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5Charge Carrier Dynamics andOptical Gain in NanocrystalQuantum Dots: FromFundamental Photophysics toQuantum-Dot LasingVictor I. KlimovLos Alamos National Laboratory, Los Alamos, New Mexico, U.S.A.I. INTRODUCTIONSemiconductor materials are widely used in both optically and electricallypumped lasers. The use of semiconductor quantum-well (QW) structures asoptical gain media has resulted in important advances in laser technology.QWs have a two-dimensional (2D), steplike density of electronic states thatis nonzero at the band edge, enabling a higher concentration of carriersto contribute to the band-edge emission and leading to a reduced lasingthreshold, improved temperature stability, and a narrower emission line. Afurther enhancement in the density of the band-edge states and an associatedreduction in the lasing threshold is, in principle, possible using quantum wiresand quantum dots (QDs) in which the confinement is in two and threedimensions, respectively. In very small dots, the spacing of the electronicstates is much greater than the available thermal energy (strong confinement),inhibiting thermal depopulation of the lowest electronic states. This effectshould result in a lasing threshold that is temperature insensitive at anexcitation level of only one electron-hole (e–h) pair per dot on average [1,2].Additionally, QDs in the strong confinement regime have an emissionwavelength that is a pronounced function of size, adding the advantage ofCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
continuous spectral tunability over a wide energy range simply by changingthe size of the dots.Quantum-dot lasing was initially realized in an optically pumped deviceusing relatively large (f10 nm) CdSe nanoparticles fabricated via high-temperatureprecipitation in glass matrices [3]. Later, this effect was observed forepitaxially grown nanoislands (epitaxial or self-assembled QDs) using bothoptical and electrical (injection) pumping [4,5]. As predicted, lasers basedon epitaxial QDs show an enhanced performance in comparison with, forexample, QW lasers and feature reduced thresholds, improved temperaturestability, and high differential optical gain (important for achieving highmodulation rates).The success of the laser technology based on epitaxial QDs has been astrong motivation force for the development of laser devices based onultrasmall, sub-10-nm chemically synthesized nanoparticles. Such nanoparticles,known also as colloidal or nanocrystal QDs (NQDs) can be routinelygenerated with narrow size dispersions (f5%) using organometallic reactions[6–8]. In the sub-10-nm size range, electronic interlevel spacings can exceedhundreds of millielectron volts and size-controlled spectral tunability over anenergy range as wide as 1 eV can be achieved. Furthermore, improvedschemes for surface passivation by, for example, overcoating NQDs with ashell of a wide-gap inorganic semiconductor [9] allow significant suppressionof surface trapping and produce room-temperature photoluminescence (PL)quantum efficiencies greater than 50%. Additionally, due to their chemicalflexibility, NQDs can be easily prepared as close-packed films [10] (NQDsolids) or incorporated with high densities into polymers or sol–gel glasses.[11,12]. NQDs are, therefore, compatible with existing fiber-optic technologiesand are useful as building blocks for the bottom-up assembly of variousoptical devices, including optical amplifiers and lasers.Despite a decade of research that provided some indication of opticalgain performance [13], strongly confined NQDs had failed to yield lasing innumerous efforts. Difficulties in achieving lasing have often been attributed tohigh nonradiative carrier losses due to trapping at surface defects, a directconsequence of the large surface-to-volume ratio characteristic of sub-10-nmparticles. Another concern raised in several theoretical articles is the stronglyreduced efficiency of electron–phonon interactions in the case of discrete,atomiclike energy structures, which are characteristic of small-size dots[14,15]. For discrete spectra, the availability of pairs of electronic statessatisfying energy conservation in phonon-assisted processes is drasticallyreduced compared to the quasicontinuous spectra of bulk materials. This deficiencyhas been expected to significantly lower the efficiency of carriercooling due to phonon emission (the effect known as a ‘‘phonon bottleneck’’),leading to reduced carrier flows into the lowest ‘‘emitting’’ states and, hence,reduced PL efficiencies. However, the difficulties anticipated due to carrierCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Copyright 2004 by Marcel Dekker, In
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This book covers several topics of
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esult, some exciting topics were no
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3. Fine Structure and Polarization
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9. III-V Quantum Dots and Quantum D
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ContributorsUri BaninThe Hebrew Uni
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1‘‘Soft’’ Chemical Synthesi
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structure of energy states leads to
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growth can proceed by Ostwald ripen
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Figure 3 Transmission electron micr
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Figure 4 Temporal evolution of the
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No. 26, are f85% (Fig. 6) [21]. Alt
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tion spectra and broad PL spectra.
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ing surface-to-volume ratio with di
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Figure 8 Photoluminescence spectra
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lattice mismatch. Such a large latt
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match between InAs and ZnS of f11%.
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successfully repeated for up to thr
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Under a different growth regime, on
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Figure 14 Atomic model of the CdSe
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‘‘Teardrop-shaped’’ particl
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Figure 17 High-resolution TEMs of C
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allows isolation of tetrapods in f8
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ligand concentrations yield a reduc
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een determined and quantitatively c
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tion volumes were also shown to be
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synthesis temperatures of z400jC ar
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Figure 23 (a) Photoluminescence spe
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levels (fV1 Mn per NQD). Despite in
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Figure 25 X-ray diffraction pattern
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Figure 27 Transmission electron mic
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An additional factor that strongly
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Figure 30 (a,b) Schematics illustra
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Slow, controlled precipitation of h
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Figure 34 Schematic illustrating th
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achieving biological compatibility
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47. Yu H.; Gibbons P.C.; Kelton K.F
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2Electronic Structure inSemiconduct
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Figure 2 (a) Simple model of a nano
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electron and hole to be treated as
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independently, Eq. (13) is commonly
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a better description of the bulk ba
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investigated. For optical experimen
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Figure 4 (a) Absorption (solid line
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Figure 6 Normalized PLE scans for s
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Figure 8 A simplistic model for des
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Figure 10 Theoretically predicted p
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Figure 12 Schematics depicting the
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Figure 14 Calculated band-edge exci
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Figure 15 Absorption (solid line) a
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Figure 18 (a) Calculated band-edge
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IV.BEYOND CdSeA. Indium Arsenide Na
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the six-band Luttinger Hamiltonian.
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11. Norris, D.J.; Efros, Al.L.; Ros
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69. Gaponenko, S.V.; Woggon, U.; Sa
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formation of a long-lived dark exci
Page 123 and 124: where the constant A is determined
Page 125 and 126: In crystals for which the function
Page 127 and 128: The respective wave functions areC
Page 129 and 130: passive, as was shown in Ref. 12. T
Page 131 and 132: square of the matrix element of the
Page 133 and 134: where e F = e F ieV and e F F = e x
Page 135 and 136: see that for all nanocrystal shapes
Page 137 and 138: optical recombination of the excito
Page 139 and 140: B. Recombination of the Dark Excito
Page 141 and 142: where x = cos h and f = l B g e H/3
Page 143 and 144: The theory of the polarization memo
Page 145 and 146: Figure 7 The size dependence of the
Page 147 and 148: state would have an infinite lifeti
Page 149 and 150: crystal axis [see Eq. (40)]. As a r
Page 151 and 152: time of the exciton momentum relaxa
Page 153 and 154: One must also account for the influ
Page 155 and 156: observed in one of the first studie
Page 157 and 158: REFERENCES1. Bawendi, M.G.; Wilson,
Page 159 and 160: 4Intraband Spectroscopyand Dynamics
Page 161 and 162: The solid line in Fig. 1 shows the
Page 163 and 164: Figure 2 FTIR spectra of n-type CdS
Page 165 and 166: of the center frequency. The experi
Page 167 and 168: limit given by radiative relaxation
Page 169 and 170: from a long lifetime due to the pho
Page 171 and 172: are two natural approaches to study
Page 173: 32. Inoshita, T.; Sakaki, H. Physic
Page 177 and 178: function and envelope function mome
Page 179 and 180: ottleneck’’ [14,28]. Further re
Page 181 and 182: in NQDs is dominated by nonphonon e
Page 183 and 184: Figure 4 Dynamics of the IR postpum
Page 185 and 186: Figure 5 (a) Time-resolved PL spect
Page 187 and 188: Figure 6 (a) The time delay of the
Page 189 and 190: and due to Auger-type e-h interacti
Page 191 and 192: Figure 9 Dynamics of the 1S bleachi
Page 193 and 194: significantly greater than the fast
Page 195 and 196: Figure 11 (a) Pump-intensity-depend
Page 197 and 198: NQD size. For small NQD sizes (R =
Page 199 and 200: Figure 14 Nonlinear absorption/gain
Page 201 and 202: sorption change associated with a s
Page 203 and 204: where n h em is the hole ‘‘emit
Page 205 and 206: ultrafast (subpicosecond to picosec
Page 207 and 208: Figure 18 Schematic of transitions
Page 209 and 210: Figure 19 Dynamics of pump-induced
Page 211 and 212: where n i (i=1, 2 . . . , N ) is th
Page 213 and 214: Figure 21 (a) Two-e-h-pair (biexcit
Page 215 and 216: the volume fraction (filling factor
Page 217 and 218: intensity dependence of this peak (
Page 219 and 220: Copyright 2004 by Marcel Dekker, In
Page 221 and 222: can contribute to the saturation of
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coupling between ‘‘volume’’
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numerous discussions on the photoph
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53. Kang, K.; Kepner, A.; Gaponenko
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the ‘‘on-off ’’ emission in
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parallel form of data acquisition i
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Figure 3 (a) Spectral time trace of
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transition energies. In fact, these
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distribution (dark line) does not d
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exposure to only room light. In our
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statistics for the off times are in
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230Shimizu and BawendiCopyright 200
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Figure 10 (a) Time trace of a CdSe(
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and excited QD core states to fluct
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arrows indicate the on-time truncat
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W. K. Woo and V. C. Sundar for assi
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size allows the electron affinity a
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II.THEORY OF ELECTRON TRANSFER BETW
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For the specific case of charge tra
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dominates, the mobility is often fi
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where the constant A and the temper
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components of modulation which are
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nanocrystals [41], understanding ph
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and ionization potential through tw
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quantum dots. Furthermore, because
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For many applications, a host mater
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form blends with morphologies that
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Figure 11 Photoluminescence efficie
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Figure 12 (a) Room-temperature PIA
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discussed briefly in Section IV. Ch
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dithiolates to thiol-terminated DNA
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Films of passivated CdSe nanocrysta
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Figure 16 Photocurrent action spect
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decay with stretched exponential ki
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siderably larger than might be esti
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dispersing CdSe nanocrystal chromop
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memory and charge storage effects [
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all) of the optically excited elect
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composites of nanocrystals and conj
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55. Asbury, J.B.; Hao, E.C.; Wang,
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108. Morgan, N.Y.; Leatherdale, C.A
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The approaches to fabrication of se
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Figure 1 Experimental realization o
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Due to this voltage division, the m
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Figure 3 Simulated tunneling spectr
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electron charging. In both positive
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Figure 5 shows the typical features
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Figure 7 Map of levels for InAs nan
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Figure 8 Scanning electron microsco
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D CB = 0.31 eV is thus obtained. On
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Figure 11 Correlation of optical an
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atomistic approach based on pseudop
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charging indicated that the tunneli
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capacitance values were also kept t
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could not be detected in the QD/DT/
Page 333 and 334:
Figure 18 Tunneling conductanceF sp
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data in the inset of Fig. 19 repres
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corresponding to the s-like wave fu
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7. Grabert, H.; Devoret, M.H., Eds.
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66. Su, B.; Goldman, V.J.; Cunningh
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dimensional confinement are created
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Figure 1 Transmission electron micr
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The room-temperature absorption and
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narrower in samples with larger mea
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GaInP 2 QDs from a plot of the squa
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crystal, indicating lattice-matched
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Figure 5 Evolution of Stranski-Kras
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Page 361 and 362:
C. Efficient Anti-Stokes Photolumin
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Because HF treatment has been shown
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intensity of the PL when it is on a
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eV stems from recombining carriers
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Figure 16 Model to explain two-colo
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although this term is not rigorousl
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10 ps (about an order of magnitude
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electron relaxation is inhibited an
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QDs, the nature of the QD capping s
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QD solution. For an interdot distan
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emission spectra of the two individ
Page 387 and 388:
Figure 23 Change of the PL intensit
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(viz. the absorbed light intensity)
Page 391 and 392:
Figure 25Impact ionization in QDs.m
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prevent electron-hole recombination
Page 395 and 396:
36. Miller, R.D.J.; McLendon, G.; N
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91. Vurgaftman, I.; Singh, J. Appl.
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139. Mićić, O.I.; Ahrenkiel, S.P.
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10Synthesis and Fabrication of Meta
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Figure 2 (A-C) Progression of HR-TE
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Figure 3 Schematic for gold nanocry
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Nanocrystal growth can occur by two
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on the other hand, provide an ensem
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Figure 6 (a) SAXS patterns for disp
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Figure 8 The gold nanocrystal film
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of the stabilizing ligand, and the
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successfully modeled the 2D island
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2. Steric Stabilization and a Soft
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are fully extended. Moving away fro
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Figure 11 High-resolution SEM image
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Figure 13 (A) Transmission electron
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function of the density of localize
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Figure 15 High-resolution SEM image
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thiol-capped nanocrystals [2]. The
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41. Ackerson, B.J. Nature 1993, 365
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with the effect of the particle com
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Figure 1a shows the surface plasmon
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Figure 2 (a) Ultraviolet-visible ab
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agents [34]. The short-wavelength b
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Figure 4 (a) Plot of the plasmon ab
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the framework of traditional Mie’
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show that effects due to the surrou
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is located at the position of the g
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Figure 8 Ultraviolet-visible absorp
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laser pulses while being mixed in t
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Figure 10 shows HR-TEM images of go
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final irradiation product. At the s
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following the laser excitation appe
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36. Papavassiliou, G.C. Prog. Solid
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particles to expand. Because the he
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was frequency doubled in a 1-mm B-
Page 471 and 472:
Figure 2 Frequency of the acoustic
Page 473 and 474:
Figure 3 Change in radius (DR/R) ve
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In this model, the electrons couple
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Figure 5 Transient bleach data for
Page 479 and 480:
Figure 7 (a) Transient bleach data
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that the particles with >80% Au hav
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3. Del Fatti, N.; Valle´e, F.; Fly
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