Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

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than 1%. Acceleration of radiative decay in the regime of stimulated emissionas well as selective amplification of propagating waveguide modes make thecontribution of biexcitons dominant over the contribution of excitons in thesample edge emission above the ASE threshold.In order to take into account the existence of two types of emitter(excitons and biexcitons) and the fact that optical gain in NQDs is dominatedby biexcitons and spontaneous emission is primarily due to single excitons,the ASE formula can be rewritten as I = A x l+A bx (e g bxl1)/g bx , where A xand A bx are constants proportional to spontaneous emission power densitiesfor excitons and biexcitons, respectively, and g bx is the biexciton modal gain.Using this formula, one can simultaneously fit both below- and abovethresholdregions of the I-versus-l dependence (solid line in Fig. 25b), whichyields g bx = 155 cm 1 . This estimate represents effective modal gain per unitstripe length measurd for waveguide modes. To estimate the modal gain inthe absence of waveguiding effects (i.e., gain per unit propagation lengthalong the ray), we can scale g bx by a correction factor b = n gl /n c 0.83 (n gl isthe index of a glass substrate), which yields g m = bg bx c130 cm 1 (thisestimate assumes that the ASE is dominated by the mode that corresponds tothe critical angle of the total internal reflection). Because of the transientnature of optical gain in the case of pulsed excitation, the above-derivedquantities represent time-averaged characteristics of the NQD gain medium(i.e., gain average over the gain lifetime).The inset in Fig. 25b displays dynamics of normalized, pump-inducedabsorption changes ( Da/a 0 ) in the same NQD film as the one used in theASE studies discussed earlier, These dynamics indicate that optical gain( Da/a 0 > 1) only exists during time t g c5 ps following photoexcitation. Asdiscussed earlier, the fast gain decay is due to intrinsic, nonradiative Augerrecombination and, possibly, to ultrafast hole trapping [50]. The short gainlifetimes are likely responsible for saturation of the ASE intensity observedin ‘‘variable-stripe-length’’ measurements at l>l s c0.08 cm (Fig. 25b). Thelight propagation time corresponding to the ‘‘saturation’’ length is f6 ps,which is a value close to the gain lifetime (f5 ps). This observation indicatesthat under pulsed-excitation conditions, the transient nature of the NQD gainFigure 25 Room-temperature emission spectra of a CdSe NQD film recordedusing the ‘‘variable-stripe-length’’ configuration (shown schematically in the inset);pump fluence is 1 mJ/cm 2 . (b) Solid circles describe the ASE intensity as a function ofthe stripe length. The dashed line is a fit to I = A(e gl 1)/g ( g = 120 cm 1 ), whereasthe solid line is a fit to I = A x l + A bx (e g bx l1)/g bx ( g bx = 155 cm 1 ); note adifference in gain magnitudes produced by these two fits. Inset: Gain dynamicsmeasured using a TA experiment (t g is a gain lifetime).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
can contribute to the saturation of ASE intensities along with a well-knownmechanism of the gain medium depletion [61].C. Lasing ExperimentsThe lasing regime in NQDs is possible by combining high-density NQDmaterials with optical cavities to provide efficient optical feedback. Severaltypes of cavity have been utilized to demonstrate NQD lasing, including polystyrenemicrospheres [17], glass microcapillary tubes [46,50,62], and distributedfeedback (DFB) resonators [63].In Ref. 62, cylindrical NQD microcavities were fabricated by drawingconcentrated hexane solutions of TOPO-capped CdSe NQDs (R = 2.5 nm)into microcapillary tubes (the inner tube diameter was 80 lm). The solventwas then allowed to evaporate. To increase the thickness of the NQD layerdeposited on the inner wall of the tube, the above procedure was repeatedseveral times. Cylindrical NQD microcavities formed in this way (inset inFig. 26a) can support both planar-waveguide-like modes that develop alongthe tube length and whispering gallery (WG) modes developing around theinner circumference of the tube in the regime of total internal reflection. Forthe modes propagating along the tube, one can only achieve the ASE regime(no optical feedback is present), whereas WG modes can support a true lasingaction (microring lasing) [64].Depending on the sample, it is observed that as the pump level isincreased, either microring lasing or ASE along the tube can develop first.This difference is likely due to uncontrolled variations in the optical quality ofthe NQD layer along and around the tube from one sample to another. Forexample, for the data shown in Fig. 26a (sample temperature is 80 K), lightamplification along the tube (the ASE peak at f613 nm) develops before theWG lasing. As the pump level is increased further, periodic oscillations due tolasing into WG modes develop on the top of the broader ASE peak.The data in Fig. 26b, which were taken for a different sample, provide anexample of pure microring lasing. Lasing into a single, sharp WG modeFigure 26 Pump-dependent emission spectra of NQDs solids (sample temperatureis 80 K) incorporated into microcapillary tubes (NQD cylindrical microcavities)demonstrating different emission regimes: (a) ASE along the tube length developsbefore microring lasing; (b) the regime of pure microring lasing (the emission spectraare shown by lines and the gain spectrum is shown by solid circles); the emissionspectra are taken at progressively higher fluences of 1, 1.4, 1.6, and 2.8 mJ/cm 2 .Insets: (a) schematics of a NDQ microcavity; (b) pump-fluence dependence of theintensity of the 612.0-nm mode demonstrating a sharp transition to the lasing regime.Copyright 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
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where the constant A is determined
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In crystals for which the function
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The respective wave functions areC
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passive, as was shown in Ref. 12. T
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square of the matrix element of the
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where e F = e F ieV and e F F = e x
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see that for all nanocrystal shapes
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optical recombination of the excito
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B. Recombination of the Dark Excito
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where x = cos h and f = l B g e H/3
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The theory of the polarization memo
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Figure 7 The size dependence of the
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state would have an infinite lifeti
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crystal axis [see Eq. (40)]. As a r
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time of the exciton momentum relaxa
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One must also account for the influ
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observed in one of the first studie
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REFERENCES1. Bawendi, M.G.; Wilson,
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4Intraband Spectroscopyand Dynamics
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The solid line in Fig. 1 shows the
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Figure 2 FTIR spectra of n-type CdS
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of the center frequency. The experi
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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 and 174: 32. Inoshita, T.; Sakaki, H. Physic
Page 175 and 176: continuous spectral tunability over
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: Copyright 2004 by Marcel Dekker, In
Page 223 and 224: Copyright 2004 by Marcel Dekker, In
Page 225 and 226: coupling between ‘‘volume’’
Page 227 and 228: numerous discussions on the photoph
Page 229 and 230: 53. Kang, K.; Kepner, A.; Gaponenko
Page 231 and 232: the ‘‘on-off ’’ emission in
Page 233 and 234: parallel form of data acquisition i
Page 235 and 236: Figure 3 (a) Spectral time trace of
Page 237 and 238: transition energies. In fact, these
Page 239 and 240: distribution (dark line) does not d
Page 241 and 242: exposure to only room light. In our
Page 243 and 244: statistics for the off times are in
Page 245 and 246: 230Shimizu and BawendiCopyright 200
Page 247 and 248: Figure 10 (a) Time trace of a CdSe(
Page 249 and 250: and excited QD core states to fluct
Page 251 and 252: arrows indicate the on-time truncat
Page 253 and 254: W. K. Woo and V. C. Sundar for assi
Page 255 and 256: size allows the electron affinity a
Page 257 and 258: II.THEORY OF ELECTRON TRANSFER BETW
Page 259 and 260: For the specific case of charge tra
Page 261 and 262: dominates, the mobility is often fi
Page 263 and 264: where the constant A and the temper
Page 265 and 266: components of modulation which are
Page 267 and 268: nanocrystals [41], understanding ph
Page 269 and 270: 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/
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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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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
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Figure 23 Change of the PL intensit
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(viz. the absorbed light intensity)
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Figure 25Impact ionization in QDs.m
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prevent electron-hole recombination
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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-
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Figure 2 Frequency of the acoustic
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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
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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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