CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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145 Earlier authors [5.4] reported charge transfer dynamics in type II CdSe/ZnTe coreshell QD using time-resolved emission spectroscopy. However in the present studies first time we are reporting time-resolved absorption studies where we have observed exciton peak shift in the red region in TA spectrum with the formation of shell and further on increasing thickness. However it is interesting to see that that relative intensity of positive absorption and exciton bleach intensity does not change with increasing the thickness of shell material as the lattice mismatch between CdSe and ZnTe are < 1 %. Here on photoexcitation of CdSe/ZnTe core-shell by 400 nm majority of the photon excite CdSe QD and electron and holes are generated in CdSe core. 0 CdSe Core ( pr =460nm) CdSe/ZnTe3 ( pr =480nm) CdSe/ZnTe4 ( pr =520nm) m O. D. -5 -10 m O. D. 0 -5 -10 0 2 4 6 Time Delay (ps) 0 100 200 300 400 Time Delay (ps) Figure 5.8: Normalized kinetic traces of a) CdSe at 460 nm, b) CdSe/ZnTe3 at 480 nm, and c) CdSe/ZnTe4 at 520 nm after excitation at 400 nm laser light. Inset: Same kinetics are shown at shorter time scale.
146 However, due to staggered band alignment between CdSe and ZnTe, photoexcited hole which was generated in CdSe core will migrate to ZnTe shell. Now to understand charge carrier cooling dynamics and charge transfer dynamics we have monitored the kinetics at the bleach wavelengths for CdSe core and CdSe/ZnTe core-shell of different thickness and shown in Figure 8. In the inset of Figure 8 we have shown the bleach kinetics at shorter time scale. We can clearly see that the growth time of the bleach from core to core-shell and further on increasing shell thickness. The growth time can be fitted with time constants of 155fs for CdSe core, 180 fs for CdSe/ZnTe3 and 300 fs for CdSe/ZnTe4 core-shell (Table 2). This growth time of the bleach can be attributed cooling of the charge carriers [5.8]. In a bulk semiconductor the cooling of carriers is expected to be pulse width limited. This can be rationalized as follows, on photoexcitation the carriers are initially populated in the higher exciton level as 400nm laser light populates the higher excitons which finally populates the lowest excitonic state with the formation of 1S bleach [5.20]. In QD bleach formation is delayed due to quantized nature of the individual bands [5.1]. The main contribution to cooling of the carriers comes from Auger mediated or surface mediated processes as reported earlier and not by multi-phonon emission. Since density of states (DOS) of holes are much larger compared to electrons because of higher effective mass and degenerate valence band, faster cooling of hole is achieved by of electron-hole (e-h) mediated energy transfer and subsequent electron cooling [5.8, 5.22-5.23]. So in the present investigation we attribute that the cooling of the charge carrier mainly to electron cooling.
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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
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 186 and 187: 139 1000 c b PL Intensity 100 10 a
Page 188 and 189: 141 samples due to a very weak exci
Page 190 and 191: 143 dynamical spectrum is negligibl
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 236 and 237: 187 transfer from CdSe core to ZnTe
Page 238 and 239: 189 However the position of bands i
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