J. Phys. Chem. 1992, 96,9933-9939 The ionization potential of the Pd atom (8.33 eV) is known to be lower than that of the Au atom (9.223 eV).23 Thus, the reduction of the Au ions proceeds more easily than that of the Pd ions, so the Au core strWure is formed sooner than the Pd core structure is. Moreover, the electronic interaction between the Au awe and the Pd layer could provide an unevendistribution of electrons. Then, the Pd in the surface layer becomes poorer in electron density than in the Au core, which might make the Au/W( 1/4) bimetallic clusters more active than Pd clusters since the substrate having a double bond favors the electron-deficient surface.
Cdwioll (1) The polymer-protected gold/palladium bimetallic clusters
can be prepared by the simultaneous reduction of the corresponding ions in the presence of the poly(N-vinyl-2-pyrrolidone). The clusters were used as the catalysts for the selective partial hydrogenation of 1,3-cyclooctadiene with higher activities than those of the monometallic clusters. (2) From the EXAFS analysis as well as TEM observation, the Au core structure, in which the Pd atoms are on the surface of the cluster particles, is prrsented as a model for the Au/Pd( 1/4) bimetallic clusters prepared by the simultaneous reduction. (3) For the Au/Pd(l/l) bimetallic cluster, it is suggested to use the cluster-in-cluster model, in which seven Au cores are located in the cluster and Pd atoms play a role to combine the Au cores.
Acknowledgment. We gratefully acknowledge the assistance of Drs. Atsushi Oyama and Masaharu Nomura at the National Laboratory for High Energy Physics (KEK) for the EXAFS measurements and of Drs. Kouichi Adachi and Satoru Fukuda at the University of Tokyo in taking electron micrographs. This study was supported by a Special Grant by the Asahi Glass Foundation and a Grant-in-Aid for Scientific Research in the Priority Area of 'Macromolecular Complexes" (01612002) from the Ministry of Education, Science and Culture, Japan.
R-m
9933
NO. PVP, 9003-39-8; HAuC14, 16903-35-8;PdClz, 7647-
10-1; Au, 7440-57-5; Pd, 7440-05-3.
Referema d Nom (1) Shfelt, J. H.; Via, G. H.; Lytle, F. W. J. Chem. Phys. 1980, 72,4832. (2) Sinfelt, J. H.; Via, G. H.; Lytle, F. W.; Gieegor, R. B. J. Chem. Phys. 1981, 75, 5527. (3) Meitmer, G.; Via, G. H.; Lytle, F. W.; Sinfelt, J. H. J. Chem. Phys. 1983, 78, 882. (4) Toshima, N.; Harada, M.; Yonezawa, T.; Kuehihashi, K.;Asakura, K. J. Phys. Chem. 1991,95, 7448. (5) Liu, H.; Mao, G.; Meng, S. 1. Mol. Catal. 1992, 74, 275. (6) Eaumi, K;Shiratori, M.; Ishimka, H.; Tano, T.; Torigoe, K.; Meguro, K. Lungmuir 1991, 7, 451. (7) Hirai, H.; Toshima, N. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; D. Reidel Pub.: Dordrecht, 1986; pp 87-140. (8) Huai, H.; Nakao, Y.; Toshima, N. Chem. h i t . 1978,545. (9) Hirai, H.; Chawanya, H.; Toshima, N. Reactive Polym. 1985,3, 127. (IO) Hirai, H.; Chawanya, H.; Toahima, N. Bull. Chem. Soc. Jpn. 1985, 58, 682. (11) Toshima, N.; Kuriyama, M.; Yamada, Y.; Hirai, H. Chem. Lctt. 1981, 793. (12) Toshima, N.; Takahashi, T.; Hirai, H. J. Mucromol. Sci.-Chem.1988, A25(5-7), 669. (13) Toahima, N.; Kushihashi, K.;Yonezawa, T.; Hirai, H. Chem. h i t . 1989, 1769. (14) Zhao, B.; Toshima, N. Chem. Express 1990, 5(10), 721. (15) Toshima, N. J. Mucromol. Sci.-Chem. 1990, A27, 1225. (16) Teo, B. K.EXAFS Basic Principles and Data Analysis, Inorganic Chemistry Concepts;Springer-Verlag: Berlin, 1986; Vol. 9. (17) Teo, B. K.;Lce, P. A. J . Am. Chem. Soc. 1979,101, 2815. (18) Via, G. H.; Drake, K. F., Jr.; Meitzner, G.; Lytle, F. W.; Sinfelt, .I. H. Catal. Lett. 1990, 5, 25. (19) Renaud, G.; Motta, N.; Lancon, F.; Belakhovsky, M. Phys. Rev. B 1988,38, 5944. (20) Lytle, F. W.; Sayers, D. E.; Stem, E. A. In X-Ray Absorption Fine Structure-y;Leon, J. M., Stern, E. A., Sayers, D. E., Ma, Y., Rehr, J. J., Eds.; Elscvier/North-Holland New York, 1988; pp 701-722. (21) Wyckoff, R. W. G. Crystal Structures, 2nd 4.; Interscience Pub lishers: New York, 1963; Vol. 1. (22) Foger, K. In Catalysis; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1984; pp 227-305. (23) Lange, N. A. Handbmk ofChemistry; McGraw-HiU Book Company, Inc.: New York, 1961.
Epitaxial Formation of PbS Crystals under Arachidic Acid Monolayers X. K. Zhao,' J. Yang? L.D.McCormick,' and J. H.Fendler**2 Department of Chemistry, Syracuse University, Syracuse, New York 13244-41O0, and Jdt D Scientific, 1815 West 1st Avenue, Mesa. Arizona 85202 (Received: June 5, 1992; In Final Form: August 14, 1992) Lead sulfide (PbS) particulate films composed of highly oriented, equilateral-triangularcrystals have b n in situ generated by the exposure of arachidic acid (AA)-monolayer-coated aqueous lead nitrate [Pb(NO&] solutions to hydrogen sulfide (H2S). The AA-coated PbS particulate films, at different stages in their growth, were transferred to solid substrates and characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM),and electron diffraction measurements. Each individual crystal had its [ l l l ] axis perpendicular and its [TT2], [T2'f],and [21T] axes parallel (arranged in 3-fold symmetry at 120° angles) to the AA monolayer surface. The rate of HIS infusion influenced the size of crystals rown under the AA monolayer. The mean length of equilateral-triangular PbS crystals decreased from 607 A (U = 133 to 297 A (a = 91 A) upon diminishing the H2S exposure time from 30 to 5 min. The epitaxial growth of PbS crystals has been rationalized in terms of an almost perfect fit between the (11 1) plane of the cubic crystalline PbS and the (100) plane of the hexagonally closepacked AA monolayer. The presence of a monolayer and the slow encounter of the precursors have been found to be essential requirements for the oriented growth of PbS crystals. Exposure of Pb(N03)2solutions to HzS in the absence of monolayer surface coverage furnished only irregular PbS crystals at the air-water interface.
f)
Iatroductioa
Construction of advanced electronic devices with desired electric, optical, and electrooptical properties requires the availability of semiconductor nanostructures with controllable purities, sizes, shapes, and orientations. A variety of techniques, including molecular beam epitaxy (MBE),chemical vapor deposition (CVD), and sputter and vapor depositions, have been developed for semiconductor superlattice formation and band-gap engi0022-3654/92/2096-9933$03.00/0
neering. Indeed, systems with atomic dimensions and smoothness have been grown in ultrahigh v a c ~ u m . ~ ' ~ In situ generation of semiconductor particles and semiconductor particulate films at the interfaces of organized surfactant assemblies represents an alternative approach. Versatility, maneuverability, and relative simplicity are the advantages of this colloid chemical method. Particularly successful has bcen the formation of sizequantized metaliwlfide semiconductor particulate films 0 1992 American Chemical Society
9934 The Journal of Physical Chemistry, Vol, 96, No. 24, 1992
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under surfactant monolayers floating on aqueous solution^.^-'^ Cadmium sulfide (CdS) particulate frlms were grown, for example, by the slow infusion of hydrogen sulfide (H2S) across arachidic acid monolayers which were well compressed on an aqueous cadmium chloride subphase. Nucleation, started at numerous distinct sites, was followed by the downward growth of nanocrystalline CdS. With continued H2S infusion, CdS crystallites grew in height and width and coalesced into interconnected arrays. Further growth resulted in the formation of the “first layer” of the porous CdS particulate film which, in turn, seeded the formation of a new set of clusters and produced the “second layer” of CdS film. The film continued to grow, layer-by-layer (downward below the monolayer), up to a plateau thickness beyond which no CdS particle formation was observed. Importantly, at any stage of its growth, the CdS particulate film could be transferred to a sdid substrate without any structural reorganization.’ Molecular recognition between the monolayer headgroups and the incipient semiconductor nanocrystallites mimics biomineralization” and would represent an important milestone in the realization of the potential of a colloid chemical approach to band-gap engineering. Encouraged by the recently reported oriented crystallization of CaC03,12-114 NaC1,lSa - g l y ~ i n e , ’ and ~’~ BaS0418-20 under monolayers, we have initiated systematic studies in the epitaxial growth of advanced materials in this medium. Evidence is presented in the present report for the epitaxial growth of lead sulfide (PbS) crystals, in situ generated by the controlled infusion of H2S across an arachidic acid monolayer, well-compressed upon an aqueous lead nitrate solution subphase. Molecular recognition is shown to be the consequence of lead ion accumulation at the monolayer interface and of the correspondence between the dloospacing of arachidic acid and the P b P b interionic distance in the PbS(111) plane.
Experimental Section Arachidic acid (AA); lead nitrate (Pb(N03)2), and sodium sulfide monohydrate (98% ACS reagent) were obtained from Sigma and were used as received. Water was purified by means of a Millipore Milli-Q filter system provided with a 0.22-pm Millistack filter at the outlet. The in situ generation of lead sulfide (PbS) crystallites, in a circular trough or in a Lauda film balance, followed the established procedure^.^ In the circular trough, the subphase contained an aqueous 5 X lo4 M solution of Pb(N03)2at pH = 5.5. The surface of the subphase was cleaned by sweeping it with an aspirator. An appropriate amount of the spreading solution (1.5 X M AA in CHC13)was careful1 injected onto the clean subphase to give a coverage of 20 per molecule of AA. Subsequent to equilibration of the monolayer (taking typically 20 min), freshly prepared 5.00 X M Na2S solution (40 mL, pH = 12) was placed in a small open dish (5.0 cm2surface area) next to the trough and both the trough and the Na2S-containing-bottle were hermetically covered by a glass jar. The H2S, liberated from the Na2S-containingbottle, filled the available air space and slowly penetrated the AA monolayer. The surface of the aqueous Pb(N03)2subphase in the Lauda film balance was also cleaned by sweeping it with an aspirator. Surface pressure of the spread AA monolayer was maintained at 20 A2 per molecule. PbS formation was initiated by the slow injection of an appropriate amount of H2S above the monolayer surface. The speed of injection was varied only between two extremes: at one extreme, the barrel of the syringe was slowly (2-5 s) pushed in; in the second extreme, the barrel of the syringe was not pushed at all. AA-coated PbS particulate films, at different stages of their growth, were transferred to solid substrates by horizontal lifting. The excess liquid adhering to the particulate film was removed immediately by using a filter paper. The samples were dried in air prior to their characterization. Amorphous-carbon- and Formvar-coated 2Wmesh copper grids and freshly cleaved mica were used as substrates for transmission electron microscopy (TEM) and atomic force microscopy (AFM), respectively.
l2
Figure 1. Transmission electron micrograph of PbS crystals, generated by exposing an aqueous 5.0 X lo4 M Pb(N03)2solution (in the absence of AA monolayers) to H2S. The PbS crystallites, formed in the bulk solution, were scooped up by a Formvar-coated, 200-mesh copper grid.
Transmission electron micrographs (TEM) were taken either on a JEOL 2000-FX electron microscope operating at 200 keV or on a JEOL 2OOO-EX electron micrmpe operating at 120 keV. Electron diffraction patterns of individual crystals were also examined in selected areas. Either Nanoscope I1 or Tak 3.0 AFM instruments were used for the investigation of PbS surfaces. AFM determinations were obtained in the constant deflection mode. Images were obtained at a scan rate of 5-10 lines per second by silicon nitride cantilevers (100 pm long and with a spring constant value of 0.58 N/m). Images were plotted on a CP-200U Mitsubishi color videoprocessor.
Results PbS Formation in Homogeneous Solutions-Control Experiments. PbS is formed in the interaction of Pb2+with sulfide (S2-) and/or bisulfide (HS-) ions. H2S, the source of S2-and HS- ions, either was introduced directly or was generated from Na2S. Importantly, in the absence of monolayers, crystallization occurred in an uncontrollable and irreproducible manner at the air-water interface. Formation of only large, irregular clumps of PbS could be observed. Transmission electron micrographs indicated the presence of randomly oriented, intergrown aggregate crystals in heterogeneous size distribution (Figure 1). Only powder-type electron diffraction could be observed for PbS crystals, grown in the absence of monolayers, in the control experiments (Figure 2). Changing the concentration of Pb2+and/or H2S or the method of H2S generation or that of its introduction did not alter the heterodispersity of PbS crystals formed in the absence of monolayers. PbS Formation under AA Monolayers in the Circular Trough. Exposure of an AA-monolayer-coated Pb(N03)2solution to H2S resulted in the formation of fairly monodisperse crystals which constituted the PbS particulate film. A typical electron micrograph of a PbS particulate film is shown in Figure 3. This film
Epitaxial Formation of PbS Crystals
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9935
40-
33 Q
30-
m F4
20-
10
-
0
1 3 8 5 4 6 0 5 3 0 615 6 9 0 7 7 0 8 4 5 925
LENGTH, Figure 2. Electron diffraction pattern of PbS crystals. See Figure 1 legend for details.
Figure 4. Size distribution (in length of equilateral triangular PbS crystals) of PbS crystals formed by the infusion of H2S to an AA monolayer, floating on an aqueous 5.0 X lo4 M Pb(N03)2 solution in a circular trough. See Figure 3 legend for details.
ire 5. Two-dimensional, 450 nm X 450 nm AFM image of a micawrted PbS particulate film. The film was formed by the infusion of to an AA monolayer, floating on an aqueous 5.0 X 10-4 M Pb(NO& tion in a circular trough, for 50 min. Figure 3. Transmission electron micrograph of a PbS particulate film. The film was formed by the infusion of H2Sto an AA monolayer, floating on an aqueous 5.0 X lo4 M Pb(N0,)2 solution in a circular trough, for 45 min. The PbS particulate film was deposited on an amorphous-carbon-coated, 200-mesh copper grid. Scale: 26.5 mm = 200 nm.
was generated by the exposure of a well-packed AA monolayer, floating on an aqueous 5 X lo4 M Pb(N03)2solution in the circular trough, to H2S for 45 min. The presence of triangular crystals is clearly discernible. TEM analysis of 15 separately prepared samples revealed the PbS crystals to be equilateral and (some) right angle triangles, some of which were aligned. The distribution of crystal length was narrow (mean = 509 A, = 97 A; Figure 4). Additional information on the morphology of PbS crystals was obtained by atomic force microscopy (AFM). A typical twodimensional 450 nm X 450 nm image of a PbS particulate film, formed by the 50-min exposure of a well-packed AA monolayer M aqueous Pb(N03)*to H2S, is shown in floating on 5 X Figure 5. The thickness of the crystals (indicated by the gray scale on the z axis) was estimated to be between 100 and 150 A. Seven separately prepared samples were examined by AFM. Fine
structures could be readily observed for all of the crystals in all of the preparations. A two-dimensional 2750 nm X 2750 nm image of a mica-supported PbS particulate film is shown in Figure 6. Thicknesses of the PbS crystals were determined to be between 100 and 160 A. Some 30-120 A diameter and 30 f 4 A deep holes could be seen in the AFM images. A typical three-dimensional view of an individual equilateral-triangular PbS crystal is illustrated in Figure 7. The crystal appears to have a threefold symmetry in the [fT2], [T2T], and [2iT] axes parallel to the AA-monolayer surface. Electron diffraction patterns of PbS particulate films, prepared under AA monolayers in the circular trough, showed mostly polycrystalline powder patterns similar to those obtained for PbS crystallites generated in aqueous Pb(N03)2in the absence of monolayers (Figure 2). The diffraction rings were indexed as (1 1l), (200), (220), (31 l), (222), (400), (331), (420), and (422) for a simple NaC1-type cubic structure with a lattice constant of a = 5.94 A. In a few cases, individual crystals, selected by choosing appropriate TEM apertures, showed well defined Bragg spots, indicating the presence of oriented crystals. Most of these PbS crystals were found to grow with their [ 1111 axis perpendicular to the AA-monolayer surface, while some of them were
Zhao et ai.
9936 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
8,
I
\-Fig.
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AREA,
K
2
Figure 8. Surface pressure-surface area isotherm of an arachidic acid monolayer spread over an aqueous 5.0 X 10-4 M Pb(N03)*solution. The arrows mark the monolayer surface areas under which the PbS crystals were grown for the given experimentsillustrated in the figures indicated.
Figure 6. Two-dimensional 2750 nm X 2750 nm AFM images of a mica-supported PbS particulate film. See Figure 5 legend for details.
Figure 7. Three-dimensional AFM view of an equilateral PbS crystal. See Figure 5 legend for details.
in the [loo] orientation (not shown). PbS Formation under AA Monolayers in the Lauda Film Balance. Spreading of AA in the Lauda film balance permitted a better surface pressure control. Well behaved surface pressure-surface area isotherms were obtained upon spreading AA monolayers on aqueous Pb(N03)2solutions (see Figure 8 for a typical surface area-surface pressure plot). In the absence of any applied surface pressure, the phase of the monolayer corresponded to that of the gaseous state in three dimensions. AA molecules, even in their gaseous state, were found to aggregate into several disconnected domains. These domains served, in fact, as the sites for oriented growth of PbS nanocrystallites (Figure 9). Applying increasingly greater surface pressure resulted in a decrease in the area which each AA molecule occupied. Two distinct regions
could be observed. The first region, up to an approximate surface pressure of 22 mN m-', corresponded to the liquid expanded state of the monolayer. At the second region, coinciding with the solid state of the monolayer, increasing the surface pressure caused very little change in the area which each AA molecule occupied. The rate of H2S infusion profoundly influenced the size of the PbS crystals constituting the particulate film formed under the AA monolayer. Decreasing the rate of hydrogen sulfide exposure resulted in the formation of larger crystals. A typical transmission electron micrograph of a PbS particulate film, formed by exposing the AA-monolayer-coated aqueous Pb(N03)2solution to H2S for 30 minutes, is illustrated in Figure 10 (top). The reciprocal lattice vectors of PbS crystals are shown in Figure 10 (bottom). The distribution of the lengths of the equilateral triangles is quite narrow (mean = 607 A, CT = 133 A; Figure 11). Significantly, taking the transmission electron micrographs at a limited aperture revealed a high degree of orientational alignment of the equilateral-triangular PbS crystals (Figure 10). Transmission electron micrographs of PbS particulate films, formed by exposing the AA-monolayer-coated aqueous Pb(N03)2solution to H2S for a relatively short period of time ( 5 min), indicated the presence of less dense and smaller PbS crystals (Figure 12). Indeed, measurements of the crystals led to a mean length of the equilateral triangles of 297 A (with CT = 91 A), as illustrated in Figure 13. PbS particulate films, generated in the Lauda film balance, exhibited coherent diffraction structures. All crystallites had [1111 orientation aligned normal to the monolayer-water interface. Rotation about the [ 1111 axis led to the reciprocal lattice of the PbS axis; each reciprocal lattice spot extending to a concentric arc of Debye rings (Figure 14). The set of concentric arcs gave a hexagonal symmetry in the order of (220), (422), (440), etc., with increasing radius. The (1 1l), (200), (31 l), (222), (400), (3 1l), and (420) rings, associated with polycrystalline NaC1-type crystal lattices, were absent in the electron diffraction patterns. The angular length of the intense arcs, (220), was estimated to be 21O. Rotation angles of less than 60° indicated an orientation coherence of the PbS crystals in the direction perpendicular to the [ 1111 axis. This can also be seen in the electron micrographs of the equilateral PbS crystals (Figures 10 and 12). Discussion It is important to state at the onset of discussion that the present system is distinctly different from bulk semiconductorsand from dispersed semiconductor particles. It is a porous particulate film which consists of a large number of reasonably uniform equilateral-triangular PbS crystals. The presence of monolayers on aqueous Pb(N03)2solutions has been shown to be an essential requirement for growing PbS crystals, constituting the particulate film, in a reproducible manner by the infusion of H2S. Reproducible aligned growth of PbS crystals could only be accomplished under monolayers which had been compressed to their solid states.
~
Epitaxial Formation of PbS Crystals
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9937
Figure 9. (a) Transmissionelectron micrograph of PbS crystals formed under an AA monolayer at a surface pressure of 40 A2/molecuie (compressed from 57 A2/molecule in a Lauda film balance; subphase = 1.0 X M Pb(N03)2) by the infusion of H2S. (b) Electron diffraction of the PbS particulate film domain in Figure 9a. Limiting aperture was applied to cover an area of 2 pm in diameter.
Indeed, AFM images showed the presence of pinholes (in 30 f 4 A depths) in mica-supported PbS particulate films (Figure 6), which had been prepared under AA monolayers in the circular trough in the absence of rigorous surface pressure control. Similarly, holes (on the order of 10 nm depths) have been recently obsemed in cadmium-arachidatemonolayers aged for three months prior to imaging?l Pinholes have been attributed to an insufficient number of water molecules for hydrating the substrate.22 Thus, careful control of monolayer surface pressure is an essential requisite for oriented crystal growth. AA monolayers in their solid states consist of CH3(CH2)& OOH molecules, two-dimensionally arrayed at the air water interface. Spread over the aqueous subphase, the carboxyl or the carboxylate groups of AA are aligned perpendicularly to the water surface. The alkyl chains of AA, fully extended in the air in a
Figure 10. (top) Transmission electron micrograph at limiting aperture coverage of PbS crystals formed by the slow (30 min) infusion of H2S to an AA monolayer in the Lauda film balance (kept at u = 26 mN m-l surface pressure) floating on an aqueous 5.0 X lo4 M Pb(NO& solution. The PbS particulate film was deposited on a Formvar-coated, 200-mesh copper grid. (bottom) Diagrams indicating the 3-fold symmetry of equilateral-triangular PbS crystals. Reciprocal lattice vectors are indicated.
planar zigzag conformation, are oriented approximately normal to the surface in a triangular lattice of hexagonal close packing with a lattice constant of a = 4.85 A and dloosTcing of 4.20 Combined synchrotron X-ray reflection and diffraction data established a structural model for AA monolayers at air-water
9938 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
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Figure 11. Size distribution (in length of equilateral triangular PbS crystals) of PbS crystals formed by the slow (30 min) infusion of H2S to an AA monolayer. See Figure 10 legend for details.
227
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333
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440
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LENGTH,
LENGTH.
Figure 13. Size distribution (in length of equilateral triangular PbS crystals) of PbS crystals formed by the fast infusion of HIS to an AA monolayer floating on an aqueous 5.0 X lo4 M Pb(N03)2solution. See Figure 12 legend for details.
Figure 14. Electron diffraction pattern of PbS crystals formed by the fast infusion of H2S to an AA monolayer floating on an aqueous 5.0 X loa M Pb(N03)2 solution. See Figure 12 legend for details.
A2/molecule, see Figure 8) permitted assessment of the lattice constant to be 4.81 A (a) and the d(loo)spacing to be 4.16 A (d(lm) = a sin 60’). These values are in good agreement with those determined for AA monolayers by synchrotron X-ray scattering (a = 4.85 A and d = 4.13 They are also similar to those determined for cadmium stearate (a = 4.89 A and d(loo)= 4.20 i 0.10 and other fatty a ~ i d ~ ’monolayers. *~* Reliable assessment of the arrangement and crystallinity of Pbz+ at the AA-monolayer interface is equally elusive. Our data is best accommodated in terms of an AA:Pb2+ = 3:4 ratio (Figure 15). Grazing incidence X-ray diffraction measurements of lead arachidate monolayers demonstrated the existence of long-range ordering (250 A) of Pb2+.27Evidence has also been obtained for the presence of an ordered Cd2+layer, with a 1000-8, coherence length, under uncompressed cadmium arachidate monolayer^.^^ Data obtained in the present work are in accord with such orderings of arachidate monolayers (vide infra). PbS is known to crystallize in a cubic crystalline lattice with a lattice constant of a = 5.9458 A.30 Atomic coordinates are (O,O,O) and (0,1/2,1/z)for Pb and they are (1/2,1/2,1/2) and (1,1,1/2) for S. Distances between the closest Pb-Pb and S-S atoms, 4.20 A, fit the d(loo)network spacing of the AA monolayer. This fit implies the alignment of PbS along its (1 11) plane to the plane of the AA monolayer (Figure 15). A comparison of the interatomic Pb to Pb distance of the (1 11) plane of the PbS crystal 4.20 A) with that of the d(,oo)spacing of the AA monolayers (4.16 revealed a mismatch of only 1% between these two crystals. Epitaxial growth of PbS under wellampressed AA monolayers is explicable in terms of the geometrical complementarity between A).23325
A)23924
Figure 12. Transmission electron micrograph (limiting aperture coverage) of a PbS particulate film. The film was formed by the fast ( 5 min; pushing the barrel of the syringe) infusion of H2S to an AA monolayer (kept at 26 mN m-’ surface pressure in the Lauda film balance) floating on an aqueous 5.0 X lo-” M Pb(N03)2solution. The PbS particulate film was deposited on a Formvar-coated, 200-mesh copper grid.
interface^.^^ The model required the hydrocarbon chains to be well packed in a pseudohexagonal lattice and tilted toward their nearest n e i g h b ~ r . ~ ~ * ~ ~ Rationalization of the packing of the AA headgroups at the aqueous subphase-air interface is, unfortunately, less than straightforward.26 The absence of information on the extent of headgroup ionization (at a bulk pH of 5.5; see Experimental Section), counterion binding, and water hydration hinders the interpretation of experimental results and the development of a reliable theoretical approach for predicting headgroup organization at the monolayer-subphase interface. Using the experimentally determined value for the surface area of one AA molecule (20.0
a,
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9939
Epitaxial Formation of PbS Crystals
[oo 11
Recognition between well-compressed arachidate ions and incipient lead sulfide crystallites at the molecular level has shown a remarkable amplification in producing microscopic equilateraltriangular lead sulfide crystals in a fairly uniform size distribution. This approach opens the door to the colloid chemical generation of semiconductors in unusual crystal structures and in controllable dimensions with unique electric, optical, and electro-optical properties. Monolayer-directed generation of semiconductor crystals and semiconductor particulate films continues to occupy our attention and will be the subject of subsequent communications from these laboratories.
Acknowledgment. Support of this work by a grant from the National Science Foundation is gratefully acknowledged.
\1 [ l o 0 1 a u = 4.8s
I
?I
dpb-pb
4.20
Figure 15. (a) Three-dimensional representation of the Pbs crystal lattice and (1 11) plane. (b) Schematic two-dimensional representation of the proposed overlap between Pb2+ions and AA headgroups: 0 = AA headgroup; 0 = Pb2+; 0 = Pb2+ and AA headgroups. A unit cell is highlighted by the dotted area which is enclosed by heavy lines.
PbS and the AA headgroups. Strong electrostatic interaction results in a very high Pb2+ concentration at the monolayer interface. The extremely low solubility of Pbs in water (Ksp = 8.81 X at 25 "C)will drive its rapid and random nucleation at the monolayer interface unless the rate of reaction is drastically diminished. Encounter of the PbS precursors has been controlled by interposing the monolayer between them and by limiting the rate and amount of H2Sexposure. These measures have ensured the formation of a critically sized nucleus and the subsequent ion-by-ion heteroepitaxial growth of the PbS crystals. Significantly, equilateral-triangular PbS crystals have been grown under compressed monolayers in the same orientation (see Figure 10). Maintaining the monolayer surface pressure at 26 mN m-' led to the formation of PbS crystals arranged in the same direction, with a variation of &8O. Transferring the PbS crystals to a solid support and alteration in the surface pressure may cause changes in crystal orientation.
Conclusion Demonstration of epitaxial growth of semiconductor crystals under monolayers is the most significant result of the present work.
Registry NO. PbS, 1314-87-0; AA, 506-30-9.
References and Notes (1) J&D Scientific.
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