J. Phys. Chem. 1995,99, 5505-5511
5505
Morphology Control of PbS Nanocrystallites, Epitaxially Grown under Mixed Monolayers Jianping Yang and Janos H.Fender* Department of Chemistry and the W.M.Keck Center For Molecular Electronics, Syracuse University, Syracuse, New York 13244-4100 Received: September 29, 1994; In Final Form: November 21, 1994@
Surface pressure (Il) vs surface area (A) isotherms and Brewster-angle microscopic (BAM) images have been recorded for monolayers, spread on aqueous 1.0 x M Pb(N03)~solutions and prepared from arachidic acid (AA) and octadecylamine (ODA) mixtures. Identical IJ vs A isotherms and BAM images were obtained for monolayers, prepared from AA: ODA = 1:0, AA:ODA = 5: 1, and AA:ODA = 2: 1, while Il vs A isotherms and BAM images for AA:ODA = 1:1 and AA:ODA = 1:2 monolayers increasingly resembled those obtained from pure ODA (AA:ODA = 0:1). Electron micrographs and diffraction patterns established the epitaxial growth of equilateral triangular PbS nanocrystallites, with their { 11l} planes parallel to the monolayer, upon exposing AA:ODA = 1:0 (at any saxface pressure) and AA:ODA = 5: 1 (kept at Il = 30 mN/m) monolayers M Pb(N03)~solutions] to H2S. Similar treatment of AA: [which were covered with aqueous 1.0 x ODA = 2: 1 monolayers (kept at Il = 30 mN/m) resulted in the formation of laminated, irregular shaped, and right-angle triangular PbS nanocrystallites with their (001) planes parallel to the monolayer. In contrast, exposing AA:ODA = 2:l monolayers (kept at IJ = 0 and A = 23 A2/molecule) to H2S led to the formation of mostly right-angular PbS nanocrystallites, epitaxially grown on their { 1lo} faces. TEM images of PbS, grown under AA:ODA = 1:l (kept at Il = 30 "/m), appeared to be quite similar to images of those that were grown under AA-rich monola;yers, but they contained proportionately more right-angular nanocrystallites. PbS, formed under AA:ODA = 1:2 monolayers (kept at Il = 30 "/m), were randomly oriented, rectangularly shaped, and irregularly sized. Findly, no epitaxy occured under AA:ODA = 1:5 monolayers; only irregular brownish-colored particles formed in the bulk solution.
Introduction
by using a Millipore Milli-Q system which was provided with a 0.22-pm Millistack filter at the outlet. Demonstration of epitaxial growth of lead sulfide (Pki!S),l lead Monolayers were formed and compressed in a commercial selenide (PbSe),2 and cadmium sulfide (CdS)3 semiconductor Lauda Model P film balance. The surface of the aqueous 1.0 nanocrystallites under arachidic acid (AA) monolayers, floating x M Pb(NO3)2 solution, kept at pH = 5.8 (the subphase), on aqueous lead nitrate and cadmium chloride solutions, was cleaned by repeated compressiodaspiration and expansion represented an important milestone in our membrane-inimetic cycles. The surface was deemed clean when the surfaceapproach to the preparation of advanced material^.^. Slow pressure increase was less than 0.2 dydcm upon compression infusion of H2S (or H2Se) across the arachidic acid monolayer to 1/20th of the original area and when this surface-pressure resulted in the nucleation and subsequent growth of the increase remained the same subsequent to aging for several semiconductor nanocrystallites. The observed epitaxy was hours.' rationalized in terms of lattice matching between the ;LIachidic Chloroform solutions of AA (1 x M) and ODA (1 x acid headgroups in the monolayer and the semicoiiductor M) were mixedin different proportions (l:O, 5:1, 2:1, 1:1, nanocrystallites which were formed under the mono1a ~ e r . ~ . ~ 1:2, 1 5 , and 0:l) and applied to the air-subphase interface to Molecular information which was available in the twogive a surface area coverage of ca. 35 A2/molecule. Subsequent dimensional monolayer was recognized by the incipient semito 15 min of incubation, the monolayer was compressed to a conductor nanocrystallites and was utilized in orientin €:threedimensional crystallization. Oriented growth of semicondi~ctor~~~surface pressure of 20 mN/m (to give a surface area of ca. 20 A2/molecule; with the exception of the monolayer which was as well as glycine,6 sodium ~hloride,~ barium sulfate,* ccalcium formed from AA:ODA = 0:1, for which compression at 30 ~arbonate,~ and silver propionate,1° crystallites under monolayers mN/m produced a surface area of 18 A2/molecule). Surface has, of course, been inspired by biomineralization-the selective pressure (n)vs surface area (A) isotherms were obtained, at crystallization at cell membrane surfaces. 1-13 room temperature (20 "C)and at a subphase pH of 5.8, by An additional level of monolayer-directed epitaxy control is compressing the surface (typically from 35 to 15 A2/molecule) the subject of the present work. Introduction of octadecylamine at compression rates of 0.8 A2/(molecule/min). (ODA) into the mamx of arachidic acid monolayers has changed Brewster-angle microscopy (BAM) was carried out by means the morphology of the PbS nanocrystallites which were formed of a home-constructed system. Monolayers were compressed upon H2S infusion. in a rectangular (95 min x 350 mm) Teflon trough which was equipped with a Wilhelmy sensor. The beam of a polarized Experimental Section argon ion laser (11 mW at , I= 488 nm) was directed to the Arachidic acid (AA; Aldrich), octadecylamine (ODA; Aldwater surface by mirrors which were attached to precision rich), lead nitrate (Fisher), and chloroform (Sigma) and hydrogen x-y-z rotation mounts. The phase-polarized laser beam sulfide (Matheson) were used as received. Water was purified reached the monolayer surface at the Brewster-angle (ca. 54") at a spot size of 1.2 mm in diameter. The refracted beam was 'Abstract published in Advance ACS Abstracts, January 15, '1995. absorbed by a piecc of black plastic which was placed at the 0022-365419512099-5505$09.00/0
0 1995 American Chemical Society
5506 J. Phys. Chem., Val. 99, No. 15, 1995
Yang and Fendler
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Figure 1. Surface pressure vs surface area isotherms of monolayers prepared from AA on pure water (a) and from AA:ODA = 1:0 (b). AA:ODA = 5:l (b), AA:ODA = 2:1 (b), AA:ODA = 1:l (c), AA: ODA = I :2 (d), and AA:ODA = 0: 1 (e) mixtures floating on aqueous 1 .O x IOF3 M Pb(N03)~solutions. The insert gives a closeup view of the isotherms in the region where phase transitions occur.
bottom of the trough. A lens (focal length = 2.4 cm) was used to collected the reflected light and focus it onto a CCD camera (MTI CCD 72; sensitivity = 0.002 lux). All structures appeared to be compressed in the vertical direction of the actual image. The middle part of the image was well focused, while the upper and bottom parts were out of focus. The images were videotaped by a VHS VCR (JVC HRS 6700Un). A frame grabber (Imaging Technology PC Vision Plus) was used to digitize images which were of interest. These images were then printed by using a video printer (Mitsubishi Video Copy Processor W 1U). Injection of H2S over the monolayer-coated aqueous Pb(NO3)z solution (5 cm above the monolayer surface) led to the gradual (30 min or longer) development of PbS under the monolayer surface. The mixed monolayer-covered PbS nanocrystallites were transferred to Formvar-coated, 200-mesh copper grids for TEM horizontal lifting (subsequent to vertical dipping of the substrate through the monolayer and turning it parallel to the monolayer). The excess liquid which adhered to the surfactantcoated PbS nanocrystallites was removed immediately by gently touching the edge of the grids with a filter paper. Transmission electron micrographs were taken on a JEOL 2000-EX transmission electron microscope (TEM) operating at 120 and 200 kV. Electron diffraction patterns were recorded at 80-cm focal length.
Results Monolayers Formed from Mixtures of Arachidic Acid (AA) and Octadecylamine (ODA). The surface pressure (n) vs surface area (A) isotherms of an AA monolayer on an aqueous 1.0 x M Pb(N03)2 solution at pH = 5.8 was found to have a sharp liquid-to-solid state transition and a headgroup area of 20 f 0.5 A2/molecule (see curve b in Figure 1). This value is in good agreement with previously reported headgroup areas of lead and cadmium arachidate monolayer~.l-~At pH = 5.8, the carboxyl groups were mostly ionized and the monolayer which was floating on an aqueous Pb(N03)2 solution was predominantly lead arachidate. Metal-arachidate monolayers are, in fact, more stable than their carboxylic acid
Figure 2. Brewster-angle microscopic images ( 1.3 mm x I .3 mm) of monolayers floating on aqueous 1.0 x IO-' M Pb(N03)Z solutions. Pure AA at A := 25 A2/molecule and ll = 0 (a) and after collapse (b); AA: ODA = :!:I at A = 25 A*/molecule and ll = 0 (c) and after collapse (d); AA:ODA = 1:5 at A = 35 A2/moleculeand ll = 3 mN/m (e) and at A = 1.8 A*/molecule after collapse (f).
counterparts (see curve a in Figure I), as was evidenced by the sharp inflection in the n vs A isotherm of lead arachidate (curve b, Figure 1). The phase transition of an octadecylamine (ODA) monolayer, floating; '11 -n aqueous 1.O x lo-' M Pb(N03)2 solution at pH = 5.8, was observed to be more gradual (see curve e in Figure 1) than that of AA. The determined headgroup area of octadecylamine, 28 f 0.5 A2/molecule, agreed well with the literature value (28.8 f 0.5 A2/molecule).I4 It is interesting to observe the n vs A isotherms of monolayers which were formed from mixtures of AA and ODA (Figure 1). Significantly, when the AA concentration was kept higher than that of ODA, the Il vs A isotherms of mixed AA-ODA monolayers became indistinguishable from the ~t vs A isotherm of pure: AA (Le., identical n vs A isotherms were observed for monolayers which were formed from AA:ODA = 1:0,5:1, and 2:l mixtures; see curve b in Figure 1). These results imply that OITA, when present in the mixed monolayer in amounts which 'were smaller than those of AA, adopts the compressiondependent structure of the arachidic acid matrix. Increasing the amount of ODA in the surfactant mixture to amounts which were equal to or greater than those of AA (see curve c in Figure 1 for AA:ODA = 1:1 and curve d in Figure 1 for AA:ODA = 1:2) resulted in a gradual decrease of the sharpness of the liquidto-solid state phase transition. Additional information on the behavior of monolayers was 0btaine.d by means of BAM. Time- and composition-dependent nucleation and growth of three-dimensional crystallites, phase
Morphology Control of PbS Nanocrystallites
J. Phys. Chem., Vol. 99, No. 15, 1995 5507
i
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Figure 3. 1 r a i i w i i \ w m clcctron micrograph\ and clcctron diffraction patterns (obtained from 2-pm-diameter areas) of PbS nanocrystallites, generated under monolayers prepared from mixtures of AA:ODA = 1 :O (a, top left), AA:ODA = 5: 1 (b, top right), AA:ODA = 2: 1 (c, middle left), AA:ODA = 1:l (d, middle right), and AA:ODA = 1:2 (e, bottom left), on Formvar-coated 200-mesh copper grids. The monolayers were spread in a Langmuir trough over aqueous 1.O x M Pb(N03)2 solutions. PbS nanocrystallites were formed under the monolayers, compressed to = 30 mN/m, upon exposure to H2S (see Experimental Section). The substrates (Formvar-coated, 200-mesh copper grids) were inserted vertically through the PbS-coated monolayer, turned parallel to it, and then lifted horizontally to effect the transfer. The scale bars = 100 nm (a and b), 200 nm (c and d), and 250 mm (e).
transition, and collapse of the monolayer could be visualized by BAM. Thus, monolayers which were formed from pure AA
(at n = 0 and A = 25 A2/molecule)appeared to contain domains of small spherical "dots" with little or no light reflection among
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5508 J. Phys. Chem., Vol. 99, No. 15, 1995
TABLE 1: PbS NanocrystallitesGrown under Monolayers Prepared from AA and ODA Mixtures AA:ODA" 1 :0 5: 1
2: 1 2:lh I:l 1:2 1 :5 0: 1
isotherm in Figure 1
diffraction pattern
6-fold (220). (420}, (440},... same as in the 1:0 case, but dispersed spot b I2-fold (200). {220}, (400}, ... 6-fold { 1 1 1 }, (200}, (222}, (31 l}, (400}, ... C 12-fold { 200). (220). {400}, ..., but spots are dispersed d powder ring; most (200). (220). (400) between d and e" NA e NA b b
If Surface pressure of the monolayer was kept at 30 m", mN/m. c. Not shown in Figure 1.
orientation preference
morphology
epitaxy on { 11 I } epitaxy on { 11 I }
equilateral triangles, 45 f9 nm indented triangles, 45 nm
epitaxy on (001) epitaxy on { 1 IO}
irregular (> 100 nm) and some right angular (50 nm) mostly right-angle triangles
epitaxy on (001)
mostly right-angle triangles
no epitaxy; square shape (ca. 80 nm), sparsely distributed but { 001} preferred NA crystals grew in bulk solution rather than under monolayers NA crystals grew in bulk solution rather than under monolayers inless stated otherwise. Surfactant mixture spread in a circular trough, kept at 0
Figure 5. High-resolution transmission electron micrograph and electron diffraction pattern (obtained from a 2-pm area) of PbS nanocrystallites, generated under a monolayer prepared from mixtures of AA:ODA = 2: 1, on a Formvar-coated 200-mesh copper grid.
Figure 4. Transmission electron micrograph and electron diffraction pattern (obtained from a 2-pm area) of PbS nanocrystallites, generated under a monolayer prepared from mixtures of AA:ODA = 2: 1. on a Formvar-coated 200-mesh copper grid. PbS nanocrystallites were formed under the monolayer, compressed to A = 23 A*/molecule and n = 0 mN/m, upon exposure to H2S (see Experimental Section). The substrate (Formvar-coated, 200-mesh copper grids) was inserted vertically through the PbS-coated monolayer, turned parallel to it, and then lifted horizontally to effect the transfer. The scale bar = 50 nm.
the much larger and brighter domains which covered approximately 70% of the image (see image a in Figure 2). Compression-dependent images of AA monolayers (not shown) were found to be entirely analogous to those which were reported previously. I5 BAM images (at n = 0 and A = 25 A2/molecule),obtained for monolayers prepared from AA:ODA = 5:l and AA:ODA
= 2:1, were quite similar to those which were taken of AA monolayers (compare images a and c in Figure 2). At the collapse pressure, images of monolayers which were prepared from pure AA and those which were obtained from mixtures of AA and ODA were quite different, however (see images b and d in Figure 2). Fractured lines, extending in lengths longer than that of the image (1.3 mm), developed rather abruptly upon the compression to collapse of monolayers which were formed from AA:ODA = 1:l and AA:ODA = 2:l (see d in Figure 2 for a typical image of a collapsed AA:ODA = 2: 1 monolayer). Apparently, addition of ODA enhances the rigidity of the mixed monolayer. Increasing the ODA content in the mixed monolayers to AA:ODA = 1:5 resulted in n vs A isotherms which resembled those obtained for pure ODA monolayers (Figure 1) and in BAM images which reflected the light uniformly and only moderately (see images e and f in Figure 2). Epitaxial Growth of PbS-Characterization by TEM. Exposing an arachidic acid monolayer-coated aqueous 1.O x M Pb(N03)2 solution to H2S led to the formation of equilateral-triangular PbS nanocrystallites (Figure 3a), as reported previous1y.I Electron diffraction patterns, taken from a 2-pm-diameter area (Figure 3a), showed a 6-fold symmetry. The 6-fold symmetry, from the {220}, {420}, {WO},etc., planes, indicated epitaxial growth of the PbS nanocrystallites with their { 111) planes parallel to the AA monolayer surface. It should be noted that equilateral triangular PbS nanocrystallites were generated even when AA was spread in a circular trough and were not compressed (i.e., at ll = 0).
J. Phys. Chem., Vol. 99, No. 15, 1995 5509
Morphology Control of PbS Nanocrystallites The size and orientation preference of PbS, grown under a monolayer prepared from AA:ODA = 5:l and kept at JJ = 30 mN/m, was quite similar to that grown under a monolayer formed from pure AA. A closer look reveals, however, subtle morphological changes. Most of the equilateral triangular crystals, grown under the AA:ODA = 5:l monolayer, had indentations in the middle of their edges (Figure 3b). Electron diffraction of a 2-pm-diameter image of this sample displayed a 6-fold symmetry pattern with somewhat dispersed spots from the {220}, {420}, { U O } , etc., planes, in addition to a powdertype diffraction ring originating from the (200) plane (Figure 3b). Changes in the surface pressure profoundly affected the morphology of PbS nanocrystallites which had been grown under monolayers prepared from mixed AA and ODA. This is most clearly in the case of the AA:ODA = 2:l mixed monolayer. Growing PhS under this monolayer, at ll = 30 mN/m, resulted in the formation of laminated, irregular-shaped, and right-angle triangular PbS nanocrystallites (Figure 3c). Electron diffraction displayed a well defined, 12-fold symmetric pattern with sharp spots from the {200}, {220}, {400), {420}, etc., planes, indicating that all of the PbS crystallites had their (001) planes parallel to the monolayer. Using a small beam of electrons, covering either a right-angular triangle or a small portion of the laminated PbS crystallite, gave a perfect 4-fold symmetry pattern. Overlapping the three 4-fold diffraction patterns by rotating O", 60°, and 120" generated an exact 12fold diffraction pattern (as shown in Figure 3c), substantiating that all PbS nanocrystallites were lying in the Ool 60°, and 120" directions, respectively. PbS nanocrystallites which had been grown at A = 23 A*/ molecule and JJ = 0 under an AA:ODA = 2: 1 mixed monolayer were mostly right-angle triangular with mean particle sizes of 20 nm and angles of 75 f 8' (Figure 4). More importantly, the 6-fold diffraction is indexed a5 { 11l}, {200}, (222)- (31 l), {400}, etc. (see Figure 4), which corresponds to an epitaxial growth of PbS on its { 110) plane. A high resolution TEM of PbS, prepared under the same monolayer, reveals a lattice image with a 3.39-A spacing (Figure 5). This value is in a good agreement with that of the d spacing of the { 111) plane (3.43
L-
(1 I 1) plane in PbS lattice
,
(001) plane in PbS lattice
A). TEM images of PbS, grown under monolayers kept at JJ = 30 mN/m and prepared from AA:ODA = 1:l (Figure 3d), appeared to be quite similar to those grown under the AA-richer monolayer (Figure 3c), but they contained fewer irregularshaped crystals and more right-angular ones. The corresponding electron diffraction pattern (Figure 3d) had a 6-fold symmetry and somewhat more dispersed spots, indicating a decreased order in the particle alignment. PbS, grown under monolayers prepared from AA:ODA = 1:2, were randomly oriented (as suggested by the powder-type diffraction pattern), rectangularly shaped, and irregularly sized at ll = 30 mN/m (Figure 3e). Nevertheless, the most intense electron diffractions ({200) ,{220) ,{400), and {420)) indicated that the orientation of the (001) planes of most crystallites are oriented parallel to the monolayer. PbS crystallite formation could not be observed under monolayers, prepared from AA:ODA = 1 5 , upon the infusion of H2S at any surface pressure. Instead, irregular brownishcolored particles formed in the bulk solution, which ultimately precipitated. Results which were obtained for the formation of PbS under the different monolayers are summarized in Table 1.
I
(1 10) plane in PbS lattice Figure 6. Schematic diagrams of the (1 11) (top), the (001) (middle), and the (1 10) (bottom) planes of cubic crystalline PbS. The dark filled circles represent Pb and the shaded ones correspond to S.
Discussion The ability of the amine headgroups in ODA to hydrogen bond with the carbonyl group in AA prompted the selection of Hydrogen bonding between neighboring this surfactant pair. AA molecules and between AA and ODA molecules had contributed to the organization and structural integrity of the mixed monolayer. Depending on the concentration ratios of the two surfactants, the hydrogen-bonded AA-to-ODA pair may
Yang and Fendler
5510 J. Phys. Chem., Vol. 99, No. 15, 1995
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Figure 7. (a) Schematic representation of the packing pattern of Pb2+ (filled circles) in the (001) plane. Sulfide ions, being in the same plane, are not shown. The shaded area indicates the unit cell of the PbS crystal. The intercation spacing, the d( 110). is shown to be 4.20 A. (b) A possible hexagonal packing of a pattern of the monolayer headgroups, ignoring the difference between AA and ODA (prepared from AA and ODA mixtures). The shaded circles represent the AA positions, and the shaded area indicates a unit cell. (c) Proposed matching between the (001) plane of PbS and the monolayer headgroups. Small filled circles represent the packing of Pb, while the lightly shaded circles correspond to the monolayer headgroups. The d( 110) of PbS is the same as the d( 10) spacing of headgroups in the direction shown.
be distributed statistically in the monolayer or, alternatively, may be segregated into domains. Domain formation is the usual event in mixed surfactants.17 Formation of segregated domains in monolayers, prepared from mixtures of N-eicosanoyl-3aminopropionic acid and eicosanamide or behenamide, has been shown to govern to the oriented three-dimensionalcrystallization of silver propionate.I6 The determined ll vs A isotherms and BAMs are best rationalized in terms of a delicate balance between the interactions of Pb2+, dissolved in the subphase, and the -COzheadgroups of AA in the monolayer and those between the oppositely charged headgroups in the mixed monolayers at different surface pressures. Evidence has been obtained, by in situ infrared reflectiodabsorption spectrometry, for the substantial effect of Pb2+ in ordering carboxylic acid monolayers into domains at water surfaces.Ig Realization of epitaxial PbS growth under AA monolayers, even at zero surface pressure,l provides further evidence for the presence of crystalline monolayer domains. In fact, oriented crystallization under uncompressed AA monolayers was found to be synergetic.19 Small domains of well-packed AA provided the initial template for the nucleation and oriented growth of PbS nanocrystallites. Each equilateral-triangular PbS formed, then attracted additional surfactants, and, hence increased the size of the AA domains. The increased AA domain permitted, in turn, the further growth
of the equilateral-triangular PbS. This process continued as long as the mismatch between the surfactant headgroups and Pb2+Pb2+ distances in the (111) plane of PbS did not increase to the extent that oriented growth of PbS was no longer possible. Addition of ODA initially disrupted the AA domains. Subsequently, hydrogen-bonded AA-ODA surfactant pairs were formed and distributed in the matrix of the predominant surfactant. Finally, when the concentration of ODA increased far above that of AA, the crystal structure of the monolayer became identical to that of pure ODA. Oriented growth of PbS was rationalized by comparison of the structures of the AA monolayer and the PbS crystals.' Synchrotron X-ray studies of AA monolayers in their solid states showed that they comprise fully extended molecules bearing a planar zigzag conformation and that they are oriented approximately normal to the liquid surface in a hexagonal closepacked array with a lattice constant of a = 4.85 A.20*21An experimentally obtained lattice constant of AA monolayers on Pb(N03)2 of a = 4.81 A, as derived from ll vs A isotherms,' can be considered to be in good agreement with the data that were obtained in synchroton X-ray studies (a = 4.85 A) and utilized in the analysis. PbS possesses a NaC1-type cubic structure with a lattice constant of a = 5.9458 A. Epitaxial growth of PbS from the (111) face was a result of the geometrical complementarity between the AA monolayer and
Morphology Control of PbS Nanocrystallites the { 111) PbS face. The Pb-Pb interionic distance of 4.20 8, in the PbS { 111) plane geometrically matched the d{lo}spacing of 4.16 8, for arachidic acid; the spatial mismatch between the crystals is only on the order of 1%. It is instructive to visualize the different planes in the PbS lattice (Figure 6). Monolayers prepared exclusively from AA, regardless of their surface pressure, have been seen to direct the growth of PbS from its (111) plane. This oriented crystallization was the consequence of the excellent match between the headgroup spacing in the AA monolayer and the { 111) plane of PbS. Introduction of a second surfactant (ODA) in the monolayer initially disturbed, then altered, and ultimately destroyed this matching. Compression of the monolayers, prepared the mixtures of AA and ODA, became important since it helped to conserve the hexagonal close packing of the surfactant headgroups. Indeed, AA:ODA = 2: 1 monolayers, kept at l-I = 30 mN/m, oriented the growth of PbS from its (001) plane (Figure 3c), whereas those formed in a circular dish, with l-I = 0, directed PbS growth from its { 110) plane (Figure 4). It is somewhat simpler, therefore, to focus attention on the effects of introducing increasing amounts of ODA into the AA:ODA monolayers which were compressed at 30 mN/ m. The two-dimensional crystal structures of monolayers, prepared from 1:l mixtures of stearic acid and octadecylamine, on pure water and on aqueous CdC12 solutions have been investigated by grazing incidence X-ray diffraction.22 The determined rectangular lattice on CdCl2 solution (a = 4.87 8, and b = 8.44 8,; Figure 10d in ref 22) represents, in fact, hexagonal close packing (tan-'[8.44/4.87] = 60") and the lattice constant a = 4.87 8, is quite close to the value that we used (4.85 A, vide supra). The indistinguishable l-I vs A isotherms for monolayers, formed from AA:ODA = 1:0, 5:1, and 2:l mixtures (see curve b in Figure l), strongly imply that the hexagonal close packing is maintained in the monolayers as long as the concentration of AA predominates. Nevertheless, PbS epitaxial growth was observed to change from the (111) to the (001) plane. This must be the result of diminished concentration of Pb2+ ions under a given area of monolayer and, hence, their altered geometrical alignment with the monolayer headgroups. Packing of Pb2+ ions in the (001) plane of an fcc PbS crystal, along with a unit PbS cell, is illustrated in Figure 7a. The Pb2+ to Pb2+ distance, d(110) = 4.20 A, has been derived from the known PbS lattice constant ( a = 5.936 8,). The packing pattem of headgroups in the monolayers was drawn by assuming structural similarities between AA and ODA and hexagonal close packing (Figure 7b). Comparing the arrangements of Pb2+
J. Phys. Chem., Vol. 99, No. 15, 1995 5511 in the (001) plane of PbS (Figure 7a) with those of the monolayer headgroups (Figure 7b) resulted in the possible matching pattem drawn in Figure 7c. Similar matching can be obtained when the monolayer headgroup packing pattern (Figure 7b) is turned 60" and 120" with respect to Pb2+ (Figure 7a). This is in accord with the alignment pattem of the PbS nanocrystallites grown under AA:ODA = 1:0, 5:1, and 2:l monolayers.
Acknowledgment. Support of this work by the W.M. Keck Foundation and by a grant from the National Science Foundation is gratefully acknowledged. References and Notes (1) Zhao, X. K.; McCormick, L. D.; Fendler, J. H. Adv. Mater. 1992, 4 , 93.
(2) Yang, J.; Fendler, J. H.; Jao, T.-C.; Laurion, T. Microsc. Res. Technol. 1993, 27, 402. (3) Yang, J.; Meldrum, F. C.; Fendler, J. H. J. Phys. Chem. 1995, 99, 5500. (4) Fendler, J. H. Membrane-Mimetic Approach to Advanced Materials; Advances in Polymer Science Series; Springer-Verlag: Berlin, 1994; Vol. 113. (5) Fendler, J. H.; Meldrum, F. C. Adv. Mater., submitted for publication. (6) Weissbuch, I.; Addadi, L.; Leiserowtiz, L.; Lahav, M. J . Am. Chem. SOC. 1988, 110, 561. (7) Landau, E. M.; Popovitz-Biro, R.; Levanon, M.; Leiserowitz, L.; Lahav, M.; Sagiv, J. Mol. Cryst. Liq. Cryst. 1986, 134, 323. (8) Heywood, B. R.; Mann, S. Adv. Mater. 1992, 4 , 278. Heywood, B. R.; Mann, S . J. Am. Chem. SOC. 1992, 114, 4681. (9) Heywood, B. R.; Mann, S . Chem. Mater. 1994, 6, 311. (10) Weissbuch, I.; Majewski, J.; Kjaer, K.; Als-Nielsen, J.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. 1993, 97, 12848. (11) Mann, S. Nature 1993, 365, 499. (12) Heywood, B. R.;Mann, S . Adv. Mater. 1994, 6, 9. (13) Addadi, L.; Weiner, S . Angew. Chem., In?. Ed. Engl. 1992, 31, 153. (14) Mingotaud, A.-F.; Mingotaud, C.; Patterson, L. K. Handbook of Monolayers; Academic Press, Inc.: San Diego, California, 1993; Vol. 1, p 390. (15) Honig, D.; Mobius, D. J. Phys. Chem. 1991, 95, 4590. (16) Jacauemain, D.; Graver Wolf. S.: Leveiller. F.: Deutsch. M.; Kim. K.; Als-Nielsen, J.; Lahav, M.;Leiserowitz, L. Angew. Chem., In?.Ed. Engl. 1992, 31, 130. (17) Mowald, H. Rep. Prog. Phys. 1993, 56, 653. (18) Gericke, A.; Hiihnerfuss, H. Thin Solid Films 1994, 245, 74. (19) Yang, J.; Fendler, J. H., unpublished results, 1994. (20) Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulos, P.; Ehrlich, S. Phys. Rev. Lett. 1987, 58(21), 2228. (21) Kjaer, K.; Als-Nielsen, J.; Helm, C. A,; Tippmann-Krayer, P.; Mohwald, H. Thin Solid Films 1988, 159, 17. (22) Gidalevitz, D.; Weissbuch, I.; Kjaer, K.; Als-Nielsen, J.; Leiserowitz, L. J . Am. Chem. SOC. 1994, 116, 3271. JP942638 1
