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9III–V Quantum Dots andQuantum Dot Arrays:Synthesis, Optical Properties,Photogenerated Carrier Dynamics,and Applications to PhotonConversionArthur J. NozikThe National Renewable Energy Laboratory, Golden, Colorado,and University of Colorado, Boulder, Colorado, U.S.A.Olga I. MićićThe National Renewable Energy Laboratory, Golden, Colorado, U.S.AI. INTRODUCTIONAs is very well known and discussed in this book, semiconductors showdramatic quantization effects when charge carriers (electrons and holes) areconfined by potential barriers to small regions of space where the dimensionsof the confinement are less than the deBroglie wavelength of the chargecarriers or, equivalently, less than twice the Bohr radius of excitons in the bulkmaterial. The length scale at which these effects begin to occur in III–V semiconductorsis less than about 25 nm.In general, charge carriers in semiconductors can be confined by potentialbarriers in one spatial dimension, two spatial dimensions, or in threespatial dimensions. These regimes are termed quantum films, also more commonlyreferred to as quantum wells (QWs), quantum wires, and quantumdots (QDs), respectively. These three regimes exhibiting one-, two-, and three-Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
dimensional confinement are created in semiconductor structures that areoften described as having a geometric dimensionality of 2D, 1D, and 0D, respectively.They can be formed either by epitaxial growth from the vaporphase [molecular beam epitaxy (MBE) or metallo-organic chemical vapordeposition (MOCVD) processes] or via chemical synthesis (colloidal chemistryor electrochemistry). Here, we will be discussing three-dimensional confinement(0D structures) in III–V semiconductors; the emphasis is on materialsformed via colloidal chemistry, but we will also present some interestingresults on QDs produced by epitaxial growth using low-pressure MOCVD.The former structures are frequently referred to as nanocrystals and the latteras quantum dots; however, for colloidal nanostructures, we will use the terms‘‘quantum dots’’ (QDs) and ‘‘nanocrystals’’ (NCs) interchangeably.In this chapter, we will first discuss the synthesis of various III–Vcolloidal QDs (InP, GaP GaInP 2 , GaAs, GaN) (with an emphasis on InP), aswell as colloidal InP–CdZnSe 2 core-shell QDs and GaAs QDs formed fromGaAs quantum wells produced by MOCVD growth; the formation of InPQD arrays will also be discussed. Then, we will present results of interestingand unique optical properties of III–V QDs and QD arrays, including highefficiencyband-edge photoluminescence (PL), size-selective PL, efficient anti-Stokes photoluminescence (PL upconversion), PL intermittency (PL blinking),anomalies between the absorption and the photoluminescence excitationspectra, and long-range energy transfer. Next, we will discuss the photogeneratedcarrier dynamics in QDs, including the issues and controversiesrelated to the cooling of hot carriers in QDs. Finally, we will discuss thepotential applications of QDs and QD arrays in novel photon conversiondevices, such as QD solar cells and photoelectrochemical systems for fuelproduction and photocatalysis.II.SYNTHESIS OF QUANTUM DOTSA. Colloidal NanocrystalsThe most common approach to the synthesis of colloidal QDs is thecontrolled nucleation and growth of particles in a solution of chemicalprecursors containing the metal and the anion sources (controlled arrestedprecipitation) [1–3]. The technique of forming monodisperse colloids is veryold and can be traced back to the synthesis of gold colloids by MichaelFaraday in 1857. A common method for II–VI colloidal QD formation is torapidly inject a solution of chemical reagents containing the group II andgroup VI species into a hot and vigorously stirred solvent containingmolecules that can coordinate with the surface of the precipitated QDCopyright 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
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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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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
Page 305 and 306: The approaches to fabrication of se
Page 307 and 308: Figure 1 Experimental realization o
Page 309 and 310: Due to this voltage division, the m
Page 311 and 312: Figure 3 Simulated tunneling spectr
Page 313 and 314: electron charging. In both positive
Page 315 and 316: Figure 5 shows the typical features
Page 317 and 318: Figure 7 Map of levels for InAs nan
Page 319 and 320: Figure 8 Scanning electron microsco
Page 321 and 322: D CB = 0.31 eV is thus obtained. On
Page 323 and 324: Figure 11 Correlation of optical an
Page 325 and 326: atomistic approach based on pseudop
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Page 329 and 330: capacitance values were also kept t
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Page 333 and 334: Figure 18 Tunneling conductanceF sp
Page 335 and 336: data in the inset of Fig. 19 repres
Page 337 and 338: corresponding to the s-like wave fu
Page 339 and 340: 7. Grabert, H.; Devoret, M.H., Eds.
Page 341: 66. Su, B.; Goldman, V.J.; Cunningh
Page 345 and 346: Figure 1 Transmission electron micr
Page 347 and 348: The room-temperature absorption and
Page 349 and 350: narrower in samples with larger mea
Page 351 and 352: GaInP 2 QDs from a plot of the squa
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Page 355 and 356: Figure 5 Evolution of Stranski-Kras
Page 357 and 358: Figure 7 Photoluminescence spectra
Page 359 and 360: Figure 8 Photoluminescence spectra
Page 361 and 362: C. Efficient Anti-Stokes Photolumin
Page 363 and 364: Because HF treatment has been shown
Page 365 and 366: Copyright 2004 by Marcel Dekker, In
Page 367 and 368: intensity of the PL when it is on a
Page 369 and 370: eV stems from recombining carriers
Page 371 and 372: Figure 16 Model to explain two-colo
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Page 377 and 378: electron relaxation is inhibited an
Page 379 and 380: Figure 18 Transmission electron mic
Page 381 and 382: QDs, the nature of the QD capping s
Page 383 and 384: QD solution. For an interdot distan
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Page 387 and 388: Figure 23 Change of the PL intensit
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Page 391 and 392: 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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