CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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139 1000 c b PL Intensity 100 10 a CdSe Core CdSe/ZnTe1 CdSe/ZnTe3 1 0 5 10 15 20 25 30 Time Delay (ns) Figure 5.5: Time resolved emission decay traces of a) CdSe, b) CdSe/ZnTe1, and c) CdSe/ZnTe3. As we have mentioned in earlier section that shell formation is not complete similarly from time resolved emission data we can see that the emission kinetics still dominated by CdSe core emission. However we can still see that contributions of shorter components decrease and longer component increases for CdSe/ZnTe1 core-shell as compared to CdSe core. Finally we have shown the emission kinetics decay trace for thicker shell CdSe/ZnTe3 in Figure 5.3c, which can be fitted bi-exponentially τ 2= 2.19 ns (41%), τ 3= 18 ns (59%) with average time (τ av) of 11.5 ns as given in the Table 5.1. Interestingly we can observe that the fast component (τ 1) is missing in the kinetic decay trace in CdSe/ZnTe3 sample.
140 Sample t 1 (ns) t 2 (ns) t 3 (ns) t avg (ns) CdSe 0.5 (59%) 3.8 (34%) 17.4(7%) 2.8 CdSe/ZnTe1 0.6(65%) 3.7(18.6) 16.3(16.4%) 3.75 CdSe/ZnTe3 2.19 (41%) 18 (59%) 11.5 Table 5.1: Fits for emission decay traces for core and core shell samples We can observe a fourfold increase of average lifetime in CdSe/ZnTe3 core-shell as compared to CdSe core. The spatial separation of electron and hole can result in a decrease of the wave function overlap and thus longer lifetime of charge recombination is expected in type II heterostructures. Our observation indicates that the formation of shell leads to an increase in lifetime which can be concluded as the clear evidence of reduced electron hole wave function overlap in core-shell material. Furthermore present study confirms the Marcus theory predictions which indicate an inverted regime in the present case which would in turn indicate that the reorganization energies are very small. In case of quantum solvent seldom plays an important role as the excitons or the charge carriers are well localized within the bulk of the quantum dot. Therefore the nuclear reorganization energy arising from excitonphonon coupling plays an important role in the charge recombination dynamics. To ascertain the role of nuclear reorganization we calculate the reorganization energy and free energy change in the case of CdSe/ZnTe3 as described by Scholes et al [5.19]. We find that the reorganization energy, λ= 90meV and ΔG= -0.36eV. This clearly shows that for that sample – ΔG> λ, inverted regime in terms of Marcus theory. This is also in accordance with previous study by Scholes et al [5.19]. Such a situation arises in the case of quantum dot core shell
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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
Page 136 and 137: 93 electron transfer times. This re
Page 138 and 139: 95 r B a * 0 3.1 me / me Now the
Page 140 and 141: 97 nonadiabatic case the electron t
Page 142 and 143: 99 drastically reduced leading to a
Page 144 and 145: 101 While the study of injection dy
Page 146 and 147: 103 study. Based on the injection d
Page 148 and 149: 105 17. L. E. Brus, J. Phys. Chem.
Page 151: 107 Chapter 4
Page 154 and 155: 109 of the QDs. As a result surface
Page 156 and 157: 111 4.2.2. Synthesis: The CdTe QDs
Page 158 and 159: 113 lower hole state. In CdSe the l
Page 160 and 161: 115 4.2 inset. The kinetic traces a
Page 162 and 163: 117 growth of the bleach kinetics (
Page 164 and 165: 119 previous section we have discus
Page 166 and 167: 121 lived charge transfer complex w
Page 168 and 169: 123 100 times concentration of the
Page 170 and 171: 125 4.5. References 4.1. Efros, Al.
Page 173: 127 Chapter 5
Page 176 and 177: 129 been made in the synthesis of t
Page 178 and 179: 131 metallic synthesis by arrested
Page 180 and 181: 133 5.3. Result and discussion: 5.3
Page 182 and 183: 135 indicating a confinement induce
Page 184 and 185: 137 absorption studies that on form
Page 188 and 189: 141 samples due to a very weak exci
Page 190 and 191: 143 dynamical spectrum is negligibl
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
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187 transfer from CdSe core to ZnTe
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189 However the position of bands i
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