Dynamics of Recombination Processes in PbI2 Nanocrystals

The present paper discusses the dynamics of recombination processes in PbI2 nanocrystals embedded in porous silica films. The photoluminescence (PL) ...
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J. Phys. Chem. 1996, 100, 8962-8972

Dynamics of Recombination Processes in PbI2 Nanocrystals Embedded in Porous Silica Films I. Dag and E. Lifshitz* Department of Chemistry and Solid State Institute, Technion, Haifa 32000, Israel ReceiVed: September 26, 1995; In Final Form: January 26, 1996X

The present paper discusses the dynamics of recombination processes in PbI2 nanocrystals embedded in porous silica films. The photoluminescence (PL) spectrum of the samples consists of three bands: an exciton band near 2.5 eV and two deeper bands centered at 2.44 eV (L band) and 2.03 eV (G band). The L band relates to bulk defects in the internal volume of the particles, while the G band relates to surface defects. The dynamics of the different recombination events was investigated by continuous and time-resolved PL techniques. All three peaks exhibit a complex decay, which consists of several multiexponential components, progressing from nanoseconds to microseconds. The exciton has an additional fast intrinsic decay component in the sub-nanosecond time scale that may be superradiative in nature. The analysis of the decay dynamics in the nanosecond regime requires a distributed kinetic model, based on the Kohlraushch-Williams-Watts (KWW) stretched exponential function. The experimental results are consistent with detrapping and repopulation processes, in which excited carriers can relax to lower lying surface states (associated with the G band). Thermal detrapping from these states and repopulation of the exciton and L band states results in a long multiexponential decay. The microsecond decay of L and G bands obeys a donor-acceptor recombination characteristic dynamics.

I. Introduction Semiconductor nanocrystals (nanoparticles) have recently become a subject of increasing interest. Their unique electronic, optical, and photochemical properties are being intensively investigated by spectroscopic methods. Among the investigated properties, two aspects of particular importance were found in nanocrystalline systems: (1) The exciton spatial confinement within the semiconductor particles causes a blue shift of the optical transition, creating the so-called “quantum size effect”.1-3 (2) The large ratio of surface atoms to bulk atoms results in a numerous surface trapping sites. These traps strongly influence the entire electronic and optical properties, and they affect, in particular, the dynamics of the optical transitions involved.4,5,13-18 This paper describes the study of PbI2 nanocrystals embedded in SiO2 films. The corresponding bulk PbI2 crystallizes in a layered structure, possessing strong intralayer bonding, yet only weak, so-called van der Waals interlayer interactions.6 The layered structure creates a strong anisotropy in the physical and electronic properties. The bulk layers can be stacked in a variety of ways to form different polytypes. The PbI2 nanocrystals investigated in this work have a 2H polytype, confirmed by X-ray diffraction (XRD) measurements. These samples show an exciton emission band near 2.5 eV and two deeper bands associated with trapped states. Recently, we reported our investigation of quantum confinement of the exciton, associated with an intermediate confinement incident.7 In this regime, the exciton properties can be described by a model of an acceptorlike exciton, due to the unique incident of three-dimensional confinement, in an anisotropic material. The preparation procedure and quantum size effect of PbI2 nanocrystals have been studied in only a few cases before.8-10 In these cases the nanocrystals were grown in colloidal solutions, in zeolite cages, or in copolymer films. We report the investigation of PbI2 nanocrystals embedded in silica glass, prepared by the sol-gel technique. This technique enables the * Corresponding author. X Abstract published in AdVance ACS Abstracts, April 15, 1996.

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growth of stable nanocrystals with a controlled size, within a solid transparent medium. In this paper we will report on the influence of surface states on the dynamic properties of the various recombination processes. These properties have been studied by continuous photoluminescence (PL) measurements and time-resolved photoluminescence methods (TRPL). To the best of our knowledge, the dynamics of recombination processes of PbI2 nanocrystals has not yet been thoroughly investigated. One related work was recently published, concerning the luminescence decay of lead ion clusters in a solid solution of Pb1-xCdxI2.11 The dynamics of excitons and trapped states in other semiconductor nanocrystals, such as CdSxSe1-x,12-14 CdSe,15,16 CdS,17-19 Cd2As3,18 and CuCl,20,21 has been extensively investigated. It appears that the emission decay is composed of two components: a fast component in a pico- to nanosecond time scale and a slow component in a nano- to microsecond time scale. Both components are strongly multiexponential, and they differ in their temperature, emission energy, and surface environment dependence. In some cases, a three-level model was offered to explain the experimental observations.13-18 Henneberger et al.16 suggested that the system undergoes a transition from intrinsic recombination at an early stage to a defect related emission below the absorption edge at a later stage. O’Neil et al.18 and Eychumller et al.17 offered a thermal repopulation mechanism. According to this model, photoexcited charge carriers, populating a band-edge state, have several routes of relaxation: they can relax directly, creating a sub-nanosecond excitonic decay, or they can relax nonradiatively to lower lying surface traps. Pairs recombining from these shallow trapped states produce donor-acceptor type distributed kinetics, responsible for the slow decay component. Some of the trapped carriers can be thermally detrapped, thus repopulating the higher excited state again. Recombination from the thermally repopulated band-edge state results in a multiexponential delayed decay in the nanosecond time scale. This repopulation can take place among electronic states close to the conduction band17,18 or © 1996 American Chemical Society

PbI2 Nanocrystals Embedded in Porous Silica Films among hole states close to the valence band, as suggested by Bawendi et al.15 Several authors explain the decay process in nanocrystals by means of superradiant decay as suggested by Hanamura.22 The fast decay of excitons is due to the fact that they have a coherent excitation over the quantum sphere and a macroscopic polarization. The most dominant experimental evidence of superradiance is an increase of excitonic lifetime with increasing temperature.19-21 Misawa et al. explains the effect of quenching of the excitons’ superradiance by acoustic phonons, confined in the nanocrystallites.19 The PL decay measurements in the present study show that each one of the emission bands has several decay components, progressing from nanoseconds to microseconds, and are strongly multiexponential in any time regime. The exciton decay has an additional faster exponential component in the subnanosecond time scale. The results summarized below suggest that the radiative recombination in PbI2 nanocrystals is composed of several overlapping processes. The sub-nanosecond component originates from an intrinsic excitonic recombination that may be superradiative in nature. The nanosecond multiexponential decay reveals a mechanism of detrapping and repopulation. This process involves electrons and holes trapped at surface sites. The long microsecond decay of the trapped states is a typical donor-acceptor recombination. The dynamic behavior and the role of the surface states will be discussed in more detail, in section IV. II. Experimental Section II.1. Sample Preparation. The nanocrystals of PbI2 embedded in SiO2 films were prepared using a sol-gel technique, similar to the procedure suggested by Nogami et al.23 and Chen et al.24 The sol was prepared by mixing 0.5 mL of a methanol solution of Si(OC2H5)4 (1 M), 0.5 mL of H2O, and a catalytic amount of HNO3 (0.05 mL) and diluting the mixture with methanol to a final volume of 4.5 mL. Then, a 0.5 mL methanol solution of Pb(CH3COO)2‚3H2O (0.26 M) was added to the mixture. The SiO2 gel was formed by hydrolysis and polycondensation reactions. The sol-gel films, incorporated with lead reagent, were either deposited onto a silicon substrate or prepared in a suspended form. The gel was dried at room temperature and then annealed in air, at 450 °C, for 3 h. The annealing conditions control the porosity of the SiO2 films, enabling the formation of lead oxides. The lead substituent is converted mainly into Pb3O4 at 450 °C and in the presence of oxygen. Upon cooling, a mixture of Pb3O4 and PbO oxides was observed. In the present case, the existence of Pb2+ and Pb4+ ions was confirmed by chemical analysis, after the annealing process. Following this, the films were reacted with iodine vapors at 250 °C under a flow of argon gas, for various durations, varying between 10 and 90 min. This stage enabled solid-gas interface reaction between the iodine molecules and lead oxides. It involves the oxidation-reduction process, including the I2w2I- (reduction) and Pb2+wPb4+ (oxidation) half-reactions. The remaining Pb2+ ions within the medium interact with the iodides, to form PbI2 particles. The mean particle radius was controlled by the initial Pb2+ concentration, the annealing temperature, and the duration of the reaction with the iodine vapors. II.2. Surface Treatments. Several surface treatments were carried out in order to investigate the role of the surface area and its influence on the optical spectra. The influence of excess iodide was examined, either by reaction with iodine vapors at 250 °C under a flow of argon gas or by immersing the sample in a diluted KI solution. The influence of iodide deficiency

J. Phys. Chem., Vol. 100, No. 21, 1996 8963 was examined by evaporating a controlled amount of iodide from the sample. This was done by annealing the sample either in air at 100 °C or under vacuum at room temperature, for different lengths of time. II.3. Elimination of By-Product Contribution. PbI2 synthesis may be accompanied by the formation of Inm- species as by-products.25-27 This is most probable under conditions of excess iodide and iodine, created in the surface treatments described above. A control experiment was performed in order to exclude a possible contribution of these by-products to the PL spectrum of PbI2 nanocrystals. Several specimens of solgel SiO2 glass were prepared without the incorporation of lead ion precursors. One of these specimens was reacted with iodine vapors at 250 °C, under a flow of argon gas, for 45 min. Another specimen was immersed in a concentrated I3- solution for 45 min. The PL spectra of these two specimens were detected in the energy range 1.90-2.60 eV and were compared to the PL of a third sample of pure SiO2. All three samples exhibit an identical weak background over the spectral range 1.90-2.60 eV. This suggests that the adsorption of I-, I2, or Inm- species to the SiO2 pores or surface does not affect the PL spectrum of the PbI2 nanocrystals. The background, originating from the SiO2 itself, is too weak to be noticeable in most of the PL spectra, as presented below. The synthesis of PbI2 nanocrystals may be accompanied by the formation of additional minor by-products. Few control experiments (described in ref 28) eliminate the contribution of optical transitions associated with intermediate reagents (lead oxides) and chemical by-products (Pb3O2I2, PbI42-, PbI64-, and PbI3-), in the spectral energy range 1.90-2.60 eV. Thus, the optical transitions of the specimens are associated only with PbI2 particles, embedded within the SiO2 films. II.4. Sample Characterization and Mean Particle Size Determination. PbI2 nanocrystals were prepared with a mean particle radius ranging from 25 to 100 Å. The mean particle radius of specimens, whose particles are larger than 40 Å, and their size distribution were initially derived from transmission electron microscopy (TEM) micrographs. For example, a mean particle radius of 60 Å exhibited a size distribution of (20 Å. However, the mean particle radius of smaller particles could not be determined accurately from TEM micrographs since the small particles are unstable under the high-energy electron beam. We therefore used the exciton peak energy as a measure for change in the mean particle radius. Due to the quantum size effect, the blue shift of the exciton peak increases with decreasing particle size. The correlation between the particle mean radius and the exciton blue shift had been previously calculated, using a unique model for the intermediate confinement regime. This calculation has already been described elsewhere.7 Therefore, the samples that have been used in this work were not investigated by TEM again, and their mean particle radii were determined according to their blue shifts. XRD patterns verified that the majority of the PbI2 nanocrystals exhibit 2H polytype with D3d symmetry. II.5. Optical Measurements. In reference to the optical measurements, samples were mounted in a variable-temperature 12CNDT Janis cryogenic dewar. The PL spectra were carried out by exciting the samples with a continuous 4579 Å (2.71 eV) Ar+ laser (Coherent, Innova 70). The emitted light passed through a holographic grating monochromator (John Yvon Model THR1000) and was detected by a Hamamatsu R666 photomultiplier tube. Time-resolved measurements in the microsecond time regime were performed in the aforementioned system. The continuous Ar+ laser was pulsed with an acoustic modulator (Isomet Model

8964 J. Phys. Chem., Vol. 100, No. 21, 1996

Dag and Lifshitz

Figure 1. Photoluminescence (PL) spectra of PbI2 nanocrystals with mean particle radii of 60 Å (dashed) and 47 Å (solid).

1206C), with a pulse duration of 1 µs to 1 ms, at a frequency of 1-1000 Hz. The emitted luminescence was amplified by a Stanford Model SR445 gain amplifier and recorded with a certain time delay, after an applied excitation pulse by a gated photon counter (Stanford Model SR400). For time-resolved measurements in the nanosecond time regime the samples were mounted in an Air-Product LT110 cryogenic dewar. As an excitation source, we used a CW modelocked Nd:YAG pumped dye laser (Coherent Nd:YAG Antares and a 702 dye laser) providing a high repetition rate ( ND. O’Neil et al.18 claim that for a finite volume there is an upper limit to the integral, which is restricted by the cluster diameter. Therefore, we use eq 6 with an upper limit R, as an additional fitting parameter. The PL decay of the L and G bands of the 60 Å sample, in the microsecond time range, was simulated utilizing eq 6. The results are represented as a solid line in Figure 10. The parameters used for the fit of the L band are R ) 200 Å, Wmax ) 6 × 105 µs-1, RJ ) 34 Å, NA ) 1 × 1016 cm-3, and for the G band R ) 160 Å, Wmax ) 1 × 106 µs-1, RJ ) 26 Å, NA ) 1 × 1017 cm-3. The Wmax, R, and RJ parameters are very similar in the two simulations. The Wmax values indicate that the L and G bands’ lifetime is on the order of 1 µs. R, the upper limit of the integral, represents the largest possible distance of the D-A pair, limited by the particle’s diameter. For a sample with a mean particle radius of 60 Å and a size distribution of (20 Å, the largest diameter of 160-200 Å is reasonable. RJ ≈ 30 Å means that the effective pair distance is ≈60 Å, enabling the existence of several D-A pairs within one particle. The acceptor density, NA, was found to be an order of magnitude larger for the G band. This reveals that the defects’ density on the particles’ surface is much larger than the defects’ density in their internal volume. IV.3. A Suggested Model. On the basis of our experimental results and the previously suggested models, we can derive the following model for the dynamics of recombination processes in PbI2 nanocrystals. Figure 12 shows a diagram of the energy levels of the exciton and the L and G bands. It illustrates schematically a mechanism for the various recombination processes. The quantitative decay analysis of the microsecond decay dynamics according to Thomas and Hopfield’s theory supports the identification of the L and G bands as typical donor-acceptor recombinations. The L band’s levels are represented in Figure 12, as shallow acceptor and donor states. These levels may overlap trapped exciton components, as explained in section IV.1. The G band’s levels are represented as rather shallow donor states and relatively deep acceptor states. This is based on the association of the G band with lead

Figure 12. Schematic model for the various electronic transitions in PbI2 nanocrystals, including radiative recombinations (solid straight arrows), trapping processes (curved solid arrows), and repopulation (curved dashed arrows).

vacancies, known to act as deep acceptors in PbI2.37 While the exciton levels are drawn as relatively discrete states, the donor and acceptor states are represented as an ensemble of levels. This originates from the distribution in the particles’ size, as well as from a distribution of donor and acceptor sites in each particle. The dynamics of the processes in the excited state initiates immediately after an excitation pulse. Some of the photoexcited carriers create excitons, while other free carriers relax into the donor and acceptor states. In addition, some of the excitons relax nonradiatively into L and G associated levels, as represented in Figure 12, by the solid curved arrows. The trapped carriers in each one of the states can recombine directly, creating a fast PL component in all energies. The fast intrinsic exciton recombination is in the subnanosecond time scale, as predicted for spatially confined excitons.22 The fast component of the L and G bands is in the nanosecond regime and is multiexponential due to the distribution in the L and G bands’ related states. However, the fast component of all three peaks is followed by an additional multiexponential component in the nanosecond regime. Quantitatively, none of the nanosecond decay components could be fitted to the Thomas and Hopfield donoracceptor theory. Therefore, we conclude that the donoracceptor recombination of the L and G bands overlaps an additional nonradiative process which dominates the dynamics in the nanosecond time scale. We suggest the existence of a detrapping and repopulation mechanism as the origin of these components. The repopulation among the donor levels is represented in Figure 12 by the dashed curved arrows. The laser power dependence, described in section IV.1, reveals that carriers detrapped from the G band surface states repopulate the exciton and L band states. Carriers detrapped from the L band states can also, possibly, repopulate the excitonic states, though this is less pronounced in the laser power dependence. An alternative model can be based on detrapping and repopulation among the acceptor states, as was suggested by Bawendi et al.15 The latter is less reasonable in our case, because the G acceptor levels are rather deep, and detrapping from such deep states will be less efficient. The above model can explain the lifetime temperature dependence of the various recombination events. The complex temperature dependence of trapped states’ dynamics in nanocrystals was discussed by Eychmuller et al.17 They suggest the

PbI2 Nanocrystals Embedded in Porous Silica Films existence of two competing effects. With increasing temperature, the thermal detrapping rate from a given trap becomes faster, thus decreasing the trapped carriers’ lifetime. On the contrary, the trapped carriers’ population shifts to deeper states at elevated tempertures, resulting in an increase in the carriers’ lifetime. As is shown in Figure 4, a shift in the trap population with increasing temperature appeared in the CW-PL as a red shift in the L and G bands’ central energy. Despite this fact, the G band recombination rate in the nanosecond regime was not affected by the temperature. Possibly, the two aforementioned effects are compensating each other for the G levels in the measured temperature range. The L band’s recombination lifetime becomes longer from 12 to 35 K and then shorter again. The red shift of the L band in the CW-PL appeared only above 50 K. This means that the increase in decay time between 12 and 35 K does not originate from the shift in the L band state’s population. Alternatively, the delay in the L band’s decay may originate from repopulation of the L donor states by electrons detrapped from G donor states. Although this process is not dominant yet at 12 K, it is enhanced with an increasing temperature of up to 35 K. Above 35 K, the L band decay becomes faster because of an additional fast radiationless transition, which is dominant above this temperature. This is manifested in the L band’s intensity quenching appearing in the CW-PL spectrum (Figure 4). The exciton dynamics’ temperature dependence can also be explained by the model represented in Figure 12. Figure 7 and Table 1 show that the weight of the sub-nanosecond exponential component (component A in the KWW fit) decreases from 50% at 12 K to 0% at 77 K. This results in a slower overall decay of the exciton when the temperature increases, despite the temperature independence of the time constants τ1 and 〈τ2〉. We suggest that at low temperatures more excited carrier can recombine directly from the exciton states without relaxation into trapped states. With increasing temperature, the repopulation from deeper traps becomes more efficient. Consequently, the number of intrinsic exciton recombinations decreases (component A), and the number of delayed recombinations increases (component B). Another experimental support for the above model is the particle size dependence of the lifetime. Figure 8 shows a faster decay of the exciton and the L band in the sample with larger mean particle size. It was shown in section III.1 that the G band intensity is drastically suppressed when the surface to volume ratio decreases. Therefore, in larger particles, only a small number of carriers can be trapped by G states and repopulate the exciton and L band states. As a result, the faster intrinsic recombination of the exciton and L band becomes dominant, and their lifetime decreases. The latter observations are consistent also with the unique phenomenon of exciton superradiance that was observed before in nanocrystals of CuCl and CdS.19-21 In these cases, the giant excitonic transition strength and the picosecond dynamics indicated an exciton superradiance. Hanamura22 explained that an exciton has a macroscopic transition dipole moment because it is a coherent excitation over the whole crystal. The interaction of this exciton with a radiation field results in a rapid radiative decay in low-dimensional systems. Misawa19 reported a constant lifetime of about 60 ps below a threshold temperature. Above this threshold, the lifetime increases with increasing temperature. This dependence suggests the quenching of superradiance of excitons by acoustic phonons. Itoch et al.21 and Nakamura et al.20 have shown that in CuCl nanocrystals the radiative lifetime of excitons is inversely proportional to the volume of the crystallite. This fact was interpreted as a

J. Phys. Chem., Vol. 100, No. 21, 1996 8971 result of the change in the coherent volume of excitons, whose motion is restricted by the crystallite size. The similarity between our temperature and particle size dependence of the excitonic decay and the above description may suggest the possibility of a superradiative recombination of excitons in PbI2 nanocrystals, in the picosecond time regime. At longer times, the exciton recombination loses its coherence and becomes strongly multiexponential. This is probably because the exciton states are repopulated by detrapped carriers, as previously explained. The decay of the various peaks extends into the microsecond time regime. In this regime, the L and G bands’ recombination can be described fairly well by a donor-acceptor distributed kinetics. In this time regime, the decay of all three bands becomes faster with increasing temperature. This can be explained by a competition of the radiative decay with a longlived thermally activated nonradiative process.5,46 The appearance of long and short recombination components in PbI2 nanocrystals is not surprising. In some complementary works in the literature concerning bulk PbI2, there are reports of a sub-nanosecond component for the exciton decay,39 a nanosecond component for all three peaks,34,39 and a microsecond component for all three peaks.33,34,57 A similar phenomenon was recently observed in Pb1-xCdxI2.11 The existence of several decay components in bulk PbI2 can be explained by a model similar to that used in PbI2 nanocrystals. As mentioned earlier, surface-like defects are very common in layered compounds, since intralayer defects can effectively act as electron traps. These states are equivalent to the surface defects in the nanocrystals and can play a role in the above model. The appearance of such intralayer defects is strongly dependent on the sample and its preparation conditions. Therefore, in some cases it affects the dynamics of the recombination processes, while in other cases its contribution is negligible. V. Summary The PL spectrum of PbI2 nanocrystals consists of three bands: a spatially confined exciton, a shallow trapped state band (L), related to internal defects in the particle volume, and a deeper trapped state band (G), related to surface defects. The dynamics of the different recombination processes in PbI2 nanocrystals was investigated by continuous and time-resolved PL techniques. It was found that as the particles become smaller, the importance of surface states becomes more prominent. The temporal behavior of the emission and its temperature dependence may be explained by a detrapping and repopulation mechanism. We suggest that the exciton decay, in the subnanosecond regime, is dominated by a fast intrinsic recombination that may be superradiative in nature. At longer times, the exciton lifetime increases, and its decay becomes strongly multiexponential. This is probably due to the exciton states that are repopulated by carriers (electrons) detrapped from G band states (donors). The L and G bands’ recombinations are strongly multiexponential in the nanosecond time regime. This is due to the repopulation process, which creates a distribution in the decay times, as well as the distribution in trap energies, creating several overlapping decay components. Both decays continue to the microsecond time regime, where they obey a donor-acceptor characteristic dynamics. Acknowledgment. This project was supported by the Israel Science Foundation, Contract No. 394/94, by the Israel Ministry of Science, Contract No. 4178-1-93, and by the Center of Advanced Opto-Electronics at the Technion. The authors would

8972 J. Phys. Chem., Vol. 100, No. 21, 1996 like to express their deep gratitude to Prof. D. Huppert, R. Davidi, and E. Poles for their contribution and help with the nanosecond time-resolved measurements, to Prof. R. Chaim for performing the TEM and XRD measurements, to Dr. M. Yassen for the help in the sample preparation, and to L. Bykov for his help in the computer simulations. References and Notes (1) Brus, L. E. J. Chem. Phys. 1984, 80, (9), 4403. (2) Ekimov, A. I.; Efros, Al. L.; Onushchenko, A. A. Solid State Commun. 1985, 56, 921. (3) Ekimov, A. I.; Efros, Al. L.; Ivanov, M. G.; Onushchenko, A. A.; Shumilov, S. K. Solid State Commun. 1989, 69, 565. (4) Carter, A. C.; Majetich, S. A. Mater. Res. Soc. Symp. Proc. 1993, 286, 81. (5) Chestony, N.; Harris, T. D.; Hull, R.; Brus, L. E. J. Phys. Chem. 1986, 90, 3393. (6) Evans, B. L. In Physics and Chemistry of Materials with Layered Structures: Optical and Electrical Properties; Lee, P. A., Ed.; Reidel Publishing: Dordrecht, 1979; Vol. 4, pp 1-143. (7) Lifshitz, E.; Yassen, M.; Bykov, L.; Dag, I.; Chaim, R. J. Phys. Chem. 1994, 98 (5), 1459. (8) Sandroff, C. J.; Kelty, S. P.; Hwang, D. M. J. Chem. Phys. 1986, 85, 5337. (9) Nozue, Y.; Tang, Z. K.; Goto, T. Solid State Commun. 1990, 73, 531. (10) Goto, T.; Saito, S. Proc. SPIE-Int. Soc. Opt. Eng. 1992, 1675, 128. (11) Hayashi, T.; Gu, P.; Watanabe, M. J. Phys. Soc. Jpn. 1994, 63 (6), 2089. (12) Bugayev, A.; Kalt, H.; Kuhl, J.; Rinker, M. Appl. Phys. 1991, A53, 75. (13) Mitsunaga, M.; Shinojima, H.; Kubodera, K. J. Opt. Soc. Am. B 1988, 5 (7), 1448. (14) Tomita, M.; Matsumoto, T.; Matsuoka, M. J. Opt. Soc. Am. B 1989, 6 (2), 165. (15) Bawendi, M. G.; Carroll, P. J.; Wilson, W. L.; Brus, L. E. J. Chem. Phys. 1992, 96, 946. (16) Henneberger, F.; Puls, J.; Spiegelberg, Ch.; Schulzgen, A.; Rossman, H.; Jungnickel, V.; Ekimov, A. I. Semicond. Sci. Technol. 1991, 6, A41. (17) Eychmuller, A.; Hasselbarth, A.; Katsikas, L.; Weller, H. BunsenGes. Phys. Chem. 1991, 95, 79. (18) O’Neil, M.; Marohn, J.; McLendon, G. J. Phys. Chem. 1990, 94, 4356. (19) Misawa, K.; Yao, H.; Hayashi, T.; Kobayashi, T. J. Chem. Phys. 1991, 94 (6), 4131. (20) Nakamura, A.; Yamada, H.; Tokizaki, T. Phys. ReV. B 1989, 40, (12), 8585. (21) Itoch, T.; Furumiya, M.; Ikehara, T.; Gourdon, C. Solid State Commun. 1990, 73 (4), 271. (22) Hanamura, E. Phys. ReV. B 1988, 38 (2), 1228. (23) Nogami, M.; Nagasaka, K. J. Non-Cryst. Solids 1990, 126, 87. (24) Chen, K. C.; Tsuchiya, T.; Mackenzie, J. D. J. Non-Cryst. Solids 1986, 81, 227. (25) Micic, O. I.; Zongguan, L.; Mills, G.; Sullivan, J. C.; Meisel, D. J. Phys. Chem. 1987, 91, 6221.

Dag and Lifshitz (26) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 5005. (27) Peterson, M. W.; Nozik, A. J. In Photoelectrochemistry and PhotoVoltaics of Layered Semiconductors; Aruchamy, A., Ed.; Kluwer Academic Publishers: Dordrecht, 1992; pp 297-317. (28) Lifshitz, E.; Yassen, M.; Bykov, L.; Dag, I.; Chaim, R. J. Phys. Chem. 1995, 99, 1245. (29) Skolnick, M. S.; Bimberg, D. Phys. ReV. B 1978, 18 (12), 7080. (30) Kleim, R.; Raga, F. J. Phys. Chem. 1969, 30, 2213. (31) Warnock, J.; Awschalom, D. D. Phys. ReV. B 1985, 32 (8), 5529. (32) Lifshitz, E.; Bykov, L. J. Phys. Chem. 1993, 97 (37), 9288. (33) Yamazaki, S.; Goto, T. J. Phys. Soc. Jpn. 1982, 51 (10), 3228. (34) Baltog, I.; Piticu, I.; Constantinescu, M.; Ghita, C.; Ghita, L. Phys. Status Solidi A 1979, 52, 103. (35) Physics and Chemistry of Materials with Layered Structures: Structural Chemistry of Layer Type Phases; Levy, F., Ed.; Reidel Publishing, Dordrecht, 1976; Vol. 5. (36) Blashkiv, V. S.; Gamernik, R. V.; Grigorovich, G. M.; Krochuk, A. S.; Nimetsky, V. P.; Franiv, A. V. Ukr. Fiz. J. 1992, 37 (3), 425. (37) De Blasi, C.; Galassini, S.; Manfredotti, C.; Micocci, G.; Ruggiero, L.; Tepore, A. Solid State Commun. 1978, 25, 149. (38) Arends, J.; Verwey, J. F. Phys. Status Solidi 1967, 23, 137. (39) Watanabe, M.; Hayashi, T.; Kato, R. J. Phys. Soc. Jpn. 1990, 59 (12), 4526. (40) Bibik, V. A.; Davydova, N. A. Phys. Status Solidi A 1991, 126, K191. (41) Ehrlich, S. H. J. Imaging Sci. Technol. 1993, 37, (1), 73. (42) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41, and references therein. (43) Spanhel, L.; Arpac, E.; Schmidt, H. J. Non-cryst. Solids 1992, 147, 657. (44) Wang, Y.; Suna, A.; McHugh, J.; Hilinski, E. F.; Lucas, P. A.; Johnson, R. D. J. Chem. Phys. 1990, 92 (11), 6927. (45) Fong, F. K. Theory of Molecular Relaxation. Applications in Chemistry and Biology; Wiley: New York, 1975. (46) Wang, Y.; Herron, N. J. Phys. Chem. 1988, 92, 4988. (47) Julien, C.; Mavi, H. S.; Chevy, A. Phys. Status Solidi B 1993, 177, 143. (48) Thomas, D. G.; Hopfield, J. J.; Augustyniak, W. M. Phys. ReV. 1965, 140 (1A), A202. (49) Williams, G.; Watts, D. C.; Dev, S. B. North Am. Trans. Faraday Soc. 1970, 66, 80. (50) Kim, E. G.; Mattice, W. L. J. Chem. Phys. 1994, 101 (7), 6242. (51) Fukao, K.; Miyamoto, Y. Polymer 1993, 34 (2), 238. (52) Bartsch, E.; Antonietti, M.; Schupp, W.; Sillescu, H. J. Chem. Phys. 1992, 97 (6), 3950. (53) Funke, K. Prog. Solid. State Chem. 1993, 22, 111. (54) Cheville, R. A.; Halas, N. J. Phys. ReV. B 1992, 45 (8), 4548. (55) Chandler, R. R.; Coffer, J. L.; Atherson, S. J.; Snowden, P. T. J. Phys. Chem. 1992, 96, 2713. (56) Baedie, G.; Sauvain, E.; Gomes, A. S. L.; Lawandy, N. M. Phys. ReV. B 1995, 51, 2180. (57) Mikhailovskaya, E. V.; Kostyshin, M. T.; Sharyi, V. M. Zurnal PriVlandnoi Spek. 1979, 31 (2), 336.

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