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

More documents

Recommendations

Info

Figure 17 Comparison of absorption (curve i) and PLE spectra (curves ii, iii, and iv)of 38 A˚ InP QDs at 298 K. The PLE for curve ii is detecting emission at 675 nm; PLEcurve iii is the total integrated emission over all wavelengths; and PLE curve iv isemission integrated under PL spectra taken at 20-nm intervals between 400 and 660nm. (From Ref. 74.)where the absorption spectrum of 38-A˚ QDs is shown along with PLE spectraobtained by three different procedures (described in the caption). Whereas theabsorption spectrum continues to rise at higher photon energies above thefirst excitonic peak at about 630 nm, the PLE spectra saturate at energiesabove the first excitonic transition and do not follow the shape of theabsorption spectrum. Pump-probe transient absorption measurements (excitingwell above the bandgap and probing at the band edge) also show thatfor InP QDs, the electrons (or holes) photogenerated at energies above thefirst excitonic transition do not appear at the band edge in proportion to theirphotogenerated density [75]. Thus, it appears another nonradiative channelopens for the higher-energy carriers besides relaxation to the band edgefollowed by radiative emission. The nature of this new channel is presently notunderstood. However, recent studies indicate that the PLE anomaly is notintrinsic in QDs and can be eliminated under certain conditions [76].IV.RELAXATION DYNAMICS OF PHOTOGENERATEDCARRIERS IN QDsWhen the photon energy absorbed in semiconductor QDs is greater than thelowest-energy excitonic transition (frequently termed ‘‘the QD bandgap,’’Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
although this term is not rigorously correct because of electron–hole interactions),photogenerated electrons and holes (usually in the form of excitons)are created with excess energy above the lowest exciton energy; these energeticcarriers are termed ‘‘hot carriers’’ The fate of this excess energy can followseveral paths: (1) It can be dissipated as heat through electron–phononinteractions or Auger processes as the carriers relax to their lowest state; (2)a second electron–hole pair can be created by the process of impact ionizationif the excess energy is at least twice the QD bandgap; (3) the electrons andholes can separate and the excess energy can be converted to increasedelectrical free energy via a photovoltaic effect or stored as additional chemicalfree energy by driving more endoergic electrochemical reactions at the surface[11]. The efficiency of photon conversion devices, such as photovoltaic andphotoelectrochemical cells, can be greatly increased if path 2 or 3 can dominateover path 1. Path 1 is generally a fast process in bulk semiconductorsthat occurs in a few picoseconds or less if the photogenerated carrier densityis less than about 5 10 17 cm 3 [77–79]. The hot electron relaxation, orcooling time can be increased by two orders of magnitude in semiconductorsquantum wells when the photogenerated carrier density is increased aboveabout 5 10 18 cm 3 by a process termed ‘‘hot phonon bottleneck’’ [77,79,80].QDs are intriguing because it is believed that slow cooling of energeticelectrons can occur in QDs at low photogenerated carrier densities [81–87],specifically at light intensities corresponding to the solar insolation on Earth.The first prediction of slowed cooling at low light intensities in quantizedstructures was made by Boudreaux et al. [81]. They anticipated thatcooling of carriers would require multiphonon processes when the quantizedlevels are separated in energy by more than phonon energies. They analyzedthe expected slowed cooling time for hot holes at the surface of highly dopedn-type TiO 2 semiconductors, where quantized energy levels arise because ofthe narrow space charge layer (i.e., depletion layer) produced by the highdoping level. The carrier confinement is this case is produced by the bandbending at the surface; for a doping level of 1 10 19 cm 3 the potential wellcan be approximated as a triangular well extending 200 A˚ from the semiconductorbulk to the surface and with a depth of 1 eV at the surface barrier. Themultiphonon relaxation time was estimated froms c x 1 exp DE ð2ÞkTwhere H c is the hot carrier cooling time, N is the phonon frequency, and DE isthe energy separation between quantized levels. For strongly quantizedelectron levels, with DE > 0.2 eV, H c could be >100 ps according to Eq. (2).However, carriers in the space-charge layer at the surface of a heavilydoped semiconductor are only confined in one dimension, as in a quantumCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Page 1:
Copyright 2004 by Marcel Dekker, In
Page 4 and 5:
Page 6 and 7:
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 34 and 35:
Figure 8 Photoluminescence spectra
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:
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
Page 84 and 85:
Figure 2 (a) Simple model of a nano
Page 88 and 89:
electron and hole to be treated as
Page 90 and 91:
independently, Eq. (13) is commonly
Page 92:
a better description of the bulk ba
Page 95 and 96:
investigated. For optical experimen
Page 97 and 98:
Figure 4 (a) Absorption (solid line
Page 99 and 100:
Figure 6 Normalized PLE scans for s
Page 101 and 102:
Figure 8 A simplistic model for des
Page 103 and 104:
Figure 10 Theoretically predicted p
Page 105 and 106:
Figure 12 Schematics depicting the
Page 107 and 108:
Figure 14 Calculated band-edge exci
Page 109 and 110:
Figure 15 Absorption (solid line) a
Page 111 and 112:
Figure 18 (a) Calculated band-edge
Page 113 and 114:
IV.BEYOND CdSeA. Indium Arsenide Na
Page 115 and 116:
the six-band Luttinger Hamiltonian.
Page 117 and 118:
11. Norris, D.J.; Efros, Al.L.; Ros
Page 119 and 120:
69. Gaponenko, S.V.; Woggon, U.; Sa
Page 121 and 122:
formation of a long-lived dark exci
Page 123 and 124:
where the constant A is determined
Page 125 and 126:
In crystals for which the function
Page 127 and 128:
The respective wave functions areC
Page 129 and 130:
passive, as was shown in Ref. 12. T
Page 131 and 132:
square of the matrix element of the
Page 133 and 134:
where e F = e F ieV and e F F = e x
Page 135 and 136:
see that for all nanocrystal shapes
Page 137 and 138:
optical recombination of the excito
Page 139 and 140:
B. Recombination of the Dark Excito
Page 141 and 142:
where x = cos h and f = l B g e H/3
Page 143 and 144:
The theory of the polarization memo
Page 145 and 146:
Figure 7 The size dependence of the
Page 147 and 148:
state would have an infinite lifeti
Page 149 and 150:
crystal axis [see Eq. (40)]. As a r
Page 151 and 152:
time of the exciton momentum relaxa
Page 153 and 154:
One must also account for the influ
Page 155 and 156:
observed in one of the first studie
Page 157 and 158:
REFERENCES1. Bawendi, M.G.; Wilson,
Page 159 and 160:
4Intraband Spectroscopyand Dynamics
Page 161 and 162:
The solid line in Fig. 1 shows the
Page 163 and 164:
Figure 2 FTIR spectra of n-type CdS
Page 165 and 166:
of the center frequency. The experi
Page 167 and 168:
limit given by radiative relaxation
Page 169 and 170:
from a long lifetime due to the pho
Page 171 and 172:
are two natural approaches to study
Page 173 and 174:
32. Inoshita, T.; Sakaki, H. Physic
Page 175 and 176:
continuous spectral tunability over
Page 177 and 178:
function and envelope function mome
Page 179 and 180:
ottleneck’’ [14,28]. Further re
Page 181 and 182:
in NQDs is dominated by nonphonon e
Page 183 and 184:
Figure 4 Dynamics of the IR postpum
Page 185 and 186:
Figure 5 (a) Time-resolved PL spect
Page 187 and 188:
Figure 6 (a) The time delay of the
Page 189 and 190:
and due to Auger-type e-h interacti
Page 191 and 192:
Figure 9 Dynamics of the 1S bleachi
Page 193 and 194:
significantly greater than the fast
Page 195 and 196:
Figure 11 (a) Pump-intensity-depend
Page 197 and 198:
NQD size. For small NQD sizes (R =
Page 199 and 200:
Figure 14 Nonlinear absorption/gain
Page 201 and 202:
sorption change associated with a s
Page 203 and 204:
where n h em is the hole ‘‘emit
Page 205 and 206:
ultrafast (subpicosecond to picosec
Page 207 and 208:
Figure 18 Schematic of transitions
Page 209 and 210:
Figure 19 Dynamics of pump-induced
Page 211 and 212:
where n i (i=1, 2 . . . , N ) is th
Page 213 and 214:
Figure 21 (a) Two-e-h-pair (biexcit
Page 215 and 216:
the volume fraction (filling factor
Page 217 and 218:
intensity dependence of this peak (
Page 219 and 220:
Page 221 and 222:
can contribute to the saturation of
Page 223 and 224:
Page 225 and 226:
coupling between ‘‘volume’’
Page 227 and 228:
numerous discussions on the photoph
Page 229 and 230:
53. Kang, K.; Kepner, A.; Gaponenko
Page 231 and 232:
the ‘‘on-off ’’ emission in
Page 233 and 234:
parallel form of data acquisition i
Page 235 and 236:
Figure 3 (a) Spectral time trace of
Page 237 and 238:
transition energies. In fact, these
Page 239 and 240:
distribution (dark line) does not d
Page 241 and 242:
exposure to only room light. In our
Page 243 and 244:
statistics for the off times are in
Page 245 and 246:
230Shimizu and BawendiCopyright 200
Page 247 and 248:
Figure 10 (a) Time trace of a CdSe(
Page 249 and 250:
and excited QD core states to fluct
Page 251 and 252:
arrows indicate the on-time truncat
Page 253 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 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
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
Page 349 and 350: narrower in samples with larger mea
Page 351 and 352: GaInP 2 QDs from a plot of the squa
Page 353 and 354: crystal, indicating lattice-matched
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: Figure 16 Model to explain two-colo
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 and 422: are fully extended. Moving away fro
Page 423 and 424:
Figure 11 High-resolution SEM image
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:
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:
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
Page 473 and 474:
Figure 3 Change in radius (DR/R) ve
Page 475 and 476:
In this model, the electrons couple
Page 477 and 478:
Figure 5 Transient bleach data for
Page 479 and 480:
Figure 7 (a) Transient bleach data
Page 481 and 482:
that the particles with >80% Au hav
Page 483 and 484:
3. Del Fatti, N.; Valle´e, F.; Fly
show all

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

Create successful ePaper yourself

Delete template?

Save as template?