J. Phys. Chem. 1996, 100, 19927-19932
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Chemical and Size Characterization of Layered Lead Iodide Quantum Dots via Optical Spectroscopy and Atomic Force Microscopy R. Mu, Y. S. Tung, A. Ueda, and D. O. Henderson* Chemical Physics Laboratory, Department of Physics, Fisk UniVersity, NashVille, Tennessee 37208 ReceiVed: February 27, 1996; In Final Form: August 6, 1996X
Lead iodide (PbI2) clusters were synthesized from the chemical reaction of NaI (or KI) with Pb(NO3)2 in H2O, D2O, CH3OH, and C3H7OH solvents. The observation of absorption features between the 550 and 350 nm region obtained with an integrating sphere strongly suggests PbI2 quantum dot formation in solution. Comparison of spectra of PbI2 clusters in solution with PbI2 clusters formed by impregnation of PbI2 in four different pore-sized porous silica substrates indicates that the PbI2 cluster size in solution is less than 2.5 nm in the lateral dimension. Atomic force microscopy (AFM) measurements of PbI2 solutions deposited on mica and highly oriented pyrolytic graphite surfaces indicate that the clusters are single layered. The measured height is 1.0 ( 0.1 nm, which is ∼0.3 nm larger than the layer thickness observed for the bulk materials. The swollen layer thickness can be attributed to the intralayer contraction from the strong lateral interaction among PbI2 molecules, which is supported by ab initio calculations. Raman scattering measurements of the LO and TO modes of PbI2 in bulk and in the confined state were also conducted in 50-150 cm-1 region. Three bands observed at 74, 96, and 116 cm-1 for the confined materials are assigned to the TO2, LO2, and LO1 modes, respectively. The relatively small red shift in the LO modes for PbI2 in the porous hosts may be caused by the surface phonon of PbI2 nanoparticles confined in the porous silica.
Introduction The study of nanophase materials physically confined in various hosts has attracted much attention in recent years.1-9 Depending upon the physical properties of the confining hosts and the nature of the confined materials, many unique mechanical, thermal, and optical properties have been observed and are related to different types of confinement effects. For example, (1) the physical confinement, the interfacial interaction between guest and host, and the reduction of the physical size of the confined particle can lead to (a) hardness modification of materials; (b) depression of the melting and freezing transition temperatures; and (c) alteration of crystal nucleation and growth characteristics; (2) the quantum confinement of free electrons in metals and excitons in semiconductors can result in the observation of the surface plasma resonance (SPR) and the shifts of the band gap; (3) dielectric confinement gives rise to surface phonons observed in a wide range of nanophase materials. Due to the fact that quantum and dielectric confinement effects typically occur simultaneously, it is difficult to identify the relative contributions to each of these effects. Heavy metal halides,4 such as PbI2, form a unique series of layered semiconductor compounds. Besides having potential application for γ-ray detection, the strong intralayer chemical bonding and the weak interlayer van der Waals interaction have made these materials good candidates for investigating cluster formation and growth in confining media. Because of their layered structure, this class of compounds are particularly wellsuited for studying confinement along different crystallographic axes. Sandroff et al.1 investigated layered semiconductor clusters in various solvents. Based upon the optical absorption spectra of the solutions containing PbI2 and BiI3 clusters and transmission electron microscopy (TEM) characterization of the cluster size and size distribution, a single-layered and platelet-like * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, December 15, 1996.
S0022-3654(96)00605-3 CCC: $12.00
cluster model was proposed. The abrupt blue shift in absorption spectra below 350 nm for PbI2 in different solvents was interpreted as charge carrier confinement for different-sized crystallites with a single layer thickness. Other research groups 2,3 have reexamined similar systems. Disagreements have been raised concerning band assignments of the UV-vis spectra. It is suggested that the bands below 350 nm may result from the possible I- or I3- presence in solution. However, no efforts have been made to understand how the band gap changes when PbI2 semiconductor changes its physical dimension and size from small clusters to its bulk. No solid explanation has been put forward to account for the discontinuous blue shift of the band gap for the PbI2 clusters in solution. In addition, there has been no direct experimental evidence reported in the literature to support the proposed disklike and single-layered semiconductor clusters being formed in solution. Therefore, the motivation of the present research is (i) to study cluster formation and growth mechanism(s) in solution, which can provide information on crystal growth on earth and may ultimately be used to better understand the growth in microgravity;5 (ii) to understand how the confining geometry modifies the electronic and vibrational spectra of the clusters in terms of the particle size; (iii) to provide direct experimental evidence of the size and morphology of these layered semiconductor clusters formed in solutions; (iv) to study quantum confinement effects when PbI2 is physically confined in four different poresized porous glasses. The band gap shifts observed for PbI2 clusters in porous glasses are used to estimate the cluster size in solution and are compared with the AFM results. Experimental Section Synthesis of PbI2 Clusters in Solutions. As reported by others,1,2 the starting materials for PbI2 cluster synthesis are Pb(NO3)2 and NaI (or KI). These reagents were purchased from Aldrich with a purity of 99+%. No effort was made to further purify these materials. Although the PbI2 clusters can be synthesized by mixing either NaI or KI with Pb(NO3)2 with © 1996 American Chemical Society
19928 J. Phys. Chem., Vol. 100, No. 51, 1996 the end products being PbI2 and NaNO3 or KNO3, there are two reasons why we chose NaI rather than KI in most of our experiments: (1) It is known that both PbI2 and KNO3 are insoluble in anhydrous alcohols, i.e., MeOH, EtOH, etc. It is conceivable that mixing of KI and Pb(NO3)2 in pure alcohol can lead to both PbI2 and KNO3 cluster formation, which complicates the study of PbI2 clusters in solution. On the other hand, NaNO3 is soluble in pure alcohol so that only PbI2 clusters are formed. (2) In order to unambiguously identify PbI2 cluster formation and cluster sizes by AFM, it is very critical to completely remove NaNO3 or KNO3 species from the substrate surface to avoid interference from the formation of alkali nitrate clusters so that AFM images can reflect the particle size and shape as well as other structural information on PbI2 clusters themselves. Therefore, using NaI compound as the starting material is highly desired. To obtain PbI2 clusters in various solvents, both Pb(NO3)2 and NaI solutions were first prepared in the same solvent with a known molar concentration. Then, lead iodide clusters were synthesized by simply mixing the two solutions in a liquid cell before being subjected to optical and AFM measurements. Impregnation of PbI2 into Porous Glasses. Details concerning impregnation of porous glasses can be found in ref 8. The procedure is summarized here. Gelsil porous substrates with pore diameters of 2.5, 5, 10, and 20 nm were chosen for impregnation of bulk PbI2. The porous glass substrates were first cleaned and dried. Then, they were transferred into a 10 mm quartz tube with one end sealed. The tube was placed into a vertical furnace. The substrates were slowly heated up to 450 °C for over a 6 h period and then were cooled to 110 °C under a dynamic vacuum to desorb the water in pores and adsorbed on the silica surface. A sufficient amount of PbI2 powder was first loaded into a 6 mm quartz tube, and then this tube was placed in the 10 mm tube where the porous substrates were dried. This 10 mm tube was sealed at 110 °C under vacuum and was again slowly heated to 50 °C above the melting temperature (Tm ) 402 °C) of PbI2. The liquid PbI2 was passed out the inner tube and emersed into the porous substrates. After 1 h soaking in liquid PbI2, the system was slowly cooled back to room temperature. The impregnated porous substrates were imbedded in residual bulk PbI2 and kept sealed until measurements were performed. Optical Characterization. In order to understand the solution structure and cluster formation kinetics, time resolved and static UV-vis spectral measurements were carried out on an Olis rapid scan monochromator (RSM) and a Hitachi 3501 spectrophotometer. With the Olis RSM spectrophotometer, a series of spectra with a time interval of 1-10 ms were collected in the spectral range 300-550 nm. The spectral intensity and frequencies were monitored as a function of time. The static electronic absorption and extinction spectra were collected with a Hitachi 3501 spectrophotometer with and without an integrating sphere accessory, respectively. All the spectra were measured in 1000-185 nm region with 1 nm resolution. Atomic Force Microscopy (AFM). AFM images in tapping (TMAFM), constant force, and lateral force (LFM) modes were obtained with a Nanoscope III scanning probe microscope from Digital Instruments. All measurements were performed under ambient conditions with relative humidities ranging from 50 to 60%. A well-calibrated E-scanner with a scan range of 12 × 12 µm2 and an A-scanner with a scan range of 1 × 1 µm2 were used in the present experiments. With the A-scanner, atomic scale resolution was obtained on the cleavage planes of Muscovite mica revealing the hexagonal structure of the SiO4 sheet. By comparing the measured value of the nearest-neighbor
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Figure 1. Optical absorption spectra of NaI (A) and Pb(NO3)2 in methanol (B) and pure methanol (C).
distance of the SiO4 tetrahedra with the literature value, the lateral resolution of the A-scanner was determined to be better than 0.02 nm on an atomically flat surface. The vertical resolution of both the A- and E-scanners in both tapping and constant force modes is better than 0.1 nm. Additional AFM measurements were made on a freshly cleaved PbI2 crystal that revealed steps with heights of 0.7 ( 0.1 nm. This height is in good agreement with the value of 0.698 nm reported in literature.4 The step height was then used as a internal height calibration during AFM measurements. Raman Measurement. Raman scattering measurements of PbI2 in its bulk and the confined forms were conducted with a Spex Raman spectrometer equipped with a double-grating monochromator, a water-cooled photomultiplier tube (PMT) detector. An Ar+ pumped Ti:saphire laser was used for excitation. A 90° scattering configuration was used to collect the Raman signal. The typical excitation energy of the laser was ∼100 mW at 770 nm. Each spectrum was obtained with 2 cm-1 resolution and a 1 s integration time. Results and Discussion Optical Characterization of Solution Structure. In order to understand solution structure and to identify various possible species present in solution, efforts were made to study (i) solvent effects on reactants, i.e., NaI and Pb(NO3)2; (ii) solvent effects on the reaction products, i.e., PbI2 clusters and NaNO3; (iii) Pb:I molar ratio effect on reaction products; and (iv) concentration effects on PbI2 cluster size formation. Figure 1 illustrates the optical absorption spectra of methanol as well as NaI and Pb(NO3)2 dissolved in methanol. Both the solvent and Pb(NO3)2 do not show any absorption bands above 250 nm. However, the NaI solution does show two absorption bands at 270 and 320 nm, respectively. As pointed out by Wang and Herron,3 these two peaks at ∼325 and 270 nm are primarily due to the absorption of I- and I3- species in methanol solution. Similar spectra were also observed for NaI (or KI) dissolved in H2O, D2O, and propanol exposed to white light (room light) irradiation although the spectra were not presented in this paper. Figure 2 exemplifies a set of optical absorption spectra of reaction products formed by mixing NaI and Pb(NO3)2 together with different molar ratioes (Pb:I ) 1:2, 1:3, and 1:4). As the number of iodine ions increases, the peaks at 270 and 320 nm increase accordingly, while a weak peak at 420 nm remained stationary. This observation is in good agreement with earlier work of Wang and Herron,3 who assigned these peaks at 270 and 320 nm to I- and I3- and not to transitions associated with PbI2 clusters.
Layered Lead Iodide Quantum Dots
J. Phys. Chem., Vol. 100, No. 51, 1996 19929 in an appreciable blue shift from its bulk band gap. Based on the effective-mass approximation (EMA) theory proposed by Brus,9 the absorption energy E(R) for a nanosized semiconductor particle (or quantum dot) can be estimated by the following equation:
E(R) ) Eg +
Figure 2. Optical absorption spectra of the reaction products from mixing of NaI and Pb(NO3)2 with different molar ratios. (A) 1:2, (B) 1:3, and (C) 1:4.
Figure 3. Absorbance spectra of PbI2 quantum dots synthesized in methanol with solution concentrations of (A) 5.0, (B) 2.5, (C) 1.0, and (D) 0.5 mM. The inset shows a scale expansion of 25× in the 350550 nm region.
In order to unambiguously identify PbI2 cluster formation and to clarify which electronic absorption bands are due to PbI2 clusters in solution, we have conducted two different optical measurements. One is the extinction spectra, and the other is absorption spectra. Extinction spectra carry information resulting from scattering losses by the PbI2 clusters and from the energy absorption by the solution. These spectra were obtained via simple transmission measurements. The absorption spectra were recorded with an integrating sphere. The extinction spectra are consistent with work reported in the past1-3 that showed two bands at 270 and 320 nm. However, the absorption spectra, shown in the inset of Figure 3, indicate additional bands above 350 nm, which are absent in the extinction spectra. From the inset of Figure 3, it is clear that there exists an absorption edge (band gap) at ∼505 nm for the 5.0 mM PbI2 solution. As the PbI2 concentration decreases from 5.0 to 1.0 mM, the absorption edge of the PbI2 clusters is blue-shifted from 505 to 420 nm. For the lowest concentration of 0.5 mM, a larger blue shift would be expected as compared to the solutions with a concentration greater than 0.5 mM. However, an increased blue shift for this concentration could conceivably fall below 370 nm where the I- and I3- have strong absorptions. Clearly, a shift into this region would result in overlapping of bands, and this would account for the apparent absence of a band for the 0.5 mM solution. It is known that when the physical size or dimension of semiconductors is comparable to or smaller than its exciton Bohr radius, quantum confinement of the electron and hole will result
(
)
p2π2 1 1.8e2 1 + + small term 2 m * mh* R 2R e
(1)
where E(R) is the absorption band gap, R is the size of the quantum dot, me* and mh* are the effective masses of electrons and holes, and is the dielectric function of the semiconductor. The first term in the eq 1 is the band gap of bulk semiconductor. The second term is due to the quantum confinement of the electron and hole pair. And the third term is Coulomb interaction between the electron and hole. Clearly, the band gap of the confined semiconductor is modified by the second (∝1/R2 ) and the third (∝1/R) terms. Therefore, as the particle size changes, the band gap is changed accordingly. As illustrated in Figure 3, the absorption edge of PbI2 clusters synthesized in methanol solutions does blue shift gradually as PbI2 concentration decreases. If it is assumed that the lowest solution concentration has the smallest clusters, then one would expect to observe the largest blue shift in the band gap. Further, as the solution concentration increases, larger clusters are formed that result in an increasing red shift that would ultimately approach the band gap of the bulk material. This trend is consistent with what is predicted by eq 1, and therefore, the absorptions between 550 and 420 nm for different concentrations are attributed to quantum confined PbI2 clusters. It is noteworthy that, in the case of heavy metal halides, quantum confinement is predominantly in the 〈001〉 plane since the weak interlayer interaction, i.e., van der Waals force, results in the exciton confinement along the c axis even in the bulk material. Therefore, quantum confinement effects for PbI2 clusters in solution is dominated by the lateral (2D) confinement instead of 3D confinement. This is consistent with anisotropy observed for the bulk free exciton where aB| ) 1.9 nm and aB⊥ ) 0.7 nm. Confirmation of the Existence of Single-Layered PbI2 Clusters Formed in Solution. As mentioned in the Experimental Section, we have used the step height of a PbI2 single crystal as a internal reference to obtain consistent results with the AFM measurements. Figure 4 shows an AFM image of a 10-4 M solution of PbI2 clusters deposited onto a freshly cleaved mica surface. It is interesting that all of the PbI2 clusters have almost the same height, i.e., 1.0 ( 0.1 nm. Although the lateral size of these clusters is difficult to estimate due to the tip convolution, 12 the lateral size distribution appears to be narrow and is in the nanometer range. Figure 5 illustrates a representative example for AFM measurements at a higher concentration (∼10-3 M). The AFM image of the PbI2 clusters seems to suggest that some of the clusters may have aggregated together at a high concentration. More interestingly, the height of these aggregated clusters remains at 1.0 ( 0.1 nm, suggesting that the aggregation process did not result in colloidal stacking, that is, one cluster on the top of another. No large clusters were observed with heights greater than ∼1.0 nm. In fact, these clusters do not appear to be assembled into a contiguous single structure. Figure 6 displays the image from the lateral force microscopy (LFM) measurement. The results clearly show that the frictional force between the PbI2 clusters and AFM tip is much weaker than that between the tip and mica surface. The ratio of the frictional force between tip-PbI2 cluster and tipmica is 0.7. The lower friction between the tip-PbI2 interface
19930 J. Phys. Chem., Vol. 100, No. 51, 1996
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Figure 4. A typical AFM image of the PbI2 clusters deposited on a mica surface from a 10-4 M PbI2 solution. The average height of the clusters is 1.0 ( 0.1 nm, indicating that the clusters are single-layered.
Figure 5. A contact AFM image of aggregated clusters. The height of the “aggregate” is ∼1.0 ( 0.1 nm.
is not surprising. It is known that PbI2 single crystal consists of I-Pb-I layers, and the interlayer bonding is through van der Waals forces. Therefore, the interaction between the AFM tip and the PbI2 surface may also be of van der Waals type. On the other hand, the interaction between tip and mica is electrostatic in nature because the mica surface is negatively charged. This tip-mica interaction is larger than van der Waals force and therefore could account for the higher friction. Tapping mode AFM images were also obtained by depositing the solution onto a freshly cleaved graphite surface. However, attempts to image the PbI2 clusters on graphite with constant force AFM were unsuccessful. When the cluster-substrate interactions are weaker than tip-cluster interactions, the system is not stable for imaging, and the clusters are displaced out of the imaging field. This is apparently the case for PbI2 clusters-
graphite interaction and explains why no reliable images were obtained in the constant force mode. Sandroff et al.1 have attempted to utilize optical, TEM, and STM techniques to confirm their proposed model of a disklike clusters for PbI2, HgI2, and BiI3 layered compounds. However, all the techniques employed have had limited success in obtaining the disk height information. STM images of BiI3 on graphite revealed a honeycomb structure of what is believed to be the Bi atoms. However, the tip-cluster interactions and possible chemical oxidations to BiI3 surface may impose large perturbations to the true cluster structure that makes the image analysis uncertain. In addition, it is difficult to obtain reliable height information for the sample with very different electrical conductivity from the substrates. Goto et al.6 have investigated PbI2 nanoparticles confined in zeolite and in ethylene meth-
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J. Phys. Chem., Vol. 100, No. 51, 1996 19931
Figure 6. A LFM image of PbI2 clusters on mica. The normal (loading) force used in the image is 86 nN calculated from the force curve and provided cantilever parameters. The lateral force on PbI2 clusters is 70% lower than that on a virgin mica surface.
acrylic acid (E-MAA) copolymer. TEM micrographs revealed that the nanocrystals formed in the copolymer are platelets. On the other hand, TEM studies7 of the nanometer-sized PbI2 particles embedded in SiO2 films suggest that these nanocrystals have an equiaxial polyhedron morphology, irrespective of their physical size. It is possible that the causes of the shape discrepancy could be attributed to (i) the nature of the guesthost interaction at the interface, (ii) the differences in thermal (thermal expansion coefficients) and mechanical properties (elastic modulii) of the confining host and the confined materials; (iii) differences in lattice constants between the substrate and PbI2. Therefore, the structural and physical information obtained from one confining matrix is not necessarily valid in the others. This argument is especially true when the confining matrix is a solid. Therefore, in order to understand the nature of the cluster formation itself, it is necessary to study these clusters in solution phase since the geometrical perturbations in solution are expected to be minimal. Based on our hundreds of AFM images of PbI2 clusters on mica, graphite, and a CH3-terminated SAM surfaces, they all suggest that the clusters have the same height, i.e., ∼1.0 ( 0.1 nm, which is about 1.4 times of the single layer thickness of the bulk single crystal. This considerable thickness deviation can be attributed to four possible causes: (1) An intralayer contraction of the clusters that is predicted by ab initio calculations1 for PbI2 and BiI3 clusters. This intralayer contraction results in a height increase as much as 40% for Pb6I122-. Therefore, it is not surprising to observe a 40% height expansion of the clusters on the atomically flat surfaces. Further, STM images1 of BiI3 clusters on graphite do indicate a lateral contraction with respect to its bulk phase and are consistent with the results presented here. (2) It is known that the operating principle of the AFM techniques including constant force and tapping modes is to maintain a constant static or time-averaged contact force between the tip and sample surface. The contact force can also depend on the physical and chemical nature of the tip used and the properties of the sample surface. Therefore, the specific tip-sample interaction can also result in different height
information. However, the current literature indicates that the deviation due to the tip-sample interaction is expected to be no more than 0.1 nm. In addition, PbI2 clusters deposited on the hydrophilic (mica), hydrophobic self-assembled monolayer (SAM), and semimetallic HOPG surfaces at different contact forces provide the same height information. Therefore, tipsample interaction induced height deviation is considered to be very small. Constant force measurements of PbI2 on HOPG failed because the tip-sample interactions exceeded those of sample-HOPG interaction. (3) In the case of the disklike structure of the PbI2 on mica, the interaction between the cluster and mica can be complicated by surface-adsorbed water molecules, potassium ions, and SiO4 tetrahedral layer on the mica surface. It is also conceivable that some of the methanol molecules from the solvent and surface-adsorbed water molecules from the air can be trapped between the PbI2 clusters and the mica surface during the deposition. However, a height increase due to molecule trapping can be ruled out based on the following argument: PbI2 clusters deposited on graphite, CH3-terminated SAM, and mica surfaces give the same height of 1.0 ( 0.1 nm, indicating that the sample-substrate surface interaction probably does not contribute significantly to the height increase. (4) The solvent effect can be the other cause to explain the swollen layer thickness. However, as we mentioned earlier, the electronic absorption spectra for PbI2 clusters synthesized at higher concentrations did resemble the spectrum of bulk PbI2 single crystals. The considerable thickness increase is primarily due to the inherent nature of the finite-sized PbI2 clusters, as suggested in the ab initio calculations.1 The interlayer distance increase caused by both surface and internal geometric reconstruction will result in the modification of the optical and physical properties as compared to the bulk. In view of the significant structure modification, we note that the use of EMA theory proposed by Brus9 and others1,11 can only serve as a rough or qualitative approximation since the calculation is done by employing the bulk parameters, such as effective masses of electrons and holes.
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Figure 7. Electronic spectra of bulk and the confined PbI2 in four different pore-sized silica glasses.
STM measurements of PbI2 on graphite surfaces were also conducted. However, no reliable experimental images were obtained. This is expected to be due to the fact that the band gap of the bulk PbI2 is about ∼2.4 eV. It is also expected that the quantum confinement of the exciton in the small clusters will result in a blue shift of the band gap. Based on the results obtained by Sandroff et al. and others,1-3,11 the band gap of the cluster may be even higher than 4 eV, making STM measurements very difficult. In addition, the STM tip may remove or destroy the PbI2 clusters on the graphite surface when the bias voltage is not high enough, which has been observed in a few cases in imaging SAMs on a Au surface.12 However, more experiments are presently underway to elucidate the lateral dimension or atomic resolution of these clusters. Optical Characterization of PbI2 Impregnated in Porous Glass. Figure 7 shows the electronic absorption spectra of bulk PbI2 and the PbI2 physically confined in 2.5, 5, 10, and 20 nm pores of silica substrates. As expected, the PbI2 confined in large pores showed a little blue shift in the band gap. However, a noticeable blue shift was observed at ∼505 nm when the PbI2 was confined in 2.5 nm pore, which is consistent with the quantum confinement theory proposed by Brus.9 By comparing the optical spectrum of PbI2 impregnated in 2.5 nm pores in Figure 7 with the spectrum of PbI2 clusters synthesized in a methanol solution at a 5 mM concentration in Figure 3, the same absorption band edge was observed. This observation provided a strong indication that the PbI2 clusters synthesized in the methanol solution were less than 2.5 nm in size. As the concentration decreases, the cluster size gets smaller so that the absorption band of the PbI2 clusters is further blue-shifted. Figure 8 illustrates the Raman spectra of PbI2 bulk and the confined in four different pore-sized silica hosts. There were three bands observed in the 60-150 cm-1 region. They are at 75, 96, and 116 cm-1, which have been assigned to the TO2, LO2, and LO1 optical phonon modes of PbI2 crystals, respectively. As the pore size decreases, the LO modes are broadened, and the center frequencies seem to be red-shifted. The red shift of the band at 116 cm-1 is more pronounced. We suggest that the red shifts in LO modes may be due to the surface phonon modes resulting from the local electric field at guest-host interface for small particles. However, the experimental results are far from being conclusive to make a claim that surface phonons are observed in this system. More experiments are currently underway to study surface phonons in layered semiconductor quantum dots. Conclusion Optical characterization of PbI2 clusters synthesized in methanol solution with an integrating sphere has unambiguously
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Figure 8. Raman spectra of the bulk and the confined PbI2 in four different pore-sized silica glasses.
confirmed the PbI2 cluster formation. A spectral comparison of the electronic transitions between the PbI2 clusters synthesized in solution and physically confined in porous glasses suggests that the cluster size in solution is less than 2.5 nm in lateral dimension. This observation is consistent with the concept that strong quantum confinement sets in when the particle size is comparable or smaller than the exciton radius (aB| ) 1.9 nm). AFM measurements of PbI2 clusters on mica, graphite, and CH3 surfaces suggest that the clusters formed from solution synthesis are disklike. A thickness of 1.0 ( 0.1 nm observed in 0.5 mM solutions confirms, for the first time, that these clusters are single-layered. The 40% expansion of the interlayer distance can be attributed to the finite size effect of the clusters. At small sizes, a strong intralayer chemical bonding can result in the lateral contraction with respect to the bulk value and can lead to the expansion in layer thickness. The LO and TO modes of bulk and the confined PbI2 were also characterized via Raman scattering measurements. The observed red shifts and spectral band broadening of the LO modes (LO1 and LO2) may be attributed to surface phonons of PbI2 nanophase in the porous host. However, the experimental results are not yet conclusive. Acknowledgment. This work was supported in part by NASA under Grant NAG8-1066 and in part by NASA funded Center for Photonic materials and Devices. References and Notes (1) (a) Marino, M. M.; Sawamura, M.; Ermler, W. C.; Sandroff, C. J. Phys. ReV. B 1990, 41, 1270. (b) Sawamura, M.; Ermler, W. C. J. Phys. Chem. 1990, 94, 7805. (c) Sandroff, C. J.; Hwang, D. M.; Chung, W. M. Phys. ReV. B 1986, 33, 5953. (d) Sandroff, C. J.; Kelty, S. P.; Hwang, D. M. J. Chem. Phys. 1986, 85, 5337. (e) Sarid, D.; Henson, T.; Bell, L. S.; Sandroff, C. J. J. Vac. Sci. Technol. A 1988, 6, 424. (2) Roy, A.; Sarma, D. D.; Sood, A. K. Spectrochim. Acta 1992, 48A, 1779. (3) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 5005. (4) Grasso, V.; Mondio, G. Electronic Structure and Electronic Transitions in Layered Materials; Grasso, V., Ed.; D. Reidel: Dordrecht, 1986; p 191. (5) For example: (a) Feigelson, R. S. J. Cryst. Growth 1988, 90 1. (b) Rosenberger, F. J. Cryst. Growth 1993, 76, 618. (6) (a) Tang, Z. K.; Nozue, Y.; Goto, T. J. Phys. Soc. Jpn. 1992, 61, 2943. (b) Saito, S.; Goto, T. J. Lumin. 1994, 58, 127. (c) Goto, T.; Saito, S. Solid State Commun. 1991, 80, 331. (7) Lifshitz, E.; Yassen, M.; Bykov, L.; Dag, I. J. Phys. Chem. 1994, 98, 1459. (8) Henderson, D. O.; Mu, R.; Ueda, A.; Burger, A.; Chen, K. T.; Frazier, D. O. Mater. Res. Soc. Proc. 1995, 366, 283. (9) Brus, L. J. Chem. Phys. 1984, 80, 4403. (10) For example: Xu, S.; Arnsdorf, M. F. J. Microsc. 1994, 173, 199. (11) Micic, O. I.; Li, Z.; Mills, G.; Sullivan, J. C.; Meisel, D. J. Chem. Phys. 1987, 91, 6221. (12) Schonenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611.
