Langmuir 1993,9, 32-35
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Preparation and Structural Characterization of Fullerene c 6 0 Langmuir Film Yasushi Tomioka,' Masayoshi Ishibashi, Hiroshi Kajiyama, and Yoshio Taniguchi Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-03, Japan Received June 25,1992. In Final Form: September 14,1992
A Langmuir-Blodgett f i i of pure Cw is prepared by using a subphase of an aqueous solution of phenol. The Cw molecules form a relatively homogeneous Langmuir film at the air-water interface without any matrix molecules. This homogeneity apparently results from the phenol molecules effectivelypromoting the spread of benzene solution containing Cw molecules on the water surface. These Langmuir f h are transferred onto a hydrophobic substrate using horizontal lifting method. Structural characterization of the Cw Langmuir films is carried out using high-resolution transmission electron microscopy. The Cw Langmuir film is found to have polydomain polymorphic structures, such as hexagonal and distorted hexagonal forms. The average size of these domains is several tens of nanometers in diameter.
Introduction The recent success in preparing macroscopic quantities of fullerenes has stimulated numerous studies of its chemical and physical properties,lV2 especially ita hightemperature superconductivity in alkali-metal-doped crystale3 So far, its physical properties have primarily been explored by using single crystals and polycrystalline films. Well-ordered films are required for a more precise and systematic investigation. Thin filmsof Cwmolecules have been fabricated using the epitaxial growth method on various substrates, including Si, MoSz, and alkali halide single These thin films have been reported to have closed-packing structures, such as hexagonal-closepacking (hcp) and/or face-centered-cubic (fcc) structures on most of the substrates, and also a corrugation structure of Cw molecules on the GaAs (l10).7 The LangmuirBlodgett (LB) technique is also considered to have the potential to construct organic thin films controlled on the order of the molecular level. Floating layers of CSOat the air-water interface were fiist reported by Obeng e t al.lo Mixed Langmuir films of Cw with icosanoic acid were then reported by Nakamura et al." Very recently, the lattice image of Cw filmsprepared on the pure water was reported
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Area per molecule / A 2
Figure 1. P A isotherms of fullerene C ~ on O pure water (dotted line) and phenol aqueous solution (solid line) subphaaes. by Long e t del2These fiis, however, are inadequate as a Langmuir f i because of their visible patchiness and inhomogeneities. In this letter, we will report on the preparation and structural characterization of relatively homogeneous CSOLangmuir films without any matrix molecules.
Experimental Section (1)KrHtschmer, W.; Fostiropolous, K.; Huffman, D. R. Chem. Phys. Lett. 1990,170,167. (2)Kriitschmer, W.; Lamb, L. D.; Fostiropolous, K.; Huffman, D. R. Nature 1990,347,354. (3)(a) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D.
Pure fullerene Cm was obtained from the Texas Fullerenes Corp. for use in our experiments. Its purity as checked by highpressure liquid chromatography (HPLC) was more than 98%. The CBOmolecules was dissolved in benzene to a concentration W.;Glarum,S.H.;Palstra,T.T.M.;Ramirez,A.P.;Kortan,A.R.Nature, of 1.0 X lo-' M. Pure water and an aqueous solution of phenol 1991,350,600. (b) Holczer, K.; Klein, 0.; Huang, S.; Kaner, R. B.; Fu, ((0.5-2.0) X M) were used as the subphase. K.; Whetten, R. L.; Diederich, F. Science 1991,262,1151.(c) Roseeinsky, The Langmuir films were prepared using a Langmuir trough M. J.; Ramirez, A. P.; Galarum, 5. H.; Murphy, D. W.; Haddon, R. C.; (Joyce Leobl, Langmuir Trough IV). The benzene solvent Hebard, A. F.; Palstra, T. T. M.; Kortan, A. R.; Zahurak, S. M.; Makhija, A. V. Phys. Reo. Lett. 1991,66,2830.(d) Tanigaki, K.; Ebbeeen, T. W.; was allowed to evaporate for at least 16 min prior to comSaito, S.; Mizuki, J.; Tsai, J. S.; Kubo, Y.; Kuroshima, S. Nature 1991, pressing. The surface pressure versus area per molecule (FA) 352,223. isotherm was measured at a compressing speed of about 4 (4)Ren,S.L.;Wang,Y.;Rao,A.M.;McRae,E.;Holden,J.M.;Hager,A2.molecule-l.min-l. The prepared film was transferred onto a T.; Wang, K.; Lee, W.; NI, H. F.; Selegue, J.; Eklund, P. C. Appl. Phys. hydrophobic surface using a horizontal lifting method. Optically Lett. 1991,59,2678. (5)Tong, W. M.; Ohlberg, D. A. A.; You, H. K.; Williams, R. S.; Anz, polished quartz and CaF2 plates, treated hydrophobically with S. J.; Alvarez, M. M.; Whetten, R. L.;Rubin, Y.; Diedrich, F. N.J . Phys. hexamethyldisilazane, were used for UV-visible and FT-IR Chem. 1991,95,4709. spectroscopy, respectively. (6)Sakurai, M.; Tada, H.; Saiki, K.; Koma, A. Jpn. J. Appl. Phys. UV-visible absorption spectrawere obtained by usinga Hitachi 1991,30,L1892. U-3400spectrometer. In situ UV-visible absorption spectra of (7)Li, Y. Z.;Chander, M.; Patrin, J. C.; Weaver, J. H.; Chaibanta, L. P. F.; Smalley, R. E. Science 1991,253,429. Langmuir films at the airwater interface were obtained with an (8)Achiba, Y.; Nakagawa, T.; Mataui, Y.; Suzuki, S.; Shiromaru, H.; Otsuka Electronics IMUC 7000 multichannel spectrometer, as Yamauchi, K.; Nishiyama, K.; Kainosho, M.; Hoshi, H.; Mitani, T. Chem. described elsewhere.lS FT-IR spectra were obtained by the Lett. 1991,1233.
(9) Ichihare, T.; Tanigaki, K.; Ebbensen, T. W.; Kuroshima, S.; Iijima, S . Chem. Phys. Lett. 1992,190,179. (10)Obeng, Y. S.;Bard, A. J. J. Am. Chem. SOC.1991,113,6279. (11) Nakamura, T.; Tachibana, H.; Yumura, M.; Mataumoto, M.; Azumi, R.; Tanaka, M.; Kawabata, Y.Langmuir 1992,8,4.
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(12)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. (13)Tomioke, Y.; Tanaka, N.;Imazeki, S. J. Chem. Phys. 1989,91, 5694.
Q 1993 American Chemical Society
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Langmuir, VoZ. 9, No. 1, 1993 33
Figure 2. Optical micrographs of fullerene CsoLangmuir films on the hydrophobic substrates, which were prepared on (a) pure water and (b) phenol aqueous solution. The arrow indicates one of the crystalline particles in the thick layers.
standard transmission method, using a Nicolet FT-IR SX-60 Fourier transform infrared spectrometer. Optical micrographs of the Langmuir films were obtained using a Nikon microscope FX-epi-pol. High-resolution transmission electron microscopy was done using a Hitachi H-9000UHR transmission electron microscope operating a t 300 kV on Langmuir film deposited on a holey carbon grid.
Results and Discussion The r-A isotherms of fullereneCm obtained on the pure water and on the phenol aqueous solution subphases are shown in Figure 1. In both cases, the monolayers exhibit condensed phases which are characterized by a steep rise of the isotherms upon compression. The Langmuir film on the pure water is remarkably rigid and visibly patchy, as seen in the optical micrograph (Figure 2a) of the film on the hydrophobic substrate. The area per molecule is about 15A2,which is obviously smaller than the ideal area per molecule, about 100 A2, as calculated from X-ray diffractiond a h 2 We could easily find out the morphology of the two-dimensionalcrystalline islands, such as aggregations formed immediately after spreading on the pure water surface. This behavior at the air-water interface resembles that of nonsubstituted phthalocyanine dye compounds.14 It was extremely difficult to reproduce the formation of a monolayer having the same r-A isotherm as reported by Obeng et al.l0 On the other hand, Cm molecules easily formed relatively homogeneousLangmuir film on the phenol aqueous solution. The limiting area per molecule was about 30 A2, which is still less than the above-mentionedideal value. The Langmuir film visibly collapsed at more than 35 "em-l. It seems that it is not a monolayer but has about three molecular layers. This estimate of the Langmuir film thickness is supported approximately by soft X-ray reflection measurement.15 The details of this will be described elsewhere. The spreadingbehavior over the phenol aqueoussolution was distinct from that of the pure water subphase and had little dependence upon the concentration of desolved phenol in the water subphase. It could be therefore ~~
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~
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(14) Baker, S.;Petty, M. C.;Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983,99, 53.
(15)Momose, A.; Hirai, Y.; Waki, I.; Imazeki, S.; Tomioka, Y.; Hayakawe, K.; Naito, M. Thin Solid Films 1989,178, 519.
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Wavelength /nm Figure 3. UV-visible absorption spectra of fullerene CM: (a) in situ spectrum of Langmuir film on phenol aqueous solution; (b) spectrum of LB films on a hydrophobic quartz substrate; (c) spectrum in hexane solution.
thought that the phenol molecules concentrate at the airwater interface and effectively promote the adsorption of Cm molecules and the spreading of the benzene droplet containing Cm molecules on the water surface. It will also be understood by the following. The so-called spreading parameter S for benzene droplet on the water subphase is described as S = yw - (yo+ ywo),where yw,yo,and ywo are interfacial tensions of air/water, air/benzene, and benzene/water, respectively. The dissolvingof phenol into pure water implies a substantial decrease of ywo,which is more than a decrease of yw. Thus, in this case of a phenolaqueous solution, an increase of the S reflects that Cm molecules which dissolved in benzene droplet over the phenol aqueous solution effectively spread before their aggegations. This would give the Cm Langmuir film more homogeneity than the pure water subphase, as shown in Figure 2b.
34 Langmuir, Vol. 9, No. 1, 1993
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Figure 4. High-resolution transmission electron micrograph of a fullerene Cm Langmuir film. The direction of the arrow indicates one of the linear grain boundaries. Original microscope magnification 20OOOOX. Table 1. UV Absorption Maxima ,A, States
of CSO in Various
states
hexane solution" Langmuir film on phenol aqueous solution" LB film" mixed LB film with icosanoic acidb vapor deposition film on NaCl(100)C vapor deposition film on MoSzd vapor deposition film on Si(l00)e film coated on quartzf
213 221 220 221.5 220 216
X,.,/nm 257 270 265 270.2 280 262.6 264
329 347 347 340 347.3 340 339.6 339
Taken from this experiment. b Taken from ref 11. Taken from ref 8. Taken from ref 6. e Taken from ref 4 (units are converted from eV into nm). /Taken from ref 2. a
The in situ UV-visible absorption spectrum of the C a Langmuir film on the phenol aqueous solution is shown in Figure 3a. This in situ spectrum is similar to that of the pure water subphase and is the same as that of the quartz substrate, as shown in Figure 3b. As the result, no obvious influence of phenol on the above-mentioned spreading behavior could be detected from the viewpoint of electronic interaction. The absorption spectra of the Cw LB film exhibited three strong bands at the red-shift positions of 347,270, and 221 nm and a broad band between 420 and 530 nm, compared with that of a hexane solution as shown in Figure 3c. The overall spectral features and the positions of the red-shifted bands resemble those of vapor deposition films on an NaCl(100) ~ u r f a c e . ~They, ,~ however, are slightly different from those of a mixed Langmuir film with icosanoic acid,ll deposition films on quartz,2and MoS2,6as shown in Table I. These distinctions might be attributable to the crystalline structures of fullerene films, as pointed out by Kritschmer et aL2and Achiba et aL8 In the FT-IR absorption spectra of the Cm LB films on the CaFz substrate, two strong lines at 1182 and 1428cm-l were observed. This FT-IR result was the
same as that of the vapor deposition film reported by Bethune et al.16 No signs attributable to the -OH stretching of the remained phenol and/or water in the film could be observed between 3000 and 3500 cm-l. Thus, it was confirmed that the Langmuir films consist mostly of Cm molecules and also that the remaining phenol or water are rarely found in the transferred Langmuir films. The typical high-resolution transmission electron micrographs of the Cm Langmuir film are shown in Figure 4. It can be seen that there are polydomain structures with such polymorphisms as hexagonal packing and distorted hexagonal packing, as well as amorphous-like disordered forms. The average size of these domains is several tens of nanometers in diameter. However, the homogeneity of this film was achieved on the macroscopic scaleas mentioned above, compared with the f i i prepared on pure water. Most vapor-deposition films are reported to have closely packed crystalline structures, such as hexagonal and/or face-centered cubic structures, due to their epitaxy and large migration energies. Compared to these vapor-deposition films, it should be noted that the Cm Langmuir film polymorphisms include an amorphouslike disordered form. The existence of these polymorphisms indicates that the Cm Langmuir film has nucleated separately and then grown together. These forms may result from the rapid cooling upon vaporization of the spreading benzene solvent which produces Cm molecular disordered structures as a metastable phase on the water surface. Moreover,the linear domain boundaries,such as a dislocation,can frequently be found in the Cm Langmuir film. This phenomenon is probably attributable to the nature of c60. In summary, we have demonstrated that a relatively homogeneous Cm Langmuir film can be prepared by using (16)Bethune, D. S.; Meijer, G.; Tang,W. C.; Rosen, H. J.; Golden, W. G.; Seki, H.; Brown, C. A.; de Vries, M. S. Chem. Phys. Lett. 1991,179, 181.
Langmuir, Vol. 9, No.1, 1993 36
Letters a subphase of phenol aqueous solution. The produced Langmuir films were confirmed to have polydomain polymorphic structures that include an amorphous-like disordered form. The thickness and surface smoothness of the Cm Langmuir film are now under detailed investigation by usingsoftX-ray reflection measurement.15 The superconductivity and other electric properties of the Cm Langmuir film doped alkali metal are also now in progress.
Acknowledgment. We wish to express our gratitude to Drs. Naoki Tanaka and Ataushi Momoae for their assistance with the FT-IR and soft X-ray reflection measurements, respectively.
Registry number supplied by author: Cm, 9968596-8.
