward by the grating coupler. However, with metal gate on top of BP layer, less than 10% of light is directed upward to get absorbed by BP. This can be attributed to the ohmic loss introduced by the metal gate. Therefore, photoconductors present higher responsivity compared to phototransistors. Yet, photoconductors do not provide on/off capability and gate tuinng capability. The solution could be replacing the Au metal gate by graphene gate or indium tin oxide (ITO) gate which bring less ohmic loss.

Conclusion For surface illuminated BP based photodetectors, photoresponse could be improved by either increasing the efficiency of photocarrier collection or enhancing the weak light-matter interaction. Field-effect transistors based on the detection material could be adopted to lower doping concentration in order to supress dark current and carrier scattering, modulate the fermi level for trap-induced photoconductive gain and adjust the barrier height for photogenerated carriers, all of which serves to achieve more efficient photocarrier collection. High gain induced by traps usually compromises the speed of the photodetector, as a result of the slow de-trapping process of photocarriers. Forming p-n junctions is an effective approach to supressing dark current as well as facilitating the separation of photogenerated carriers, which contributes to low noise and high detectivity. To extend the cutoff wavelength of BP, the bandgap could be reduced by either applying a vertical electrical field or introducing arsenic into BP to form b-AsP as the detecting material. Detection up to 8 µm wavelength has been demonstrated. Apart from narrow bandgap which enables 148

MIR photodetection, the anisotropic in-plane lattice structure of BP is another advantage over the other 2D materials. Polarization resolved BP based photodetectors have been demonstrated with potential for infrared polarization imaging which could be employed in various applications such as automotive monitoring and bio-molecule sensing. Thanks to the ease of integrating BP on varied substrate, waveguide-integrated BP based photodetectors are envisaged for dense telecommunication application in the NIR and on-chip sensors in the MIR. Especially for the MIR, integration of photodetectors with Si photonics is currently a bottleneck for the development of chip-scale sensors. BP is a promising candidate to fill this gap owing to its small direct bandgap and the feasibility of extending the detection wavelength by electric field tuinng or arsenide doping. In the MIR on-chip sensor, spiral waveguide can be used for light/analyte interaction while a PDMS gas cell can be bonded to the chip for gas input/output. However, it is noteworthy to point out the integration of silicon photonics with light source such as LED and light source is still a grand challenge for full integration. The major challenge of BP based photodetectors is the synthesis of wafer-scale thin-film BP. As suggested in this chapter, the most commonly used method is tape exfoliation. Such method is efficient and useful for small-scale production for fundamental research purpose but not scalable and controllable. Sonication assisted liquid phase exfoliation is extensively studied (56). Size-control can be realized by centrifugation. However, continuous BP films cannot be formed using this method, hindering its application in wafer-scale processing. Some other top-down methods including electrochemical exfoliation, laser irradiation, thermal annealing, and bottom-up methods including pulsed laser deposition and hydrothermal are also investigated (57). In particular, CVD-type bottom-up method is most promising for wafer-scale thin film BP synthesis from the authors’ perspective. In 2015, X. Li et al. reported the synthesis of thin-film BP on a flexible polyester substrate with a diameter of 4 mm (58). The field-effect mobility is relatively poor at 0.5 cm2 V-1 s1. In 2018, the same group advanced the technique and demonstrated a thin-film BP on a sapphire substrate with 5 mm diameter (59). The field-effect mobility is 160 cm2 V-1 s-1 at room temperature, reaching the same order as bulk BP. Larger scale thin-film BP synthesis method is desired and anticipated to achieve real wafer-scale thin-film BP formation. Mass production is then feasible using wafer-scale processing. The challenge of waveguide-integrated BP based photodetector lies in its mechanical robustness. The mechanical robustness of BP is weaker than some other 2D materials such as graphene. It poses a challenge during BP/waveguide integration because BP needs to climb across the waveguide ridge. Two solutions have been provided. In the first solution, the photoresist is remained on top of the waveguide after Si waveguide etching. SiO2 is deposited to the same height as the waveguide followed by lift-off to remove the photoresist. The resultant structure is a flat surface with waveguide in the middle and SiO2 plethoras at two sides. Yet, such method requires strict control of SiO2 deposition rate (45). The second solution involves SiO2 upper cladding deposition followed by chemical-mechanical planarization to provide a flat and smooth surface (11). Besides these two reported solutions, another solution could be the utilization of waveguide with subwavelength grating (SWG) cladding. The SWG is a periodic structure with period much less than the bragg wavelength of light. It serves a homogeneous material when light propagates through (60). Such periodic cladding structure provides stronger mechanical support than the traditional ridge waveguide.

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Chapter 8

Photocatalytic Property of Phosphorus Yang Liu,1 Jie Li,1 Zhuofeng Hu,2 and Jimmy C. Yu1,* 1Department of Chemistry, The Chinese University of Hong Kong,

Shatin, New Territories 999077, Hong Kong, China 2School of Environmental Science and Engineering, Guangdong Provincial Key Laboratory

of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou 510275, China *E-mail: [email protected].

Elemental phosphorus photocatalysts, usually red phosphorus (RP) and black phosphorus (BP), feature wide-range visible-light absorption, excellent charge separation and transportation, and customizable band-edge potentials. These merits make RP and BP photocatalysts compelling for clean energy production and pollutant removal. In this chapter, we review advances of RP and BP photocatalysts and propose roadmaps for their future development in material design and photocatalytic application.

Introduction The excessive consumption of non-renewable fossil fuels not only causes the energy crisis but also increases greenhouse gas emissions to endanger the environment. Solar energy is a sustainable, green energy-resource alternative to fossil fuels. Photocatalysis driven by inorganic semiconducting materials provides a promising approach of utilizing solar light to produce clean energy and to degrade pollutants. The ground-breaking study in semiconductor photocatalysis is the use of TiO2 as electrode to catalyze water photolysis reported by Fujishima and Honda in 1972 (1). Since then, TiO2-based materials emerged as the first generation of semiconductor photocatalyst, transition metal-based oxides, sulfides, and nitrides, such as TiO2 (2), CdS (3), and BiVO4 (4), as the second generation, and metal-free materials with visible-light-responsive band-gap, such as g-C3N4 and nonmetal elemental photocatalysts (e.g. phosphorus (5), sulfer (6) and boron (7)) as the third generation. Despite these advances, developing semiconductor photocatalysts having deep visiblelight absorption while maintaining high charge separation and tunable band potentials, remains challenging for the field. Elemental phosphorus photocatalysts offer the promise to address the challenges, as they feature wide-range visible-light absorption, excellent charge separation and transportation, and customizable band-edge potentials. Among the phosphorus photocatalysts, red phosphorus (RP) © 2019 American Chemical Society

and black phosphorus (BP) are most widely studied. The pioneering study on RP dates all the way back to 1669 (8). However, its photocatalytic property was discovered only recently. In 2012, Wang et al. reported that RP could harvest visible light to split water for hydrogen evolution (5). The systematic study on BP was initiated in 1914 (9). The potential of BP as a photocatalyst did not attract much attention until the exfoliation of the bulk BP into the few-layered ones (10). The demonstration by Lee’s group that BP-based photocatalysts are promising for bacterial inactivation and organic pollutant removal, pushes BP into the limelight of semiconductor photocatalysis field (11). This review details the remarkable advances in the development of RP and BP photocatalysts. Strategies for further performance improvement and applications in energy and environment are also discussed.

Red Phosphorus (RP) General Different allotropes of RP have been synthesized and explored for photocatalytic applications, including hexagonal (type-II) P (12, 13), Hittorf P (type V) (5, 14–16), and fibrous P (type VI) (17, 18). Among them, the structures of Hittorf and fibrous phase have been characterized in detail (14, 17). (Figure 1) Both of them are composed by covalently linked phosphorus pentagonal tubes that comprise sequences of P8 and P9 cages linked by a P2 chelating bridge. The P tubes in Hittorf P are perpendicular to each other, whereas the counterparts in fibrous P are parallel. All of them are p-type semiconductors with direct band-gap ranging from 1.4 to 1.7 eV (5, 16, 18, 19). This narrow band-gap endows RP with broad-range, tunable light absorption across the visible light, of which the maximum threshold is up to 700 nm (Figure 2). Illumination on RP by the light with the energy higher than the band-gap excites electrons from the valence band (VB) to the conduction band (CB), and, meanwhile, creates holes residing on the VB. The photo-generated charge carriers could either participate directly in the reaction or induce the generation of reactive species such as 1O , •O -, •OH, and H O . RP can be well dispersed in H O. When applied for photocatalytic 2 2 2 2 2 water splitting, it can easily bond with H2O to lower intermediate energetics to enhance electrondonating/-accepting abilities and thus the catalytic efficiency. RP has strong adsorption toward organic pollutant and bacteria, which facilitates photocatalytic pollutant degradation and disinfection.

Figure 1. The molecular structures of (a) Hittorf phosphorus and (b) fibrous phosphorus. Reproduced with permission from reference (16). Copyright 2019, Elsevier.

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Figure 2. The absorption edge of RP in different crystal type. Strategies To Enhance the Photocatalytic Performance The photocatalytic applications of RP focus primarily on water splitting, organic pollutant degradation, and disinfection. The efficiency achieved so far by RP photocatalysts is comparable to those of the traditional photocatalysts such as TiO2 and CdS, but remains below the requirement for practical, large-scale implementation (20, 21). The performance obstacles usually originated from the rapid recombination of the photogenerated electron-hole pairs, band-edge potential mismatching, and sluggish surface catalytic dynamics. Recently, a number of strategies have emerged as promising solutions to these challenges. Crystal Structure and Morphology Control The charge carrier mobility, a parameter of extensive significance to photoreactivity, is closely related to the crystal structure and the morphology of photocatalysts. Given the fast charge transfer and low charge trapping, crystalline RPs are expected to showcase higher photocatalytic performances than the amorphous counterpart. Usually, phase transformation would not occur in RP crystal until the temperature rises above 500°C, from which the material becomes flammable. A chemical vapor deposition (CVD) method was applied to avoid oxidation and obtain the crystalline RP (17). Amorphous RP and specific substrate material were sealed in a vacuum quartz tube and thereafter underwent a heating process. The amorphous RP was vaporized under high temperature. The RP vapor then deposited onto the substrate as the temperature decreased. After cooling to the room temperature, the crystals were collected. Different substrates were employed to achieve a controllable growth of different crystalline RP. RP submicron fibers with lengths of 20–30 µm and diameters of 400–600 nm can be obtained on the p-Si (100) wafer at 550°C (13). (Figure 3a-c). Its X-ray diffraction (XRD) pattern (Figure 3d) was indexed to the triclinic fibrous phosphorus (Figure 3e, based on crystallographic data CSD 391323). Its high-resolution transmission electron microscopy (HRTEM) image (Figure 3g) displayed clear parallel lattice fringes of (001) and (400) facets. The mechanism for the crystal growth was found to be the lattice matching effect between the fibrous phosphorus and substrates. The p-Si (100) nanowire wafer showed a lattice fringe of 5.43 Å for (001) and (010) facets. The value matched well with the interlayer spacing of 5.67 Å for fibrous phosphorus, and thus the growth along the long axis of fibers is energetically favorable. Similarly, type II RP submicron rods (3–5 µm in length with a diameter of 300–400 nm) preferentially grew on the α-Ti wafer. When p-Si (111) nanowire wafer was employed, there was no obvious lattice match between the substrate and the RP allotropes. A mixture of fibrous phosphorus submicron fibers and type II phosphorus submicron rods was thus obtained. Rhodamine B (RhB) degradation tests were conducted to evaluate the photocatalytic 157

efficiencies of the two allotropes. After 6 hours of visible light irradiation, 46.4% and 28.8% of the dye were degraded respectively by fibrous P and type II RP (2 mg catalyst per 20 mL of 10 ppm RhB solution). The apparent reaction rate of fibrous RP was ~1.6 times larger than that of type II RP. Colorless 2,3,6-trichlorophenol (TCP) was also degraded over the fibrous phosphorus by 41.9% under the same condition. (2 mg catalyst per 20 mL of 20 ppm TCP solution). The photoactivity of fibrous RP was superior to that of amorphous RP particles because the crystalline structure is beneficial for the charge separation. Micro-fibrous RP can also grow on SiO2 fibers (18). RP in the micro-fibrous P/SiO2 acted as the active component showing a record high photocatalytic H2 evolution rate (633 μmol h-1 g-1), which was over one thousand times that of amorphous RP. The same allotrope was also obtained when CuCl2 was added to the precursor. But the bulk crystal showed a lower efficiency of only 21 μmol h1 g-1. The activity could be dramatically improved to 684 μmol h-1 g-1 after the bulk RP was smashed

ultrasonically for 10 hours. The SEM and XRD patterns are shown in Figure 4a-d. The hydrogen evolution performances of micro-fibrous P/SiO2 and smashed-fibrous RP are displayed in Figure 4e. The superior activity was attributed to the RP microstructure in the micro-fibrous P/SiO2 and the smashed fibrous RP, the desired features favorable for efficient charge transfer. The micro-fibrous RP can be well dispersed in water to ensure sufficient contact with water. The high conduction band bottom (-0.9 eV versus normal hydrogen electrode (NHE)) of fibrous-phase P also contributed to their superior activity. The hydrogen evolution efficiency of the smashed fibrous P is superior among elemental photocatalysts. (Figure 4f) Liquid bismuth (Bi) was demonstrated to initiate the growth of RP microbelts (PMBs) (16). The crystal was indexed to Hittorf P according to the XRD data. A unique charge polarization effect was found in the crystal. (Figure 5c) Under visible light illumination, the photoexcited electrons preferentially migrated to the short edges of the belt, while the holes tended to locate in the middle. This behavior promoted the separation of the charge carriers and increased the electron density on the short edges functioning the active sites for H2 evolution. The edges were identified as (013) facets by HRTEM image. The charge polarization was mapped by photodeposition of Pt and MnOx. (Figure 5a and b) After the PMBs was irradiated with the metal precursors, Pt and MnOx particles were selectively deposited onto the short edges and the middle of the belts, tracking the positions, to which the photogenerated electrons and holes transferred, respectively. The charge distribution in the PMBs was also confirmed by the Kelvin probe force microscope (KPFM) and theoretical calculation. (Figure 5d and e) The H2 evolution rate of PMBs was calculated to be 513.3 μmol h-1 g1. The material also showed a six-fold improvement in the removal of methyl orange than amorphous

RP, in which the photoexcited charge transported randomly.

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Figure 3. (a–c) Representative SEM images, (d) XRD patterns, the blue curve was the simulated pattern based on the crystallographic data, (e) atomic structure and (f–h) TEM, HRTEM and SAED analysis of fibrous phosphorus submicron fibers obtained at -0.06 MPa, 100 mg RP and 550 °C. Reproduced with permission from reference (13). Copyright 2014, The Royal Society of Chemistry.

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160 Figure 4. SEM images of (a) micro-fibrous P/SiO2, (b) smashed-fibrous P, and XRD patterns of (c) micro-fibrous P/SiO2 and (d) smashed-fibrous P, bulk-fibrous P. The standard XRD patterns of fibrous P are placed in (c) and (d) as the references. (e) Time course of the hydrogen evolution on micro-fibrous P/SiO2 and smashed-fibrous P. (f) Comparison of the activity of photocatalytic hydrogen evolution on different elemental photocatalysts. The light source used or referred here are all visible light, except the mesoporous crystalline Si (full spectrum). Reproduced with permission from reference (18). Copyright 2016, Wiley-VCH.

Figure 5. SEM images of photo-deposition of Pt (a) and MnOx (b) on the PMBs. (c) Schematic for H2 evolution from H2O over the highly charge polarized PMB. (d) AFM image of a PMB. (e) Surface potential change after the introduction of 532 nm laser. (ΔCPD is the contact potential difference of the PMB section between in light and dark, detected by KPFM). Reproduced with permission from reference (16). Copyright 2019, Elsevier. Cocatalyst Loading Cocatalyst can improve the activity of a photocatalyst, although itself is photoinactive or has a negligible photoactivity. Platinum (Pt) is acknowledged as an effective cocatalyst for H2 evolution through modulating the electronic structure and suppressing the charge carrier recombination (22). It can trap electrons and provide proton reduction sites (23, 24). The Pt loaded RP was prepared by photochemical reduction of H2PtCl6 in the RP-contained aqueous suspension. The H2 production rate increased dramatically after 1 wt% Pt was loaded (5). The Pt was demonstrated to lower the activation potential of H2 formation. However, an excess amount of Pt would increase the number of electron-hole recombination centers, thereby deteriorating the photoactivity. With 1 wt% of Pt loaded, the activity of RP was increased by 12 times. Ni(OH)2 nanoparticles were reported by Dang, et al. to be an alternative to the Pt cocatalyst (25). The Ni(OH)2/RP prepared by a facile precipitation method at room temperature. The potential of Ni2+/Ni (0.23 V versus standard hydrogen electrode (SHE), pH=0) is between the CB of RP and the H+/H2 potential so that the excited electrons from CB of RP transfer to Ni(OH)2 where protons were reduced. The composite containing 0.5 wt% of NiOH exhibits 1.12 times enhancement than the Pt/RP. A composite photocatalyst of graphene quantum dots and RP (GQDs/RP) fabricated by ball milling was found to have an improved visible-light-driven (VLD) photocatalytic degradation performance of RhB (26). The deposition of GQDs onto RP lowered the surface potential in water, so the catalyst was more hydrophilic and contacted well with water. The ultra-small GQDs could also 161

facilitate the interfacial charge transfer. Photogenerated electrons preferentially moved towards RP because of the more negative CB of GQDs (about -0.9 V versus NHE) than that of RP. The apparent rate constant of the RhB degradation over GQDs/RP (0.071 min-1) is 1.8 times larger than over the ball-milled RP (0.039 min-1). Construction of Composite Materials The hybridization of two photocatalysts with different electronic structure will form an electric field between their interfaces (27, 28). The interfaces are favorable for effective spatial separation of photogenerated carriers. Several studies of RP-based hybrid photocatalysts were carried out to improve the photocatalytic performance of RP. Hierarchical RP/YPO4 hollow microspheres were developed for photocatalytic H2 evolution (29). (Figure 6a-d) The composites, which were fabricated by the reaction between amorphous RP and YCl3 aqueous solution via a hydrothermal method (Figure 6e), exhibited higher activity than each individual component. The hybrid was composed of crystalline YPO4 nanosheets and amorphous RP. The CB and VB of YPO4 were more negative than the reduction and oxidation potential of H2O, respectively. The photogenerated holes from RP were transferred to the VB of YPO4 in the RP/YPO4 composite. Thus the photogenerated charge carriers could be well separated. The rate of H2 evolution for the composite with 55 wt% YPO4 was 6 times higher than that for RP.

Figure 6. (a, b) SEM and (c, d) TEM images of resulted hierarchical YPO4/P hollow microspheres with different magnifications. Inset is the HRTEM image of the marked frame region in (d). (e) The possible formation process of hierarchical P/YPO4 hollow spheres. Reproduced with permission from reference (29). Copyright 2012, Elsevier. A single elemental heterostructure composed of black and red phosphorus (BP–RP) was reported to have a high photocatalytic activity, which could rival CdS (19). The hybrid was prepared by a facile mechanical ball milling method using commercial RP as the precursor. The XRD patterns of RP, BP, and their heterostructure are shown in Figure 7a. The in situ grown BP has an excellent interfacial contact with the RP component. The heterostructure exhibited stronger light absorption from 600 to 800 nm than pure RP. The CB of BP was calculated to be 0.26 eV versus NHE, which was lower than the CB of RP. That would facilitate the transfer of the electrons from the CB of RP to

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the CB of BP. (Figure 7b) The BP–RP heterostructure (0.0690 min-1) had higher RhB degradation activity than CdS (0.0105 min-1), especially during the first 30 min. (Figure 7c)

Figure 7. (a) The XRD patterns of RP, BP, and their heterostructure; (b) Schematic illustration showing the mechanism of its enhanced charge separation and transfer; (c) VLD photocatalytic degradation of RhB over CdS, red phosphorus and BP–RP heterostructure and their dark control curves. Reproduced with permission from reference (19). Copyright 2014, The Royal Society of Chemistry.

Figure 8. (a) Schematic representation of the proposed charge trapping model in g-C3N4 (left) and RP/gC3N4 (right). Normalized fs-TA decay kinetics (dotted lines) with exponential fitting curves (solid lines) of the sample dispersions (0.1 mg mL-1) in H2O probed at 560 nm under 400 nm excitation: (b) short time scale, (c) long time scale; (d) ns-TA decay kinetics of the sample dispersions (0.1 mg mL-1) in H2O probed at 460 nm under irradiation of 400 nm laser. Reproduced with permission from reference (34). Copyright 2017, Wiley-VCH. Graphitic-like carbon nitride (g-C3N4) is a good partner to couple with RP for photocatalytic reaction due to its moderate bandgap (~2.7 eV) and high stability (30–33). Ansari et al. synthesized RP/g-C3N4 composite by ball milling, and found its high activity when using it as a photocatalyst for the degradation of organic compounds and as a supercapacitor electrode. Jing et al. obtained the RP/g-C3N4 using a CVD method. The P-C and P-N bonds induced intimate contacts between RP nanoparticles and g-C3N4 nanosheets, resulting in effective charge separation and significantly 163

enhanced photo(electro)catalytic activity. The RP/g-C3N4 composite showed an H2 evolution rate of 2,565 μmol h-1 g-1, which is sixfold of the pristine g-C3N4 and 28 times of the RP. The proposed charge transfer process is shown in Figure 8a. In bare g-C3N4, the photogenerated charges would be trapped in a trap state, and cannot participate in the photocatalytic process. However, in the RP/gC3N4, the charge could be extracted by RP from g-C3N4, due to the energy band difference and the closely chemical interaction between the two components. Thus the charge trapping process can be effectively suppressed. The femtosecond time-resolved transient absorption (fs-TA) was carried out to investigate the dynamics of the charges. (Figure 8b-d) The active charges in RP/g-C3N4 have a tenfold longer lifetime (1,122 ps) than in the bare g-C3N4 (100 ps). Other RP-based hybrid composites have also been reported, such as RP/CdS (35), MoS2/RP (36), RP/TiO2 (37–39), core-shell structured Cr2O3:P@fibrous phosphorus (40), and RP modified ZnIn2S4 hollow microspheres (41), which render RP-mediated photocatalysis promising. Photocatalytic Mechanism During the photocatalytic reaction over RP, •OH radicals were detected by measuring the generated 2-hydroxyterephthalic acid from terephthalic acid (5). The increasing fluorescence intensity of the solution would confirm the formation of •OH radicals during photocatalysis. The reactive oxygen species (ROSs) generated by the photocatalysts could serve as powerful oxidants to inactivate the bacteria. A detailed investigation of the generated ROSs was carried out by Xia et al. (42) Furfuryl alcohol, nitroblue tetrazolium, p-chlorobenzoic acid, and coumarin were applied as the specific probes to detect 1O2, •O2-, •OH, and H2O2, respectively. Only the last three ROSs were detected. (Figure 9b-e) Each of these scavengers were added individually to the system to isolate the bactericidal contribution from a specific reactive species, including Cr(VI) for electron (e-), sodium oxalate for hole (h+), L-histine for 1O2, isopropanol for •OH, 2, 2, 6, 6‐tetramethypiperidine‐N‐oxyl‐4‐ol (TEMPOL) for •O2- and Fe(II) for H2O2. As shown in Figure 9a, after the addition of Cr(VI) to capture e-, the h+- e- separation was enhanced, so the inactivation process was elevated slightly. L-histidine made no significant change in the inactivation kinetics, so the 1O2 had a negligible role in the disinfection. After adding the scavengers to capture •O2and •OH, which could be generated through e- transfer, the E. Coli inactivation were prohibited. H2O2 can be generated by either reduction of surface-adsorbed O2 or oxidation of surface-adsorbed H2O. Its involvement was affirmed by the efficiency decrease after adding Fe(II). Therefore, the dominant effective species for photocatalytic disinfection performed by RP was suggested to be the photogenerated e- and e--derived •OH, •O2- and H2O2.

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Figure 9. (a) Photocatalytic inactivation efficiencies, (b) Level of 1O2, (c) Level of •O2–, (d) Level of •OH, (e) Level of H2O2, were measured with red phosphorus in the presence of various scavengers (l-histidine, 0.5 mM; Fe-EDTA, 0.1 mM; Cr(VI), 0.05 mM; TEMPOL, 1 mM; Sodium oxalate, 0.5 mM; Isopropanol, 0.5 mM) under a xenon lamp irradiation. Reproduced with permission from reference (42). Copyright 2015, American Chemical Society.

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Photostability Since the RP is used in aqueous solution, PO43− and HPO42− may be generated during the photocatalytic reaction. The stability of RP can be studied by the determination of these phosphate ions. Trace levels of PO43− and HPO42− were detected when putting Hittorf RP in the solution for 12 h and, even throughout three days of irradiation, their concentrations were kept at almost unchanged constants of 26.4 and 48.5 ppm, respectively (5). The photocatalytic cycling tests were also carried out to check whether the catalyst could be maintained well. For the pollutant degradation and the disinfection experiments, the catalyst was usually recollected by centrifuging or filtering, and then dried for reuse. 85% of original RhB degradation activity of the BP-RP was maintained after three cycles (19). The photocatalytic disinfection efficiency of RP maintained well after the fifth cycle (42). For H2 evolution test, the system was vacuumed at specific intervals to exclude the already generated H2. The activity of RP/g-C3N4 could last over 40 h without obvious decay (34). Type II, type V, and type VI RP have similar stability and their photoactivities can be maintained for at least four cycles of hours of measurements (13, 18).

Black Phosphorus (BP) General Black phosphorus usually has an orthorhombic phase and is a thermodynamically stable allotrope of phosphorus. Its bulk crystal has a layer structure. Its monolayers (termed phosphorene) are stacked together by weak van der Waals forces. BP has a shiny black appearance and a broad light absorption spanning from ultraviolet to infrared. BP has been proved to be a typical p-type semiconductor with positive Hall coefficient (43). It has a thickness-dependent tunable direct bandgap, which increases with the decrease of layer number. The band-gap energies of the bulk BP and phosphorene range from 0.10 to 0.36 eV and 1.0 to 2.0 eV, respectively (44–48). The bandgap can be tailored to absorb a wide range of solar light. BP has a high charge mobility of 1,000 cm2 V-1 s-1, which is beneficial for charge transfer (49). The good photostability also contributes to its robust photocatalytic activity. Strategies To Enhance the Photocatalytic Performance Although BP has a wide light absorption, good conductivity, and suitable band structure, its photocatalytic activity requires further improvement to meet the real-world execution. Many efforts have been made to promote the carrier mobility and separation and to offer more active sites. Morphology Modulation The bandgap of BP can be adjusted by the layer numbers. Compared with bulk BP, phosphorene with few layers and small size has a more suitable bandgap, less interlayer coupling, and higher carrier mobility, thereby enabling better photocatalytic performance (45). Since the first exfoliation of BP by sticky-tape microcleavage method in 2014 (10), different methods have been used to synthesize the few-layered phosphorene, such as liquid exfoliation (50), wet-chemical self-assembly (51), and CVD method (52).

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Hu’s group first demonstrated that two-dimensional (2D) BP could be used as a catalyst for solar-to-chemical energy conversion. In terms of the oxidation of trimethylamine (TEA), 2D BP showed a 40-fold enhancement than bulk BP. The schematic of the photocatalytic reaction is shown in Figure 10a. The improvement was attributed to the quantum confinement of the 2D structure, which increased the oxidizing and reducing abilities of the photogenerated charge carriers (53). As indicated by the density functional theory calculation results, the VB shifted more than CB with the increase of the quantum confinement. (Figure 10b) Charge recombination was found to occur at the flake edges and step defects of the 2D BP. BP in few-layer nanosheets was also reported to catalyze the photo-reduction of nigrogen (54).

Figure 10. (a) Light-driven redox reactions on 2D BP. Chloro(triphenylphosphine)gold(I) (AuITPP) is converted to AuIBP and reduced to Au nanoparticles (NPs), while triethylamine (TEA) is oxidized to TEA+. (b) An energy diagram shows the band edges of the monolayer (2D) and bulk BP, redox potentials of each couple, and an indication of whether charge transfer is energetically favorable. The line accompanying AuITPP/Au0 is not a formal reduction potential; rather, it marks the onset of an irreversible reduction of AuITPP to Au0 as measured by us and others. Ees represents interband edge states, but the energy of these states is presently unknown. Reproduced with permission from reference (53). Copyright 2016, American Chemical Society. BP quantum dot (BPQD) has many distinct properties that make it a promising catalyst, such as size-dependent light absorptions, long exciton lifetimes, and high photoluminescence quantum yields. Yuan and his co-workers synthesized the BPQDs in different size by a facile solution-based method. The prepared BPQDs have tunable bandgaps due to the quantum size effect. The BPQDs with a bandgap of 2.82 eV were found to remove RhB with a high apparent degradation constant of 0.81 h-1 (55). Cocatalyst Loading The plasmonic metal nanoparticles with the localized surface plasmon resonance (LSPR) are promising cocatalysts, as they can boost light harvesting in the visible region. Besides, the interface between the metal and BP is favorable for charge separation. Ag/BP was synthesized by a chemical reduction approach in Liu’s group (56). When the BP layer thickness was decreased from multiple (m-BP) to few layers (f-BP) and the Ag particle size was increased from 20 to 40 nm, a significant enhancement of 20-fold in the visible-light-driven RhB degradation was observed in the 5 wt% Ag/f-BP. The reasons for the activity rise are the modified bandgap and the enhanced local field 167

amplification of Ag/BP nanohybrid. A ternary BP-sensitized Au/La2Ti2O7 was reported to process enhanced photocatalytic activity in the visible and near-infrared light regions because the plasmonic effect of Au broadened the light absorption of BP. The H2 evolution rates over the material were about 0.74 and 0.30 mmol h-1 g-1 under the irradiation of the light with wavelengths longer than 420 and 780 nm, respectively (57). Zhu et al. combined BP nanoflakes with Pt loaded reduced graphene oxide (Pt/RGO) and found efficient charge transfer between excited BP nanoflakes and Pt/RGO by photoelectrochemical measurements and transient absorption (58). Non-noble metal phosphide and sulfide nanoparticles were also applied to boost the photocatalytic H2 evolution activity of BP. Amorphous Co-P nanoparticles loaded BP nanosheets exhibited an apparent quantum efficiency of 42.55% at 430 nm and an energy conversion efficiency of over 5.4% at 353 K for H2 production (51). CdS/BP-MoS2 could produce H2 at a remarkably improved rate of 186.32 mmol h-1 g-1, due to the rich active sites and abundant structural and compositional features (59). Construction of Composite Materials The construction of photocatalyst composites is a successful strategy to enhance the charge separation and prolong the lifetime of the photogenerated electrons and holes. Many efforts have been made to develop BP-based heterostructures for photocatalysis. BP@TiO2 is the first reported BP-based hybrid photocatalyst (11). TiO2 nanoparticles were loaded on the BP by replacing some P atoms on BP the surface, and this makes BP more stable. Figure 11A and B show the structures of the few-layered BP and BP@TiO2. The components of the composite were confirmed by elemental analysis shown in Figure 11C. The hybrid showed higher efficiency than bare BP and TiO2 on the organic dyes removal and bacteria disinfection. As an efficient metal-free photocatalyst, g-C3N4 has been employed to couple with phosphorene to form BP/CN composite. Zhu et al. studied the efficient charge transfer in the BP/ CN by time-resolved diffuse reflectance spectroscopic measurement (60). The P−N coordination served as the site to trap electrons, while holes were rapidly quenched by the scavenger. The lifetime of the photogenerated electrons was prolonged in BP/CN compared to bare BP under the irradiation of either 400 or 780 nm light. (Table 1) Therefore, the photocatalytic H2 evolution performance was significantly enhanced. Compared to the single components, the optimum H2 yield for BP/CN reached 1.93 μmol and 0.46 μmol for 3 h of visible light and infrared irradiations, respectively. A metal-free 2D/2D Van der Waals heterojunction of few-layer phosphorene/g-C3N4 nanosheet (FP/ CNS) was prepared by a facile self-assembly approach in Qiao’s group (61). The nanocomposite produced at a dramatically increased rate of 571 μmol h-1 g-1 in 18 v% lactic acid aqueous solution under visible light. The improved performance was attributed to the intimate electronic coupling at the 2D/2D interface. Schematic illustration of the charge transfer process and photocatalytic H2 production in the FP/CNS system is shown in Figure 12. Under illumination, the photoinduced electrons in the CB of g-C3N4 transferred to the CB of phosphorene for the reduction of protons to H2. The photoinduced holes in the VB of g-C3N4 migrated to the VB of phosphorene for the oxidation of lactic acid. Besides, BPQD was also applied as an excellent cocatalyst of collecting or trapping holes when it was loaded on the g-C3N4 nanosheets (62). BPQDs/g-C3N4 was also applied for photocatalytic carbon dioxide reduction (63). 168

Figure 11. Modeled structures and elemental mapping analysis. Modeled mono-, bi-layer, and tri-layered BP structures (A), TiO2 substitution on BP structure (B), and line profile (top panel) and its elemental mapping (bottom panel) of P, Ti, and O elements in BP@TiO2 hybrid photocatalyst (C). Reproduced with permission from reference (11). Copyright 2015, Nature Publishing Group. Table 1. Lifetimes of TDR Decays of CN, BP, and BP/CN under 400 and 780 nm Irradiation, Respectively, Calculated from the Time Profiles at 950 nm. Reproduced with permission from referemce (60). Copyright 2017, American Chemical Society. τ1 (ps) Sample

400 nm

τ2 (ps) 780 nm

400 nm

τav (ps)a 780 nm

195 (49%)

400 nm

780 nm

CN

11 (51%)

BP

0.6 (90%)

0.8 (50%)

3.0 (10%)

0.8 (50%)

0.8

0.8

BP/CN

8.4 (45%)

2.7 (27%)

127 (55%)

60 (73%)

73

44

169

101

Figure 12. (a) The charge separation and transfer in the FP/CNS system under visible‐light irradiation (λ > 400 nm). (b) Schematic illustration of photocatalytic H2 production in the FP/CNS system under visible‐light irradiation (λ > 400 nm). The red, green, gray, blue, and black spheres denote H+, H, C, N, and P atoms, respectively. Reproduced with permission from reference (61). Copyright 2019, Wiley-VCH.

Figure 13. Photocatalytic a) H2 and b) O2 production from the water with and w/o sacrificial agents on different catalysts under >420 nm light irradiation. Photocatalytic water splitting without (w/o) any sacrificial agents by using c) BP/BiVO4 and d) BP/BiVO4/Co3O4 under >420 nm light irradiation. e) Effect of ratio of BP in BP/BiVO4/Co3O4 on photocatalytic water splitting under >420 nm light irradiation. f) Cycle stability test on BP/BiVO4/Co3O4 photocatalytic water splitting under >420 nm light irradiation. Reproduced with permission from reference (66). Copyright 2017, Wiley-VCH.

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Zero-dimensional (0D) ZnxCd1-xS (ZCS) nanoparticles were used to decorate phosphorene nanosheets due to their large surface area and short charge-diffusion length (64). The 0D/2D ZCS/ phosphorene nano-composite had strong electronic coupling between the components. Under visible-light irradiation, the photoinduced electrons would transfer from the ZCS to the 2D phosphorene to reduce protons to H2, while the photo-induced holes would stay in the ZCS to oxidize the sacrificial reagent. The optimized 0D/2D nano-composite showed an excellent photocatalytic H2-production activity of 9326 mmol h-1 g-1, with an apparent quantum efficiency (AQE) of 21.5% at 420 nm. Although the phosphorene is a good photocatalyst for H2 evolution, its VB potential is negative for O2/H2O and thus cannot catalyze O2 evolution from water without modification (65). The overall water splitting was achieved on the Z-Scheme of black phosphorus/bismuth vanadate heterostructure (BP/BiVO4) with a Co3O4 cocatalyst under visible light. The H2 and O2 were generated simultaneously by using BP as the water reduction site and BiVO4 as the water oxidation site (Figure 13a and b). As shown in Figure 13b and c, the cocatalyst enhanced the gas evolution efficiencies. The evolved gas radio was related to the mass ratio of BP and BiVO4 (Figure 13e). The material was stable in three cycling tests. (Figure 13f) The lifetime of the photoinduced electrons in the Z-scheme system was shorter than pure BiVO4, due to the fast charge transfer from the CV of BiVO4 to the VB of BP. In contrary, the lifetime of the photoinduced holes was prolonged in the composite, because a portion of electrons in the CB of BiVO4 combined with the holes in the VB of BP, resulting in less recombination of charge carriers within the hybrids. Photocatalytic Mechanism Ultrathin BP nanosheets were found by Xie’s group as effective photosensitizers for the generation of 1O2 with a high quantum yield of about 0.91 (67). The amount of 1O2 was detected by diphenylisobenzofuran (DPBF) as the probe molecule in ethanol. The species could not be detected in the control experiment carried out in N2 atmosphere, which indicated that 1O2 is generated by from the ground-state oxygen (3O2). The photocatalytic activity of the BP nanosheets was evaluated by the MO degradation. 90% of MO molecules could be degraded after 20 min under visible light (λ ≥ 600 nm). After sodium azide (NaN3) as a scavenger for 1O2 was added to the system, no obvious degradation of MO was observed. The 1O2 was also identified by electron spin resonance (ESR) spectrum. An optical switching effect of this material was also discovered in their experiments, which suggested that 1O2 and •OH were generated under visible- and ultraviolet-light excitations, respectively (68). The singlet oxygen sensor green (SOSG) and terephthalic acid were used to detect 1O and •OH, respectively. 1O was only detected under visible light illumination (λ ≥ 420 nm). 2 2 However, the 2D BP could still degrade MO under UV irradiation (λ < 420 nm), due to the generation of •OH. A sub-band structure was proposed to explain the excitation-energy-dependent optical switching effect, as shown in Figure 14. Under visible light illumination, 1O2 was generated by the energy transfer from the excited excitons of BP in its internal band to 3O2. Upon UV-light illumination, the photoexcited holes would rapidly relax to the top of VB in the external band, which 171

matches the redox potential of H2O/•OH. Hence H2O reacted with the holes and produce •OH. The two sub-band systems of BP nanosheets responded to different excitation light and generated different ROS.

Figure 14. Schematic illustration of the subband structure of ultrathin BP nanosheet as well as the photoexcitation and photocatalytic processes occurring therein. The optically switchable ROS generation of 1O and •OH are depicted corresponding to the internal and external band systems, respectively. CB and 2 n VBn (n =1, 2) denote the n-th conduction band and valence band, respectively. Eex and Eb stand for the excitonic energy level and exciton binding energy, respectively. Reproduced with permission from reference (68). Copyright 2018, American Chemical Society. Photostability BP has higher thermal stability than other P allotropes. It cannot be spontaneously ignited until being heated to about 400 °C in air. It is a stable photocatalyst against light illumination (69). The ternary BP-Au/La2Ti2O7 material can catalyze H2 production through the four cycling tests with negligible degradation (57). The photocatalytic activity of BPQD-C3N4 could maintain after five recycles with almost similar amounts of H2 evolved. The main component of the sample did not change as confirmed by XPS measurements (62).

Comparison between RP and BP The synthesis of RP is simpler than that of BP. The production of amorphous RP has been achieved on an industrial scale. And the CVD method enables the tailoring of RP crystalline in 172

a controllable manner. Some theoretically predicted RP structures have yet to be synthesized experimentally, which may bring new possibilities for enhancing the photocatalytic efficiency. Compared with RP, BP has better charge property and thus higher photoactivity benefiting from its layered structure. However, the synthesis of BP needs more energy and is time-consuming. The tough requirements (high temperature, high pressure, and vacuum) in BP syntheses make the mass production still challenging. The exfoliation of layered BP is also a technology-required process. The thickness is difficult to be controlled in mechanical exfoliation. And BP is easily oxidized during the long-duration liquid exfoliation. Exploring new approaches for BP synthesis and exfoliation merits further attention.

Summary and Outlook Research interest in phosphorus photocatalysts is soaring, stimulated by their merits of environmental friendliness, low cost, elemental abundance, appreciable chemical stability, widerange light absorption, tunable redox potential, and excellent compatibility with other chemical and biological materials. Endeavors in their synthetic exploration, structural design, band modulation, and mechanistic investigation have led to the skyrocketing of their photocatalytic performances to a level that far surpasses those of other elemental photocatalysts including boron, silicon, selenium, and a-sulfur and even rivals those of some mainstream photocatalysts. As a consequence, phosphorus photocatalysts have been, and are being, leveraged considerably in energy and environment applications. These include water splitting, organic pollutant degradation, bacteria disinfection, and cancer treatments. Finally, we would like to highlight the roadmaps for the future development of phosphorusdriven photocatalysis: 1) Crystal structure and morphology control: Synthesis of highly crystalline phosphorus photocatalysts with architectures of one-dimensional nanowires, two-dimensional nanosheets with few-layer thickness, three-dimensional hierarchical nanospheres, and hollow porous nanostructures are extremely desirable. These structures enable higher photoreactivity and provide platforms for the elucidation of the structure-performance relationship. 2) Cocatalyst grafting: Introducing carbon-based nanomaterials (i.e. graphene, carbon nanotube, graphdiyne, or carbon nitride), transition metal dichalcogenides (i.e. MoS2, WS2, or MoSe2), and single-atom catalysts of noble metals (Pt, Rh, Ru, Au, Ag, or Ir) as cocatalysts could improve the photoactivity of phosphorus photocatalysts. These cocatalysts enhance both the charge separation and surface catalytic dynamics. 3) Epitaxial growth: It is highly appealing to utilize elemental phosphorus as both the support and phosphorus source to epitaxially grow transition metal phosphides (CoP, Cu3P, FeP, MoP, or Ni2P) on their surfaces. These epitaxially grown phosphides are expected to outperform the exotically grafted cocatalysts by intensifying the interfacial interaction between cocatalyst and support. New applications: More efforts should be dedicated to leveraging phosphorus photocatalysts for the reduction of nitrogen to ammonia and converting CO2 to value-added chemicals.

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Chapter 9

Electronic Applications of Black Phosphorus Thin Films Arnob Islam1 and Philip X.-L. Feng1,2,* 1Department of Electrical Engineering and Computer Science, Case School of Engineering,

Case Western Reserve University, Cleveland, Ohio 44106, United States 2Department of Electrical and Computer Engineering, Herbert Wertheim College of Engineering, University of Florida, Gainesville, Florida 32611, United States *E-mail: [email protected].

Black phosphorus (P), an allotrope of elemental phosphorus, and its crystaline atomic layers, has attracted significant attention as a two-dimensional (2D) semiconductor for its extraordinary electronic properties. Black P ultra-thin body (UTB) 2D field effect transistors (FETs) have exhibited excellent hole mobilities with very good on-off ratios, which endorses its suitability for logic applications and superiority over many other 2D materials. In addition, 2D black P has been used in realizing diodes, tunneling field effect transistors (TFETs), complementary logic circuits, vibrating channel transistors for nanoelectromechanical system resonators.

Introduction Black phosphorus (P) is one of the three allotropes of phosphorus along with white and red phosphorus, which is thermodynamically stable. In 1914, American physicst Percy W. Bridgmen first synthesized this black P allotrope from white phosphrous by using high pressure techniques in an apparatus at greater than 1 GPa (1). Since then various methods for the synthesis of black P have been developed. Later, in 1982, single crystal bulk (mm size) of black P was first synthesized (1) from red P by phase transformation under high pressure (1 GPa), 550°C and subsequent cooling down from 900°C to 600°C at a rate of 30°C/h. However, research activities related to black P have been very limited thereafter, until its rediscovery in 2014 as an exciting 2D material derived from its bulk (2, 3). 2D layered black P is a semiconductor (4) with thickness dependent bandgap (5, 6) (Figure 1b) from 2eV (1L) to 0.3eV (bulk) (7, 8). It displays extraordinary electronic (9, 10) and photonic (11, 12) properties. Figure 1a shows the crystal structure of 2D black P, which indicates two in-plane crystal directions, armchair (AC) and zigzag (ZZ). The in-plane anisotropy of black P makes this material quite unique and special in comparison to most of the other 2D materials, which can lead to unprecedented nanoscale device applications (13). © 2019 American Chemical Society

In this Chapter, we particularly focus on the electronic device applications of black P nanomaterial. The field effect transistors (FETs) fabricated by using black P has already been demonstrated with excellent performance, including higher carrier mobility (9) compared to 2D transition metal dichalcogenides (TMDCs) FETs and very good on-off ratios thanks to its non-zero bandgap unlike graphene. In addition, other electronic devices, e.g., complementary logic circuits, diodes, tunnel field effect transistors (TFETs) have also been realized by utilizing black P thin films or atomic layers. Moreover, due to the high carrier mobility of black P, this material is an excellent candidate for realizing radio frequency (RF) transistors. Black P vibrating channel transistors have also been demonstrated for resonant nanoelectromechanical systems (NEMS) and their potential resonant sensing applications.

Electronic Properties of Black Phosphorus

Figure 1. (a) Three-dimensional (3D) illustration of the crystal structure of black P and its layered nature. (b) Thickness dependent electronic bandgap of black P reported in the theoretical works, and optical bangap values obtained from experimental studies. (a) is adopted with permission from reference (18). Copyright 2018 American Chemical Society. Black P possesses a thickness dependent electronic bandgap from ~1.5eV at monolayer (1L) to ~0.3eV at multilayer or bulk form (>8L or 4nm) (Figure 1). Therefore, its small and tunable bandgap in its multilayer and thin film forms nicely bridges the gap (on the scale of bandgaps of various materials) between the zero bandgap of graphene and and wider bandgaps of TMDC materials (which often have bandgaps ranging from 1.5eV to 2.5eV). Single atom thick, ultrathin graphene has showed ultrahigh electron mobility up to 30,000cm2/(V·s) at room temperature (14). However, due to its zero bandgap, graphene FETs face major challenges toward adoption for logic applications. Therefore, its promises and potential in the electronics apolications and industry, have been tuned down recently and become limited mostly to radio-frequency (RF) electronics. On the other hand, although TMDC materials exhibit great promises and potential for future transistors and logic applications with ultrahigh on-off ratios of ~108, TMDC transistors suffer from low field effect carrier mobilities (often only ~10–200cm2/(V·s)). Fortunately, thanks to the advent of black P, Hall mobility along the armchair (AC) direction of a 15nm thick black P thin film has been found to be above 600cm2/(V·s) at room temperature and above 1000cm2/(V·s) below 120K (15). Field effect hole mobility along AC direction in bulk black P, exceeds 1000cm2/(V·s) at 300 K and 55,000cm2/(V·s) at 30K, respectively (16). The electron mobility measured along the AC-direction 180

is also close to 1000cm2/(V·s) at 300K and is above 10,000cm2/(V·s) at 50K (16). Therefore, in term of field effect mobility, black P FETs have a great superiority over TMDC FETs to date. Again, due to non-zero bandgap of black P, FETs also show very good on-off ratios of 105, comparable to many TMDC and other 2D FETs, and orders of magnitude higher than typical values attained by graphene FETs (9). Further, due to lower bandgap and higher carrier mobility, it has been observed that black P has lower ionization threshold compared to TMDC materials. Ahmed et al. have also recently reported that ionization threshold field in hexagonal boron nitride (hBN) encapsulated black P devices (17) is lower (≈5.5–7V/μm) and thus allowing them to operate at a bias voltage, where carrier concentration can be increased via impact ionization, without breaking down, which will result in non-saturating behavior in current.

Black Phosphorus Field Effect Transistors

Figure 2. Black P FETs: (a) Three-dimensional (3D) illustration of a black P FET with silicon global backgate. (b) Measured sheet conductivity as a function of gate voltage for devices with different thicknesses: 10 nm (black solid line), 8 nm (red solid line) and 5 nm (green solid line), with field effect mobility values of 984, 197 and 55 cm2/(V·s), respectively. Inset shows the thickness dependent mobility and drain current modulation. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nature Nanotechnology, Ref. (9), Black phosphorus field-effect transistors, L. Li et al., Copyright (2014). Field effect transistors (FETs) have been the essential building blocks of modern integrated circuits. Thanks to the advent of 2D semiconductors combined with their ultrathin nature, they naturally become attractive candidates for new channel materials for enabling new FETs with 2D ultrathin body (UTB) that are expected to offter greater immunity to short channel effects. This in turn holds strong promises for enabling deeply scaled FETs with sub-5nm channel lengths, thus further extending Moore’s law, which has been confronting the advanced Si and even modern IIIV MOSFETs. As we have mentioned in the last section, black P is one of the best 2D materials candidates for realizing 2D UTB FETs. In 2014, 2D p-type black P FETs on SiO2/Si substrate with heavily doped Si as a backgate (Figure 2a) were first demonstrated, which show excellent field effect hole mobility of µh = 1000cm2/(V·s) and very good on/off ratio, IOn/IOff ~105 (Figure 2a) (9). Thickness (tBP) dependent µh and IOn/IOff ratios have been observed in these black P FETs (Figure 2b). I has also been found that µh is low for thinner black P FETs due to the effect of charge impurity 181

and interface scattering. The experiments show that µh monotonically increases to reach the peak value of 1000cm2/(V·s) for tBP ≈ 10nm. For tBP > 10nm, µh again decreases with the increase of thickness due to the fact that backgate only induces free carriers at the bottom layers of thicker black P samples because of charge screening effect. IOn/IOff ≈ 105 is obtained from the thinnest black P sample, tBP ≈ 5nm; and it decreases monotonically in thicker black P devices. This is because backgate does not modulate the carriers at the top atomic layers of thicker black P films, which dictates higher IOff for thicker black P samples. This trend of IOn/IOff with respect to the thickness of black P has also been found consistent in other studies (19). Another study (20) on black P FETs with ~15nm-thick HfO2 dielectric top gate investigates the thickness dependence nature of subthreshold swing (SS) in black P FETs. It has been found that IOff and SS are degraded with increasing tBP and increased source-to-drain bias voltage (VDS). The best FET reported in this study exhibits a saturated subthreshold slope of SS = 161mV/decade and IOn/IOff = 2.84 × 103, for VDS = 1V (20). The reason behind this higher SS compared to the ideal room temperature value, SS=60mV/decade, is the existence of Schottky barrier (SB) contacts and interface traps between the black P thin layers and the dielectric films.

Figure 3. Device physics of Black P FETs with SB contacts: (a) Simulated transistor characteristic of a black P FET with SB contacts. (b) Band diagrams ((i)-(iv)) corresponding to different operating regimes on the transistor characteristics. Reproduced with permission from reference (21). Creative Commons Attribution 4.0 International License. In order to understand the effect of SB contacts on SS, we can focus on the underlying device physics of 2D FETs. For 2D FETs with SB, carrier transport and polarity is controlled by the relative SB heights of electrons and holes. In these black P FETs, transistor switching mechanism is basically controlled by SB modulation (21) (Figure 3b). For negative gate voltage (VGS), due to the band bending, SB for holes become narrow and holes tunnel through the SB (Figure 3b(i)) and hole conduction dictates the carrier transport. When we increase the VGS, SB width for holes begin to increase and at one point, holes can only be injected to the channel by thermionic emission (Figure 3b(iii)), which contributes to very small current. With the increase of VG, simultaeously, electrons start to tunnel through the SB (Figure 3b(ii)). At high enough positive VG, electron conduction begins as SB width for electron becomes narrower (Figure 3b(iv)), electron conduction dominates the carrier transport in the channel. Now, if the Fermi level is located near the mid-gap instead of very 182

close to the valance band, the electron conduction begins before the hole conduction becomes fully thermionic limited, which leads to higher off-current and nonideal SS (Figure 3a). This also explains the reason behind ambipolar carrier transport behavior. Besides, the degradation of SS and IOn/IOff is more prominent in thicker flakes or devices due to the reduced energy bandgap. Black P FETs can exhibit p-type, n-type and ambipolar behavior depending on the proper choice of metal contacts and the thickness of the black P flakes, due to the thickness dependent bandgap of black P without any doping (22). If we rule out Fermi-level pinning, then for a thinner black P flake (below 10nm), by using low work-function metal contacts (e.g., Al, Sc, Ti, etc.), it is possible to realize unipolar n-type black P FETs; and conversely, high work-function metal (e.g., Pd, Au, Ni, etc.) will yield p-type unipolar behavior. Again, unipolar p-type or p-type dominated ambipolar behavior is observed for thicker flakes (>10nm) with high work-function metal contacts; and ambipolar behavior is found for low work-function metal contacts. However, Fermi-level pinning can play a major role in determining the carrier polarity. Therefore, irrespective of choice of metal contacts, carrier polarity can be determined by the position at the bandgap where the Fermi-level is pinned, and the corresponding SB heights for electrons and holes. Most of the cases, multilayer black P FETs show p-type ambipolar behavior because of the unintentional p-type doping and Fermi-level pinning at the metal-semiconductor interfaces that cause smaller (larger) SBs for holes (electrons). Thermal annealing has been demonstrated as an effective method for Fermi-level depinning and reduction of contact resistance.

Figure 4. Effect of the intrinsic Electronic anisotropy on the performance of black P FETs. (a) Measured Hall mobilities along armchair (AC, x-direction) and zigzag (ZZ, y-direction) at varying temperature. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature, Nature Communications, Ref. (15), Rediscovering black phosphorus as an anisotropic layered material for optoelectronics and electronics, F. Xia et al., Copyright (2014). (b) Comparison of the performance of black P naoribbon FETs oriented along AC and ZZ directions. Reproduced with permission from reference (25). Copyright 2018 Wiley & Sons. The works so far mentioned about black P FETs have not used any external doping, rather they rely on the intrinsic unintentional doping of black P. However, in order to further enhance the performance it is imperative to investigate appropriate doping mechanisms. Chemical doping has been employed to enhance the performance of black P FETs. P-type doping has been achieved by using F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) layer on BP FETs (23). The doping technique has caused an average of 2.5 times decrease in sheet resistance, 1.4 times reduction in contact resistance, and 1.7 times enhancement in field effect mobility, statistically. By 183

doping with Te (0.1% atomic concentration), black P FETs with hole mobility up to 1850cm2/(V·s) was achieved (24). In this method, most of the fabricated black P FETs with the thickness range between 6–15nm, not only exhibit excellent mobility, but also display very good IOn/IOff of 103–106 with p-type behavior. Te-doped BP devices show an extraordinary ability to resist ambient degradation with a retained mobility of >200cm2/(V·s) and IOn/IOff of 500, after 21 days of ambient exposure. The in-plane Electronic anisotropy of black P can be utilized to engineer and further improve the performance of black P FETs. It has been theoretically found that the hole effective mass along armchair (AC) direction is 6–8 times smaller than that along zigzag (ZZ) direction (3). Therefore, carrier mobility will be significantly higher if carrier transport occurs along armchair direction. Angle-resolved DC conductance measurement reveals Hall mobility along armchair and zigzag directions, 1000cm2/(V·s) and 600cm2/(V·s), respectively (15) (Figure 4(a)). Later in another study (25), black P nanoribbons oriented along AC direction display higher mobility compared to nanoribbons oriented along ZZ directions (Figure 4(b)). These studies suggest that it is important to consider the crystal orientation of black P during the fabrication of black P FETs.

Figure 5. p-n homojunction diodes: (a) p-n homojunction diode realized by Al-doped n-type black P and corresponding diode characteristics shown in (b). (a) and (b) Reproduced with permission from reference (32) Copyright 2017 WILEY-VCH Verlag GmbH. (c) B-doped n-type black P for fabricating p-n diode, with its diode characteristics presented in (d). (c) and (d) Reproduced with permission from reference (33). Copyright 2019 American Chemical Society. In perspective, although black P FETs display outstanding mobility and IOn/IOff, they suffer from poor SS. In order to improve SS, high-k dielectric materials can be used as the gate dielectric. In a recent study, high-k dielectric hafnium oxide (HfO2) has been used as gate dielectric for a black P FET (26), which shows near-ideal SS of ~69 mV/dec with hole mobility µh > 400cm2/(V·s). In another recent study, ferroelectric capacitor, Hf ZrO has been employed to realize black P FETs with 184

negative capacitance for achieving sub-60mV/decade SS (27). However, still more studies need to be performed to minimize interface trapping to achieve sub-60mV/decade SS, which requires better growth techniques of ferroelectrics on black P. In addition, realizing unipolar n-type black P FETs has been found challenging due to Fermi level pinning. Recently, one study demonstrates the use of Sc or Er metal contacts to realize unipolar n-type black P FETs, which seems promising (28). From the standpoint of scaling down, top gated black P FETs (~10nm-thick) with 20nm channel length (29) has already been demonstrated, which shows IOn =174μA/μm and IOn/IOff ≈ 102. The lower IOn/IOff is stemmed from short channel effects like drain induced barrier lowering (DIBL), which can be improved by using thinner black P flakes. In another study, h-BN encapsulated black P FETs demonstrate ultrahigh hole mobility of µh = 5200cm2/(V·s) at room temperature and moderate vacuum (30), which is the highest mobility reported to date for black P FETs. Another study (31) deals with the electrical breakdown of black P with increasing bias voltage, which is caused by fracture in the black P layer at 600K. This study estimates a maximum power density of ~500mW/µm3 that a 50 nm-thick black P FET (length = 1500nm, width = 900nm) can sustain before breakdown or failure occurs.

Black Phosphorus Diodes and Tunnel Field Effect Transistors Diodes are indispensable components for a wide spectrum of electronic and optoelectronic applications. However, for 2D materials, due to the lack of effective doping strategy, it is always challenging to realize p-n homojunction diodes (i.e., in the same contructing material). Without any doping, black P FETs in most of the cases show p-type or ambipolar behavior irrespective of the work-function of metal contacts due to Fermi-level pinning. Therefore, a systematic method of ntype doping is necessary to realize p-n homojunction diodes. Table 1. p-Type and n-Type Doping Strategies Doping Type

p-type

n-type

Doping Strategy

Comments

F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane) (23)

Chemical doping

Tellurium (Te) doping (24)

Synthesis of black P bulk crystal with 0.1% Te

Doping by integrating MoS2 Nanoparticles (34)

Charge transfer doping by work function differences between BP and MoS2

Aluminum (Al) doping (32)

ALD deposition of Al2O3

Boron (B) doping (33)

Pulsed-plasma process using B

Potassium (K) doping (41)

Potassium (K) capping layer

Copper (Cu) doping (35)

Band structure change due to deposited Cu adatoms in the top layers of black P

Magnesium Oxide (MgO) doping (36)

Surface charge transfer doping due to MgO deposition

A study (32) in the literature reported spatially controlled Al-doping to realize black P p-n homojunction diodes. In this doping process, 1nm Al and 10nm Al2O3 layers were deposited on 185

top of black P at 120°C using atomic layer deposition (ALD) with e-beam lithography patterning for spatial control of deposition. After the deposition, Al atoms thermally diffuse into the black P, donating electrons to convert the covered black P portion from p- to n-type (Figure 5a). The diode achieves a near unity ideality factor along with a rectification ratio, IFB/IRB of ≈5.6 × 103 at bias voltages of ±2V (Figure 5b). This work demonstrates a way to fabricate p-n homojunction diode in a controlled fashion. Recently, another method (33) incorporating a pulsed-plasma process using boron with rapid processing time (30s for ~13nm black P thickness) at room temperature exhibits n-type doping of black P. p-n homojunction diodes fabricated using this method also display high rectification ratio (up to 2.8 ×104) (Figure 5c-d). Here, in table-1, we have presented p-type and ntype doping strategies.

Figure 6. Black P TFET: (a) Device structure of black P TFET with p-i-n regions. (b) False color SEM top view of the fabricated TFET. (c) Device physics of the black P TFET. Reproduced with permission from reference (38). Copyright 2018 American Chemical Society. In order to achieve sub-60mV/dec SS for a switching device, which is the “holy grail” for future low power integrated circuits, tunneling field effect transistors (TFETs) are considered as one of the promising options. Among all 2D materials, black P is considered as the most suitable material for fabricating TFETs, because of its moderate bandgap, light effective mass and high mobility (37). Wu et al. demonstrates p-i-n TFET by using black P (38), where, p and n-doped regions of black P are electrostatically defined by two separate local gates (Figure 6). In that work, by using a top gate over the intrinsic region of black P, it is possible to modulate carrier transport via controlling the bandto-band-tunneling (BTBT). The TFETs exhibit “on”-currents of up to 0.6μA/μm and SS ~170mV/ dec at room temperature for Vds = 0.8V, which is well-above 60mV/dec at room temperature. The authors claim that the high SS is due to the choice of thickness of the flake (8–13nm) and the dielectric (5.6nm). By aggressively scaling down of channel thickness to monolayer and dielectric thickness to 0.5nm (effective oxide thickness, EOT), theoretically it has been predicted to achieve SS = 12mV/dec at room temperature (38). However, these TFETs did not show lower SS, while a record high On current of 15.2μA/μm is obtained, which is the highest among all 2D TFETs reported (39). Another interesting feature of these TFETs is the ability to reconfigure those TFETs to p-type or n-type FETs by applying appropriate gate voltage at gate electrodes (G1 and G2) (Figure 6c). For example, by applying positive gate voltage both at G1 and G2 will enable us to reconfigure the TFET device as a n-type MOSFET. 186

Black Phosphorus Complementary Logic Circuits Complementary logic circuits are one of the major components in modern day digital integrated circuits. In complementary logic circuits, we have to realize both p-FET and n-FET. For 2D materials, it is possible to use two different 2D materials for p-type and n-type FETs to realize hybrid complementary logic circuits. However, here, we only focus on monolithically integrated black P complementary logic circuits. In the last section, two different doping mechanisms have been discussed to achieve n-type doping. By Al-doping as mentioned in the previous section complementary inverter has been realized (Figure 7a, 7c) (40), which shows a distinctive logic inversion with a high voltage gain of up to ~11 at a supply voltage, VDD = 1.5V. At the same time, a high noise margin of 0.27 × VDD is obtained for both low (NML) and high (NMH) input voltages. Moreover, oscillation frequency of ~1.8GHz has been observed from a three-stage ring oscillator with those black P complementary inverters (Figure 7d) (40). In addition the previously mentioned two n-type doping strategies of black P, selective potassium (K) capping layer has been used to reduce the bandgap and effectively electron-dope the black P flake (Figure 7b) (41). In this method, from the transistor perspective, ambipolar behavior is observed, where hole conduction is not completely suppressed. Using this method, monolithic black P complementary inverter has also been realized with gain of ~5. In addition, Cu-doping has been used to realize complementary logic inverter with a gain of 46 (35). Moreover, MgO surface charge transfer doping technique has also been employed to realize n-type doping and subsequently complementary logic inverters (36).

Figure 7. Complementary logic circuits in black P. (a) Logic inverters fabricated by using black P with Al doping. (b) Potassium (K) layer capping for n-type doping to fabricate monolithic black P inverters. (c) Inverter characteristics measured from the Al doped black P inverter. (d) 1.9GHz oscillations obtained from the ring oscillator by using three Al doped black P inverters. (a), (c) and (d) Reproduced with permission from reference (40). Copyright 2018 WILEY-VCH Verlag GmbH. (b) Reproduced with permission from reference (41). Copyright 2017 American Chemical Society. 187

Other Emerging Electronic Applications of Black Phosphorus Radio frequency (RF) transistors are used for RF amplifiers at radio and microwave frequencies and are very important elements in RF/microwave engineering and many other related industry sectors. Previously, graphene transistors were expected to have great potential for RF applications due to the ultrahigh mobility of carriers and very high cut-off frequencies promised by graphene FETs. However, again plagued by the zero bandgap of graphene, graphene FETs lack current saturation, which can lead to challenges such as reduction of voltage and power gains, thus compromising RF performance (42). On the other hand, because of limited carrier mobility of 2D TMDCs, they are not ideal materials for realizing RF transistors. As black P FETs have demonstrated excellent field-effect mobility and black P has non-zero bandgap, they are outstanding candidates for RF transistors. Wang et al. presented a black P RF transistor (43) with the channel length and thickness of 300nm and 8.5nm, respectively. They also align the channel of the FET along the AC direction of the crystal, in order to exploit the high mobility along that in-plane direction, given the intrinsic Electronic anisotropy of black P discussed earlier. They have obtained a peak short-circuit current gain cutoff frequency of fT = 12GHz and a maximum oscillation frequency of fmax = 20GHz. Another study (44) has investigated a black P RF transistor (channel length is 0.5µm and black P thickness is 13nm) fabricated on flexible Kapton polyimide substrate, which shows slightly better performance, with fT = 17.5GHz (Figure 8). Specifications and performance of such RF transistors may be further improved by scaling down the channel length.

Figure 8. An RF transistor fabricated on flexible Kapton polymide (PI) and its peak short circuit current gain (in dB) as a function of frequency. Reproduced with permission from reference (44). Copyright 2016 American Chemical Society. Nanoelectromechanical systems (NEMS), especially those operating in their various engineerable resonant modes (45, 46), are mechanical resonators shrunk to sub-µm scale, often exhibit resonant frequencies and multimode responses in the high frequency (HF, 3–30MHz), very high frequency (VHF, 30–300MHz) and even ultra high frequency (UHF, 300MHz–3GHz) bands, which enables improvements in responsivities and sensitivities (for detecting physical quantities, such as force, mass, energy, etc.), and superb displacement sensitivities near the standard quantum limit (SQL) and quantum mechanical behavior when cooled down to very low temperatures. Due to very large fracture strain limit (up to 30%), strain tunable electronic and optoelectronic properties, black P is an excellent candidate for 2D NEMS resonator (47). However, all-electrical actuation and detection of resonance motion of a 2D NEMS resonator is very challenging due to its ultra small size. Vibrating channel transistor (VCT) scheme can be used for all-electrical transduction of black P NEMS resonators. Given the excellent electrical conductivity (7) of black P, VCTs can be realized by 188

employing capacitive or electrostatic actuation and amplitude or frequency modulation (AM or FM) down-mixing schemes for the readout of device resonance motion (48). In this transduction scheme, the gate voltage induced conductance modulation of the VCT (Figure 9b) is exploited to efficiently transuduce and read out the mechanical motion of the black P resonator. Frequency downmixing is employed to obtain better signal-to-background ratio (SBR) for the detection to evade the often large and overwhelming background responsnce due to large parasitic capacitance and electrical feedthrough coupling between the drive and sense ports (Figure 9c-d).

Figure 9. Experimental demonstration of black P vibrating channel transistor (VCT): (a) Transport characteristics, (b) Transistor characteristics. (c) All-electrical transduction scheme of detection of mechanical resonance of black P drumhead resoantors by utilizing VCT characterstics and FM downmixing. (d) Detected mechanical resonance frequency by using this method. Copyright 2018 IEEE. Reprinted, with permission, from reference (48), Islam, A.; Lee, J.; Feng, P. X.-L. All-electrical transduction of black phosphorus tunable 2D nanoelectromechanical resonators. Proceedings of the 31st IEEE International Conference on Micro Electro Mechanical Systems (MEMS’18) 2018, 1052-1055, Belfast, UK, Jan. 21-25, 2018. Besides the aforementioned applications, being a promising layered semiconductor, black P has also been utilized for nonvolatile memory devices, e.g., floating gate transistors (49), which are considered as critical building blocks in modern solid state memory applications. Nonvolatile memory devices also have many emerging electronic applications, including neuromorphic computing, efficient data storage, dynamically reconfigurable digital circuits, etc. (49)

Perspectives and Challenges From the above mentioned discussions in this Chapter, it is evidential and encouraging that black P has exhibited tremendous potential in a broad range of electronic applications. For future energy efficient transistors with sub-thermionic steep-slope switches, it has been already discussed that black P is probably to date the best candidate among 2D materials for TFETs. Besides, from the scaling down perspective, it has been theoretically predicted that for a ultrashort channel monolayer black P FET, where ballistic transport dominates, the ballistic IOn along the AC transport direction outperforms that of monolayer MoS2 FETs by a factor of 1.57 and 1.89 for n-type and p-type devices 189

respectively, and ultrathin body (3nm) Si FETs (i.e., tri-gate transistor or FinFETs) by 1.69 and 2.41, respectively (50). Moreover, a few doping strategies have already been developed for black P, which show important progress towards monolithic complementary logic circuits, homojunction diodes, etc. Considering these wonderful potential and perspectives of black P, there are two major challenges to overcome for black P, toward realizing future electronic devices. First challenge is the large-scale growth of high-quality black P. Toward large-scale growth of black P, it has been demonstrated that black P can be reliably inkjet printed, enabling scalable development of nanodevices (51). In this inkjet printing method, ultrasound-assisted liquid phase exfoliation (UALPE) of bulk BP crystals along with appropriate stable dispersions or solvents are used. By using this method, black P is inkjet printed on untreated ultrathin PET (1.5μm) over an area of 100mm × 63mm. Average thickness of black P as low as 3.37nm has been achieved by using this method. This study shows future large-scale production of black P. However, it requires further effort to investigate and improve the quality of inkjet printed black P thin films, in order to make sure the outstanding and unique properties of black P remain intact in samples prepared by this technique. Table 2. Solutions for Preventing Degradation of Black P Prevention Solutions h-BN encapsulation (53)

Black P thickness (nm)/Layer number 4.5nm

Degradation Time

Limitations

>2months

Complex, low throughput and yield.

Al2O3 ALD Passivation (52)

8.9nm

>2weeks

Not suitable for monolayer and few layer black P flakes.

Oxygen Plasma Etching (55)

1L

~3days

Not fully prevent degradation.

Oxygen Plasma Etching+Al2O3 ALD (55)

1L

>6days

BV Doping (56)

>10days

5-18nm

Effectiveness of the method for mono and few-layer black P is not demonstrated.

Secondly, the degradion of black P under the ambient conditions can be a major concern. Among the passivation techniques to prevent degradation, atomic layer deposition (ALD) of Al2O3 has been successfully employed in several studies (52). However, as thin black P samples (< 5nm) are very unstable, they could be rapidly oxidized by the ALD process itself. Therefore, ALD passivation is not automatically suitable for monolayer or few-layer black P samples. Encapsulation using 2D hexagonal boron nitride (h-BN) can be another option, but to date it still has low throughput and yield (53). In order to understand the degradation process and environmental factors of black P devices, non-encapsulated and encapsulated by h-BN in comparison, recently a systematic study (54) has been performed to investigate environmental effects on degradation, by continuously monitoring and measuring the devices over several weeks, under various controlled conditions that include factors such as ambient air, humidity, light, and their combinations. This study shows, even for h-BN covered black P devices, top surface may not be immune to, but show deferred degradation 190

through ‘leaking’ exposure to environment, when the h-BN may not ideally conformally cover the black P devices, due to finite thickness of h-BN and non-planar top of the device. Another passivation technique employing oxygen plasma has been employed to oxidize the top most layer of black P in order to protect the layers underneath it. Eventually it has been found that although this process can delay degradation process (~3 days), it is not sufficient to prevent degradation, as O2 and air molecules can still penetrate through the holes in the oxide layer (55). Later, oxygen plasma etching followed by ALD of Al2O3 was found to be more robust in preventing degradation. From photoluminescence (PL) measurement, it has been confirmed that there is no significant decay of PL peaks over 6 days for a monolayer black P (55). Recently, a study reported passivation of few-layer black P samples using benzyl viologen (BV) doping, which shows the air stability of few-layer black P about ~10 days by observing no significant deterioration in measured electrical characteristics (56). We have summarized these degradation prevention techniques in Table 2.

Conclusions In conclusion, crystalline black P thin films and atomic layers have rapidly emerged as a promising candidate in the family of 2D materials and devices, and have demonstrated excellent promises and potential as a direct-bandgap semiconductor for future electronic, optoelectronic, transducers, and other applications. The proof-of-concept electronic devices reviewed in this Chapter demonstrates the versatility of black P as a new nanomaterial with unique properties for nanoelectronics and beyond. In addition to its potential for future energy efficient logic switches, black P electronic devices may also promise and lead to a number of important niche applications, including different types of unconventional ultralow-power computing architectures, sensing applications with resonance, electronic, and photonic modalities, as well as flexible and wearable electronics.

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Editor’s Biography Hai-Feng (Frank) Ji Dr. Hai-Feng (Frank) Ji is current a professor of Department of Chemistry, Drexel university in Philadelphia, PA, USA. His research interests focus on novel nanomaterials for energy and environmental applications, MEMS devices, polymers, drug discovery, materials for 3D printing, and surface chemistry. He is currently a co-author of 170 peer-viewed journal articles and book chapters, 6 patents. He has an H-index of 34. He is an editorial board member of several chemistry, materials, and medical journals.

© 2019 American Chemical Society

Indexes

Author Index Alalq, I., 61 Amaral, P., 27 Ang, K., 135 Castellanos-Gomez, A., 47 Chen, M., 1 Cui, D., 1 Dong, B., 135 Feng, P., 179 Frisenda, R., 47 Gao, J., 61 Hu, Z., 155 Huang, L., 135 Islam, A., 179 Ji, H., x Ji, H., 27

Kong, X., 79 Lee, C., 135 Li, Z., 1 Li, J., 155 Liu, Y., 1 Liu, Y., 155 Mu, L., 79 Ryu, Y., 47 Shatruk, M., 103 Wang, B., 61 Yang, S., 79 Yu, J., 155 Zhou, C., 1 Zhou, M., 79

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Subject Index introduction, 179 perspectives and challenges, 189 black P, solutions for preventing degradation, 190t tunnel field effect transistors, black phosphorus diodes, 185 black P TFET with p-i-n regions, device structure, 186f doping strategies, p-type and n-type, 185t

B Black phosphorus, degradation upon environmental exposure and encapsulation strategies to prevent it, 47 encapsulation and functionalization, passivation methods, 50 black phosphorus, aluminum doped, 53 encapsulated and unencapsulated BP FETs, hole mobility, 51f few-layer phosphorene enriched aqueous dispersions, preparation process, 53f monolayer phosphorene samples, time dependence of normalized PL intensity, 52f instability under ambient conditions, origin, 48 BP flakes in air after exfoliation, AFM scans, 48f Ag2 Raman mode as a function of time under different exposure conditions, integrated intensity, 49f introduction, 47 Black phosphorus thin films, electronic applications, 179 black phosphorus, electronic properties, 180 crystal structure of black P and its layered nature, three-dimensional illustration, 180f black phosphorus, emerging electronic applications, 188 black P vibrating channel transistor, experimental demonstration, 189f flexible Kapton polymide, RF transistor fabricated on, 188f complementary logic circuits, black phosphorus, 187 black P, complementary logic circuits, 187f conclusions, 191 field effect transistors, black phosphorus, 181 black P FETs, effect of the intrinsic electronic anisotropy, 183f black P FETs with SB contacts, device physics, 182f black P FET with silicon global backgate, three-dimensional illustration, 181f p-n homojunction diodes, 184f

P Phosphides, synthesis, 103 conclusion, 112 binary metal phosphides, synthetic procedures, 113t general methods, 104 chemical vapor transport, 105 introduction, 103 phosphides, synthesis actinide phosphides, synthesis, 110 metal phosphides, alkali and alkaline-earth, 106 metal phosphides, main-group (posttransition), 111 phosphides, alkali metal, 106 phosphides, lanthanide and actinide, 109 phosphides, transition metal, 108 Phosphorus, photocatalytic property, 155 black phosphorus CN, BP, and BP/CN under 400 and 780 nm irradiation, lifetimes of TDR decays, 169t cocatalyst loading, 167 composite materials, construction, 168 2D BP. Chloro(triphenylphosphine)gold(I) (AuITPP), light-driven redox reactions, 167f FP/CNS system under visible-light irradiation, charge separation and transfer, 170f general, 166 modeled mono‑, bi-layer, and tri-layered BP structures, modeled structures and elemental mapping analysis, 169f morphology modulation, 166 201

photocatalytic mechanism, 171 photocatalytic performance, strategies to enhance, 166 photostability, 172 subband structure of ultrathin BP nanosheet, schematic illustration, 172f water with and w/o sacrificial agents, photocatalytic H2 and O2 production, 170f introduction, 155 red phosphorus cocatalyst loading, 161 composite materials, construction, 162 fibrous phosphorus submicron fibers, TEM, HRTEM and SAED analysis, 159f general, 156 Hittorf phosphorus and fibrous phosphorus, molecular structures, 156f measured with red phosphorus in the presence of various scavengers, photocatalytic inactivation efficiencies, level of 1O2, 165f micro-fibrous P/SiO2, smashed-fibrous P, SEM images, 160f morphology control, crystal structure, 157 photocatalytic mechanism, 164 photocatalytic performance, strategies to enhance, 157 photo-deposition of Pt and MnOx on the PMBs, SEM images, 161f photostability, 166 proposed charge trapping model in g-C3N4 and RP/g-C3N4, schematic representation, 163f RP, BP, and their heterostructure, XRD patterns, 163f RP in different crystal type, absorption edge, 157f RP microbelts, growth, 158 YPO4/P hollow microspheres, SEM and TEM images with different magnifications, 162f RP and BP, comparison, 172 summary and outlook, 173 Phosphorus, physical and chemical properties, 61 black phosphorus, structure and properties, 65

black-P and blue-P structures, top and side view, 69f bulk black-P, lattice and electronic structures, 66f multi-layer and monolayer black-P, atomic structure, 65f nitrene moiety, functionalization of monolayer black-P, 68f phosphorene, band structure, 66f single crystals, black-P, 70 thin film, black-P, 67 conclusion, 71 introduction, 61 red phosphorus, structure and properties, 63 violet and fibrous red phosphorus, comparison, 63f violet phosphorus, fibrous phosphorus, high-resolution electron micrograph, 64f white phosphorus, structure and properties, 62 Phosphorus clusters and quantum dots, 79 binary clusters, phosphorus related metal phosphide clusters, 92 relative clusters, 93 introduction, 79 phosphorus cluster, 80 anionic phosphorus clusters, mass spectra, 86 CID masss pectrum, 82 ions obtained at low collisional energies, CID mass spectra, 81f ions obtained at relatively high collisional energies, CID mass spectra, 84f neutral phosphorus clusters, 91 odd-numbered cluster anions of P-2m+1, lowest-energy configurations, 89f odd-numbered cluster cations of P+2m+1, lowest-energy configurations, 82f phosphorus cluster anions, 85 phosphorus cluster anions obtained by laser ablation of RP, mass spectra, 85f phosphorus cluster anions obtained by laser ablation of RP, mass spectrum, 85f phosphorus cluster cations, 80 phosphorus cluster cations obtained by laser ablation of RP, mass spectra, 80f P-n-clusters, photoelectron spectra, 87f

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Pn+1 clusters, lowest energy structures and their corresponding isomers, 91f selected anions, CID mass spectra, 90f selected ions obtained at relatively high collisional energies, CID mass spectra, 88f selected ions obtained at relatively low collisional energies, CID mass spectra, 88f phosphorus quantum dots BPQDs, morphology characterization, 95f brief overview, 94 PQDs, applications of laser ablation, 96 PQDs, TEM image, 97f summary and outlook, 97 Phosphorus nanomaterials, introduction and characterization, 27 black phosphorus and phosphorene, introduction and characterization armchair direction on a sideview, structure of phosphorene, 35f black phosphorus, orthorhombic, 33 black phosphorus and phosphorene, introduction, 33 black phosphorus and phosphorene, Raman, AFM, photoluminescence, and powder XRD characterization, 37f BP and phosphorene, characterization, 36 few-layer phosphorene FET device, electrical characterization, 38f liquid phosphorus, 39 phosphorus clusters, 39 phosphorus cage-like structures; phosphorus ring-like structures, proposed structures, 40f phosphorus vapor, 38 red phosphorus, introduction and characterization, 27 amorphous red phosphorus, powder XRD, Raman, and HRTEM characterization, 30f amorphous red phosphorus type I, characterization, 30 different types of red phosphorus and crystals, morphologies, 28f fibrous and violet phosphorus, powder XRD, Raman, and HRTEM characterization, 33f fibrous phosphorus and violet phosphorus, characterization, 32 fibrous phosphorus and violet phosphorus, structural differences, 29f

introduction, 27 red phosphorus type II, characterization, 30 red phosphorus type II, powder x-ray diffractogram and Raman spectrum, 31f red phosphorus type II, XRD powder patterns of interplanar spaces, 31t red phosphorus type III, characterization, 31 red phosphorus types II and III, XRD powder patterns of interplanar spaces, 31t Photodetectors, black phosphorus based, 135 BP based photodetectors, waveguideintegrated BP based photodetectors in the MIR, waveguide-integrated, 146 BP based photodetectors working in the NIR, waveguide-integrated, 145f BP based photodetector working in the MIR, waveguide-integrated, 147f near-infrared, waveguide-integrated photodetectors, 143 photodetectors, waveguide-integrated, 143 waveguide-integrated Ge based photodetector, schematic, 144f conclusion, 148 photodetectors, BP based, 149 introduction, 136 photodetectors, surface illuminated BP based avalanche BP photodetectors, 142 bowtie antenna and bowtie apertures, BP based photodetector with plasmonics, 141f BP based dual-gate field-effect transistor for bandgap tuning, cross section schematics, 140f BP based photodetectors, plasmonic enhanced, 141 BP based photodetector with a ring-shaped electrode, microscope image, 139f BP homojunctions, photodetectors based on, 138 BP/MoS2 hetero p-n diode, schematics, 140f BP-TMDCs heterojunctions, photodetectors based on, 139 metal-BP-metal structures with or without gate, photodetectors based on, 137 photodetector with bottom gate, typical surface-illuminated BP based, 137f

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polarization resolving, BP based photodetectors, 142 polarization resolving, BP/MoS2/BP/Au photodetector, 142f reducing bandgap, enabling longer wavelength detection, 140 surface illuminated BP based photodetectors, overview, 136

high-pressure conversion, 16 low-pressure transport route synthesized large black phosphorus single crystals in a silica ampoule, optical image, 8f other methods, 18 phosphorene, electrochemical exfoliation process using tetrabutylammonium hexafluorophosphate and DMF as electrolyte, 12f phosphorus nanosheets, wet-chemical synthesis, 19f crystal structures, phosphorus, 1 white phosphorus, red phosphorus, and black phosphorus, crystal structures, 2f red phosphorus nanomaterial, synthesis fibrous red phosphorus synthesized with iodine as a catalyst, high-resolution TEM image and diffraction pattern, 6f iodine doped red phosphorus nanosphere, SEM image, 7f mechanical methods, 2 other methods, 7 red phosphorus, crystalline, 4 red phosphorus-carbon nanotube composite, EM images, 3f red phosphorus nanodots on reduced graphene oxide, vaporization-condensation synthesis method, 5f thermal growth strategies, 3

R Red and black phosphorus nanomaterials, synthesis, 1 black phosphorus nanomaterial, synthesis, 8 ball-milling synthesized black phosphoruscarbon black nanocomposite as a lithiumion battery electrode material, TEM image, 17f black phosphorus, amorphous, 14 black phosphorus, CVD growth, 15f black phosphorus, liquid exfoliation, 10 black phosphorus, solvent exfoliation in various solvents via tip ultrasonication, 11f black phosphorus and blue phosphorus, crystal structure, 16f black phosphorus flake, AFM images, 9f black phosphorus nanoribbon, production process, 20f exfoliation, 9 growth strategies, bottom-up, 13

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