NANO LETTERS
Carrier Relaxation Dynamics in GaSe Nanoparticles
2002 Vol. 2, No. 9 1015-1020
V. Chikan and D. F. Kelley* Department of Chemistry, Kansas State UniVersity, Manhattan Kansas 66506-3701 Received July 2, 2002; Revised Manuscript Received August 2, 2002
ABSTRACT Hole relaxation dynamics have been studied in approximately 4 nm diameter GaSe nanoparticles in solution. The particles are photoexcited with femtosecond pulses at 387.5 nm and subsequently probed at 500 to 700 nm. A strong absorption is observed in this range, which is assigned to hole intraband transitions. This assignment is made on the basis of the decay kinetics, compared with the emission decay kinetics. The assignment is also confirmed by carrier quenching studies in which the nanoparticles are capped with pyridine ligands that act as hole acceptors and thereby quench the transient absorption. The interfacial hole transfer time to the pyridine ligands is about 2.5 ps. The 500 to 700 nm absorption exhibits a 20 ps rise component, having a wavelength-dependent amplitude. This transient is assigned to a hole intraband relaxation. The comparison of this hole relaxation rate with carrier relaxation rates in other types of semiconductor nanoparticles is discussed.
Introduction. The spectroscopy and charge carrier dynamics in semiconductor nanoparticles have received a great amount of attention in recent years. It is the spectroscopic and electronic properties of any type of semiconductor nanoparticles that determine the technological application for which it will be suited.1 Central to the dynamics are the rates of carrier relaxation and trapping. These dynamics have been studied in several different types of semiconductor nanoparticles. The nanoparticles in which carrier trapping rates have been determined include CdS,2-5 InP,6-8 SnO,9 and several others.10-12 In most cases, carrier trapping occurs in less than a few picoseconds. The excited-state dynamics of MoS2 and related types of nanoparticles have also recently been studied.13,14 These studies involve the use of timeresolved absorption and emission polarization spectroscopies. The electron and hole dynamics are separated through carrier quencher and carrier injection studies. These studies yield carrier trapping times that are much slower than in most other types of nanoparticles. Electron and hole trapping were found to occur on the 300 and 30 ps time scales, respectively.15-18 Due to well-developed synthetic methods, the most thoroughly studied type of nanoparticle is CdSe. In this case, electron intraband relaxation rates have been extensively studied. The results show that electron relaxation is facilitated by Auger interactions with the hole and occurs on the subpicosecond time scale.19-22 However, in the absence of the hole and therefore electron-hole Auger processes, much slower relaxation (picoseconds to hundreds of picoseconds) is observed.22,23 Subsequent electron and hole trapping processes have also been studied, and occur on longer time * Corresponding author:
[email protected] 10.1021/nl025678m CCC: $22.00 Published on Web 08/24/2002
© 2002 American Chemical Society
scales.23-25 Carrier quenching and time-resolved emission results complement transient absorption studies and have permitted assignment of the observed transients to electron or hole processes. The results indicate the trapping of holes into shallow, intrinsic surface states occurs on the 2 ps time scale, while electron trapping into surface defect states occurs on the 30 ps time scale. These processes are followed by subsequent relaxation to deeper trap states.25 GaSe has interesting structural and electronic properties and our initial studies indicate that these properties are reflected in the nanoparticles.26,27 GaSe has a hexagonal layered structure consisting of Se-Ga-Ga-Se tetra-layered sheets.28 Each of the selenium sheets forms a twodimensional hexagonally close packed plane, with the selenium atoms in each of the planes aligned with those in the opposing plane. The gallium atoms are in pairs, aligned along the z axis (perpendicular to the selenium planes) in the trigonal prismatic sites between the selenium sheets. Bulk GaSe consists of Se-Ga-Ga-Se tetra-layer sheets, separated by a relatively large gap, and having only weak van der Waals interactions holding the Se-Ga-Ga-Se tetralayer sheets to each other. There are three different crystal structures of GaSe (β, γ, and ), differing in how the Se-Ga-Ga-Se layers stack on each other. Bulk GaSe is a photoluminescent, indirect band gap semiconductor, having a 2.11 eV direct band gap.29,30 The energetic separation between the direct and indirect band gap is quite small, about 25 meV. In high quality crystals, emission is seen almost entirely from direct band edge exciton.31,32 Depending on the defect density of the crystal, emission can also be observed from the indirect band edge
exciton and from trap states. The electronic structure is such that GaSe nanoparticles can be extremely photostable. In the bulk material, the direct band gap transition is at Γ and is characterized by having an electronic nodal plane between the layers of gallium atoms.29-31 Production of a node in the electronic wave function at the plane midway between the planes of gallium atoms corresponds to considerable GaGa, σ* character in the band edge states, which does little to weaken the Ga-Se bonds. This is an important point because it suggests that this excited state may be far less reactive (and hence more photostable) than the band gap states of other types of semiconductor nanoparticles involving metal-chalcogenide antibonding character. We have very recently synthesized and characterized GaSe nanoparticles.26 The synthesis is similar to that used to obtain CdSe nanoparticles33 and results in particles having diameters between 2 and 6 nm. Electron diffraction results indicate that the particles are disks, exactly four atoms thick; that is, they have a single tetra-layer (Se-Ga-Ga-Se) morphology. Like CdSe nanoparticles, these particles are capped with trioctyl phosphine and/or trioctyl phosphine oxide. GaSe nanoparticles are strongly luminescent with an emission quantum yield of about 15%. We have recently studied their wavelength-dependent emission kinetics.27 These studies have shown that following 400 nm photoexcitation, there are three distinct emission decay components, having decay times of 80 ps, 400 ps, and 2.4 ns. Polarization and spectral shift results, along with comparison to the known spectroscopy of bulk GaSe, allow assignment of the 80 ps transient to an electron relaxation corresponding to a direct to indirect band edge relaxation. The subsequent 400 ps and 2.4 ns transients are assigned to hole trapping in shallow and deep traps, respectively. The extent of the spectral shifts associated with each of these components indicates that the trap depths are similar to those in the bulk material. In this paper, we continue and expand the study of GaSe nanoparticle photophysics. Specifically, we use time-resolved spectroscopy to detect absorptions that can be assigned to intraband hole transitions. Holes are produced by nanoparticle photoexcitation, and the time and wavelength dependence of the absorption is used to elucidate the hole relaxation dynamics. Experimental Section. GaSe nanoparticles are synthesized using a method previously described.26 Briefly, the synthesis is based on the reaction of trimethyl gallium with trioctyl phosphine selenium in a high-temperature solution of trioctyl phosphine (TOP) and trioctyl phosphine oxide (TOPO). In this synthesis, a solution of 15 g of TOPO and 5 mL of TOP is heated to 150 °C overnight in a nitrogen atmosphere. Commercial TOPO is typically wet, and this heating is to remove any water. Prior to making this solution, the TOP (technical grade from Aldrich) has been vacuum distilled at 0.75 Torr, taking the fraction from 204 °C to 235 °C. A solution consisting of 12.5 mL of TOP with 1.579 g Se (99.999%) is then added to the mixture. The selenium dissolves to form TOPSe. The above TOP/TOPO/TOPSe reaction mixture is then heated to 278 °C. This is followed by the injection of 0.8 mL of GaMe3 dissolved in 7.5 mL of 1016
distilled TOP. The temperature is then held at 266-268 °C for about 2 h. The presence of nanoparticles is indicated by the appearance of a 400-450 nm absorption. The reaction mixture is then cooled to room temperature and the particles can be extracted with methanol, with the nanoparticles going into the methanol phase. Following the extraction, the methanol is evaporated and GaSe nanoparticles are redissolved in toluene or pyridine. Time-resolved absorption measurements were made using the femtosecond absorption spectrometer described previously.34 Briefly, the femtosecond light source is based on a Clark-MXR 2001. This produces 775 nm, 130 fs, 800 µJ pulses at a repetition rate of 1 kHz. About 4% of the pulse intensity is split off, attenuated, and used to generate a white light continuum. Wavelength selection is accomplished using (25 nm band-pass interference filters. The remainder of the 775 nm beam is doubled and attenuated to give ca. 15 µJ, 387.5 nm, 120 fs pulses at the sample. The pump beam is typically focused to a spot size of about 1.0 mm at the sample. The power density can be varied by changing the position of the sample with respect to the focal point of the pump beam. In the results presented here, an increase of the power density by a factor of 5 has no detectable effect on the observed kinetics. The low intensity probe beam is split into reference and sample probe components. The intensity of the probe pulses is less than 1 µJ at the sample. Prior to the sample, the probe beam is linearly polarized at a 45° angle with respect to the (vertical) polarization of the excitation pulse. After the sample, the probe beam is split into horizontal and vertical polarization components. These beams and the I0 beam are imaged onto UDT Sensors PIN 13DI photodiodes, biased at -15 V. The photodiode outputs are amplified and input into an Stanford Research Systems gated integrator. The gated integrator output is measured using a National Instruments 16 bit A/D converter in the data acquisition computer. The A/D converter and gated integrator triggering and reset are synchronized with a CPA 2001 Q-switch and controlled by home-built timing electronics. Data acquisition is controlled by LabView software running on a Pentium II computer. In this configuration, the probe intensities are sensitive to both polarized transient absorption and transient birefringence (the optical Kerr effect). Alternatively, the polarization of the probe beam can be set to either 0° or 90° with respect to the pump polarization. From the combination of these two polarizations, time-resolved absorption anisotropies can also be determined. The advantage of the latter configuration is that the signals are not affected by optical Kerr effect transients.35 Comparison of the results obtained in the two different configurations (at 45° versus parallel and perpendicular polarizations) permits determination of the duration of the optical Kerr effect transient. This turns out to be about 10-15 ps for the samples studied here. Results and Discussion. The absorption spectrum of 4 nm GaSe nanoparticles in room temperature toluene is shown in Figure 1. An absorption onset occurs in the 440 nm region, with a distinct shoulder at about 400 nm. There is very little static absorption to the red of about 450 nm. This spectrum Nano Lett., Vol. 2, No. 9, 2002
Figure 1. Absorption spectrum of GaSe nanoparticles in toluene solution. The arrow indicates the excitation wavelength of 387.5 nm.
is essentially identical to those reported in our original papers describing the synthesis and time-resolved emission spectroscopy of GaSe nanoparticles.26 Much of the width of the absorption onset is due to the finite particle size distribution. In the present studies, the sample is photoexcited with 387.5 nm polarized light, which puts as much as 3500 cm-1 of energy into the particles above band gap excitation. Following photoexcitation, the sample is subsequently probed with white light that is linearly polarized at a 45° angle with respect to the pump light polarization. The photoexcited nanoparticles exhibit a broad absorption in the 500 to 700 nm region. In addition to the transient absorption, a transient birefringence, i.e., an optical Kerr effect, is also observed. Averaging the parallel and perpendicular polarizations eliminates the transient birefringence signal. Since the birefringence only rotates the plane of polarization, taking the average of the parallel and perpendicular absorption components results in a transient that depends only on the absorption signal. The parallel, perpendicular, and average absorbances are shown in Figure 2, with a probe wavelength of 600 nm. Figure 2 shows that at short times (less than a few picoseconds) the apparent polarized absorbance changes are dominated by the transient birefringence. Most of the birefringence transient decays within a few picoseconds, with a small tail extending out to about 10-15 ps. In this paper, we will ignore the optical Kerr effect transient and focus on the transient absorption and its polarization characteristics at long times. The (polarization averaged) transient absorption exhibits fairly complicated kinetics. When probed at 600 nm, about 60% of the absorption appears within the instrument response and the remaining 40% of the absorption appears with a 20 ps time constant. The absorption subsequently decays on a longer time scale. These kinetics can be fit to a partial 20 ps rise followed by a biphasic (400 ps, 2.4 ns) decay, as shown in Figure 3. Assignment of the relaxation processes responsible for these transients is facilitated by comparison to time-resolved emission results,27 which are briefly summarized below. The total (unpolarized) emission exhibits decay components of 80 ps, 400 ps, and 2.4 ns, while the emission anisotropy exhibits 400 ps and 2.4 ns decay components. The emission polarization is Nano Lett., Vol. 2, No. 9, 2002
Figure 2. Polarized transient absorption kinetics at 600 nm. The apparent absorptions parallel and perpendicular to the polarization of the excitation light, and the average of these curves are shown.
Figure 3. 600 nm absorption kinetics. The kinetics taken over a longer time scale (0-700 ps) are shown in the inset. Also shown is a calculated curve corresponding to a partial (40%) 20 ps rise and a 400 ps and 2.4 ns decay.
determined by the mixing of several different valence bands.31,36 Because the different emission polarizations involve only differences in the valence band (i.e., the hole state), polarization results allow assignment of these transients. The 80 ps total emission decay component results in no emission depolarization, and because the emission polarization is independent of the electron state, the observed 80 ps emission transient must be assigned to an electron relaxation. In analogy to the bulk material, GaSe nanoparticles are an indirect band gap semiconductor, and this relaxation corresponds to an direct to indirect band edge relaxation. Reconstructed time-resolved spectra allow spectral shifts to be assigned to each of the decay components. These results indicate that the indirect and direct band gaps in 4 nm nanoparticles are separated by about 450 cm-1 (about a factor of 2 more than in bulk GaSe). The 400 ps and 2.4 ns transients are associated with depolarization decays and are therefore assigned to hole relaxations. The spectral shifts 1017
Figure 4. 550 nm absorption kinetics for pyridine-capped GaSe nanoparticles. Also shown is a calculated curve corresponding to 2.5 ps (67%), 20 ps (20%), and constant (13%) decay components.
indicate that these decays can be assigned to hole relaxations into shallow and deep traps, respectively. It is important to note that the time-resolved emission polarization results show that following the 80 ps transient the hole is at the top of the valence band and the electron is at the bottom of the conduction band; neither carrier is trapped. Subsequent hole trapping occurs on the 400 ps and 2.4 ns time scales.27 The absorption kinetics in Figure 3 show no 80 ps transient. If this absorption was due to any transition involving the electron, then a change in the electron state would be expected to give rise to a change in the absorption intensity. This is not observed, indicating that the absorption is independent of the electron state. Because the emission polarization results show that hole trapping occurs on the 400 ps and 2.4 ns time scales, the 500-700 nm transient absorption is assigned to a hole intraband transition. Consistent with this assignment, the absorption decay can be fit to the same decay times as the hole trapping times derived from the emission studies. This assignment is further confirmed by hole quenching studies. Pyridine is known to act as a hole acceptor,22,25 and hole transfer to adsorbed pyridine ligands therefore quenches any transition associated with the hole. The absorption kinetics for pyridine-capped nanoparticles are shown in Figure 4. The absorption decays rapidly as a result of hole quenching. Most of the absorption decay occurs with a 2.5 ps time constant, indicative of the hole transfer time. A relatively small portion decays with an approximately 20 ps time constant, to be discussed below. There is also a small long-lived (.100 ps) component that decays by the next laser pulse, 1 ms. The probe wavelength dependence of the transient absorption kinetics is shown in Figure 5. The maximum absorbance occurs in the 550 to 600 nm region. The 600 nm kinetics shown in Figure 5 are indistinguishable from those presented in Figures 2 and 3. A fast, instrument-limited rise is followed by a 20 ps rise, which subsequently undergoes the slow (400 ps, 2.4 ns) decay. (The 500 nm and, to a lesser extent, 600 nm kinetics also show a very short-lived transient absorption in the first few picoseconds. This transient is real and not an artifact of the birefringence, but will not be analyzed in 1018
Figure 5. Wavelength-dependent absorption kinetics. Results at probe wavelengths of 500 nm, 600, 650, and 700 nm are shown.
this paper.) The 500 nm kinetics show that the relative magnitude of the slowly rising (20 ps) component is larger than at 600 nm, and that the time constant remains constant at 20 ps. The magnitude of the 20 ps rise is very small at 650 nm. At 700 nm, the initial transient is a small amplitude, 20 ps decay. Taken together, these data indicate that the transient absorption starts out peaked in the near-infrared region of the spectrum and that this peak shifts back to a maximum at 550-600 nm with a 20 ps time constant. Since the absorption is assigned to valence band holes and the relaxed state has the hole at the top of the valence band, this 20 ps transient must correspond to an intraband hole relaxation. From the above arguments, we conclude that the state populated following the 20 ps relaxation, but prior to hole trapping, corresponds to the hole being at the top of the valence band and exhibits an absorption in the 500 to 700 nm region. Polarization results indicate that this band may not be assigned to a single broad intraband transition. The polarization of the transient absorption can be characterized by its anisotropy, defined as r ) (Apar - Aperp)/(Apar + 2Aperp) For photoselection from a randomly oriented ensemble of oscillators, the anisotropy is determined by the angle between the pump and probe oscillators. Specifically, r ) 1/5 (3cos2θ - 1), where θ is the angle between the oscillators. In these two-dimensional nanoparticles, the x-y plane is defined as the nanoparticle plane and the z-axis is taken to be the unique axis. We have previously shown that absorption in the 350 to 440 nm region is primarily z-axis polarized in these nanoparticles27 (as is the direct band gap absorption in bulk GaSe31). Thus, if the transient absorption oscillator is also z-axis polarized, the absorption anisotropy will have a positive value, approaching 0.4. Alternatively, if the transient absorption oscillator is x-y polarized, the anisotropy will be negative, approaching -0.2. Intermediate anisotropies indicate mixed or overlapping transitions. Figure 6 shows the probe wavelength-dependent emission anisotropy kinetics. The short time behavior is dominated Nano Lett., Vol. 2, No. 9, 2002
Figure 6. Wavelength-dependent absorption anisotropy kinetics. Results at probe wavelengths of 500 nm, 600, 650, and 700 nm are shown. The first several picoseconds are dominated by an optical Kerr effect signal and therefore do not reflect the actual absorption anisotropy. Table 1: Relaxed Absorption Anisotropies as a Function of Probe Wavelength 500 nm -0.01
550 nm -0.01
600 nm .017
650 nm .040
700 nm 0.052
by the optical Kerr effect transient and is therefore not indicative of the absorption polarization. These results are obtained with 45° pump/probe relative polarizations. Comparison with results obtained at 0° and 90° pump/probe relative polarizations indicates that the optical Kerr effect transient has almost completely decayed after about 10-15 ps. Furthermore, the unrelaxed hole absorption has an anisotropy different (negative) than the relaxed hole absorption. As discussed above, hole relaxation takes place on the 20 ps time scale. Thus, the anisotropies obtained after about 40 ps are indicative of the relaxed hole absorption. These values are collected in Table 1. The absorption has a negative anisotropy when probed to the blue of 600 nm and a positive anisotropy at 600 nm and further red. This indicates the presence of two overlapping transitions that form the observed band. The blue edge is dominated by an in-plane absorption while the red edge is dominated by an absorption that is polarized perpendicular to the plane of the nanoparticle. While it is possible to speculate about the assignment of these transitions and the assignment of the unrelaxed hole transition, no definite assignments can be made on the basis of the results presented here. To make such assignments, data from different sizes of nanoparticles is necessary. This is because changing the size (the x-y dimension) of these single tetra-layer nanoparticles will affect the extents of x-y and z quantum confinement differently, and therefore change the relative energetics of x-y and z polarized transitions in a predictable way. These studies are currently in progress and will be reported in a later paper. As mentioned above, in the case of the pyridine capped nanoparticles (Figure 4), the hole intraband transient absorption has a small component that can be fit to a 20 ps decay. The amplitude of this component is very small, making accurate determination of the decay time problematic. However, within the accuracy with which it can be measured, Nano Lett., Vol. 2, No. 9, 2002
the decay time seems to match the 20 ps hole intraband relaxation time. Because of the difficulty of accurately measuring the decay time, any conclusions based on the coincidence of decay times are necessarily tentative. If this match of decay times is correct, it implies that for the particles in which photoexcitation produces a hole in an excited state, hole relaxation and transfer to a pyridine ligand are coupled. This suggests that hole transfer to the pyridine occurs rapidly (2.5 ps) only if the hole is in the relaxed valence band state. Holes in excited valence band states do not undergo rapid transfer to the pyridine ligands; they persist until they undergo relaxation within the valence band. These results therefore suggest that excited holes undergo slower interfacial charge transfer than relaxed ones. This is an interesting conclusion, because it implies that hole transfer to the pyridine ligands is in the Marcus inverted regime. It is of interest to compare the time scale of the intraband hole relaxation observed here with carrier relaxation times in other types of semiconductor nanoparticles. Intraband relaxation dynamics have been studied in CdSe and, to a lesser extent, InP nanoparticles.20,22,23,37 In the CdSe case, the effective mass of the hole is much greater than that of the electron. As a result, the energy gaps between the quantum-confined hole levels are much less than for the electron levels. Electron relaxation in CdSe nanoparticles occurs from a 1P to a 1S level. These levels are separated by an energy of about 2500 cm-1 to 4000 cm-1, depending on particle size.23 Following photoexcitation of the particle, this relaxation is found to occur very quickly, on a subpicosecond time scale. However, if the hole is removed from the nanoparticle by trapping into a surface state, or if the electron is put in the conduction band by chemical reduction, then much slower electron relaxation is observed. In both cases, there is no hole for the electron to interact with and 1P to 1S relaxation times of 3 ps21 and 200 ps23 have been reported. These results indicate that electron relaxation is facilitated by an Auger interaction between the excited electron level and the manifold of closely spaced hole levels. In the absence of the hole, the electron relaxation can occur only by a multiphonon mechanism, and is much slower. Similar photophysics are observed for InP nanoparticles.37 In the GaSe nanoparticle case, the effective mass treatment of quantum confinement is slightly more complicated because of the disk-like shape of the particles and the fact that the electron and hole effective masses are anisotropic. The z-axis effective masses30,31,38 are mh* ) 0.2, me* ) 0.3, and the x-y plane effective masses mh* ) 0.8, me* ) 0.17. However, the effective mass treatment of quantum confinement is expected to break down, especially for quantum confinement along the z axis, in these single tetra-layer particles. The exact spacing of quantum confined electron and hole energy levels is therefore difficult to predict. However, an important qualitative point follows from the effective mass considerations: much of the quantum confinement is along the z-axis, and the z-axis hole effective mass is fairly small, somewhat less than that of the electron. As a result, the hole levels are expected to be much more widely spaced than in the CdSe case, and their spacing is expected to be comparable to the 1019
electron levels. Comparably spaced electron and hole levels preclude the possibility of very fast hole relaxation, facilitated by electron/hole Auger interactions. Furthermore, in these small particles, the hole energy levels are expected to be sufficiently widely spaced such that multiphonon relaxation is inefficient. These qualitative considerations explain why slow (20 ps) hole intraband relaxation is observed. We also note that the hole intraband relaxation time (20 ps) is of the same order of magnitude as the electron relaxation corresponding to the indirect to direct band edge transition (80 ps). In the electron relaxation case, the energy dissipated is fairly small (about 450 cm-1), but the situations are similar because Auger interactions are not expected in either case. Thus, slow relaxations are observed for both the electrons and holes. Conclusions. Several conclusions can be drawn from the results presented here, and are summarized as follows. (1) Photoexcitation of GaSe nanoparticles results in a transient absorption in the red (500 to 700 nm). This absorption exhibits the same decay kinetics as the emission, except lacking the 80 ps component assigned to an electron relaxation. These results indicate that the transient absorption can be assigned to hole intraband transitions. (2) Pyridine can be used to displace the TOPO ligands attached in the particle synthesis. The presence of pyridine ligands quenches valence band holes with an interfacial hole transfer time of about 2.5 ps. The observation that pyridine quenches the transient absorption confirms the assignment to hole intraband transitions. (3) Absorption is positively polarized on the red edge and negatively polarized on the blue edge of the transient absorption band. This is consistent with the presence of overlapping hole intraband transitions. (4) There are pulse width limited and 20 ps rise time components to the transient absorption. The relative amplitudes of these components are wavelength dependent, indicating that the absorption shifts to the blue as relaxation proceeds. This indicates the presence of relaxed and unrelaxed hole intraband transitions, with a relaxation time of 20 ps. We note that this is faster, but of the same order of magnitude as electron relaxation in the absence of an Auger mechanism in CdSe. Acknowledgment. This work was supported by a grant from the Department of Energy (Grant # DE-FG0300ER15037). References (1) Semiconductor Nanoclusters - Physical, Chemical and Catalytic Aspects; Kamat, P. V., Meisel, D., Eds.; Elsevier: Amsterdam, 1997. (2) Eychmu¨ller, A.; Ha¨sselbarth, A.; Katsikas, L.; Weller, H. Ber. Bunsen.-Ges. Phys. Chem. 1991, 95, 79.
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(3) Skinner, D. E.; Colombo, D. P.; Cavaleri, J. J.; Bowman, R. M. J. Phys. Chem. 1995, 99, 7853. (4) Zhang, J. Z.; O’Neil, R. H.; Roberti, T. W. J. Phys. Chem. 1994, 98, 3859. (5) Zhang, J. Z.; O’Neil, R. H.; Roberti, T. W.; McGowen, J. L.; Evans, J. E. Chem. Phys. Lett. 1994, 218, 479. (6) Kim, S. H.; Wolters, R. H.; Heath, J. R. J. Chem. Phys. 1996, 105, 7957. (7) Micic, O. I.; Cheong, H. M.; Fu, H.; Zunger, A.; Sprague, J. R.; Mascarenhas, A.; Nozik, A. J. J. Phys. Chem. B 1997, 101, 4904. (8) Langof, L.; Ehrenfreund, E.; Lifshitz, E.; Micic, O. I.; Nozik, A. J. J. Phys. Chem. B 2002, 106, 1606. (9) Cavaleri, J. J.; Colombo, D. P.; Bowman, R. M. J. Phys. Chem. 1998, 102, 1341. (10) Cherepy, N. J.; Liston, D. B.; Lovejoy, J. A.; Deng, H.; Zhang, J. Z. J. Chem. Phys. B 1998, 102, 770. (11) Sengupta, A.; Jiang, B.; Mandal, K. C.; Zhang, J. Z. J. Phys. Chem. B 1999, 103, 3128. (12) Zhang, J. Z. J. Phys. Chem. B 2000, 104, 7239. (13) Kelley, D. F. In Semiconductor Photochemistry and Photophysics; Schanze, K. S., Ramamurthy, V., Eds.; Marcel Dekker: New York, 2002; Vol. 10, in press. (14) Chikan, V.; Kelley, D. F. J. Phys. Chem. B 2002, 106, 3794. (15) Doolen, R.; Laitinen, R.; Parsapour, F.; Kelley, D. F. J. Phys. Chem. B 1998, 102, 3906. (16) Huang, J. M.; Kelley, D. F. J. Chem. Phys. 2000, 113, 793. (17) Huang, J. M.; Laitinen, R.; Kelley, D. F. Phys. ReV. B 2000, 62, 10995. (18) Chikan, V.; Waterland, M. R.; Huang, J. M.; Kelley, D. F. J. Chem. Phys. 2000, 113, 5448. (19) Woggon, U.; Geissen, H.; Gindele, F.; Wind, O.; Fluegel, B.; Peyghambarian, N. Phys. ReV. B 1996, 54, 17681. (20) Klimov, V. I.; McBranch, D. W. Phys. ReV. Lett. 1998, 80, 4028. (21) Klimov, V. I.; Schwarz, C. J.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Phys. ReV. B 1999, 60, R2177. (22) Klimov, V. I.; Milhailosky, A. A.; McBranch, D. W.; Leatherdale, C. A.; Bawendi, M. G. Phys. ReV. B 2000, 61, R13349. (23) Guyot-Sionnest, P.; Shim, M.; Matranga, C.; Hines, M. Phys. ReV. B 1999, 60, R2181. (24) Underwood, D. F.; Kippeny, T.; Rosenthal, S. J. J. Phys. Chem. B 2001, 105, 436. (25) Burda, C.; Link, S.; Mohamed, M.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 12286. (26) Chikan, V.; Kelley, D. F. Nano Lett. 2002, 2, 141. (27) Chikan, V.; Kelley, D. F. J. Chem. Phys., submitted. (28) Levy, F. Crystallography and Crystal Chemistry of Materials with Layered Structures; Reidel: Holland, 1976. (29) Grasso, V. Electronic structure and electronic transitions in layered materials; Reidel: Dordrecht, 1986. (30) Wieting, T. J.; Schluter, M. Electrons and Phonons in Layered Crystal Structures; Reidel: Dordrecht, 1979; Vol. 3. (31) Mooser, E.; Schluter, M. NuoVo Cimento 1973, 18B, 164. (32) Capozzi, V. Phys. ReV. B 1989, 40, 3182. (33) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (34) Waterland, M. R.; Kelley, D. F. J. Phys. Chem. A 2001, 105, 4019. (35) Fleming, G. R. Chemical Applications of Ultrafast Spectroscopy; Oxford: New York, 1986. (36) Schluter, M. NuoVo Cimento 1973, 13B, 313. (37) Ellingson, R. J.; Blackburn, J. L.; Fu, H.; Micic, O. I.; Nedeljkovich, J.; Nozik, A. J. In Twenty-fifth DOE Photochemistry Research Conference, 2002. (38) Ottaviani, G.; Canali, C.; Nava, F.; Schmid, P.; Mooser, E.; Minder, R.; Zschokke, I. Solid State Comm. 1974, 14, 933.
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Nano Lett., Vol. 2, No. 9, 2002
