CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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143 dynamical spectrum is negligible [5.5]. The broad feature beyond the exciton bleach feature can arise from trapped carriers or intraband transitions [5.18]. The assignment of this feature to intraband transition is highly unlikely as the intraband transition show very fast decay (few ps) and highly wavelength dependent kinetics which is not observed in the present investigation. So from the above observation we can safely conclude that the feature most likely comes from trapped carriers. To monitor the charge carrier dynamics in CdSe QD we have monitored the bleach recovery kinetics at 460 nm and transient decay kinetics at 720 nm. Figure 6A show the bleach recovery kinetics of CdSe QD at 460 nm (1S Excitonic position). The growth kinetics of the bleach at 460 nm can be fitted 155 fs and the recovery kinetics can be fitted multiexponentially with time constants 2.55 ps (45%), 27 ps (17%) and > 400 ps (38%). Here 155 fs time constant can be attributed to state filling to 1S excitonic level [5.8]. The bleach recovery time constants suggest the charge recombination time constants between photogenerated electrons and holes. We have also monitored decay kinetics of excited state absorption (ESA) of photo excited CdSe at 690 nm (Figure 5.4 inset) which can be fitted multi-exponentially 1.3ps (30%), 10 ps (45%) and >200 ps (25%). It is interesting to see that growth time of the signal can be fitted single exponentially with pulse-width limited time constant (< 100 fs). Earlier investigations suggest that the positive absorption band in NIR region is mainly due to intraband transition of the photo excited holes [5.21]. In our early studies of CdTe QD we have observed that positive absorption can be due to both the trapped charge carriers (electron and hole).
144 5.3.5. Transient absorption studies of CdSe/ZnTe core-shell QD: In the present studies we have intended to study charge carrier dynamics (cooling and trapping) and charge transfer dynamics in core-shell nano-structure. For this purpose we have chosen two different QD core-shell samples CdSe/ZnTe3 and CdSe/ZnTe4 where core size of CdSe QD kept same but the ZnTe shell thickness was different. Figure 7 shows the transient spectra for the above two core-shells. Figure 7 shows the excitonic bleach peak at 490 nm for CdSe/ZnTe3 core-shell and at 510 nm for CdSe/ZnTe4 core-shell which matches with position of exciton from optical absorption. A(mOD) 1.0 0.0 -1.0 -2.0 0.3 0.0 -0.3 -0.6 CdSe/ZnTe3 CdSe/ZnTe4 0.1ps 0.4ps 1ps 5ps 10ps 80ps 500 550 600 650 700 Wavelength (nm) 0.1ps 0.4ps 1ps 5ps 10ps 80ps Figure 5.7: Transient absorption spectra of CdSe/ZnTe3 (top panel) and CdSe/ZnTe4 (bottom panel) Quantum dot core-shell at different time delay after excitation at 400 nm laser light.
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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
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 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 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 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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