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Direct Observation of Molecular Arrangements in Fatty Acid Monolayers with an Atomic Force Microscope Tisato Kajiyama,' Yushi Oishi, Fuminobu Hirose, Kenshiro Shuto, and Taishi Kuri Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan Received November 15,199P Molecular-resolution images of fatty acid molecules in the monolayers on mica substrate were successfully observed with an atomic force microscope (AFM) for the first time. The AFM image of a lignoceric acid monolayer prepared at a surface pressure of 5 mN.m-' showed a two-dimensional periodic structure with locally disordered molecular arrangements. Also,the nondestructive AFM image observation was successful for a stearic acid monolayer which was prepared by a multistep creep method, indicating that a high mechanical stability of the monolayer is inevitably required for the nondestructive AFM observation.
Introduction Langmuir-Blodgett (LB) films have been applied to molecular electronics,l nonlinear optics in the field of integrated circuits,2 and biosen~ors.~ The ultimate properties in their applications can be attained by using a defect-freemonolayer which is the precursor of LB films.4 In order to prepare a defect-free or defect-diminished monolayer, it is indispensable to estimate the molecular arrangements and structural defects in a monolayer. An atomic force microscope (AFM) allows one to characterize the surface structure of organic materials on a molecular level. The molecular arrangements611 and structural defects12J3 of multilayered LB films were observed by using an AFM. On the other hand, t h e molecular-resolution AFM image of monolayers was successfully taken by using only limited samples such as a polymerized amphiphilic monolayer,14JS dialkyl amphiphilic monolayer,6*16 and f a t t y acid salt monolayer7J0J1J7 which were formed owing to a relatively
* To whom correspondence should be addressed. e Abstract
published in Advance ACS Abstracts, February 15, 1994. (1) Sugi, M. J. Mol. Electron 1986, 1, 3. (2) Khanarian, G.Thin Solid F i l m 1987,152, 265. ( 3 ) Reichert, W. M.; Bruckner, C. J.; Joseph, J. Thin Solid Films 1987, 152. 346. (4)Yuda, E.; Uchida, M.; Oishi, Y.; Kajiyama, T. Rep. Prog. Polym. Phys. Jpn. 1989,32,161. ~~~
(6)Meyer, E.; Howald, L.; Ovemey,R. M.; Heinzelmaun,H.; Frommer, J.; Glintherodt, H. J.; Wagner, T.; Schier, H.; Roth, S. Nature 1991,349, ma
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(6) Weisenhom,A. L.; Egger, M.;Ohnesorge,F.; Gould, S. A. C.; Heyn, S. P.; Hansma, H. G.;Sineheimer, R. L.; Gaub, H. E.; Hansma, P. K. Langmuir 1991, 7, 8. (7) Hansma, H. G.; Gould, S. A. C.; Hansma,P. K.; Gaub, H. E.; Longo, M. L.; Zasadzinski, J. A. N. Langmuir 1991, 7, 1061. (8) Schwartz, D. K.; Garnaea, J.; Viswanathan, R.; Zasadzinski, J. A. N. Science 1992, 257, 608. (9) Schaper,A,;Wolthaus,L.; MBbius,D.; Jovin, T. M. Langmuir 1993,
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(10) Schwartz, D. K.;Viswanathan, R.; Garnaes,J.; Zasadzinski,J. A. J. Am. Chem. SOC.1993,115,7374. (11) Viwanathan,R.;Zasadzinski,J. A.; Schwartz,D.K. Science 1993, 261, 449. (12) Bourdieu, L.; Silberzan, P.; Chatenay, D. Phys. Reu. Lett. 1991, 67, 2029. (13) Peltonen, J. P. K.; He, P.; Rosenholm, J. B. J. Am. Chem. SOC. 1992,114,7637. (14) Marti, 0.;Ribi, H. 0.; Drake, B.; Albrecht, T. R.; Quate, C. F.; Hamma, P. K. Science 1988,239,60. (16) Radmacher, M.; Eberle, K.; Gaub, H. E. Ultramicroscopy 1992, 42-44,968.
(16) Josefowicz, J. Y.; Maliszewskyj,N. C.; Idziak, S. H. J.; Heiney, P. A.; McCauley, Jr., J. P.; Smith, A. B., 111. Science 1993,260, 323. (17) Radmacher. M.: Tillmann. R. W.: Fritz.. M.:. Gaub. H. Science 1992, k57, 1900. I
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stronger aggregation force. There has been no observation of the molecular-resolution image of the fatty acid monolayer with a weaker aggregation force. We present here the molecular-resolutionAFM images of lignoceric and stearic acid monolayers on mica for the first time and also propose a method for obtaining the nondestructive AFM image of monolayers with a weaker aggregation force.
Experimental Section Benzene solutionsof lignoceric(CHs(CH.J&OOH) and stearic (CHs(CHZ)&OOH) acids with concentrations of 1 X 1W and 3 x 1V mol.L-l, respectively,were spread on the pure water surface of 293 K. Since Tw is below the at a subphase temperature, T,,,, melting temperatures, Tm,of the lignoceric acid (Tm= 347 K) and the stearic acid (Tm= 317 K) monolayers,laa those monolayers are in a crystalline state. The subphase water was purified with the Milli-QIIsystem. The lignocericacid monolayer was prepared at a surface pressure of 5 mN.m-I by a continuous compressionat a rate of 1.7 X 1V nm2.molecule-1.s-1. The stearic by the continuous acid monolayer was prepared at 23 "am-' compression method or a multistep creep methodsa1 The multistep creep method is a monolayer preparation method for which the monolayer is stepwisely compressed up to a higher surface pressure by alternating the compression and area creep. This procedure causes rearrangement of molecules in the crystalline monolayer and/or filling of the vacancies in the interfacial regions among crystalline monolayer domains, which releases the stress concentration in the monolayer. Therefore, the multistep creep method provides a mechanically stable and defect-diminished monolayer. Each monolayer was transferred onto a freshly cleaved mica (Okabe Mica, Fukuoka, Japan) by avertical dipping method. The transfer ratio for each monolayer was unity, which implies that a mica substrate is completely covered with eachmonolayer. The AFM imagesof the monolayers were obtained with a SFA300 (Seiko Instruments, Inc.) in air at room temperature, using a 0.8-pm scanner and a silicon nitride tip on a cantilever with a small spring constant of 0.027Nmm-l. Images were recorded within 20 s in the "constant-height" mode; that is, feedback electronics and software were used to keep the sample height constant and measure the cantilever deflection. The applied force on imaging was evaluated to be about 10-lO N in an attractive force region, from the magnitude of cantilever deflection. This attractive force may not be the actual applied force between the tip and sample but an apparent composite (18) Kajiyama, T.; Oishi,Y.; Uchida, M.; Morotomi, N.; Ishikawa, J.; Tanimoto, Y . Bull. Chem. SOC.Jpn. 1992,65,864. (19) Kajiyama, T.; Oishi,Y.; Uchida, M.; Tanimoto, Y.; Kozuru, H.
Langmuir 1992,8, 1663. (20) Oishi, Y.; Kozuru, H.; Shuto, K.; Kajiyama, T. Mater. Res. SOC. Symp. Proc., in press. (21) Kuri, T.; Oishi, Y.; Kajiyama, T. Mater. Res. SOC.Symp. Proc., in press.
0743-746319412410-1297$04.5010 0 1994 American Chemical Society
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Figure 2. Nonfiltered AFM image of a mica substrate on a scan area of 4 X 4 nm2. The image exhibited a periodic hexagonal array with the (10)spacing of 0.46 nm which was in disagreement with the (10)spacing of the lignoceric acid monolayer (Figure 1).
Figure 1. (a, top) Nonfiltered AFM image of a lignoceric acid monolayer on a scan area of 4 X 4 nm2. The monolayer was prepared at a surface pressure of 5 mN0m-l by a continuous compression method. This image was not changed even by repeated scanning. Note the periodic arrangement of the moleculeswith a hexagonal array. (b, bottom) ZD-FFT spectrum of (a). The numbers are Miller indices of (hk). The spectrum exhibited a hexagonal pattern with the (10) spacing of 0.43 nm. force (applied force between the tip and sample, adhesion force by the water molecule a t sample surface, etc.).
Results and Discussion Figure l a shows a nonfiltered AFM image of the lignocericacid crysaline monolayer, which was prepared by the cont~nuouscompress~onmethod, on at a scan area of 4 X 4 nm2. The AFM image is given in a top-view presentation in which the brighter and darker portions correspond to higher and lower regions of the monolayer surface, respectively. Though scanning was done repeatedly, the monolayer was not damaged by a tip. However, a hole could be artificially pierced through the monolayer with a stronger applied force than 10-9 N. The hole was about 3 nm deep, being comparable with the calculated molecular length based on the CPK model, in other words, the thickness of the lignocericacid monolayer. It is reasonable to expect that the brighter portion in the AFM image represents the single methyl group of the lignoceric acid molecule, because the hydrophobic of the lignoceric acid molecule was oriented toward air by the vertical dipping method, The AFM image exhibits that lignocericacid molecules are regularly arranged with a hexagonal array. Then, in order to clarify the molecular arrangement in the monolayer, a two-dimensional fast Fourier transform (2D-FFT) treatment was carried out. Figure 1b shows the 2D-FFT spectrum of the image shown in Figure la. The bright spots in the 2D-FFT spectrum exhibit a hexagonal pattern with the (10)spacing of 0.43 nm. (10)represents the two-dimensional lattice 0. The plane with Miller indices of h = 1 and 12 magnitude of (10) spacing which was evaluated from the 2D-FFT spectrum agrees well with the spacing of 0.43 nm
Figure 3. Nonfiltered AFM image of a lignocericacid monolayer on a scan area of 9 x 9 nm2. Note a two-dimensional periodic structure with locally disordered molecular arrangements.
which W a s estimated from the electron diffraction (ED) Pattern of the lignoceric acid monolayerr20 and also this magnitude is quite different from the 0.46-nm spacing of a mica substrate from the AFM image (Figure 2)- Moreover, the molecular occupied area of the liwoceric acid molecule in the monolayer which was evaluated from the AFM image and the ED pattern was nm2* to 0*25nm20 This magnitude was isotherm which was Obtained On the basis Of measurements. Therefore, it is reasonable to conclude from Figures 1and 2 that the brighter portion in the AFM image of Figure l a represents the single methyl group of the 1ignOCeriC acid molecule in the " m l a y e r and also that 1iflOCeriCacid molecules me regularly arranged with a array* Figure 3 showsa nonfiltered AFM image of the lignoceric acid monolayer on a larger area scan of 9 X 9 nm2. A regularly periodic hexagonal array in the AFM image was extended over about 10 nm. The range of the periodic hexagonal array Was comparable to the magnitude of CrYsaloPaPhical continuity which W a s evaluated by a single line based on Fourier analysis of the EDprofileofthelign~~ericacidmonolayer.Asshownby (22) Hofmann, D.;Walenta, E.Polymer 1987,28,1!298.
Molecular Arrangements in Fatty Acid Monolayers
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Figure 5. A filtered AFM image of a stearic acid monolayer on a scan area of 20 X 20 nm2. The stearic acid molecules were regularly arranged in a hexagonal array over 20 X 20 nm2.
was prepared at a fairly higher surface pressure, for example, 23 "om-', by the multistep creep method?' because stress concentration in the monolayer was almost completely released. Figure 4a shows a nonfiltered AFM image of the stearic acid crystalline monolayer which was prepared by the multistep creep method with a scan area of 5 X 5 nm2. Though scanning was done repeatedly on the stearic acid monolayer prepared at the high surface pressure of 23 mN0m-l by the multistep creep method, the monolayer Figure 4. (a, top) Nonfiltered AFM image of a stearic acid was not damaged by the tip. On the other hand, the monolayer on a scan area of 5 X 5 nm2. The monolayer was monolayer which was prepared a t the same surface pressure prepared at 23 "em-1 by a multistepcreep method. The image by the continuous compression method was easily collapsed was not changed evenby repeated scanning. (b, bottom)A filtered AFM image of (a). For (b), a digital filtering treatment for the by the tip on scanning. This collapse may be caused by Fourier-transformed image was carried out by keeping only the the stress concentration at vacancies among crystalline spatial frequenciescorresponding to the six spots of the Fourierdomains in the monolayer. In order to reduce the noise transformed image of (a). The stearic acid molecules were component in Figure 4a, a digital filtering treatment for regularly arranged in a hexagonal array with the (10) spacing of the Fourier-transformed image was carried out by keeping 0.42 nm. only the spatial frequencies corresponding to the six spots of the Fourier-transformed image. Figure 4b shows a the circle in Figure 3, the hexagonal array of lignoceric filtered AFM image of the monolayer. A higher region in acid molecules was locally disordered. Thus, the molecthe AFM image represents the single methyl group of the ular-resolutionAFM image of the lignocericacid monolayer stearic acid molecule in the monolayer. The AFM image was nondestructively obtained and exhibited a twoof Figure 4b indicates that stearic acid molecules are dimensional periodic structure with locally disordered regularly arranged with a hexagonal array with the (10) molecular arrangements. spacing of 0.42 nm. This magnitude agrees with the The nondestructive AFM observation of the lignoceric spacing of 0.42 nm which was estimated from the ED acid monolayer was successful, as shown in Figures 1and pattern of the stearic acid monolayer.l8Jg When the area 3. However, it was impossible to obtain a molecularscan was enlarged, the regularly periodic hexagonal array resolution AFM image of the lignoceric acid monolayer was extended over 20 X 20 nm2,as shown in Figure 5. This which was continuously compressed up to higher surface magnitude was comparable to the magnitude of crystalpressures than 10 mN0m-l because of the monolayer lographical continuity evaluated on the basis of the single destruction during the AFM scan. In the case of the line method. Further, no distinct molecular disordered crystalline monolayer, the crystalline domains grown right region was observed in the scan area of 20 X 20 nm2at the after spreading a solution are assembled into a morphologically homogeneous monolayer during c o m p r e s ~ i o n . ~ ~ J ~AFM image. The larger crystallographical continuity and extended regular molecular arrangement of the stearic However, the vacancies at a molecular level may remain acid monolayer are ascribed to effective sintering at at the interfaces among the crystalline domains in the interfaces among crystalline domains by the multistep monolayer when the monolayer is prepared by the creep method.l8I2l continuous compression method. These vacancies cause the stress concentration in the monolayer under a conConclusion tinuous compression. Since large stress locally concenThe molecular-resolution AFM image of the fatty acid trates in the monolayer at higher surface pressures, such monolayer was successfullytaken by using a mechanically a monolayer is mechanically unstable. This mechanical stable monolayer. The fatty acid monolayer with a unstability may be the reason why the monolayer which remarkable mechanical stability for nondestructive AFM was continuouslycompressedup to a high surface pressure, observations was prepared at a higher surface pressure by for example, 10mN-m-l, was easilycollapsed by the applied the multistep creep method and at a lower surface pressure force on the AFM scan. This argument was justified in by the continuous compression method. the nondestructive AFM image of the monolayer which
