Controlled Growth of the Ordered Cadmium Sulfide Particulate Films

China, and Institute of Acoustics and Mourden Acoustics, Nanjing University, ... We studied the relation between the reaction condition and the st...
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J. Phys. Chem. B 1997, 101, 9703-9709

9703

Controlled Growth of the Ordered Cadmium Sulfide Particulate Films and the Photoacoustics Investigation Z. Y. Pan,*,† X. J. Liu,‡ S. Y. Zhang,‡ G. J. Shen,† L. G. Zhang,† Z. H. Lu,*,† and J. Z. Liu† National Lab of Molecular and Biomolecular Electronics, Department of Chemistry, Southeast UniVersity, Nanjing, 210096, China, and Institute of Acoustics and Mourden Acoustics, Nanjing UniVersity, Nanjing, 210093, China ReceiVed: June 3, 1997X

Two kinds of cadmium sulfide (CdS) particulate films have been generated in situ by exposing stearic acid (SA) Langmuir monolayer at the air-aqueous CdCl2 interface to hydrogen sulfide (H2S) gas: particulate films composed of oriented rodlike nanocrystals (A-type particulate films), and those of dotlike nanocrystals which formed a stripelike domain with straight edges aligned with 6-fold symmetry (B-type particulate films). The SA- coated CdS particulate films were transferred to a solid substrate and examined by transmission electron microscopy (TEM) and photoacoustic spectroscopy (PAS). The dark field image and the transmission electron diffraction of the TEM were used to study the morphology and growth mechanisms of the CdS particulate films in detail. The PAS of the CdS particulate films composed of the oriented rodlike nanocrystals shows a new peak at 417.5 nm. This peak implies some new physical phenomena corresponding to the ordered alignment of the semiconductor nanocrystals, which exists in the ordered nanosystems. The novel synthesis method described here leads to the fabrication of the highly oriented semiconductor quantum wires and provides a new method to investigate the structure of the Langmuir monolayer at the air-water interface.

Introduction Semiconductor materials with nanoscopic dimensions not only have potential application in areas such as optics and electronics but also are of fundamental interest in that the properties of a material can change in the region of transition between the bulk and molecular scales.1-5 For nanocrystal materials, a modification of the size or shape of the crystallites may actually generate an equivalent change.6-8 In this field, it is a very important problem to assemble the semiconductor nanocrystals in an orderly form and, at the same time, maintain the properties of each individual nanocrystal.9,10 Fendler’s group first recognized that semiconductor nanoparticles can be synthesized by exposing a fatty acid monolayer at the air-aqueous salt solution interface to the small molecule gas.11 Some semiconductor particulate films generated by this method were reported in detail.11-23 Because of the advantages of versatility and simplicity, this method is a very interesting and attractive one to form the inorganic semiconductor nanocrystals in an orderly form. Following the preparation of the CdS monolayer within LB films and that of the copper layer at the monolayer-subphase interface,24-26 we have tried to prepare ordered CdS particulate films. The monolayer at the air-liquid interface not only provides size, geometrical control, and stabilization within a single dimension for nanocrystals but also influences the structure of the particulate films.27-29 The CdS particulate films can be easily prepared by a method similar to Fendler’s.11-24 Fendler’s group reported the preparation of CdS particulate films composed of oriented rodlike nanocrystals and suggested the possible growth mechanisms. We studied the relation between the reaction condition and the structure of particulate films in detail. It was found that two kinds of particulate films could be formed: one (A-type) is similar to that of Fendler’s; the other (B-type) is not. For the A-type particulate films, the dark field image and the selected area †

Southeast University. Nanjing University. X Abstract published in AdVance ACS Abstracts, September 15, 1997. ‡

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transmission electron diffraction of the TEM show that the composite and growth mechanism is different from Fendler’s result. In this paper, we present the two kinds of ordered CdS particulate films formed by exposing the stearic acid (SA) Langmuir monolayer-coated salt solution to the hydrogen sulfide gases. Maintaining the total area of the SA monolayer in the reaction process leads to the formation of CdS particulate films composed of oriented rodlike particles including some dotlike particles (A-type particulate films).20,21 The dark field image of the TEM and electron diffraction pattern were used to investigate the composite of CdS particulate films in detail. It was found for the first time that the electron diffraction pattern was composed of six sets of individual diffraction patterns. This implies that the CdS particulate films are composed of six sets of CdS nanocrystals. According to the electronic diffraction pattern, and the corresponding dark field image, the epitaxial growth of the rodlike CdS nanocrystals was rationalized by the matching of the nanocrystals and the monolayer (the matching of the distance of the (220) planes of the CdS crystals and that of the {101h0} planes of the hexagonal close-packed SA monolayer). However, maintaining the surface pressure of the SA monolayer in the reaction process leads to the generation of the other kind of CdS particulate films: the particulate films composed of dotlike particles, that form stripelike domains with straight edges aligned with 6-fold symmetry (B-type particulate films). The fatty acid LB films on silicon form distinct islands with straight edges which aligned with 6-fold symmetry.30 Thus, we guessed that the monolayer at the air-water interface reorganizes, leading to CdS particles that are aligned in an orderly form. Because of the quantum size effect of nanoparticles, the UVvisible absorption spectroscopy method has become a powerful tool to investigate the nanosystems.31,32 Compared with the UV-vis spectroscopy, photoacoustic spectroscopy (PAS) is more sensitive and accurate for weak optical absorption.33-35 So the PAS method can be used to study the electronic and © 1997 American Chemical Society

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optical properties of the CdS particulate films in detail. PAS shows that these two kinds of CdS particulate films are sizequantized. A peak was excited at 417.5 nm of the PAS of A-type particulate films. This peak implies some new physical phenomena are related to the ordered alignment of the semiconductor nanocrystals. Experiment Section The solution (in chloroform, 1 × 10-3 mol/L) of SA was spread on a four-times distilled water subphase containing CdCl2 in a concentration of about 2 × 10-4 mol/L and NaHCO3 in a concentration of about 3 × 10-4 mol/L at pH ) 6.42. The generation of monolayer-supported CdS semiconductor particulate films was achieved as follows: In a rectangular trough the surface of the subphase was cleaned by sweeping it with an aspirator. The SA monolayer were compressed to the solid phase to give a coverage area of 20 Å2/molecule. Injection of 100 µL/h of H2S to the air-liquid interface led to the slow growth of CdS particulate films at the monolayer-metal ion interface. In the whole reaction process, maintaining the total area of the monolayer leads to the formation of A-type particulate films; maintaining the surface pressure leads to the formation of B-type particulate films. These CdS particulate films were prepared through the reaction of H2S and Cd2+ at the monolayer-subphase interface. The presence of the SA monolayer plays an important role in the formation of the CdS particulate films. Because significant changes would be involved in the deposition procedures,36 in order to decrease the changes in the depositing procedure, the horizontal transferring method was used to transfer the monolayer-supported CdS particulate films to substrate after the reaction. After an hour the monolayer-supported CdS particulate films were transferred to solid substrates. Amorphous-carbon and formvar-coated 300-mesh copper grids and freshly well-cleaned silica were used as the substrates for transmission electron microscopy and photoacoustic spectroscopy, respectively. The TEM observation were carried out by a JEOL-2000EX electron microscope operating at 160 kV. Electron diffraction patterns and the dark field image were also taken in the selected area. The PAS measurement was based on a single-beam photoacoustic spectroscopy system.35 An intensity-modulated beam of monochromatic light from xenon was illuminated on CdS films placed in a closed cell. The periodic heating of the CdS films produces periodic pressure fluctuations, which were measured by a sensitive gas microphone. In order to limit the possible thermal effects and the sample degradation, the intensity of input light was kept as low as possible. Typically, a light beam with a power of 0.2 mW was incident upon a 0.5 × 0.5 cm2 sample. Result and Discussion The present system is different from bulk semiconductors and from dispersed semiconductor particles. It is ordered particulate films. The TEM is a tremendously powerful tool because it allows us to see the arrangements, sizes, and the important physical characteristics of the crystallites.1 The combination of the TEM dark field image with an electron diffraction pattern enables us to investigate the structure and the growth mechanisms of these particulate films. Maintaining the Total Area of the Monolayer in the Reaction Process. Figure 1 is a typical TEM image of a CdS particulate film formed in these conditions. The rodlike CdS particles of length about 100 nm aligned in three directions. These three directions are parallel to the edge directions of an

Figure 1. Typical TEM image of the CdS particulate films.

Figure 2. Electron diffraction pattern of the CdS particulate films in Figure 1.

equilateral triangle. Some dotlike CdS particles without any regular shape have also been found. Because the film is very thin, the bright light image is not very clear. Long reaction time could make the films thicker and easier to observe, leading to the formation of the layer by layer structure; such a multilayer structure would be very difficult to analyze.11-23 Fortunately, the combination of the electron diffraction pattern and the corresponding TEM dark field image enables us to see the morphology and composition of the particulate films clearly. Figure 2 is the transmission electron diffraction pattern from the area shown in Figure 1, displaying a symmetry pattern with somewhat dispersed and elongated spots. This implies that the CdS nanocrystals are not oriented randomly. The orientation with distribution is limited in a small range; that is, there is pronounced texture. The diffraction pattern shows that the diffraction arches should be indexed as (220) (outer circle) and (111) (inner circle) faces of the CdS cubic lattice.37 To analyze the composition and the growth mechanisms of CdS nanocrystals under the SA Langmuir monolayer, the TEM dark field image technique was used. It was found that the adjacent arches in one diffraction circle and in different diffraction circles were contributed by different CdS nanocrystals. Only the two arches (a-a, b-b, c-c, etc.) symmetrical about the diffraction center (symmetric inversion) were contributed by the same CdS nanocrystals, as shown in Figure 3 (O). These two symmetrical arches compose a set of diffraction patterns. Six sets of individual electron diffraction patterns compose the diffraction

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Figure 3. (O) Composite diffraction pattern composed of six sets of diffraction spots. Spots a-f were contributed by the crystals shown in parts a-f, respectively.

pattern of CdS particulate films; that is, the diffraction pattern is actually a composite one. The different dark field images, corresponding to the different sets of diffraction arches, are

shown in Figure 3a-f, respectively. In each part of the Figure 3a-c the CdS nanocrystals were aligned in only one direction, respectively, but these three directions were different. The

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Figure 4. Hexagonal close-packed structure of the SA monolayer: (a) plan view; (b) three-dimensional representation.

morphologies of nanocrystals in Figure 3d-f were dotlike. In this experiment, the magnetic rotation angle of the TEM was 186°, the long axis of the rodlike CdS crystals in each part of Figure 3a-c were parallel to the line connecting the two correspondent diffraction arches, respectively, and, at the same time, these three directions were parallel to the edges of an equilateral triangle, as shown in Figure 1 and Figure 3. Because the particulate films were very thin and the nanocrystals were very small, we could not get more information about the threedimensional structure of CdS, such as the crystal data in the other direction, by rotating the sample. Therefore, we cannot define the CdS crystal face parallel to the monolayer surface. However, at least, we can confirm that the [220] crystal zone of the CdS is parallel to the long axis of the rodlike nanocrystals. In other words, the (220) planes of the CdS crystals are parallel to the electron beam and perpendicular to the surface of the monolayer. It was mentioned above that the CdS nanocrystals were formed by exposing the SA monolayer at the air-aqueous CdCl2 interface to H2S gas. The surface charge density due to the head groups of the monolayer molecule is important. Because of ionization of the carboxyl head groups of SA, the negatively charged SA monolayer in the solid state consists of CH3(CH2)16COO- ions which are ordered two dimensionally at the air-aqueous interface. The electrostatic attraction between positively charged Cd2+ and the negatively charged head group of the monolayer will lead to a very high Cd2+ concentration at the monolayer-subphase interface, where the nucleation and the aggregation of CdS nanocrystals are initiated. The preferential two-dimensional growth of the CdS crystallites is also reasonably attributed to the high local reagent concentration at the monolayer-subphase interface. Therefore, the structure of head group (monolayer) plays an important role in the fabrication of the CdS particulate films. If the SA monolayer is compressed to its solid state, the carboxyl or the carboxylate groups are aligned perpendicular to the water surface. The alkyl chains of SA, fully extended in the air in a planar zigzag conformation, are oriented approximately normal to the surface in a hexagonal close-packed lattice.11-23 Many references about the hexagonal close-packed structure of the monolayer at the air-water interface have been discussed previously.38,39 Figure 4 shows the proposed hex-

Figure 5. (a) Crystal structure of the cubic CdS crystal and (b) schematic diagram of the different CdS crystals’ growth directions: (f) the growth direction of the rodlike crystals; (--->) the growth direction of the dotlike crystals.

agonal close-packed structure of the monolayer. From the structure of the monolayer and the areas per molecule (20 Å2), it is shown that the lattice constant a equals 4.81 Å. The electron diffraction pattern of Figure 2 shows that the d220 and d111 spacing of cubic CdS crystal are 1.94 Å and 3.16 Å, respectively. This is in good agreement with some reported reference.37 A comparison of the double d220 spacing of the CdS crystallites with d101h0 of the SA monolayer (4.16 Å) revealed a 6.8% mismatch between the template and the crystals. The morphology of these rodlike CdS nanocrystals (Figure 1, Figure 3a-c) is rationalized by the small mismatch (good fit) between d220 spacing of CdS crystals and the d101h0 spacing of the hexagonal close-packed SA monolayer. This is illustrated in Figure 5 in detail. Thus the preferred orientation of the [110] axis of the CdS crystal is parallel to the monolayer and perpendicular to the electron beam. Therefore, the rodlike CdS nanocrystals parallel to the edge directions of an equilateral triangle are reasonably attributed to the three equivalent (101h0) planes of the hexagonal close-packed SA monolayer. There is 30° angle between the line connecting the two diffraction arches produced by the corresponding rodlike CdS nanocrystals and that of the adjacent diffraction arches produced by the corresponding dotlike CdS nanocrystals. This is shown in Figure 3(O) in detail. This result implies that the dotlike CdS nanocrystals were induced by the {112h0} planes of the hexagonal close-packed SA monolayer because the same angle is found between the [101h0] and [112h0] axes of the SA monolayer. The mismatch between the d111 spacing (3.16 Å) of the CdS crystal and the d112h0 spacing (4.81 Å) of the SA monolayer is 31.7%. On the other hand, the mismatch between the double d111 spacing of the CdS crystal and the d112h0 spacing of the SA monolayer is 34.2%. In other words, the match between these two kinds of crystal planes is difficult to accommodate. The greater mismatch compared with that of

Ordered Cadmium Sulfide Particulate Films

Figure 6. Typical TEM image of the CdS particulate films formed in the condition of maintaining the surface pressure in the reaction process.

the d220 spacing of the CdS crystal and the d101h0 spacing of the SA monolayer (6.8%) resulted in the morphology of the dotlike nanocrystals (Figure 3d-f). Although the preferred growth orientation of the dotlike CdS nanocrystals is the [111] direction of the CdS crystal and this direction is perpendicular to the electron beam, the crystallites cannot grow long enough to produce rodlike morphology. This result is shown in Figure 5, which shows the hexagonal close-packed structure of the Langmuir monolayer. The size (width 5-10 nm; length 100 nm) of each individual rodlike CdS nanocrystal is size-quantized; their sizes are comparable to the de Broglie electron wavelength, the mean free path of excitons.40 This implies that the rodlike CdS is a kind of quantum wire, which would lead to novel applications and devices. The reaction of the monolayer-covered subphase with small gaseous molecules provides a potential method for nanofabrication of quantum dots and quantum wires. The most important factor in the preparation of this quantum confinement system is the match of the crystal and the template (the monolayer on the surface of the subphase), in other words, the match of the face distance of the two kinds of crystal structures. If the semiconductor and the monolayer are suitable, the fabrication of perfect and highly oriented quantum wires is very possible. This result opens the door to the colloid chemical generation of semiconductors with unusual crystal structures, controllable dimensions, and unique electric, optical, and electro-optical properties. The other important phenomenon is that diffraction spots were observed instead of diffraction arches. This was due to the structure of the compressed monolayer, which was not a strict hexagonal close-packed structure. It was also the reason why the rodlike CdS particles were not aligned in one direction; there were small direction differences for the different CdS rodlike crystallites. This implies that the structure of the monolayer is the other important and necessary factor for the fabrication of highly oriented quasi-two-dimensional quantum wire structures. Maintaining the Surface Pressure in the Reaction Process. In this condition, the surface pressure of the monolayer was maintained as high as 37.5 mN/M (the surface pressure corresponding to 20 Å2/molecule). Figure 6 shows the typical TEM image of the particulate films formed in this condition. The dotlike particles in the size range 10-20 nm form a

J. Phys. Chem. B, Vol. 101, No. 47, 1997 9707 stripelike domain. The edges of the stripes align with 6-fold symmetry, i.e. align in the edge directions of an equilateral triangle. D. K. Schwartz et al. had reported that fatty acid LB films on silicon formed distinct islands with straight edges.30 The inset Fourier transform shows that the island edges had approximate 6-fold symmetry. This was due to the reorganization of the LB films. In our experiment, the H2S gas infused into the subphase through the chinks and defects among the molecules, where the nucleation and aggregation of the CdS were initiated. The CdS nanocrystals adhered to the hydrophilic group of the SA molecules. Therefore, the morphology of the CdS particulate films was an illustrative mirror of the structure of the monolayer. For the monolayer on silica, the monolayer will detach in certain regions and reorganize because of the dewetting.30 This dewetting can be attributed to the force resulting from the head group-head group interaction of the SA monolayer and the head group-water interaction. Having compared the two reaction conditions in our experiment, we suggest that this force comes from the high surface pressure (37.5 mN/M). The continuous process to equilibrium includes two steps: at first, the macroscopic “crystallites” whose straight edges are explicit corresponded to the SA molecular arrangement formed, and this arrangement is related to the close-packed hexagonal structure of the monolayer. Then, the monolayer formed distinct island domains with straight edges aligned with 6-fold symmetry.30 Here, the strength that comes from the high surface pressure must be considered as a possible driving force for the reorganization of the monolayer. A similar process in LB films was observed,30,41,42 but there are few reports about the ordered reorganization of the monolayer at the air-water interface due to the difficulties of investigation by normal technology, especially by TEM, because the organic small molecule monolayer is almost transparent in TEM. The method we used enables us to study the reorganization of the monolayer by investigating the morphology of the particulate films formed under the monolayer. We guess the initial CdS nuclei formed before the generation of the stripelike domain of SA monolayer. After the formation of the “stripe” of the monolayer, the occurrence of defects became much more possible at the edge of the island. The H2S gas was easy to infuse into the subphase through these defects, where the aggregation of CdS particles was initiated. Because of the hexagonal close-packed structure of the SA monolayer in the small domain, the dotlike CdS particles formed dendritelike morphology. This is shown in Figure 6. There are several stripes composed of CdS dendritelike particles. The whole process of reorganization is very possibly finished at the airwater interface; if not, the same result would occur in A-type particulate films. This result provides a method to synthesize semiconductor quantum dots aligned in one direction and leads to a novel approach to investigate the structure of the Langmuir monolayer and the process of reorganization of the Langmuir monolayer. The Photoacoustic Spectroscopy Investigation of the Two Kinds of CdS Particulate Films. In these nanostructures, the dimensions of the wave function of the electron-hole (exciton) in the lowest excited state are comparable to the physical size of the particles. This quantum confinement of the exciton means that the continuous band of energies becomes more molecular in character, with narrow ranges of energy and line structure in the optical spectra.40,43-48 The changes in the absorption spectrum of semiconductors with changing particle size make the threshold of absorption shift toward the short wavelength direction. The UV-visible spectrum investigation of these

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Figure 7. PAS spectroscopy of the (a) A-type CdS particulate films (s) and (b) B-type CdS particulate films (- - -).

effects is reported widely.46-51 We will find whether photoacoustic spectroscopy can provide more information. Figure 7 shows the PAS of the CdS particulate films. It is shown that structured absorption spectra of the CdS particulate films are obtained. Curve a and b are the absorption spectrum of the A- and B-type CdS particulate films, respectively. The absorption edge of A-type particulate films was at about 487.5 nm, shifting about 0.15 eV with respect to bulk CdS. The absorption edge of B-type particulate films was observed at about 450 nm, shifting about 0.4 eV with respect to bulk CdS. From this result, we can conclude that the two kinds of particulate films are in a quantum confinement state. For the A-type particulate films, the threshold of the absorption corresponded to 5-10 nm on Henglein’s published Eg vs particles size curve.52 The result is a tally of the size of the rodlike CdS particles (width 5-10 nm, length 100 nm) and the dotlike particles (diameter 5-10 nm). For the B-type particulate films, the observed range of the threshold of absorption corresponded to 3-6 nm on Henglein’s published Eg vs particle size curve. The size of the particles is 10-20 nm, but the shape of the particles is not like a disc or a sphere. The particles of B-type particulate films are dendritelike particles with the “branches” radiating outward from the center of the particles. It is more clearly at the edge of “islands” or “stripes”, as shown in Figure 6. Every branch is very small. Therefore, the blue shift is reasonably attributed to the shape of particles; in other words, the blue shift of the threshold in the absorption spectrum is related to the shape of the particles. The qualitative features of curves a and b in Figure 7 are the same except for the peak at 417.5 nm of curve a. This implies that there are exciton states or some other interaction in the A-type particulate films. The semiconductor nanocrystals were studied in both experiment and theory.43-54 Many factors, such as size quantum effect and surface modification, influence the absorption spectrum of the nanocrystals. A- and B-type particulate films were formed in similar conditions. On the other hand, the size of the particles in the two kinds of particulate films is larger than 5 nm. For the CdS particles, the structured exciton absorption could not be observed if the size of the particles is larger than 5 nm.43-54 The size quantum effect and the surface modification cannot explain the presence of the PAS peak at 417.5 nm. The difference between A- and B-type particulate films is that the rodlike nanocrystals of A-type particulate films are aligned in an orderly form. The peak is possibly due to the ordered alignment of the nanocrystals. For the nanocrystals, the interaction among the nanocrystals is too weak to observe; we guessed that the ordered alignment of the nanocrystals would enhance this interaction and the “enhance effect” makes the

Two kinds of CdS particulate films have been generated at the monolayer-subphase interface. The epitaxial growth of the CdS particles was rationalized by the matching of the CdS crystals and monolayer. Generating films at the monolayerliquid interface provides a new method to synthesize ordered semiconductor nanocrystals or quantum wires and leads to a novel approach to investigate the reorganization of the Langmuir monolayer. The PAS investigation of the CdS particulate films shows that the blue shift of the threshold in the absorption spectrum not only depends on the size of particles but also depends on the shape of the particles. On the other hand, it is very possible that the interaction of the nanosystems is enhanced by the ordered alignment of the nanocrystals. We suggest that this interaction came from the coupling effect of exciton states between nanocrystals. References and Notes (1) Alivisatos, A. P. MRS Bull. 1995, xx, 23. (2) Heath, J. R. Science 1995, 270, 1315. (3) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (4) Martin, C. R. Science 1994, 266, 1961. (5) Gref, R.; et al. Science 1994, 263, 1600. (6) Kayanuma, Y. Phys. ReV. B 1988, 38, 9797. (7) Braun, P. V.; Osenar, P.; Stupp, S. I. Nature 1996, 380, 325. (8) Brus, L. Appl. Phys. A 1991, 53, 465. (9) Covin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (10) Ashoori, R. C. Nature 1996, 379, 413. (11) Zhao, X. K.; Yuan, Y.; Fendler, J. H. J. Chem. Soc., Chem. Commun. 1990, 1248. (12) Zhao, X. K.; McCormick, L. D.; Fendler, J. H. Chem. Mater. 1991, 3, 922. (13) Zhao, X. K.; Fendler, J. H. Chem. Mater. 1991, 3, 168. (14) Zhao, X. K.; Xu, S. Q.; Fendler, J. H. Langmuir, 1991, 7, 520. (15) Zhao, X. K.; McCormick, L. D.; Fendler, J. H. Langmuir 1991, 7, 1255. (16) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1991, 95, 3716. (17) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1992, 96, 9933. (18) Yang, J.; Fendler, J. H.; Jao, T. C.; Laurion, T. Microsc. Res. Techn. 1994, 27, 402. (19) Tian,Y.; Wu, C.; Kotov, N.; Fendler, J. H. AdV. Mater. 1994, 6, 959. (20) Yang, J.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5505. (21) Yang, J.; Meldrum, F.C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (22) Fendler, J. H. Supramol. Chem. 1995, 5, 1. (23) Fendler, J. H.; Meldrum, F. C. AdV. Mater. 1995, 7, 607 (24) Zhu, R.; Min, G.; Wei, Y. J. Phys. Chem. 1992, 96, 8210. (25) Luo, G. P.; Ai, Z. M.; Hawkes, J. J.; Lu, Z. H.; Wei, Y. Phys. ReV. B 1993, 48, 15337, and references therein. (26) Pan, Z. Y.; Peng, X. G.; Wu, Z. H.; Li, T. J.; Zhu, M. Liu, J. Z. Langmuir 1996, 12, 851. (27) Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87, 727. Heywood, B. R.; Rajam, S.; Mann, S. J. Chem. Soc., Faraday Trans. 1991, 87, 735. (28) Ulman, A. AdV. Mater. 1993, 5, 55. (29) Heywood, B. R.; Mann, S. J. Am. Chem. Soc. 1992, 114, 4681. (30) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 10444. (31) Peng, X.; Lu, R.; Zhao, Y.; Li, T. J. Phys. Chem. 1994, 98, 7052. (32) Jr. Foss, C. A.; Horuyak, C. L.; Stockert, J. A.; Martin, C. R. J. Phys. Chem. 1994, 98, 2963. (33) Rosenwaig, A.; Photoacoustic and photoacoustic spectroscopy; John Wiley & Sons Inc.: New York, 1980. (34) Qiu, S. Y.; Zhang, S. Y.; Wei, L. H. J. Appl. Sci. (in China) 1986, 207. (35) Jiang, Y. S.; Zhang, S. Y.; Qian, F.; Shao, H. P. J. Phys. IV 1994, c7. (36) Gupta, V. K.; Kornfield, J. A.; Ferencz, A.; Wegner, G. Science 1994, 265, 940. (37) Osugi, O. ReV. Phys. Chem. Jpn. 1966, 36, 59; J. Committ. Power Diffract. Stands. 1971.

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