CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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166 absorption in the present studies. Plausible conclusion that one can make from the relaxation dynamics is that the charge carrier diffusion is hindered along the sheet due to the presence of trap sites, which facilitate energy relaxation by alternate pathways. Presence of surface (defect) states is further supported by recent STM study which showed that atomic arrangement in GO sheets composed of regions of graphene like hexagonal lattice separated by disordered regions composed of topological defects and oxidized parts [6.26]. This conclusion is further supported by electrical measurements which show hopping and tunneling mediated conductivity [6.27] in GO. In our earlier investigations [28] the role of individual carriers in relaxation dynamics has been ascertained in quantum dots by use of electron/hole quenchers like quinones, pyridine, and amines. So we adopted a similar strategy to throw some light on the electron and hole contribution to the trapping dynamics. In the present study we have used benzoquinone (BQ) as electron quencher and pyridine (Py) as hole quencher to find out the contribution of trapped electrons and trapped holes in the transient signal, where the trapped carriers can be quenched completely or partially depending upon the trapped depth of the carrier. Earlier Charlier et al [6.29] have shown that carbon nano tube (CNT) can interact effectively with π –cyclic organic molecules although CNT surface has curvature. Charlier et al [6.29] probed the electronic interaction between CNT and different π –cyclic organic molecules like benzene and dichloro dicyano quinone (DDQ). In case of benzene-CNT system the interaction found to be weak as benzene physically adsorbed on CNT. However in case of CNT-DDQ system, DDQ acted like electron acceptor and CNT act as electron donor, which was evident from the formation of mid gap states in CNT+DDQ composite system. Benzoquinone with similar quinone structural unit, can act similar to DDQ as an efficient
167 electron acceptor for Graphene. As GO is a planar surface both benzoquinone and pyridine are expected to interact with GO through π-ring of GO surface more effectively. In this condition it is expected that individual charge carriers will interact with the quenchers and will facilitate faster recombination with the complementary carriers. Figure 6.6 shows the transient decay for GO (trace a), GO in presence of electron quencher (benzoquinone, BQ) (trace b) and GO in presence of electron hole quencher (pyridine, Py) (trace c) at 670 nm. A(mO.D) 0.4 0.3 0.2 0.1 0.0 -0.1 GO GO-BQ GO-PY 0 2 4 6 8 100 200 Delay Time (ps) Figure 6.6: Transient decay kinetics of graphene oxide (GO) at 670 nm after exciting at 400 nm laser pulse in absence of any quencher (GO), in presence of electron quencher (GO-BQ) and in presence of hole quencher (GO-PY). In presence of BQ the kinetic trace decays faster as compared to that of pure GO. The kinetics can fitted multiexponentially with time constants of τ 1 = 0.85 ps (61.8%), τ 2 = 10 ps (9.8%), τ 3 = > 400 ps (28.4 %) (Table 6.1). Additionally contribution of 0.9ps component
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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 (
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 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: 165 GO as shown in the Figure 3 rev
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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