CHEM01200604009 Sreejith Kaniyankandy - Homi Bhabha ...

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164 carriers on laser excitation. It is interesting to observe that a sizeable contribution (26.7%) of transient absorption signal does not decay beyond 400 ps. 5.0 A (mO.D) 0.4 0.2 A*10 4 4.0 3.0 2.0 1000 1500 2000 2500 Excitation Energy Density (J/cm 2 ) 0.0 0 2 4 6 8 100 200 Delay Time (ps) Figure 6.5: Transient decay kinetics of graphene oxide (GO) at 670 nm after exciting at 400 nm laser pulse. Inset: Absorbance change (Δ O. D.) graphene oxide (GO) at 670 nm at different excitation energy density (pump intensity). This observation is in contrast to transient absorption signal of pure graphene as observed earlier [6.22]. Earlier studies on graphene or graphene oxide show ultrafast relaxation dynamics of photoexcited charge carriers with time constants ranging from ~ 100fs-2ps. So the slower decay time constants for transient signal observed in the present investigation might be due to slow recombination of the trapped charge carriers. Confirmation of this assignment is discussed in the later parts of this contribution. The dynamics at 670 nm for
165 GO as shown in the Figure 3 reveals a pulse width limited rise after excitation indicating the formation of the state within pulse-width time scale (< 50 fs). The relaxation of photoinduced charge carriers in the present investigation could due to a number of processes like carriercarrier scattering, auger recombination, carrier-phonon relaxation or carrier trapping. It is reported in the literature [6.22] that the carrier-carrier interactions are complete within 30 fs in graphene. So it is reasonable to conclude that a quasi equilibrium state can be attained in photoexcited GO and subsequent relaxation process can take place with the mediation of optical phonons and which relax within a few picoseconds. To ascertain the role of carrier-carrier interactions, we have carried out intensity dependence studies by monitoring the transient signal at 670 nm. The normalized signals showed an intensity independent kinetics which clearly indicates that the charge carrier dynamics does not depend on the carrier density. This observation excludes carrier-carrier interactions which have non trivial carrier density dependence (n 2 for carrier-carrier interaction and n 3 for Auger recombination, where n is the carrier density) [6.22-6.25] on lifetime. So from our observation we can safely conclude that carrier-carrier interactions are complete within the pulse width time (< 100fs) in the present studies. The shortest lifetime component (0.9ps) which contributes 33.3% to the relaxation may be assigned to a fast detrapping. Majority (66.7%) of the transient signal still remain which has lifetimes of 9ps and >400ps. This suggests that unlike graphene on SiC, free GO in solution follows a different relaxation mechanism. Interestingly signal intensity of the photoinduced transient absorption signal was found to vary linearly with the intensity (Figure 6.5 Inset). In case of defect induced relaxation this is expected to saturate, but the possible reason for non saturation of intensity could be due to lower light intensity which is insufficient for saturation for transient
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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
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 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: 163 dynamics of graphene oxide and
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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