CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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187 transfer from CdSe core to ZnTe shell. This study further highlighted the importance of having a proper interface in enabling an efficient transfer of charges. Aim of the work described in the chapter 6 is to investigate role of graphene in charge separation from a QD. We chose chemically synthesized reduced graphene oxide as the template to decorate it with CdTe QDs. The samples were characterized by XRD, HRTEM and optical techniques. Since graphene oxide and reduced graphene oxide is also known to absorb 400nm light we also studied their dynamics as control. Our analysis of relaxation dynamics revealed involvement of phonon and trap states. Involvement of electrons traps in relaxation dynamics was ascertained by use of quenchers. Furthermore long time component observed in relaxation were still present after reduction procedure indicating presence of traps. However we observed a dramatic decrease in traps with higher reduction time. The dynamics of 24h reduced Graphene oxide was found to be close to previous reports on pristine graphene. The CdTe decorated graphene was synthesized by colloidal methods and characterized. The TCSPC studies and transient absorption studies confirmed an efficient charge separation. 7.2. Outlook Focus of the present is on investigation of charge carrier dynamics in confined systems with particular emphasis on charge separation behavior. Insights on how charge carrier dynamics in QD is mediated by different factors like surface, carrier interactions etc have been drawn from dynamics measurements. Major conclusion of the present thesis is on tunability of charge transfer properties by creation of heterostructures by simple synthesis modifications of a QD.
188 Study of interfacial electron transfer dynamics revealed a better charge separation behavior in a confined system. There is still scope for doing further research in this area. The possibility of variation in dynamics by varying the size of TiO 2 remains unexplored. How much such smaller TiO 2 can improve the actual photovoltaic efficiency is a question that needs to be explored. The next chapters describe dynamics in QDs and QD based heterostructure. In chapter 4 we have studies the dynamics in CdTe QD. The main inferences drawn indicate significant size dependent dynamics. In fact in smaller QD the recovery at bleach was found to be much faster as compared to larger QD. This indicates significant role played by surface. Previous study on QD using TOPO or oleic acid capping showed relaxation in time scales of ns. This hints at the role of capping agent and crystallinity in relaxation. Therefore synthesis of good crystalline QD with better surface capping agent needs to be investigated. This might help in overcoming the drawbacks arising from trapping of carriers. This is also true in case of CdSe/ZnTe core shell QD. Here our purpose was to investigate charge transfer time. The results of this study were promising and it warrants further exploration. The characteristics of the interface are one of the areas one needs to investigate. The formation of a shell forms an interface between two different materials. This leads to a lattice mismatch which could lead to formation of defects along the interface. The types of defects and its energetics is an area which needs careful attention. Additionally the lattice mismatch for small shell thickness could lead to a strain close to the interface on core and shell. This is also known to affects band alignment at the interface. Therefore knowledge of band alignment is of utmost importance. Studies have shown role of cyclic voltammetery in obtaining band positions.
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Charge Transfer Dynamics in Quantum
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DECLARATION I, hereby declare that
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ACKNOWLEDGEMENTS It is my privilege
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1.3.5. Defect Mediated Relaxation 2
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2. 8. 4. White Light Generation- 45
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6.2. Experimental 6.2.1.Synthesis o
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devices based on QDs it has been sh
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Furthermore generation of pump (~40
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shell). This clearly indicated that
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12. V. I. Klimov, J. Phys. Chem. B,
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1.13 Reactant and Product Potential
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light. Inset: Kinetic traces monito
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6.5 Transient decay kinetics of gra
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at 670 nm after exciting at 400 nm
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ABBREVIATIONS BET BQ CB CCD CdS CdT
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1 Chapter 1
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3 size dependent optical properties
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5 1.2. Physics of Semiconductors 1.
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7 As seen from the schematic, poten
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9 Substituting this in Schrödinger
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11 ( r, r ) ( r ) ( r ) (1.13) e
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13 E E E 2 2 2 EX ne, le nh,
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15 spherical symmetry of field. The
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17 gE ( ) 2Em 2 3 3 (1.25) For a
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19 1.3.2. Electron-Hole energy tran
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21 Impact Ionization Figure 1.7. Sc
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23 understanding on mechanistic asp
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25 carriers are unable to sample th
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27 CB VB QD Metal Figure. 1. 10. Sc
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29 energy barrier. Therefore it is
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31 1 f q 2 A qB (1.30) 2 In equa
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33 Vibrational contribution can be
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35 1. 7. 1. Electron Injection ET i
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37 Under assumption of invariance o
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39 distribution. To achieve good si
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41 initially achieved monodispersit
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43 and rate of charge transfer acro
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45 1.32 P. V. Kamat, J. Phys. Chem.
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47 Chapter 2
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49 chemical species. Since a partic
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51 fluorescence forms an important
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53 eliminated by use of standard wh
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55 sample. Raman spectroscopy is ba
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57 will appear bright and region wi
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59 volatile solvent is drop casted
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61 spots arise from diffraction fro
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63 The electrical signal is then ch
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65 delayed and inverted. The two si
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67 Pump-probe technique is one of t
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69 Amplifier Jade Stretcher fs Osci
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71 Figure 2.6. Optical layout of Ti
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73 Grating Convex Mirror Concave Mi
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75 changes the polarization from ho
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77 Grating Output Input Mirror Grat
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79 μJ has a very high peak power.
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81 2.6. Dynamical Theory of X-Ray D
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83 Chapter 3
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85 Therefore the study of interfaci
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87 3. 2. Experimental Section 3.2.1
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89 and intensity show absence of ot
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91 which is also neutral; therefore
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93 electron transfer times. This re
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95 r B a * 0 3.1 me / me Now the
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97 nonadiabatic case the electron t
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99 drastically reduced leading to a
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101 While the study of injection dy
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103 study. Based on the injection d
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105 17. L. E. Brus, J. Phys. Chem.
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107 Chapter 4
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109 of the QDs. As a result surface
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111 4.2.2. Synthesis: The CdTe QDs
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113 lower hole state. In CdSe the l
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115 4.2 inset. The kinetic traces a
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117 growth of the bleach kinetics (
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119 previous section we have discus
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121 lived charge transfer complex w
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123 100 times concentration of the
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125 4.5. References 4.1. Efros, Al.
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127 Chapter 5
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129 been made in the synthesis of t
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131 metallic synthesis by arrested
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133 5.3. Result and discussion: 5.3
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135 indicating a confinement induce
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137 absorption studies that on form
Page 186 and 187: 139 1000 c b PL Intensity 100 10 a
Page 188 and 189: 141 samples due to a very weak exci
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Page 192 and 193: 145 Earlier authors [5.4] reported
Page 194 and 195: 147 Sample t r(ps) t 1(ps) t 2(ps)
Page 196 and 197: 149 In addition to the cooling dyna
Page 198 and 199: 151 pulse-width time scale followed
Page 200 and 201: 153 5.20. Sreejith Kaniyankandy, Sa
Page 203 and 204: 155 CHAPTER 6 Charge Separation in
Page 205 and 206: 157 Studies by Wang et. al. [6.15]
Page 207 and 208: 159 graphene in a 1M NaOH solution.
Page 209 and 210: 161 Intensity (a.u.) 5 4 3 2 1 CdTe
Page 211 and 212: 163 dynamics of graphene oxide and
Page 213 and 214: 165 GO as shown in the Figure 3 rev
Page 215 and 216: 167 electron acceptor for Graphene.
Page 217 and 218: 169 governs the recombination dynam
Page 219 and 220: 171 Sample τ 1 (ps) τ 2 (ps) τ 3
Page 221 and 222: 173 A (mO.D) 10.0 0.0 -10.0 -20.0 0
Page 223 and 224: 175 involved in the relaxation may
Page 225 and 226: 177 Electron quencher (benzoquinone
Page 227 and 228: 179 Scheme 6.1. Schematic of Electr
Page 229 and 230: 181 6.6. Reina, A.; Jia, X.; Ho, J.
Page 231: 183 6.26. Kaiser, A. B.; Navarro, C
Page 234 and 235: 185 assigned to injection into diff
Page 238 and 239: 189 However the position of bands i
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