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Slow, controlled precipitation of highly ordered colloidal crystals hasalso been achieved by the method of ‘‘controlled oversaturation’’ [91]. Here,gentle destabilization is induced by slow diffusion of a nonsolvent into a NQDsolution directly or through a ‘‘buffer’’ solution. The nonsolvent (e.g.,methanol), buffer (e.g., propan-2-ol, if present), and the NQD solution(toluene solvent) are carefully layered in reaction tubes (Fig. 32a). Colloidal–crystalnucleation occurs on the tube walls and in the bulk solution.Although both methods, not buffered and buffered, yield well-orderedcolloidal crystals from 100 to 200 Am in size, the latter process is relativelymore controlled and produces faceted, hexagonal platelets (Fig. 32c), ratherthan ragged, irregularly shaped colloidal crystals (Fig. 32b). The CdSe NQDbuilding blocks again yield (fcc) superlattices [91].In the self-assembly of large colloidal crystals from nano-sized crystals,the nanocrystals behave as artificial atoms. The self-assembly process islargely driven by the relative favorability of the interaction between theNQD surface ligands and the solvent. Without a favorable interaction,particle cores begin to attract via van der Waals forces. Nanoparticles canalso assemble into large crystals by a self-assembly process called ‘‘orientedattachment.’’ In contrast with self-assembly by ligand-stabilized colloids,oriented attachment entails direct interaction between ligand-free (or almostligand-free) nanocrystals [92]. The driving force for the assembly is the lack ofsurface-passivating ligands. The bare nanoparticles assemble in order tosatisfy surface dangling bonds, and the assembly process is sufficientlyreversible that oriented attachment dominates, leading to highly crystallineFigure 32 (a) Schematic illustrating the method of ‘‘controlled oversaturation’’ forthe preparation of CdSe NQD colloidal crystals. (b) Optical micrograph ofirregularly shaped colloidal crystals prepared without a ‘‘buffer–solvent’’ layer(i.e., faster nucleation). (c) Optical micrograph of faceted hexagonal colloidal crystalsprepared with a ‘‘buffer–solvent’’ layer (i.e., slower nucleation). (From Ref. 91,reprinted with permission.)Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
macrostructures. Oriented attachment is known in a variety of systems,including natural mineral systems where chains and extended sheets areformed [93], epitaxial attachment of metal nanoparticles to metal substratesthrough a process of dislocation formation/movement and particle rotationin response to interfacial strain [94], chain formation from ‘‘artificial’’ nanoscaleTiO 2 building blocks (Fig. 33) [95], ZnO rod formation from ZnOnanodots [92], and sheet formation from ‘‘artificial’’ nanoscale rhombohedralInS and InSe building blocks [63]. This crystal-growth mechanism may evenprovide an advantage compared to traditional atom-by-atom growth, asnanoscale inorganic building blocks are typically characterized by nearlyperfect or perfect crystal structure, devoid of internal defects. Constructinglarge crystals from these preformed perfect crystallites may permit growth ofextended solids having unusually low defect densities [96]. Alternatively,impurities (natural and intentional dopants) can perhaps be more easilyincorporated into large crystals by oriented attachment of nanocrystalsdecorated with surface impurities [93,96]. Various types of dislocations, fromedge to screw, can form as a result of interfacial distortions generated toaccommodate coherency between surfaces that are not atomically flat(Fig. 34). Thus, the forces driving particles to attach (i.e., the drive toeliminate unsaturated surface bonds) can induce dislocation formation(Fig. 35) [97]. Predictably, attachment occurs on high-surface-energy faces[97]. For example, the rhombohedral InS (R-InS) and InSe (R-InSe) crystalstructures feature planar, covalently bonded sheets that are four atomic layersFigure 33 Transmission electron micrograph of TiO 2 nanocrystal aggregates thathave assembled by ‘‘oriented attachment.’’ The chains are oriented by a process ofparticle docking, aligning, and fusing. (From Ref. 95, reprinted with permission.)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
Page 22 and 23: growth can proceed by Ostwald ripen
Page 24 and 25: Figure 3 Transmission electron micr
Page 26 and 27: Figure 4 Temporal evolution of the
Page 28 and 29: No. 26, are f85% (Fig. 6) [21]. Alt
Page 30 and 31: tion spectra and broad PL spectra.
Page 32 and 33: ing surface-to-volume ratio with di
Page 34 and 35: Figure 8 Photoluminescence spectra
Page 36 and 37: lattice mismatch. Such a large latt
Page 38 and 39: match between InAs and ZnS of f11%.
Page 40 and 41: successfully repeated for up to thr
Page 42 and 43: Under a different growth regime, on
Page 44 and 45: Figure 14 Atomic model of the CdSe
Page 46 and 47: ‘‘Teardrop-shaped’’ particl
Page 48 and 49: Figure 17 High-resolution TEMs of C
Page 50 and 51: allows isolation of tetrapods in f8
Page 52 and 53: ligand concentrations yield a reduc
Page 54 and 55: een determined and quantitatively c
Page 56 and 57: tion volumes were also shown to be
Page 58 and 59: synthesis temperatures of z400jC ar
Page 60 and 61: Figure 23 (a) Photoluminescence spe
Page 62 and 63: levels (fV1 Mn per NQD). Despite in
Page 64 and 65: Figure 25 X-ray diffraction pattern
Page 66 and 67: Figure 27 Transmission electron mic
Page 68 and 69: An additional factor that strongly
Page 70 and 71: Figure 30 (a,b) Schematics illustra
Page 74 and 75: Figure 34 Schematic illustrating th
Page 76 and 77: Figure 36 Transmission electron mic
Page 78 and 79: achieving biological compatibility
Page 80 and 81: 47. Yu H.; Gibbons P.C.; Kelton K.F
Page 82 and 83: 2Electronic Structure inSemiconduct
Page 84 and 85: Figure 2 (a) Simple model of a nano
Page 88 and 89: electron and hole to be treated as
Page 90 and 91: independently, Eq. (13) is commonly
Page 92: a better description of the bulk ba
Page 95 and 96: investigated. For optical experimen
Page 97 and 98: Figure 4 (a) Absorption (solid line
Page 99 and 100: Figure 6 Normalized PLE scans for s
Page 101 and 102: Figure 8 A simplistic model for des
Page 103 and 104: Figure 10 Theoretically predicted p
Page 105 and 106: Figure 12 Schematics depicting the
Page 107 and 108: Figure 14 Calculated band-edge exci
Page 109 and 110: Figure 15 Absorption (solid line) a
Page 111 and 112: Figure 18 (a) Calculated band-edge
Page 113 and 114: IV.BEYOND CdSeA. Indium Arsenide Na
Page 115 and 116: the six-band Luttinger Hamiltonian.
Page 117 and 118: 11. Norris, D.J.; Efros, Al.L.; Ros
Page 119 and 120: 69. Gaponenko, S.V.; Woggon, U.; Sa
Page 121 and 122: 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
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from a long lifetime due to the pho
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are two natural approaches to study
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32. Inoshita, T.; Sakaki, H. Physic
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continuous spectral tunability over
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function and envelope function mome
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ottleneck’’ [14,28]. Further re
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in NQDs is dominated by nonphonon e
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Figure 4 Dynamics of the IR postpum
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Figure 5 (a) Time-resolved PL spect
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Figure 6 (a) The time delay of the
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and due to Auger-type e-h interacti
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Figure 9 Dynamics of the 1S bleachi
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significantly greater than the fast
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Figure 11 (a) Pump-intensity-depend
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NQD size. For small NQD sizes (R =
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Figure 14 Nonlinear absorption/gain
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sorption change associated with a s
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where n h em is the hole ‘‘emit
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ultrafast (subpicosecond to picosec
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Figure 18 Schematic of transitions
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Figure 19 Dynamics of pump-induced
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where n i (i=1, 2 . . . , N ) is th
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Figure 21 (a) Two-e-h-pair (biexcit
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the volume fraction (filling factor
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intensity dependence of this peak (
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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/
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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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Figure 7 Photoluminescence spectra
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Figure 8 Photoluminescence spectra
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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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Figure 18 Transmission electron mic
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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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Figure 11 Transmission electron mic
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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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