660
Langmuir 1995,11, 660-665
Langmuir-Blodgett Film of Amphiphilic CSO Carboxylic Acid Mutsuyoshi Matsumoto,* Hiroaki Tachibana, Reiko Azumi, Motoo Tanaka, and Takayoshi Nakamura National Institute of Materials and Chemical Research, 1-1,Higashi, Tsukuba, Ibaraki 305, Japan
Gen Yunome and Masahiko Abe Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 277, Japan
Shigeru Yamago and Eiichi Nakamura" Department of Chemistry, Tokyo Institute of Technology, Meguro, Tokyo 152, Japan Received October 24, 1994@ An amphiphilic c60 molecule with a substituent having a hydrophilic carboxylic group at the end was found to form a monolayer at the air-water interface and the structure of the Langmuir-Blodgett (LB) film (Z-type)was elucidated. The shortest distance between the c60 was estimated to be 0.95 nm using a limiting area per molecule of 0.78 nm2 at the air-water interface, assuming the close packing of the c60 moiety in two dimensions. Adjacent c60 moieties have an electronic interaction which was shown in the red-shift of the characteristic bands in the UVlvis absorption spectrum of the LB film compared with that of a solution spectrum. Several lines of evidence suggested that the molecule has an oblique orientation in the LB film and exists in a pairwise manner due to the dimer formation of the carboxylic groups within a monolayer, not between the adjacent monolayers. The AFM observations revealed that the surface of a single-layerLB film, which consists of domains of ca. 0.1 ym in diameter, is rather smooth and that the undulation is f l nm for most of the surface except for defects such as vacancy and bilayer regions. The area fractions of the monolayer, the vacancy, and the bilayer regions were estimated to be 88 & 7 , 7 k 5, 5 k 3%, respectively. The layer structure was also confirmed using X-ray diffraction analyses which indicate the repeat distance of ca. 2.6 nm along the surface normal of the multilayer LB film.
Introduction Fullerenes, c60 in particular, have attracted much interest because of their unique physial and chemical pr0perties.l Extensive studies have been carried out to obtain thin fullerene films with ordered structures such as epitaxial films under ultrahigh vacuum,l LangmuirBlodgett (LB) f i l m ~ , ~ self-assembled -l~ films,18-21 and chemically adsorbed films,22aiming a t characterizing the ~~
Abstract publishedinAdvance ACSAbstracts, January 1,1995. (1) For example, Fullerenes; Kroto, H. W., Fischer, J. E., Cox, D. E., Eds.; Pergamon Press: Oxford, 1993, and Buckminsterfullerenes; Billups, W. E., Ciufolini, M. A., Eds.; VCH Publishers: New York, 1993. (2) (a)Obeng, Y. S.; Bard, A. J. J.Am. Chem. SOC.1991,113,62796280. (b) Jehoulet, C.; Obeng, Y. S.; Kim, Y.-T.; Zhou, F.; Bard, A. J. J.Am. Chem. SOC. 1992,114,4237-4247. (c)Bulhoes, L. 0.S.; Obeng, Y. S.; Bard, A. J. Chem. Mater. 1993, 5, 110-114. (3) (a)Nakamura, T.; Tachibana, H.; Yumura, M.; Matsumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y. Langmuir 1992, 8, 4-6. (b) Nakamura, T.; Tachibana, H.; Yumura, M.; Matsumoto, M.; Tagaki, W. Synth. Met. 1993,55-57, 3131-3136. (4) (a) Williams, G.; Pearson, C.; Bryce, M. R.; Petty, M. C. Thin Solid Films 1992,209,150-152. (b) Williams, G.; Moore, A. J.; Bryce, M. R.; Lvov, Y. M.; Petty, M. C. Synth. Met. 1993,55-57,2955-2960. ( c ) Goldenberg, L. M.; Williams, G.; Bryce, M. R.; Monkman, A. P.; Petty, M. C.; Hirsch, A,; Aoi, A. J . Chem. Soc., Chem. Commun. 1993, 1310-1312. (d) Williams, G.; Soi, A,; Hirsch, A,; Bryce, M. R.; Petty, M. C. Thin Solid Films 1993, 230, 73-77. (5) (a)Long, C.-F.; Xu, Y.; Guo, F.-X.; Li, Y.-L.; Xu, D.-F.; Yao, Y.-X.; Zhu, D.-B. Solid State Commun. 1992,82, 381-383. (b) Guo, J.; Xu, Y.; Li, Y.; Yang, C.; Yao, Y.; Zhu, D.; Bai, C. Chem. Phys. Lett. 1992, 195,625-627. (c)Xu, Y.; Guo, J.; Long, C.; Li, Y.; Liu, Y.; Yao, Y.; Zhu, D. Thin Solid Films 1994,242, 45-49. (6)Milliken, J.; Dominguez, D. D.; Nelson, H. H.; Barger, W. R. Chem. Mater. 1992, 4, 252-254. (7) Back, R.; Lennox, R. B. J. Phys. Chem. 1992,96, 8149-8152. (8) Iwahashi, M.; Kikuchi, K.; Achiba, Y.; Ikemoto, I.; Araki, T.; Mochida, T.; Yokoi, S.;Tanaka, A,; Iriyama, K. Langmuir 1992,8,2980@
2984.
fdlerene solids thoroughly or a t processing the fullerenes for practical applications. In order to fully explore the potentials of fullerenes in material science, it is imperative to fabricate fullerene systems in which the location and organization of the fullerenes are controlled precisely. In this respect, the LB technique has advantage over other methods, as it permits one to obtain monolayer films with a well-defined (9) (a) Wang, P.; Shamsuzzoha, M.; Wu, X.-L.; Lee, W.-J.; Metzger, R. M. J . Phys. Chem. 1992,96,9025-9028. (b)Wang, P.; Shamsuzzoha, M.; Lee, W.-J.; Wu, X.-L.; Metzger, R. M. Synth. Met. 1993, 55-57, 3104-3109. (c) Wang, P.; Metzger, R. M.; Bandow, S.; Maruyama, Y. J . Phys. Chem. 1993,96,2926-2927. (10)Tomioka, Y.; Ishibashi, M.; Kajiyama, H.; Taniguchi, Y. Langmuir 1993, 9, 32-35. (11) Xiao, Y.; Yao, Z.; Jin, D.; Yan, F.; Xue, Q. J . Phys. Chem. 1993, 97, 7072-7074. (12) Maliszewskyj, N. C.;Heiney,P.A.; Jones,D. R.; Strongin,R.M.; Cichy, M. A.; Smith, A. B., 111 Langmuir 1993, 9, 1439-1441. (13)Wang, Y.-M.; Kamat, P. V.; Patterson, L. K. J . Phys. Chem. 1993,97, 8793-8797. (14)Wang, J. Y.; Vaknin, D.; Uphaus, R. A.; Kjaer, K.; Losche, M. Thin Solid Films 1994, 242, 40-44. (15)Zhang, X.; Zhang, R.; Shen, J.; Zou, G. Mucromol. Rapid Commun. 1994,15,373-377. (16) Kharlamov,A. A,; Chernozatonskii, L. A,; Dityat'ev, A. A. Chem. Phys. Lett. 1994,219, 457-461. (17) Maggini, M.; Karlsson, A,; Pasimeni, L.; Scorrano, G.;Prato, M.; Valli, L. Tetrahedron Lett. 1994, 35, 2985-2988. (18) Chupa, J. A.; Xu, S.;Fischetti, R. F.; Strongin, R. M.; McCauley, J . P. Jr.; Smith, A. B., 111; Blasie, J . K. J . Am. Chem. SOC.1993, 115, 4383-4384. (19) (a) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J . Am. Chem. SOC. 1993, 115, 1193-1194. (b) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J . Langmuir 1993, 9, 1945-1947. (20) Li, D.-Q.; Swanson, B. I. Langmuir 1993, 9, 3341-3344. (21) Tsukruk, V. V.; Lander, L. M.; Brittain, W. J. Langmuir 1994, 10, 996-999. (22) Zhane, J . Z.: Geselbracht, M. J.; Ellis. A. B. J . Am. Chem. SOC. 1993, 115, 7?89-7793.
0743-746319512411-0660$09.00/00 1995 American Chemical Society
Langmuir-Blodgett Film of CSOCarboxylic Acid
Langmuir, Vol. 11, No. 2, 1995 661
in common solvents, was found to be inappropriate for the LB film fabrication. In this paper, we report the formation of a monolayer of amC60as well a s the successful fabrication of LB films on various substrates. The structure of the LB film is indeed highly ordered, as has been elucidated through spectroscopic measurements, atomic force microscope Figure 1. Chemical structure of am&,. (AFM) observations, and X-ray diffraction analysis of the LB films as well as surface active properties of the structure and molecularly controlled t h i c k n e s ~ . ~In ~-~~ molecule. To our knowledge, the present LB films are the addition, the LB technology also makes possible the best structurally defined LB films made thus far from c60 fabrication of multilayer films onto various solid subor c60 derivatives. strates. The attempts to fabricate high-quality fullerene LB films Experimental Section have so far encountered difficulties. At the initial stage Monolayer Measurements. The c 6 0 was obtained from of the LB film studies on c60, c60 by itself was used as a carbon soots as described in t h e 1iteratu1-e.~~ The amphiphilic film-forming molecule. It is yet still controversial whether c 6 0 used in this study was synthesized a s previously reported.28 or not c60 forms a monolayer a t the air-water interface. Surface pressure-area (n-A) isotherms were recorded on a pure On the basis ofthe surface pressure-area isotherms, some water (deionized, MilliQ filtered, and distilled) subphase using reported monolayer formation,2J1J2 while others noted a Lauda Langmuir trough at 290 K. The spreading solvent was multilayer f ~ r m a t i o n . ~ , ~ ” The ~ ~ - monolayer ’~J~ without benzene (Dojindo, spectroscopic grade), and t h e concentrations matrices could not be deposited onto solid substrates,2J1J2 were 1.0 x and 1.0 x mol L-’ for amC60 and c60, although the three-dimensionally aggregated multilayer respectively. The compression speeds of t h e molecules at t h e film^^,^-'^ and the mixed films of c60 with maand 1.7 x nm2 air-water interface were 4 x t r i ~ e ~ ~ , a~t the ~ , air-water ~ , ~ ~ , interface ~ ~ - ~have ~ been (molecule)-’ for amC60 and c60, respectively. The deposition of an amC60 monolayer was done with a moving-wall-type trough transferred to form LB films. (Nippon Laser & Electronics Lab.) equipped with a Wilhelmy In the light of such problems with G o , the most balance. The monolayers were transferred by using a vertical promising strategy for the fabrication of high-quality dipping method at a surface pressure of 10 mN m-l on to quartz monolayer films is to render the molecule amphiphilic by plates for W/vis absorption spectroscopy, gold films vacuumappropriate chemical modification12J7since the difficulties evaporated onto glass slides which had been hydrophobized with mentioned above seem to arise from the excessive hy1,1,1,4,4,4-hexamethyldisilazane for reflection-absorption indrophobicity Of C60. The efficacy of this strategy has been frared spectroscopy (RAIRS), CaFz plates for transmission partly proven for C60012and N-a~etylfulleropyrrolidine~~ infrared spectroscopy (TIRS), and freshly cleaved mica for AFM. molecules which form monolayers a t the air-water Characterization of LB Films. The uV/vis absorption spectra of a m & in the forms of a dichloromethane solution and interface. However, it has not been shown that the a n LB film were measured using a Shimadzu 265FS spectrometer transferred films have really well-defined monolayer with a slit width 2 nm. All infrared measurements were made structures. with a Perkin-Elmer Model System 2000 Fourier transform Since the balance between the hydrophilicity and the infrared spectrometer. The RAIRS and TIRS spectra of LB films hydrophobicity of a molecule is important in the formation were recorded at 4 cm-I resolution with t h e coaddition of 100 of a n LB film,23 it is reasonable to expect that a n scans using an MCT and a TGS detector, respectively. The amphiphilic molecule bearing a highly hydrophilic moiety incident angle of 80” was employed for RAIRS measurements. would make a good candidate for monolayer formation. AFM images were taken at ambient temperature with a Nanoscope 111, Digital Instrument. Scanner D was used for We have already reported the synthesis of such a scanning from 10 to 1 p m with applied forces of 20-60 nN. compound, a n amphiphilic water-miscible c 6 0 molecule Flattening procedures along t h e horizontal direction were done (referred to as amC60 hereafter) originally designed to to cancel t h e artificial deformations near the edges. For t h e investigate the physiological activities Of C60 (Figure 1).28 estimation of area fractions of monolayer, vacancy, and bilayer This compound bears a single polar head, exhibits a regions, 12 AFM images (2 x 2 pm2) were analyzed and the detergent-like character, and is the first compound of this average values were obtained. X-ray diffraction patterns of the class. Since this molecule meets the above criteria, we amCeo LB film were obtained by a Phillips PW 1800 using a Cu were much interested in investigating its film-forming target with a Ni filter. ability. This molecule has an aliphatic tether between the c 6 0 head and the polar which makes the molecule Results and Discussion readily soluble in common organic solvents: the molecule Monolayer of amCw at the Air-Water Interface. should be first dissolved in a spreading solvent for the LB Figure 2 shows n-A isotherms of amCso and c 6 0 on a pure film preparation. Because of this constraint, the parent water subphase a t 290 K, from which limiting areaAocan C6pbearing cyclopentanecarboxylicacid, which is insoluble be obtained by extrapolating the linear portion of the isotherm to 0 mN m-l. (23)Gaines, G. J. Insoluble Monolayers at Liquid-Gas Interfaces; It is clearly seen that the molecular area of 0.78 nm2 Wiley-Interscience: New York, 1966. (24)Kuhn, H.; Mobius, D.; Biicher, H. Physical Methods ofchemistry; for amC6o obtainable from Figure 2 is much larger than Weisserberger, A,, Rossiter, B. W., Eds.; John Wiley & Sons: New York, the value of 0.33nm2 for c60. This value for c60 is known 1972;Vol. 1, Part IIIB, pp 577-702. to depend on the concentration of the spreading solution,2J2 (25)Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990. and our current value is much larger than our previously (26)Ulman, A.A n Introduction to Ultrathin Organic Films: From reported value of 0.17 nm2 obtained with a higher Langmuir-Blodgettto Self-Assembly;Academic Press: Boston,MA, 1991. (27)Proceedings of the Sixth International Conference on LangmuirBlodgett Films. Thin Solid Films 1994,242-244. (28)Tokuyama, H.;Yamago, S.;Nakamura, E.; Shiraki, T.; Sugiura, Y. J . Am. Chem. SOC.1993,115,7918-7919. (29)Although the terms, “polar head” and “hydrophobic tail”, are often used in colloid chemistry, we like to use the terms ‘‘Cm head” and “polar tail” in the present study, taking into account the molecular shape of amCso.
(30)(a) Haufler, R. E.; Conceicas, J.; Chibante, L. P. E.; Chai, Y.; Bryne, N. E.; Flanagen, S.; Haley, M. M.; O’Brien, S. C.; Pan, C.; Xiao, 2.;Billups, W. E.; Ciufolini,M. A.; Hauge, R. H.; Margrave, J. L.; Wilson, L. J.; Curl R. F.; Smalley, R. E. J . Phys. Chem. 1990,94,8634-8636. (b)Allemand, P.-M.;Koch, A.;Wudl, F.; Rubin,Y.; Diedrich, F.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J . A m . Chem. SOC.1991,113, 10501051.
662 Langmuir, Vol. 11,No.2, 1995
Matsumoto et al.
75
r
E
z
. E
e
cn
2
P
al 25 ¶
v)
1
..-
Figure 2. Surface pressure-area isotherms of amCm (solid line) and c60 (dashed line) at the air-water interface at 290 K.
concentration of the spreading solution (1.0 x mol L-1).3a Because of its polar character, resembling a n aliphatic carboxylic acid, the amC60molecule in a monolayer a t the air-water interface is expected to have its hydrophilic carboxylic group facing the water side and the hydrophobic c 6 0 moiety protruding toward the air side. Assuming hexagonal packing of the c 6 0 moiety a t the airwater interface, as in the (111)face of face-centered-cubic lattice, we obtain the distance between neighboring c 6 0 molecules as 0.95 nm. This value is in good agreement with the value 1.00 nm for the c60 crystaP1 or the value 1.0-1.1 nm determined using STM for adsorbed layers on A ~ ( l l l ) The . ~ ~results also confirm that the area a t the air-water interface is governed mainly by the c60 moiety of the amC60molecule and not by the side chain. The monolayer of amC60were successfully transferred onto hydrophilic substrates using a moving-wall-type trough with a vertical dipping method. The deposition was carried out to form Z-type LB films and the transfer ratio was near unity. The presence of the carboxylicgroup should be responsible for the much higher transfer ratio than those previously reported for other c 6 0 derivatives having much lower hydrophilicity. The transferred films were used to investigate further the electronic, physical, and morphological structures. W/visAbsorption Spectra of the amCm LB Film. Figure 3 shows the absorption spectra of amC60 in the mol L-l form of a 10-layer LB film and a 1.0 x dichloromethane solution. In the solution spectra Of C60 derivatives lacking a single olefinic bond due to addition of organic residues, there are generally observed four absorptions around 260,320, 430, and 700 nm,33-39and this was also found to be the case for the dichloromethane solution of amC60. In the (31) Heiney, P. A.; Fischer, J. E.; McGhie, A. R.; Romanow, W. J.; Denenstein, A. M.;McCauley, J.P.,Jr.;Smith,A. B., I11 Phys. Rev. Lett. 1991, 66, 2911-2914 and references cited therein. (32)(a)Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; de Vries, M. S; Junziker, H. E.; Wendt, H. R. Nature 1990, 348, 621-622. (b) Wragg, J. L.; Chamberlain, J. E.; White, H. W.; Kratschmer, W.; Huffman, D. R. Nature 1991, 348, 623-624. ( c ) Howells, S.; Chem, T.; Gallagher, M.; Sarid, D.; Lichtenberger, D. L.; Wright, L. L.; Ray, C. D.; Huffman, D. R.; Lamb, L. D. Surf. Sci. 1992, 274, 141-146. (33) Creegan, K. M.; Robbins, J. L.; Robbins, W. K.; Millar, J. M.; Sherwood, R. D.;Tindall, P. J.; Cox, D. M.;Smith, A. B., 111; McCauley, J. P., Jr.; Jones, D. R.; Gallagher, R. T.; J . Am. Chem. SOC.1992,114, 1103-1105. (34) Prato, M.;Suzuki, T.;Foroundian, H.;Li, Q.; Khemani,K.;Wudl, F.; Leonetti, J.; Little, R. D.; White, T.;Rickborn, B.; Yamago, S.; Nakamura, E. J . Am. Chem. SOC.1993, 115, 1594-1595.
Wavelength I nm
Figure 3. UV/vis absorption spectra of amCso in the form of a 10-layer LB film (solid line) and a 1.0 x mol L-l
dichloromethane solution (dashed line).
spectrum of the am& LB film, however, the 260- and 320-nm peaks shift to longer wavelength, and the remaining two almost disappear, probably due to broadening. This change in the absorptions is a clear indication of the electronic interaction between the c 6 0 moiety of amC60 with adjacent molecules and, in turn, is in good agreement with the above conclusion that the c60 moiety is closely packed in the LB film. The red-shift of the bands has also been reported for the evaporated films of C60,40 the LB films of C60 with icosanoic and the LB films of C60012and f~lleropyrrolidine.~’ Molecular Orientation of the amCm LB Film. The IR spectroscopy provides useful information on the anisotropic arrangement of molecules in monolayers and LB f i l m ~ . ~ l The - * ~RAIRS and TIRS spectra are particularly powerful tools for the analysis of molecular orientation in the LB film, since these two spectra give us complementary information on the orientation of the transition moment for a given functional group with respect to the direction of the electric field of the incident light. For the amCGO molecule in its elongated conformation (as in Figure l ) , the transition moment ofthe asymmetric C-0-C stretch lies parallel to the molecular axis (“parallel stretch”), while those of C=O stretches, symmetric CH3 and CH2 stretches, and asymmetric CH2 stretch are nearly perpendicular to the molecular axis (“perpendicular stretches”). If the amC60 molecule in a monolayer is oriented with its molecular axis strictly orthogonal to the surface of the film, the perpendicular stretches in the RAIRS spectrum and the C-0-C “parallel”stretch in the TIRS spectrum will vanish (or be (35) Prato, M.; Lucchini, V.; Maggini, M.; Stimpfl, E.; Scorrano, G.; Eiermann, M.; Suzuki, T.;Wudl, F. J.Am. Chem. SOC.1993,115,84798480. (36) Maggini, M.; Scorrano, G.: Prato, M. J . Am. Chem. SOC.1993, 115, 9798--9799. (37)An. Y.-2.;Anderson, J. L.; Rubin, Y. J . O r-. . Chem. 1993, 58, 4799-4801. (38)Komatsu, K.; Murata, Y.; Sugita, N.; Takeuchi, K.; Wan, T.S. M. Tetrahedron Lett. 1993,34, 8473-8476. (39) Komatsu, K.;Kagayama,A.; Murata,Y.; Sugita, N.;Kobayashi, K.; Nagase, S.; Wan, T.S. M. Chem. Lett. 1993, 2163-2166. (40) Hebard, A. F.; Haddon, R. C.; Fleming, R. M.; Kortan, A. R. Appl. Phys. Lett. 1991,59, 2109-2111. (41) Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J . Chem. Phys. 1983, 78, 946-952. (42) Kimura, F.;Umemura, J.;Takenaka, T. Langmuir 1986,2,96101. (43) Mitchell, M. L.;Dluhy, R. A. J.Am. Chem. SOC.1988,110,712718. (44) Dote, J.L.;Mowery, R. L. J . Phys. Chem. 1988,92,1571-1575. (45) Gericke, A.; Hdhnerfuss, H. J . Phys. Chem. 1993,97, 1289912908.
Langmuir, Vol. 11,No. 2, 1995 663
Langmuir-Blodgett Film of CSOCarboxylic Acid
A0
0
I
:
H
H 0. ‘*.\
/”
‘H O Y O
/ w 3000
I
Figure 5. Schematic view of a facing dimer (a) and an outof-plane dimer (b).
2000 1800 1600 1400 1200 lo00
Wavenumberkm-*
Figure 4. M R S spectrumof a 10-layer LB film (a)and TIRS spectrum of a 20-layer LB film (b) of amCm. Table 1. Absorbance Ratio of “Perpendicular Stretch” to “Parallel Stretch” absorbance ratio
RAIRS
TIRS
very weak in practice). It may be noted that the elongated conformation of amC60 in a film is a reasonable assumption here in view of the organic conformational theory and the 2.6-nm interlayer distance of our LB film measured by the X-ray diffraction (vide infra). In Figure 4 are shown the RAIRS spectrum of a 10layer LB film and the TIRS spectrum of a 20-layer LB film. Most importantly, the two spectra were qualitatively the same, and all crucial bands were observed in common (and also found for a powder sample, spectrum not shown), indicating that the molecule is either tilted with respect to the film surface or takes random orientation in the film. Thus, bands due to the “perpendicular stretches” (C=O a t 1710 and 1735 cm-l, symmetric CH2 stretch a t 2850 cm-l, symmetric CH3 stretch a t 2870 cm-l, and asymmetricCH:! a t 2920 cm-l) and that due to the “parallel stretch”(C-0-C a t 1165cm-’) were observed with nearly equal relative intensity. The bands a t 1182 and 1429 cm-l were assigned to the c60 moiety. In order to obtain further information on the molecular orientation, we needed quantitative comparison of the intensity of the.absorbance. Since direct comparison of the intensity of the RAIRS and the TIRS spectra are known to be useless,46we compared the relative intensity of the bands due to “perpendicular stretches” using the intensity of the “parallel C-0-C stretch” as a reference. As seen in Table 1,the perpendicular/parallel ratios for the crucial stretches are always larger in the TIRS spectrum than in the RAIRS spectrum. Thus, this result excludes the random molecular orientation and indicates that, on average, the molecular axis, while tilted, is arranged perpendicularly with respect to the film surface. (46) Hansen, W. N. J . Opt. Soc. Am. 1968,58,380-390.
4 P Figure 6. AFM image of a single-layer LB film of amCm on mica (4 x 4 pm”.
-10
0
1
2
3
4
Distance/pm
Figure 7. Depth profile of the 1-layer LB film of amCco taken along a horizontal line in Figure 5.
In addition to the information on the molecular orientation, the IR spectra also gave information on the environment of the carboxylic terminus of amC60 in the film. As has been discussed above, the absorption due to the carboxylic acid appears at 1710 cm-l (instead of 1760 cm-l, which characterizes a monomeric carboxylic acid47). Evidently, the carboxylic acid of amC6o in the multilayer LB film is hydrogen bonded. There are two possibilities for the hydrogen-bonded s t r u c t ~ r e :one ~ ~is*due ~ ~ to “facing (47) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 4th ed.;John Wiley 8z Sons: New York, 1981; Chapter 3.
664 Langmuir, Vol. 11, No. 2, 1995
Matsumoto et al.
o\qoyO ?.ti?
o40
5i
2'"
04
0'
Po
Figure 8. Structural model of a multilayer LB film of am&,.
dimer" formation between molecules in two adjacent layers and the other due to "out-of-plane dimer" formation between two adjacent molecules in the same monolayer (Figure 5). The first possibility was ruled out, because the 1710-cm-l absorption was also observed for a singlelayer LB films (RAIRS and TIRS spectra not shown),which can only be reconciled by the hydrogen bonding within a single layer (thus out-of-planedimer formation). The same chemical environment of the carboxylic acid group in a single-layer and a multilayer LB film indicates the Z-type layer structure of the film and precludes the possibility of reorganization of the Z-type film into a Y-type one through overturning of the molecules. Although the structural details of such a dimer are not clear a t this time, the oblique orientation of the molecular axis (vide supra) together with the smaller cross section of the carboxylic group as compared with that of C60 would allow the formation of such a dimer under the closely packed monolayer environment. The total lack of absorption at 1760 cm-' indicates that all carboxylic acid groups are engaged in the dimer formation. Morphology of the amc60 LB Film. The surface of a single-layer LB film of am& was directly observed using AFM to investigate the surface roughness and the thickness of the film. Figure 6 shows a typical AFM image of the single-layer LB film of amC60. It was reproducibly found for all parts of the sample that the surface, ifnot completely smooth, was by far much smoother than the one obtained for the LB film of CEO or the mixed LB films with mat rice^.^ The quantified depth profile in Figure 7 indicates that the undulation of the surface is within f l nm for most of the surface. Close investigation of the surface reveals that the LB film consists of small domains ca. 0.1 pm in diameter. (48) Barraud, A,; Leloup, J.;Gouzerh, A,; Palacin, S. ThinSolidFilms 1985,133.117-123. (49)Jones, C.A.;Petty, M. C.; Roberts, G. G.; Davies, G.; Yarwood, J.;Ratcliffe, N. M.; Barton, J. W. Thin SolidFilms 1987,155,187-195.
There are two types of defects in the film: one is holes formed between the two-dimensionally aligned domains and the other is hills which are a three-dimensional stack of the domains. The diameters of the holes range from ca. 50 to 500 nm while the size of the hills is almost equal to that of a typical domain. As is seen from Figure 7, the depth of the hole is ca. 2-3 nm, the height of the hill also being ca. 2-3 nm, and these values agree nicely with the length of the amC60 molecule in its stretched form. This strongly suggests that the bottom ofthe hole is the surface of the substrate, mica, and that most of the area is covered with a n amC60 monolayer; the hills correspond to the bilayer regions of the film. Quantitative analyses of the surface undulation revealed that the area fractions of the monolayer, the vacancy, and the bilayer regions of the single-layer LB film are 88 f 7 , 7 f 5, and 5 f 3%,respectively. In these analyses, we assumed that the region for which the height is within fl nm with respect to the average height of the surface is a monolayer region: the regions higher and lower than that were assumed to be the bilayer and the vacancy regions, respectively. These results clearly demonstrate that the amc60molecule forms a smooth highquality LB film on a solid substrate. Layer Structure of the amc60 LB Film. The layer structure ofthe amCaoLB film was confirmed by the X-ray diffraction analyses. A broad peak at ca. 3.4"was observed in the X-ray diffraction pattern of a 14-layerLB film, which can be reduced into the repeat distance 2.6 nm ifwe assume that the peak is assigned to a (001)diffraction. This value, similar to the molecular length of am& in a stretched form, is consistent with the Z-type deposition and the AFM results.
Conclusions We have demonstrated above that the amphiphilic c60 is much superior to Cm or the previously known derivatives
Langmuir, Vol. 11, No. 2, 1995 665
Langmuir-Blodgett Film of C60 Carboxylic Acid
in film-forming ability and that high-quality LB films can be build UD on various substrates. Several features were noticed fir the LB film derived from a m C d (1) close packing of the Csomoiety, (2) oblique orientation of the molecular axis with respect to the film surface, (3) dimer formation of the carboxylic group within a monolayer, (4) smooth film surface, (5) repeat distance similar to the molecular length along the surface normal, and (6) Z-type deposition. From these observations, we propose a structural model of the amCsoLB film as shown in Figure 8. The LB film of this kind, with the c 6 0 moiety confined within a nanoscale plane, will set a stage for further
investigations into various areas such as the construction of molecular electronics. Acknowledgment. We are grateful to Dr. T. Kamata for stimulating discussion on the molecular orientation in the LB film. This research was supported in part by grants from the Ministry of Education, Scienceand Culture (the Grant-in-Aid for Scientific Research, No. 05403015, to E.N., and the Grant-in-Aid on Priority Area, No. 06224208, to S.Y.) and the Tokuyama Science Foundation to E.N. LA9408376
