Ternary Quantum Dots: Synthesis, Properties, and Applications (Woodhead Publishing Series in Electronic and Optical Materials) [1 ed.] 0128183039, 9780128183038

Table of contents :
Front Cover
Ternary Quantum Dots: Synthesis, Properties, and Applications
Copyright
Contents
Preface
Chapter One: Fundamentals of quantum dot nanocrystals
1.1. Quantum confinement
1.2. Quantum dots
1.3. Particle size-bandgap-first excitonic wavelength relationship
1.4. Electronic transitions in semiconductors
1.5. Binary semiconductor vs ternary semiconductor
1.6. Nanocrystal growth mechanisms
1.7. Magic size clusters
1.8. Shelling of QDs
1.9. Colloidal stability
1.10. Summary and outlook
A. Annexure
References
Chapter Two: Properties of ternary quantum dots
2.1. Introduction
2.2. Quantum confinement effect
2.3. Multiple exciton generation (MEG)
2.4. Optical properties of ternary quantum dots
2.5. Toxicity and biocompatibility of ternary quantum dots
2.6. Conclusions
References
Chapter Three: Synthesis of ternary I-III-VI quantum dots
3.1. Nonconventional methods
3.2. Conventional methods
3.3. Organic phase synthesis
3.3.1. Hot injection method
3.3.2. Noninjection method
3.3.3. Solvothermal method
3.3.4. Thermal decomposition method
3.3.5. Cation exchange method
3.3.6. Ligand exchange
3.4. Aqueous synthesis
3.5. Alloy synthesis
3.6. Core/shell synthesis
3.7. Size selective precipitation
3.8. Conclusions
References
Chapter Four: Ternary semiconductor nanocomposites
4.1. Polymer-based nanocomposites
4.1.1. Ex situ blending
4.1.2. In situ growth
4.1.3. In situ polymerization
4.1.4. Ligand exchange
4.1.5. Ligand encapsulation
4.1.6. Ligand grafting
4.2. Carbon materials-based nanocomposites
4.2.1. Graphene composites
4.2.2. Graphene oxide composites
4.2.3. Carbon nanotubes composites
4.2.4. Carbon dots composites
4.2.5. Graphitic carbon nitride composites
4.3. Summary and outlook
References
Chapter Five: Characterization techniques for ternary I-III-VI quantum dots
5.1. Introduction
5.1.1. Ultraviolet-visible (UV-Vis) spectroscopy
5.2. Photoluminescence (PL) spectroscopy
5.3. X-ray diffraction (XRD) technique
5.4. Transmission electron microscopy (TEM)
5.5. Dynamic light scattering (DLS) of I-III-VI QDs
5.6. X-ray photon electron microscopy
5.7. Fourier transform infrared spectroscopy (FTIR)
5.8. Conclusions
References
Chapter Six: Cytotoxicity of ternary quantum dots
6.1. Introduction
6.2. Toxicity assay
6.2.1. In vitro assay
6.2.1.1. In vitro cytotoxicity in ternary quantum dots
6.2.1.2. In vivo assay
6.2.1.3. Ternary quantum dots, in vivo assays
6.3. Mechanism of QDs toxicity
6.4. Conclusion and remarks
References
Chapter Seven: Bioimaging and therapeutic applications of ternary quantum dots
7.1. Fluorescence imaging
7.2. In vitro cell imaging
7.3. In vivo imaging
7.4. Multiphoton imaging
7.5. Multiplex imaging
7.6. Multimodal imaging
7.6.1. Magnetic resonance/fluorescence imaging
7.6.2. Computed tomography/fluorescence dual imaging
7.6.3. Positron emission tomography/fluorescence/Cerenkov luminescence imaging
7.6.4. Ultrasound/fluorescence imaging
7.7. Photodynamic and photothermal therapy
7.8. Drug/gene delivery
7.9. Summary and outlook
References
Chapter Eight: Ternary quantum dots for sensing applications
8.1. Introduction
8.2. Type of sensors and their applications
8.2.1. Biosensors
8.2.2. Chemical sensor
8.3. Mechanism
8.3.1. Frster resonance energy transfer (FRET)
8.3.2. Chemiluminescence resonance energy transfer (CRET)
8.3.3. Bioluminescence resonance energy transfer (BRET)
8.4. Sensor development
8.5. Conclusion and remarks
References
Chapter Nine: Photocatalytic applications of ternary quantum dots
9.1. Introduction
9.2. Semiconductor quantum dots photocatalysis mechanism
9.3. Applications of ternary quantum dots as photocatalysts
9.3.1. Ternary quantum dots-based photocatalyst for wastewater treatment
9.3.2. Ternary quantum dots-based photocatalyts for hydrogen evolution
9.3.3. Other photocatalytic applications of ternary quantum dot
9.4. Conclusions and perspectives
References
Chapter Ten: Ternary quantum dots for solar cell applications
10.1. Introduction
10.1.1. Ternary compounds
10.2. Concepts of efficient photovoltaic device
10.2.1. Basic working principle of QDSSCs
10.2.2. Auger generation material
10.2.3. Band gap engineering
10.3. Manufacturing
10.3.1. Synthesis and purification of ternary quantum dots
10.3.2. Preparation of quantum dots film
10.4. Conclusions and perspectives
References
Chapter Eleven: Ternary I-III-VI quantum dots for light-emitting diode devices
11.1. Introduction
11.2. Operating principle of quantum dots-based LEDs (QDs-LEDs)
11.2.1. Electrical characteristics of light emitting diodes
11.2.2. Construction of a light emitting diode
11.2.3. Architecture of quantum dots-based light emitting diodes (QDs-LEDs)
11.2.4. Charges transport layer in QDs-LEDs
11.3. Ternary quantum dots-based LEDs
11.4. Conclusions
References
Index
Back Cover

Citation preview

TERNARY QUANTUM DOTS

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Woodhead Publishing Series in Electronic and Optical Materials

TERNARY QUANTUM DOTS Synthesis, Properties, and Applications OLUWATOBI SAMUEL OLUWAFEMI

Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Doornfontein, South Africa Centre for Nanomaterials Science Research, University of Johannesburg, Doornfontein, Johannesburg, South Africa

EL HADJI MAMOUR SAKHO

Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Doornfontein, South Africa

SUNDARARAJAN PARANI

Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Doornfontein, South Africa

THABANG CALVIN LEBEPE

Department of Chemical Sciences, University of Johannesburg, Doornfontein Campus, Doornfontein, South Africa

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright ©2021 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-818303-8 (print) ISBN: 978-0-12-818304-5 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisitions Editor: Kayla Dos Santos Editorial Project Manager: Rachel Pomery Production Project Manager: Vignesh Tamil Cover Designer:, Victoria Pearson Typeset by SPi Global, India

Contents Preface

ix

1. Fundamentals of quantum dot nanocrystals 1.1 Quantum confinement 1.2 Quantum dots 1.3 Particle size-bandgap-first excitonic wavelength relationship 1.4 Electronic transitions in semiconductors 1.5 Binary semiconductor vs ternary semiconductor 1.6 Nanocrystal growth mechanisms 1.7 Magic size clusters 1.8 Shelling of QDs 1.9 Colloidal stability 1.10 Summary and outlook A Annexure References

2. Properties of ternary quantum dots 2.1 Introduction 2.2 Quantum confinement effect 2.3 Multiple exciton generation (MEG) 2.4 Optical properties of ternary quantum dots 2.5 Toxicity and biocompatibility of ternary quantum dots 2.6 Conclusions References

3. Synthesis of ternary I–III–VI quantum dots 3.1 Nonconventional methods 3.2 Conventional methods 3.3 Organic phase synthesis 3.4 Aqueous synthesis 3.5 Alloy synthesis 3.6 Core/shell synthesis 3.7 Size selective precipitation 3.8 Conclusions References

1 1 4 5 8 11 17 20 21 26 28 29 30

35 35 36 38 39 41 41 42

47 47 48 48 62 64 65 66 68 69

v

vi

Contents

4. Ternary semiconductor nanocomposites

77

4.1 Polymer-based nanocomposites 4.2 Carbon materials-based nanocomposites 4.3 Summary and outlook References

77 93 105 106

5. Characterization techniques for ternary I–III–VI quantum dots 5.1 Introduction 5.2 Photoluminescence (PL) spectroscopy 5.3 X-ray diffraction (XRD) technique 5.4 Transmission electron microscopy (TEM) 5.5 Dynamic light scattering (DLS) of I–III–VI QDs 5.6 X-ray photon electron microscopy 5.7 Fourier transform infrared spectroscopy (FTIR) 5.8 Conclusions References

6. Cytotoxicity of ternary quantum dots 6.1 Introduction 6.2 Toxicity assay 6.3 Mechanism of QDs toxicity 6.4 Conclusion and remarks References

7. Bioimaging and therapeutic applications of ternary quantum dots 7.1 Fluorescence imaging 7.2 In vitro cell imaging 7.3 In vivo imaging 7.4 Multiphoton imaging 7.5 Multiplex imaging 7.6 Multimodal imaging 7.7 Photodynamic and photothermal therapy 7.8 Drug/gene delivery 7.9 Summary and outlook References

117 117 119 122 123 125 128 130 131 133

137 137 138 148 151 151

155 155 157 173 177 178 179 188 194 195 197

Contents

8. Ternary quantum dots for sensing applications 8.1 Introduction 8.2 Type of sensors and their applications 8.3 Mechanism 8.4 Sensor development 8.5 Conclusion and remarks References

9. Photocatalytic applications of ternary quantum dots 9.1 Introduction 9.2 Semiconductor quantum dots photocatalysis mechanism 9.3 Applications of ternary quantum dots as photocatalysts 9.4 Conclusions and perspectives References

10.

Ternary quantum dots for solar cell applications 10.1 Introduction 10.2 Concepts of efficient photovoltaic device 10.3 Manufacturing 10.4 Conclusions and perspectives References

11.

Index

vii

207 207 208 213 219 220 222

225 225 227 228 232 232

237 237 240 243 245 246

Ternary I–III–VI quantum dots for light-emitting diode devices

251

11.1 Introduction 11.2 Operating principle of quantum dots-based LEDs (QDs-LEDs) 11.3 Ternary quantum dots-based LEDs 11.4 Conclusions References

251 252 256 260 261 265

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Preface We are indeed delighted in presenting the book on Ternary Quantum Dots: Synthesis, Properties, and Applications. Semiconductor quantum dots (QDs) has turned to be a special class of nanomaterials due to their peculiar size tunable optical properties. The growing interest on quantum dots comes from its wider range of applications on different sectors, such as displays, energy, sensors, lasers, water, medical, etc. QDs have already found their place in the market especially on displays. The overall quantum dot market is expected to grow more than $10B by 2025. Therefore, knowledge of QDs offers diverse opportunities. Three-four decades ago, the research on QDs started with the binary II–VI, IV–VI chalcogenide semiconductors such as CdSe, PbS, and now there is a wealth of information available on the synthesis, properties, and application of these binary QDs materials. Over this research period, the structure and optelectronic properties have been well understood. Cadmium-based QDs have been investigated especially thoroughly because of their high fluorescence and wide range of tunability from visible to near infrared. However, due to their inherent toxicity, these QDs remain a threat to the environment, which hampers their industrial production and applications. Hence, many companies are now shifting towards nontoxic alternatives. For the past decade, ternary QDs involving I–III–VI chalcopyrite semiconductor materials such as AgInSe, which are relatively nontoxic compared to binary QDs, have been emerging as safer and eco-friendly alternative. Being ternary material, the properties of these chalcopyrite QDs are not only size dependent but also composition dependent. However, unlike binary QDs, the potential of I– III–VI QD materials are still not well realized enough. Though several review papers have appeared in the last 5 years, there has been no book on ternary quantum dots produced so far and this motivated us to write this book. Thus, this book aims to introduce ternary chalcopyrite I–III–VI QDs, their principles, syntheses, properties, and different applications; however, it also extends to II-III-VI QDs such as ZnIn2S4 in some chapters. Briefly, it started with the fundamentals of QD

ix

x

Preface

nanocrystals, electronic properties with the comparison of binary and ternary QDs, different types of growth mechanisms, and the shelling of QDs have been discussed. The book is then further divided into three categories which cover (i) the synthesis of ternary QDs, their polymer and carbon nanocomposites, (ii) their properties and characterization, and (iii) their applications on biomedical, solar cells, light emitting diode, photocatalytic, and sensing applications in separate chapters. The large body of literature collections have also been included wherever it's applicable. This book is intended for readers in science and will be a valuable reference material for university professors, postdoctoral fellows, postgraduate students, industrialists, and scientists, primarily in the field of material science, applied chemistry, biology, and applied physics. The content of the book is profound, comprehensive, and aids the readers to understand and strengthen their knowledge of the subject. We hope this book will be highly useful to the intended audience. The authors, while writing the book, were highly enlightened by a number of articles and books which are gratefully acknowledged. The authors are also thankful to the referees for their valuable comments and suggestions which further advanced the book. The authors acknowledge University of Johannesburg, South Africa for their constant support and motivation. The authors extend their gratitude towards Elsevier Publisher for giving us the opportunity to write and publish this book. Efforts were made to prepare a book free from errors, however typos or other errors could have escaped from our sight. Any comments or suggestions which could help us improve the book in the future are warmly welcome. Please write to us. Samuel Oluwatobi Oluwafemi Sundararajan Parani El Hadji Mamour Sakho Thabang Calvin Lebepe

CHAPTER ONE

Fundamentals of quantum dot nanocrystals Abstract Semiconductor fluorescent nanoparticles also known as quantum dots (QDs) is an attractive member among the nanomaterials family that holds great potential for variety of applications in the field of biomedical, optoelectronic, energy, water treatment, etc. The size, composition tunable optical properties of QDs are highly unique that draw the researchers’ attention worldwide. Earlier investigations on QDs were based on binary chalcogenide materials, which usually composed of heavy metals in their structure. However, the increased awareness on the toxicity of the heavy metals, which is a major threat to the human and environment, directed the research on nontoxic/less toxic QDs that includes ternary chalcopyrite materials. This chapter provides a basic introduction to QD nanocrystals, their crystal growth mechanism, colloidal stability in general and their structural and optical properties in comparison with that of bulk semiconductor. In particular, binary semiconductor II–VI and ternary I–III–VI materials are discussed at their bulk and nanocrystal scale. CdSe and CuInS2 are taken as a representative of II–VI and I–III–VI system, respectively, for a comparative discussion. Keywords: II–IV, Optoelectronic properties, Ternary quantum dots

1.1 Quantum confinement Quantum confinement is a direct consequence of one of the basic principles of quantum mechanics, Heisenberg uncertainty principle, according to which, the position and momentum of tiny particle can never be simultaneously measured to an arbitrary precision but only with uncertainty. The standard deviation of the position in space (Δx) and momentum (Δp) is related by Heisenberg as Δx: Δp


ħ 2

(1.1)

where ħ is the reduced Planck constant. The above equation implies that when the motion of tiny particle is confined to a certain region of space by the potential barrier, the momentum of the particle increases such that the particle has the uncertainty value always higher than ħ/2. As the Ternary Quantum Dots https://doi.org/10.1016/B978-0-12-818303-8.00010-1

Copyright © 2021 Elsevier Ltd. All rights reserved.

1

2

Ternary quantum dots

magnitude of momentum, P is related to kinetic energy, E and mass, m as E ¼ P2/2m, the increase in the momentum of the confined particle greatly increases its kinetic energy. When a semiconductor is excited by energy higher than its bandgap energy, electrons in the valence band (VB) absorb the energy and “jump” to the conduction band (CB) leaving a hole in the VB. The bulk semiconductor has a fixed band gap (Eg) and their charge carriers (electron and hole) move freely across the crystal in all spatial directions. However, if the size (diameter/ width) of semiconductor crystal reduced to the order of a few nanometers, the charge carriers become highly confined and hence they cannot move freely [1, 2]. In this nanosize regime, classical physics is no longer applicable, and the system follows quantum mechanical model. Depending upon the degrees of freedom for the charge carriers, systems with 3, 2, 1 and 0 degrees of freedom (alternatively 0, 1, 2, 3 degrees of confinement) are denoted as bulk, quantum wells (QWs), quantum rods (QRs), and quantum dots (QDs), respectively, as illustrated in Fig. 1.1. When the 1D material, QRs is not so rigid but has slightly flexible structure extending to several micrometer in length, they are called quantum wires. The confinement of charge carrier allows the system to change its electronic band energy levels from continuous to discrete. Hence the increase in the energy gap between valence band and conduction band and the change of electronic density of states (DOS). Fig. 1.2 shows the representative images of QWs, QRs and QDs visualized using transmission electron microscope (TEM). No confinement Bulk (3D materials)

Bulk

1D confinement Quantum wells (2D materials)

2D confinement Quantum wire (1D materials)

Quantum well

Quantum wire

3D confinement Quantum Dots (0D materials)

Quantum dot

Fig. 1.1 Illustration of quantum confinement of charge carriers from 3D bulk to 0D quantum dots with variation in DOS [1].

3

Fundamentals of quantum dot nanocrystals

(A)

(B)

(C)

Fig. 1.2 Representative transmission electron microscope (TEM) images of quantum confined structures. (A) scanning TEM image of InGaN QWs [3], the edges of the well are not seen due to high magnification. (B) TEM image of CdSe@CdS QRs [4] and (C) TEM image of CdSe QDs, inset: high resolution TEM image of a single CdSe QD. Reprinted from D.V. Talapin, S.K. Poznyak, N.P. Gaponik, A.L. Rogach, A. Eychm€ uller. Synthesis of surface-modified colloidal semiconductor nanocrystals and study of photoinduced charge separation and transport in nanocrystal-polymer composites. Phys. E: Low-Dimen. Syst. Nanostruct. 14(1–2) (2002) 237–241. Copyright (2002) with permission from Elsevier.

The excited electron and hole in a semiconductor form a bound-pair called as “exciton” due to the Coulombic interaction (Fig. 1.3A) with a physical spatial separation defined by Bohr exciton radius (rBohr). The quantum confinement in the semiconductor nanocrystals is determined by their Bohr exciton radius and the semiconductor is said to be quantum confined where at least one of its dimensions (diameter/width) is of or smaller than Bohr exciton radius. Fig. 1.3B shows the QDs whose diameter is lesser than Bohr radius with the confinement of its exciton. The Bohr radius depends on the type of semiconductor and it is given by: rBohr ¼

ħ2 ε e2 μ

(1.2)

Conducon band `

e

e e

Bandgap

hν

h h

(A)

Valence band

h

(B)

Bulk Bohr exciton radius

Confined Exciton in zero dimensional QDs

Fig. 1.3 (A) Illustration of exciton and Bohr exciton radius (rBohr) and (B) quantum confinement in QDs.

4

Ternary quantum dots

where ħ is the reduced Plank constant, μ is the reduced mass of the exciton and defined by the relation μ1 ¼ (m∗e )1 + (m∗h)1, me and mh are the effective masses of electron and hole respectively, e is the charge of electron, and ε is the dielectric constant. Annexure lists the Bohr exciton radius and bandgap energy for the selected binary and ternary bulk semiconductors [5, 6].

1.2 Quantum dots Quantum dots are a kind of fluorescent semiconductor zerodimensional nanocrystalline particles with size typically falling in the range of 1–10 nm and confines the motion of the charge carriers in three dimensions. QDs were first observed in a glass matrix in 1981 by the Russian scientist, Alexey Ekimov [7] at the Vavilov State Optical Institute in St. Petersburg and later in colloidal solutions in 1984 by an American scientist Brus [8] while working at the at the AT&T Bell Laboratories in New Jersey. The research on QDs got boosted after Murray, Norris and Bawendi [9] in 1993 successfully reported the colloidal synthesis of nearly monodisperse CdE (where E ¼ S, Se, Te) QDs followed by perspective report by Alivisatos [10]. This opened up variety of QDs materials with interesting properties. QDs have conventionally been prepared as binary chalcogenide semiconductor nanocrystals from group II–VI (e.g., CdTe, ZnSe, etc.), group IV–VI (e.g., PbSe) or group III–V (e.g., GaAs, InP) in the periodic table. Furthermore, QDs made up of single element such as Si [11, 12], Ge [13, 14], carbon [15, 16] and graphene [17, 18] have also been reported. Among them, cadmium based QDs have been widely investigated due to their easy tunable optical properties across visible and near infrared wavelength (NIR) region. However, there comes a time to consider the inherent heavy metal toxicity of these QDs that have hampered the synthesis and use of these QDs in many fields. Application oriented studies on QDs have now deviated from heavy metal toxic QDs and the growing interest has been on nontoxic alternatives. For the past decade, QDs prepared from I–III–VI group elements (e.g., CuInS2, AgInSe2, etc.) have received great attention due to their low toxicity with size and composition tunable optical properties [19–21]. In contrast to bulk semiconductor with the fixed bandgap, QDs exhibits size dependent bandgap where the bandgap energy increases with the decrease in crystal size as illustrated in Fig. 1.4. Compared to other confined structures (2D and 1D materials), the bandgap of QDs are strongly dependent on its size (number of atoms) hence altering its size greatly alter their

5

Fundamentals of quantum dot nanocrystals

Fig. 1.4 Illustration of size dependent variation of bandgap of QDs in comparison with the bandgap of bulk and single molecule.

band gap and optical properties. For instance, the bandgap of smaller QDs is higher and when it is excited, it emits blue color whereas the bigger crystals are with low bandgap which emit red color after excitation (Fig. 1.5). Because of the strong confinement, the quantized energy levels of QDs shows similarities with the that of atoms/molecule and hence they are often represented as artificial atoms.

1.3 Particle size-bandgap-first excitonic wavelength relationship The bandgap energy of the semiconductor crystal can be related to its radius (R) based on effective mass approximation (EMA) as in the Eq. (1.3). E ¼ Eb +

ħ2 π 2 Ae2 ∗   0:248 ERy 2R2 μ εR

(1.3)

where E and Eb are the band gap energies of the crystal and that of the corresponding bulk material. The other parameters are the same as in Eq. (1.2). The first two terms on the right-hand side in the Eq. (1.3) was given

6

Ternary quantum dots

Fluorescence

2.3 nm

5.5 nm Par cle Size

Fig. 1.5 Representative image of size dependent fluorescence of quantum dots (CdSe/ ZnS). Adapted with the permission from B.O. Dabbousi, J. Rodriguez-Viejo, F.V. Mikulec, J.R. Heine, H. Mattoussi, R. Ober, K.F. Jensen, M.G. Bawendi. (CdSe) ZnS core-shell quantum dots: synthesis and characterization of a size series of highly luminescent nanocrystallites. J. Phys. Chem. B 101(46) (1997) 9463–9475. Copyright (1997) American Chemical Society.

by Brus [22]. The first term with 1/R2 denotes the quantum localization energy which is the sum of the confinement energy of electron and hole. The second term with 1/R represents the Coulombic interaction between electron and hole. The coefficient A is 1.79 for the 1Se-1Sh electronic transition state and has values between 1.6 and 1.9 for other electronic transition states. The third energy term in the Eq. (1.3) was introduced by Kayanuma [23] and is usually negligible but it can become significant for a semiconductor with a smaller dielectric constant. For bulk crystals, the influence of the particle size on its bandgap is fully negligible as there is no confinement of charge carriers. Accordingly, all the terms involving R can be omitted and hence E simply becomes equal to Eb. However, for QD nanocrystals, as mentioned earlier, the bandgap is highly dependent on their size and so their electronic transitions. As the bandgap decreases with the increase in the size of QDs, both absorption and emission spectra of the QDs will be red-shifted as depicted in the Fig. 1.6A, B and D. The relationship between the bandgap and the particle size can also be derived empirically. The bandgap Eg energy (eV) can be experimentally calculated from the absorption wavelength (λ, nm) of first excitonic transition (1Se-1Sh) using absorption spectroscopy according to the Eq. (1.4)

Fig. 1.6 Representative figures showing (A) size dependent absorption spectra and, (B) fluorescence spectra, (C) bandgap determination from Tauc’s plot, and (D) size dependent bandgap of CdSe (CS) QDs of various sizes from 2.5 to 5.2 nm [24].

8

Ternary quantum dots

E¼

hc λ

(1.4)

where h is the plank constant, c is the speed of light. However, Tauc’s plot is widely used to calculate the bandgap with more accuracy using the Tauc’s relationship given below
n αhv ¼ A hv  Eg (1.5) where α is the absorption coefficient, A is the Tauc constant, hv is the photon energy, n is the parameter which is ½ for direct bandgap and 2 for indirect bandgap. The extrapolation of the linear region of the plot of (αhv)1/n vs hv to α ¼0 yields the bandgap (Fig. 1.6C). On the other hand, average particle size can be calculated from XRD patterns using the Scherrer equation given below: D¼

Kλ β cosθ

(1.6)

where D is the crystallite size, K is a shape factor which typically 0.9 for spherical particles, λ is the wavelength of the X-rays, β is the width (fullwidth at half-maximum) of the X-ray diffraction peak in radians and θ is the Bragg angle [25]. Moreover, the exact size of the crystals and their distribution can be obtained by analyzing the samples using high resolution transmission electron microscope (HRTEM). Based on the correlation of above analyses, the empirical relationship between the particle diameter (D) and the first excitonic absorption peak (λ) or bandgap (Eg) for some of the QDs have been determined in good approximation and given in the Table 1.1. In addition, particle size can be used to calculate molar extinction co-efficient, an important parameter to determine the concentration of the QDs using Beer Lambert law. It must be noted that the above relations are only the fitting functions of experimental data which are valid only for particular range of sizes as described in the experiments.

1.4 Electronic transitions in semiconductors Exciton is a quasiparticle containing energy and it is technically neutral. Therefore, it can transport energy without transporting electric charge. The lifetime of exciton is very short and hence, the excited electrons and holes tend to recombine quickly during which photons can be emitted with

Table 1.1 Empirical functions correlating the size of QDs (D, nm) with the first excitonic peak (λ, nm) in their absorption spectra and with the molar extinction coefficient (ε, L mol1 cm1). QD Relationship Molar extinction coefficient References

CdTe CdSe CdS PbS PbSe CuInS2 (wurtzite) CuInS2 (chalcopyrite)

D ¼ (9.8127  107)λ3  (1.7147  103)λ2 + (1.0064)λ  (194.84) D ¼ (1.6122  109)λ4  (2.6575  106)λ3 + (1.6242  103)λ2  (0.4277)λ + (41.57) D ¼ (6.6521  108)λ3 + (1.9557  104)λ2  (9.2352  102)λ + (13.29) – D ¼ (λ  143.75)/281.25 Eg ¼ 1.327 + 1/(0.0125D2 + 0.225D + 0.938) Eg ¼ 1.532 + 1/(0.0882D2 + 0.587D  0.517)

ε ¼ 10,043 (D)2.12

[26]

ε ¼ 5857 (D)2.65

[26]

ε ¼ 21,536 (D)2.3

[26]

ε ¼ 3925 (D)2.32 ε ¼ 3389 (D)2.54 ε3.1eV ¼ 10,175 (D)3 –

[27] [28] [29] [29]

10

Ternary quantum dots

a specific energy corresponding to the band gap. Since a part of the energy may be released in a nonradiative way, the energy of emitted photons is always lower than the excitation energy. There are multiple relaxation/ recombination pathways for the excited electron as shown in Fig. 1.7. The relaxation process is nonradiative and fast (in picasecond range) while the recombination process is radiative and slow (in nanosecond range). Initially, excited electron relaxes within the conduction band (Fig. 1.7C) and then recombines with the hole. The direct recombination of the electrons from the conduction band and holes in the valence band is referred to as the “band edge” recombination process (Fig. 1.7D). In addition, there might be some energy states within the bandgap where the electrons or holes may get trapped nonradiatively before their recombination. These trap states are formed due to the imperfections/defects in the crystals such as dislocations, vacancies, substitutions, surface defects and the presence of interstitial atoms, (doping) impurities, etc. Electron traps are found under the bottom of the conduction band and hole traps are found above the top of the valence band. The traps can occur at varying depths (shallow and deep) based on the nature

Conduc on band c ST

e a

hν

d

jb g

DT

Donor

i

h

f

DT

Acceptor ST

Valence band Fig. 1.7 Schematic illustration of electronic transitions of photo-excited electron in a semiconductor: (A) band edge absorption, (B) sub-band edge absorption, (C) electronic relaxation within the conduction band, (D) band-edge electron-hole recombination, (E) excited electron, (F) excited hole trapping into shallow trap (ST) and deep trap (DT) states as well as further trapping from ST to DT, (G) electron-hole recombination from conduction band to trapped hole acceptor state, (H) electron-hole recombination from trapped electron donor state to valance band, (I) trapped electronhole donor-acceptor recombination, and (J) exciton-exciton annihilation.

Fundamentals of quantum dot nanocrystals

11

of the trap energy states (Fig. 1.7E and F). The trap states that lead to radiative recombination can be referred as donors (for electron traps) and acceptors (hole traps). There can be multiple donors and acceptors depending on the type of the trap states. In Fig. 1.7, only one donor (D) and one acceptor (A) state is shown for the sake of clarity. The electron can recombine with the hole through these traps in three different ways such as, conduction band to acceptor (CA, Fig. 1.7G), donor to valance band (DV, Fig. 1.7H) and donor to acceptor (DA, Fig. 1.7I) recombination. Generally, both DV and CA recombination occurs at higher energy level, while DA recombination occurs at lower energy levels as the energy gap of latter is small. The recombination involving through these traps usually results in broad emission with the less energy as the electrons and holes move to the traps of varying depths. These defect states can also cause sub-band edge absorption (Fig. 1.7B) in addition to the excitonic band-edge absorption (Fig. 1.7A) that was used to create exciton, and in such cases both absorption and emission bands appear very broad. Moreover, exciton can colloid with another exciton and annihilate in a nonradiative way (Fig. 1.7J).

1.5 Binary semiconductor vs ternary semiconductor Binary II–VI and ternary I–III–VI semiconductors are structurally and chemically different. The binary semiconductor preferably crystallizes in the zinc blende crystal structure while the ternary semiconductor crystallizes in chalcopyrite crystal structure under normal conditions [30]. Both II–VI and I–III–VI semiconductors may also crystallize wurtzite structure under certain conditions, however wurtzite phase is mostly a metastable phase except for CdSe which possess stable wurtzite phase at room temperature. In II-VI zinc blende crystal structure, each group-VI anion is tetrahedrally coordinated to four group-II cations and vice versa. The I–III–VI chalcopyrite crystal structure can be viewed as the two units of II–VI zinc blende crystal structure where group II ions of zinc blende are replaced by group I and group III ions (Fig. 1.8). Accordingly, each group-VI anion is tetrahedrally coordinated to two group-I and two group-III cations and each cation (both I and III) are tetrahedrally coordinated to one group-VI anion. However, the mismatch between the bond lengths of group VI anions to group I and III cations (RI–VI 6¼ RIII–VI) distorts the crystal structure and make the I–VI bond weaker than III-VI bond [31]. This leads to several defects including vacancies, interstitial, anti-site (substitution) defects. The defect states of these ternary I–III–VI materials are based on their stoichiometric

12

Ternary quantum dots

Zinc Blende Se

Cd CdSe

In

CuIn

VCu

Cui

InCu

Cu Cu-rich Cu-In-S

Stoichiometric CuInS2

In-rich Cu-In-S

Chalcopyrite Fig. 1.8 Structural relation between II–VI (CdSe) and I–III–VI (Cu-In-S) semiconductors (VCu denotes copper vacancy; CuIn and InCu, copper and indium antisite defects; Cui, copper interstitial defect). Adapted from O. Yarema, M. Yarema, V. Wood. Tuning the composition of multicomponent semiconductor nanocrystals: the case of I–III–VI materials. Chem. Mater. 30(5) (2018) 1446–1461. Copyright (2018) American Chemical Society. https://pubs. acs.org/doi/10.1021/acs.chemmater.7b04710. Further permissions related to the material excerpted should be directed to the ACS.

composition. For instance, Cu rich CuInS2 exhibits interstitial, CuIn antisite defect (Cu in place of indium) while Cu deficient or In rich CuInS2 exhibit Cu vacancy defect and InCu anti-site defect (In in place of Cu). Interestingly, both n-type and p-type I–III–VI semiconductor can be prepared by simply controlling the defects without adding any external impurities [32]. The stoichiometric deviation of ternary materials can go up to several percentages from the ideal stoichiometric composition (I–III–VI2). However, stoichiometric deviation is very limited for binary II–VI semiconductors, and excess carrier mobility is possible only by inducing external impurities such as doping for example Cu-doped CdSe. The bulk semiconductors of both binary (II–VI) and ternary (I–III–VI) materials exhibit dominant broad band emission in their PL spectra (Fig. 1.9A) where defect state emission is stronger than band edge emission [33]. For instance, II–VI binary semiconductor CdSe in bulk form exhibits strong and broad donor-acceptor recombination (DA) due to Frenkel pair defects where Cd interstitials acts as donors and Cd vacancies as acceptors as

13

Fundamentals of quantum dot nanocrystals

Bulk crystal

Normalized intensity

CdSe CuInS2 CuInS2 Cu-rich)

1.50 Eg (CuInS2)

1.4

1.5

1.54

1.6

Eg (CdSe)

1.8

1.7

Energy (eV)

(A)

1.5

(B)

2.0

2.5

3.0

Energy (eV)

3.5

1.5

Norm. absorbance

CdSe

normPL intensity

Norm. absorbance

normPL intensity

Nanocrystal CuInS2

2.0

2.5

3.0

3.5

Energy (eV)

Fig. 1.9 (A) Emission spectra of bulk CdSe, CuInS2, and Cu-rich CuInS2. (B) Absorption and emission spectra of CdSe nanocrystals (left panel) and CuInS2 (right panel) nanocrystals. Reproduced from A.C. Berends, M.J. Mangnus, C. Xia, F.T. Rabouw, C. de Mello Donega. Optoelectronic properties of ternary I–III–VI2 semiconductor nanocrystals: bright prospects with elusive origins. J. Phys. Chem. Lett. 10(7) (2019) 1600–1616. Copyright (2019) American Chemical Society. https://pubs.acs.org/doi/10.1021/acs.jpclett.8b03653. Further permissions related to the material excerpted should be directed to the ACS.

its band edge emission is very weak. Similarly, the I–III–VI ternary CuInS2 in bulk form displays strong and broad defect mediated recombination. It has been stated that in I–III–VI semiconductors, the defect states such as group VI vacancies (VVI), interstitial group I atoms (IntI), and group III atoms substituted at the group I sites (anti site-IIII) can act as donors while group

14

Ternary quantum dots

I vacancies (VI), group III vacancies (VIII), interstitial group III atoms (IntIII) and group I substituted at the group III position (anti site-IIII) can act as acceptors [34, 35]. These defect states can serve as recombination center for the photoexcited charge carriers. Accordingly, different types of trap emissions such as DV, CA and DA recombination are possible [36, 37]. In bulk CuInS2, DA emission is stronger and broader among the defect mediated emissions (Fig. 1.9A). As shown in Fig. 1.4, at nanoscale levels, the optoelectronic properties of both II–VI binary and I–III–VI semiconductors are different from their bulk counter parts due to the quantum confinement. The bandgap of nanocrystals is wide compared to bulk and increases with decrease in size and hence the absorption and emission occurs at higher energies than that of bulk (Fig. 1.9B). In addition, contrary to their bulk form, optoelectronic properties of binary II–VI and ternary I–III–VI nanocrystals differ from each other. This is evident from their absorption and emission spectra. The uniform sized CdSe QDs would display well defined narrow absorption peak near the band-edge, narrow emission peak with low Stokes shift, low lifetime. On the other hand, uniform-sized CuInS2 QDs typically display featureless absorption, broad emission, huge Stoke shift, and long life time (Fig. 1.9B). This is due to the fact that unlike binary QDs, ternary QDs possess intrinsic defect energy states which traps the charge carriers in many ways. In addition, the composition of ternary QDs plays important role in the light absorption and emission as explained below. The recombination pathway of the charge carriers in the binary QDs has been well demonstrated to occur via conduction band to valence band transition (band edge) in the absence of surface defects (Fig. 1.7D) [33]. However, the recombination pathway in the I–III–VI ternary QDs has been a debate and even for the single crystal, the low temperature photophysical properties is complex. This is due to the presence of various intrinsic defects as mentioned before. Accordingly, different mechanism such as DA, DC, CA (Fig. 1.7G–I), have been proposed to explain the broad emission band with large Stokes shift and long lifetime. However, based on the recent advanced studies, researcher have now come to an agreement that the recombination most likely occur involving a conduction band electron and a hole-like midgap acceptor state localized on the group I defect (CA). Thus, recombination of charge carriers in ternary I–III–VI QDs is different from that of bulk counterpart where DA recombination is dominant [31, 33, 38–41]. The recombination in CuInS2 QDs is primarily channeled at Cux sub-bandgap defects. (Fig. 1.10). These defect states are in the midgap

S Cu CuIn” In InCu•• VCu’ Near-Stoichiometric

Cu-Deficient

Fig. 1.10 (Left panel) Anti-site defects (CuIn, InCu) in Cu-rich CuInS2 nanocrystals lead to sub-bandgap absorption (hʋCu,a) in addition to bandedge absorption (hʋx,a). Sub-bandgap emission (hʋCu,PL) with the huge Stokes shift occurs. Anti-site defects distort the tetrahedral co-ordination. (Right panel) Cu vacancy defect (CuCu generated by VCu, hole trap Th) in Cu deficient CuInS2 nanocrystal leads band-edge absorption with no sub-bandgap absorption. Sub-bandgap emission (hʋCu,PL) with the relatively decreased Stokes shift occurs. Cu vacancies reduces the tetrahedral distortion. Reproduced from A. Fuhr, A. Alexandrova, P. Sautet. Stoichiometry-controllable optical defects in CuxIn2-xSy quantum dots for energy harvesting. J. Mater. Chem. A 8 (2020) 12556–12565 with permission from The Royal Society of Chemistry.

16

Ternary quantum dots

region and far from the “nonradiative” electron and hole trap states that are concentrated near conduction band and valence band edges, respectively. This emission transition is akin to "metal to ligand charge transfer" where electron is transferred from the conduction band (CB, as LUMO of a “ligand” following photoexcitation) to the Cu center (metal). Cu2 + + e ðCBÞ ! Cu + + hv

(1.7)

where hv is the photon emitted. Hence it is obvious that Cu2+ energy state is necessary for the emission. This is because Cu2+ has 3d9 configuration and have one vacant site for electron capture. However, stoichiometric CuInS2 QDs exhibits Cu+ state in the ground state with filled d shell (3d10 configuration) generated by anti-site defects. Therefore, the photoexcited hole from the conduction band is rapidly captured by Cu+ state in order to achieve Cu2+-3d9 configuration for radiative recombination. After radiative recombination by electron capture, the original Cu+ state is restored. On the other hand, Cu deficient CuInS2 nanocrystals has Cu2+ state (3d9) configuration in the ground state itself, which is originated for charge compensation due to the presence of large number of Cu vacancies (VCu). Hence this hole like 3d9-Cu2+ state is “emission ready” without prior activation [40]. However, for dominant sub-bandgap emission to occur, band edge emission must be avoided and hence the hole in the valence band must be captured. This is done by VCu defects which acts as hole trap. The recombination is then completed by transfer of electron to the VCu which restores the original oxidation state (2+) of the Cu defect. Hence, the presence of hole trap by VCu is necessary for sub-bandgap emission in Cu deficient CuInS2 QDs. It is also demonstrated that the anti-site defect states in the stoichiometric CuInS2 QDs are not only emissive but also absorptive because it is occupied below the fermi energy level (Ef). As a result, the band edge absorption together with the sub-bandgap absorption (hʋCu,a) results in the broad absorption spectra with the tail extending to lower energies [38, 42]. As these defects acts as hole traps as mentioned earlier, the abundance of these defects reduces the fluorescence intensity and hence its emission spectra (sub-bandgap emission) is obtained with the huge Stoke shift. In case of Cu deficient CuInS2 QDs, Cu2+ defect (3d9, paramagnetic, with a vacant site) is occupied above Ef. Hence no sub-bandgap absorption but only band edge absorption occurs which results in the sub-band gap emission with a relatively high fluorescence intensity and low Stoke shift. It is worth to note that, energy states and the recombination pathways of Cu-deficient CuInS2 QDs resembles that of Cu doped binary QDs (Cu:CdSe) [43].

Fundamentals of quantum dot nanocrystals

17

1.6 Nanocrystal growth mechanisms Quantum dot nanocrystals are generally synthesized through wet chemical colloidal synthesis wherein the reaction medium consists of precursor compounds dissolved in a suitable solvent in the presence of stabilizing (capping) agent. When the medium is subjected to heat, microwave irradiation or provided with some other energy sources, the precursors chemically transform into monomers. The term monomer here indicates the intermediate species in the transformation of precursors into nanocrystals. Once the concentration of the monomer attained a critical super-saturation level which is energetically unstable, a burst of nucleation occurs with the generation of a large number of nuclei in a short time. A nucleus is solely a first precipitated solid phase from solution. When the concentration of monomer falls below the critical level for nucleation, nucleation of the crystal stops, and the particles will start to grow further by the molecular addition at a slower rate determined in the growth process. The growth rate approaches zero when the monomers have depleted. This mechanism was proposed by LaMer [20, 44] and depicted in the Fig. 1.11. The growth process of nanocrystals involves two different stages namely “focusing” and “defocusing” processes [45, 46]. In the “focusing” process, the existing free monomers is not sufficient to nucleate more particles and hence they add to nucleated particles thus promote relatively large growth. As larger crystals need more atoms, longer time and lot of energy to grow than smaller crystals, the smaller crystals will grow faster than large ones which results in focusing on size distribution and yield nearly uniform sized particles (Fig. 1.12A and B). This process is called as a “reaction limited” growth process where the probability of growing particle forming the aggregates is very low. The supersaturation, fast nucleation and the effective separation of nuclei and the growth phases are the key parameters to attain monodisperse nanocrystals. When separation is insufficient, the consumption of free monomers to form nuclei is much faster than the growth of smaller nanoparticles. With the depletion of monomers, the particle growth can proceed via Ostwald ripening, where small unstable particles are dissolved back in the solvent and become the monomers for the growth of large ones. The net result of this stage is the defocusing (size broadening) of size distribution resulting in polydispersed particles (Fig. 1.12C and D). This process is called as a “diffusion limited” process where the probability of growing particle forming the aggregates is high and it is irreversible and strong [46, 47]. An inverse of Ostwald ripening, where smaller particles

18

Ternary quantum dots

Fig. 1.11 LaMer diagram depicting nucleation and growth of particles. Reproduced from W.M. Girma, M.Z. Fahmi, A. Permadi, M.A. Abate, J.Y. Chang. Synthetic strategies and biomedical applications of I–III–VI ternary quantum dots. J. Mater. Chem. B 5(31) (2017) 6193–6216 with permission from The Royal Society of Chemistry.

grow at the expense of larger ones has also been reported and this process is called as digestive ripening [38, 48, 49]. In this case, the larger crystals re-dissolve/break down in solution in the presence of digestive ripening agent and supply the monomers for the growth of smaller crystals (Fig. 1.12E–G). Coalescence is another route of particle growth [50]. In coalescence, the particle growth takes place by attachment of particles to the growing particles. They can be either random attachment (Fig. 1.12H) or oriented attachment (Fig. 1.12I) where in the latter case, the particles attach to the specific crystallographic planes [39]. In coalescence, the defects occur at the interface of the attachment which can be either kept or pop out during the self-integration process. Intraparticle ripening is a unique phenomenon

Fundamentals of quantum dot nanocrystals

19

Fig. 1.12 Different growth process of nanocrystals: Schematic representation of (A) focused growth, (C) Ostwald ripening, (E) digestive ripening. Reproduced from [45] with the permission from Copyright (2020) John Wiley & Sons, Ltd. TEM images showing the (B) focused growth. Reproduced from [46] with permission from The Royal Society of Chemistry, (D) defocused growth by Ostwald ripening process. Reproduced from [47] with permission from The Royal Society of Chemistry. TEM images showing the digestive ripening of (F) bigger crystal to (G) QDs nanocrystal [48], (H) TEM image showing the nonoriented, Reprinted with permission from [49] Copyright (2007) American Chemical Society and (I) oriented attachment Reprinted with permission from [50] Copyright (2010) American Chemical Society, (J) Schematic representation of intraparticle ripening. Reprinted with permission [51]. Copyright (2001) American Chemical Society.

20

Ternary quantum dots

where nanoparticle rearranges its shape with time due to the diffusion of monomers along the surface (Fig. 1.12J). This intraparticle ripening is different from Ostwald ripening process which is an interparticle ripening. Intraparticle ripening occurs at a monomer concentration several times higher than interparticle ripening. In this intraparticle diffusion, high energy facets of crystal dissolve to grow low energy facets which leads to the change of shapes within the crystal [51, 52]. All the above growth mechanisms are highly dependent on the monomer concentration, temperature, time, surface potential, and capping ligands. In real experimental conditions, the growth of the nanocrystals is accompanied by neither reaction controlled nor diffusion controlled but controlled by a combination of both processes according to Talapin et al. [53]. The evolution of a single particle of radius “r” in a solution of monomer with constant concentration is given by the following equation.   1 S  exp ∗ dr r∗ hαi ¼ (1.8) dτ r ∗ + K exp r∗ r∗ and τ are particle radius and time, while K is the ratio between the rates of a purely diffusion-controlled process and a purely reaction controlled one. It is a dimensionless parameter describing the type of the process involved. K < 0.01 indicates purely diffusion-controlled process, K > 100 corresponds to reaction-controlled process while the regime of mixed control with comparable contributions from both processes is represented by 0.01 < K < 100. S and α are the oversaturation of the monomer in solution and the transfer coefficient of the activated complex (0 < α < 1), respectively.

1.7 Magic size clusters Atoms can often arrange in a densely packed structures to produce magic size clusters [54, 55]. The dense pack of atoms offers a high stability and enhance optical properties. The formation of magic size clusters is quite different from that of standard crystal growth model. In the standard growth model, the growing crystal is surrounded by the monomers which undergoes dynamic bonding and unbonding onto the crystal surface. This continues till it reaches the critical size after which the bonding process predominates and there is only continuous growth of the crystals. However, in

Fundamentals of quantum dot nanocrystals

21

case of magic-sized cluster, the size of the cluster determines the bonding and unbonding of monomers. If the size of the growing cluster is slightly larger than the magic size, then the crystal releases the surface atoms at faster rate than the deposition of monomers. Consequently, if the size of the crystal is smaller, the rate of deposition is higher than that of the release, to reach the magic size. Hence magic sized clusters are extraordinarily stable than the normal crystal and this is accompanied with its high symmetry in their structure. It is suggested that the magic size crystal only undergo nucleation without any growth process and due to its high stability, Ostwald ripening and any other growth process is thus prevented. However, these clusters can undergo quantized aggregation resulting in a larger cluster. In such quantized aggregation, the number of aggregates can be easily obtained by volume of aggregates divided by their volume of magic cluster. Magic size clusters can also be produced from the etching of large crystals with various chemical etchants [56]. Magic-size clusters are of great interest as these clusters exhibit a size close to atomic species with the discrete spectral properties. In a normal growth method, CdSe QDs are usually produced in 2–8 nm range of particle size. However, magic size clusters with the size less than 2 nm with a good stability and discrete optical properties can be obtained which are difficult to obtain in the normal growth method. Though magic sized clusters have been reported for number of conventional binary quantum dots [49, 57–59], there has been no report available yet on the I–III–VI ternary QDs.

1.8 Shelling of QDs As zero-dimensional material with the average particle size falling 94%) can be achieved for CdSe/CdS QDs by a controlled shell growth with several monolayer shell thickness. Such giant shell thickness aids the core QD free of surface defects and isolates it from the environment completely [74]. However, it is the ZnS that has been widely used as a shell material even though CdSe/ZnS has lattice mismatch higher (11%) than CdSe/CdS. This is because of the fact that ZnS shell reduces the toxicity of the core material which is much needed for II– VI QDs having inherent toxicity. The epitaxial growth of the ZnS or CdS Shell on the binary QDs results in no shift or slightly red-shift of the absorption and emission spectra. (Fig. 1.15A and C). The slight red shift is due to the partial leakage of exciton from core to shell. On the other hand, ZnS exhibits only small mismatch of 2% with CuInS2. However, shelling of ZnS on the CuInS2 is not straight forward and results in the interplay between several dynamic process taking place in the solution, at the interface, and the within the core nanocrystal [77]. As a result, the combination of several processes such as surface etching, cation exchange, Zn interdiffusion can occur during the ZnS shelling of CuInS2 QDs (Fig. 1.15D) depending on the reaction conditions, and the surface chemistry of core nanocrystals thereby making epitaxial growth implausible. Consequently, size reduction and/or alloy formation occur in the core rather than actual core/shell formation at the interface. This results in the blue shift in both absorption and emission spectra (Fig. 1.15B and D) due to band gap widening. Fig. 1.16 depicts the schematic changes of the band gap with the addition of ZnS shell over the CuInS2 QDs. It has only been recently that the CuInS2/ZnS core/shell QDs have been synthesized displaying a redshift in the absorption spectra [77]. This was achieved by leaving a residual acetate (from the precursor) on the core CuInS2 QDs which was found to facilitate the epitaxial ZnS shell growth. Both CdS and ZnS shelling on CuInS2 have been found to increase the

(A)

450

550

Wavelength (nm)

650

CuInS2 CuInS2/ZnS

400

(B)

500 600 700

800

CuInS2 CuInS2/ZnS

PL intensity (a.u)

Absorbance (a.u)

Absorbance (a.u) 350

PL Intensity (a.u)

CdSe CdSe/ZnS

450 500 550 600 650 700 750 800

Wavelength (nm)

Wavelength (nm)

Alloy

Core

Core/shell

S Cd Zn

S Cu In Zn

CdSe (C)

Core/shell

CdSe/ZnS (D)

Ca on exchange

CuInS2 Etched Core/shell

Fig. 1.15 (A) Absorption and PL spectra of CdSe nanocrystals before and after ZnS shelling showing no or little red-shift. From [75]. (B) (left panel) Absorption and (right panel) PL spectra of CuInS2 nanocrystals before and after ZnS shelling showing huge blue-shift. Reproduced from [76] with permission from The Royal Society of Chemistry. (C) Schematic representation of structural change of CdSe nanocrystals after ZnS shelling. (D) Schematic representation of structural changes of CuInS2 nanocrystals that can take place during a ZnS shelling. Reproduced from [77]. Copyright (2018) American Chemical Society. https://pubs.acs.org/doi/10.1021/acs.chemmater.8b00477. Further permissions related to the material excerpted should be directed to the ACS.

26

Ternary quantum dots

ZnS

Zn-CuInS2

CuInS2

Conduc on band

Valence band +Zn2+ Fig. 1.16 Schematic of the changes of energy gap with the addition of Zn2+ in CuInS2/ ZnS QDs [78].

quantum yield upto 85% [33] due to elimination of surface traps and occupation of some of the vacant site defect states. In fact, interdiffusion in the CdS shelling of CuInS2 QDs is minimum likely due to the its larger mismatch in both crystal lattice (5%) and ionic radius. However as mentioned earlier, in order to prepare the heavy metal toxic free QDs, the use of ZnS shell is preferred over CdS shelling.

1.9 Colloidal stability QDs are colloidal nanoparticles which in solution are metastable species and naturally attract each other because of high surface to volume ratio and high reactivity. The long-range Van der Waals London (VDWL) force existing between the nanoparticles is the cause of attraction between particles which can lead to aggregation. Therefore, it is essential to provide long range repulsion between the nanoparticles for its stability. QDs colloid syntheses make use of electrostatic, steric or depletion stabilization methods to stabilize the nanoparticles from each other [79, 80]. Capping ligands play an important role in stabilizing the QDs by reducing the surface dangling bond [81]. In addition, they direct nanoparticle size, shape, and interparticle spacing. Moreover, they are used to functionalize the QDs for different applications. In the absence of capping ligands, QDs immediately aggregate and produce bulk structures. The capping ligands are usually mixed with metal cations (Cd, Cu,In) for complexation before the anion part (Se, S) is added in their synthesis. During the reaction, these ligands are in dynamic binding with the QD surface in order to control the growth process. The packing density of ligands is one of the factors

Fundamentals of quantum dot nanocrystals

27

determining the stability. The bulky, branched ligands have low packing density and thereby making QDs less stable while the simple, short ligands that largely covers the surface offers high stability. Typically, the ligands bind with the metal ion part of QDs surface either by co-ordinate bond or covalent bond. The coordinate bonding ligands are weaker, and it can be desorbed during the purification, while covalent bonding ligands exhibit strong binding. QDs can be dispersed in both organic medium and aqueous medium based on the surface chemistry of the ligands. For example, in organic synthesis of QDs, hydrophobic ligands are used where one end of the chain contain functional groups to attach to surface while the other part of the ligand is pointing radially outwards. The functional group of the ligand such as amines, phosphines, phosphine oxides which have lone pair electrons form coordinate bonding while alkylthiols, carboxylic acids form covalent bonding with the QDs surface. In organic medium, the stabilization over VDWL attraction is achieved by mutual repulsion of QDs by stearic stabilization generated by the ligands (Fig. 1.17). For aqueous solubilization of QDs, the water-soluble ligands are used which contains functional groups at both ends, one for anchoring at the surface (e.g., thiols) and the other end for solubilizing (e.g., carboxylic group, amine) in water. In addition, the solubilizing functional part of the ligands offer surface charges (negative

Fig. 1.17 Types of colloidal stabilization. (In depletion stabilization, ligands are nor drawn for the sake of clarity. In electrostatic stabilization, negatively charged QDs is depicted, capping ligands are avoided for clarity.)

28

Ternary quantum dots

for carboxylic group, and positive for amine). Since each particle contains the same type of charges, the VDWL force is counterbalanced by “repulsive Coulomb force” between the charged particles. The ionic groups present in the medium can adsorb to the particle surface to form a charged layer. In order to maintain electroneutrality in the medium, an equal number of counter-ions of opposite charge will surround the colloidal particles and it leads to produce overall charge-neutral double layers. In this electrostatic stabilization, it is this mutual repulsion of these double layers surrounding the particles that offers the stability (Fig. 1.17). The polymers which contains an anchoring ligand can also be used to provide a steric stabilization while unanchored free polymers can offer depletion stabilization that creates the repulsive forces between the particles (Fig. 1.17). A combination of electrostatic and steric stabilization leads to electrosteric stabilization.

1.10 Summary and outlook Quantum dots have emerged as a potential candidate for wide range of applications such as displays, lasers, photovoltaic cells, biomedicine, etc. thanks to their fascinating size tunable optoelectronic properties. This chapter provided insight on the fundamental concepts of quantum dots and the relation of their particle size with light absorption/emission//molar extinction/bandgap properties. Possible electronic transitions in the excited state semiconductor were deeply discussed. In addition, the binary II–VI and ternary I–III–VI bulk semiconductor was compared on the aspects of their crystal structure and optical properties by taking CdSe and CuInS2 as classical representative, respectively. Though the bulk semiconductor exhibits almost similar optoelectronic properties for both binary and ternary QDs, they largely differ from each other at nanoscale levels. The binary QDs exhibits its emission spectra with narrow FWHM and low Stokes’ shift while ternary QDs is obtained with broad emission and large Stoke’ shift. This difference is due to the fact that the three-component system of ternary QDs introduces large number of inherent defects through which the emission occurs whereas in binary QDs, the inherent defects are nonsignificant and their emission occurs at band-edge. It has been shown that the optoelectronic properties of I–III–VI QDs are not only size tunable but composition tunable across the visible and NIR wavelength range. Compared to stoichiometric I–III–VI QDs, the nonstoichiometric I–III–VI QDs have been found to exhibit excellent fluorescence properties due to the presence of defects (group-I vacancies). The further striking difference between the binary and ternary QDs has been demonstrated when the ZnS is grown

29

Fundamentals of quantum dot nanocrystals

as shell material over their core material. The binary QDs after ZnS shell passivation shows redshift in the optical spectra while the ternary QDs shows blue shift due to bandgap widening by many factors. This chapter also discussed about the nucleation and growth mechanism of QDs and their colloidal stability in solutions. Compared to binary QDs, the heavy metal toxic free ternary QDs are more attractive in terms of ecofriendly nature which signals the workability of large-scale industrial production for wide range of application. However, optical properties of ternary QDs have not reached up to the level of binary QDs. Therefore, more studies on controlling the defect chemistry and surface chemistry are required to extract the maximum potential of the ternary QDs.

A Annexure Table: Energy gap at room temperature, exciton Bohr radius of some of the semiconductors in their bulk form, crystal structure, and lattice parameter [5, 6]. Semiconductor

Eg (eV)

Exciton Bohr radius (nm)

Crystal structure

Lattice parameter (Å)

ZnO ZnS ZnSe ZnTe CdS CdSe CdTe HgS HgSe HgTe PbS PbSe PbTe CuInS2 CuGaS2 CuInSe2 CuInTe2 AgInS2 AgGaS2 AgInSe2 AgInTe2

3.35 3.61 2.69 2.39 2.53 1.74 1.50 0.54 0.00 0.00 0.37 0.29 0.26 1.54 2.43 1.09 1.00 1.80 2.73 1.24 0.96

1.8 2.5 4.1 6.7 2.9 5.6 7.3 6.2 17 40 18.0 47.0 150.0 4.1 4.0 10.6 10.5 5.5 3.3 5–6 –

Wurtzite Zinc blende Zinc blende Zinc blende Wurtzite Wurtzite Zinc blende Zinc blende Zinc blende Zinc blende Rocksalt Rocksalt Rocksalt Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite Chalcopyrite

3.236/5.161 5.41 5.668 6.104 4.136/6.714 4.3/7.01 6.482 6.043 6.03 6.45 5.936 6.117 6.462 5.528/11.08 5.359/10.49 5.782/11.62 6.161/12.36 5.828/11.19 5.755/10.28 6.102/11.69 6.406/12.56

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CHAPTER TWO

Properties of ternary quantum dots Abstract Ternary I–III–VI quantum dots have emerged as promising semiconductor materials due to their excellent characteristic that include high absorption coefficient, good photostability, size-dependent band gap, and low toxicity. They have been useful in several fields. This chapter discuss about the various properties of TQDs which include quantum confinement effect, multiple electron generation, optical properties, toxicity, chemical and physical properties. Keywords: Ternary I–III–VI quantum dots, Quantum confinement, Multiple exciton generation, Optical properties, Toxicity

2.1 Introduction Semiconductor I–III–VI quantum dots have gained significant attentions because of their excellent properties such as tunable band gaps, highquantum yield, large coefficient absorption, easy solution processability, and possibility of multiple electron generations through their impact-ionization effect [1]. So far many QDs materials have been developed for various commercial and research purposes. These are semiconductors such as ZnSe, CdTe, CdS, CdSe, CdTe, PbS, and HgS, to mention a few [2–4]. However, most of these semiconductor QDs are based on heavy metals which are highly toxic element [cadmium (Cd) or lead (Pb)] and have been shown to be harmful to the humanity with negative and dangerous environmental issues [5, 6]. In addition, major difficulties arise from the use of these toxic elements as well as their poor chemical stability in aqueous solution [7, 8]. To address these issues, scientists and researchers have focused on the development of non-toxic and eco-friendly ternary quantum dots (TQDs) also called cadmium-free quantum dots. These TQDs have outstanding properties which include broad absorption, good photo-stability, tunable and small band gaps, possibility of multiple excitons generations, good quantum yield and low toxicity [9, 10]. Thus, the chapter describes the outstanding electronic and optical, as well as the physical, chemical, and toxicity properties of Ternary Quantum Dots https://doi.org/10.1016/B978-0-12-818303-8.00004-6

Copyright © 2021 Elsevier Ltd. All rights reserved.

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TQDs. The chapter starts with the physical and theoretical properties of TQDs such as quantum confinement, and multiple exciton generations. This is followed by a thorough review on the optical properties of TQDs such optical absorption, band gap engineering and fluorescence, and their toxicity. The final section is the summary of the chapter with future perspectives on the properties of TQDs.

2.2 Quantum confinement effect Semiconductor ternary quantum dots because of their quantum confinement effects exhibit unique electronic and optical properties [11]. The most interesting characteristic in QDs is their size-dependence optoelectronic properties. The charge carriers in QDs have the same behavior as three-dimensional (3D) quantum wells. When a photon excites a semiconductor with an energy that is higher or equal to its band gap one or more electrons are generated and behave as particles confined in an infinite potential well [12]. The band gap of a QDs material always depends on its size with a discrete density of states. Fig. 2.1 shows the blue-shifted wavelength of QDs with reduced size and the concentration of the oscillator strength into just a few of transitions [13, 14]. The particle sizes of a semiconductor QDs are comparable to the excitons (electon-holes) Bohr radius of their corresponding materials. For a bulk semiconductor, radius (AB) is the Quantum Dot (QDs)

Energy

Conduction Band Band Gap Valence Band

Decreasing QDs Size

Fig. 2.1 Schematic diagram of quantum Confinement effect of QDs.

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Properties of ternary quantum dots

average distance between the excitons. Thus, the average particle size of QDs can be calculated using the following equation.
 h2 ε 1 1 AB ¼ 2 (2.1) + e me mh where e represent the charge of an electron, ħ the reduced Planck constant, ε the dielectric constant of the semiconductor QDs, me and mh are the effective mass of the electron and the effective mass of the hole, respectively. In QDs the excitons are material dependent due to their dielectric constant and effective masses. Semiconductor QDs have different exciton Bohr radius. For example, the exciton Bohr radius of ZnO, CuInS2, PbS, and PbSe are 2.3 nm, 4.1 nm, 18 nm, and 46 nm, respectively [15–18]. When the motions of the charge carriers in the semiconductor QDs are confined in all direction, within the particle, with dimensions less than the bulk, their radius approaches the exciton Bohr radius. Due to their quantum confinement effect, QDs show significant and better optoelectronic properties compared to their bulk counterparts [19–23]. QDs under strong quantum confinement exhibit discrete level of energy, which is similar to the discrete level of energy of an atom and due to that, QDs are defined as artificial atoms [24, 25]. In quantum mechanics, the wavefunction describe the behavior of a particle. Thus, the following equations describe the wavefunctions and energies of a three-dimensional box. sffiffiffiffiffiffiffiffiffiffiffiffiffiffi


 ny πy 8 nx πx nz πz ψ nx , ny , nz ¼ sin sin sin (2.2) Lx Ly Lz Lx Lz Ly "


 # ny 2 h2 π 2 nx 2 nz 2 Enx , ny , nz ¼ (2.3) + + 2m Lx Ly Lz where the three-dimensional wavevector is. knx , ny , nz ¼ knx x + kny y + knz z ¼

ny π nx π nz π x+ y+ Lx Lz Ly

(2.4)

where nx, ny, nz ¼ 1, 2 … the quantum confinement numbers, Lx, Ly and Lz are the confining dimensions [26]. The radius of the spherical particle is calculated by the following equation.

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Ternary quantum dots

E e, h ∝

1 R2

(2.5)

The band gap can also be calculated by using the following classical LaplaceYoung equation.
 1 1 2γ 1 when R1 ¼ R2 ¼ R + (2.6) Δp ¼ 2γH ¼ γ ¼ ∝ R1 R2 R R where Δp represent the difference in pressure between the fluid interface, γ is the tension of the surface, H is curvature of the mean, and R1 and R2 are the two principal radii of curvature.

2.3 Multiple exciton generation (MEG) Sometimes semiconductor QDs because of their impact’s ionization (II) effect can exhibit MEG phenomena [26]. MEG can be described as a generation of one or more additional excitons by an electron or hole having kinetic energy higher or equal to the semiconductor band gap energy [27]. The kinetic energy is created when the QDs is illuminated by a photon with energy higher than the band gap of the semiconductor. This phenomenon have been observed with different materials which include InAs, InSb, CdSe Ge, PbS, PbSe, and PbTe [28–31]. Formation of MEG can improve the conversion of quantum dots-based optoelectronic applications which include solar cells and photocatalysis. However, II effect cannot enhance the quantum yields (QY) in solar cells materials. This is due to the fact that, the efficiency of II. reach important values only if the photon energies are located in the ultraviolet region. The rate of I.I. must be competitive to the rate of energy relaxation by phonon emission via the scattering of electronphonon. It has been demonstrated that electron with kinetic energy higher than the band gap energy (Eg) induce the semiconductor QDs with competitive rate of I.I. compared to the phonon scattering rates [31]. For example, Si has I.I. efficiency of 5% with total quantum yield of 105% at hν  4 eV (3.6 Eg), and 25% at hν  4.8 eV (4.4 Eg). Semiconductor Si and GaAs based solar cells show low power conversion efficiency which is due to their large blue-shift of the threshold photon energy for I.I [32–34]. Nevertheless, The density of electron generation through the interactions of electron-phonon in QDs can be highly reduced by considering the discrete character of the electron-hole spectra, and because of the

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Properties of ternary quantum dots

e–

One Photon yield two e–h+ pairs e–

hν

e–

Egap

Multiple exciton generation (MEG) O

h+

O

h+

Quantum dot

Fig. 2.2 The mechanism of MEG in QDs [35].

increased Coulomb interaction between the excitons and the carriers confinement, Auger processes rate which include the inverse Auger process of electron-hole multiplication, is greatly improved [35]. Fig. 2.2 show the mechanism of MEG in QDs.

2.4 Optical properties of ternary quantum dots Ternary I–III–VI quantum dots show outstanding optical properties. Thus, understanding the optical and unique characteristic of ternary I– III–VI quantum dots are imperative for their application in different fields such solar cells, imaging, sensing, photocatalyts, drug delivery, phototherapy, light emitting diodes, etc. The size-dependent optical and emission properties are the most important characteristics of ternary I–III–VI quantum dots (Fig. 2.3) [36]. Ternary I–III–VI quantum dots show small direct band gap in the visible region. For example, the band gap of CuInSe, CuInS2, AgInS2, AgInSe QDs is 1.05 eV, 1.5 eV, 1.87 eV, and 1.2 eV, respectively [37–41]. CuInS2 QDs exhibit high optical absorption coefficient and good photostability [42]. The optical absorption properties of ternary I–III–VI quantum dots change with the chemical composition of the QDs. Thus, it has been reported that the molar ratios of precursors could control the nonstoichiometry of ternary I–III–VI QDs. Moreover,

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Ternary quantum dots

Fig. 2.3 (A) Size dependent fluorescence spectra of QDs and (B) different relative particle sizes [36]. Copied from J.J.H. Pijpers, E. Hendry, M.T.W. Milder, R. Fanciulli, J. Savolainen, J.L. Herek, D. Vanmaekelbergh, S. Ruhman, D. Mocatta, D. Oron, A. Aharoni, U. Banin, M. Bonn, Carrier multiplication and its reduction by photodoping in colloidal InAs quantum dots, J. Phys. Chem. C 11 (2007) 4146–4152 with copyright permission from the Royal Society of Chemistry.

passivation of the core ternary I–III–VI QDs by a wide band gap semiconductor shell such as ZnS improves their emission properties [43, 44]. In addition, aspect such as QD shape, exactions interaction, crystal structure, defects and impurities can affect the absorption and emission properties of ternary I– III–VI QDs [45–47]. Dopants or defects in ternary I–III–VI QDs can lead to a near infrared (NIR) emission or can induce magnetic features for magnetic resonance imaging (MRI) application in bioimaging field [47, 48]. The tunable optical properties of I–III–VI QDs are crucial for their use in different application such as imaging, LED, solar cells, sensing and so on. For example, Gugula et al., through surface modification prepared CuInS2/ZnS QDs with good photoluminescence QY (60%) [49]. Cai et al. reported tunable optical properties of CuInS2 QDs for high efficient QDs solar cells (5.71%). In this work, the tunable optical properties were induced by defects via intentional introduction of Cu deficiency [50]. Regulacio et al. obtained long PL lifetime (170 ns) and a high QY (20%) of AgInS2/ZnS core/shell QDs [51].

Properties of ternary quantum dots

41

2.5 Toxicity and biocompatibility of ternary quantum dots Cd-and Pb-based quantum dots have gained interesting attention owing to their unique optoelectronic properties. However, the use of hazards chemical such Cd and Pb have limited their applications in various field considering the hazard to human health and environment. Over this past decade, ternary quantum dost also called Cd-free quantum dots have been intensively studied due to their less toxicity as well as good optoelectronic properties. Ternary QDs are less toxic and hence they have been used in many biological applications such as drug delivery, bio imaging and therapy [51]. For example, Li et al. used CuSnI2/ZnS QDs in in vivo imaging as fluorescent labels [52]. Guo et al. tested the cytotoxicity of Zn doped CuSnI2/ZnS tests with normal 3T3 cells and found that the as-prepared sample were less toxic and biocompatible [53]. Yong et al. investigated the in vitro cytotoxicity of the CuInS2/ZnS QDs with pancreatic cancer cells and found that the viability of the cells was higher than 80%, suggesting a minimal cytotoxicity [54]. It has been reported that cytotoxicity of ternary Cd-free QDs is proportional to their concentration. For example, Yang et al. investigated the cytoxicity of Zn-doped AgInS2 with NIH/3 T3 cells and found that the viability of the cells at 2 mg mL1 remained at 80% after 24 h incubation [55].

2.6 Conclusions This chapter focused on the unique properties of Cd-free QDs such, as possibility of multiple exciton generation, large absorption, good photostability, tunable and band gaps, good quantum yield and low toxicity. Ternary I–III–VI QDs because of their quantum confinement effect which can be understood as the decrease in size with increasing bang gap, exhibit excellent optoelectronic properties. Their multiple exciton generation (MEG) phenomena induced from impacts ionization represent the key factor for the success of Ternary I–III–VI QDs based optoelectronic applications which include LED, solar cells, and photocatalysis. Their size-dependent optical and emission characteristic have been considered as the most valuable properties of ternary I-III–VI QDs. These properties also depend on the shape, electron-hole interaction, crystal structure, defect and impurities of the ternary III–VI QDs. Compared to conventional Cd-and Pb-based

42

Ternary quantum dots

QDs, ternary III–VI QDs are less toxic and have been considered as potential material for many biological applications. The toxicity of ternary III–VI QDs change with the concentration of the QDs material. However, more studies in the cytotoxicity of ternary III–VI QDs are crucial for wide and efficient applications in biomedical fields and environmental issues.

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CHAPTER THREE

Synthesis of ternary I–III–VI quantum dots Abstract Quantum dots (QDs) have received much attention in the field of nanotechnology because of their unique properties especially size-dependent optical properties because of their quantum confinement effect. This has led to the generation of various synthetic approaches to prepare a diverse range of QDs. Obviously, most studies are based on conventional chalcogenide binary QDs (e.g., CdSe, PbS) because of their wide tunability across the visible and near-infrared range. Nevertheless, the intrinsic toxicity of these chalcogenide QDs has raised serious health and environmental concerns. Hence the research deviate to nontoxic QDs. Luminescent chalcopyrite I–III–VI2 QDs (CuInS2, AgInS2, CuInSe2, and AgInSe2) are emerging as a safer alternative to chalcogenide QDs because of their advantage of nontoxicity and similar excellent luminescence properties compared to conventional QDs. The synthesis of chalcopyrite ternary QDs is not much different from chalcogenide binary QDs; however, the introduction of the third element plays a crucial role. This chapter discusses the different types of synthesis of I–III–VI2 QDs where conventional colloidal syntheses are given focus with CuInS2 based QDs as the major example. Keyword: Quantum dots, Ternary, Organic synthesis, Aqueous synthesis, Alloy, Core/shell

3.1 Nonconventional methods There are different approaches developed for the fabrication of semiconductor QDs. In the nonconventional methods, a lithographic process [1, 2] was used to create a two-dimensional structure that could then be etched down to isolate a QD. The major disadvantages of this method are the fabrication cost and the difficulty in positioning individual dots. QDs can also be grown epitaxially on certain substrates [3, 4]. In a familiar Stranski-Krastanov epitaxial growth, a semiconductor is subjected to grow on the surface of another semiconductor with a moderate difference in lattice constant. When the growth of the crystallized layer exceeds critical thickness, breakdown of growth occurs because of the strain which results in the nanoparticles with regular size and shape. The slow growth rate and expensive operating process are the drawbacks of this technique. Ball milling is another nonconventional approach [5, 6] where the bulk powder mixtures Ternary Quantum Dots https://doi.org/10.1016/B978-0-12-818303-8.00009-5

Copyright © 2021 Elsevier Ltd. All rights reserved.

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Ternary quantum dots

are ground to extremely fine powders because of the high energy collisions from the balls. This process of synthesis is simple, ecologically safe as no reagents nor solvents is involved, however small and uniformly sized particles cannot be achieved by this method. The above physical approaches for the fabrication of QDs are less popular and mostly, unary and binary QDs are reported by these methods [7–9]. Recently, Dutkova´ et al. [10] prepared CuInS2-ZnS nanocrystals through mechanochemical process by dry highenergy milling. In a typical reaction, CuInS2 was initially prepared by milling of Cu, In, and S in an inert atmosphere for 60 min. This was followed by co-milling of zinc acetate and sodium sulfide for 30 min to form CuInS2ZnS composites.

3.2 Conventional methods Colloidal synthesis is a conventional way for the preparation of QDs that employs a “bottom-up” approach where the precursor materials serve as a seed for nucleation and growth of crystals in a suitable solvent. Various types of QDs such as core/shell QDs, alloyed QDs, QD composites with a controlled shape and size can be synthesized by bottom-up approach, which are hard to obtain by the top-down and physical methods. QDs obtained by colloidal route have high monodispersity and photoluminescence quantum efficiency. This approach was introduced by Brus [11], an American scientist at Bell laboratories, New Jersey and carried over by his post docs notably Moungi Bawendi [12] and Paul Alivisatos [13]. Due to its mild synthetic conditions, this method was widespread among the researchers for their applications in the biomedical, photovoltaic, analytical fields, and so on. Colloidal synthesis can be broadly classified as organic phase and aqueous phase syntheses which are discussed in the following sections. Table 3.1 provides some of the reports on the conventional methods of I–III–VI based QDs.

3.3 Organic phase synthesis Organic synthesis generally involves the pyrolysis of precursors in the presence of hydrophobic solvents and capping agents/ligands at high temperatures under inert atmosphere. The synthesis can also be called organometallic synthesis as it involves organic reagents and the metals. The ligands contain hydrophobic long-chain aliphatic moiety with the functional group that can bind to the crystal surface. The solvents employed are aliphatic with

Table 3.1 Some of the reported conventional synthetic methods for I–III–VI based QDs. MaxReaction Synthesis tempa (°C) timea Material Precursors method

PL rangea (nm)

Max-QYa (%) References

CISe CIS/ZnS

CuI, InCl3, (Me3Si)2Se, TOP, OAm CuNO3, In(Ac)3, Zn(Ac)2, S, DDT, OAm, SA, OA, MA, ODE CISe/ZnS CuI, In(Ac)3, Se, ZnSt2, DDT, TBP, ODE CISe/ZnS CuI, InI3, (TMS)2Se, (TMS)2S, ZnEt2, DDT, TOPO, HDA CIS/ZnS CuAc, InSt3, S, ZnSt2, DDT, OA, OAm, ODE CISeS/ZnS CuAc, In(Ac)3, Se, Zn(EtX)2, Zn(OA)2, DDT, TBP,ODE CISe CuCl, InCl3, OAm, Se, DPP ZAISe Zn(Ac)2, AgNO3, In(Ac)3, Se, DDT, OA, OAm, ODE CIS/ZnS Cu(NO3)2, In(Ac)3, S, Zn(Ac)2, OA, SA, DDT, MA, OAm CISe/ZnSe CuCl, InCl3, TOPSe, ZnEt2, LiN(SiMe3)2 AISe/ZnS AgNO3, In(Ac)3, Se, S, ZnSt2, DDT, OAm, OA, ODE AZIS AgNO3, InCl3, ZnSt2, DDT, OA, ODE CISe/ZnS CuI, In(Ac)3, Zn(Ac)2 Se, DDT, TOP, OA, OAm, ODE CIS/ZnS CuAc, InSt3, S, DDT, Zn(Ac)2, OA, ODE

Hot injection Hot injection

280 200

12 min >45 min

640–975 695

25 35

[14] [15]

Hot injection Hot injection

200 270


120 min 600–850
120 min 700–900

26 60

[16] [17]

Hot injection

210

>30 min

500–950

30

[18]

Hot injection

220

90 min

610–750

40

[19]

Hot injection Hot injection

180 175

60 min 30 min

735–800 660–800

– 50

[20] [21]

Hot injection

200

>55 min

650–820

30

[22]

Hot injection Hot injection

320 175

18 min 120 min

700–1200 700–820

60 40

[23] [24]

Hot injection Hot injection

170 200

>30 min >40 min

632–739 709

62 60

[25] [26]

Hot injection

230

5h

642–708

50

[27] Continued

Table 3.1 Some of the reported conventional synthetic methods for I–III–VI based QDs—cont’d Maxtemp Reaction Synthesis (°C) time Material Precursors method

PL range (nm)

Max-QYa (%) References

CIS/ZnS CIS/ZnS CIS/ZnS

CuI, In(Ac)3, Zn(Ac)2, DDT, OAm, ODE CuI, In(Ac)3, S, ZnSt2, DDT, OAm, ODE CuI, InI3, Zn(dedtc)2, S, DDT, OA, TOP, ODE CIS CuAc, In(Ac)3, DDT, ODE CISe CuI, InCl3, Se, TOP, TOOP, ODE CIS/ZnS CuI, In(Ac)3, ZnSt2, Zn(EtX)2 DDT, ODE CISe/ZnS CuCl, InCl3, SeU, Zn(EtX)2, Zn(OA)2, DDT, TOP, DOA, ODE CIS CuI, In(Ac)3, DDT, OA, ODE CISe/ZnSe CuI, InCl3, Se, Zn(Ac)2, TOP, TOOP, HDA, OAm, ODE CIS/ZnS CuI, In(Ac)3, Cd(OA)2, ZnSt2, S, DDT, TOP CIS/CdS CIS/ZnS CuI, In(Ac)3, Zn(Ac)2, DDT,MA, ODE CIS/ZnS CuI, In(Ac)3, ZnSt2, DDT, ODE CIS/ZnS CuI, In(Ac)3, Zn(Ac)2, DDT, OA, OAm, ODE ZAIS AgNO3, In(Ac)3, S, DDT, TOP,OA, ZnSt2, ODE AIS/ZnS AgNO3, InSt3, Zn(EtX)2, S, DDT, OA, ODE AIS/ZnS AgNO3, In(Ac)3, ZnSt3, S, DDT,TOP, OA, ODE CIS/ZnS CuI, In(Ac)3, Zn(Ac)2, DDT, OA, OAm, ODE AIS-ZnS AgNO3, In(Ac)3, ZnSt2, DDT, OA, ODE

Hot injection Hot injection Noninjection

220 240 240

90 min 8h 20 min

682–738 530–642 688–775

65 85 15

[28] [29] [30]

Noninjection Noninjection Noninjection

240 320 270

2.5 h 130 s 290 min

600–750 838–918 650–830

2 5 60

[31] [32] [33]

Noninjection

250

>1 h

700–1030

50

[34]

Noninjection Noninjection

200 320

120 min >15 min

700–900 808–929

– 16

[35] [36]

Noninjection

230

80 min

630–780

86

[37]

Noninjection Noninjection Noninjection

230 230 230

270 min 570–760 >150 min 638–743 >30 min 542–760

65 – 65

[38] [39] [40]

Noninjection

210

>30 min

520–680

41

[41]

Noninjection

180

360 min

520–570

60

[42]

Noninjection

210

30 min

602–657

32

[43]

Noninjection

220

>150 min 614

75

[44]

Noninjection

210

2h

38

[45]

510–670

ZCIS/ZnS CuI, In(Ac)3, ZnSt3, S, DDT, OA, OAm, ODE CIS CuI, In(Ac)3, DDT ZAIS AgNO3, In(Ac)3, S, ZnSt2, DDT, TOP, OA,, OAm, ODE ZAISe AgAc, In(Ac)3, Zn(Ac)2, SeU, OAm CIS/ZnS CuI, In(Ac)3, Zn(Ac)2, DDT, OAm, ODE CIS/ZnS Cu(OA)2, In(OA)2, Zn(OA)2, DDT, ODE CIS/ZnS CuI, In(Ac)3, Zn(St)2, DDT, OA, ODE CIS/ZnS CuI, In(Ac)3, Zn(Ac)2 DDT, ODE, CGSe/ CuI, Ga(Ac)3, ZnI2, DDT, OAm, ODE ZnSe ACIS/ZnS: CuI, In(Ac)3, AgAc, Zn(Ac)2, Al(IPA)3, S, DDT, OAm, OA, ODE Al CIS/ZnS CuI, In(Ac)3, ZnSt2, DDT, ODE CIS CuCl, InCl3, S, OAm CIS/ZnS CuI, In(Ac)3, Zn(EtX)2, ZnSt2, DDT, ODE CIS/ZnS CuI, In(Ac)3, ZnSt2, DDT CIS/ZnS Cu(Ac)2, In(Ac)3, Zn(Ac)2, TAA, ODA CIS/ZnS CuI, In(Ac)3, Zn(Ac)2, DDT, OA, ODE CIS (PPh3)2CuIn(SEt)4, HT, TOPO, DOP ZCIS ZAIS CIS ZCIS

Noninjection

290

130 min

600–815

50

[46]

Noninjection Noninjection

210 130

30 min 20 min

550–840 539–644

18 35

[47] [48]

Noninjection Noninjection Noninjection

250 220 250

10 min 170 min 14 h

650–800 667 559–670

– 52 80

[49] [50] [51]

Noninjection Noninjection Noninjection

220 250 240

>60 min >7 h >1 h

650–740 681–850 500–630

80 38 78

[52] [53] [54]

Noninjection

250


7 h

534–868

91

[55]

Noninjection Solvothermal Solvothermal

230 170 200

>48 h 1h 17 h

560–600 525–750 545–614

70 1.2 65

[56] [57] [58]

200 200 180 200


20 h 14 h 6h 5h

558–659 620–650 651–775 nm 625–663

81 – 85 4.4

[59] [60] [61] [62]

280

300 s

630

5

[63]

180

>30 min


520–780

66

[64]

180

1h

706–820

2

[65]

180

20 min

680–760

5

[66]

Solvothermal Solvothermal Solvothermal Thermal decomposition CuI, InCl3, Zn(dedtc)2,TOP, OAm, ODE Thermal decomposition Thermal (AgIn)xZn2(1-x) (dedtc)4, OAm decomposition Thermal Cu(dedtc)2, In(dedtc)3, OAm decomposition Cu(dedtc)2, In(dedtc)3, Zn(dedtc)2, OAm Thermal decomposition

Continued

Table 3.1 Some of the reported conventional synthetic methods for I–III–VI based QDs—cont’d MaxSynthesis temp Reaction Material Precursors method (°C) time

PL range (nm)

Max-QYa (%) References

CIS ZAIS/ZnS CIS CIZS CIS CIS CIS/ZnS

CuCl2, InCl3, TU, MPA AgAc, Zn(Ac)2, In(Ac)3, TAA, Cys Cu(NO3)2, In(NO3)3, Na2S, GSH CuCl2, InCl3, ZnCl2, TU, MPA CuCl2, InCl3, Cys, TU CuCl2, InCl3, MPA, TU CuI, In(Ac)3, Zn(Ac)2, S, DPP, OAm, ODE CIZS CuI, In(Ac)3, ZnSt2, DDT CIS/ZnS CuCl2, InCl3, ZnCl2, Na2S, TU, sodium citrate, TGA CIS/ZnS CuCl2, InCl3, Zn(Ac)2, Na2S, TU, sodium citrate, GSH AIS/ZnS AgNO3, In(NO3)3, Zn(NO3)2, Na2S, sodium citrate, PAA, TGA, GSH AZISe AgNO3, In(NO3)3, Zn(NO3)2 NaHSe, MPA ZCIS CuCl2, InCl3, Zn(Ac)2, Na2S, MPA AISe-ZnSe AgNO3, In(Ac)3, Zn(Ac)2, NaHSe, MPA AIS AgNO3, InCl3, Na2S, sodium citrate, TGA CIS/ZnS CuCl2, InCl3, Zn(Ac)2, Na2S, TU, sodium citrate, GSH AIS/ZnS AgNO3, In(NO3)3, Zn(Ac)2, TU, sodium citrate, GSH

Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Hydrothermal Microwave

150 110 100 180 150 175 255

25 h 270 min 5h 12 h 23 h 20 h >85 min

660 560–650 654–675 450–664
660 450–680 600–880

3.3 26 13.4 11.3 3.4 10 65

[67] [68] [69] [70] [71] [72] [73]

Microwave Microwave

230 95

5 min
4 h

591–674 533–666

77 38

[74] [75]

Reflux

95

175 min

543–625

38

[76]

Reflux

100


140 min 525–640

20

[77]

Reflux

100

504–585

15

[78]

Reflux Reflux Reflux

100 90 95

>45 min 532–700 >210 min 625–940 45 min 680

4.7 31 10.3

[79] [80] [81]

Reflux

95

2h

708–730

12.3

[82]

Reflux

95

2h

623–665

49.5

[83]

ZIS CIS/ZnS CIS CIS AIS/ZnS CISe/ZnS

Zn(Ac)2, InCl3, Na2S, gelatin, TGA Reflux CuCl2, InCl3, Zn(Ac)2, Na2S, TU, sodium Reflux citrate, GSH Cation Cu2-XS, In(Ac)3, TOP, DDT, ODE exchange Cation Cu2-XS, In(NO3)2, TOP exchange AgNO3, In(OH)3, ZnCl2 (NH4)2S, TU, Pressure gelatin, TGA cooker CuCl2, InCl3, NaHSe, ZnO, ammonium Pressure thioglycolate, gelatin TGA cooker, 180 kPa

95 95

120 min 105 min

509–512 740–695

1.12 28

[84] [85]

125

200 min

954–984

1

[86]

–

Days

665–870

10

[87]

>80

2h

571–613

57

[88]

120

2h

612–686

23.3

[89]

(Me3Si)2Se, bis(trimethylsilyl)selenide; OAm, oleylamine; TOP, tri-n-octylphosphine; Ac, acetate; OA, oleic acid; SA, stearic acid; ODE, octadecene; DDT, dodecanethiol; MA, myristic acid; TBP, tri-n-butylphosphine; St, Stearate; TMS, trimethylsilyl; TOPO, tri-n-octylphosphine oxide; HDA, hexadecyl amine; Et, ethyl; EtX, ethyl xanthate; DPP, diphenyl phosphine; dedtc, diethyldithiocarbamate; TOOP, trioctylphosphite; SeU, Selenourea; DOA, Di-n-octylamine; ODA, octadecylamine; TAA, thioacetamide; Ph, phenyl; HT, hexanethiol; DOP, dioctylphthalate; TU, thiourea; MPA, 3-mercaptopropionic acid; Cys, L-cysteine; GSH, L-glutathione; PAA, polyacrylic acid; DDA, dodecylamine. a ¼maximum temperature, time, PL range and maximum photoluminescence quantum yield extracted from the references are given. For accurate informations, please refer to the relevant articles.

54

Ternary quantum dots

either coordinating or noncoordinating nature. The presence of inert atmosphere is the standard procedure for organometallic synthesis because of the high reactivity of precursors with air at elevated temperatures. The organic synthesis yields high-quality crystals because of annealing of the crystals at high temperature. As a result, good optical properties especially the near unity PLQY can be obtained using this method. Synthesis of QDs in organic phase can be broadly classified into the following:

3.3.1 Hot injection method This method usually involves the rapid injection of a “cold” solution of anionic precursors into hot reaction mixture containing the cationic precursors in a suitable organic solvent (Fig. 3.1). This method became popular after the successful synthesis of CdSe QDs by Murray et al. in 1993 [12]. This injection leads to the instant formation of the nuclei of nanocrystals accompanied with the decrease in the temperature of reaction solution and as a temperature decreases, formation of additional nuclei is prevented. Later, heating will be restored to allow for the growth of nanocrystals.

Fig. 3.1 Schematic diagram of hot-injection method for the synthesis of I–III–VI QDs. Reprinted from N. Tsolekile, S. Parani, M.C. Matoetoe, S.P. Songca, O.S. Oluwafemi, Evolution of ternary I–III–VI QDs: synthesis, characterization and application. Nano-Struct. NanoObjects 12 (2017) 46–56 with the permission from Elsevier.

Synthesis of ternary I–III–VI quantum dots

55

However, unlike II–VI QDs, I–III–VI QDs system involves two differently charged cations (M+, M3+) of different reactivity and hence the synthesis requires special care. To balance the reactivity and prevent the formation of impurity phases (from I–VI or III–VI), hard and soft acid and bases (HSAB) concept needs to be considered. According to HSAB concept, hard acid-hard base and soft acid-soft base configuration form stable bond than the mixed hard-soft configuration. For example, in CuInS2 ternary system, Cu+ is a soft acid and In3+ is a hard acid while S2 is a soft base. As a result, S2 complexes strongly with Cu+ than In3+ and forms Cu-S phases. However, the careful selection of capping agent(s) that can coordinate to these metal ions aids the formation of hard-hard, soft-soft acid base configurations. Long chain alkane thiol such as dodecane thiol (DDT) is usually employed as a capping agent for the I–III–VI QDs in organic phase synthesis. DDT is a soft base which preferentially complexes with M+ (Cu+, Ag+) soft acid and suppresses its high reactivity toward another soft base, hence the reactivity of M+/M3+ toward VI soft base anion (S2/Se2/ Te2) is balanced. In order to achieve this, an excess amount (>fivefold) of thiol is required. Thus, all kinds of synthesis of I–III–VI nanocrystals including hot-injection mediated synthesis has been designed by considering HSAB theory. Liu et al. [22] reported the synthesis of DDT stabilized CuInS2 and AgInS2 QDs using hot-injection method. In a typical reaction, the cationic precursors indium acetate, copper nitrate (or silver nitrate for AgInS2 QDs) and oleic acid, stearic acid were mixed in octadecene and heated to 90°C in an argon flow followed by injection of excess amount of DDT. Long chain fatty acids such as oleic acid, stearic acid were identified as to decrease the reaction between In3+ and thiol (RSH). The solution mixture was then heated to 170°C for 10 min under argon flow and subsequently 3 mL oleylamine-sulfur solution as sulfur source was injected into the solution. The reaction mixture was held at 170°C for another 10 min to obtain red and NIR emitting QDs. Another successful way to balance the reactivities of I/III cations is the use of amide superbase [23] which is typically lithium bis(trimethylsilyl)amide, LiN(SiMe3)2. This superbase injected in excess along with the VI precursor to the I/III salt solution at high temperature forms I and III-metal-amide intermediate complex and accelerate the nucleation step. This amide mediated approach is reported to produce the stoichiometric composition of I–III–VI QDs linearly proportional to the I, III, VI precursors ratio [90].

56

Ternary quantum dots

3.3.2 Noninjection method Though hot injection yields QD nanocrystals with good optical and structural properties, it is unrealistic for the large-scale synthesis. This paves way for noninjection method. In the noninjection method also called as "heatup" method, both cationic and anionic precursors together with the capping agents are mixed together in the solvent at certain temperature for nucleation and heated up for the growth of nanocrystals [91] (Fig. 3.2). This method can proceed at relatively low temperature than the hot-injection method and it is energy efficient. Unlike hot-injection method, nucleation and growth of nanocrystals is not distinct in noninjection method which might lead to aggregation of particles. However, proper selection of precursors based on HSAB concept would prevent the aggregation. Zhong et al. [31] employed heating up method to produce gram scale synthesis of CuInS2 nanocrystals. In a typical synthesis, they mixed copper acetate, indium acetate, DDT in octadecene to form an intermediate complex which was decomposed at 240°C into CuInS2 nanocrystals as the reaction progresses. DDT here acts as sulfur source as well as capping agent. They observed that when the size of the CuInS2 nanocrystals exceeded 3 nm, the particles aggregated which could be attributed to the fast reaction between the metal ions and the sulfur decomposed from DDT. In another reaction, they

Fig. 3.2 Schematic representation of synthesis of QDs using heating up method. Adapted from Z. Zhuang, X. Lu, Q. Peng, Y. Li. A facile “dispersion–decomposition” route to metal sulfide nanocrystals. Chem. Eur. J. 17(37) (2011) 10445–10452 with the permission from John-Wiley and Sons.

Synthesis of ternary I–III–VI quantum dots

57

replaced copper acetate (CuAc2) with copper iodide to mitigate this issue [35]. According to the HSAB, Cu+ is a soft acid and Ac is a hard base, whereas I and thiols (R-SH) are soft bases, and I is more softer than R-SH. Therefore, binding between Cu+ and I is stronger than Cu+ and R-SH, and thereby reducing the reaction rate. They have obtained 2.3 g of CuInS2 nanoparticles in a single batch for which the product yield is 80%–90%. Numerous synthesis on I–III–VI QDs using noninjection method has been reported indicating the success of noninjection method.

3.3.3 Solvothermal method Solvothermal method is an alternative effective method for the preparation of semiconductor nanocrystals. This method uses the autoclave as a reaction vessel wherein all the precursors, capping agent, and solvent are added followed by heating at the desired temperature. This method doesn’t require an inert atmosphere nor refluxing set up. Furthermore, it is easy to operate compared to the other methods and it is suitable for large-scale production. However, the solvothermal method often produces the nanoparticles with broad size distribution. Chuang et al. [59] reported the solvothermal synthesis of CuInS2 QDs in an autoclave at 180°C for 5 h 30 min, using copper iodide, indium acetate, and DDT as solvent and sulfur source. In another report, Jindal et al. [60] synthesized CuInS2/ZnS QDs using copper acetate, indium acetate, as cationic precursors and thioacetamide as sulfur source, octadecyl amine as capping agent and ethanol as solvent in an autoclave at 160°C for 6 h. At high temperature, thioacetamide decomposes to release sulfide ion for the formation of CuInS2 nanocrystals. Zinc acetate dissolved in ethanol was later added and the reaction continued further at 200°C for 8 h to prepare CuInS2/ZnS QDs (Fig. 3.3).

3.3.4 Thermal decomposition method This method involves the decomposition of the single molecule precursors to obtain nanocrystals by controlling the temperature of decomposition. In single molecule precursors, the source of both the metal and nonmetal component of the target ternary compound is contained within a single precursor species such as metal complex of thiosemicarbazides, dithioocarbamates, alkylxanthates [92–95]. Though the preparation of these compounds is an additional step, having a preformed metal chalcogenide bond (e.g., CudS, IndS), single source precursors mitigate the problem of reactivity difference between M+ and M3+ ions. Furthermore, it offers the benefit of ease of use,

58

Ternary quantum dots

Fig. 3.3 Schematic representation of QDs synthesis using autoclave. Reprinted from S. Jindal, S.M. Giripunje, S.B. Kondawar, P. Koinkar. Green synthesis of CuInS2/ZnS core-shell quantum dots by facile solvothermal route with enhanced optical properties. J. Phys. Chem. Solids 114 (2018) 163–172 with the permission from Elsevier.

provide a convenient reactive intermediate for growth and produce high quality monodispersed products under relatively mild reaction conditions. Sun et al. [96] prepared the organometallic single source precursor (Ph3P)2Cu-(μ-SEt)2In(SEt)2 for the preparation of CuInS nanocrystals through the series of reaction of triphenyl phosphine, copper chloride, indium chloride with sodium ethanethiolate (NaSEt) in benzyl acetate solvent (Fig. 3.4). Decomposition of this precursor in the presence of thiol at 150–190°C for 3 h under nitrogen atmosphere produced 1.8–5.2 nm CuInS2 nanocrystals in a gram scale. Although, this method has been successful for the preparation of I–III–S ternary system, it is difficult to prepare I–III–Se and I–III–Te nanocrystals because of the challenges in the preparation of single source precursors, containing MdSe and MdTe bonds.

3.3.5 Cation exchange method Cation exchange method is a method of producing the nanocrystals where cationic part of the parent crystal (precursor) is replaced partly or fully by

Fig. 3.4 A general illustration for the formation of CuInS2 nanoparticles via single source precursor method. Reproduced from C. Sun, Z. Cevher, J. Zhang, B. Gao, K. Shum, Y. Ren. One-pot synthesis and characterization of chalcopyrite CuInS2 nanoparticles. J. Mater. Chem. A 2 (27) (2014) 10629–10633 with permission from The Royal Society of Chemistry.

60

Ternary quantum dots

another cation in solution. The advantage of this method is the formation of the crystal with the new composition without changing the size, morphology, or crystal structure. The cation exchange process is highly directional, and it leaves the anionic part unaffected. The driving force for the exchange is the solvation energy of the releasing cation in the solution. Higher solvation energy results in faster exchange. Depending on the cations involved and the nature of the solution, this process requires a temperature much lower than the temperature required to form crystalline particles, thereby making this process energy efficient. However, the disadvantages of this process are that it requires a monodispersed, high purity parent crystal and it may take longer reaction time. Using this method, metal sulfides, selenides and tellurides were prepared keeping the morphologies and crystal structures of the parent crystals. Stam et al. [87] reported the preparation of luminescent CuInS2 QDs by partial cation exchange in Cu2 xS nanocrystals. In a typical reaction, methanolic solution of indium nitrate was added to the solution of Cu2 xS in toluene in an equimolar ratio followed by the addition of trioctylphosphine as copper extracting ligand. Stirring of the solution for several days resulted in the transformation of Cu2 xS into CuInS2 QD with luminescent properties emitting in NIR region (Fig. 3.5). In another work, Fenton et al. [97] reported the cation exchange mediated synthesis of the CuInS nanoparticles from Cu2 xS nanoparticles. Briefly, hexane solution of Cu2 xS seed was injected into the mixture of indium acetate in oleylamine and octadecene preheated at 110°C under nitrogen atmosphere.

Fig. 3.5 Schematic illustration of the elementary kinetic steps involved in the conversion of Cu2 xS nanocrystals into CuInS2 nanocrystals by Cu+ for In3+ cation exchange. Cu+ extraction (mediated by trioctylphosphine, TOP) and In3+ incorporation into Cu+ vacancies took place at the NC surface and set in motion two solid state cation diffusion fluxes (inwards diffusion of In3+, Inin, and outwards diffusion of Cu+, Cuout). Reprinted with permission from W. Van Der Stam, A.C. Berends, F.T. Rabouw, T. Willhammar, X. Ke, J.D. Meeldijk, … C. de Mello Donega. Luminescent CuInS2 quantum dots by partial cation exchange in Cu2–x S nanocrystals. Chem. Mater. 27(2) (2015) 621–628. Copyright (2015) American Chemical Society.

Synthesis of ternary I–III–VI quantum dots

61

The mixture was then heated to 140°C followed by the dropwise addition of t-DDT. The heating was continued for another 20 min to produce the CuInS nanoparticles.

3.3.6 Ligand exchange The ligand exchange process is often carried out to convert the hydrophobic QDs to water-soluble QDs. The ligand exchange reaction is based on the affinity of the ligands to coordinate to metal atom on the surface of the QDs. Generally, thiols have high affinity toward metal surface. Consequently, they were employed to replace the weakly bound ligands. MPA, a mercapto-carboxylic acid is often used to replace TOPO from TOPO capped QDs [98, 99]. This also converts the hydrophobic nature of the QDs into hydrophilic. For instance, Zhao et al. reported the efficient ligand exchange strategy (Fig. 3.6) for aqueous phase transfer of hydrophobic oleyl amine capped CuInS2/ZnS QDs by using glutathione (GSH) and MPA for bioimaging applications [100]. The simple thiols as capping agents are prone to oxidation for numerous reasons. Hence, functionalized polymers have been exploited to substitute the native capping ligands. The polymeric ligand must contain a functional anchor group with high affinitive to QD surface. Polymer coating is usually performed by direct mixing of the QDs with functionalized polymers as a single step. On the other hand, multistep exchange reaction can also be carried out. The most common polymer used to exchange the existing ligand is functionalized polyethylene glycol (PEG) because of its ease of preparation, handling, water solubility, low toxicity, and biocompatibility [101, 102].

Fig. 3.6 (A) Schematic illustration of ligand exchange process of hydrophobic QDs using thiols for water solubility. (B) QD solutions before and after ligand exchange showing change of phase. Reprinted with permission from C. Zhao, Z. Bai, X. Liu, Y. Zhang, B. Zou, H. Zhong. Small GSH-capped CuInS2 quantum dots: MPA-assisted aqueous phase transfer and bioimaging applications. ACS Appl. Mater. Interfaces 7(32) (2015) 17623–17629. Copyright (2015) American Chemical Society.

62

Ternary quantum dots

The suitable functional groups, such as thiols [103], amines [104], carboxylic acid [105] can also be introduced in the polymeric chain. These functional groups offer the possibility for additional functionalization steps. The polymers with the different chain length and binding dendates are also used based on the application. Using the polymer with multidentate ligands to bind the QD surface forms strong chelate type bonds at the surface with their multiple anchoring points. Hence, desorption of the polymer from the QD surface is considerably slower when compared to the simple ligands and provides higher stability to the QDs. Amine and carbodithioate containing polymers [106, 107] with the multidentate ligands are effective in exchanging the TOPO ligands. Ligand exchange can also be carried out by using functionalized dendrimers, which are three dimensional, highly branched macromolecules. However, these macromolecules are different from classical polymers, and they have specific size with narrow molecular weight distribution, good degree of molecular uniformity and a highly functionalized surface [108, 109]. Despite the above advantages, the exchange of the native capping ligands may damage the surface because of imperfect coverage which affects the photophysical properties of the QDs such as PL maximum, photostability, and quantum yield. Therefore, more efforts are needed to preserve the properties of QDs during the ligand exchange. The ligand exchange methods using polymer were detailed with examples on ternary QDs in Chapter 4.

3.4 Aqueous synthesis In order to make the hydrophobic QD to be water soluble, QD surface needs to be modified into hydrophilic via ligand exchange or surface encapsulation as mentioned above. These processes are tedious, involving multiple steps to change the solvent from organic to aqueous and during this course, photoluminescence of the QDs is markedly decreased. Instead, I–III–VI QDs can also be synthesized in aqueous medium directly by arrested precipitation reaction. Typically for CuInS QDs synthesis, the Cu2+ and In3+ ions are dissolved in water medium in the presence of water-soluble thiol capping ligands followed by reaction with a S2 ion. The thiols act as capping agent and also as reducing agent to convert Cu2+ to Cu+. This precipitation reaction can be induced by reflux/hydrothermal/ microwave method/pressure cooker method [77, 83, 89, 110–112]. As it is an aqueous reaction, pH plays important role in the preparation of QDs. Arshad et al. [112] reported an aqueous synthesis of CuInS2 QDs through

Synthesis of ternary I–III–VI quantum dots

63

hydrothermal method using copper nitrate, indium nitrate, and sodium sulfide precursors with glutathione as capping agent at pH 8.0, at 100°C for 8 h. However, the use of dual stabilizer is suggested to balance the reactivities of copper and indium in aqueous solution. Water-soluble short chain thiols [MPA, thioglycolic acid (TGA), and GSH] are recommended to control the reactivity of copper while excess citrate ions are for indium ions. For example, Parani et al. [83] have employed GSH and sodium citrate as dual stabilizers and reported the aqueous synthesis of AgInS2/ZnS QDs at 95°C. In another work, Kang et al. [89] reported the aqueous synthesis of watersoluble CuInSe2/ZnS and AgInSe2/ZnS core/shell QDs using TGA and gelatin as dual stabilizers. The reaction was carried out in a commercial kitchen pressure cooker (Fig. 3.7) and thus large-scale product was obtained. Compared with organic phase synthesis, aqueous synthesis is low toxic, inexpensive, and especially the products have excellent water solubility and biological compatibility. The prepared QDs can be directly used or

Fig. 3.7 (A) Schematic for the large-scale preparation process of CISe/ZnS and AISe/ZnS core/shell QDs; (B) the digital photographs of a commercial electric pressure cooker (C) and the crude dispersion of the as-prepared CISe/ZnS and AISe/ZnS core/shell QDs. Reproduced from X. Kang, Y. Yang, L. Huang, Y. Tao, L. Wang, D. Pan. Large-scale synthesis of water-soluble CuInSe2/ZnS and AgInSe2/ZnS core/shell quantum dots. Green Chem. 17(8) (2015) 4482–4488 with permission from The Royal Society of Chemistry.

64

Ternary quantum dots

conjugated with biomolecules for biological applications like bioimaging. However, this method often yields polydispersed QDs with low photoluminescene QY as the surface is prone to oxidization in solution. In some cases, the as-prepared QDs have to be stored in inert conditions since the capping thiols are least protective.

3.5 Alloy synthesis Ternary I–III–VI QDs systems are often extended to produce quaternary QDs by alloying with the other elements. Both cations mixed (e.g., ZnCu-In-S) and anion mixed (e.g., CuInSe(S)) compounds have been reported [113–115]. Alloying is mainly conducted to extend the bandgap toward the alloy material, improve photoluminescene properties, etc. However, the tunable range of bandgaps is limited by lattice mismatch. A minimal lattice mismatch between the alloy material and the base material enables the wide tunability. Another advantage of alloying is that it eliminates the interior traps, minimize structural defects, and results in a more stable crystal structure. Zinc alloying has been the most popular one which can be done in two ways i) in situ mixing the zinc precursor with the other precursors during core synthesis, and ii) via cation exchange of the QDs. Gabka et al. [113] recently reported the synthesis of Ag-In-Zn-S nanocrystals via hot-injection method where the precursors silver nitrate, indium chloride, zinc stearate, DDT are mixed in octadecene solvent and heated to 150°C followed by the injection of oleyl amine solution of sulfur and increasing the temperature to 180°C. Zhang et al. [114] also reported the cation exchange method for the preparation of (Zn)CuInS QDs (Fig. 3.8) Typically, zinc stearate dissolved in octadecene and trioctylphosphine was added drop wisely to the as-synthesized, DDT capped CuInS2 solution preheated at 120°C followed by heating up to 200°C. The In3+ ions were slowly

Fig. 3.8 Schematic representation of synthesis of alloy and core/shell QDs [114].

Synthesis of ternary I–III–VI quantum dots

65

replaced by the added Zn ions during the reaction, thus forming alloy type (Zn)CuInS QDs. Park et al. [115] recently reported the anion mixed alloy synthesis of DDT capped CuInSxSe2 x QDs via hot injection method where DDT serves as a sulfur source and TOP-Se mixture supplies Se2 ions. However, there are only few reports so far on the aqueous synthesis of quaternary QDs. For example, Song et al. [116] reported the Ag-ZnIn-S QDs via reflux method. Initially, GSH capped Zn3In2S6 QDs were prepared in water medium at 100°C using zinc acetate, indium acetate, and thioacetamide as precursors with GSH as capping agent. Then silver acetate and GSH were added to dope Ag ions in the crystal structure. In another work, Zhang et al. [79] reported water-soluble MPA stabilized Zn-Cu-In-S QDs at pH 9–10 via simple reflux method at 100°C for 40 min.

3.6 Core/shell synthesis As discussed in Chapter 1, because of its high reactivity, the core QDs dispersed in solution or exposed to air is not stable, prone to oxidize and lose its optical properties. Hence the use of core QDs is extremely limited for many applications and thus the development of shell around the core has become conventional. Nearly all the QDs that have been reported for decades are modified for core/shell structure which has became the standard structure for the representation of QDs. The shell material acts as physical barrier between the core and the surrounding medium. In addition, the growth of the shell eliminates the surface defect sites and enhances the fluorescence of the core QDs. The shell material in this case is usually another semiconductor material with the wider bandgap than that of core material. Most importantly, epitaxial growth of the shell needs to exhibit only little lattice mismatch with that of core material to reduce the strain and the defects between the core/shell interface. ZnS is the popular choice of shell material for CuInS2, AgInS2 core QDs as its lattice mismatch for these QDs is typically only
2%. The growth of the shell materials is usually carried out by injecting the shell precursors to the core solution and heated at temperature lower than the growth temperature for the core QDs. This is necessary to avoid the growth of the core QDs while growing the shell material. In addition, the shell precursors are often injected at a very slow rate (few ml/hour) using syringe pump. Zhang et al. [114] performed epitaxial growth of ZnS on (Zn)CuInS alloyed QDs where the solution of Zn and S precursors were injected into the hot solution of (Zn)CuInS alloyed QDs (220°C) at a rate of 2 mL/h (Fig. 3.8). With the varied amount of

66

Ternary quantum dots

injection and time, it is possible to obtain the core/shell QDs with the desired shell thickness. Such a slow injection is critical for epitaxial shell growth to minimize the lattice mismatch. Both single step and two step shell coating methods [117–119] have been reported. The single pot approach involves the injection of the shell precursors in the crude core reaction solution, whereas in the two-step approach, the QDs purified from the reaction solution is re-dispersed in solvent to which the shell precursors are injected. The one pot approach is time saving and mostly investigated whereas the two-pot approach eliminates the unwanted side products before the growth. Chen et al. [119] synthesized DDT capped AgInS2 QDs followed by ZnS shell growth using one pot approach heating up method. Unlike binary QD core/shell systems [118, 120] such as CdTe/CdS, CdSe/ZnS where growth of the ZnS shell causes red shift in the optical spectra because of the partial leakage of core confined exciton wavefunctions to the shell, the growth of ZnS over I–III–VI QD cores usually results in the blue shift. This blue shift with the ZnS analogue shell coating is a typical feature of I–III–VI QDs [120, 121] and can be attributed to multiple mechanisms such as i) surface etching which leads to smaller crystals, ii) cation exchange between the Zn ions from the shell precursors and I/III metal ions in the QDs, and iii) diffusion of Zn ions from the surface to core. These mechanisms involves the widening of the core band gaps because of the size reduction or introduction of wider bandgap of ZnS to the narrow band gap of I–III–VI crystals. In the cation exchange and diffusion mediated process, the graded interface is expected rather than clear boundary between core and shell. The synthesis of core/shell I–III–VI based QDs is an active area of research. So far, the vast reports are based on CuInS2/ZnS core/shell QDs. Nam et al. [122] reported the synthesis CuInS2/ZnS QDs where the zinc precursors (zinc ethylxanthate and zinc stearate) was added dropwisely by syringe pump to the crude solution of DDT capped CuInS2 QDs by noninjection method. Zinc stearate has been the widely employed precursor for organic synthesis while zinc acetate for aqueous synthesis. Zhang et al. [79] obtained water-soluble, core/shell CuInS2/ZnS QDs by the injection of zinc acetate and sodium sulfide to the as-prepared solution of MPA capped CuInS2 QDs.

3.7 Size selective precipitation QDs can be precipitated from the crude reaction mixture by the addition of polar nonsolvent. The well-timed reaction solution may contain

Synthesis of ternary I–III–VI quantum dots

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particles of broad size distribution. The gradual addition of a polar nonsolvent to the crude solution can produce size-dependent precipitation of the QDs from its dispersion because of increased polarity. Since bigger particles in the dispersion are greatly attractive to the nonsolvent, they tend to flocculate first and can be separated by centrifugation. This process can be repeated to collect different sizes of QDs starting from bigger sized QDs to small sized QDs. This size selection can be performed using a proper solvent/nonsolvent combination (e.g., hexane/ethanol, water/isopropanol, etc.). This procedure also allows washing out of the reaction byproducts and excessive stabilizing agents. Nam et al. [58] carried out the size selective precipitation on the purified DDT capped CuInS2 QDs using the chloroform (solvent)/methanol (nonsolvent) combination. By the addition of methanol to the chloroform solution of QDs followed by centrifugation and collection step by step, they were able to obtain nine fractions of

Fig. 3.9 Normalized PL emission spectra of a series of size-sorted 9 CuInS2 QD fractions from the largest QD fraction of QD #1 to the smallest one of QD #9, showing gradual blue-shifts in both absorption and emission spectra as the sorting process proceeded repeatedly. Representative QD fraction samples (dispersed in chloroform) illuminated by a 365 nm multiband UV lamp are shown in the inset. Reproduced from D.E. Nam, W. S. Song, H. Yang. Facile, air-insensitive solvothermal synthesis of emission-tunable CuInS2/ZnS quantum dots with high quantum yields. J. Mater. Chem. 21(45) (2011) 18220–18226 with permission from The Royal Society of Chemistry.

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QDs attributed to different sizes of QDs. Fraction 1 containing the much bigger particles shows PL maximum at 664 nm, whereas the final fraction 9 containing the least sized QDs emit at 622 nm (Fig. 3.9). Similarly, Akdas et al. [123] precipitated the DDT capped CuInS2 QDs into three major fractions using the combination of toluene (solvent)/acetone (nonsolvent) where the third fraction was obtained as a residue by evaporating the toluene as it could not be precipitated. They observed different optical properties and structural properties for these three fractions while the residue fraction was found to be rich in organic molecules that cannot be separated by precipitation.

3.8 Conclusions In summary, I–III–VI ternary QDs are emerging as an important class of semiconductor nanocrystals because of their toxic heavy metal free components. However, the synthesis of I–III–VI QDs is quite different and complex compared to II–VI binary QDs because of their multicomponent structure and their difference in reactivity. The HSAB concept theory provides an idea to choose the suitable precursors and capping agent with the balanced reactivity between I/III ions in the synthesis. DDT has been the successful and widely studied capping agent in the organic phase synthesis, which also serves as a sulfur source. Good quality crystals are obtained by hot-injection method while heat up method produces the product with high yield (80–90%). The stoichiometric composition of I–III–VI QDs based on their precursor ratio is reported to be successful with amide superbase mediated approach. The problem of reactivity difference in the synthesis can be bypassed in the thermal decomposition method using single source precursor and in the cation exchange method. In aqueous phase synthesis, the use of dual stabilizers such as TGA and sodium citrate is suggested to balance the reactivity of I/III ions. Alloying the I–III–VI QDs with another metal ion (Zn2+) reduces the interior structural defects while growing the shell (ZnS) over these core QDs reduces the surface defects and thus increasing the optical properties. However, unlike in II–VI binary QDs, growing the (ZnS) shell over I–III–VI QDs causes undesirable blueshift because of multiple reasons including cation exchange. To conclude, though ternary I–III–VI QDs are a safe alternative to binary II–VI QDs, their synthetic strategies still needs lot of improvement for the better control of size, composition, and optical properties with the thorough understanding in the mechanism.

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CHAPTER FOUR

Ternary semiconductor nanocomposites Abstract Composite materials are the products of two or more materials with superior properties and greater advantages than those of individual materials. In the composites, the individual materials work together but they don’t dissolve into others. Nanocomposites are the type of the composites where at least one of its components are at nanoscale. Nanocomposites are prepared with the particular applications in mind such as increased mechanical, optical and electrical properties, increased structural, chemical, and thermal stabilities, etc. In this chapter, nanocomposites of ternary semiconductor I (II)–III–VI materials with polymer and carbon-based materials are considered. The methods of the preparation of these ternary nanocomposites and their application with the recent investigations are deeply discussed and the corresponding comprehensive collection of research works are provided in tabular form. Keywords: Ternary QDs, Polymer, Nanocomposites, Carbon-based, Synthesis

4.1 Polymer-based nanocomposites The combination of polymer and nanoparticles has been carried out for plenty of requirements. In this polymer-nanocomposite system, both nanoparticles and polymers share the advantages of having each other. For polymers, the addition of quantum dots (QDs) in their matrices introduces fascinating properties of QDs. QDs can play many roles in the fabrication of polymer-based optoelectronic devices [1, 2] such as photovoltaic cells, light emitting diodes (LED). In photovoltaic (PV) cells, QDs as a semiconductor can be directly blended with a conducting polymer and used as a photodetector to prepare polymer-QDs hybrid solar cell which can convert solar energy into electrical energy. In addition, as excellent fluorophores, QDs have received great attraction to be used as luminescent downshifting materials (LDS) which, instead of PV cells, absorbs incident radiation, and re-emits to the PV cell at its specific wavelength. This LDS layer can enhance trapping efficiency of absorbed photons and decrease the radiation loss at the PV surface. Moreover, broad absorption and large Stoke’s shift Ternary Quantum Dots https://doi.org/10.1016/B978-0-12-818303-8.00002-2

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features of QDs make it an excellent material to be used in luminescent solar concentrator (LSC) which can harness the solar energy in the ultravioletvisible (UV-Vis) region and re-emits to the PV cell [3]. This is advantageous unlike the conventional silicon-based PV cells which lacks absorption in UV-Vis region. Compared to binary QDs, ternary QDs are reported to exhibit large stoke shift (>150 nm) and are expected to produce better LSC. The LDS and LSC layers are mostly prepared by embedding QDs in non-conducting polymers. On the other hand, polymers serve as a stabilizing agent for QDs in many biological, biomedical applications, etc. In a colloidal synthesis of nanoparticles, capping ligands are used to control the growth and prevent aggregation. However, they undergo dynamic binding and unbinding with the QDs surface which causes them to get desorbed from the surface [4]. This can occur during the reaction, washing, irradiation, storing at open atmosphere or interaction with another incoming ligand. Furthermore, changes in pH, high salt concentration also result in the agglomeration of the particles [5]. Therefore, mere capping ligands are insufficient in stabilizing the nanoparticles. In such cases, polymers can offer the stability. Long-chain polymer molecules can wrap the QDs, thus interacting with the capping groups or defects and metal atoms on the surface. This could reduce the toxicity of the QD cores. Additionally, by using polymers, multiple and diverse chemical functionalities can be introduced at the QD surface which helps the hydrophobic QDs to be soluble in aqueous medium and further functionalized for biological applications. A variety of polymers, such as copolymers, multidentate polymers, dendrimers, and biopolymers have been employed as stabilizing agents for QDs. Polymer-based nanocomposites can be prepared by one or combination of the following techniques: (a) ex situ blending, (b) in situ growth, (c) in situ polymerization, (d) ligand exchange, (e) ligand encapsulation, and (f) ligand grafting. In the methods (a)–(c), QDs are embedded in the polymer matrices and in (d)–(f), ligand interactions at the QDs surface are involved. The schematic representations of these methods are shown in Fig. 4.1 and each of these methods are discussed with few examples in the following sections.

4.1.1 Ex situ blending In this ex situ method, nanocomposites are prepared by blending the polymer with the prepared nanomaterials. The compatibility of the nanomaterials with the chosen polymer is of great importance in forming

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Fig. 4.1 Different methods of polymer-based nanocomposite preparation. (A) ex situ blending, (B) in situ growth, (C) in situ polymerization, (D) ligand exchange, (E) ligand capping, and (f) ligand grafting.

the homogeneous composite without phase separation. Ex situ blending method has been popular to produce transparent fluorescent thin film, solar cells, and light emitting diodes (LED) because of its ease of preparation. Polymer-based solar cells are mostly processed in organic medium and hence hydrophobic QDs are used for blending to produce polymer-QD hybrid solar cells. Hybrid solar cell fabricated by using the blend of polymer and I–III–VI ternary nanoparticles was first reported by Arici et al. [6]. Initially, triphenyl phosphate capped CuInS2 nanocrystals were prepared in an organic medium which were then blended with a conducting polymer PEDOT:PSS (Abbreviations of all polymers are expanded under Table 4.1) to fabricate PEDOT:PSS-CuInS2 hybrid solar cell. A power conversion efficiency (PCE) of 0.07% was obtained with bilayers of CuInS2-PEDOT:PSS and PCBM in the PV device. Thereafter, ex situ blending of different types of conducting polymers such as PEDOT:PSS, P3HT, MEH-PPV, MDMOPVV, PCDTBT with the ternary QDs such as CuInS2, CuInSe2, AgInS2, AgInSe2 QDs have been reported to produce hybrid solar cells as seen in Fig. 4.2 and Table 4.1. Recently, Perner et al. [7] synthesized dioleamide capped CuInS2 QDs by hot-injection method using metal xanthate precursors. This was followed by ligand exchange process with 1-hexanethiol for polymer compatibility before blending with PCDTBT conjugated polymer for solar cell application. The fabricated solar

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Table 4.1 Database of polymer-based nanocomposites with ternary semiconductor materials. Nanocrystal Polymer Applications References Ex situ blending

CuInS2 CuInSe2 CuInS2 CuInS2 CuInS2/ZnS CuInS2/ZnS CuInS2 CuInS2 CuInS2/ZnS CuInS2/ZnS CuInS2/ZnS CuInS2/ZnS/ZnS CuInZnS CuInS2/ZnS CuInS2/ZnS CuInS2 CuInS2/ZnS AgInZnS CuInS2/ZnS CuInGaS2/ZnS AgInS2/ZnS AgInSe2 CuInS2/ZnS

PCDTBT P3HT P3HT PEDOT:PSS/PCBM PEUA EVA MEH-PPV MEH-PPV:POSS PS PS ADS233YE PMMA PMMA PS, PMMA, PVP PVK PMMA PMMA PVA CEC PVA PVA MDMOPVV PVP

Photovoltaic Photovoltaic Photovoltaic Photovoltaic LDS LDS Photovoltaic PLED LED WLED PLED WLED WLED LED Memory device Memory device Memory device Sensor Fluorescent displays WLED WLED Photovoltaic WLED

[7] [8–10] [11, 12] [6, 13] [14] [15] [16–20] [21] [22] [23] [24] [25, 26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

In situ growth

CuInS2 CuInS2 CuInS2 ZnS-AgIn5S8 ZnIn2S4 ZnIn2S4 AgInS2/ZnS CuInS2/ZnS AgInS2 AgInS2/ZnS AgInS2 CuInS2 AgInSe2/ZnSe ZnCuInS-ZnS

P3HT PSiF-DBT Cellulose fluoropolymer Polypyrrole PEDOT:PSS Gelatin PMMA PEI Gelatin CMC P3EBT Gelatin PDMS

LDS

[38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51]

LDS LSC

[52] [53]

Photovoltaic Photovoltaic Photocatalyitc Photocatalytic Photocatalytic Bioimaging LSC Sensing and bioimaging WLED Bioimaging Photovoltaic

In situ polymerization

CuInS2/ZnS CuInS2/CdS

PS/PLMA PLMA

Table 4.1 Database of polymer-based nanocomposites with ternary semiconductor materials.—cont’d Nanocrystal Polymer Applications References

Cu-ZnInSe CuInSeS/ZnS CuInS2 AgInS2-ZnS

PLMA PLMA PSi PAAm

LSC LSC WLED WLED

[54] [55] [56] [57]

TMM-PEG SPH His-PIMA-PEG/NH2 DHLA-PEG-SuccRGD cRGD-PME TA-PEG-Suc-RGD TA-PEG-Suc-RGD

Bioimaging Bioimaging Quenching Bioimaging

[58] [59] [60] [61]

Bioimaging Bioimaging Bioimaging

[62] [63] [64]

Bioimaging Bioimaging Bioimaging Bioimaging Temperature sensing Bioimaging

[65] [66] [67] [68] [69] [70] [71] [72] [73] [74]

Ligand exchange

CuInS2/ZnS ZnAgInSe/ZnS CuInS2/ZnS QDs ZnAgInSe/ZnS ZnCuInSe/ZnS ZnAgInS ZnAgInSe Ligand encapsulation

CuInS2/ZnS CuInS2/ZnS CuInS2/ZnS CuInS2/ZnS CuInS2/ZnS AgInS2-ZnS CuInS2/ZnS CuInS2/ZnS CuInS2/ZnS CuInS2

Pluronic F127 FA-SOC PEGMA PMAO-Jeffamine PEG-POES PMAO PMAL PMAO-DMAPA Glycol-Chitosan PGA

AgGaInS

LDL-PDDA

Bioimaging Bioimaging, drug delivery Bioimaging

[75]

Ligand grafting

ZnCuInS/ZnS ZnCuInS/ZnS

Cellulose Bacterial cellulose

[76] [77]

P3HT, Poly(3-hexylthiophene); PEDOT:PSS, poly(styrenesulfonate)-doped poly(3,4-ethylenedioxythiophene); PCBM, Phenyl-C61-butyric acid methyl ester; MEH-PPV, poly[2-methoxy-5-(20 -ethylhexyloxy)-p-phenylene vinylene; POSS, polyhedral oligomeric silsesquioxane; ADS233YE, poly [(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,10,3}-thiadiazole)] end-capped with dimethylphenyl; PS, polystyrene; PMMA, polymethylmethacrylate; PVP, polyvinylpyrrolidone; PVK, poly(N-vinylcarbazol); CEC, cyanoethyl cellulose; PVA, poly(vinyl alcohol); MDMOPVV, poly[2-methoxy-5-(30 , 70 -dimethyloctyloxy)-1,4-phenylenevinylene; PEUA, polyester-urethane acrylate; PCDTBT, poly[[9(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophene-diyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl-l]; EVA, ethylene-vinyl acetate; PSiF-DBT, poly[(2,7-silafluorene)-alt-(4,7-di-2-thienyl-2,1, 3-benzothiadiazole)]; CMC, carboxymethylcellulose; P3EBT, poly(3-(ethyl-4-butanoate)thiophene); PEI, polyethylenimine; PDMS, poly(dimethylsiloxane); PLMA, poly(lauryl methacrylate); PSi, polysiloxane; PAAm, polyacrylamide; PIMA, poly(isobutylene-alt-maleic anhydride); SPH, sulfobetaine-PIMA-histamine; PEG, polyethylene glycol; DHLA, dihydrolipoic acid; Suc, succinic anhydride; cRGD, cyclic arginine glycine-aspartate; TMM, tris(mer-captomethyl); PME, cysteamine hydrochloride (MEA) and n-boc-ethylenediamine (EDA) grafted poly(acrylic acid) (PAA); TA, thioacetamide; FA-SOC, folate-modified N-succinyl-N0 -octyl chitosan; PMAO, poly(maleic anhydride-alt-1-octadecene); PMAL, poly(maleic anhydride-alt-1-tetrade-cene); 3-(dimethylamino)-1-propylamine; DMAPA, 3-(dimethylamino)-1-propylamine; PEGMA, poly(ethylene glycol) methacrylate; POES, polyoxyethylene stearate; LDL, low-density lipoprotein; PDDA, poly(diallyldimethylammonium chloride); PGA, poly(L-glutamic acid).

(a)

(b) PMMA+ POPOP

Al

PMMA+ CIZS-20

PMMA+POPOP CIZS-20 composite

PEDOT:PSS ITO (c)

Taylor cone formaon QD+PS in Chloroform

(d)

High CRI

Apply 20kV QD-PS Nanofiber

Ground connecon

Blue LED + QDEN with LGP

Fig. 4.2 Polymer-QDs composites prepared by ex situ blending method and some of their applications (A) Architecture of the MEH-PPV/ CuInS2 solar cells. (B) The photo images of POPOP+ PMMA, CIZS-20 + PMMA, and POPOP +CIZS-20 + PMMA composite films under normal and UV-LED backlit illumination. White LED films are produced from POPOP+ CIZS-20 + PMMA composites. (C) schematic diagram of the ex situ electro-spinning process for QDs embedded in nanofibers for color conversion film (D) corresponding color image with high color rendering index (CRI). Panel (A) reprinted from W. Yue, M. Lan, G. Zhang, W. Sun, S. Wang, G. Nie. Size-dependent polymer/CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots. Mater. Sci. Semicond. Process. 24 (2014) 117–125. Copyright (2014) with permission from Elsevier, (B) from P. Ilaiyaraja, P.S. Mocherla, T.K. Srinivasan, C. Sudakar. Synthesis of Cu-deficient and Zn-graded Cu–In–Zn–S quantum dots and hybrid inorganic–organic nanophosphor composite for white light emission. ACS Appl. Mater. Interfaces 8(19) (2016) 12456–12465. Copyright (2016) American Chemical Society, and (D) from N. Kim, W. Na, W. Yin, H. Jin, T.K. Ahn, S.M. Cho, H. Chae. CuInS2/ZnS quantum dot-embedded polymer nanofibers for color conversion films. J. Mater. Chem. C 4(13) (2016) 2457–2462 with permission from The Royal Society of Chemistry.

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cell exhibited PCE of 0.23%, which is the highest in the polymer-hybrid cell prepared by ex situ method. It should be noted that few reports are available on the aqueous processed polymer-QDs hybrid solar cell using vinylene polymer derivatives, however they involved binary CdTe QDs [78–80] and so far, no reports on the aqueous processed ternary QDs-polymer hybrid solar cells are available. Ex situ method has also been applied to prepare (nonconducting) polymer-QD composites as LDS and LSC thin films which can be placed on the top of different types of PV cells such as, CdTe, silicon, and polymer solar cells. For instance, Draaisma et al. [14] synthesized dodecane thiol (DDT) capped CuInS2/ZnS QDs with 15% PLQY and performed ligand exchange reaction with oligo-caprolactone ligands for polymer compatibility followed by ex situ blending with the UV curable resin matrix. This composite film was then used as LDS layer on the top of multicrystalline silicon solar cell. Recently, Nakamura et al. [15] synthesized DDT capped CuInS2/ZnS QDs with a maximum of 60% PLQY. This was then blended with EVA copolymer to produce LDS nanocomposite film on single crystalline Si solar cells. Perfect white display can be produced by proper mixing of red, green, and blue lights or simply yellow and blue lights as yellow is the combination of red and green. QDs are excellent choices as color converters for LED application as QDs can produce different color emission across visible region based on the size, composition, and type. By proper mixing/coating of yellow, red, green emitting QDs on blue emitting chips, white light emitting diodes (WLEDs) can be easily obtained. Like PV devices, the fabrication of QDs-based LEDs majorly involves ex situ blending of QDs and polymers. The non-conducting polymers such as PMMA, PVP, PVA, PS have been used to embed the QDs within their matrices by simple mixing for LED fabrication (Fig. 4.2 and Table 4.1). However, mixing of two different colored emitting QDs in a single matrix is susceptible to re-absorption and consequent loss in efficiency. Wang et al. [35] synthesized green and red emitting water-soluble AgInS2/ZnS QDs which were then embedded in PVA matrices separately. The highly luminescent transparent composite films were placed (red under green) on the top of the commercial InGaN blue LED chips to produce WLED with color rendering index (CRI) of 90.2. On the other hand, Ilaiyaraja et al. [27] synthesized bright yellow emitting DDT capped CuInZnS QDs and blue emitting POPOP organic fluorophores. Ex situ blending of these two fluorophores in PMMA matrix yielded organic–inorganic hybrid polymer composite which are not susceptible to re-absorption as expected for the mixture of two different color

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emitting inorganic QDs. The composite film produces white light when placed under UV-LED with high CRI of 92.0. Ternary QDs have been shown to increase the efficiency of polymer LEDs (PLED) where the light emission originates from the conducting polymer and QDs are used as dopant. For example, Demir et al. [24] prepared a yellow emitting polymer on which as-synthesized CuInS2/ZnS QDs were blended in small quantities. This hybrid composite exhibited fourfold enhancement in brightness and twofold enhancement in power efficiency compared the undoped PLED device. Ternary QDs-polymer composites prepared via ex situ blending method have also been reported for sensing and memory device fabrication [29–32].

4.1.2 In situ growth Polymers are used as a matrix for growing nanoparticles in this method. This in situ preparation of polymer nanocomposite has some advantages over ex situ method such as absence of separate nanoparticle synthesis, absence of capping ligand, and the use of polymer as stabilizing agent and directing template for the nanoparticles. Kharkwal et al. [38] investigated in situ and ex situ polymeric stabilization of CuInS2 QDs synthesized in organic medium and observed that in the in situ method, the polymer ligands are anchored on the QDs providing better polymer steric stabilization compared to the ex situ method where mostly free and unanchored polymers are found around QDs. Rath et al. [39] developed polymer/CuInS2 QDs nanocomposite solar cell where the homogeneous nanocomposite film was directly prepared via thermal decomposition of copper and indium xanthates in PSiF-DBT polymer. The solar cell fabricated using this in situ PSiF-DBT-CuInS2 polymer hybrid exhibited PCE upto 2.8% which is higher than ex situ method reported so far. Recently, Reishofer et al. [40] employed organically soluble bio-based trimethylsilyl cellulose and metal xanthates to fabricate cellulosicCuInS2 nanocomposites based solar cell with PCE of 1%. As metal xanthates are poorly soluble in common organic solvents, suitable modification of xanthates is often carried out prior to decomposition. Alternatively, dissolving agents such as pyridine, butyl amine can also be added in small quantities to prepare homogeneous polymer solution [38, 81]. In situ synthetic route have also been carried out in the aqueous medium to prepare the polymer-QDs nanocomposites which are used for photocatalysis, LED, and bioimaging applications. For instance, He et al. [41] prepared ZnSAgIn5S8/fluoropolymer fiber composites via hydrothermal synthesis and

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investigated their photocatalytic H2 evolution from splitting of water under sunlight radiation. The results revealed that the rate of H2 evolution using the polymer nanocomposite is higher when compared with ZnSAgIn5S8 nanoparticles alone because of the excellent stabilization of the nanoparticles in polymer matrix which enhances its photocatalytic activity. Conducting polymers are advantageous in separating the charge carriers for photocatalysis. For example, polypyrrole [42] and PEDOT:PSS [43] conducting polymers were used to prepare polymer-ZnIn2S4 nanocomposite with enhanced photocatalytic activities in the removal of total organic carbon. For biological applications of polymer-QD composite, biocompatibility of the polymer with cells and tissues is important. Kang et al. [44] prepared biopolymer gelatin stabilized AgInS2/ZnS QDs nanocomposite via in situ method in aqueous medium at large scale. Due to the presence of many functional groups such as –COOH, –NH2, –OH, gelatin can effectively chelate to Ag+ and In3+ ions to balance their reactivity toward sulfur ions along with thioglycolic acid (TGA) capping agent and impede the phase separation during the reaction. The as-prepared nanocomposite exhibited high photostability, buffer stability and was used for cell imaging application (Fig. 4.3A–D). Alternatively, Wang et al. [45] from the same research group reported the use of polyethylenimine (PEI) to prepare in situ PEIAgInS2 QDs nanocomposite for bioimaging and sensing application. Kang et al. [46, 47] have also prepared series of gelatin stabilized AgInS2/ZnS, AgInSe2/ZnS, CuInSe2/ZnS QDs nanocomposites using the same method and reported their performance in WLED applications. Li et al. [82] reported the in situ fabrication of LSC (Fig. 4.3E and F) using PMMACuInS2/ZnS composite with optical efficiency as high as 26.5%. The PCE of the PV device constructed with LSC-QDs reached more than threefolds than that of bare PMMA and the fabricated LSC-QDs PV cells was able to harvest aboutfivefold solar energy than that of PV cells without LSC. Fabrication of such LSC paves an ecofriendly way for the high PCE with low cost solar energy.

4.1.3 In situ polymerization Unlike attaching the preexisting polymers to the nanoparticle surface by other methods to prepare polymer nanocomposite, the “in situ polymerization” is a unique technique where the polymerization is initiated in the presence of previously prepared nanoparticles. To obtain a homogeneous

(e)

(a) 1h

1h AIS

AIS/ZnS

Under daylight

(b)

Under UV light

(d)

(f) PCE = 2.73 %

PL Intensity (a.u.)

(c)

450

500

550

600

650

700

PCE = 8.71 %

Harvest Area (cm*cm)

PCE = 13 %

Wavelen th nm Fig. 4.3 (A) Schematic for the large-scale preparation process of gelatin stabilized AgInS2 core QDs and AgInS2/ZnS core/shell QDs via in situ growth method. (B) The luminescence photographs for samples with different Ag:In ratios under daylight. (C) The corresponding photoluminescence spectra (D) aqueous solution of stabilized AgInS2/ZnS core/shell QDs. (E) Photograph of the LSC prototype (22  22  3 mm) non-containing and containing CuInS2/ZnS QDs illuminated by UV lamp and (F) Different schematic of LSC-PV devices constructed with bare PMMA and LSC-QDs [82]. Panel (D) reprinted with permission from X. Kang, L. Huang, Y. Yang, D. Pan. Scaling up the aqueous synthesis of visible light emitting multinary AgInS2/ZnS core/shell quantum dots. J. Phys. Chem. C 119(14) (2015) 7933–7940. Copyright (2015) American Chemical Society PMMA-QDs composites prepared by in situ growth method.

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composite, the compatibility of nanoparticles surface ligands with the polymer matrix is required. Lesyuk et al. [52] synthesized DDT capped CuInS2/ ZnS QDs in an organic medium and performed ligand exchange with vinylaniline and zinc methacrylate where the latter was a co-ligand used to minimize the surface defects during ligand exchange. Compared with native DDT capped QDs, vinylene ligand capped QDs were readily soluble in styrene which was then polymerized to form polystyrene-QDs composite. The resulting polystyrene-QDs composite exhibited higher photostability under photoirradiation for a long time compared to the QDs alone with significant enhancement of photoluminescence because of light soaking effect. Such composites could be beneficial for the application in low-cost solar-energy devices. Similarly, polylaurylmethacrylate (PLMA)-QDs composite was prepared by using laurylmethacrylate as monomer, ethylene glycol dimethacrylate (EGDM) as cross-linking agent and Irgacure 651 as radical initiator [53]. It is important to note that while using initiator, its amount should be kept as low as possible to lower the free radical formation which deteriorates QDs photophysical properties. Using PLMA, Cu-ZnInSe/ ZnSe, CuInS2/CdS, CuInSe/ZnS QDs-polymer composites have been reported for the fabrication of liquid solar concentrators [53–55]. Recently, Hu et al. [56] reported the fabrication of WLED using the tricolor QDs (blue carbon, yellow and red CuInS2/ZnS QDs) embedded into polysiloxane (PSi) matrix. The PSi-QDs composite are highly luminescent compared to QDs alone and also exhibited high photostability and thermal stability with the significant fluorescence recovery after the heat system was removed. The fabricated PSi-QDs LED exhibited greater performance than the commercial LED, with high CRI of 97. Similarly, Su et al. [57] reported the preparation of red, yellow, and green emitting polyacrylamide-AgInS2/ ZnS QDs hydrogel composites (Fig. 4.4) for the fabrication of WLED on blue emitting InGaN chip.

4.1.4 Ligand exchange The ligand exchange process is often carried out to convert the hydrophobic nanoparticles to hydrophilic [83, 84]. The ligand exchange reaction is based on the affinity of the ligands to coordinate to metal atom on the crystal facets and the molecular geometry of the ligands covering the facet. Strongly binding ligands form a dense layer stabilizes particles better than weakly binding ones. Hence, functionalized polymers have been exploited to replace the native capping ligands. The polymeric ligand must contain a functional

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Fig. 4.4 Synthetic illustration of the preparation of AgInS2/ZnS QDs and QDs/PAAm composite film prepared via in situ polymerization method for WLED. Reprinted from D. Su, L. Wang, M. Li, S. Mei, X. Wei, H. Dai, Z. Hu, F. Xie, R. Guo. Highly luminescent water-soluble AgInS2/ZnS quantum dots-hydrogel composites for warm white LEDs. J. Alloys Compd. 824 (2020) 153896. Copyright (2020) with permission from Elsevier.

anchor group that attaches to the inorganic nanoparticle surface, an outer functional group and a spacer in between. The functional anchor groups introduced into the polymer main chain or side groups are commonly thiols and amines as these groups exhibit high affinity toward nanoparticles metal surface. Also, these polymer ligands allow a larger number of functionalities to be introduced to the assembly, as compared to a procedure with simple organic ligands. Depending on the need, varying polymer chain lengths and number of binding dentate are used. Using the polymer with multidentate ligands to bind the nanoparticle surface forms strong chelate type bonds at the surface with their multiple anchoring points. Hence, desorption of the whole polymer ligand from the nanoparticle surface is considerably slower when compared to simple ligands and provides higher stability to the nanoparticles. For biological applications, functionalized polymers are required to possess biocompatibility, in addition to water solubility. Gravel et al. [58] developed PEGylated tridendate ligand using PEG2000 monoethyl ether and tris(mercaptomethyl) chelating group (TMM). The asprepared TMM-PEG2000 ligand was used to replace the native hydrophobic ligands (oleic acid/oleyl amine) of visible CdSe/CdS/ZnS QDs and near infrared (NIR) CuInS2/ZnS QDs in chloroform. The three thiol units in TMM group offers high chelating effect to the QDs surface and the PEG

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provides water solubility for the QDs. Compared to simple dihydrolipoic acid (DHLA) ligand, TMM-PEG2000 polymer ligand offers high stability for QDs at different pH values and in the presence of molecular iodine which is able to oxidize thiols into disulfides. In addition, long circulating PEG and NIR CuInS2/ZnS QDs served as a better pair for in vivo bioimaging. Recently, Deng et al. [59] designed a multifunctional polymer ligand (RGD-SPH) which involves histamine molecule for anchoring, sulfobetaine to provide water solubility, RGD peptide for tumor targeting molecule, and poly(isobutylene-alt-maleic anhydride) (PIMA), biocompatible polymer backbone with carboxyl groups to aid anchoring and water solubility. The as-synthesized RGD-SPH polymer ligand was used to replace the native DDT ligand of ZnAgInSe/ZnS QDs to produce RGD-SPH QDs composite (Fig. 4.5). The imidazole ring in the histamine is an excellent anchor group for coordinating to QDs surface, less prone to oxidation and does not get easily replaced in bioconjugation reactions or by sulfur containing molecules in biological media. The obtained polymer-QD multifunctional nanocomposite exhibited NIR emission, long time circulation and dual targeting ability, suitable for in vivo imaging. Ligand exchange can also be carried out by using functionalized dendrimers, which are three-dimensional, highly branched macromolecules. Unlike classical polymers, dendrimers have a high degree of molecular uniformity, narrow molecular weight distribution, specific size and shape characteristics and a highly functionalized terminal surface. Poly(amidoamine) dendrimer (PAMAM) with terminal functional groups can be successfully applied to modify QDs surfaces [85]. However, the use of dendrimer/ hyperbranched polymer for ligand exchange has not been exploited yet on the ternary QDs. Despite the above advantages, the ligand exchange methods often resulted in QDs with surface defects compared to native QDs because of incomplete coverage and imperfect exchange which affects the photophysical properties of the QDs. The parameters influenced are the photoluminescence wavelength, quantum yield, and photostability. Therefore, more efforts should be concentrated on the preservation of the brightness of the QDs during ligand exchange.

4.1.5 Ligand encapsulation The alternate strategy to fabricate polymer nanocomposites is to encapsulate polymers onto nanoparticle surface through physical interaction, such as hydrophobic interaction, electrostatic interaction, or hydrogen bonding

Fig. 4.5 Ligand exchange assisted scheme for water-soluble RGD-SPH ZnAgInSe/ZnS QDs-clusters, and their applications for bioimaging. Reprinted with permission from T. Deng, Y. Peng, R. Zhang, J. Wang, J. Zhang, Y. Gu, D. Huang, D. Deng. Water-solubilizing hydrophobic ZnAgInSe/ZnS QDs with tumor-targeted cRGD-sulfobetaine-PIMA-histamine ligands via a self-assembly strategy for bioimaging. ACS Appl. Mater. Interfaces 9(13) (2017) 11405–11414. Copyright (2017) American Chemical Society.

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between native ligands of the nanoparticles and polymers. The ligands are not exchanged in this process and thus the photophysical properties of the nanoparticles are not significantly affected. Several amphiphilic polymers, micelles, and polyelectrolytes have been synthesized and employed to modify nanoparticles surface through encapsulation method. In this method, the added polymer or micelle intercalates its aliphatic chains between the aliphatic chains of the surface ligands of nanoparticles through spontaneous hydrophobic interaction. This encapsulation forms a polymeric coating over the surface while the hydrophilic parts of the polymer promote the water solubility and chemical functionality to the nanoparticles. Liu et al. [65] reported the optimization of the organic synthesis of DDT capped CuInS2/ZnS and AgInS2/ZnS QDs and hydrophilized the QDs using Pluronic F127 block copolymer micelles. The hydrophobic part of the micelles stabilized the QDs while hydrophilic part made them water soluble. They have observed no significant changes in the optical properties and studied the QDs for in vitro and in vivo imaging. In another report, N-succinyl-N0 -octyl chitosan [66] and amphiphilic PEG polymer [67] was used as micelles to prepare water soluble ternary QDs-polymer nanocomposites. Similarly, amphiphilic PMAO and its derivatives (Fig. 4.6) also served as an alternative to lengthy PEG chains to prepare the nanocomposites [68, 70–72]. Hung et al. [72] synthesized zwitter ionic-amine derivative of PMAO polymer to encapsulate hydrophobic CuInS2/ZnS QDs. The prepared polymer-QDs composite was highly stable over wide range of pH and high salinity solutions due to its zwitter ionic nature and was also used for cell imaging. Polyelectrolytes can “cap onto” charged QDs surface by electrostatic interaction. When QDs capped with ionic ligands are made to interact electrostatically with opposite charged polyionic polymers, the QDs are tightly capped with polymer because of the multi-interaction between surface ligands and a single polymeric chain which offers a better stability to the composite. This technique can be repeated several times to obtain layerby-layer (LbL) polymer/QD assembly [86]. Successive layers are assembled by keeping the excess charge on the surface of each deposition steps. The excess charge can be controlled by changing the ratio of charged and non-charged ligands on the surface. Smooth, uniform, defect-free, highquality with controlled thicknesses of polymer-QD thin films can be fabricated by this LbL technique and exploited for optoelectronic and biosensing applications. This type of composite is usually achieved via simple mixing of positively or negatively charged QDs with its counter charged polymer.

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Fig. 4.6 (A) Scheme of hydrophobic QDs encapsulation with PMAO-Jeffamine M1000 polymer (QDs 1) and subsequent conjugation of the obtained QDs with Jeffamine M1000 (QDs 2) and Jeffamine ED-2003 (QDs 3) using EDC chemistry. (B) Fluorescence image of Gel migration of CuInS2/ZnS QDs modified with Jeffamin. Reprinted with permission from E.S. Speranskaya, C. Sevrin, S. De Saeger, Z. Hens, I.Y. Goryacheva, C. Grandfils. Synthesis of hydrophilic CuInS2/ZnS quantum dots with different polymeric shells and study of their cytotoxicity and hemocompatibility. ACS Appl. Mater. Interfaces 8(12) (2016) 7613–7622. Copyright (2016) American Chemical Society.

Gao et al. [74] synthesized electrostatic complex of positively charged CuInS2 QDs and negatively charged poly(L-glutamic acid) loaded with doxorubicin drug. The as-prepared complex was used for the drug delivery and imaging. Recently, Song et al. [75] prepared water-soluble negatively charged AgInS2 QDs followed by cation exchange with Ga to form AgGaxIn1 xS2 QDs. This was conjugated to low-density lipoprotein via electrostatic coupling using PDDA, a cationic polyelectrolyte. The as-prepared composite was used as selective 3D fluorescence imaging of cancer stem cells.

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4.1.6 Ligand grafting In the grafting/conjugation method, QDs are covalently coupled with suitably functionalized polymer. Wang et al. [76, 77] reported the grafting of (ZnS)x(CuInS2)1-x/ZnS QDs on the surface of cellulose and bacterial cellulose (BC) using 3-(mercaptopropyl) trimethoxysilane as coupling agent in ionic liquid medium. This was followed by coagulation and freeze drying to form QDs fluorescent-polymer aerogel composite. This fluorescent aerogel composite could be used for smart sensing, catalysis, or 3D applications (Fig. 4.7).

4.2 Carbon materials-based nanocomposites Carbon is a conducting material and so are its allotropes. Carbon materials such as graphene, carbon nanotubes (CNT), and carbon dots have received greater attraction in optoelectronic, photocatalytic, and other applications because of their excellent conducting properties, high surface area, etc. Nanocomposites of carbon materials have been demonstrated with the improved properties. For example, ternary semiconductor materials have been investigated as photocatalyst, however they are limited by their charge recombination. By combining them with the carbon materials, their photocatalytic performance can be increased as carbon material are good electron acceptors which reduces the charge recombination. Carbon allotrope materials are naturally hydrophobic, but they can be modified to exhibit aqueous solubility and biocompatibility for their use in biological applications. They can be used to carry the nanoparticles/drugs to the cellular tissue target where the nanoparticles perform their intended role. The preparation of nanocomposites of ternary semiconductor materials with various carbon materials such as graphene, graphene oxide, carbon dots, graphitic carbon nitride with their applications are discussed below.

4.2.1 Graphene composites Graphene is a single atomic layer of carbon atoms which are sp2 hybridized and tightly arranged in 2D honeycomb lattice structure [87]. Natural graphite is nothing but the multiple stacks of graphene with the interlayer distance (d-spacing) of 3.4 A˚, bonded by van der Waals force. Graphene possess excellent properties such as high thermal and electrical conductivity, high mechanical strength yet it is a very lightest material. The composite of graphene nanocomposites is the promising candidate for photovoltaic and

Fig. 4.7 (A) Preparation scheme of BC-templated ZCIS/ZnS QD565 aerogels. (B) A schematic diagram of the BC fibers grafted with QD565. (C,D) Photos of the BC-ZCIS/ZnS QD565 aerogels under a UV lamp made via (C) chemical binding (grafting) and (D) physical absorption. Reprinted from H. Wang, H. Qian, Z. Luo, K. Zhang, X. Shen, Y. Zhang, M. Zhang, F. Liebner. ZCIS/ZnS QDs fluorescent aerogels with tunable emission prepared from porous 3D nanofibrillar bacterial cellulose. Carbohydr. Polym. 224 (2019) 115173. Copyright (2019) with permission from Elsevier.

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Fig. 4.8 Schematic of the electron-transfer process of the AgInS2/GNW electrode under illumination. Reproduced from J. Nong, G. Lan, W. Jin, P. Luo, C. Guo, X. Tang, Z. Zang, W. Wei. Eco-friendly and high-performance photoelectrochemical anode based on AgInS2 quantum dots embedded in 3D graphene nanowalls. J. Mater. Chem. C 7(32) (2019) 9830–9839 with permission from The Royal Society of Chemistry.

photocatalytic applications as graphene can effectively transport the photogenerated charge carriers from the nanoparticles. Kumari et al. [88] have reported that the incorporation of graphene in P3HT: CuInS2 composite enhances the efficiency of the PV by increasing the conducting pathways. Recently Nong et al. [89] prepared graphene nano-wall (GNW) which is a three-dimensional graphene network and deposited on the silica substrates followed by immersing in AgInS2 QDs solution (Fig. 4.8). The results revealed that the prepared composite exhibits rapid photo-response, high photo-current density and excellent stability. The pure form of graphene is obtained via chemical vapor deposition (CVD) methods by decomposing carbon sources usually a mixture of methane and hydrogen [90]. This physical method is expensive and complicated and hence simple chemical method is majorly employed to produce graphene from graphite with the compromise in quality. Typically, graphite is oxidized with strong oxidizing agents using Hummer’s method to form graphite oxide [91]. The introduction of oxygen functionalities such as epoxy, hydroxyl, carboxyl, groups partially distorts sp2 hybridized carbons, increase the d-spacing and reduces the van der Waal forces. Separation of these layers using ultrasonication results in a single or few layers of graphite oxide which is called graphene oxide (GO). Introduction of these oxygen functionalities makes graphene oxide hydrophilic and soluble in water. The reduction of these graphene oxide sheets can yield graphene. The reduction can be carried out by chemical or thermal treatment. In a chemical

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treatment, reducing agents such as hydrazine or sodium borohydride are used. However, these reducing agents are corrosive and introduces some additional functional groups such as CdN bonds (by hydrazine) and CdOH bonds (by sodium borohydride) in graphene sheets. Thermal treatment of graphene oxide can yield good quality graphene, however high temperature (>550°C) is required which leads to increased cost and energy requirement. Hence, solvothermal and hydrothermal approaches which doesn’t require reducing agents or high temperature is the most successful way to produce graphene sheets at large scales with low cost. Organic solvents such as N-methyl pyrrolidone (NMP), dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO) are used [91] in solvothermal reduction of graphene oxide while in hydrothermal reduction, only water is used, a much-preferred solvent for eco-friendlier synthesis. Alcohols are also used in place of water which is sometimes called as alcohothermal reaction. Addition of sulfuric acid in these solutions are reported to increase the rate of reduction by opening the epoxy ring followed by dehydration. Nevertheless, solvents such as DMF, NMP are found to introduce CdN bonds while DMSO introduces CdS bonds in the graphene sheets and hydrothermal reactions leaves some hydroxyl groups in the graphene sheet. Compared to pure graphene produced by physical method, chemical oxidation and reduction methods yield impure or defective graphene sheets as graphene oxide sheets cannot be completely reduced back with all carbons sp2 hybridized in the hexagonal arrangement with actual d-spacing of pure graphene. Hence, this form of graphene is rather referred to as reduced graphene oxide (rGO). Depending on the degree of reduction, the thermal and electrical conductivities of rGO vary. High degree of reduction of GO produces rGO with high conductivity though it cannot compete with that of pure graphene. Due to its ease of preparation with low price, rGO have been used as an alternative to graphene in the preparation of graphene-based composites for optoelectronic applications. In situ hydrothermal or solvothermal synthesis is mostly performed to prepare the rGO-ternary QDs composites. Typically GO is added to the mixture of QDs precursors dissolved in water/solvent which is then transferred to Teflon autoclave. By heating this mixture at high temperature depending upon the type of solvents, rGO-QD composites are obtained. For example, Meng et al. [92] carried out Hummers method for the synthesis of GO and ultrasonically dispersed into ethanol to which copper acetate, indium acetate, and 1octadecylamine were added. A uniform dispersion was obtained by additional ultrasonication after which thiourea was added. This precursor mixture was

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transferred to Teflon lined autoclave and heated for 160°C for 12 h to produce rGO-CuInS2 QDs aggregated composites. The hydroxyl and epoxy groups (but not the carboxyl groups) in GO serves as nucleation and growth center for CuInS2 QDs which in turn aids for the better reduction of GO and prevents their aggregation in solvothermal conditions simultaneously (Fig. 4.9A). The as-prepared rGO-CuInS2 QDs composites proved to be a

Fig. 4.9 (A) Schematic illustration for the formation of rGO/CuInS2-QDs hybrid. (B) Formation of CNTs/ZIS, CQDs/ZIS, and RGO/ZIS nanocomposite with different morphologies. Panel (A) reprinted from W. Meng, X. Zhou, Z. Qiu, C. Liu, J. Chen, W. Yue, M. Wang, H. Bi. Reduced graphene oxide-supported aggregates of CuInS2 quantum dots as an effective hybrid electron acceptor for polymer-based solar cells. Carbon 96 (2016) 532–540. Copyright (2016) with permission from Elsevier and (B) from Y. Xia, Q. Li, K. Lv, D. Tang, M. Li. Superiority of graphene over carbon analogs for enhanced photocatalytic H2-production activity of ZnIn2S4. Appl. Catal. B Environ. 206 (2017) 344–352. Copyright (2017) with permission from Elsevier.

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good electron acceptor compared to rGO alone for polymer-based hybrid solar cell. Similarly, rGO-CuInS2 QD composites have been reported with high efficiencies in dye sensitized solar cells (Table 4.2). Semiconductor nanocrystals have been widely investigated as photocatalysts for the past two decades. Among them, ternary ZnIn2S4 nanocrystals with unique layered structure have gained great interest because of its narrow bandgap (2.34–2.48 eV) which falls well under visible region. ZnIn2S4 has been known to generate hydrogen by splitting of water through photocatalysis under solar radiation. Unlike non-renewable fossil fuels, sunlight assisted-photocatalytic water splitting shows the way to generate clean and renewable hydrogen fuel. However, in ZnIn2S4, fast recombination of charge carriers and photo-corrosion limits its efficiency for photocatalytic hydrogen production. By incorporating ZnIn2S4 in graphene sheets, recombination of charge carrier can be prevented as graphene is efficient electron acceptor, and photo-corrosion of the catalyst can be minimized. In addition, graphene as a mediator enhanced rate of electron transport and also acts as co-catalyst in hydrogen production. Ye et al. [101] synthesized rGO-ZnIn2S4 composites using in situ solvothermal reaction which exhibited visible light assisted photocatalytic hydrogen evolution 4 times as that obtained by ZnIn2S4 alone. Yang et al. [107] recently reported photocatalytic hydrogen production with the simultaneous degradation of organic amines by rGO/ ZnIn2S4 composites. It is also to be noted that photocatalytic activity of the composite is dependent on the reduction degree and the size of rGO [102, 103]. Xia et al. [108] have reported high photocatalytic performance of rGO-ZnIn2S4 composites for hydrogen production in comparison with other carbon analogs, that is, carbon QDs-ZnIn2S4 and carbon nanotube (CNT)-ZnIn2S4 composites. This is because of the difference in their morphology. During in situ hydrothermal reaction, zero-dimensional carbon QDs and one-dimensional CNT are wrapped in the microspheres of ZnIn2S4 which are self-assembled by their nanosheets. However, due to steric hindrance of two-dimensional rGO, self-assembly of ZnIn2S4 was prevented and hence ZnIn2S4 sheet-on-rGO sheet structure was formed instead of characteristic ZnIn2S4 spheres (Fig. 4.9B). This enhances the active sites for photocatalysis making rGO-ZnIn2S4 composites superior to other carbon analog composites. Similarly, Tang et al. [94, 100] prepared rGO-CuInS2 and rGOAgInS2 composites with excellent photocatalytic activity for hydrogen evolution from splitting of water. Recently, Xie et al. [99] synthesized rGO-CuInS2 QDs using solvothermal method with DMF as a solvent which exhibited the excellent photocatalytic performance compared to

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Table 4.2 Database of carbon materials-based nanocomposites with ternary semiconductor materials. Nanocrystal Carbon material Applications References

CuInS2 AgInS2 AgInS2

Graphene GNW rGO

Photovoltaic device Photoelectrochemical device Electrochemical sensing and H2 production AgInZnS rGO Photocatalytic H2 production Dye sensitized solar cell CuInGaSe2 rGO rGO Dye sensitized solar cell CuInS2 rGO Photovoltaic device CuInS2 rGO Photocatalytic water treatment CuInS2 CuInZnS rGO Photocatalytic H2 production rGO Photocatalytic H2 production ZnIn2S4 rGO Photocatalytic degradation ZnIn2S4 rGO Photocatalytic H2 production and ZnIn2S4 pollutant degradation rGO, CNT, QD Photocatalytic H2 production ZnIn2S4 GO Memory device CuInS2 GO Fluorescent sensor CuInS2 Fluorescent Sensor CuInS2/ZnS GO AgInZnS GO Fluorescent sensor GO Bioimaging AgInS2 GO Photoelectrochemical sensors AgInS2 SWCNT Photovoltaic CuInS2 MWCNT Photocatalytic H2 production ZnIn2S4 CQD Photocatalytic degradation ZnIn2S4 CQD Photocatalytic reduction ZnIn2S4 CDs Photocatalytic degradation ZnIn2S4 CND Photocatalytic H2 production ZnIn2S4 Carbon sphere Photocatalytic reduction ZnIn2S4 Carbon Photocatalytic H2 production ZnIn2S4 nanofiber g-C3N4 Photocatalytic degradation ZnIn2S4 g-C3N4 Photoelectrochemical sensor ZnIn2S4 g-C3N4 Photocatalytic H2 production ZnIn2S4 Photocatalytic H2 production Zn-AgIn5S8 g-C3N4 g-C3N4 Photocatalytic degradation Zn3In2S6 g-C3N4 Photocatalytic degradation CuInS2 g-C3N4(O) photocatalytic H2 production ZnIn2S4 rGO/g-C3N4 Photocatalytic H2 production ZnIn2S4 rGO/g-C3N4 Photoelectrocatalytic degradation ZnIn2S4

[88] [89] [93] [94] [95] [96–98] [92] [99] [100] [101–105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123, 124] [125] [126, 127] [128] [129] [130] [131] [132] [133]

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CuInS2 QDs alone. The as-prepared composite was used in the wastewater photocatalytic treatment and it was found that even 1% rGO- QDs composite effectively treated the pharmaceutical wastewater with 86.5% chemical oxygen demand (COD) removal efficiency.

4.2.2 Graphene oxide composites Unlike reduced graphene oxide (rGO) or graphene, graphene oxide (GO) is hydrophilic, water soluble and exhibits very low toxicity and biocompatibility. Hence major applications of GO find their place in biological application, sensing, drug delivery, etc. As previously mentioned, GO is prepared via Hummer’s method and it can be readily used for the preparation of GOQDs composite. However, in situ preparation is not preferred for the GOQDs composites as GO gets reduced to rGO at high temperatures and lose its hydrophilicity. In addition, in situ method often results in GO-QDs with low photoluminescence due to the surface defects introduced to QDs by GO which is not desirable for fluorescence applications. Hence GO-QDs composite is usually prepared by ex situ interactions. GO because of its hydrophilicity, can be used to convert hydrophobic QDs to hydrophilic QDs which then can be used for biological applications. Sheng et al. [113] prepared hydrophobic Zn doped AgInS2 QDs (AIZS) which was then converted into hydrophilic by oleyl amine modified GO via emulsion method (Fig. 4.10). While the oleyl amine groups in GO forms hydrophobic interaction with hydrophobic capping groups of QDs, the carboxyl and hydroxyl groups of GO made the whole composite water-soluble and biocompatible for cellular imaging. Liu et al. [110] reported water-soluble composite of GO with DNA (Ky2 aptamer) conjugated CuInS2 QDs where the DNA aptamer forms π-π stacking interactions with GO which resulted in quenching of the QDs fluorescence. The Ky aptamer binds well with the antibiotic kanamycin than GO and as a result GO was released from the composite which restores the QDs fluorescence. Hence this GO-KyCuInS2 QDs composite was used as a turn-on fluorescence sensor for the detection of kanamycin.

4.2.3 Carbon nanotubes composites Carbon nanotubes (CNT) are the cylindrical form of graphene with the single wall (SW) or multiwall (MW) consisting of several concentric SWCNTs bonded by van der Waals forces. CNTs are usually obtained by CVD techniques though other physical techniques are available [134]. Though CNT is

Fig. 4.10 (A) Schematic illustration of the procedure for synthesizing fluorescent AIZS-GO nanocomposites and their (B) bio-imaging application. Reproduced from Y. Sheng, X. Tang, E. Peng, J. Xue. Graphene oxide based fluorescent nanocomposites for cellular imaging. J. Mater. Chem. B 1(4) (2013) 512–521 with permission from The Royal Society of Chemistry.

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one-dimensional material compared to two-dimensional graphene, it exhibits good electrical and thermal conductivities like graphene. There are numerous reports on the synthesis and applications of CNT-QDs available in the literatures [135–139], however only very few of them involved ternary QD nanomaterials. For instance, Chai et al. [116] prepared MWCNT-ZnIn2S4 nanocomposites via in situ hydrothermal method as visible driven photocatalyst for the hydrogen production from water splitting. Later, Ahmad et al. [115] prepared SWCNT-CuInS2 QDs nanocomposite where octadecylamine-functionalized SWCNT were bonded to DDT capped CuInS2 QDs via hydrophobic interaction. The addition of CNT to the QDs increased the charge separation and electron transport.

4.2.4 Carbon dots composites Carbon dots (CDs) are zero-dimensional carbon materials with few nanometers in size where all carbon atoms are bonded to each other with sp3 hybridization [140]. Amorphous carbon dots that lacks quantum confinement are called as carbon nanodots (CNDs) while crystalline carbon dots with quantum confinement are called as carbon quantum dots (CQDs). These two are different from graphene quantum dots (GQDs) which are “dot sized nanosheets” consisting sp2 hybridized π-conjugated carbons. CQDs and GQDs exhibit fluorescence because of quantum confinement [141]. Depending on the precursors and the synthesis methods, these dots can be obtained [142–144]. CQDs are popular among carbon dots and can be easily obtained by hydrothermal synthesis involving pyrolysis of carbon rich sources such as polysaccharides, or plant sources such as leaves, seeds, etc. The obtained QDs are soluble in water because of the hydroxyl, carboxyl groups introduced in the dots during the reaction which is beneficial for biological applications. CQDs possess unique optical properties such as size dependent and excitation dependent luminescence. The oxygen groups on the surface of the dots causes defect emission. The higher the oxygen, higher the defects, and the shifting of the emission toward red region. In addition, CQDs exhibit up-conversion fluorescence property where long wavelength light is absorbed to emit short wavelength light. Since both CDs and QDs are zero-dimensional materials, their 0D-0D composite usually require dispersing medium like polymer or silica [145] to be incorporated or it can be obtained by successive or co-deposition on the substrates [146]. However, to the best of our knowledge, there has been no report available for 0D-0D CDs—ternary QDs composites. On the other

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hand, 0D CDs can be incorporated in the other dimensional materials (1D, 2D, 3D). In the bulk form, ternary I(II)–III–VI materials usually exhibit 3D microspheres morphology and very few studies are reported on ternary semiconductor-CDs composites. Recently, Xu et al. [117] prepared ZnIn2S4-CQDs composite via in situ hydrothermal methods where CQDs were added to the solution containing ZnIn2S4 precursors and heated at 160°C for 6 h. The obtained composite has CQDs trapped in ZnIn2S4 microspheres. The addition of CQDs increased the photocatalytic property of ZnIn2S4 under UV-Vis light because of the increased charge separation. In addition, as CQDs exhibited up-conversion, it absorbs NIR-light and emits short wavelength light which is used to excite ZnIn2S4 to generate more electron hole pairs. The ZnIn2S4-CQDs exhibited high photocatalytic performance compared to individual ZnIn2S4. Similarly, Liu et al. [118] prepared ZnIn2S4-CQDs composite for photocatalytic reduction of Cr(VI). Recently, Lei et al. [120] reported the preparation of CNDs and its incorporation in ZnIn2S4 microspheres in three different ways; (i) in situ hydrothermal as discussed above, (ii) ex situ hydrothermal where CNDs and ZnIn2S4 were separately prepared and heated together in hydrothermal condition, and (iii) stirring of as-prepared CNDs with ZnIn2S4. The results revealed that ZnIn2S4-CNDs composites prepared via in situ hydrothermal method exhibited high photocatalytic activity due to the high loading of CNDs in ZnIn2S4 compared to other methods. The composites of ZnIn2S4 with other carbon structures such as carbon sphere [121] and carbon nanofiber [122] have also been prepared in a similar way and reported to show the enhanced photocatalytic performance.

4.2.5 Graphitic carbon nitride composites Graphitic carbon nitride (g-C3N4) resembles graphite in structure with sp2 hybridization, but it is composed of alternate CdN bonds. g-C3N4 is usually obtained by pyrolysis of organic substances containing carbon and nitrogen bonds such as urea, melamine, ethylene diamine, etc. and few chemical synthesis routes are also attempted [147]. However, unlike graphite, g-C3N4 contains vacancy defects surrounded by nitrogen atoms (Fig. 4.11A). g-C3N4 exists in two major structures, that is, triazine and heptazine based structures depending on the number of nitrogen atoms that forms vacancy [148]. As a result of these defects, crystalline, long-range ordered g-C3N4 is difficult to prepare and it is often obtained as amorphous powders. Consequently, g-C3N4 is a poor conductor compared to graphite

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Fig. 4.11 (A) Postulated planar structure of g-C3N4 with characteristic vacancies surrounded by nitrogen atoms. (B) Ternary Z-scheme composite composed of g-C3N4, rGO and ZnIn2S4 for photocatalytic applications. Panel (A) reprinted from M. Inagaki, T. Tsumura, T. Kinumoto, M. Toyoda. Graphitic carbon nitrides (g-C3N4) with comparative discussion to carbon materials. Carbon 141 (2019) 580–607. Copyright (2019) with permission from Elsevier and (B) from T. Yu, W. Wu, L. Liu, C. Gao, T. Yang. Novel ternary p-ZnIn2S4/ rGO/ng-C3N4 Z-scheme nanocatalyst with enhanced antibiotic degradation in a dark selfbiased fuel cell. Ceram. Int. 46 (2019) 9567–9574. Copyright (2020) with permission from Elsevier.

which limits its optoelectronic applications. Nevertheless, g-C3N4 receives immense attentions in the field of photocatalysis because of its visible light absorption with its narrow bandgap. Much work has been reported for the use of g-C3N4 for the photocatalytic splitting of water for hydrogen production, degradation of organic pollutants in water under visible light [148]. As ZnIn2S4 ternary semiconductor is also a visible light driven photocatalyst, incorporation of ZnIn2S4 in the g-C3N4 could result in a photocatalytic heterojunction composite with higher efficiency due to the increased charge

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separation. Accordingly, such composites can be prepared through in situ precipitation methods. Ding et al. [126] prepared ZnIn2S4/ g-C3N4 composites where g-C3N4 was added to the ZnIn2S4 precursors dissolved in water followed by microwave treatment. The optimized ZnIn2S4/g-C3N4 composite exhibited better photocatalytic activity with increased rate of hydrogen production than the individual ZnIn2S4 and g-C3N4. Similarly, g-C3N4 composites with ZnIn2S4, CuInS2 and Ag-In-S have been reported for the photocatalytic hydrogen production, decomposition of organic pollutants and sensing of glucose (Table 4.2). Recently, Yu et al. [133] developed ternary hybrid composite ZnIn2S4/ rGO/g-C3N4 which served as Z-scheme nanocatalyst (Fig. 4.11B) for the degradation of antibiotics in a fuel system. The introduction of rGO in the composite further enhanced the charge separation in the p-type ZnIn2S4 and n-type g-C3N4 semiconductor heterojunction. As a result, the novel ternary composite exhibited much higher photocatalytic activity than unary ZnIn2S4, g-C3N4, and binary ZnIn2S4/g-C3N4 for the degradation of triclosan antibiotic pollutant.

4.3 Summary and outlook Nanocomposites have found their applications in multi-disciplinary fields. Ternary semiconductor nanocomposites are emerging as better alternative to their cadmium or lead based binary analogues because of their relatively eco-friendly nature. In this chapter, we have discussed two different types of ternary material composites namely, polymer-based and carbon material-based composites. Their various synthesis methods and their applications were thoroughly investigated. These composites have been majorly used for photovoltaic, photocatalyst, sensing and biomedical applications. Though these ternary nanocomposites are promising potential candidates, their research is still at early stages and much works are needed to be carried out to investigate and improve their properties. For example, the power conversion efficiency of photovoltaic devices prepared by using ternary I–III–VI nanocomposites are quite low compared to that of binary II–VI nanocomposites. ZnIn2S4-graphene nanocomposites appear to be a good eco-friendly photocatalyst to produce clean renewable hydrogen energy from water splitting. However, their H2 production rate is low because of characteristic microsize of ZnIn2S4. Recent research developments [149–151] have promised the synthesis of ZnIn2S4 in nanoscale sizes which

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can be used to enhance photocatalytic performances because of increased active catalytic surfaces. While the graphene oxide increases the aqueous stability of the embedded QDs in its matrix, on the other hand, it significantly quenches the QDs fluorescence which is still one of the problems that needs to be addressed on the use of graphene oxide-QDs composites for effective bioimaging applications. Hence to mitigate these issues and improve the performance, it is necessary to optimize the composites according to their applications. Specifically, the effect of size, shape, concentration of nanomaterials, dispersion in the composites and the relationship between these factors with the mathematical model are the key areas to be focused.

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CHAPTER FIVE

Characterization techniques for ternary I–III–VI quantum dots Abstract Cd-free quantum dots (QDs) have been considered as excellent and cost-effective materials for different applications from biological to sensor, water purification, fabrication of devices, to mention a few. Recently, I–III–VI QDs have attracted tremendous interests because of their unique properties such as low toxicity, tunable optical absorption, and near-infrared fluorescence emission. These properties have been investigated with different characterization techniques. This chapter focuses on the various characterization tools used for determining the optical and structural properties of I–III–VI QDs such as ultraviolet-visible spectroscopy (UV-Vis), photoluminescence spectroscopy (PL), X-ray diffraction (XRD) technique, transmission electron microscopy (TEM), dynamic light scattering (DLS) technique, X-ray photon electron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR). Finally, the chapter ends with the challenges and future perspectives on the characterization techniques of I–III–VI QDs. Keywords: Ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, Photoluminescence spectroscopy, X-ray diffraction, Dynamic light scattering, X-ray photon electron microscopy

5.1 Introduction Over the last few years, I–III–VI QDs because of their unique optoelectronic properties, low toxicity, environmentally friendliness, low cost, and easy processing have generated enormous interest in scientific research [1–5]. Because of these excellent characteristics, I–III–VI QDs have been useful for various high-performance applications which includes solar cells, sensors, photocatalysis, light emitting diode (LED), drugs delivery, bioimaging and therapy [5–12]. These properties are known via the use of several characterization techniques or instrumentation. This has opened up our understanding on the optical, chemical, structural, and magnetic properties of these I–III–VI QDs. Some of these techniques includes X-ray diffraction technique, ultraviolet-visible (UV-Vis) spectroscopy, photoluminescence spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray photon electron spectroscopy (XPS), and Fourier Ternary Quantum Dots https://doi.org/10.1016/B978-0-12-818303-8.00007-1

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transform infrared spectroscopy (FT-IR), to mention a few [12–15]. This chapter is a general description on different characterization techniques used for I–III–VI QDs.

5.1.1 Ultraviolet-visible (UV-Vis) spectroscopy UV-Vis spectroscopy is a qualitative and nondestructive technique which measures the absorption or reflectance properties of an optical material in the UV-Vis spectral region as seen in Fig. 5.1. This technique measures electronic transitions from the ground state to the excited state of a molecule after absorption of electromagnetic waves in the ultraviolet and visible region. Thus, π-electrons or nonbonding electrons molecules (n-electrons) can absorb the energy in the form of UV-Vis waves to excite these electrons to higher anti-bonding molecular orbitals. The more easily excited the electrons, the longer the wavelength of light that can be absorbed [17]. In semiconductors, the ground state and excited states are replaced by valence band and conduction band. Thus, this technique has been widely used for characterization of ternary QDs. These QDs exhibit different absorption spectra based on their chemical compositions and structures [18]. For example, CuInS2 (CIS) core shows broad absorption spectra from 400 to 550 nm as shown in Fig. 5.2A [19]. Controlling the [Cu]/[In] ratios enable the tuning of the absorption spectrum of CIS QDs. By increasing the composition of Cu, CIS core exhibits absorption spectra toward visible region from 400

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Fig. 5.1 Schematic diagram of UV-Vis spectrometer [16].

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Wavelength (nm)

Fig. 5.2 Absorption spectra of (A) CIS QDs, (B) CIS/ZnS QDs at various [Cu]/[In] molar ratios [18], (C) AIS QDs at different reaction time, and (D) AISe and AISe/ZnSe.

to 600 nm as shown in Fig. 5.2B [19]. In comparison to CIS core absorption spectra, the CuInS2/ZnS core/shell QDs absorption spectra are blue shifted. This blue shift is attributed to the cationic interdiffusion between the CuInS2 core and the wide band gap of the ZnS shell [19–22]. Other common I–III–VI QDs which include CuInSe2 [23], AgInS2 (AIS) [24], and AgInSe2 (AISe) [25] show similar UV-Vis absorption properties like CIS QDs Fig. 5.2C and D [26].

5.2 Photoluminescence (PL) spectroscopy Photoluminescence spectroscopy (PL) is a very useful, contactless technique used to analyze the emission properties of fluorescent materials. Fluorescence emission is the most important characteristic of TQDs. In a PL process, light is directed onto a material, where it is absorbed and imparts excess energy into the sample. By emitting the light, the sample can dissipate this excess energy. In other words, photoluminescence can be seen as the spontaneous emission of light from a material under optical excitation [27]. Thus, because of their outstanding fluorescence properties, I–III–VI QDs have been used for many high-performance optical applications like bioimaging, photodetectors, and photocatalysis, to mention a few. In comparison to the fluorescence emissions behavior of Cd-based QDs, the PL

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emission spectra of I–III–VI QDs show broad emission as seen in Fig. 5.3 [28, 29]. Core/shell TQDs have better quantum yield and higher emission lifetime compared to their corresponding core QDs materials [30, 31]. As seen in Fig. 5.4A and B, core/shell TQDs materials have better PL emission compared to their core materials. As shown in Fig. 5.4C, the emission wavelength of CISe/ZnS QDs was tuned from 612 to 686 nm by increasing the amount of In in the Cu/In ratio. Furthermore, large stokes shifts and broad PL full width at half maximum (FWHM) were observed. In Fig. 5.4D, the emission wavelength of AgInSe/ZnS was tuned from 575 to 775 nm by increasing the amount of In in the Ag/In ratio. A blue shift in PL peak position is usually observed by I–III–VI QDs core materials after ZnS shell coating. This shift in wavelength has been attributed to the cationic interdiffusion between the core and the shell as mentioned earlier [32]. It was demonstrated that for CIS/ZnS QDs, the effective cationic exchange of Cu with Zn cations decreased the nonradiative recombination of the QDs by reducing the internal traps [33]. TQDs can also have emission at the near-infrared (NIR) region for biological applications. NIR emission of TQDs can be obtained by doping or functionalization of ternary QDs [34, 35]. For example, Pons et al. used CuInS2/ZnS core/shell QDs for NIR in vivo imaging of two regional lymph nodes. In this work, organic solvent was used to prepare CuInS2/ZnS under inert condition before ligand exchange process to make the material hydrophilic with emission at around 800 nm [36] (Fig. 5.5).

excitation photon

band gap energy

excited states nonradiative relaxation

conduction band

luminescence photon valence band

electrons Fig. 5.3 Working principle of photoluminescence spectroscopy.

Fig. 5.4 (A) PL spectra of AgInSe2 and AgInSe2/ZnSe QDs [26], (B) CIS and CIS/ZnS, (C) CISe/ZnS, and (D) AISe/ZnS core/shell QDs synthesized at different Cu/In and Ag/In ratios.

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Video monitor for sample alignment

X-rays

a Sc

red

tte

s ay x-r

Optics

X-ray source

Sample

2D detector Goniometer Fig. 5.5 Schematic diagram of XRD microscopy [37].

5.3 X-ray diffraction (XRD) technique X-ray powder diffraction is performed to analyze the molecular crystal structure and composition of ternary QDs. Sample is excited by beam of monochromatic X-rays which will be scattered by the sample atoms. The scattered X-rays interfere with each other. The crystallinity of the material is determined with the interference of the scattered X-rays by using Bragg’s Law [37]. As seen in Fig. 5.6A, CIS core show lattice planes of the tetragonal

Fig. 5.6 (A) XRD diffraction of CIS QDs and CIS/ZnS Core/shell QDs [35], (B) XRD of Gddoped CIS [38].

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crystal structure while its corresponding core-shell QDs exhibits similar diffraction peaks which are red shifted when compared with the corresponding peaks of the CIS QDs [39]. Doping strategy does not change the lattice planes of CIS but shift the diffraction peaks toward higher 2θ values compared with the diffraction peaks of pristine CIS as observed in Fig. 5.6B. This shift has been attributed to the increase in d-spacing because of the inclusion of ions having bigger size than Cu and In ions [35]. CuInSe2, AgInS2, and AgInSe QDs show several crystal structures majorly chalcopyrite. XRD technique is also used to determine particle size of I–III–VI QDs (D) using Scherrer equation. D ¼ 0:9λ=ðβ cos θÞ

(5.1)

where, λ, β, and θ represent the X-ray wavelength, full width at the half maximum (FWHM), and diffraction angle, respectively.

5.4 Transmission electron microscopy (TEM) TEM is a well-known imaging technique that uses beam of electrons as seen in Fig. 5.7. In TEM, a beam of electrons is transmitted through an ultra-thin specimen and interact with the specimen as it passes through. The beam of electron with very short wavelength are emitted from a gun at the top of a cylindrical column of about 2 m high. Electrons are emitted by the heated filament and accelerated with an anode voltage. A higher anode voltage will provide the electrons at higher speed with a smaller de Broglie wavelength according to the equation, λ ¼ h/mυ. The resolving power of a microscope is directly related to the wavelength of radiation, which is used to create a TEM image. The resolution increases with decreasing wavelength of radiation [41]. Fig. 5.8 shows the TEM image of CIS QDs with average particle size of 3.5 nm and d-spacing of 3.2 A˚ corresponding to (112) lattice fringes from diffraction peak at 27.9 degree [42]. While TEM is used to measure the particle size of QDs, it can also provide information about the size distribution of the corresponding QD materials. The inset in Fig. 5.9A and B show the particle size distribution of AgInSe2 and AgInSe2/ZnSe QDs with average particle size of 2.8 and 3.2 nm, respectively. The crystallinity of TQDs can be also investigated by looking at the lattice fringes through high resolution transmission microscope (HRTEM) as seen in Fig. 5.9A and B inset [26]. TEM instrument is also used to find the chemical composition of I–III–VI QDs as seen in

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First Condenser Lens

Second Condenser Lens Condenser Aperture

Sample Objective Lens Objective Aperture Select Area Aperture

First Intermediate Lens Second Intermediate Lens

Projector Lens Screen

Fig. 5.7 Schematic diagram of TEM [40].

Fig. 5.8 (A and B) Typical TEM images CIS QDs with different Cu/In ratios [39].

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Fig. 5.9 TEM images of (A) AgInSe2 QDs, (B) AgInSe2/ZnSe QDs, (Insets: HRTEM images showing crystallinity of the QDs and the corresponding particle size distribution). EDS patterns of (C) AgInSe2 QDs, (D) AgInSe2/ZnSe QDs [27].

Fig. 5.9C. This analysis was perfumed through Energy-dispersive X-ray spectroscopy (EDAX or EDS) attached to TEM equipment.

5.5 Dynamic light scattering (DLS) of I–III–VI QDs DLS is noninvasive technique used for measuring the hydrodynamic size distribution profile of I–III–VI QDs in a suspension (Fig. 5.10). DLS calculates the Brownian motion of the particles or molecule and relates this movement to an equivalent hydrodynamic diameter, with the motion of smaller particles being overestimated. In fact because of the Brownian motion of particles or molecules in suspensions, laser light is scattered at different intensities, and DLS measures the time-dependent fluctuations in scattering intensity created by constructive and destructive interference which come from the relative Brownian movements of the particles or molecules within a sample. Light scattering involves three domains based on a dimensionless size parameter, α which is defined as: α¼

πDP λ

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Sample Laser 45 or 100 mW θ

Photon Detector

Lens

Correlator

Lens

Fig. 5.10 Working principle of DLS [42].

where πDp ¼ circumference of a particle, and λ ¼ wavelength of incident light. Based on the value of α, these domains are: α ≪ 1: Rayleigh scattering (small particle compared to wavelength of light); α  1: Mie scattering (particle about the same size as wavelength of light); and, α ≫ 1: Geometric scattering (particle much larger than wavelength of light) [43]. Fig. 5.11 shows the size distribution of glycol-chitosan-coated mercaptoundecanoic acid—CIS/ZnS QDs with average particle size of 203.8  7.67 [45]. DLS measures the Brownian motion which is equivalent to hydrodynamic diameter. Thus, the hydrodynamic diameter from the DLS analysis is always larger than the particle size determined with XRD or TEM. For example, Martynenko used DLS to find the aggregated AIS/ZnS QDs with average particle size of 5.5 nm. In this work, the TEM image showed particle size varying from 3.0 (0.9) to 4.5 (0.5) nm [46]. Lui et al. through DLS showed monodisperse mercaptopropionic acid (MPA) modified CIS QDs with diameters ranging from 1 to 5 nm and average particle size of 3.20 nm (Fig. 5.11A) [47]. The hydrodynamic diameter (HD) of carboxymethylcellulose sodium salt (CMCel)-AIS QDs was smaller than the HD of poly-L-arginine (PolyArg)-(CMCel)-AIS nanoconjugates through DLS measurement as seen in Fig. 5.11B. In this

Fig. 5.11 (A) Size distribution of CIS/ZnS [36] (B) Plot of HD versus Zeta Potential of AIS-CMCel, AIS-CMCel_PolyArg and AIS-CMCel_Cys [44]. (C, D) hydrodynamic size distribution of CuInS2/ZnS after coating with 540 PEGMA and 1080 mL PEGMA, respectively.

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work, the reduction of hydrodynamic diameter of PolyArg-(CMCel)-AIS nanoconjugates, compared to (CMCel)-AIS HD were attributed to the electrostatic interactions between the positive charge PolyArg and the negative charge CMCel which will reduce the distances between CMCel chains [44]. In contrast to TEM, DLS measurement always show bigger particle size due to hydrodynamic diameter while TEM shows the net corresponding particle size. As seen in Fig. 5.11C and D, DLS measurements on aqueous samples of Poly (ethylene glycol) methacrylate (PEGMA)-coated CuInS2/ZnS QDs with 540 or 1080 mL showed hydrodynamic mean diameters of 114 nm and 122 nm, respectively, which indicates that the concentration of PEGMA dominates the hydrodynamic mean diameters of the QDs.

5.6 X-ray photon electron microscopy XPS spectroscopy (Fig. 5.12) is a nondestructive spectroscopic technique in which the energies of photoelectron injected by bombarding the sample with a beam of monochromatic X-rays are analyzed. In XPS technique, photons with specific energy will excite the electronic states of atoms which emit an electron. The photoexcited electrons give information that the photon energy of the incident photon is in excess of the binding of the electron for targeting atom nucleus while the remaining energy stands itself

photon source

energy analyser

• X-ray tube • UV lamp • Synchrotron

electron optics z

hν

–

e–

detector

sample y + x

Fig. 5.12 schematic diagram of XPS [48].

UHV - Ultra High vacuum ( p < 10–7 mbar

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as the kinetic energy of the photogenerated electrons. XPS was used to analyze the chemical elements of I–III–VI QDs (Fig. 5.13). The XPS graph show that the valence of the elements Ag, S, In, and Zn were +1, 2, +3, and +2, respectively, indicating that AgInS2 I–III–VI QDs were

Fig. 5.13 XPS spectra of Zn-AgInS2 QDs: the XPS spectra of (A) S(2p), (B) Ag(3d), (C) In (3d), (D) Zn(2p), (E) Cu (2p3/2), and (F) Cu (2p). Reprinted with permission from Y. Liu, M. Deng, T. Zhu, X. Tang, S. Han, W. Huang, Y. Shi ,A. Liuc, The synthesis of water-dispersible zinc doped AgInS2 quantum dots and their application in Cu2 + detection, J. Lumin. 192 (2017) 547–554. Copyright (2019) Elsevier.

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successfully prepared and doped with zinc [49]. In another development, Raevskaya group used XPS to observe the In to Ag ratio and the Zn to Ag ratio in AIS and AIS/ZnS. They also found that all the constituents of AIS and AIS/ZnS QDs were in their valence states Ag+, In3+, Zn2+, and S2 [50]. Yang et al. conduct XPS measurements in order to further determine the chemical states of mixed-ligand (MPA/l-cysteine (Cys) and Cys modified AIS/ZnS QDs. In this work, they noticed that Ag 3d, In 3d, Zn 2p and 2S peaks for MPA/Cys-AIS/ZnS QDs was blue shifted compared with Cys- AIS/ZnS QDs [10]. Kobosko et al. used XPS technique to find the ratio of element present in AIS and AIS/ZnS QDs. Their XPS results show that the stoichiometry of the QDs was (Ag1.2 In0.8 S2)0.67(ZnS)0.33. In this work, it was also reported through the XPS measurements show that AIS/ZnS QDs contained 2.4 times more Ag than Zn and 1.5 times more Ag than In [51].

5.7 Fourier transform infrared spectroscopy (FTIR) FT-IR spectroscopy analysis is used to investigate the chemical functional groups present in a sample (Fig. 5.14). FT-IR spectroscopy is the study of the interactions between electromagnetic (EM) fields in the infrared

Fig. 5.14 Schematic diagram of FT-IR spectrometer [52].

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region (IR) and matter. The molecular vibrations mainly couple with the EM waves in the infrared region. In other words, by absorbing IR radiation, molecule can be excited to higher vibrational state. The main objective of FT-IR spectroscopy analysis is to find the chemical functional groups in the sample. Characteristic frequencies of IR radiation are absorbed by different functional groups [53]. Fig. 5.15 shows FT-IR spectra of AgInSe2/ZnSe, Gelatin, and thioglycolic acid (TGA). In this work, FT-IR was used to demonstrate that AISe/ZnSe was functionalized with carboxyl and hydroxyl groups from TGA and gelatin, respectively. Both AISe and AISe/ZnSe QDs exhibit similar characteristic peaks which are CdO stretching, OdH, NdH stretching, SdH stretching, and NdH bending. These characteristics are functional groups of gelatins (CdO stretching, OdH, NdH stretching, and NdH bending) and TGA (SdH stretching and g CdO stretching) [26]. It is well known that the characteristics peaks observed in FT-IR for I–III–VI QDs belong to their corresponding ligands characteristics peaks. For example, the FTIR spectra of CIS QDs and its ligand molecule 6-mercaptohexanol (MCH) were reported showing similar characteristics peaks which are OdH and CdO stretching at 2945 cm1 and 1010 cm1, respectively, for MCH and slightly shifted at 3272 cm1 and 1049 cm1, respectively for MCH-CIS [5]. As seen in Fig. 5.15B, the CuInS2/ZnS spectra after PEGMA coating showed no new peak in the FT-IR spectrogram indicating that the coating process occurred via physical adsorption without a chemical reaction [54].

5.8 Conclusions To summarize, this chapter described the common techniques such as UV-Vis spectroscopy, PL spectroscopy, XRD, X-ray photon electron microscopy, TEM, FT-IR, and DLS microscopy. The principle and purpose of these methods for characterizing I–III–VI QDs were established in this chapter. Thus, understanding the excellent properties of I–III–VI QDs by using these tools is crucial for their applications in various highperformance applications. These methods give information on optical properties, structures, size, shape, and chemical compositions of III– VI QDs.

C=O

O-H

C-H C-S

XCH

XSH

XOH & XNH GN-H

AlSe/ZnSe TGA TGA

Gelatin 4000

(A)

%Transmitance

Transmittance (%)

PEGMA

3500

3000

2500

Xco Xco

2000

1500

CulnS2/ZnS

PEGMA coated CulnS2/ZnS

gelatin

1000

500

Wavelength (nm)

3500

(B)

3000

2500

2000

1500

1000

500

Wavenumber (cm–1)

Fig. 5.15 FT-IR spectra of (A) AISe/ZnSe, Gelatin and TGA [25] and (B) PEGMA, CuInS2/ZnS QDs, and PEGMA-coated CuInS2/ZnS QDs.

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CHAPTER SIX

Cytotoxicity of ternary quantum dots Abstract The use of ternary quantum dots (TQDs) for biological application is widely increasing. However, their cytotoxicity and their potential interference with cellular processes are primary issues in both in vitro and in vivo applications. This chapter will be looking at the cytotoxicity of different TQDs. The chapter starts by looking into different cytotoxicity assays which can be used to analyze the toxicity or biocompatibility of quantum dots with emphasis on the factors that can influence the cytotoxicity of quantum dots in general. This was followed by mechanisms which have been postulated to be responsible for ternary quantum dots cytotoxicity while the chapter ends with the future perspectives of TQDs in the biological applications. Keywords: Ternary quantum dots, Cytotoxicity, In vitro assay, In vivo assay, Toxicity mechanism

6.1 Introduction Ternary quantum dots (TQDs) are currently replacing conventional dyes for different biological application due to their distinctive features, sucsh as narrow emission spectra, size-tunable emission, broad absorption spectra, superior brightness, and durability to photobleaching [1, 2]. However, information about biocompatible profiling of TQDs still need to be elucidated. Cell cytotoxicity and proliferation assays are generally used for nanoparticles screening to detect whether the test particles have effects on cell proliferation or display direct cytotoxic effects [3, 4]. Cytotoxicity assays are broadly classified as in vivo and in vitro tests. The in vivo involves the cell-based assay. However, they are time-consuming and expensive and involve ethical issues. While in vitro methods are cell culture-based assay, which includes approaches for assessment of the integrity of the cell membrane and the metabolic activity of viable cells. In vitro toxicity tests are faster, convenient, and less expensive [5]. Different types of in vivo and in vitro tests will be discussed in Section 6.2 to show how different techniques can be employed to test quantum dots (QDs). Ternary Quantum Dots https://doi.org/10.1016/B978-0-12-818303-8.00011-3

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Cytotoxicity testing of QDs is of critical importance for their practical biological and biomedical applications [6]. However, QDs cytotoxicity can be mystifying because each individual type of QDs possesses its own unique physicochemical properties, which in turn regulates its impending toxicity or lack [7, 8]. The cytotoxicity of QDs is mostly attributed to the release of toxic ions such as heavy metals into the cytoplasm after the elimination of the ligand shell of QDs in the intracellular environment. Therefore, this parameter is highly dependent upon the composition of the QDs utilized. For example, ternary quantum dots prepared from CdHgTe using 3-mercaptopropionic acid as ligand was toxic when the quantum dots were exposed to mouse fibroblast cell line, due to the release of cadmium (Cd) ions into the cell [9]. Currently, TQDs are synthesized with metals such as copper (Cu) [2, 10–13], silver (Ag) [14–17], zinc (Zn) [18, 19] to replace toxic heavy metal like Cd and mercury (Hg). The high surface-to-volume ratio of QDs is also another concern, as many QDs surface atoms with unsaturated bonds are available to form free radicals. Different localization of the particles intracellularly in vesicles or extracellularly on the cell membrane may also involve different mechanisms of toxicity [20]. There are numerous factors which have been reported in many reviews that can influence cytotoxicity of quantum dots. This includes the core composition, size, color, presence of protective inorganic shell, nature and even the chirality of capping ligands, presence of surface modification, surface charge and colloidal stability, cell type and incubation conditions [2, 6, 8, 20, 21] (Fig. 6.1). Several mechanisms have been postulated to be responsible for QDs cytotoxicity. These include desorption of free metal ion (QDs core degradation), free radical formation, and interaction of QDs with intracellular components [21]. These mechanisms will be discussed later in Section 6.3 of this chapter. This chapter is divided into three sections the first section we will be discussing on different cytotoxicity assay. In the second section, different mechanisms associated with QDs cytotoxicity will be discussed while the last section will involve future perspective.

6.2 Toxicity assay 6.2.1 In vitro assay In vitro cell culture systems are major requirement to assess endpoints of cellular health such as growth, replication, and morphology following exposure to a test material. Although in vitro cytotoxicity assay using a mammalian cell culture has been adopted for safety evaluation in numerous national

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Fig. 6.1 Factors influencing cytotoxicity of quantum dots.

and international standards, the recommended testing methodologies vary, and accordingly, cytotoxicity results for the same device may vary according to the standards. The types of materials being tested ranges from solid to liquid to gas, single to multi-piece material(s), and stand-alone to combination products and thus, the process of test article preparation could significantly contribute to variability between datasets. The in vitro cytotoxicity assays can be divided into quantitative methods (e.g., tetrazolium (MTT/XTT), neutral red (NR), or colony formation assays (CFA)) and qualitative methods (e.g., elution, agar overlay/diffusion, or direct) [3, 22, 23]. Typically, several major cell types are used in vitro for testing including phagocytic, neural, hepatic, epithelial, endothelial, red blood cells, and various cancer cell lines. In each case, the specific cell line selected for in vitro assay is intended to model a response or phenomenon observed or sensitized by particles in vivo [23–25].

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The MTT assay is a commonly used colorimetric assay in TQDs to measure the cytotoxicity. The reagent used is 3-(4,5-Dimethylthiazol-2-yl)2,5-Diphenyltetrazolium Bromide. This reagent is yellow in color and is light sensitive. In MTT assay the reagent is reduced by mitochondrial succinate dehydrogenases in viable cells [3, 5, 22]. For example, Carvalho et al. used this technique to investigate AgInS2 conjugated to L-cysteine or polyL-arginine against HEK 293T and U-87 MG cells [26]. This colorimetric assay is far superior to many in vitro assay methods, as it is easy to use, and has a high reproducibility. The Trypan blue exclusion assay is used to investigate the number of viable cells present in a cell suspension [27]. It is based on the principle that live cells have intact cell membranes that exclude certain dyes, such as trypan blue, while dead cells do not. In this test, a cell suspension is simply mixed with dye and then visually examined to determine whether cells take up or exclude dye. This technique has been used in TQDs such as CuInS2 (CIS), to investigate the effect of chlorin e6 conjugated to CIS coated with hyaluaronic acid against mouse melanoma cell (B16F1) [11]. The neutral red assay is also used to measure cell viability. It has been used as an indicator of cytotoxicity in cultures of different cell lines. It this assay living cells take up the neutral red, which is concentrated within the lysosomes of cells [22, 28]. This method is not commonly used in the QDs cytotoxicity as assay. A flow cytometer can quickly perform many quantitative and sensitive measurements on each distinct cell within a large, heterogeneous population. The modern commercially available analytical instruments, which can be found in most hospitals, pathology laboratories, and cell biology research laboratories in the industrially developed countries, can now routinely measure fluorescence simultaneously at four different wavelengths, in addition to light scatter in two directions, at rates of thousands of cells per second [3]. Table 6.1 is showing some of the in vitro assay examples and their mode of action. 6.2.1.1 In vitro cytotoxicity in ternary quantum dots Most papers on TQDs are still based on the synthesis alone, with MTT assay being the widely used method to check the cytotoxicity of these TQDs (group I–III–VI2 elements). Among this group, CuInS2 (CIS) QDs are the most evaluated. They have been evaluated in different cell lines such as human mesenchymal stem cells and Mouse cancer cell line etc. CIS has been shown to be less toxic to many of this cell lines. Wang et al. evaluating CIS using MCF-7, Human cervical cancer cells (Hela) and HepG2

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Table 6.1 Different types of in vitro cytotoxicity assays and their mode of actions. In vitro assay Instrumentation used types Stain/dye Mode of action for quantitation

MTT

Trypan blue exclusion NR

Calcein AM

Tetrazolium conversion to insoluble purple formazan in live cells Blue dye stains dead cells Lysosomal uptake in live cells

Green fluorescent dye stains live cells Live or dead Green fluorescent dye stains live cells: Red fluorescent dye stains dead cells Cell titer-96 MTS conversion to Aqueous soluble purple formazan in live cells CellTiter-Blue A kit form of Alamar Alamar Blue blue Resazurin reduction to red fluorescent dye resorufin by live cells CytoTox one LDH enzymatically homogeneous reduces resazurin into membrane resorufin integrity Flow Green fluorescent dye cytometry with stains live cells: Red live/dead fluorescent dye stains dead cells

Mitochondrial Culture medium by a metabolism spectrophotometer; high throughput Cell membrane Lysosomal membrane Cell membrane Live, cell membrane; dead, nucleic acid Cell membrane

Cell membrane

Live, cell membrane; dead, nucleic acid

Point signal (cell) by microscope Culture medium by a spectrophotometer; high throughput Point signal by microscope or fluorescence by spectrophotometer

Culture medium by a fluorescent spectrophotometer; high-throughput

Culture medium by a fluorescent spectrophotometer; high-throughput Point signal; automatic counter; high throughput

cell lines, demonstrated that CIS has limited impact on the cell viability, and most of the cells still survived (>70%) after the treatment even at 500 μM (Fig. 6.2C) [31]. Yong et al. increased the concentration of CIS to 1 mM using HeLa cells, and this showed similar results with Wang et al. report. As an extension of this work HUVEC cells was also used and similar high cell viability was also reported [17].

A

B

80 60 40 20 0 Control 0.03125 0.0625 0.125 0.25 0.5 Concentration (mg mL–1)

C

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0.8 1.6

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6.4 12.8

Concentration (mg/ml)

D

1.0 Cell viability (%)

Cell viability (%)

60

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100 80 60 40 20 0

CIS

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0 Control

1.0

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CIS@ZnS

100 Cell viability (% of control)

Cell viability (%)

100

0.8 0.6 0.4 0.2 0.0

0

1.5 3.75 7.5 15 30 60 120 250 500 CIS-ZnS-TTAB (mM)

0 0.04 0.4 4 40 400 Concentration of the loaded QDs (mg/mL)

Fig. 6.2 Cell cytotoxicity of CIS/ZnS in (A) BEL-7402 [29], (B) HEK-293 [30] (C), HepG2,HeLa, and MCF-7 [31], and (D) HELF [12].

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Chen et al. (2015) used human hepatoma (HepG2) cell line to evaluate the cytotoxicity of CIS/ZnS QDs using MTT assay. At concentrations of 0– 40 μg/mL QDs, they observed low cytotoxicity of less than
25% making the material suitable for bioimaging application [10]. Later, the cytotoxicity and in vitro phototherapy efficacy of the CIS/ZnS QDs in the mouse mammary carcinoma cell line 4 T1 was performed by Lv et al., at a concentration of 200 μg/mL of QDs. The result showed 90% cell viability which suggested low cytotoxicity [32]. Recently Zhang et al. assessed the cytotoxicity of CIS/ZnS QDs on BEL-7402 cells (human hepatoma cell line) using an MTT assay (Fig. 6.2A). The viability of the BEL-7402 cells maintain a high level (over 85%) upon addition of CIS/ZnS QDs up to 0.25 mg/mL. However, the cells viability was merely above 60% when the concentration of CIS/ZnS QDs was increased to 1.0 mg/mL [29]. Mir et al. also evaluated CIS and CIS/ZnS QDs against human kidney embryo (HEK-293 cells) (Fig. 6.2B). There was no significant cytotoxicity seen with the group treated with 100 and 200 μg/mL. In addition, cytotoxicity of QDs was negligible even after treating the cells at high concentration of nanoparticles (3.6 mg/ml). Core-shell QDs are highly biocompatible than core QDs. The obtained IC50 values for CIS and CIS/ZnS QDs were found to be 2.2 and 8.5 mg/mL. This directly signifies the very good biocompatibility of QDs after formation of core-shell structures [30]. In another development, Deng et al. used MTT assay to investigate the oil-soluble CIS/ZnS QDs functionalized with biodegradable folatemodified N-succinyl-N0 -octyl chitosan (FA-SOC) micelles cytotoxicity on human embryonic lung fibroblast (HELF) cells. They tested different ranges of concentrations from 0.004 to 400 μg/mL. The result showed that the as-synthesized CIS/ZnS-FA-SOC was not toxic to the HELF cells, however, there was a decrease in cell viability by 5% to 15% when the concentration of CIS/ZnS-FA-SOC was increased from 40 to 400 μg/mL (Fig. 6.2D), which suggest that the material may be toxic at the high concentrations [12]. In a recent development CIS/ZnS stabilized with glutathione (GSH) and sodium citrate has been reported to be biocompatible at 500 μg/mL against normal kidney fibroblasts (BHK21) and leukemia cancer cell line (THP-1) using WTS assay [2]. All these results demonstrated that CIS and it derivative cytotoxicity is concentration dependent. Tan et al. examined the cytotoxicity of the multi-dentate polymer (MDP)-capped AgInS2 (AIS) QDs before applying them for biological imaging (Fig. 6.3A). Their results showed that at a high concentration of

120

24 h 48 h

100 80 60 40 20 0

B 120 Cell Viability (%)

Cell Viability (%)

A

Control

0.05

0.01

80 60 40

0.1

0.5

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Control 10

D

60 40 20

25 50 100 150 300 Concentration (mg/mL)

E

CZIS/ZnS QDs CZIS QDs CdTe QDs

100

Cell Viability (%)

CZIS/ZnS QDs

80

0

AIS/ZnS@OA-FA (MCF7)

20

Cell Viability (%)

Cell Viability (%)

100

AIS/ZnS@OA (MCF7)

AIS/ZnS@OA-FA (HeLa)

100

Concentration (mg/mL)

C

AIS/ZnS@OA (HeLa)

80 60 40 20

22.5

55 110 165 Concentration (mg/mL)

220

0

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SMMC-7721

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MCF-7

A549

80 60 40 20

2

4 6 8 Incubation time (h)

24

0

0

10

25 50 100 250 500 Concentration (mg/mL)

1000

Fig. 6.3 Cell viability of AIS/ZnS using MTT assay in (A) HeLa [17] and (B) HeLa and. MCF7 cells [15]. (C) Cell viability of CZIS/ZnS using MTT assay in Hep-G2. (D) Effect of incubation time on the cytotoxicity of CZIA and CZIS/ZnS against Hep-G2 [18], (E) Cell viability assay of SMMC7721, MCF-7 and A549 cells incubated with various concentrations of AgInSe2-ZnSe QDs [14].

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1.0 mg/mL, over 90% of the cells survived compared to the control group [17]. AIS-ZnS QDs encapsulated with oleylamine (AIS/ZnS@OA) was also evaluated for cytotoxicity by Fahmi et al. In their study, MTT assay was used to evaluate (i) cytotoxicity of the as-synthesized AIS/ZnS@OA on HeLa and MCF7 cancer cells and (ii) the efficacy of adding folic acid (FA) on AIS/ZnS@OA (Fig. 6.3B). They found out that the amount of live HeLa cells after incubation with both AIS/ZnS@OA and AIS/ZnS@OA-FA was still over 80% even when the concentration was increased up to 300 μg/mL. The same result was also reported for MCF7 cancer cells where both AIS/ ZnS@OA and AIS/ZnS@OA-FA caused no harm to the living cells [15]. Jiao et al. also reported that AIS stabilized with polyethyleneimine and GSH is also biocompatible with HeLa showing cell viability that is over 70% even when the concentration reached 45 μM [33]. AISe/ZnSe has also been shown to be nontoxic towards noncancerous normal kidney cell line (BHK21) with cell viability above 75% even at 100 μg/mL concentration [16]. The cytotoxicity of Cu-doped Zn-In-S/ZnS (CZIS/ZnS) was also carried out by Jiang et al., using MTT assay on Hep-G2 liver cancer cells (Fig. 6.3C). They tested the different concentration of CZIS/ZnS QDs ranging from 22.5 to 220 μg/mL. In their results they were able to show that CZIS/ZnS QDs was less toxic with a cell viability that is >90%. They also looked at cell exposure time from 2 to 24 h at 230 μg/mL of CZIS/ZnS QDs and reported that even after 24 h of exposure only 10% decrease in cell viability was observed (Fig. 6.3D). The doped core ZIS also showed similar results [18]. In another current development, Zikalala et al. had shown that ZnInS (ZIS) capped with thioglycolic acid (TGA) and gelatin were biocompatible towards both BHK-21 and HeLa cells even at 1000 μg/mL concentration [19]. Cell viability assay of Human hepatoma cells (SMMC-7721), breast carcinoma cells (MCF-7) and lung cancer cells (A549) cells incubated with various concentrations of AgInSe2-ZnSe QDs was reported by Che et al. using MTT assay. The cells were incubated for 24 h with AgInSe2-ZnSe QD concentrations ranging from 0 to 1000 μg/mL (Fig. 6.3E). Their results showed decrease in cell viability with increasing AgInSe2-ZnSe QDs concentration. At 100 μg/mL, the cell viability was over 95%, however at 1000 μg/mL, the cell viability decreased to approximately 85%. They concluded that the AgInSe2-ZnSe QDs can be a good alternative for future biological applications [14].

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The Ag and Zn TQDs have shown to be less toxic than Cu TQDs, however these materials have posed some challenges during synthesis and ZIS in particular has a low quantum yield. It is still early to give a solid conclusion on these materials because the toxicity depends on many factors such as the size, surface charge, metal ion, and surface modifications. Most of the TQDs have less toxicity at 100 μg/mL, which is a typical concentration for cellular staining with QDs under in vivo conditions. Nevertheless, more investigations still need to be carried out on these materials. In conclusion non-cadmium-based TQDs have been shown to be less toxic to many cell lines and can be applied in vivo testing to evaluate the toxicity in the real host model. The next sections will focus on in vivo assays and the mechanisms involved in the toxicity of the QDs. 6.2.1.2 In vivo assay The in vivo toxicity assay is an important procedure, it is done pre to clinical trials of any substance to be consumed or even applied by humans. This assay involves numerous testing in animal models mostly mice, hamsters, worms, pigs, and monkey to name a few. An in vivo toxicity study based on TQDs is very limited as many researchers only reported in vivo imaging without looking at the toxicity. The lack of information on the in vivo toxicity, especially the long toxicity is stopping the TQDs from clinical trials nonetheless there are few reports. 6.2.1.3 Ternary quantum dots, in vivo assays The CIS as a Cd free TQD has been widely investigated in in vitro assay as discussed in the last section, some researchers have already started looking at their impact in in vivo assay. Chen and co-workers investigated the fate, degradation, and exposure time of CIS, CIS/ZnS and both QDs coated with Chitosan in Caenorhabditis elegans (C. elegans) as a model organism in biology. Their aim was to use different oxidation states of copper and zinc ions of QDs to achieve chemical stability in C. elegans. They used X-ray absorption near-edge structure to find the oxidation state of CIS and CIS/ZnS QDs in various exposure times. Prior to the in vivo experiment they first investigated the intracellular uptake and cell viability tests in Hela and OECM-1 cancer cell lines using AlamarBlue assay to calculate the cellular viability and confocal microscopy to see the method of cell entry. They found that both the cell viability of HeLa and OECM-1 were more than 90% (Fig. 6.4A), which indicated that CIS/Chitosan and CIS/ZnS/Chitosan QDs are nontoxic to cells. The confocal microscopy showed that for Hela

Fig. 6.4 (A) The cytotoxicity of CIS/ZnS/Chitosan in Hela and OECM-1 cancer cell lines using AlamarBlue assay. (B) confocal microscopy imaging of CIS/ZnS/Chitosan in Hela and OECM-1 cancer cell lines [10]. (C) Images of C. elegans exposed to CIS/ZnS/Chitosan. (D) Histology of RALNs resected after 7 days post injection [13].

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cells, the QDs entered into the cells and localized around the nucleus, whereas in OECM-1, the QDs did not only enter the cell membrane but also entered into the nucleus (Fig. 6.4B). Their results showed that the QDs can be applied in biological systems and this was proved with the inoculation of the QDs in C. elegans. They exposed C. elegans to QDs in their food source and observed post to digestion at different time intervals of 12, 48, 72, and 96 h (Fig. 6.4B). At 72 h, they observed that QDs penetrated from the alimentary system to the reproductive system and localized at both distal gonad and the uterus [10]. Pons et al. investigated the effect of CIS/ZnS compared to notorious CdTeSe/CdZnS QDs when functionalized with phospholipid micelles, using female balb/c mice for imaging lymph nodes (LN). They administered CdTeSe/CdZnS QDs (20, 10, and 5 pmol) and CIS/ZnS QDs (100, 75, 50, and 20 pmol) in mice and sacrificed them 7 days post-injection. Right axillary LN and right lateral thoracic LN were removed and weighed for histological analysis. They reported that both QDs showed a clear dosedependent increase in weight of the Right axillary LN (RALN), reflecting the increasing toxicity of the QDs. However, the inflammation was more visible in the lower dosage of CdTeSe/CdZnS QDs than CIS/ZnS QDs. They added that the minimal dose of CIS/ZnS QDs needed to detect the RALN was about 50 times smaller than the onset of RALN inflammation [13]. The in vivo assays done on CIS, clearly showed that the Cd-Free TQDs are less toxic as discussed earlier, however more testing on these materials needs to be done, looking at the long-time exposure to animal models and it means of leaving the animal body after it has served its purpose. The AIS and ZIS TQDs in vivo and in vitro toxicity need to be explored, as they have shown to be less toxic than CIS.

6.3 Mechanism of QDs toxicity Mechanism of QDs toxicity is similar to all the other nanoparticles. As discussed earlier, it is due to the release of toxic ions such as heavy metals (Cd, Pb, Hg, Te, As, etc.) into the cytoplasm after the elimination of the QDs passivating agent into the intracellular or extracellular environment [34]. Therefore, it is important to consider parameters as such core composition, size, color, presence of protective inorganic shell, nature, and even the chirality of capping ligands, presence of surface modification, surface charge and colloidal stability, cell type and incubation conditions during

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the synthesis [6, 8, 20, 21, 35]. These parameters have been demonstrated by various research groups as the key to many nanoparticles toxicity and can also help in predicting the possible toxicity of the materials. The mechanisms involved in the quantum dots cytotoxicity are desorption of free metal ion (QD core degradation), free radical formation, and interaction of QDs with intracellular components [21]. These mechanisms are the common principles responsible for the toxic action of QDs and can also be used to predict the TQDs mechanisms by looking at the common traits of the material in hand [5]. The Metal ions cause toxic effects through several routes such as interference with DNA repair and substitution for physiologic Zn. This process is known as free metal ions desorption. Heavy metal ions increase oxidative stress; however, they do not directly generate free radicals. QDs with heavy metals are extremely reactive, hence, photoactivation of these QDs through visible or UV light leads to their oxidation. A light photon may excite the QDs and thus produces an excited electron that transfers to molecular oxygen, forming singlet oxygen. Reaction of singlet oxygen with water/other biological molecules results in the production of free radicals. Most of these QDs mechanisms have been found to generate oxidative stress and inflammation in both in vitro and in vivo assays. The oxidative stress is caused by oxidant-producing properties of QDs themselves along with their capability to stimulate generation of reactive oxygen species (ROS) as a part of cellular response to QDs, transition metal-based or transition metal contaminants QDs used as catalysts during the production of nonmetal QDs, relatively stable free radical intermediates present on reactive surfaces of particles and lastly the redox active groups resulting from functionalization of nanoparticles [36]. • • ROS such as superoxide anion (O• 2 ), hydroxyl (HO ), peroxyl (RO2), • and alkoxyl (RO ) radicals, as well as O2-derived nonradical species (H2O2) are widely known [36]. They are molecules containing one or more oxygen atoms and they are highly reactive than the normal molecular oxygen. ROS are found in a normal cellular metabolism and play vital roles in the stimulation of signalling pathways in cells in response to changes in intra- and extracellular environmental conditions. The major intracellular source of ROS is the mitochondrion; however, this happens due to sustained environmental stress [37, 38]. This process leads to ER stress, mitochondrial dysfunction, redox imbalance, lipid peroxidastion, protein oxidation, DNA damage, interference of signal transduction, which stimulate necrosis, apoptosis, autophagy, and pyroptosis (Fig. 6.5).

Reactive oxygen species Oxidative stress

Release of metal ion and special properties of QDs

Inflammation

Fig. 6.5 Cytotoxicity mechanism of quantum dots.

ER stress •Mitochondrial dysfunction •Redox imbalance •Lipid peroxidation •Protein oxidation •DNA damage •Interference of signal transduction Inflammatory cytokines Adhesion factors Chemotactic factors Activation or inhibition of signal transduction

Stimulation of necrosis, apoptosis, autophagy and pyroptosis

Cytotoxicity

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6.4 Conclusion and remarks Although non-cadmium-based TQDs have shown to be less toxic in cells and mice at the low injection dose used. A detailed acute and chronic toxicity study is needed to broadly determine the toxic effects of these new materials. Hence, the use of TQDs for in vivo imaging experiments is still limited to mice or other small animals at low doses. More in vitro assays need to be done especially on AIS and ZIS, to identify the mechanism of toxicity. This will assist in knowing the interaction of QDs with the plasma membrane, lysosomes, mitochondria, and the nucleus. The CIS QDs are well studied materials in the group of free heavy metals TQDs, but they still show some toxicity at higher concentrations.

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[12] D. Deng, Y. Chen, J. Cao, J. Tian, Z. Qian, S. Achilefu, Y. Gu, High-quality CuInS2/ ZnS quantum dots for in vitro and in vivo bioimaging, Chem. Mater. 24 (15) (2012) 3029–3037. [13] T. Pons, E. Pic, N. Lequeux, E. Cassette, L. Bezdetnaya, F. Guillemin, F. Marchal, B. Dubertret, Cadmium-free CuInS2/ZnS quantum dots for sentinel lymph node imaging with reduced toxicity, ACS Nano 4 (5) (2010) 2531–2538. [14] D. Che, X. Zhu, H. Wang, Y. Duan, Q. Zhang, Y. Li, Aqueous synthesis of high bright and tunable near-infrared AgInSe2–ZnSe quantum dots for bioimaging, J. Colloid Interface Sci. 463 (2016) 1–7. [15] M.Z. Fahmi, J.-Y. Chang, Potential application of oleylamine-encapsulated AgInS2ZnS quantum dots for cancer cell labeling, Proc. Chem. 18 (2016) 112–121. [16] O.S. Oluwafemi, B.M.M. May, S. Parani, N. Tsolekile, Facile, large scale synthesis of water soluble AgInSe2/ZnSe quantum dots and its cell viability assessment on different cell lines, Mater. Sci. Eng. C 106 (2020) 110181. [17] L. Tan, S. Liu, X. Li, I.S. Chronakis, Y. Shen, A new strategy for synthesizing AgInS2 quantum dots emitting brightly in near-infrared window for in vivo imaging, Colloids Surf. B: Biointerfaces 125 (2015) 222–229. [18] T. Jiang, J. Song, H. Wang, X. Ye, H. Wang, W. Zhang, M. Yang, R. Xia, L. Zhu, X. Xu, Aqueous synthesis of color tunable cu doped Zn–In–S/ZnS nanoparticles in the whole visible region for cellular imaging, J. Mater. Chem. B 3 (11) (2015) 2402–2410. [19] N.Z. Zikalala, P. Sundararajan, N. Tsolekile, O.S. Oluwafemi, Facile green synthesis of ZnInS quantum dots: temporal evolution of its optical properties and cell viability against normal and cancerous cells, J. Mater. Chem. C (2020), https://doi.org/ 10.1039/D0TC02098B. [20] I. Martynenko, A. Litvin, F. Purcell-Milton, A. Baranov, A. Fedorov, Y. Gun’ko, Application of semiconductor quantum dots in bioimaging and biosensing, J. Mater. Chem. B 5 (33) (2017) 6701–6727. [21] T. Jamieson, R. Bakhshi, D. Petrova, R. Pocock, M. Imani, A.M. Seifalian, Biological applications of quantum dots, Biomaterials 28 (31) (2007) 4717–4732. [22] G. Fotakis, J.A. Timbrell, In vitro cytotoxicity assays: comparison of LDH, neutral red, MTT and protein assay in hepatoma cell lines following exposure to cadmium chloride, Toxicol. Lett. 160 (2) (2006) 171–177. [23] E. Caballero-Diaz, M.V. Cases, Analytical methodologies for nanotoxicity assessment, TrAC Trends Anal. Chem. 84 (2016) 160–171. [24] C.F. Jones, D.W. Grainger, In vitro assessments of nanomaterial toxicity, Adv. Drug Deliv. Rev. 61 (6) (2009) 438–456. [25] R.C. Borra, M.A. Lotufo, S.M. Gagioti, F.D.M. Barros, P.M. Andrade, A simple method to measure cell viability in proliferation and cytotoxicity assays, Braz. Oral Res. 23 (3) (2009) 255–262. [26] I.C. Carvalho, A.A. Mansur, S.M. Carvalho, R.M. Florentino, H.S. Mansur, Lcysteine and poly-L-arginine grafted carboxymethyl cellulose/Ag-in-S quantum dot fluorescent nanohybrids for in vitro bioimaging of brain cancer cells, Int. J. Biol. Macromol. 133 (2019) 739–753. [27] M. Vidal, J. Granjeiro, Cytotoxicity tests for evaluating medical devices: an alert for the development of biotechnology health products, J. Biomed. Sci. Eng. 10 (09) (2017) 431. [28] G. Repetto, A. Del Peso, J.L. Zurita, Neutral red uptake assay for the estimation of cell viability/cytotoxicity, Nat. Protoc. 3 (7) (2008) 1125. [29] F. Zhang, P. Ma, X. Deng, Y. Sun, X. Wang, D. Song, Enzymatic determination of uric acid using water-soluble CuInS/ZnS quantum dots as a fluorescent probe, Microchim. Acta 185 (11) (2018) 499.

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[30] I.A. Mir, K. Das, T. Akhter, R. Ranjan, R. Patel, H. Bohidar, Eco-friendly synthesis of CuInS 2 and CuInS 2@ ZnS quantum dots and their effect on enzyme activity of lysozyme, RSC Adv. 8 (53) (2018) 30589–30599. [31] M. Wang, X. Liu, C. Cao, L. Wang, Highly luminescent CuInS 2–ZnS nanocrystals: achieving phase transfer and nuclear homing property simultaneously through simple TTAB modification, J. Mater. Chem. 22 (41) (2012) 21979–21986. [32] G. Lv, W. Guo, W. Zhang, T. Zhang, S. Li, S. Chen, A.S. Eltahan, D. Wang, Y. Wang, J. Zhang, Near-infrared emission CuInS/ZnS quantum dots: all-in-one theranostic nanomedicines with intrinsic fluorescence/photoacoustic imaging for tumor phototherapy, ACS Nano 10 (10) (2016) 9637–9645. [33] M. Jiao, Y. Li, Y. Jia, C. Li, H. Bian, L. Gao, P. Cai, X. Luo, Strongly emitting and long-lived silver indium sulfide quantum dots for bioimaging: insight into co-ligand effect on enhanced photoluminescence, J. Colloid Interface Sci. 565 (2020) 35–42. [34] X. Michalet, F. Pinaud, L. Bentolila, J. Tsay, S. Doose, J. Li, G. Sundaresan, A. Wu, S. Gambhir, S. Weiss, Quantum dots for live cells, in vivo imaging, and diagnostics, Science 307 (5709) (2005) 538–544. [35] G. Xu, S. Zeng, B. Zhang, M.T. Swihart, K.-T. Yong, P.N. Prasad, New generation cadmium-free quantum dots for biophotonics and nanomedicine, Chem. Rev. 116 (19) (2016) 12234–12327. [36] E. Frohlich, Cellular targets and mechanisms in the cytotoxic action of nonbiodegradable engineered nanoparticles, Curr. Drug Metab. 14 (9) (2013) 976–988. [37] K. Apel, H. Hirt, Reactive oxygen species: metabolism, oxidative stress, and signal transduction, Annu. Rev. Plant Biol. 55 (2004) 373–399. [38] M.L. Circu, T.Y. Aw, Reactive oxygen species, cellular redox systems, and apoptosis, Free Radic. Biol. Med. 48 (6) (2010) 749–762.

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CHAPTER SEVEN

Bioimaging and therapeutic applications of ternary quantum dots Abstract I–III–VI-based quantum dots have received tremendous attention over the past decade due to their excellent properties such as composition tunable fluorescence across the visible and near infrared region, low toxicity, high photostability, longer excited-state lifetime, etc. These salient features presented them as a potential candidate and safer alternative to the conventional cadmium based binary QDs for several biomedical and analytical applications. In this chapter, we have provided some fundamental concepts of imaging techniques and cancer therapy and summarized recent advances of the use of I–III–VI QDs as a fluorescent probe in these techniques. Furthermore, we also focused on the application of these fluorescent QDs in in vitro and in vivo cancer imaging as well as their utilization with other imaging contrast agents for multimodal imaging of tumor/organ. In addition, recent investigations on the phototherapeutic and drug delivery applications using these QDs for cancer treatment are also provided. The comprehensive collection of research works based on the application of these ternary QDs for bioimaging and therapy are provided in a tabular form. Keywords: Ternary QDs, Bioimaging, Therapeutic, Cytotoxicity, In vivo, In vitro

7.1 Fluorescence imaging Imaging plays a crucial role in clinical protocols thus providing morphological, structural, metabolic, and functional information of cells and tissues. Various biomedical imaging techniques such as optical imaging, computed tomography (CT), ultrasonography (USG), magnetic resonance imaging (MRI), positron emission tomography (PET) are being used in the diagnosis of diseases especially in cancer [1–3]. Among them, optical imaging is the one with the highest sensitivity and is in routine use at research institutions. Optical imaging instruments make use of absorption, fluorescence, or a scattering phenomenon of light to produce images [4,5]. Among them, fluorescence technique has emerged as one of the most sensitive imaging techniques [6,7]. Ternary Quantum Dots https://doi.org/10.1016/B978-0-12-818303-8.00006-X

Copyright © 2021 Elsevier Ltd. All rights reserved.

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Fluorescence occurs by the recombination of charge carriers in the excited fluorophore. In general, light source are used to excite the fluorophore. To obtain a high-quality image by a fluorescent based microscope, the ideal fluorophores should meet the following characteristics: bright fluorescence, adequate resistance to photobleaching, high molar extinction coefficient, longer excited state lifetime and large Stokes shift. Organic fluorophores such as rhodamine, fluorescein isothiocyanate, alexa are the most commonly used fluorophore in bioimaging. However, these organic fluorophores lack some of the above features. Fluorescent semiconductor QDs overcome many of these limitations and emerge as a useful alternative for studies [8]. The key advantages of using inorganic QDs over organic fluorophores are listed below. (i) The properly passivated inorganic QDs are thousands of times more photostable than organic dyes and are more resistance to photobleaching under continuous high-energy illumination [9]. On the contrary, organic dyes on continuous irradiation, loses their photoluminescence (PL) due to a combination of several processes such as photobleaching, photoisomerization, change of conformations by internal rotation, intramolecular transfer of an electron/proton. (ii) QDs possess large molar extinction coefficient about 10–50 times larger than that of organic dyes [8,10]. Therefore, QDs are able to absorb photons more than organic dyes at the same excitation photon flux (that is, the number of incident photons per unit area) making them brighter probes under photon-limited in vivo conditions where light intensities are severely attenuated by scattering and absorption. (iii) QDs have broad excitation spectra because its molar extinction coefficient gradually increases towards shorter wavelength. This allows the multicolor QDs to be simultaneously excited by a single high-energy light source thus eliminating the need for multiple excitation sources and reducing the cost of imaging instrumentation. In contrast, organic dyes have narrow excitation spectra and therefore unique wavelength is needed for excitation. It becomes more complicated when multiple organic fluorophores are used for labeling since it requires multiple excitation sources. (iv) QDs have longer excited state lifetime and emit light slowly than organic dyes. Most of the background auto-fluorescence is over by the time QDs emission occurs. Consequently, fluorescence signal by a QDs can be easily separated in a technique known as time-domain imaging. (v) The large Stokes shift of QDs can be utilized to improve its detection with sensitivity. Even red emitting QDs can be excited with blue/ ultraviolet (UV) lamp. This is unlikely in the case of organic dyes.

Bioimaging and therapeutic applications of ternary quantum dots

157

(vi) The QDs can be made in different sizes so that it can be internalized in cells through multiple mechanisms. This is useful in labeling different cellular targets. With the above excellent properties, different types of QDs have been synthesized and applied for fluorescent bioimaging. The conventional QDs made of binary elements from group II–VI and IV–VI such as CdSe, PbS have been widely investigated for bioimaging since their invention as these QDs can be tuned for emission across the visible and NIR region. There are numerous reviews available in the literature that employed these conventional QDs for cellular imaging [11–16]. However, the inherent heavy metal toxicity of these QDs has been a source of concern even though they are protected with thick shells like silica or enveloped by polymers. Hence, the shift towards materials that are toxic free like ternary I–III–VI QDs [17,18]. These QDs have emerged as alternate safer fluorescent materials which exhibits similar photophysical properties and excellent biocompatibility compared to conventional QDs.

7.2 In vitro cell imaging Cellular labeling is important in biomedical applications since it provides the information about the several molecular processes at the subcellular level. QDs can be delivered to various cell lines and applied as a fluorescent label. The intracellular delivery of QDs can be performed via specific or non-specific endocytosis route, a process whereby external material (here QDs) is internalized within the cell. Non-specific labeling is based on pinocytosis in which small particles are brought into cells within small vesicles. Pinocytosis can be facilitated [11, 12] by (i) direct microinjection of QDs; however, this process is time consuming and also limits the number of cells labeled, (ii) chemically creating pores in the cells; this method is “harsh” and mainly limited to "fixed cells," (iii) electroporation which uses electrical charge to physically deliver QDs through the membrane in both live and fixed cells; however, this method results in aggregation of QDs in the cytoplasm and generally results in widespread cell death. On the other hand, the specific labeling can be performed by a process called receptor-mediated endocytosis (RME). In this method, QDs are conjugated with biomolecules usually proteins or peptides which has high specific affinity to the cell receptors. When the QD conjugates are added to the cells, an affinity complex between the receptor and QD-bioconjugates occurs which internalizes

158

Ternary quantum dots

the conjugates inside the cells via clathrin or caveolin assisted RME. QDs once internalized will then act as a fluorescent label for cellular imaging. Most intracellular organelles such as endosomes and lysosomes are acidic (pH 4–6) and hence QDs are more likely to aggregate inside the cells and are often trapped in endocytotic vesicles. Hence the fluorescence of the internalized QDs in cytoplasm depends strongly on their pH stability which in turn depends on the surface capping of QDs. Generally, QDs capped with small ligands are internalized non-specifically while large QD-bioconjugates are internalized through RME. Shortly after introduction within the cell, the former gets aggregated, due to their poor stability in acidic conditions. This leads to PL quenching which limits the cellular imaging. On the other hand, sufficient protein or peptide coating over the surface of QDbioconjugates ensures the stability of QDs in the cellular environment and prevents the aggregation of QDs inside the cells. Consequently, QDs maintain their fluorescence significantly within the cells thereby offering bright and longtime imaging. Another major advantage in RME is the tracking of membrane receptor inside the cell. As the membrane receptor diffuses along with the QD-bioconjugate, its traffic pathways inside the cell can be identified by monitoring QDs fluorescence. It must be noted that various factors such as surface charge, hydrophobicity, size, shape, surface chemistry influence the interactions of cells with QDs and their viability [13]. Several membrane receptors such as transferrin, folic acid, Her2, and so on [19] have been labeled with the corresponding antibody conjugated QDs. Not only membrane receptors, the intracellular organelles and intranuclear proteins can also be labeled with the specific “QD marker”. Interestingly, different targets in the same cell can be detected with different colored QDs. QDs can also be employed to probe the cell division [20] as daughter cells itself transfer equal number of cellular components from mother cells. Furthermore, the migration of daughter cell can be tracked. Cancer cells proliferate greatly than normal cells during metastasis and hence their membrane receptors are over-expressed in the cancer cells. This allows them to uptake large numbers of QD-bioconjugates through RME whereas little or no conjugates are taken by normal cells. Thus, cancer cells can be specifically fluorescent labeled and tracked which is beneficial in cancer diagnosis and therapy. As I–III–VI QDs possess low toxicity compared to conventional II–VI QDs, attempts have been made to use I–III–VI based QDs especially CuInS2 and AgInS2 QDs for bioimaging applications. In the case of hydrophobic QDs prepared in organic medium, they will be first converted into water soluble QDs using surface modification for bioapplications (Table 7.1). For example, Liu et al. [21] synthesized dodecanethiol

Table 7.1 Database of bioimaging and therapeutic applications using I-III-VI based ternary QDs. Synthesis medium/ phase Targeting transfer Nanocrystal Capping agent agent Biological applications

MPA TGA

W W

CuInS2 CuInS2-ZnS

W W W

PAA, MAA AgInS2–ZnS Fe3O4@SiO2-CuInS2-ZnS MPA, BSA MPA CuInS2-ZnS

W

AgInSe2–ZnSe

MPA

W W

AgInS2 AgInS2-ZnS

PEI CMC

W

AgInS2

L-Cys

W W

AgInS/2ZnS Ag-In-Zn-S

and poly-LArg grafted CMC TGA MPA

W

CuInS2/ZnS

Cys and Thr

W

Zn-Ag-In-S

GSH, DHLAPEG2000-NH2

FA or RGD peptide

In vitro In vitro fluorescence imaging (HepG2) In vitro fluorescence multicolor imaging (HEPG2) In vitro labelling (baculoviral vectors) In vitro fluorescence imaging (HepG2) In vitro fluorescence imaging (RAW264.7 macrophages) In vitro fluorescence imaging (SMMC-7721) In vitro fluorescence imaging (HeLa) In vitro fluorescence multiplexed imaging (U-87 MG) In vitro fluorescence imaging (U-87MG, HEK 293T) In vitro fluorescence imaging (A549) In vitro fluorescence imaging (MCF-7, HEPG2) In vitro fluorescence multicolor imaging (A549) In vitro fluorescence targeted imaging (BEL-7402 or MDB-MB-231)

References

[85] [23] [86] [87] [25] [26] [24] [88] [89] [90] [91] [92] [93] Continued

Table 7.1 Database of bioimaging and therapeutic applications using I-III-VI based ternary QDs—cont’d Synthesis medium/ phase Targeting transfer Nanocrystal Capping agent agent Biological applications

References

W

Zn-Ag-In-S/ZnS

L-Cys

Anti-AFP

[94]

W

CuInS2@SiO2

APTMS

anti-PSCA

W W

Cu-Zn-In-S/ZnS Cu-In-Zn-S/ZnS

GSH GSH

anti-AFP APBA

W

CuInS2

GSH

RGD

W

Ag-Zn-In-S/ZnS

GSH

anti-AFP

W

Ag-Zn-In-S

GSH

anti-AFP

W

CuInS2

GSH

RGD

W W

Cu-In-S/ZnS Au@SiO2 @CuInS2/ZnS

GSH GSH

WGA anti-AFP

O➔W

Zn-Ag-In-S @SiO2

O➔W O➔W

CuInS2/ZnS Zn-Ag-In-S/ZnS

APTMS, mPEG2000 PEGMA MPA, TGA, sugar chains

In vitro fluorescence targeted imaging (Hep G2) In vitro fluorescence targeted imaging (PC-3M) In vitro fluorescence Imaging (Hep-G2) In vitro fluorescence targeted imaging (HeLa) In vitro fluorescence targeted imaging (HeLa) In vitro fluorescence targeted imaging (Hep G2) In vitro fluorescence targeted imaging (Hep G2) In vitro fluorescence targeted imaging (HeLa) In vitro fluorescence imaging (CAL-27) In vitro fluorescence targeted imaging (Hep G2) In vitro fluorescence imaging (NIH/3T3) In vitro fluorescence imaging (HepG2) In vitro fluorescence imaging (THP-1, J774.A1, HepG2)

[95] [96] [29] [97] [98] [99] [100] [101] [102] [103] [104] [105]

O➔W O➔W

AgInS2-ZnS (nanorods) CuInS2/ZnS

GO GSH

In vitro fluorescence imaging (NIH/3T3) In vitro fluorescence imaging (MCF10CA1a) In vitro fluorescence imaging (KB) In vitro fluorescence multiplex and life time imaging (RAW264.7) In vitro fluorescence imaging (A549) In vitro fluorescence imaging (NIH/3T3) In vitro fluorescence targeted 2D and 3D imaging (colon cancer stem cells) In vitro fluorescence imaging (Glioma C6)

O➔W O➔W

Ag-Zn-In-S/ZnS Zn-Ag-In-S @SiO2

MPA

O➔W O➔W O➔W

CuInS2/MnS/ZnS CuInS2-ZnS Ag-Ga-In-S

Pluronic F127 Pluronic F127 PDDA-LDL

O➔W

Zn-Cu-In-S/ZnS QDs

O➔W

AgInS2–ZnS

Hexanethiol, PMAT PMAO

FA

O➔W

AgInS2/ZnS

dodecanoic acid

FA

O➔W

Zn-Ag-In-S/ZnS

MES

R8 peptide

O➔W

Zn-Ag-In-Se

SOC

FA

O➔W

CuInS2/ZnS@SiO2

GPTMS

Tf

O➔W

Ag-In-S/ZnS

CTX

O➔W

CuInS2/ZnS

PEG-PLGA micelles PMAO

O➔W

AgInS2-ZnS

Oleyl amine

LTVSPWY peptide FA

In vitro fluorescence targeted (HepG2) In vitro fluorescence targeted (HeLa) In vitro fluorescence imaging Tissue-Derived Stem Cell) In vitro fluorescence targeted (Bel-7402) In vitro fluorescence targeted (HeLa) In vitro fluorescence targeted (U87) In vitro fluorescence imaging

[47] [22] [106] [107] [108] [109] [31] [110]

imaging

[27]

imaging

[111]

(Adipose

[112]

imaging

[113]

imaging

[114]

imaging

[115]

(SKBR3)

[116]

In vitro fluorescence imaging (HeLa and MCF-7)

[117] Continued

Table 7.1 Database of bioimaging and therapeutic applications using I-III-VI based ternary QDs—cont’d Synthesis medium/ phase Targeting transfer Nanocrystal Capping agent agent Biological applications

References

O➔W

Zn-Cu-In-S

[118]

O➔W

CuInS2-ZnS

O➔W

Zn-Ag-In-S/ZnS

O➔W

CuInS2/ZnS

W

AgInS2

W

CuInS2

W

Zn-Ag-In-Se

O➔W O➔W

CuInS2/ZnS CuInS2/ZnS

MPA

iRGD peptide In vitro fluorescence imaging (HeLa and SKBR3 cell lines and their threedimensional multicellular tumor spheroids) GSH anti-AFP In vitro fluorescence targeted imaging (HepG2) MPA R8 peptide In vitro fluorescence imaging (adipose tissue-derived stem cells) PASP-Na-ganti-Ki-67 In vitro fluorescence targeted imaging PEG-DDA (MD-MBA-231) In vitro and in vivo PAA-g-MEA In vivo NIR fluorescence imaging (mouse) RNase A In vivo NIR fluorescence imaging (mouse stomach) RGD In vitro fluorescence targeted imaging GSH, GSH, (U87MG) and in vivo NIR fluorescence DHLA-PEG1000Suc targeted tumor imaging (U87MG tumor bearing mouse) DHLA In vivo fluorescence imaging (mouse) In vivo NIR fluorescence imaging (lymph DPPE-PEG2000nodes in mice) (methyl ether)/ DPPE-PEG2000(COOH)

[119] [120] [121]

[35] [36] [122]

[123] [33]

O➔W

Cu-In-Se/ZnS

Dithiol sulfobetaine DSPE-mPEG2000

O➔W

Cu-In-Se/ZnS

O➔W

CuInS2/ZnS and AgInS2/ ZnS

Pluronic F127

O➔W

AgInS2

Pluronic F127

O➔W

AgInS2/ZnS

O➔W

CuInS2/ZnS

octylamine-PAAPEG750-(NH2) PMMA

O➔W

AgInTe2 (rods)

DSPC

O➔W

CuInS2-ZnS

MUA

O➔W

Zn-Ag-In-S @GO

GO

O➔W

CuInS2/ZnS

SOC

FA

In vivo NIR fluorescence imaging (lymph nodes in mice) In vivo NIR fluorescence imaging (mouse) In vitro fluorescence imaging (human macrophages) and in vivo NIR fluorescence tumor imaging (mouse) In vitro fluorescence targeted imaging (MDM) and in vivo NIR fluorescence targeted tumor imaging (RIF tumor bearing mouse) In vivo NIR fluorescence imaging (Mammary Tumor bearing mouse) In vivo NIR fluorescence imaging (mouse) In vivo NIR fluorescence imaging (mouse) In vitro fluorescence imaging (L929, Vero, MCF7) and Intravital fluorescence imaging (Zebrafish-Embryos) In vitro fluorescence imaging (SK-BR-3) and in vivo fluorescence imaging (SKBR-3 tumor bearing mouse) In vitro fluorescence targeted imaging (Bel-7402) and in vivo NIR fluorescence targeted tumor imaging (Bel-7402 tumor bearing mouse)

[34] [124] [21]

[125]

[126] [127] [128] [129]

[130]

[28]

Continued

Table 7.1 Database of bioimaging and therapeutic applications using I-III-VI based ternary QDs—cont’d Synthesis medium/ phase Targeting transfer Nanocrystal Capping agent agent Biological applications

O➔W

CuInS2/ZnS

O➔W

Zn-Ag-In-Se

O➔W

Zn-Cu-In-S/ZnS

O➔W

AgInSe2/ZnS

O➔W

CuInS2/ZnS

O➔W

CuInS2/ZnS

O➔W

CuInSe2/ZnS

DSPEmPEG5000, DSPE-PEG5000 (COOH) RGD-SOC micelles

FA

RGD

In vivo fluorescence, multiplex targeted tumor imaging (Panc-1 tumor-bearing mice)

In vitro fluorescence targeted imaging (MDB-MB-231) and in vivo NIR fluorescence targeted tumor imaging (MDB-MB-231 tumor bearing mouse) PMAO cRGD peptide In vivo NIR fluorescence targeted tumor imaging (U87MG tumor bearing mouse) OctylamineRGD peptide In vitro fluorescence targeted imaging modified PAA (MDA-MB-231) In vivo NIR fluorescence targeted tumor imaging (MDA-MB-231 tumor bearing mouse) HSA-PEG FA In vivo fluorescence targeted tumor imaging (HeLa tumor bearing mouse) BSA-PCL cRGD peptide In vitro fluorescence targeted imaging (HeLa, U87) and In vivo NIR fluorescence imaging (mouse) Amphilic (amino) CGKRK In vitro fluorescence targeted imaging thiol ligand, PEG peptide (MCF10CA1a, GFP-CT2A) and In vivo NIR fluorescence targeted tumor imaging (MCF10CA1a, GFP-CT2A tumor bearing mouse)

References

[44]

[131]

[132] [37]

[39] [30]

[40]

O➔W

Zn-Ag-In-Se/ZnS

SOC

RGD peptide

O➔W

Zn-Cu-In-Se/ZnS

TG, PME

cRGD

O➔W

CuInS2/ZnS

RGD

O➔W

Zn-Ag-In-Se/ZnS

MUA, glycolchitosan sulfobetainePIMA-histamine

O➔W

CuInS2–ZnS

MUA

FA

O➔W

CuFeS2/ZnS

Liposomes

macrophage membrane

PEG W

ZnCuInS/ZnS (CuInSexS2
x)/ZnS

MPH-PEG 400 PLGA

Invasin

W

Gd-Cu-In-S/ZnS

GSH

FA, APBA

cRGD

In vitro fluorescence targeted imaging (U87MG) In vivo NIR fluorescence targeted tumor imaging (U87MG tumor bearing mouse) In vitro fluorescence targeted imaging (U87MG) in vivo NIR fluorescence targeted tumor imaging (U87MG tumor bearing mouse) In vivo NIR fluorescence targeted tumor imaging (RR1022 tumor-bearing mouse) In vitro fluorescence targeted imaging (U87MG) and In vivo NIR fluorescence targeted tumor imaging (U87MG tumor bearing mouse) In vitro fluorescence imaging (pa1) and In vivo NIR fluorescence targeted tumor imaging (B16F10 tumor-bearing mouse) In vivo NIR fluorescence targeted tumor inflamed tissue imaging (H22 tumor bearing mouse) In vivo fluorescence imaging (mouse) In vivo whole animal fluorescence imaging and monitoring orally administered particles Multimodal imaging In vitro fluorescence/T1-weighted MR targeted imaging (HeLa)

[133]

[134]

[135] [136]

[137]

[138]

[139] [140]

[141] Continued

Table 7.1 Database of bioimaging and therapeutic applications using I-III-VI based ternary QDs—cont’d Synthesis medium/ phase Targeting transfer Nanocrystal Capping agent agent Biological applications

References

W

Ag-Mn-In-S/ZnS

GSH

[142]

O➔W

CuInS2–ZnS/MnS

CTAB

O➔W

Zn-Cu-In-Se/Mn-ZnS

DHLA-PEG

O➔W

CuInS2/ZnS@SiO2-Gd chelates

APS-DTPA

O➔W O➔W

Gd-Cu-In-S/ZnS Gd-Zn-Cu-In-S/ZnS

MPA BSA

O➔W

CuInS2/Mn-ZnS

DHLA-PEG2000

O➔W

Gd-Cu-In-S/ZnS

PEG2000-DS-SA polymeric lipid vesicles

HA

In vitro fluorescence/in vitro T1weighted MR imaging (B16F1 cells) In vitro fluorescence imaging (BXPC-3)/ in vitro T1-weighted MR imaging (cell lysates) In vivo NIR fluorescence/T1-weighted MR imaging (lymph nodes in mouse) In vitro fluorescence imaging (BXPC-3)/ in vitro T1-weighted MR imaging (cell lysates) In vitro fluorescence imaging (HeLa) In vitro fluorescence/T1-weighted MR imaging (HeLa) and In vivo NIR fluorescence/T1-weighted MR imaging (mouse) In vivo fluorescence/T1-weighted MR targeted tumor imaging (subcutaneous and intraperitoneal tumor bearing mouse) In vivo NIR fluorescence/T1-weighted MR tumor targeted imaging (U87 tumorbearing mice)

[143]

[51] [144]

[145] [146]

[147]

[50]

O➔W

CuInS2/ZnS-Gd chelates

DTDTPA

O➔W

CuInS2/ZnS-Gd chelates

PMAO-DTPA

FA

O➔W

CuInS2/ZnS-Gd chelates

BSA-DTPA

anti-CD133 monoclonal antibody

O➔W

Fe3O4-CuInS2 @SiO2-Gd APS, DTPA chelates

W

[64Cu]CuInS/ZnS

GSH, mPEG5000SH

W

AgInS2/ZnS

GSH

W

CuFeSe2

PTMP
PMAA

W

CuInS2/ZnS ZnIn2S4

MPA

RGD

ALA

In vitro fluorescence imaging (HeLa) and in vivo NIR fluorescence/T1-weighted MR targeted tumor imaging (HeLa cells bearing mouse) In vitro fluorescence targeted imaging (HeLa) In vitro fluorescence/T1-weighted MR targeted imaging (SU2 stem cells) and In vivo fluorescence/T1-weighted MR targeted imaging (CD133 + tumor bearing mouse) In vitro fluorescence targeted imaging (BXPC-3)/in vivo T1/T2 weighted MR targeted tumor imaging (pancreatic adenocarcinoma bearing mouse) In vivo PET/NIR CRET targeted tumor imaging (U87MG tumor-bearing mice) Phototherapy In vitro fluorescence imaging (Candida fungal cells) in vitro PDT (Candida fungal cells) in vivo SPECT/CT, PA, CT T1weighted MRI imaging (4T1 tumor bearing mouse) In vitro PTT (4T1 cells) In vitro NIR PDT (MCF-7) In vitro PDT (HEPG2)

[54]

[52] [53]

[56]

[66]

[76]

[60]

[74] [75] Continued

Table 7.1 Database of bioimaging and therapeutic applications using I-III-VI based ternary QDs—cont’d Synthesis medium/ phase Targeting transfer Nanocrystal Capping agent agent Biological applications

References

O➔W O➔W

CuFeS2 CuInS2-ZnS

mPEG2000-SH mPEG2000-DSPE

[78] [81]

O➔W

CuInS2/ZnS-rGO

Liposome

O➔W

Fe3O4@PB@PEI@ CuInS2-ZnS

PEI

HA

In vitro PTT (HeLa) In vitro PDT/PTT (4T1) In vivo NIR fluorescence imaging (4T1 tumor bearing mouse) In vivo MSOT imaging (4T1 tumor bearing mouse) In vitro tumor PDT/PTT (4T1 tumor bearing mouse) In vitro fluorescence imaging (Eca-109) In vivo fluorescence tumor imaging (Eca109 tumor bearing mouse), in vitro phototherapy (Eca-109), in vitro tumor PTT/PDT(Eca-109 tumor bearing mouse) In vitro fluorescence targeted imaging (HeLa), In vivo NIR fluorescence targeted tumor imaging (HeLa tumor bearing mouse), In vitro T2 weighted MR targeted imaging (HeLa) In vivo T2 weighted MR targeted tumor imaging (HeLa tumor bearing mouse) In vitro targeted PTT (HeLa) In vivo targeted tumor PTT (tumor bearing mouse)

[79]

[80]

O➔W

Ce6-CuFeS2

BSA

W

CuInS2 QDs

MPA

W

UCNP/AgInS2-ZnS

GSH, MEA

W

CuInS2

L-cysteine

W

CuFeSe2

PTMP-PMAA

O➔W

CuInS2-ZnS/Fe3O4@ SiO2

APS, MPEGS

FA

In vitro fluorescence targeted imaging (HeLa), in vitro targeted PDT/PTT (HeLa) Drug Chemotherapy/gene delivery MUC1 DNR In vitro fluorescence targeted aptamers imaging (PC-3M) In vitro chemotherapy (PC3M) FA DOX In vitro fluorescence targeted imaging (HeLa) In vitro chemotherapy delivery PGA DOX In vitro NIR fluorescence imaging (PC-3M, HEPG2) In vitro chemotherapy (PC3M, HEPG2) In vivo SPECT/CT, PA, IR NH2-PEG2000- DOX thermal imaging (4T1 tumor NH2, bearing mouce) In vivo Chemo/PTT Pt(IV) In vitro fluorescence imaging anticancer (MCF-7) drug In vitro chemotherapy (MCF-7)

[148]

[149]

[150]

[151]

[152]

[55]

Continued

Table 7.1 Database of bioimaging and therapeutic applications using I-III-VI based ternary QDs—cont’d Synthesis medium/ phase Targeting transfer Nanocrystal Capping agent agent Biological applications

References

O➔W

AgInS2-ZnS/MnFe2O4

BSA

[83]

O➔W

MB@CuInS2–ZnS

PEI

O➔W

AgInS2-ZnS

MPA

O➔W

CuFeS2

CTAB

FA

PEI,

DOX

In vitro fluorescence targeted imaging (HeLa) In vitro T2 weighted MR targeted imaging (HeLa) In vitro DOX delivery (HeLa) pDNA In vitro NIR fluorescence imaging (HeLa) In vivo ultrasonogram (rabbit kidney) In vitro delivery of pDNA (HeLa) siRNA In vitro fluorescence imaging (U87, hMSCs) In vitro delivery of siRNA cis-Pt pro- In vivo NIR Thermal/PA drug imaging (A549 tumor bearing mice) In vivo chemotherapy/PTT (A549 tumor-bearing mouse)

[69]

[84]

[153]

O➔W

Gd-Zn-Cu-In-S/ZnS @SiO2

APTMS, PEG

CD326 DNA aptamer

DOX

Ex vivo NIR fluorescence [154] imaging (4T1 tumor-bearing mouse), in vivo T1 weighted tumor targeted imaging In vivo targeted drug delivery (4 T1 tumor-bearing mouse)

W, Aqueous medium; O➔W, phase transfer from organic medium to aqueous medium; MPA, 3-mercaptopropionic acid; PAA, polyacrylic acid; MAA, mercaptoacetic acid; BSA, Bovine serum albumin; CMC, Carboxymethyl cellulose; PEI, polyethyleneimine; L-Cys, L-Cystein; L-Arg, L-Arginine; Thr, threonine; TGA, thioglycolic acid; GSH, glutathione; DHLA, Dihydrolipoic acid; PEG, polyethylene glycol; FA, folic acid; AFP, alpha-fetoprotein; APBA, 3-aminophenyl-boronic acid; APTMS, (3-amino propyl)trimethoxysilane; PSCA, prostate stem cell antigen; mPEG, methoxyPEG; WGA, wheat germ agglutinin; PEGMA, poly(ethylene glycol) methacrylate; GO, graphene oxide; PDDA, poly(diallyldimethylammonium chloride); LDL, low-density lipoprotein; PMAT, poly(maleic anhydride-alt-1-tetradecene); MES, 2-mercaptoethanesulfonic acid; SOC, N-succinyl-N0 -octyl-chitosan; PMMA, poly(methyl methacrylate); GPTMS, (3-glycidyloxypropyl)-trimethoxysilane; PLGA, poly(lactic-co-glycolic acid); Tf, Transferrin; CTX, chlorotoxin; PASP-Na-g-PEG-DDA, poly(aspartate)-Na-graft-poly(ethylene glycol)-dodecylamine; MEA, 2-mercapto-ethylamine; RNase A, ribonuclease A; Suc, Succinic acid; DPPE, dipalmitoyl phosphotidylethanolamine; DSPE, 1,2-distearoyl-sn-glycero-3-phosphoethanolamine; DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine; MUA, 11-mercaptoundecanoic acid; MPH, 6-sulfanyl-1-hexanol; PMAO, poly(maleic anhydride-alt-1-octadecene); HSA, Bovine serum albumin; PCL, poly(ε-caprolactone); TG, Thioglycerol; PME, cysteamine hydrochloride and n-boc-ethylenediamine grafted poly(acrylic acid); PIMA, poly(isobutylene-alt- maleic anhydride); HA, hyaluronic acid; CTAB, cetyltrimethylammonium bromide; APS, (3-aminopropyl) triethoxysilane; DTPA, diethylenetriaminepentaacetic acid; DS, dextran; SA, stearyl acid; DTDTPA, 2-[bis[2-[carboxymethyl-[2-oxo-2-(2-sulfanylethylamino)ethyl]amino]ethyl]amino]acetic acid; ALA, 5-aminolevulinic acid; PTMP-PMMA, trithiol-terminated poly-(methacrylic acid); PB, Prussian blue; Ce6, chlorin e6; DNR, daunorubicin; DOX, doxorubicin; UCNP, upconversion nanoparticle; PGA, poly(L-glutamic acid); MPEGS, 2-(Methoxypoly(ethyleneoxy)propyl)trimethoxysilane; MB, microbubbles; pDNA, plasmid DNA.

172

Ternary quantum dots

(DDT) capped CuInS2 and AgInS2 QDs in organic medium and later encapsulated them with Pluronic F127 block copolymer micelles to disperse them in water medium. These micelle encapsulated QDs were then used for the in vitro imaging of human macrophages. The result showed that these QDs were accumulated within the cells and did not cause any damage to the morphology of the cells indicating the low toxicity of these QDs. In another work, Zhao et al. [22] synthesized oleyl amine capped organically soluble CuInS2 QDs and performed ligand exchange reaction using a mixture of glutathione (GSH) and mercaptopropionic acid (MPA) for phase transfer to water medium. The QDs were then successfully used for fluorescent staining of MCF10CA1 breast tumor cells. Most of the QDs were found to be accumulated in the cytoplasm. There are fewer reports available for I–III–VI QDs directly synthesized in water medium and directly used for in vitro imaging without any further modification/conjugation to biological targets. Chen et al. [23] reported large scale synthesis of Cu-In-S/ZnS core/ shell quantum dots using dual stabilizers, thioglycolic acid (TGA), and sodium citrate for the labeling of HepG2 cells. Recently, Wang et al., [24] synthesized water soluble polyethylenimine-coated AgInS2 QDs with no additional capping agents in gram scale for HeLa cell imaging. In another development, Zhang et al. [25] reported the synthesis of MPA capped ZnCu-In-S/ZnS core/shell QDs and investigated their imaging characteristics. The as-prepared QDs after incubation with RAW264.7 macrophages exhibited delocalized emissions from the cytoplasm. The reports on the use of I-III-Se/Te QDs such as CuInTe, AgInSe QDs for bioimaging applications are very rare but they have found their application on solar cells enormously. Recently, Che et al. [26] reported the synthesis of MPA capped AgInSe2-ZnSe QDs for the fluorescent labelling of SMMC-7721 cells. Quantum dots are often functionalized to perform many biological functions such as high cellular uptake, tumor targeted imaging and delivery etc. Compared to bare QDs, the functionalized QDs exhibit higher stability under physiological conditions. The functionalization strategies mainly involve electrostatic attraction, hydrophobic-hydrophobic interaction, covalent bonding, etc. For example, Chang et al. [27] prepared DDT capped organically soluble AgInS2-ZnS QDs modified with poly(maleic anhydridealt-1-octadecene) (PMAO) for their aqueous solubility. This was followed by conjugation with folic acid (FA) using covalent coupling reaction. They observed that the FA-conjugation facilitated the specific intracellular uptake by HepG2 cancer cells via RME and QD-FA conjugates are internalized in the cytoplasm. In another work by Deng et al., [28] DDT capped CuInS2/ZnS QDs were transferred into water using encapsulation by

Bioimaging and therapeutic applications of ternary quantum dots

173

folate-modified N-succinyl-N0 -octyl-chitosan micelles. The bioimaging studies revealed FA receptor-positive Bel-7402 cells were selectively labeled by QDs conjugates compared to the FA receptor-negative A549 cells. Recently, Vinayagam et al. [29] synthesized CuInZnS/ZnS QDs coated with dual stabilizers poly (styrenesulfonic acid-co maleic acid) and GSH, and covalently conjugated to aminophenyl boronic acid (APBA) using coupling reaction. APBA moieties is an ideal ligand to selectively bind to sialic acids that are overexpressed in cancer cells compared to normal cells. Comparative analysis confirmed that HeLa cells had a high fluorescence with QD-APBA conjugates than unconjugated QDs indicating the receptor mediated targeting. Peptides are known to exhibit hydrophilicity/ amphilicity, high affinity to the receptors, excellent permeability and hence they are often conjugated with QDs to selectively label the cells. Liu et al. [30] prepared CuInS2/ZnS quantum QDs in organic medium and converted them into aqueous medium by surface modification with bovine serum albumin (BSA)-poly(ε-caprolactone) (PCL) conjugate which was then conjugated to cyclic RGD peptide for targeted cell imaging. The RGD peptide is known to be highly selective to the tumors expressing αvβ3 integrin receptors such as MDA-MB-231, U87MG, etc. Comparative in vitro imaging studies proved that peptide conjugated QDs were readily and largely internalized than unconjugated QDs by U87 and HeLa cells that overexpresses integrin αvβ3 binding sites (Fig. 7.1A). In another recent work, Song et al. [31] synthesized Gd-doped AgInS2 (AgGaxIn1
xS2) QDs via cation exchange reaction and conjugated to low density lipoprotein (LDL) via polyelectrolyte mediated electrostatic interaction. The assynthesized LDL-QD conjugates were delivered to selectively target cancer stem cells that overexpress LDL receptor, which was demonstrated in threedimensional imaging of cancer stem cells (Fig. 7.1B).

7.3 In vivo imaging The underlying basic concept of in vitro imaging can also be applied for in vivo imaging. However, in vivo imaging is not so easy in reality. Compared to unicellular or thin tissue section in in vitro, the thick tissue sections in the live animal limit the optical imaging processes in multiple manners. Delivery, distribution, metabolism, blood clearance of QDbioconjugate probe (pharmacokinetics), penetration of optical signals (excitation and emission) through deep tissues and autofluorescence of tissues are the major issues of “in vivo” imaging. Near Infrared (NIR) emitting QDs

DAPI

A Ultrasonicaon

Evaporaon

HeLa Cells

(a) cRGD

BSA-PCL soluonEmulsionBSA-PCL coated cRGD-BSA-PCL coated QDs QDs

BSA-PCL

B

QDs

Merge

(b)

(d)

PDDA

CIS/ZnS QDs

(c)

cRG D

QDs

3D Mapping

(e)

ApoB-100 3D MIP

LDL

DIC Overlay

LDL-PDDA

LDL-PDDA-QDs

(f)

PL

Fig. 7.1 (A) (a) Construction of the NIR fluorescence nanoprobe with amphiphilic BSA-PCL conjugate and CuInS2/ZnS QDs, (b) HeLa cells treated with nontargeting nanoprobe, and (c) HeLa cells treated with cRGD-functionalized nanoprobe measured by confocal laser scanning microscope. (B) (a) Schematic illustration of polyelectrolyte modification of LDL and electrostatic self-assembly of LDL-QDs nanoprobe, (b) Differential interference contrast, PL and the overlay images of colon cancer stem cell lines (CCSCs), incubated with LDL-QDs NCs, (c) Three-dimensional surface mapping of the CCSCs. Inset: Magnified 3D image of a representative cell. (d) Three-dimensional maximum intensity projection of the view along the x, y, and z-axis. Panel (A) adapted with permission from Liu, Z., Chen, N., Dong, C., Li, W., Guo, W., Wang, H., Wang, S., Tan, J., Tu, Y. and Chang, J., 2015. Facile construction of near infrared fluorescence nanoprobe with amphiphilic proteinpolymer bioconjugate for targeted cell imaging. ACS Appl. Mater. Interfaces, 7(34), pp.18997-19005. Copyright (2015) American Chemical Society and (B) from Song, J., Zhang, Y., Dai, Y., Hu, J., Zhu, L., Xu, X., Yu, Y., Li, H., Yao, B. and Zhou, H., 2019. Polyelectrolyte-mediated nontoxic AgGaxIn1–xS2 QDs/low-density lipoprotein nanoprobe for selective 3D fluorescence imaging of cancer stem cells. ACS Appl. Mater. Interfaces, 11(10), pp.9884-9892. Copyright (2019) American Chemical Society.

Bioimaging and therapeutic applications of ternary quantum dots

175

offer more advantage in “in vivo” imaging applications [32]. In general, excitation of fluorescent probes in biological media at UV or visible region is attenuated by absorption and scattering of blood and water molecules. Also, the strong green autofluorescence of fluorophores like structural proteins, amino acids, NADH, FAD present in the skin, viscera, small intestine and bladder especially the gallbladder is astoundingly high, which precludes the use of visible light for in vivo imaging. Moreover, visible fluorescent tags could not be used to detect deep organs such as the liver and spleen because of poor penetration of visible light. On the other hand, wavelengths beyond NIR band results in the generation of heat due to vibrational excitation of water and other biomolecules. For these reasons, NIR excitation and emission is preferable for in vivo exogenous fluorescent tagging. NIR light emitting QDs can achieve deep penetration in tissues and also minimizes autofluorescence background effectively. Though some organic NIR emitting dyes like Cyanin and Alexa are available, they exhibit poor biological and photochemical stability [8]. QDs fluorescent probes can be administered into live animals via either systemically or locally for in vivo imaging and targeting. It is well known that the drug, probe, or any external material delivered systemically could be taken and rapidly cleared by reticulo-endothelial system (RES) consisting of liver, spleen, and lymph nodes. The stimulation of immune response (opsonization) towards the foreign substances by phagocytic cells of these organs is responsible for this non-specific uptake followed by clearance. Fluorescence imaging by QDs probes is useful while there is an issue with this RES and especially to map the lymph node [33,34] pathways and their underlying anatomy. QDs without targeting molecules are best suit for the whole body imaging, which is a significant improvement over the dye or radioactivity method currently used. Tan et al. [35] reported the synthesis of multidentate polymer-capped AgInS2 at room temperature administered to the pre-anesthetized mice via tail vein injection. After 4 h of post injection, they were able to detect the fluorescence from most parts of the body indicating that QDs were distributed all over the body under the act of blood circulation. In another work, Xi et al. [36] developed the synthesis of ribonuclease (RNase) capped CuInS2 QDs and delivered to the stomach of the mouse using gastric syringe. They were able to detect strong NIR fluorescence signal from the mouse stomach besides the depth and acidic condition of the stomach. In order to target and image the other organs or tumor, QD must be linked to target specific antibodies. Tumors are over-expressed with certain

176

Ternary quantum dots

receptors or antigen. Hence, QDs conjugated with the receptor substrates are very useful in targeting and imaging of cancer at its various stages especially metastasis. Deng et al. [37] prepared oil soluble highly fluorescent AgInSe2/ZnS core/shell quantum dots. An amphiphilic polymer octylamine-modified polyacrylic acid was then used to wrap the QDs to make it hydrophilic in nature. The water soluble QDs was later conjugated to RGD peptide using covalent coupling. The results of in vivo study using mice indicated that NIR emitting AgInSe2/ZnS QDs were able to target MDA-MB-231. The comparison study with the MCF-7 tumor bearing mouse revealed weak fluorescence signal as they do not express these specific receptors (Fig. 7.2). Similar reports are available for the peptide-modified/ conjugated ternary QDs (Table 7.1). Amphiphilic polymer

(a)

Animal Imaging RGD

EDCl, NHS AgInSe2/ZnS QDs

(b)

Amphiphilic polymer wrapped QDs

(c)

Cell Imaging RGD-AP QDs

(d)

High

Low Fig. 7.2 (A) General scheme for water transfer of the prepared oil-soluble AgInSe2/ZnS core/shell QDs with the PAA-based amphiphilic polymer and subsequent surface modification with the monomeric cyclic RGD peptide. (B) NIR fluorescence images of tumorbearing mice before tail-vein injection and after intravenous injection with 775 nmemitting RGD-amphiphilic-QDs for 6 h: (C) αυβ3-positive MDA-MB-231 tumor and (D) αυβ3-negtive MCF-7 tumor (λex ¼ 660 nm, a 700-nm long-pass filter). Reproduced from D. Deng, L. Qu, Y. Gu. Near-infrared broadly emissive AgInSe2/ZnS quantum dots for biomedical optical imaging. J. Mater. Chem. C 2(34) (2014) 7077-7085 with permission from The Royal Society of Chemistry.

Bioimaging and therapeutic applications of ternary quantum dots

177

Surface modification of nanomaterials with polyethylene glycol (PEGylation) and their bioconjugates is often used for in vivo imaging application. This is because PEG is resistant to protein interactions and is therefore an ideal surface functional moiety to offer extended stability to QDs in biological environment. In addition, PEG is the long circulating molecule, which avoids nonspecific phagocytosis and reduces rapid clearance by blood [38]. Several reports are available for PEGylated ternary QDs for bioimaging and are given in Table 7.1. Lee et al. [39] synthesized folic acid conjugated PEGylated serum stabilized CuInS2/ZnS QDs and investigated its imaging performance in tumor bearing mice. In vivo and ex vivo imaging revealed that the QDs were largely accumulated in the tumor and very weak fluorescent signal was detected in the liver as PEGylated nanoparticles are hardly recognized and cleared by the macrophages distributed in the liver. In another study, Liu et al. [40] reported peptide-modified PEGylated CuInSe2/ZnS core/shell QDs with the reasonably long circulating half-life of 6.6 h to avoid rapid renal clearance.

7.4 Multiphoton imaging QDs owing to their unique optical properties have been investigated for many other imaging techniques. Multiphoton imaging has received attraction in biomedical field due to its advantage of deep tissue imaging [41]. Multiphoton absorption is a nonlinear process where two or more photons with same or different frequencies are absorbed. The sum of the energies of photon can excite the molecule with higher bandgap energy. Due to this property, multiple NIR photons can be used to excite the molecule exhibiting higher bandgap in UV/visible region which is not possible in single photon imaging. For example, two photons of 700 nm laser light carry the same energy as one photon of 350 nm and can excite the molecule with the UV/visible absorption. Multiphoton absorption is a complex process and hence simple two-photon absorption has been widely studied on many fluorophores. Compared to organic fluorophores and fluorescent proteins, QDs possesses very large two-photon absorption cross section (104 Goeppert-Mayer (GM) units) which make them an excellent candidate for two-photon imaging. Most of the reports on two-photon imaging are based on binary QDs. Though ternary QDs such as CuInS2 and AgInS2 QDs have been tested successfully for two-photon absorption [21, 40, 42–48], to the best of our knowledge, there has been only one article on ternary QDs used for two photon imaging reported by Liu et al. [40]

178

Ternary quantum dots

where PEGylated and peptide conjugated CuInS2/ZnS and CuInSe2/ZnS QDs have been used to demonstrate in vivo two-photon imaging.

7.5 Multiplex imaging Unlike organic fluorophores that can be excited only at specific wavelength, the broad continuous absorption of QDs allows them to be excited at varying wavelength. This allows the use of single light source to excite multicolored QDs whereas organic dyes emitting at different wavelength require multiple light sources. Hence, QDs can be effectively used as multicolored fluorescent marker for multiplex imaging. There are few reports available in the literature that demonstrates the multiplex imaging of ternary QDs. For example, Deng et al. [28] administered two different NIR emitting (720 and 800 nm) CuInS2/ZnS QD loaded chitosan micelles into the mouse via subcutaneous injection and investigated their multiplexed imaging performance (Fig. 7.3). The exciting multiplexed imaging behavior of QDs can be simply achieved by selecting the appropriate excitation wavelength or filter. The results showed that at λexc ¼ 660 nm with 700 nm long pass filter, fluorescence from both QDs was detected whereas at λexc ¼ 766 nm with 800 nm long pass filter, only the fluorescence from 800 nm emitting QDs was detected.

Fig. 7.3 Multiplex NIR fluorescence imaging of mouse administered with two different NIR-emitting QDs-loaded micelles by subcutaneous injection (the right leg, 720 nmemitting nanocrystals; the left leg, 800 nm-emitting nanocrystals): (A) before injection, (B) λexc ¼ 660 nm, an 700-nm long pass filter, (C) λexc ¼ 660 nm, an 800-nm long pass filter, (d) λexc ¼ 766 nm, an 800-nm long pass filter. Reprinted with permission from D. Deng, Y. Chen, J. Cao, J. Tian, Z. Qian, S. Achilefu, Y. Gu. High-quality CuInS2/ZnS quantum dots for in vitro and in vivo bioimaging. Chem. Mater. 24(15) (2012) 3029–3037. Copyright (2012) American Chemical Society.

Bioimaging and therapeutic applications of ternary quantum dots

179

7.6 Multimodal imaging Imaging the biological targets like tumor is always complex and hence single modality imaging technique is inefficient to provide accurate information. Multimodal imaging techniques are being developed to address these challenges. Recent advances in QDs include the use of QDs for multimodal imaging techniques which combine the fluorescence imaging of QDs with other imaging techniques, such as MRI, PET, CT imaging, etc.

7.6.1 Magnetic resonance/fluorescence imaging Magnetic resonance imaging (MRI) is a non-invasive medical imaging technique that produces three dimensional images using strong magnetic field and radio waves. The most common imaging modes of MRI are longitudinal (T1)-weighted and transverse (T2) weighted scans which are based on the relaxation behavior of excited protons of the tissue under magnetic field perturbed by radiofrequency. The relaxation rate of T1 and T2 are different in the normal tissue and the affected tissue (for example, tumor). Contrast agents are used to enhance the contrast between relaxation rate in normal cells and cancer cells which distinguish them clearly in the imaging. Contrast agents can be categorized into positive contrast agents and negative contrast agents [49]. Positive contrast agents mostly consist of paramagnetic metal ions with many unpaired electrons such as Gd3+, Mn2+. These contrast agents shorten the T1 relaxation thus the contrast enhanced tissue parts appear brighter in T1 weighted images. The negative contrast agents involve the use of dysprosium or super-paramagnetic nanoparticles such as Fe3O4 nanoparticles where the enhanced parts appear darker in T2 weighted images. Unlike fluorescent imaging, MRI is not limited by deep tissues. However, advanced MRI requires a targeted contrast agent to achieve better sensitivity. Hence the magnetic QDs with both fluorescent and magnetic property could be a potential candidate in targeted molecular imaging and therapy. Several synthetic strategies are reported to induce the magnetic functionality in the fluorescent QDs for MRI, by doping, conjugating or encapsulating the contrast agents with QDs. Initially, cadmium based magnetic QDs were investigated for this bimodal imaging, however, because of its toxicity, they are being replaced by alternative low-toxic QDs including I-III-VI QDs. Doping technique provides the structural stability of the multimodal agent, co-localization of fluorescent and magnetic species and offers

180

Ternary quantum dots

ultra-small size. For bimodal MR/fluorescence imaging, Yang et al. [50] synthesized Gd doped CuInS2/ZnS QDs with high longitudinal relaxivity (r1 ¼ 9.45 mM [Gd]
1 S
1) in water. In vivo and ex vivo fluorescent imaging of this bimodality QDs conducted on the tumor bearing mouse showed that, after 4 h post-injection, strong fluorescence signals were revealed due to passive targeting in tumor in addition with liver and kidney due to the macrophage uptake of QDs in these organs. Similarly, in vivo T1 weighted MR imaging indicated significant contrast enhancement in tumor after 4 h post injection. Comparative MRI studies with the Magnevist (commercial contrast agent) showed that enhanced signal intensities were observed for bimodal QDs than the commercial agent (Fig. 7.4A). Alternatively, Sitbon et al. [51] synthesized NIR emitting Zn-Cu-In-Se QDs followed by the growth of paramagnetically doped MnxZn1
xS shell where the contrast agent Mn2+ ions were directly incorporated in the inorganic shell rather than core to minimize the quenching of core fluorescence. By optimizing the reaction condition, they were able to obtain 3000 Mn atoms/QD with the optimum longitudinal relaxivity r1 ¼ 0.5 mM [Mn]
1 S
1. They demonstrated the use of these QDs for MR/NIR fluorescence imaging of regional lymph nodes in mice. On the other hand, Cheng et al. [52] covalently conjugated Gd-diethylenetriaminepentaacetic acid (DTPA) complex to CuInS2/ZnS QDs and further conjugated with folic acid. The fluorescence of the QDs were retained even after the conjugation with Gd-DTPA complex and folic acid. MR/fluorescence imaging analyses indicated that the as-prepared QDs dual model probe is an effective T1 contrast agent with r1 relaxivity of 3.72 mM [Gd]
1 s
1 and exhibited higher selectivity towards targeted imaging of HeLa cells over HepG2 and MCF-7 cells. Similarly, Zhang et al. [53] synthesized CuInS2/ZnS QDs and covalently conjugated to DTPA-coupled BSA with Gd3+ chelation. This was followed by linking the conjugate to anti-CD133, monoclonal antibody. The obtained bimodal probe exhibited very high longitudinal relaxivity (r1 ¼ 15.2 mM [Gd]
1 s
1) in MR imaging and selectively targeted glioma stem cells when seen in fluorescent imaging. In a recent report, Yang et al. [54] emphasized a simple method for introducing the paramagnetic property in the NIR emitting CuInS2/ZnS quantum dots for bimodal MR/NIR imaging. Typically, they modified the QDs with 2-[bis[2-[carboxymethyl-[2-oxo-2-(2sulfanylethyl-amino)ethyl]amino]ethyl]amino]aceticacid (DTPA) followed by simply chelating Gd3+ ions under stirring. They observed only negligible quenching by Gd-chelation. The longitudinal relaxivity (r1) of the prepared QDs@DTPA-Gd nanoparticles was reported as high as 9.91 mM

(A)

(B) Pre

2h

4h

6h

18h

High

(b)

(a)

(a)

Heart Liver Spleen

Control

Tumor

1h 12 h

3h

1h 12 h

3h

6h

Lung Kidney Tumor

Low

Pre

High

2h

4h

6h

Low

18h (b)

(c)

Control

Low

6h

High

Fig. 7.4 (A) In vivo fluorescence imaging (a), ex vivo imaging (b), and T1-weighted imaging (c) (18 h postinjection) of U87 tumor-bearing mice (red circles) after tail-vein injection of GCIS/ZnS@PLVs. (B) (a) and (b) T1- and T2-weighted magnetic resonance images of pancreatic adenocarcinoma before injection, and 1 h, 3 h, 6 h and 12 h post injection. Panel (A) Reprinted with permission from W. Yang, W. Guo, X. Gong, B. Zhang, S. Wang, N. Chen, W. Yang, Y. Tu, X. Fang, J. Chang. Facile synthesis of Gd–Cu–In–S/ZnS bimodal quantum dots with optimized properties for tumor targeted fluorescence/MR in vivo imaging. ACS Appl. Mater. Interfaces, 7(33) (2015) 18759–18768. Copyright (2015) American Chemical Society and (B) from J. Shen, Y. Li, Y. Zhu, X. Yang, X. Yao, J. Li, G. Huang, C. Li. Multifunctional gadolinium-labeled silica-coated Fe3O4 and CuInS2 nanoparticles as a platform for in vivo tri-modality magnetic resonance and fluorescence imaging. J. Mater. Chem. B 3(14) (2015) 2873–2882 with permission from The Royal Society of Chemistry.

182

Ternary quantum dots

[Gd]
1 s
1. In vivo fluorescence experiments revealed the passive targeting effect of HeLa cells by QDs@DTPA-Gd. Compared to T1 imaging, little work has been done in the T2 MR/fluorescence multimodal imaging with ternary QDs. Earlier, Hsu et al. [55] prepared CuInS2 QDs and superparamagnetic Fe3O4 nanoparticles and embedded them together in a silica matrix. The as-synthesized nanohybrids were water soluble, emitting in the red region and exhibited high spin-spin (T2) relaxivity (r2 ¼ 214 mM [Fe]
1 s
1) which made them novel contrast agent with dual imaging modalities. Later Shen et al. [56] reported the fabrication of trimodality MR/fluorescence imaging probes which composed of both T1 and T2 MR contrasting agents along with the fluorescent QDs. In a typical reaction, first, silica nanohybrids containing embedded CuInS2 QDs and superparamagnetic Fe3O4 nanoparticles were prepared and later functionalized with DTPA for chelating with Gd3+ ions. It was further conjugated to RGD peptide for active tumor targeting. In this trimodal imaging, Gd3+ ions serve as T1 MRI contrast agent, Fe3O4 nanoparticles as T2 contrast agent while CuInS2 QDs acts as fluorescent probe. MR relaxivity studies showed r1 of 1.56 mM
1 s
1 for T1 and r2 of 23.22 mM
1 s
1 for T2 relaxivity against [Fe] in the T1 weighted positive contrast and T2 negative contrast MR imaging respectively. The low r1 and r2 values compared to the bare Gd-DTPA complex and Fe3O4 nanoparticles could be attributed to the interference of T1 and T2 relaxations with each other. In vitro fluorescence imaging indicated that peptide conjugated nanoprobes were readily internalized in human pancreatic cancer cell lines (BXPC-3). In vivo targeted MRI imaging performed on pancreatic adenocarcinoma mouse model showed the significant brightened and darkened enhancement effect in T1 and T2 MR images respectively after 12 h post injection (Fig. 7.4B). These results indicated the advantages of combination of positive and negative MR contrast agents with fluorescent QDs in the targeted imaging.

7.6.2 Computed tomography/fluorescence dual imaging Computer tomography (CT) imaging is a popular medical imaging technique which uses X-rays to obtain the images of anatomical structures. The denser the tissue, the higher the X-rays absorption which appears white in the CT image. However, CT is unable to detect the changes in the cellular and molecular level and lacks specificity and sensitivity. Hence the combination of CT with cellular sensitive fluorescent imaging techniques

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can improve the tissue structure imaging. The most common CT contrast agent is based on iodinated molecules due to its high X-ray absorption coefficient [57]. However, major issues with these contrast agents are i) requirement of high concentration of contrast agent, ii) rapid clearance of contrast agents by kidney leading to very short imaging time iii) difficulty in imaging specific organs and disease. With the use nanobiotechnology, these issues could be overcome, and the improved imaging could be obtained. For example, Ding et al. [58], dispersed hydrophobic red emitting CdSe/ZnS QDs in the iodinated oil to produce nanoemulsion droplets which were stabilized using liposome for dual CT/fluorescence imaging of rabbits bearing atherosclerotic plaques. The nano-emulsion acts as a carrier and enhances the circulating time before it is cleared from the body. They observed significant increase in CT value (20 HU) in both atherosclerotic plaques and liver after 2 h of post injection indicating long circulating time of the contrast agent. Likewise, in vivo NIR fluorescent imaging studies revealed the intake of the contrast probes by plaque-loaded areas confirming the dual imaging functionality of the nanoprobes. Researchers have developed inorganic nanomaterials such as Bi2S3, gold nanorod, TaOx, WS2 QDs to replace iodine-based contrast agent for CT imaging to enhance the circulating time (>2 h) and sensitivity. For instance, Chen et al. [59] synthesized hybrid clusters composed of hydrophobic Bi2S3 nanoparticles and CdSe/ZnS QDs. The cluster was then made hydrophilic by using polyethylene glycolphospholipid bilayer for combined in vivo, CT/fluorescent mouse imaging. The nanohybrid exhibited long circulation time (>4 h) and high CT values were observed for heart, spleen and liver (513) after 2 h post injection. Ex vivo fluorescent imaging with the mouse revealed that spleen, liver and kidneys were well detected after 1 h post injection in agreement with CT imaging. Though the study claims the biocompatibility of the nanoprobe and reported no significant harmful effects to the organs, it is still not desirable to use for clinical studies because of inherent heavy metal toxicity due to Cd. Regarding the use of ternary I–III–VI QDs for the integrated CT/fluorescent imaging, to the best of our knowledge, there has been no report so far in the literature at the time of writing. In a recent development, Jiang et al. [60], reported the synthesis of water soluble CuFeSe2 ternary nanocrystals, analog of I–III–VI QDs, with ultra-small size (75%); however, they are limited by poor photostability, visible light absorption, narrow absorption band [70]. In contrast, QDs which does not have such issues seem to be attractive. Nevertheless, the generation of ROS by QDs is extremely low (