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Figure 8 Photoluminescence spectra for CdSe NQDs and (CdSe)ZnS (core)shellNQDs. Core diameters are (a) 2.3, (b) 4.2, (c) 4.8, and (d) 5.5 nm. (Core)shell PLQYs are (a) 40%, (b) 50%, (c) 35%, and (d) 30%. Trap-state emission is evident inthe (a) core–particle PL spectrum as a broad band to the red of the band-edge emissionand absent in the respective (core)shell spectrum. (From Ref. 30, reprinted withpermission.)Photoluminescence spectra can be used to indicate whether effectivepassivation of surface traps has been achieved. In poorly passivated nanocrystals,deep-trap emission is evident as a broad tail or hump to the red ofthe sharper band-edge emission spectral signal. The broad trap signal willdisappear and the sharp band-edge luminescence will increase followingsuccessful shell growth (Fig. 8a). Note: The trap-state emission signal contributionis typically larger in smaller (higher relative surface area) nanocrystalsthan in larger nanoparticles (Fig. 8a).Homogeneous nucleation and growth of shell material as discrete nanoparticlesmay compete with heterogeneous nucleation and growth at core–particle surfaces. Typically, a combination of relatively low precursor concentrationsand reaction temperatures is used to avoid particle formation.Low precursor concentrations support undersaturated-solution conditionsand, thereby, shell growth by heterogeneous nucleation. The precursors,diethylzinc and bis(trimethylsilyl) sulfide in the case of ZnS shell growth,for example, are added dropwise at relatively low temperatures to preventthe buildup and supersaturation of unreacted precursor monomers in theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Figure 9 Absorption spectra for bare (dashed lines) and one- to two-monolayerZnS-overcoated (solid lines) CdSe NQDs. (Core)shell spectra are broader andslightly red-shifted compared to the core counterparts. Core diameters are (a) 2.3, (b)4.2, (c) 4.8, and (d) 5.5 nm. (From Ref. 30, reprinted with permission.)growth solution. Further, employing relatively low reaction temperaturesavoids growth of the starting core particles [20,30]. ZnS, for example, cannucleate and grow as a crystalline shell at temperatures as low as 140jC [30],and CdS shells have been successfully prepared from dimethylcadmium andbis(trimethylsilyl) sulfide at 100jC [20], thereby avoiding complications dueto homogeneous nucleation and core-particle growth. Additional strategiesfor preventing particle growth of the shell material include using organiccapping ligands that have a particularly high affinity for the shell metal. Thepresence of a strong binding agent seems to lead to more controlled shellgrowth; for example, TOPO is replaced with TOP in CdSe shell growth onInAs cores, where TOP (softer Lewis base) coordinates more tightly thanTOPO (harder Lewis base) with cadmium (softer Lewis acid) [33]. Finally, theratio of the cationic to anionic precursors can be used to prevent shellmaterialhomogeneous nucleation. For example, increasing the concentrationof the chalcogenide in a cadmium–sulfur precursor mixture hinders formationof unwanted CdS particles [20].Successful overcoating is possible for systems where relatively largelattice mismatches between core and shell crystal structures exist. The mostcommonly studied (core)shell system, (CdSe)ZnS, is successful despite a 12%Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Page 1: Copyright 2004 by Marcel Dekker, In
Page 4 and 5: Copyright 2004 by Marcel Dekker, In
Page 6 and 7: Copyright 2004 by Marcel Dekker, In
Page 8 and 9: This book covers several topics of
Page 10 and 11: esult, some exciting topics were no
Page 12 and 13: 3. Fine Structure and Polarization
Page 14 and 15: 9. III-V Quantum Dots and Quantum D
Page 16 and 17: ContributorsUri BaninThe Hebrew Uni
Page 18 and 19: 1‘‘Soft’’ Chemical Synthesi
Page 20 and 21: 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 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 72 and 73: Slow, controlled precipitation of h
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
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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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Copyright 2004 by Marcel Dekker, In
Page 221 and 222:
can contribute to the saturation of
Page 223 and 224:
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coupling between ‘‘volume’’
Page 227 and 228:
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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