Langmuir 1993,9, 2178-2184
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Surface Morphology and Stability of Langmuir-Blodgett Mono- and Multilayers of Saturated Fatty Acids by Scanning Force Microscopy A. Schaper, L. Wolthaus, D.MBbius, and T.M. Jovin* Max-Planck-Institut fur Biophysikalische Chemie, Postfach 2841,37018 Giittingen, FRG Received October 25, 1992. In Final Form: June 8, 1993
The molecular architecture of Langmuir-Blodgett (LB) films of saturated fatty acids of different lengtha transferred onto flat solid supports (mica, SiO, highly oriented pyrolytic graphite) was investigated with the scanning force microscope (SFM) under ambient conditions. Intrinsic defects in the LB monolayer have been used to measure the film thickness, which changes by an apparent increment of 0.2 nm per methylene group. The influence of different solid supports on monolayer morphology was determined. The investigations were extended to three-dimensional superstructures by deposition of successive monolayers. Reorganization of the bilayer in contact with the aqueous subphase forms regions with different thickness. Molecular resolution was achieved with the SFM on films with more than two layers. We have also determined unit cell parameters of the LB film surface of different fatty acid multilayers. The lattice constants did not change significantly,but the amplitude of the surface corrugationsincreases with the length of the aliphatic chain. Energetic contributions to monolayer stiffness are discussed.
I. Introduction The growing interest in Langmuir-Blodgett (LB) films' for application in surface finishing is mainly due to two features. The films can be made ultrathin with defined architecture, and they can easily incorporate subunits providing a specific functionalization of the interface and new surface properties.2 Ultrathin, ordered two-dimensional organic monolayers of molecules of low molecular weight can be prepared via the Langmuir-Blodgett (LB) technique.' The latter permits the assembly of amphiphilic molecules, like fatty acids at the air-water interface into a solidlike state under controlled surface pressure, and the transfer of the monolayer to a solid substrate. Multilayers can be generated by stepwise deposition of monolayers. The molecular arrangement and the morphology of LB films on solid substrates have been extensively studied by diffraction methods? electron microscopy,4 and scanning tunneling microscopy.6 It has been shown that monolayers on solid substrates are not always free of defects (e.g. holes) even on a microscopic scale.4 The scanning force microscope! SFM, provides a new tool for studying a variety of surface phenomena on a molecular level. It has been (1) Blodgett, K. B. J. Am. Chem. SOC.1935,57,1007. Blodgett, K. B., Langmuir, I. Phys. Res. 1937, 51, 964. Kuhn, H. J.Pure Appl. Chem.
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(2) MBbius, D. Ber. Bunsen-Ges. Phys. Chem. 1978,82,848. Kuhn, H. Thin Solid Film 1983, 99, 1. Petty, M. C. Thin Solid F i l m 1992, 2101211,417. (3) Fiecher, A., Sackmann, E. J. Phys. (Paris) 1984,45,517. Fischer, A., Sackmann, E. Nature 1986, 313, 299. Vogel, V. WGU, C. J. Chem. Phys. 1986,84,5200. Skita, V., Filipkowski, M., Garito, A. F., Blasie, J. K. Phys. Rev.B 1986,34,5826. Garoff, S., Deckman, H. W., Dunsmuir, J. H., Alvarez, M. S., Bloch, J. M. J. Phys. (Paris) 1986,47,701. Kjaer,
K., Ala-Nieleen, J., Helm, C. A,, Tippmann-Krayer, P., MGhwald, H. J.
Phys. Chem. 1989,93, 3200. (4) Kopp, F., Fringeli, U. P., Miihlethaler, K., Ghthard, Hs. H. Z . Naturforsch. 1976, 30C, 711. Uyeda, N.; Takenaka, T., Aoyama, K., Matsumoto, M., Fujiyoehi,Y .Nature 1987,327,319. Fereshtehkhou, S., Neuman, R. D., Ovalle, R. J. Colloid Interface Sci. 1986, 109, 385. (5) Fuchs,H., Schrepp, W.,Rohrer, H.Surf.Sci. 1987,181,391. Smith, D. P. E., Bryant, A., Quate, C. F., Rabe, J. P., Gerber, C., Swalen, J. D. hoc. Natl. Acad. Sci. U.S.A. 1987, 84, 969. Eng. L., Hidber, H.-R., Rwnthaler,L.,Staufer, U., Wiesendanger,R., Gtintherodt, H.-J.; Ta", L. J. Vac. Sci. Technol. 1988, A6, 358. Lang,C. A., HBrber, J. K. H., Hhech, T. W.; Heckl, W. M.; MBhwald, H. J. Vac. Sci. Technol. 1988, A6,368. Braun, H. G.; Fucha, H.; Schrepp, W. Thin Solid F i l m 1988, 159, 301. (6) Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986,56,930.
shown that the SFM is capable of resolving the molecular structure of LB monolayers operating in air7 and under water.8 The SFM can also be used for nondestructive investigation of LB mono- and multilayer structure on solid supportss and their mechanical and frictional properties.10-12 In this study we have used SFM to investigate the film architecture of transferred mono- and multilayers in the range of microscopic to molecular dimensions. We show that the SFM is capable of resolving changes in monolayer thickness with subnanometer accuracy. In addition, we present a study of the dependence of the molecular arrangement on the length of the aliphatic chain of various saturated fatty acids. 2. Materials and Methods Fatty acids (>99% purity, E. Merck, Darmstadt, FRG) dissolved in HPLC grade chloroform stabilizedwith 1% ethanol up to a final concentrationof 1 mM were spread at the air-water interface of a Fromherz-type round trough.la The pH of the subphase (water treated in a Milli-Q system, Millipore Corp.) wasadjuetedto6.0withsodium bicarbonate(>99.6%,E. Merck), and CdClz (>99% purity, E. Merck) was added to a concentration (7) Marti, 0.; Ribi, 0. H.; Drake, B.; Albrecht, T. R.; Quate, C. F.; Hansma, P. K. Science 1988,239,50. Meyer, E.; Howald, L.;Ovemey, R. M.; Heinzelmann, H.; Frommer, J.; Gtiutherodt, H.-J.; Wagner, T.; Schier, H.; Roth, S. Nature 1991,349,398. Garnaee, J.; Schwartz, D. K.; Viswanathan, R.; Zasadzinski,J. A. N. Nature 1992,357,54. Bourdieu, L.;Silberzan, P.; Chatenay, D. Phys. Rev. Lett. 1991,67,2029. (8) Zasadzinski, J. A. N.; Helm, C. A.; Longo, M. L.; Weisenhorn, A. L.;Gould, S. A. C.; Hansma, P. K.Biophys. J. 1991,59,766. Weieenhom, A. L.;Egger, M.; Ohnesorge, F.; Gould, S. A. C.; Heyn, S.-P.;Hamma, H. G.; Sinsheimer, R. L.; Gaub, H. E.; Hansma, P. K. Langmuir 1991,7, 8.
(9)Hansma,H.G.;Gould,S.A.C.;Hansma,P.K.;Gaub,H.E.;Longo, M. L.; Zasadzinski, J. A. N. Lungmuir 1991, 7,1051. Viswanathan, R.; Schwartz, D. K.; Gama-, J.; zaeadzinski, J. A. N. Lungmuir 1992,8, 1603. Fuchs, H.; Chi, L.F.; Eng, L.M.; Graf, K. Thin Solid Film 1992, 210/211,655. Peltonen, J. P. K.; He, P.; Roeenholm, J. B. J. Am. Chem. SOC.1992,114,7637. (10) Meyer, G.; Amer, N. M. Appl. Phys. Lett. 1990,57, 2089.
(11) Meyer, E.; Howald, L.; Overney, R.; Brodbeck, D.; Lothi, R.; Haefke, H.; Frommer, J.; Gtintherodt, H.-J. Ultramicroscopy 1992,4244,274. Ovemey, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lothi, R.; Howald, L.;Gtiutherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y.
Nature 1992, 359, 133.
(12) Radmacher, M.; Till", R. W.; Fritz, M.; Gaub, H.E. Science 1992,257,1900. (13) Fromhen, P. Rev. Sci. Instrum. 1975,46, 1380.
0743-7463/93/2409-2178$04.00/0 0 1993 American Chemical Society
LB Films of Fatty Acids
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Figure 1. SFM topographic images of LB monolayers of different saturated fatty acids (left, Cd myristate (C14); middle, Cd behenate (C22); right, Cd hexacosanoate (C26), on mica in air (all values in nm). (a) Top view. The gray scale encodes the height (for nm scale see bar on the right side). Defects in the film structure appear dark while higher regions are light. (b) Cross-sectional surface profile along the line drawn in (a). Film layer thickness is given by the vertical distance of markers (c). Histogram of depth in the area enclosed by the box in (a). The difference between the two maxima is equal to the mean monolayer thickness, i.e. 1.2 nm for C14,2.7 nm for C22, and 3.6 nm for C26; the deviation from the mean is a measure of the surface roughness (all scales in nm). of 0.3 mM. After the chloroform was allowed to evaporate for several minutes, the monomolecular film was slowly compressed up to a final surface pressure of 30 mN/m. I t was left to relax for 10min. The film was transferred to a flat solid substrate. We used freshly cleaved mica (Plano GmbH, Marburg, FRG), freshly cleaved highly oriented pyrolytic graphite (HOPG, ZYB grade, Union Carbide Corp., Cleveland, OH), and a Si(100) wafer (Wacker Chemie, Burghausen, FRG). The Si wafer surface exposed to air is always covered with an amorphous oxide layer, SiO, of nanometer thickness." T o remove adsorbates from the surface the Si wafer was ultrasonicated in distilled acetone followed by treatment with Milli-Q water. Alternatively, the Si wafer was treated with a mixture of 30% H202 and 95% H2SO4 (1:2 vol) a t 110 "C for 30 min. The film was transferred to the solid support by vertically lifting the support with a speed of 13 mm/min through the interface a t constant surface pressure. For HOPG (hydrophobic surface) the first monolayer was transferred during the downstroke. The transfer ratio of each layer was always about unity. For fatty acids shorter than myristate, C14 (for convenience, we use this abbreviation for the number of carbons of the monocarboxylic acid), the monolayer preparation fails because of the solubility of the fatty acid. For chain lengths larger than lignocerate (C24) the transfer is hindered by film stiffness. Therefore, for hexacosanoate ((226) we used a horizontal transfer technique placing the substrate horizontally in the subphaseand lowering the level of the subphase buffer afterward. LB multilayers were generated by alternately lifting and dipping the solid support through the interface. For SFM measurements in liquid, the mica sample remained in a small dish after the transfer of the final monolayer which was placed in the subphase of the trough. During the adjustment of the fluid cell, the samples were continuously covered with a drop of water. The samples were examined soon after preparation with a commercially available Scanning Probe Microscope (NanoScope XI, Digital Instruments, Santa Barbara, CA). Measurements were done in air a t room temperature. For SFM experiments in liquid (14) Raider, S. L.; Flitach,R.; Palmer, M. J. J. Electrochem. SOC.1975, 122,413.
we used a fluid cell from Digital Instruments. Imaging with the STM was with a D-scanner with a 12 X 12(xy) X 5(2) micrometer scan range. The tip (Pt/Ir 80%/20%) wascut mechanically and tested for resolution on HOPG. Imaging with the SFM was with a D-scanner with a 14 X 14(xy) X 5 ( 2 ) micrometer scan range. The SFM was operated in the repulsive (contact) mode and with an optical readout. The feedback system was used in the constant force mode, in which the constant deflection of the cantilever is maintained by changing the extension of the piezo tube scanner normal to the surface. Calibrations of the piezo tube scanner were performed with a ruling (Digital Instruments) for lateral displacements and with polystyrene beads (Serva, Heidelberg, FRG)lS for vertical displacements. For nanometer calibrations in x , y, and z directions we used mica. Measurements were done with three different D-scanners. We used microfabricated pyramidal shaped Si~Nd-tips integrated into a rectangular cantilever with a typical force constant of 0.08 N/m (ML-100, Park Scientific Instruments, Sunnyvale, CA). Only tips giving atomic resolution on mica were selected. The force of the cantilever against the surface was always adjusted to a minimum and was typically lower than 1 nN. The total repulsive force between the surface and the tip is usually much larger because it has to balance attractive forces comprising van der Waals and capillary forces, which are usually in the range of 10-100 nN in air and 30-100-fold less in solution.16 All images (400 X 400 pixels) were taken within 50 s, a t a scan rate of 8 lines/s. Image analysis was with the NanoScope I1 software.
3. Results and Discussion Monolayer Thickness from Topographic Defects. Topographic SFM images in air of LB monolayers formed from different saturated fatty acids on mica are shown in Figure l a (left, C14; middle, C22; right, C26). It is known that aliphatic chains in monolayers of Cd salts of fatty (15) Li, Y.; Lindsay, S. M. Rev. Sci. Instrum. 1991,62,2630. (16) Iaraelachvili,J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992, Chapter 11.
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Figure 2. Film layer thickness of monolayers of saturated fatty acids measured with the SFM ( 0 )in air on mica taken from histogram data. X-ray data of monolayer thickness ( 0 )from ref 18. The straightlines are obtainedfrom a least-squaresfit: SFM data z [nm] = -1.63 [nm] + 0.20 [nm/-CH& correlation coefficient r = 0.998,and X-ray data z [nm] = 0.39[nm] + 0.12 [nm/-CH& with r = 0.999 with x = number of carbons in the range of C14426. The slope is equal to the incrementalchange of the monolayer thickness per methylene residue. Az (01, amplitude of the surface corrugations (Table I).
acids are not tilted.17 Defects (holes) were always present in such samples, with a size of several hundred nm2 to several pm2and a large variation in shape and distribution. The actual force applied by the tip was sufficiently small so that the same surface area could be repeatedly imaged without creating new defects. Inherent defects are formed during transfer of the film to the mica support and/or exposure to air. Cross sections across the monolayer defects enable the determination of the monolayer thickness (Figure lb). Similar results can be obtained from surface height histograms in which depth is referred to the lowest point in the selected area. A depth histogram for a two-level system is shown in Figure IC. The lower peak in these plots corresponds to the level of the mica substrate and the higher to the monolayer surface. The difference between both maxima corresponds to the actual mean monolayer thicknees. In addition, the deviation from the mean at half maximum in the histogram of Figure IC is a measure of the maximum surface roughness of the corresponding level, which is typically 10.1 nm on the monolayer surface. Dependence of Monolayer Thickness on Length of Fatty Acids. SFM measurements of the monolayer thickness were made of single monolayers formed from saturated fatty acids varying in length from 14 to 26 carbons. The results are summarized in Figure 2 (circles) andcompared withX-ray data (ref 18,Figure 2 (squares)). X-ray data were obtained from thickness measurements of multilayers where the periodic arrangement of the cations was used for the determination of the mean monolayer thickness.18 In contrast to the longer chain fatty acids, the shorter chains (
