J. Phys. Chem. 1995,99, 1245-1250
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Photodecomposition and Regeneration of PbI2 Nanometer-Sized Particles, Embedded in Porous Silica Films E. Lifshitz," M. Yassen, L. Bykov, and I. Dag Department of Chemistry and Solid State Institute, Technion, Ha ifa 32000, Israel
R. Chaim Department of Materials Engineering, Technion, Haifa 32000, Israel Received: February 9, 1994; In Final Form: October IO, 1994@
The photoluminescence (PL) spectrum of nanometer-sized particles of PbI2, at 77 K, is dominated by an acceptor-like exciton emission. When the specimens were irradiated with visible light at temperature of 70 "C, the exciton energy was blue-shifted and its intensity decreased. This blue shift is a manifestation of a quantum size effect, and it indicates the reduction of the particle's size. The changes in the PL spectra showed strong dependence on the radiation conditions. These results suggest that the irradiation produces chemical decomposition of Pb-I bonds. Moreover, retreatment of the particles with iodine vapors nearly regenerated the original PbIz particle's size
I. Introduction Over the past decade, there has been a significant increase of interest in nanometer-sized semiconductor particles (Qparticles), embedded in transparent media.1-11 These particles exhibit unique optical properties created due to quantum size effect^.'^^'^ These optical properties are pronounced in the excitonic transitions associated with the fundamental absorption edge. In a recent paper," we have described the structural and exciton properties of lead iodide (PbI2) Q-particles, embedded in Si02 films. This paper will elaborate on the photodecomposition (PD) process in PbI2 particles. This process is induced by visible light radiation at temperatures between 25 and 100 "C and results in the chemical decomposition of Pb-I bonds and reduction of particle size. Retreatment of the Q-particles with iodine vapors nearly regenerates the original particle's size. The lead iodide crystallizes in a layered structure, possessing strong intralayer bonding and only weak, so-called, van der Waals interlayer intera~ti0ns.l~The layers can be stacked in a variety of ways to form different polytypes. The most common polytype is named 2H with (AbA), stacking (A = I, b = Pb). Several reports have been published in the past concerning the photochemical decomposition process of P ~ I z . ' ~ - ~These O have concentrated on the PD process in single crystals, evaporated films, or emulsion particles (few microns), induced by visible light and electron bombardment. Dawood et al.15 have suggested that the PD induced by visible light involves excitation of an exciton followed by an ionization process. This process was most efficient at temperatures above 180 "C. Heating plays an important part in the decomposition observed by electron bombardment.18 Forty et al.15 have presented the utilization of the PD effect of PbI2 in photographic imaging processes. Verwey et a1.22and Arends et al.21have shown that visible and UV radiation lead to the desorption of iodine atoms or molecules from the sample surface and to the formation of Pb+ or Pb aggregates. The suggested mechanisms of the PD process will be discussed in more detail in section IV. Most of the PD effects of the layered iodides have been studied in samples with bulk properties. To the best of our @
Abstract published in Advance ACS Absrrucfs, January 1, 1995.
knowledge, the influence of visible light radiation on semiconducting Q-particles has been studied only in ~ o l l o i d s . ~ ~ - ~ ~ Weller and Henglein et al.23-25have proposed that radiation of CdS, ZnS, and Cd3P2 under an oxygen environment results in a photoanodization process, while in the absence of oxygen a photodissociation and reduction of particle size may occur. Micic et aLZ6have reported photolysis studies of colloidal solutions of the layered iodides: PbI2, HgI2, and Bi13. In the present work, the particle's crystal structure and size distribution have been determined by X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The changes in optical properties of the specimens, induced by the visible light radiation, were followed by examination of the changes in the PbI2 characteristic photoluminescence (PL) spectrum. These changes have been examined as a function of radiation energy, radiation intensity, duration of radiation, temperature of the specimens, and environmental conditions. It should be noted that the synthesis of PbI2 Q-particles may be accompanied by the formation of several byproducts such as Pb(OH)I,26,2713-,26327P b 3 0 ~ 1 2 ,PbL2-?9 ~~ P ~ I s ~ - , ~and O Pb13-.31-32The present work describes several control experiments (section 111.1) that either eliminate the existence of the above byproducts or exclude the possible influence of any traces amounts.
11. Experimental Section 1. Sample Preparation. The Q-size particles of PbI2 embedded in Si02 films were prepared by a sol-gel technique, in a manner similar to the procedure suggested by Nogami et al.' and Chen et al.33 The sol was prepared by mixing 0.5 mL methanol solution of Si(OCzH5)4 (1 M), 0.5 mL of HzO, and a catalytic amount of HN03 (0.05 mL) and diluting the mixture with methanol to a final volume of 4.5 mL. Then, 0.5 mL methanol solution of Pb(CH3COO)y3H20 (0.26 M) was added to the mixture. The Si02 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
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annealing conditions control the porosity of the Si02 films and enable the formation of lead oxides. According to refs 29 and 30 at 450 "C and in the presence of oxygen, the lead substituent is converted mainly into Pb304, and upon cooling, a mixture of the oxides Pb304 and PbO is observed. In the present case, the existence of Pb(I1) and Pb(1V) 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 a duration between 10 and 90 min. This stage enabled a solid-gas interface reaction between the iodine molecules and the lead oxides. This stage involves the oxidation-reduction process, including the I2 21- (reduction) and Pb(I1) Pb(1V) (oxidation) half-reactions. Then, the remaining Ph(I1) within the medium interacts with the iodides to form the PbI2 particles. The PD process was carried out by placing the specimens in a quartz reactor with an optical window. The samples were covered by an aluminum foil mask containing a 2 mm diameter hole. The exposed areas of the specimens were illuminated mainly by 4579 8, Ar+ laser with output power ranging between 2 and 5 Wlcm2. The illumination energy dependence was examined by exposing the specimen to a high-pressure xenon arc lamp in the energy range 2.5-3.0 eV, utilizing different combinations of optical band-pass filters. The environmental conditions were examined by performing the reaction in air under vacuum (S10-3 Torr) or under a flow of He gas. In addition, the PD effect was examined in the temperature range between 25 and 100 "C. Following the PD process, the specimens were annealed at 250 "C under iodine vapors and argon flow for a duration of 90 min. This is the regeneration process. 2. Physical Measurements. The mean particle size and the distribution of the PbI2 crystallites were determined using a transmission electron microscope (TEM), Model JEM 2000FX, operated at 200 kV. "EM specimens were prepared by deposition of the films on 3 mm diameter Si wafers, containing a 0.3 mm central hole. In order to minimize electron beam heating effects on the PbI2 crystallite,18 the specimens were cooled within a cold stage to the liquid nitrogen temperature, and a relatively weak electron beam intensity was applied. The crystal structure of the particles was determined using a X-ray diffractometer (XRD), Model PW-1820, operated at 40 kV and 40 mA, and monochromatized Cu K a radiation. The XRD spectra were recorded over the 2 0 = 10"-60", using a scanning speed of 0.4 deglmin. The PbI2 particles exhibited a characteristic luminescence spectrum in the temperature range between 1.4 and 180 K, as reported before." Most of the PL spectra reported below were recorded at 77 K. This low temperature prevented decomposition during the PL measurements. The PL measurements were carried out by placing the samples in a cryogenic Dewar and exciting them with a continuous 4579 8, Ar+ laser (Coherent, Innova 70). The emitted light was passed through an holographic grating monochromator (Jobin Yvon, Model THR1000) and was detected by an Hamamatsu R666 photomultiplier tube. The emission was filtered by a color glass filter in order to eliminate the excitation scattered light. The emission was recorded in the energy range 1.90-2.60 eV.
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111. Results 1. Elimination of the Byproducts Contributions. It was previously pointed out that reactions with iodine may lead to a formation of 13- and Pb(0H)I as the major chemical byproducts. The latter, Pb(OH)I, is eliminated in the present work by performing the solid-gas interface reaction at elevated tem-
peratures (250 "C) under the flow of argon gas. However, the studied specimens contained trace amounts of 13-, as was confirmed by Raman spectroscopy analysis (not shown here). In order to verify that 13- does not contribute optical contamination, we have prepared a control solution containing high concentration of 13-. The latter solution has not shown any luminescence band in the energy range 1.90-2.60 eV.36 While Pb(0H)I and 13- are the most likely contaminants to be concerned, it is possible that other byproducts may be formed under solid state reactions. The latter may include the formation of oxoiodides (e.g., Pb30212, Pb40413.6, and Pb50412) or Pb-I complexes (e.g., Pbb2-, Pb164-, and PbI3-). However, in the following paragraph several arguments and control experiments are discussed which eliminate any contribution of oxoiodides or Pb-I complexes to the forthcoming experimental data. The XRD spectra of the specimens prior to and after the photodecomposition (PD) were analyzed carefully. Profile deconvolution was used in order to separate several overlapping peaks. No traces of the oxoiodides, Pb3O2I2, Pb40413.6, and Pb5O&, were found. The phases prior to and after PD contained mainly PbI2, while after PD there are small amount of other phases. The latter phases are identified with lead oxides and metallic lead, which are the reaction products of the PD process (discussed below). Moreover, the phase diagram of the PbI2PbO mixture reported in ref 37 indicates that none of the aforementioned oxoiodides can be formed below 400 "C. However, the preparation conditions (section 11.1) permit the coexistence of PbI2, PbO, and Pb304 only at 250 "C, thus excluding the possibility of oxoiodides formation as the reaction byproduct. As mentioned in section 11.1, the last stage in the preparation of PbI2 Q-particles includes a solid (lead oxides)-gas (iodine) reaction. This stage may lead to the formation of Pbb2-, Pb13-, and PbIa4- as chemical byproducts. Although we cannot exclude the possibility of their formation by chemical analysis, we have checked carefully that some of them do not contribute any optical contamination. A control sample of K2Pb4 has been prepared. This sample has not shown any luminescence band in the energy range 1.9-2.6 eV, in the studied temperature range. According to ref 30 the PbIs4- complex has an absorption initiating at 3.469 eV, while in the present work the specimens were excited with energy in the range 2.50-3.0 eV, thus eliminating the possibility of Pb164- excitation. On the contrary, we are aware that Pb13- in APbI3 (A = K, Cs, Rb, T1, RNH3+) compound^^^^^^ do exhibit excitonic effect in the studied energy range. However, the XRD and Raman measurements of the studied specimen excluded the existence of the Pb-I complexes in major amounts. Thus, we conclude that traces amounts of PbI3- do not contribute optical contamination. As discussed in section 11.1, the Q-particle specimens contain lead oxides within the Si02 medium. The optical contribution of the latter was examined by recording the PL spectrum of a control specimen. This specimen contained PbO and Pb304 embedded within the Si02 before their reaction with iodine. The PL of this control specimen exhibits a weak background over the spectral range 1.90-2.60 eV. As will be seen below, this background is not noticeable in the PL spectrum of the PbI2 particles. This suggests only minor optical contaminant of the lead oxides to the optical properties of the Q-particle specimens. Thus, the aforementioned control experiments and the relevant previous works exclude any possible contribution of any byproducts to the experimental data. Thus, the forthcoming measurements and discussion are associated solely with the PbI2 Q-particles.
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, -2.55
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Energy (eV) 2.45 2.40
4850 Wavelength,
6)
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Figuree2. PL spectrum of PbI2 Q-particles with mean particle radius of 60 A at 4.2 and 77 K. Inset: the exciton energy blue shift as a
function of mean particle radius (triangles, experimental; solid line. theoretical).
Figure 1. (A) TEM micrograph of virgit? PbIz Q-particles embedded in Si02 films. (B) TEM micrograph of visible light radiated (at 70 "C)PbIz Q-particles.
2. The Characterization of PhIt Q-Particles Prior to Visible Light Radiation (Virgin Samples). A representative TEM picture oi virgin PbI2 particles embedded in Si02 films is shown in Figure 1A. The mean particle radius of various samples, derived from the TEM micrographs, ranged between 25 and 100 A. The mean particle radius of the specimen in Figure IA is 60 A. The virgin particles mean size could be altered by controlling the time of treatment with iodine and initial concentration of the Pb2+ ions in the sol-gel solution. The Q-particle samples were stable over a period of months. In general, the TEM pictures exhibited polyhedral morphology of the nanoparticles, irrespective of their size.' I The XRD patterns supplied qualitative verification that the majority of the PbI2 Q-particles exhibit 2H-polytype with D3n symmetry. The PL spectra of the PbI2 Q-particles, embedded in Si02 films. were recorded in the temperature range 4.2-120 K. Figure 2 shows two represenyive spectra of a specimen with a mean particles radius of 60 A, recorded at 4.2 and 77 K. The spectrum at 4.2 K consists of a shoulder at 2.516 eV, intense band at 2.506 eV, and a broad band centered at 2.440 eV. In the spectrum recorded at 77 K, the 2.506 eV band has disappeared. and the 2.516 eV band dominates the spectrum. The observed PL patterns and their thermal deactivation were found to be similar to the corresponding PL spectrum of the bulk.3*8-4' Based on this comparison and the identification of 2H-PbIz structure by XRD,the 2.516 eV band is associated with the free exciton (FE) emission and the 2.506 eV band is
identified with the bound exciton (BE), while the broad band (labeled L in Figure 2) at 2.440 eV is associated with impurities or defects. Although the PL patterns of the PbI2 Q-particles resemble that of the bulk, the center energies of the bands are blue-shifted. The amount of the blue shift was measured on different specimens with various mean particle radii, and it was found that this shift increased with decreasing particle size. For example, a plot of the FE blue shift versus mean particle radius is drawn in Figure 2 (inset). The triangles in the inset represent the experimental blue shifts while the solid line is a theoretical curve (vide infra). For comparison, the energy of the bulk FE, at 77 K, is centered around 2.501 eV. 3. The Characterization of the PbI2 Particles after Visible Light Radiation. The effect of visible light radiation was examined by exposing a small area of the film and masking the rest of the specimen. Subsequently, the structural and optical properties of the specimens were characterized and compared with those of the virgin samples. The nonradiated sites had identical properties to those of the virgin sites. The radiated area retained the 2H crystal structure, as determined from the XRD spectrum. However, the TEM picture (Figure IB) indicates that the particles have been partitioned into smaller fragments, which are organized inside the polyhedral frame. The PL spectra of the radiated areas exhibited several changes with respect to the virgin samples. These changes were mainly pronounced as a blue shift of the FE band, with a reduction of its intensity. The levels of change are dependent on light exposure conditions. In the following paragraphs the influence of the environment, temperature of the specimens, duration of excitation, energy, and the power of the excited source will be discussed. Each parameter was varied gradually. Following each variation, the specimen was removed from the reactor and was placed in a cryogenic dewar in order to record the PL spectrum. A microcontrol enabled repetition of a nearly identical optical alignment for all the recorded PL spectra. The results reported below ytilized initial virgin samples with a mean particle radius of 60 A. When the visible light treatment was carried out under atmospheric conditions, there was no change in the PL spectra. Light exposure under the flow of He gas produced a small change, with a strong dependence on the flow rate. However, the largest effect was observed when the reactor was evacuated
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1248 J. Phys. Chem., Vol. 99, No. 4, 1995
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Energy (eV) 2.45 240 I
I
2.35
I
32k' I
40
I
I
I
I
I
5050 5150 5250 Wavelength, (%I Figure 3. PL spectra of a specimen after visible light irradiation (at Torr, 2 W/cmZ)for the indicated durations. 70 "C, 5
4850
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to 110-~ Torr. A control experiment was performed by evacuating the specimen for a certain length of time at 70 "C in the absence of visible light radiation. The PL spectrum of this specimen was identical to the corresponding virgin spectrum. Thus, the changes in optical properties are associated with light radiation, and this process may be enhanced under vacuum conditions. The changes in the PL spectra became more pronounced when specimen temperature was increased between 25 and 100 "C. Higher temperatures were eliminated in order to avoid thermal decomposition or evaporation. At 100 "C, the changes were already occumng extremely fast, preventing the determination of the radiation effect. Hence, most of the experiments were carried out at an optimal temperature of 70 "C. The effect of light radiation energy was examined over several stages. Initially, the samples were radiated with a high-pressure xenon arc lamp, and the energy was selected by a combination of band-pass filters. Changes in the PL were observed only when the samples were irradiated with energy >2.5 eV. With this fact in mind, most of the experiments were performed by utilizing visible laser (4579 A), which supplied a focused beam and a variable output intensity. The FE blue shift increased with increasing laser intensity between 2 and 5 W/cm2. However, the influence of the laser intensity could not be separated from that of the other experimental conditions. For example, exposure for 10 min at 5 W/cm2 resulted in complete desorption of PbI2 molecules from the porous silica films, while exposure for the same length of time at 2 W/cm2 had no effect at all. Thus, the effect of radiation laser power could not be evaluated quantitatively. The duration of visible light radiation was found to play a strong part in the changes in optical properties. Figure 3 represents the PL spectra, recorded after 0, 20, and 95 min of radiation with a 4579 8, laser (2 W/cm2) at 70 "C and under vacuum (110-3 Torr). It is clear from the figure that a blue shift of the FE energy and a reduction in its intensity occurred upon visible light treatment. The FE energy blue shift and its intensity are plotted versus duration of radiation in Figure 4 by the circles and squares, respectively. The solid lines in Figure 4 are drawn as a guide. The changes increase substantially with increasing duration of exposure, up to about 50 min, and become more gradual beyond that length of time. Interestingly, when the light-treated samples were reannealed under iodine atmosphere, this resulted in red shift of the exciton
I
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80
I
I20
I
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I
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Time (min) Figure 4. FE energy blue shift (circles)and the FE intensity (squares) versus duration of irradiation. The solid lines were drawn as a guide. band almost up to the virgin sample position (curve 4 in Figure 3) and an increase of the exciton luminescence intensity. These changes occurred regardless of prior light exposure conditions. The visible light effect on the Q-particles was compared with the corresponding effect on bulk single crystals, under identical exposure conditions. The single crystals exhibited a unique appearance following visible light radiation. The radiated areas were blackened, and the XRD of these sites revealed the existence of Pb atoms. It should be noted that when the pressure in the reactor was raised > Torr, additional byproducts were formed. The XRD spectrum of the specimens radiated under the aforementioned conditions revealed the coexistence of PbO and Pb atoms. Since most of the exposure condition dependencies of the bulk and the Q-particles were identical, we tend to believe that Pb atoms and PbO are also formed in the nanosized particles. However, due to the extremely low concentration of the exposed sites within the Si02 media, blackening is invisible and the corresponding XRD signals are not resolved.
IV. Discussion The first part of this section discusses the quantum size effect in the virgin samples, while the following paragraphs propose a mechanism which explains the influence of the visible light radiation on the properties of PbI2 nanosized particles. The blue shift of the FE energy in the PL spectrum of the virgin sample is a manifestation of the quantum size effect. Although a detailed description of the quantum size effect in the virgin PbI2 particles has been reported elsewhere," we have summarized several important points below. The quantum size effect is observed when photoinduced electron (e)-hole (h) pairs are spatially confined within the particles. This effect induces variations in the exciton properties of the Q-particles. The amount of the FE blue shift depends on the relative dimensions of the mean particle radius (a) and the exciton Bohr radius (uB). When a < UB, the confinement is strong. In this limit, the e-h Coulomb interaction is negligible, and the optical transitions are determined mainly by the motion confinement of the individual electron and h ~ l e . ' ~ .When ' ~ a > 3@3, the confinement is weak. In this limit, the motion confinement energy is smaller than the e-h Coulomb interaction energy, so an exciton may be confined as a whole.12 Neither of the last two confinement regimes exists in the present case. Instead, this work is associated with an inteormediateconfinement case in which U B < a < 3aB (UB = 19 A in the bulk), and the exciton confinement mechanism is more complex. In addition, PbI2 represents a case in which m, >> mh (m,l= 2.lm0, mell = 0.48m0, mh = 0.2mo). According to Ekimov et a1.,4* the intermediate
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confinement regime in combination with a substantial difference in the effective masses results in a strong electron localization close to the particle center and free motion of the hole. An e-h pair with an electron localization is named an acceptorlike exciton.42 The localization of the electron is further enhanced due to the potential of the hole, whose motion is confined by the Q-particle boundaries. According to Ekimov$2 the transition energy of an acceptorlike exciton can be evaluated by a variation method with a single parameter @), which minimizes the following equation:42
occeotor 0-porticles
Figure 5. Schematic drawing of the photodecomposition mechanism: (a) the formation of the acceptor-like exciton, (b) the capturing of the electron by the PbZ+ion and the hole by the iodine ion at the surface, and (c) the desorption of the iodine atoms and the fragmentation of the particle.
Here Jocv) and Jib) are spherical Bessel functions, R is the Rydberg constant, and a h is the hole radius. This equation has been utilized in the present work in order to calculate the theoretical FE emission energy for specimens with different mean particle radius (a). These calculations were done by substituting the bulk parameters R = 70.8 meV and a h = UB = 19 A and then searching for minimum value in Em@) via iteration over the /3 variable parameter, for each value of a. The theoretical blue shift [A& = (EE)Q-pmicles - (Em)bulk] versus mean particles' radius (a) is shown in Figure 2 (inset) by the solid line, while the solid triangles in the figure represent the experimental blue shifts. It should be noted that the XRD and the PL measurements indicated that the Q-particles preserved the bulk structural properties, and therefore the bulk parameters could be utilized in the above calculations. The quantum size effect was found to be a sensitive tool in the determination of the visible light irradiation effect. Thus, the gradual blue shift of the FE energy upon visible light irradiation (Figures 3 and 4) is a consequence of a decrease in the mean particle size. The PL and TEM measurements suggest that visible light treatment results in a partial photodecomposition (PD) and photofragmentation (PF) of the particles. As will be discussed below, these processes are associated also with the reduction of the amount of PbI2 material. Therefore, the PL intensity decreases with increasing irradiation duration, as plotted in Figure 4. As mentioned before, the heating or evacuation alone has no effect, and light irradiation was essential to observe changes in the PL spectra. However, these changes were most effective when the specimens were heated to temperatures between 25 and 100 "C and the reactor was evacuated to Torr. In addition, pronounced variations were observed only when the radiation energy was above the band gap energy. This suggests that the visible light treatment initially involves a valence to conduction band excitation, followed by decomposition process. Since the light radiation at 77 K has no influence over long periods of exposure, it can be assumed that the relatively elevated temperatures supply the thermal activation for decomposition. Evacuation avoided the presence of oxygen and enhanced the withdrawal of decomposed chemicals. It should be noted at this stage that the preservation of the 2H structure of the remaining particles after irradiation excluded the possibility of visible light radiation causing structural phase transition. Several mechanisms of the PD process in lead halides have been suggested in the past. Dawood, Forty, and Tubbs15 have proposed a reaction of two excitons localized at an anion site. Their model eventually leads to the production of an iodine molecule, a lead atom, and two anion vacancies next to the
original site. De and Schoonman et a1.% have pointed out the importance of the transport of anion vacancies during the thermal decomposition of relevant materials, PbC12 and PbBr2. Verwey et also emphasize the importance of hole transport as an intermediate step in the decomposition process. Verwey suggested that the PD of the lead halides consists of the transport of photogenerated holes to the surface where they are trapped by halogen ions. Verwey proposed that the halogen atoms are then desorped from the surface and that the anion vacancies created in this way subsequently diffuse into the crystal. In addition, it has been suggested21that the photogenerated electrons react with Pb2+ ions to give Pb atoms in an aggregated state. The diffused anion vacancies enable the accommodation of the large Pb atoms. Moreover, a previous ESR study of the PD p r o ~ e s shas ~ ~shown , ~ ~ that Pb+ ions are an intermediate product that tends to aggregate in pairs or triples and, as so, react to form Pb and Pb2+ ions. On the basis of our experimental results and the previously suggested models, the following mechanism can be derived for the PD of the PbI2 Q-particles. The suggested stages of the PD process are drawn schematically in Figure 5 . Primarily, the visible light radiation creates an acceptor-like exciton (Figure 5a). Previous electronic band structure calculations of the bulk have implied that the excitonic absorption at the fundamental band edge is associated with the Pb(6s) Pb(6p) trar~sition.~' Thus, the localized heavy electron is tightly bound to the cation (Pb2+)site, momentarily converting it into Pb+ (Figure 5b). The close vicinity of the hole to the surface and its relatively free motion enable its annihilation by iodine ions at the surface (Figure 5b). The atomic iodine can then be desorped, either as a single atom or as a diatomic molecule, leaving an anion vacancy at the surface (Figure 5c). The iodine desorption is enhanced by evacuation over the specimen in the reactor. It is more likely that two adjacent Pb+ ions undergo a disproportionate reaction, converting these ions into Pb and Pb2+.21,22In the presence of small traces of oxygen, PbO may be formed. Indeed, the XRD analysis has indicated the existence of Pb metal and PbO compound after the PD process. However, the relatively larger Pb atom probably has caused internal stresses within the particles. The latter has resulted in the particle fragmentation, as indicated by the TEM picture in Figure l b and shown schematically in Figure 5c. Schoonman et aLM have proposed that in bulk metal halides the anion vacancies migrate from the surface into the bulk to accommodate the large Pb atoms. However, the observed fragmentation excluded the occurrence of vacancies' migration in the Q-particles. The moderate decomposition rate above 50 min of exposure, as seen in Figure 4,can be explained by a saturation effect of the PD process.
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Retreatment of the radiated samples with iodine vapors results in a red shift and in an increase of the emission intensity of the exciton band. This is due to the conversion reaction of PbO/ Pb203 into PbI2, a process which nearly regenerates the original PbI2 particles’ size. It should be noted that cycles of photodecomposition-regeneration processes were successfully performed on the same specimen, and the changes in the corresponding PL spectra were reproducible on subsequent cycles.
V. Summary Quantum size effects manifest themselves in the exciton properties of PbI2 nanometer-sized particles. The exciton emission energy is blue-shifted with decreasing mean particle size. In addition, the photogenerated electron-hole pairs form a special acceptor-like exciton within the particles. The occurrence of a blue shift upon exposure to visible light radiation was found to be a sensitive tool in the determination of the irradiation effect. The experimental results suggest chemical decomposition of the Pb-I bonds, resulting in the fragmentation of the particles. In addition, retreatment of the photodecomposed particles with iodine vapors nearly regenerates the original PbI2 particles’ size.
Acknowledgment. This research was partially supported by the U.S. -Israel Binational Science Foundation (BSF) under Contract 89-00316/3 and by the center of Opto-Electronics at the Technion. R. Chaim acknowledges the Sachs Center at the Department of Materials Engineering. References and Notes (1) Nogami, M.; Nagasaka, K. J . Non-Cryst. Solids 1990, 126,87. (2) Weller, H. Ber. Bunsen-Ges. Phys. Lett. 1990, 174,241. (3) Weller, H. Ber. Bunsen-Ges. Phys. Chem. 1991, 95,1361. (4) Potter, B. G.; Simmons, J. H. Mater. Res. SOC. Symp. 1991, 206, 134. (5) Wang, Y.; Herron, N. J. Phys. Chem. 1988, 92,4988. (6) Potter, B. G.; Simmons, J. H. Phys. Rev. B 1988, 37, 10838. (7) Chepic, D. I.; Efros, Al. A.; Ekimov, A. I.; Ivanov, M. G.; Kharchenko, V. A,; Kudriavtsev, I. A,; Yazeva, T. V. J . Lumin. 1990,47, 113. ( 8 ) Micic, 0. I.; Rajh, T.; Comor, M. E.; Zec, S.; Nedeljkovic, J.; Patel, R. C. Mater. Res. SOC. Symp. 1991, 206, 127. (9) Kuczynski, J.; Thomas, J. K. J . Phys. Chem. 1985, 89,2720. (10) Stramel, R. D.; Nakamura, T.; Thomas, J. K. J . Chem. SOC.,Faraday Trans. 1988, 84, 1287. (11) Lifshitz, E.; Yassen, M.; Bykov, L.; Dag, I.; Chaim, R. J . Chem. Phys. 1993, 98, 1463.
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