10444
J. Phys. Chem. 1992, 96, 10444-10447
Reorganlzatlon and Crystallite Formation in Langmulr-Blodgett Fllms D. K. Schwartz, R. Viswanathan, and J. A. N. Zasadzinski* Department of Chemical and Nuclear Engineering, University of California, Santa Barbara, California 93106 (Received: August 4, 1992; In Final Form: September 14, 1992)
The atomic force microscope (AFM) was used to monitor the reorganization and approach to equilibrium of Langmuir-Blodgett monolayers and multilayers of cadmium fatty acid salts submerged under aqueous subphase for extended periods. We have found that uniform, constant thickness monolayer and multilayer films are unstable to bilayer step defects that originate at isolated sites and quickly spread to cover the entire film (30 min for cadmium arachidate). Although the network of defects originally appears random and meandering, within 10 h features with straight edges begin to form and eventually (48 h submerged) the entire film comprises high islands with straight edges aligned with sixfold symmetry. The direction of island edges are explicitly shown to correspond to a particular molecular lattice direction by comparison of molecular resolution images with lower magnification images. The driving force behind the reorganization is the formation of headgroupheadgroup interfaces stabilized by the strong interaction of cadmium ions at the cost of substrate-headgroup and water-headgroup interfaces.
Introduction Langmuir-Blodgett (LB) films, layered assembles of amphiphilic molecules, have applications in the areas of nonlinear optics, molecular electronics, and biosensors, and as models for cell membranes.' Most of these applications rely upon the self-organization of amphiphilic molecules to form very thin and and essentially perfect films. Although a few experimenters have warned that the flat, featureless LB film may not be the equilibrium structure,2little work has been done to study the approach to equilibrium. Any metastability is detrimental, for example, to applicationsof LB films in nonlinear optics where interdiffusion and molecular rearrangement can severely diminish second harmonic g e n e r a t i ~ n . ~Microscopy ,~ techniques have clear and significant advantages over scattering and spectroscopy when it comes to exploring such inhomogeneities and defects.s7 The atomic force microscope (AFM) has emerged as the preferred technique for imaging LB filmP7 because it can probe LB films quickly, directly, and nondestructively without the need for high vacuum or metal replicas as is the case for electron or scanning tunneling microsc0py.S Furthermore, we can vary the magnification from molecular resolution images to a field of view of 15 pm, allowing the correlation of structures on extremely different length scales. We have chosen cadmium arachidate (and other cadmium fatty acid salts) as the material for this study as it is often taken to be the pratotypical LB system. In previous workSwe have shown that the headgroupheadgroup linkage (in the presence of cadmium) is the crucial ingredient necegPary to obtaining stable and well-ordered LB films. The conclusion of the present work is that a homogeneous and uniformly thick Langmuir-Blodgett film, given the opportunity, will detach in certain regions from the substrate (a form of "dewetting") and reorganize to form additional headgroupheadgroup interfaces, thereby forming step and holes with heights corrmponding to integral numbers of bilayers. This dewetting can be attributed to the strength of the headgroupheadgroup interface relative to the headgroupsubstrate and headgroupwater interfaces. The continued approach to equilibrium involves the formation of macroscopic crystallites whose straight edges are explicitly shown to correspond to molecular lattice symmetry directions. The crystallites in an aged film, although they are isolated from each other by amorphous substrate or monolayer areas, have orientations related to each other in a near-hexagonal arrangement. The crystallites appear to retain the hexagonal symmetry of the original LB films, indicating that the reorganization is a bulk folding of layers rather than molecular scale events. This is most likely related to the hexagonal 'HKinning" we have observed on homogeneous LB films6 and electron diffraction measurements9that show the orientational order in monolayers and multilayers can extend for tens of mi*Towhom correspondence should be addressed.
crometers. The minimum size of the folded domains likely depends on the competition between the increase in adhesion energy of the newly folded regions and the extra energy d a t e d with the increase in hydrocarbowwater contact at newly formed step edges. The implications of this study are especially relevant to researchers interested in making multilayer films of different composition. It is often necessary to allow the film coated substrate to be immersed in the subphase while one monolayer is removed from the interface and another is put in its place, or the surface pressure is adjusted. During this time, the multilayers under the subphase may undergo at least a partial reorganization, thereby destroying the desired layered structure. Although not investigated here, this same process likely occurs in air, although with greatly reduced kinetics. Hence, though we did not observe any appreciable reorganization of films in air for periods up to 30 days, in an earlier study,lScadmium arachidate monolayers left in air for periods of several months showed a significantly greater density of pinhole defects than did fresh films.
Materials and Methods Arachidic acid (CH,(CH&,,COOH, Aldrich, 99%) was spread from chloroform (Fisher spectranalyzed) solution (1.85 mg/mL) onto an aqueous (water from a Milli-QlO system was used) subphase in a commercial NIMA" trough. The subphase water included 5 X lo4 M CdC12 (Aldrich 99.99%) and was adjusted to a pH of 6.5 by addition of NaHC03 (Aldrich 99.95%). Substrates were freshly cleaved mica or polished silicon wafer@ (orientation (loo), 3 ohmecm, n-type) with a rms roughness of approximately 3 A as measured by AFM. Prior to deposition, the silicon wafers were cleaned in a hot solution of H202/H2S04 (3:7 ratio) to remove any organic contaminants while leaving the amorphous native oxide intact, and then stored in clean water until use. The mica substrates were cleaned by continuous rinsing with ethanol for 5 min. After removal from the ethanol bath, the mica was cleaved using ordinary adhmive tape and inserted into the subphase. Since all f i i were Y type (Le., adjacent layers stack head to head or tail to tail), films deposited on a hydrophilic substrate and imaged in air had an odd number of layers (1. 3, or 5 ) with the methyl end of the alkyl chain at the interface. Isotherm and film deposition were done on the Nima trough at 22.0 f 0.5 OC and a surface pressure of r = 30 f 0.1 dyn/cm. Film transfer was accomplished by vertical dipping at a speed of approximately 1.6 mm/min. Transfer ratios were approximately unity. Aging under aqueous subphase was achieved by interrupting the dipping proceso with the film submerged, waiting for a given time, and then drawing the substrate through the air-water interface. This last pass through the interface could be done with or without a monolayer at the interface. Films were stored in closed containers in air for times ranging between 1 and 30 days before imaging. This length of time in air between deposition and
0022-365419212096-10444$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10445
Crystallite Formation in LB Films
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Figure 1. AFM images (15 pm X 15 pm) of LB films of cadmium arachidate after being aged for various period of time under aqueous subphase. (a-c, upper row, left ro right; d-f, lower row, left to right) (a) A 3-layer film on silicon after 4 min aging between deposition of layers 2 and 3. Only small pinholes mar the perfection. (b) A 3-layer film on silicon after 10 min of aging between layers 2 and 3. Several nucleation sites for the reorganization are visible. (c) A 3-layer film on silicon aged for 30 min between layers 2 and 3. The reorgani~ationis extensive. (d) A 5-layer film on silicon aged 10 h between layers 4 and 5. Long, straight features begin to appear. The inset is the two-dimensional Fourier transform of the image. (e) A 5-layer film on silicon aged 25 h between layers 4 and 5. Islands with straight edges are present. The inset is a Fourier transform. (0 A Clayer film on silicon pulled through the air-water interface after being aged for 48 h. The film consists completely of distinct islands with straight edges. The inset Fourier transform shows that the island edges have an approximate sixfold symmetry.
imaging did not affect the images, although in an earlier study cadmium arachidate monolayers left in air for several months showed a significantly greater number of pinhole defects.15 AFM measurements were performed with a Nanoscope III3 FM in air at room temperature, using a 15-pm or a 1-pm scanner and a silicon nitride tip on a cantilever with a spring constant of 0.12 N/m. Most imaging was done using "height" mode (utilizing feedback to keep the spring at constant deflection and measuring sample motion); however, the best molecu!ar resolution was achieved by using "force" mode, i.e., scanning the tip at approximately constant height and measuring spring deflection.
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AFM images were obtained of cadmium arachidate films of 3-5 layers deposited on mica or silicon wafers after they had been aged under aqueous subphase for various periods of time. Neither the choice of substrate nor the number of layers deposited altered the basic results of the experiment. Figure 1 shows an overview of the time evolution of a cadmium arachidate LB film when allowed to stay submerged under aqueous subphase. Figure la shows a film submerged for only 4 min, the minimum time necessary for dipping to be completed. It appears to be flat and homogeneous except for some very small h01es.l~ After being submerged 10 min (Figure 1b), small rough patches were observed which appeared to be nucleation sites for reorganization. Three heights can be observed, corresponding to 1-, 3-, and 5-layer regions. The nucleation site appears as though small bilayer sections have simply peeled off the first monolayer ("dewetted") and flipped over onto already covered areas of the film to form multilayer patches. The sizes and shapes of the holes
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Figure 2. Height histogram of the image shown in Figure IC. It shows that the surface consists of terraces separated by vertical distances of -6 nm, the thickness of a bilayer. The finite widths of the histogram peaks are due to the slope at the edges of terraces as well as a slight overall artificial curvature introduced into the image because of nonlinearity of the piezoelectric scanner.
in the film are consistent with the sizes and shapes of the multilayer regions. After 30 min (Figure IC),the process is so advanced that nearly the entire film is involved in the reorganization. There is an extensive random network of multilayered terraces, separated by heights that are a multiple of the bilayer spacing of 5.6 f 0.2 nm.s Figure 2 shows a height histogram of Figure IC, demonstrating how the heights are partitioned into bilayer steps. The finite widths of the peaks in the histogram are due to the boundaries between steps as well as an artificial overall curvature of the entire image due to nonlinearity of the piezoelectric scanner. From a rough analysis of a number of images, and from the histogram, it is also
10446 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
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Figure 3. Higher magnification images of portions of the image shown in Figure Id (a, top; b, bottom). The fibrous crystallites that are growing over the random film are readily apparent.
apparent that the relative areas with less than the average number of layers is roughly equal to those with more than the average
Schwartz et a!. number of layers. Hence, none of the cadmium arachidate is being lost or solubilized, it is merely reorganizing on the surface. After 10 h of submersion (Figure 1d), an interesting new feature begins to form; long fibrous shapes with straight edges overlay the rest of the film. This can be seen more clearly in the higher magnification images (Figure 3a,b) which are taken from within the area shown in Figure Id. The inset in Figure Id is a two-dimensional Fourier transform, which in this case is featureless since the image is still largely isotropic. After 25 h of submersion (Figure le), the film has segregated into isolated islands, separated by regions of monolayer coverage. Not only do the islands have straight, sharp boundaries, but, as shown by the approximately hexagonal symmetry of the Fourier transform, the island edges have well-defined absolute orientations. After 48 h of submersion (Figure 1f), the hexagonal symmetry is even more pronounced and the islands can essentially be identified as isolated crystallites related by hexagonal "twinning". Because of the large range of magnificationsavailable to the AFM we have been able to explicitly establish a correspondence between the direction defined by the edge of an island and the molecular lattice direction within that island. Figure 4b shows a 1 pm X 1 pm image containing parts of a 3-bilayer island and a 5-bilayer island. The region between islands corresponds to a single monolayer. Figure 4d shows a height cross section along the line marked in Figure 4b, which shows the island heights to be 3 and 5 times the bilayer spacing of 5.6 f 0.1 nm, respectively, on the left and the right. By taking higher magnification images from atop the respective domains we can determine the orientation of the molecular lattice in each location (Figure 4a, 3-layer island; Figure 4c, 5-layer region). Details of molecular lattice mea~ . ~lattice of cadmium surements have been given e l s e ~ h e r e ;the arachidate has been determined to be defined by a nearly-centered rectangular 2-molecule unit cell with a herringbone type packing of the alkane chains. By analyzing the Fourier transforms of the lattice images, we can determine which lattice direction in each image corresponds to the particular direction which we have previously given the
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Lateral distance (nm) Figure 4. (b, center) A 1 pm X 1 pm image of a 5-layer film aged for 25 h showing parts of 3-bilayer- and 5-bilayer-high islands on the left and right, respectively. The straight edges of the two islands have a relative angle of 60°. (a, left) A high magnification image showing the orientation of the molecular lattice on the 3-bilayer-high island on the left of Figure 4b. Note that the direction of the [IO] lattice row corresponds to the direction of the edge of the corresponding island. (c, right) A high-magnification image showing the orientation of the molecular lattice on the 5-bilayer-high island on the right of Figure 4b. The direction of the [IO]lattice row again corresponds to the edge direction of the island. (d, bottom) A height cross section along the line marked on Figure 4b, showing the heights of the islands explicitly. The bilayer spacing is 5.5 nm.
Crystallite Formation in LB Films indices [Ol].576 In two dimensions this vector uniquely defines the orientation of the lattice. In the particular case shown (and in the two other cases we have observed), the [Ol] direction is identical to the edge direction observed. In this image the situation is especially dramatic since the lattice orientation in the 3- and 5-layer domains are not identical but related by a rotation of 60°, as is the island edge direction. The large anisotropy of the crystallitesformed during the reorganization process suggests that this particular lattice direction has a substantially lower edge energy than does the other crystallographic directions. In the rectangular packing of cadmium arachidate, the [Ol] direction has the shorter of the two lattice vector^^.^ and hence is the close-packed direction of the lattice.
Dimmion In previous work5 we have noted the importance of the headgroupheadgroup interface in providing stability and long-range order to the LB film. The strength of the headgroup-headgroup interaction must also be considered as a possible driving force for the reorganization seen here. We can rule out purely entropic effects (which are important in thermal crystal roughening14) because the state the system appears to be approaching in equilibrium is one of thick, but flat, crystallites. Figure If shows a film consisting of isolated flat crystallites, the vast majority of which are 7 layers high on a film which should be 3 layers thick on average. If the reorganization were driven by entropic considerations we would expect the film to approach a much rougher final state. By stacking higher, the film can increase the number of internal layers while minimizing both substrate and surface interactions. In effect, the cadmium headgroup interface prefers to "wet" itself as opposed to water or substrate.16 An additional energetic cost are the island edges that have a repulsive hydrophobic interaction between the exposad alkyl chains of the cadmium arachidate and the subphase water. However, it is possible that the island edges are closad off in a sort of semimicellar or other arrangement to " hthis cost. The cost of edges is also not uniform;the large shape anisotropy of the crystallites suggests that the [Ol] direction is preferred to all others. This edge energy serves to explain, however, why both nucleation and crystal growth takes place by the movement and folding of arcas large in comparison to molecular dimensions. The ratio of surface area to edges is roughly proportional to the square root of the surface area. Hence, the edge energy for a small island would be diqmptionately large. As a consequence, there is likely to be a minimum size island required to initiate folding and nucleate the reorganization, similar to the case in three dimensions for condensation of a liquid from its vapor in which a minimumsize droplet is necessary to initiate nucleation of a liquid in a vapor due to the competition between the latent heat of condensation and surface tensional6 Changing the length of the hydrocarbon chain, and hence the strength of the hydrophobic interaction and resulting edge energy, alters the kinetics of the reorganization dramatically. Cadmium stearate (18 carbons) reached a degree of reorganization comparable to a 30-min-aged cadmium arachidate (20 carbons) film after only 5 min aging. Cadmium behenate (22 carbons) needed approximately 60 min aging to reach the stage of first nucleation that was seen in cadmium arachidate after 10 min aging. Decreasing the edge energy by decreasing the length of the fatty acid hydrocarbon chain clearly decreases the minimum size of a nucleation site, resulting in the faster kinetics.
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10447 Conclusion We have observed the evolution of LB thin LB films of cadmium arachidate when allowed to age under aqueous subphase. The uniform film p r d s to reorganize by forming terraces of bilayer steps initially in a random network. After more time the film forms into separate island crystallites (with straight edges) that have orientations related by an approximate hexagonal symmetry. The predominant direction of the island edges corresponds to the [Ol] molecular lattice direction. The crystallites appear to retain the nearly hexagonal symmetry of the original LB films, indicating that the reorganization is a bulk folding of layers rather than molecular scale events. The driving force for the reorganization can be attributed to the strength of the headgroupheadgroup interface relative to the headgroupsub strate and headgroupwater interfaces. This effect is of great practical significanceto those researchers interested in building up multilayers of different chemistry or functionality using Langmuir-Blodgett deposition. The ordering of the layers can be severely disrupted if the filmaated substrate is allowed to rest in the subphase for any length of time, for example, during the time required to aspirate one type of monolayer from the surface and replace it with another. If such alternate layered systems are required, cadmium behenate, with its substantially slower reorganization kinetics, should be chosen over cadmium arachidate or cadmium stearate. Acknowledgmenr. This work was supported by the Office of Naval Research under Grant N00014-90-J- 1551, the National Science Foundation under Grant CTS90-15537, the National Institutes of Health under Grant GM 47334, and the donors of the Petroleum Research Foundation. We also thank Frank W e l d of NIMA Technologies for his assistance with the trough and software.
References and Notes (1) Roberts, G. G. Adv. Phys. 1985, 34, 475.
(2) Gaines, G. L. ThinSolid F i l m 1980,68, 1. Kopp, F.; Fringeli, U.P.; Miihlethaler, K.; Giinthard, Hs. H. Biophys. Struct. Mechanism 1975,1,75. (3) Hayden, L. M.; Kowel, S. T.; Srinivasan, M. P. Opt. Commun. 1987, 61, 351. (4) Shimomura, M.; Song, K.; Rabolt, J. F. Langmuir 1992, 8, 887. (5) Schwartz, D. K.; Games, J.; Viswanathan, R.; Zasadzinski, J. A. N. Science 1992, 257, 508. Schwartz, D. K.; Gamaes, J.; Viswanathan. R.; Chiruvolu, S.; Zasadzinski, J. A. N. Quantitative Lattice Measurement of Thin Langmuir-Blodgett Films by Atomic Force Micmwopy: Phys. Rev. A, in press. (6) Gamaes, J.; Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. Nature 1992, 357, 54. (7) Meyer, E.; Howald, L.; Overney, R. M.; Heinzelmann, H.; Frommer, J.; Giintherodt, H.-J.; Wagner, T.; Schicr, H.; Roth, S. Nature 1991,349,398. Zasadzinski, J. A. N.; Helm, C. A.; Longo, M. L.; Weisenhorn, A. L.; Gould, S. A. C.; Hansma, P. K. Biophys. J . 1991,59,755. Bourdieu, L.; Silbcrzan, P.; Chatenay, D. Phys. Rev. Lett. 1991, 67, 2029. (8) Zasadzinski, J. A. N.; Schneir, J.; Gurley, J.; Elings, V.; Hansma, P. K. Science 1988, 239, 1013. (9) Garoff, S.; Deckman, H. W.; Dunsmuir, J. H.; Alvarez, M. S. J. Phys. (Fr.) 1985, 47, 701. (10) Millipore Corp., Bedford, MA. (1 1) NIMA Technology, Ltd., Warwick Science Park, Coventry CV4 7EZ, England. (12) Semiconductor Processing, Boston, MA. (13) Digital Instruments, Inc., Goleta, CA 93117. (14) Wortis, M. In Chemistry and Physics of Solid Surfaces VIk Vanselow, R., Howe, R., Eds.; Springer-Verlag: Berlin, 1988; pp 367-405. (15) Hansma, H. G.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.; Longo, M. L.; Zasadzinski, J. A. N. Lmgmuir 1991, 7, 1051. Viswanathan, R.; Schwartz, D. K.; Gamaes, J.; Zasadzinski, J. A. N. Langmuir 1992,8,1603. (16) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1992; Vol. 2.
