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

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Many features observed for superlattice growth are consistent with themodel developed by Burton et al. for the growth of crystals from atoms in thevapor phase that describes the growth of crystal layers based on the steps andkinks along the crystal surface that serve as adsorption sites for additionalatoms [49,50]. Slight differences in interparticle attraction can lead to ratherdramatic changes in superlattice film morphology. For example, the Hamakerconstant for gold in hexane is approximately 20% higher than for gold inchloroform due to the lower dielectric screening in hexane. Even though bothchloroform and hexane are considered good solvents for alkanethiol cappingligands with v c 0, the films deposited from hexane are very rough with roundsuperlattice domains, whereas the films from chloroform are relativelysmooth, as shown in Figs. 2E and 2F. HR-SEM images of films evaporatedfrom these two solvents, using the same nanocrystal size and capping ligands,show that the superlattice crystallizes preferably in the lowest energy [110] SLdirection, whereas superlattice growth from hexane is isotropic: The increasedinterparticle attraction in the solvent leads to similar crystal growth rates in allcrystallographic directions.Homogeneous superlattice nucleation can be induced by adding a relativelysmall amount of antisolvent to the dispersion. The antisolvent increasesthe supersaturation of the dispersion and promotes interparticle attractions.If too much antisolvent is added, the nanocrystals will simply flocculate. Theactivation barrier to homogeneous nucleation, however, is significant. Theexternal addition of energy, for example, by sonication can induce homogeneousnucleation as shown in Fig. 11, but the rapid evaporation of thesolvent under relatively poor solvent conditions results in a disordered nanocrystalfilm. Apparently, the balance between solvation and desolvation of thenanocrystals during the crystallization process does not provide sufficiententropic freedom for crystallization to occur in this timescale.In addition to isotropic van der Waals forces and steric repulsion betweenparticles, researchers have identified the potential importance of chain–chain interactions and shape effects [9,13,51]. For example, the superlatticestructure depends on the ratio of the capping ligand chain length hLi, to theradius of the nanocrystal core R: v = hLi/R. [9,13]. Whetten and co-workers[9] found that fcc packing is preferred below v c 0.73, whereas body-centeredcubic (bcc) order occurs above this value. As v increases, the energeticinterparticle attraction decreases, which induces the structural transition tothe more entropically favored bcc structure. Ligand–ligand interactions canalso lead to other deviations from fcc packing. For example, Luedtke andLandman performed molecular dynamics simulations, finding that dodecanechains adsorbed to gold nanocrystals could form interlocking bundles incontrast to shorter butyl groups which did not exhibit bundling [51]. Based onTEM investigations, nanocrystals have been found to form a variety ofCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Figure 11 High-resolution SEM image of gold nanocrystals deposited from adispersion of 10% ethanol and 90% hexane by volume (A) without and (B) withsonication. Without sonication, the superlattices exhibit rough films with significantareas of dendritic growth and some areas of homogeneous aggregation. Sonicationprovides the energy necessary to induce homogeneous nucleation resulting in nanocrystalsuperlattice crystallites that settle onto the substrate.structures including spheres, ellipsoids, tetrahedron, truncated octahedron,decahedron, and icosahedron that affect packing geometry [32,52,53].C. Solvent Effects During Nanocrystal Depositionand Thin-Film FormationOn a macroscopic scale, solvent evaporation can lead to convective instabilitiesand substrate dewetting phenomena that affect nanocrystal filmformation [54–56]. On a local microscopic scale, the nanocrystals organizeinto close-packed superlattices according to the guidelines discussed earlier.However, very interesting macroscopic patterning can occur, particularlywhen depositing nanocrystals by evaporation of a volatile solvent.An evaporating drop of a concentrated nanocrystal dispersion exhibitsvarious flow patterns due to the temperature gradient that occurs between thecenter of the droplet and the outer droplet edge [57,58]. Solvent evaporatesfaster at the edge of the droplet due to the boundary conditions above thedroplet and the evaporative flux, for example, can be expressed as J(r) = J 0 (1r/R) 2 , where k = (p 2h c )/(2p 2h c ). In fact, the flux diverges at the edge ofthe droplet, leading to a significant mass, thermal, and surface tension gradientsbetween the center of the drop and the outer rim. These gradients, inturn, pull the nanocrystals to the edge of the droplet. The evaporating dropletsgenerally exhibit a stick–slip motion during evaporation, where they becomepinned for a short time as nanocrystals accumulate at the droplet edge andCopyright 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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Figure 36 Transmission electron mic
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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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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
Page 371 and 372: Figure 16 Model to explain two-colo
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Page 375 and 376: 10 ps (about an order of magnitude
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
Page 385 and 386: emission spectra of the two individ
Page 387 and 388: Figure 23 Change of the PL intensit
Page 389 and 390: (viz. the absorbed light intensity)
Page 391 and 392: Figure 25Impact ionization in QDs.m
Page 393 and 394: prevent electron-hole recombination
Page 395 and 396: 36. Miller, R.D.J.; McLendon, G.; N
Page 397 and 398: 91. Vurgaftman, I.; Singh, J. Appl.
Page 399 and 400: 139. Mićić, O.I.; Ahrenkiel, S.P.
Page 401 and 402: 10Synthesis and Fabrication of Meta
Page 403 and 404: Figure 2 (A-C) Progression of HR-TE
Page 405 and 406: Figure 3 Schematic for gold nanocry
Page 407 and 408: Nanocrystal growth can occur by two
Page 409 and 410: on the other hand, provide an ensem
Page 411 and 412: Figure 6 (a) SAXS patterns for disp
Page 413 and 414: Figure 8 The gold nanocrystal film
Page 415 and 416: of the stabilizing ligand, and the
Page 417 and 418: successfully modeled the 2D island
Page 419 and 420: 2. Steric Stabilization and a Soft
Page 421: are fully extended. Moving away fro
Page 425 and 426: Figure 13 (A) Transmission electron
Page 427 and 428: function of the density of localize
Page 429 and 430: Figure 15 High-resolution SEM image
Page 431 and 432: thiol-capped nanocrystals [2]. The
Page 433 and 434: 41. Ackerson, B.J. Nature 1993, 365
Page 435 and 436: with the effect of the particle com
Page 437 and 438: Figure 1a shows the surface plasmon
Page 439 and 440: Figure 2 (a) Ultraviolet-visible ab
Page 441 and 442: agents [34]. The short-wavelength b
Page 443 and 444: Figure 4 (a) Plot of the plasmon ab
Page 445 and 446: the framework of traditional Mie’
Page 447 and 448: show that effects due to the surrou
Page 449 and 450: is located at the position of the g
Page 451 and 452: Copyright 2004 by Marcel Dekker, In
Page 453 and 454: Figure 8 Ultraviolet-visible absorp
Page 455 and 456: laser pulses while being mixed in t
Page 457 and 458: Figure 10 shows HR-TEM images of go
Page 459 and 460: Figure 11 Transmission electron mic
Page 461 and 462: final irradiation product. At the s
Page 463 and 464: following the laser excitation appe
Page 465 and 466: 36. Papavassiliou, G.C. Prog. Solid
Page 467 and 468: particles to expand. Because the he
Page 469 and 470: was frequency doubled in a 1-mm B-
Page 471 and 472: 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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