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7Electrical Properties ofSemiconductor NanocrystalsDavid S. Ginger and Neil C. GreenhamCavendish Laboratory, Cambridge, EnglandI. INTRODUCTIONThe size-dependent optical properties of semiconductor nanocrystals havebeen extensively studied, as described elsewhere in this volume. In thischapter, we concentrate instead on the electrical properties of nanocrystals.We show how electrons can be injected into nanocrystals, transported betweennanocrystals and transferred between nanocrystals and organic molecules.We then describe the potential applications of nanocrystals in electronicand optoelectronic devices. We attempt to give an overview of the field,although we cannot mention all of the articles which have been published inthis diverse and expanding area. We draw many of our examples from ourown work and from the work of those other groups with which we happen tobe most familiar. We restrict ourselves to chemically synthesized semiconductornanocrystals and concentrate mostly on II–VI nanocrystals such asCdSe in the size regime of less than 10 nm, where quantum confinement effectsare important.In a nanocrystal, the electronic states are typically confined within thenanocrystal by significant potential barriers. Transport between nanocrystalscan, therefore, be considered as a problem of hopping between localizedstates. For small nanocrystals in the strong confinement regime, charge transportis limited by interparticle electron transfer, rather than by the transportof electrons within a single particle. Therefore, the physics has more in commonwith that of charge transport in molecular systems, rather than withtraditional semiconductor transport theory. In nanocrystals, changing theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
size allows the electron affinity and ionization potential of the nanocrystal tobe tuned, which may, in principle, affect the rate of charge transfer. Inpractice, though, disorder and surface trap states are very important andmay dominate the charge transport properties of nanocrystalline systems. Wewill discuss some of the theory relevant to charge transport and chargetransfer in nanocrystalline systems in Section II.One of the major motivations for the study of nanocrystals is thepossibility that they may be useful in device applications. Some of the deviceswhich have been demonstrated in the laboratory include light-emitting diodes(LEDs) [1–4], photovoltaic devices [5,6], and single-electron transistors [7].For large-area applications such as LEDs and photovoltaics, nanocrystalshave the advantages of being processable from solution and having sizetunableabsorption and emission. Despite these advantages, nanocrystalshave not yet found their way into commercial devices and we will discuss theprospects for improvements in performance which may make commercialapplications more likely.In order to illustrate some of the charge transport and charge transferprocesses important in nanocrystal-based devices, we will briefly introducethe method of operation of LEDs and photodiodes. The simplest device inwhich light emission can occur is a thin film of nanocrystals sandwichedbetween metal electrodes, as illustrated in Fig. 1. One of the electrodes must besemitransparent; hence, indium–tin oxide (ITO) is typically used as the anode.When a positive voltage is applied to the anode, holes are injected from theanode and electrons from the cathode. These carriers can then move throughthe device under the action of the applied field by hopping from nanocrystal tonanocrystal. When an electron and hole arrive on the same nanocrystal, anexciton can be formed, which can then decay radiatively giving emission at anenergy characteristic of the size of the nanocrystal. We have found that thisFigure 1 Schematic representation of a typical structure for electrical characterizationof nanocrystalline films.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
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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
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 (
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Page 221 and 222: can contribute to the saturation of
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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: W. K. Woo and V. C. Sundar for assi
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
Page 271 and 272: quantum dots. Furthermore, because
Page 273 and 274: For many applications, a host mater
Page 275 and 276: form blends with morphologies that
Page 277 and 278: Figure 11 Photoluminescence efficie
Page 279 and 280: Figure 12 (a) Room-temperature PIA
Page 281 and 282: discussed briefly in Section IV. Ch
Page 283 and 284: dithiolates to thiol-terminated DNA
Page 285 and 286: Films of passivated CdSe nanocrysta
Page 287 and 288: Figure 16 Photocurrent action spect
Page 289 and 290: decay with stretched exponential ki
Page 291 and 292: siderably larger than might be esti
Page 293 and 294: dispersing CdSe nanocrystal chromop
Page 295 and 296: memory and charge storage effects [
Page 297 and 298: all) of the optically excited elect
Page 299 and 300: composites of nanocrystals and conj
Page 301 and 302: 55. Asbury, J.B.; Hao, E.C.; Wang,
Page 303 and 304: 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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