Molecular-scale structures of Langmuir±Blodgett lms of fatty acids observed by atomic force microscopy (II) ± cation dependence

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1 Thin Solid Films 331 (1998) 170±175 Molecular-scale structures of Langmuir±Blodgett lms of fatty acids observed by atomic force microscopy (II) ± cation dependence N. Sigiyama*, A. Shimizu, M. Nakamura, Y. Nakagawa, Y. Nagasawa, H. Ishida Toray Research Center, Inc., Sonoyama, Otsu, Shiga 520 Japan Abstract The morphologies of LB lms of arachidic acid and its salt were observed by atomic force microscopy (AFM) to reveal their cation dependence from the viewpoint of ionic radius and solution ph. The salt lms having divalent metal ions (Ba 21,Sr 21 and Co 21 ) except for Mn 21 showed similar characteristic stripe-shaped macroscopic structures. An area per molecule in the lm increased with decrease of ionic radius. Concerning the solution ph, signi cant changes in molecular arrangements and macroscopic structures were observed in the barium arachidate lms. Coexistence of 2 2 superstructures with 1 1 fundamental structures were observed at ph ˆ 8:0, but 3 1 at ph ˆ 9:8. These structures were considered to depend on the increase of the salt forming ratio increases with ph. q 1998 Published by Elsevier Science Ltd. All rights reserved. Keywords: Langmuir±Blodgett lm; Atomic force microscopy; Ionic radius; ph; Molecular arrangement; Superstructure 1. Introduction The control of molecular arrangement and orientation in organic thin lms is essential to realize the nano molecular electronics. The Langmuir±Blodgett (LB) technique is one of the promising methods to control the molecular arrangement [1]. The structure of LB lms has been studied extensively with X-ray diffraction, infrared spectroscopy, transmission electron microscopy, and scanning tunneling microscopy [1±3]. Recently, atomic force microscopy (AFM) has been successfully applied to observe the molecular arrangements of LB lms, revealing various structures [4±13]. In a previous work [14], we have studied the structures of arachidic acid and barium arachidate lms on chemically attened hydrophobic Si(111) substrates with AFM. It was con rmed that barium ion expands the size of the unit cell in comparison with the arachidic acid lms. A characteristic 2 2 superlattice was observed only on the salt lms. As for the in uence of cations, a relationship between area per molecule and the element of cation was discussed in the view of Pauling electronegativity by Schwartz et al. [8]. * Corresponding author. Tel.: ; fax: ; naoyuki_sugiyama@trc.toray.co.jp. In their report, 2 2 and 3 1 superstructures were observed in barium salt lms on hydrophilic cleaved mica or amorphous silicon oxide substrates [8]. They proposed a packing defects models to explain the coexistence of two types of superstructures. On the other hand, it has been reported that the composition ratio of salt and arachidic acid in the barium stearate LB lms varies with ph range 6±8.5 [15]. An approximately linear increase of Ba 21 content with ph over the range 5.0± 8.7 was also observed by neutron activation analysis of barium arachidate lms [16]. According to these facts, there may be another possibility that the superstructures are due to the ordering of salt and acid molecules, and may have some variation with the ph which changes their composition ratio. In this work, we therefore investigated the cation dependence of the structure of salt LB lms focusing on the ionic radius and the solution ph. We used the divalent metal ions of Ba, Sr, Co and Mn of which ionic radii differs a lot (e.g. Ba ˆ 1:35 A and Sr ˆ 0:72 A). We also varied the ph of the subphase over the range 7.0±10.0, especially for Ba. 2. Experimental Arachidic acid lms and salt lms were prepared by the /98/$ - see front matter q 1998 Published by Elsevier Science Ltd. All rights reserved. PII S (98)

2 N. Sigiyama et al. / Thin Solid Films 331 (1998) 170± Fig. 1. AFM images of (a) an arachidic acid, (b) a barium arachidate, (c) a strontium arachidate, (d) a cobalt arachidate and (e) a manganese arachidate bilayer LB lm. The size of the images is 5:0 5:0 mm 2. standard LB technique. Arachidic acid (Nacalai Tesque) was spread from a 0.5 mg/ml chloroform solution on an aqueous subphase in a commercial rectangular LB trough (MGW Lauda). Ultra-pure water and 0.5 mm MeCl 2 /nh2o (Me ˆ Ba; Sr; Co and Mn) aqueous solution were used as the subphase to prepare the arachidic acid lm and the salt lms, respectively. The lms on the subphase were compressed to a surface pressure of 30 mn/m and then transferred onto substrates by vertical immersion with a speed of 1.0±15 cm/min. The surface pressure and the immersion speed were chosen so as to obtain solid-phase monolayer lms and unity transfer ratios. Bilayer lms were deposited for all materials. The subphase ph was adjusted to 7.0±10.0 for ph dependence experiments by adding NaHCO 3. Hydrogen-terminated silicon wafers were prepared as hydrophobic substrates by the method reported previously [11]. The obtained surface was con rmed to be contaminant free and atomically at. The AFM observations were carried out using a commercial atomic force microscope, NanoScope IIIa (Digital Instruments), with a m m or a 0:8 0:8 m m piezo scanner. Tip-integrated cantilevers made of silicon nitride with a spring constant of 0.12 N/m (Olympus Opt.) were used for the observations. All the images were obtained by variable height mode in air at room temperature. 3. Results and discussion 3.1. In uence of ionic radii of cations Fig. 1 shows macroscopic AFM images (5:0 5:0 mm) of bilayer LB lms of (a) arachidic acid, (b) barium arachidate (BaA 2 ), (c) strontium arachidate (SrA 2 ), (d) cobalt arachidate (CoA 2 ), and (e) manganese arachidate (MnA 2 ). In the image for arachidic acid, stripe-shaped and arrow-shaped corrugations are observed. The height of the stripe is about 0.8 nm and the width is 50±100 nm. For BaA 2, similar stripe-shaped ridges which have some branches are observed over the images. The size of these ridges is the same as those of arachidic acid, although the density of ridges is much higher. Similar corrugations were also observed both on SrA 2 and on CoA 2, and the sizes of ridges are almost the same. Kurnaz et al. observed analogous ridges on CdA 2 trilayer lm which are explained to be due to the phase separation of acid and salt [11]. The ridges we

3 172 N. Sigiyama et al. / Thin Solid Films 331 (1998) 170±175 Fig nm 2 AFM images of (a) an arachidic acid, (b) a cobalt arachidate and (c) a manganese arachidate bilayer LB lm. observed are, however, considered to be different from them, because they faded by aging or by repetitive AFM scans, which indicates that the insertion is volatile. Moreover, the height of the ridges were 0:8 ^ 0:1nm and were independent of the size of cations. Therefore, the corrugations we observed are assumed to be due to the water layer incorporated with the interlayer carboxylic group. Remarkably different images were obtained on MnA 2 lms from other salt lms. The number of characteristic domain structures, instead of the ridges, were observed as shown in Fig. 1e. The morphology of the MnA 2 lm was almost independent of the immersion speed of the substrates in the range of 1.0±15.0 cm/min. The presumable reason why the MnA 2 shows quite different morphology is that only Mn takes many different ionic charge number which is sensitive to solution ph. There may be trace amount of contaminants located at the center of the structures, which locally cause the concentric distribution of ph. Fig. 2 shows high magni cation images of (a) arachidic acid, (b) CoA 2 and (c) MnA 2. For arachidic acid and CoA 2, well-ordered molecular arrangements are observed. On the other hand, only an unclear arrangement is partially observed for MnA 2. These facts indicate that the MnA 2 lm is unstable and the packing of the molecules is loose. Here, we compare the p±a isotherms of L lms with different cations. A typical ph value in this experiment was around 6.5 in case of Ba 21. Table 1 summarizes the phase transition points of various lms along with area per molecule obtained from AFM images. On salt lms having divalent metal ions (Ba 21,Sr 21,Co 21 and Mn 21 ) as cations, distinct tendency is not con rmed in the area-per-molecule values at phase transition from liquid to solid. On the other hand, the surface pressure of salt lms at those points is clearly lower than that of arachidic acid. Moreover, the surface pressure at the transition points is in the order of Sr 21.Mn 21.Co 21.Ba 21, which is contrary to the ionic radii of divalent metal ions. This indicates that the attractive force between the molecules increases as the radius of the metal ion increase. A similar tendency is also seen in the area per molecule values from AFM images. Zasadzinski et al. reported a general trend of increasing the area-per-molecule with decreasing cation electronegativity [17]. There seems, however, a tendency of increasing the area-per-molecule with decreasing ionic radii of cations rather than the electronegativity, although there exists a few percent deviation. For example, two alkaline earth metal Ba and Sr, have almost the same electronegativity although they show apparently different surface pressures at transition points and area per molecule value obtained from AFM images. Table 1 Transition points in p±a isotherm and area per molecule values obtained from AFM images Material Liquid±solid transition Area per molecule obtained from AFM image (AÊ 2 ) Surface pressure (mn/m) Area per molecule (AÊ 2 ) Ionic radius of cations (AÊ 2 ) Electronegativity of cations Arachidic acid ± ± Barium arachidate Cobalt arachidate Manganese arachidate Strontium arachidate

4 N. Sigiyama et al. / Thin Solid Films 331 (1998) 170± Fig. 3. The p±a isotherms of barium arachidate against various ph In uence of solution ph We investigated the in uence of subphase ph on the molecular arrangement and macroscopic morphology. Fig. 3 shows the p±a isotherms against various ph of barium arachidate. At ph ˆ 7:2, three step successive phase transition (gas-liquid-solid) is clearly observed. The range of liquid phase becomes narrower when the ph ˆ 8:0, and disappears when the ph ˆ 9:8. A similar tendency was also observed on SrA 2. It is reported that calcium stearate begins to cluster into two-dimensional aggregates termed surface micelles between ph ˆ 6:4 and 8.0, and above ph ˆ 8:0 all the material is present as a mosaic of surface micelles [1]. The change of p±a isotherm in this work is considered to result from similar behavior as the calcium stearate case. Accordingly, the p±a behavior is due to the extent of ion incorporation into the lm, i.e. salt forming ratio, at various ph values [1]. Fig. 4 shows a set of low-magni cation AFM images of BaA 2 lms formed at various ph. A remarkable difference is observed depending on the ph. At ph ˆ 7:2, both islandlike and stripe-shaped ridges are observed. The ridges are similar to those shown in Fig. 1b. At ph ˆ 8:0, fewer ridges and many pinholes are observed. At ph ˆ 9:8, very high ridges, indicating that the segregation has taken place, are also observed. Such a morphological change is in good agreement with the variation of p±a isotherms. Fig. 5 shows a set of high magni cation AFM images of the lms formed at ph ˆ 7:2, 8.0 and 9.4 along with their 2D Fourier transformed images. From these images, 1 1 fundamental structure at ph ˆ 7:2, 2 2 superstructure in addition to 1 1atpHˆ 8:0 and 3 1 superstructure at ph ˆ 9:8 is observed. Considering the fact that no superstructure was observed in arachidic acid [14] and that coexistence of 2 2 with 1 1 is observed at ph ˆ 8:0, these difference of molecular arrangement may be due to the salt forming ratio as mentioned in the low magni cation AFM images. A structural model for each superstructure is shown schematically in Fig. 6. White circles denote the acid molecules and the hatched circles denote the alkyl chains composing salt molecules. This model is based on the molecular arrangements changing with ph, i.e. variation of the salts forming ratio, and thus is different from a packing defects model proposed by Schwartz et al. [8]. The 2 2 model is composed of the acid and the salt alkyl chains alternatively. A local salt forming ratio in this model is to be 50%. The 3 1 model is composed of a arachidic acid alkyl chain existing at intervals of two salt chains. Salt forming ratio is 66.7% in this case. In both models, the coordination of arachidic acid molecules and Ba ions is assumed to be two second-neighboring acid molecules are bonded to a Ba ion. We estimated the validity of this model with semiemperical Fig. 4. 5:0 5:0 mm 2 AFM images of BaA 2 bilayer LB lms (a) at ph ˆ 7:2, (b) at ph ˆ 8:0 and (c) at ph ˆ 9:8.

5 174 N. Sigiyama et al. / Thin Solid Films 331 (1998) 170±175 Fig. 5. High magni cation AFM images of the lms formed at (a) phˆ7.2, (b) ph ˆ 8:0 and (c) ph ˆ 9:4 along with their 2D Fourier transformed images. The spots denoted F and S are originated from a fundamental structure and a superstructure, respectively. molecular orbital calculations and molecular mechanics simulations as a rst approximation 1. The simulation showed that the strain energy due to the U-shaped interconnection is comparable to the non-bonding interaction between the molecules. It means that -CZOZBaZOZCbonds are exible enough to interconnect the second-neighboring alkyl chains without energetic loss and the models are concluded to be stable. with AFM molecular images. This result is in good agreement with the tendency of the surface pressure at the transition points in p ±A isotherms for each material. This indicates that the attractive force between the molecules increases as the radii of cations and is considered to be more appropriate to explain a variety of area per molecule values rather than electronegativity suggested previously [17]. Signi cant changes are observed both in macro- and in 3. Conclusion We have examined the in uence of cations on the structure of bilayer LB lms of arachidic acid and its salt deposited on chemically- attened hydrophobic Si(111) substrates by changing ionic radii of cations and solution ph, i.e. the extent of cations incorporated with lms. The tendency that area per molecule became larger as the radii of the cations added became smaller was con rmed 1 The semiemperical MO calculations were performed with MOPAC to investigate the energy variation with the bond angles in the ±C±O±BaO±C± chain. The PM3 Hamiltonian model was used. The molecular mechanics simulations were performed with MM2 to check the stability of the models shown in Fig. 6. Fig. 6. Schematic structural models for each superstructure, (a) 2 2 and (b) 3 1. White circles denote the acid molecules and the hatched circles denote the alkyl chains composing salt molecules.

6 N. Sigiyama et al. / Thin Solid Films 331 (1998) 170± microscopic morphology of lms when the ph of the subphase was varied. Especially, notable changes were observed on the molecular arrangement, which exhibited different superstructures of 2 2atpHˆ 8:0 and 3 1at ph ˆ 9:8. These variations of molecular arrangements is considered to be due to the salt forming ratio, i.e. the content of cations incorporated with lms. Structural models were proposed to satisfy both the superstructures and salt forming ratio variation with ph. These models are quite different from packing defects models proposed previously by Schwartz et. al. [8]. Further experiments are necessary to verify these meddles. References [1] G. Roberts (Ed.), Langmuir±Blodgett Films, Plenum Press, New York, 1990, Chapters 1±4. [2] C.A. Lang, J.K.H. Horber, T.W. Hansch, W.M. Heckl, H. Mohwald, J. Vac. Sci. Technol. A 6 (1988) 368. [3] W. Mizutani, M. Shigeno, K. Saito, K. Watanabe, M. Sugi, M. Ono, Jpn. J. Appl. Phys. 27 (1988) [4] E. Meyer, L. Howald, R.M. Overney, et al., Nature 349 (1991) 398. [5] H.G. Hansma, S.A.C. Gould, P.K. Hansma, H.E. Gaub, M.L. Longo, J.A.N. Zasadzinski, Langmuir 7 (1991) [6] Y. Nakagawa, T. Takahagi, F. Soeda, A. Ishitani, Ext. Abstract 39th Spring Meeting of the Japan Society of Applied Physics and Related Societies, Chiba, March, 1992, (in Japanese). [7] A. Schaper, L. Wolthaus, D. Mobius, T.M. Jovin, Langmuir 9 (1993) [8] D.K. Schwartz, R. Viswanathan, J. Garnaes, J.A.N. Zasadzinski, J. Am. Chem. Soc. 115 (1993) [9] D.K. Schwartz, R. Viswanathan, J.A.N. Zasadzinski, Phys. Rev. Lett. 70 (1993) [10] D.K. Schwartz, R. Viswanathan, J.A.N. Zasadzinski, Langmuir 9 (1993) [11] M.L. Kurnaz, D.K. Schwartz, J. Phys. Chem 100 (1996) [12] M.L. Kurnaz, D.K. Schwartz, Langmuir 12 (1996) [13] H.D. Sikes, D.K. Schwartz, Langmuir 13 (1997) [14] Y. Nakagawa, A. Shimizu, N. Sugiyama, F. Soeda, A. Ishitani, Jpn. J. Appl. Phys. 34 (1995) [15] C. Vogel, J. Corset, M. Dupeyrat, J. Chim. Phys. 76 (1979) 909. [16] J.G. Petrov, I. Kuleff, D. Platikanoff, J. Colloid. Interface Sci. 88 (1982) 29. [17] J. A. Zasadzinski, R. Viswanathan, L. Madison, J. Garnals, D.K. Schwartz, Science 263 (1996) 1726.