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moderate aspect ratios. This suggest that further gains may be possible byusing even longer, wirelike rods [128,129] or by using other nonsphericalnanostructures which can now be reproducibly synthesized [130]. Indeed, theauthors found that devices with moderately sized (8 13-nm) nanorods couldconvert five times as much optical radiation into electrical power as devicesmade from smaller (4 7-nm) nanorods.In some cases, polymer–nanocrystal blends have also been shown toundergo phase separation in the direction perpendicular to the plane of thefilm [118]. Such vertically graded structures can help optimize charge generationand charge collection efficiency, as has been shown in solution-processedmolecular films [125]. Furthermore, adjusting the surface energyinteractions of nanorods and a host polymer can be used to control filmmorphology in a rational manner [131].Finally, although we have largely neglected quantum-confinementeffects in our discussion of nanocrystal photovoltaic devices, one can envisionscenarios where they could be used to tune the response of a detector to aspecific wavelength or to optimize energy level offsets in order to maximizeopen-circuit voltages. Even though these effects have not yet been fullyexploited, the study of quantum-confinement effects has led to the ability tocontrol the size, shape, and surface of these chemically synthesized quantumdots, properties which offer compelling advantages to the development ofsolution-processable photovoltaic devices.VII.CONCLUSIONSNanocrystals provide an interesting system in which to study the physics ofcharge transport at the nanoscopic level. Both electronic tunneling and structuralreorganization play important roles in determining the rate of chargetransport, and we have shown earlier how the charge transport is expectedto change as the nanocrystal size and spacing are varied. Experimental measurementsof charge transport in nanocrystalline films have allowed mobilitiesto be measured and have identified the importance of disorder andtrapping in determining the macroscopic charge transport properties.Nanocrystals also allow the study of photoinduced electron transferfrom organic molecules and polymers to semiconductors, because a largeinterfacial area is present at the nanocrystal surface where charge transfer cantake place. We have shown that CdSe nanocrystals act as good electronacceptors from many conjugated polymers, providing rapid electron transferfrom the photoexcited polymer to the nanocrystal, followed by slow recombinationof the charge-separated state. This charge-separation process can beexploited as the first step in the operation of a photovoltaic device based onCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
composites of nanocrystals and conjugated polymers, and we have reviewedrecent progress in developing both these devices and related structures whichact as light-emitting diodes. Nanocrystal-based electronics remains an excitingarea of research since it allows fine control of electronic energy levelsthrough changing the nanocrystal size, together with the possibility ofstructural control on nanometer length scales by exploiting the ability ofnanocrystals to assemble into useful structures.REFERENCES1. Colvin, V.L.; Schlamp, M.C.; Alivisatos, A.P. Nature 1994, 370, 354.2. Dabbousi, B.O.; Bawendi, M.G.; Onitsuka, O.; Rubner, M.F. Appl. Phys.Lett. 1995, 66, 1316.3. Schlamp, M.C.; Peng, X.; Alivisatos, A.P. J. Appl. Phys. 1997, 82, 5837.4. Gao, M.; Lesser, C.; Kirstein, S.; Mohwald, H.; Rogach, A.L.; Weller, H. J.Appl. Phys. 2000, 87, 2297.5. Greenham, N.C.; Peng, X.; Alivisatos, A.P. Phys. Rev. B 1996, 54, 17,628.6. Huynh, W.U.; Peng, X.; Alivisatos, A.P. Adv. Mater. 1999, 11, 923.7. Klein, D.L.; Roth, R.; Lim, A.K.L.; Alivisatos, A.P.; McEuen, P.L. Nature1997, 389, 699.8. Greenham, N.C.; Friend, R.H. In Solid State Physics; Ehrenreich, H. Spaepen,F., Eds; Academic Press: San Diego, CA, 1995; Vol. 49, 1.9. Friend, R.H.; Gymer, R.W.; Holmes, A.B.; Burroughes, J.H.; Marks, R.N.;Taliani, C.; Bradley, D.D.C.; dos Santos, D.A.; Bre´das, J.L.; Lo¨gdlund, M.;Salaneck, W.R. Nature 1999, 397, 121.10. Miller, R.D.; McLEndon, G.L.; Nozik, A.J.; Schmickler, W.; Willig, F. SurfaceElectron Transfer Processes; VCH Publishers: New York, 1995.11. Marcus, R.A. Annu. Rev. Phys. Chem. 1964, 15, 155.12. Marcus, R.A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265.13. Brus, L. Phys. Rev. B 1996, 53, 4649.14. Schmitt-Rink, S.; Miller, D.A.B.; Chemla, D.S. Phys. Rev. B 1987, 35, 8113.15. Shiang, J.J.; Risbud, S.H.; Alivisatos, A.P. J. Chem. Phys. 1993, 98, 8432.16. Oshiro, K.; Akai, K.; Matsuura, M. Phys. Rev. B 1998, 58, 7986.17. Jortner, J. J. Chem. Phys. 1976, 64, 4860.18. Mattoussi, H.; Radzilowski, L.H.; Dabbousi, B.O.; Thomas, E.L.; Bawendi,M.G.; Rubner, M.F. J. Appl. Phys. 1998, 83, 7965.19. Borovikov, Y.Y.; Ryltsev, E.V.; Bodeskul, I.E.; Feshchenko, N.G.; Makovetskii,Y.P.; Egorov, Y.P. J. Gen. Chem. USSR 1970, 40, 1942.20. Wolfarth, C.; Wohlfarth, B. In Landolt-Bo¨rnstein; Lechmer, M.D., Ed.;Spring-Verlag, Heidelberg, 1996; Vol. 38.A.21. Leatherdale, C.A.; Kagan, C.R.; Morgan, N.Y.; Empedocles, S.A.; Kastner,M.A.; Bawendi, M.G. Phys. Rev. B 2000, 62, 2669.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
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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
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 and 254: W. K. Woo and V. C. Sundar for assi
Page 255 and 256: size allows the electron affinity a
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: all) of the optically excited elect
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
Page 327 and 328: charging indicated that the tunneli
Page 329 and 330: capacitance values were also kept t
Page 331 and 332: could not be detected in the QD/DT/
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 and 342: 66. Su, B.; Goldman, V.J.; Cunningh
Page 343 and 344: dimensional confinement are created
Page 345 and 346: Figure 1 Transmission electron micr
Page 347 and 348: 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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Page 361 and 362:
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)
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
Page 445 and 446:
the framework of traditional Mie’
Page 447 and 448:
show that effects due to the surrou
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is located at the position of the g
Page 451 and 452:
Page 453 and 454:
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
Page 459 and 460:
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final irradiation product. At the s
Page 463 and 464:
following the laser excitation appe
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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
Page 479 and 480:
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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