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Langmuir 1992,8, 4-6
Formation of Langmuir-Blodgett Films of a Fullerene Takayoshi Nakamura,. Hiroaki Tachibana, Motoo Yumura, Mutsuyoshi Matsumoto, Reiko Azumi, Motoo Tanaka, and Yasujiro Kawabata National Chemical Laboratory for Industry, Higashi 1-1, Tsukuba, Ibaraki 305, Japan Received October 14, 1991 The Langmuir-Blodgett (LB)films of the fullerene (CW)were formed with and without matrix molecules. The pure c60 formed a collapsed film at the air-water interface. The surface of the transferred film was inhomogeneous observed by atomic force microscopy (AFM). The monolayers of a 1:l mixture with icosanoic acid were transferred onto substrates with good quality. The LB films had a smooth surface composed of a mixture of c 6 0 and icosanoic acid dispersed with flat microcrystals of CSOobserved by AFM. The 1:l mixture with dioctadecyldimethylamonium perchlorate was deposited as Z-type film onto the substrates. The absorption spectra of both of the mixed films were in good agreement with those of evaporated films. These films will be precursors of the molecular systems utilizing the peculiar properties of fullerenes and fullerene anions. Since macroscopic amounts of fullerenes were synthesized from carbon soots,1 this class of highly symmetric compounds have spurred intense interest from the viewpoint of their chemical, electronic, and physical properties. More recently, the superconducting transition was observed in the forms of evaporated films2 and powder^.^^^ The Langmuir-Blodgett (LB) technique is one of the most powerful tools to construct organic thin films whose structures are controled at the molecular level. Usually, the fib-formingmaterials should be amphiphilic, although a few molecules without long alkyl chains formed LB films.4 In some cases, the functional molecules which do not have film-forming ability at the air-water interface by themselves can be incorporated in the LB films by mixing with film-forming materials. For example, the LB films with metallic conductivity were constructed using a mixture of a metal-dmit complex and icosanoic acid.5 Various functional systems utilizing peculiar properties arising from the three-dimensional nature of the fullerenes with be realized, when fullerenes are included in the LB system, preserving its symmetry without any chemical modification. In the course of our study, Obeng and Bard reported the floating layers of c 6 0 at the air-water interface.6 Here, we will report the formation and characterization of LB films of c60 with and without matrix molecules. The fullerene, c60, was obtained from carbon soots as described in the literat~re.~*B The surface pressure-area (r-A) isotherm was obtained on a pure water using a
Mean area per molecule
/ nm2
Figure 1. r A isotherms of CW(- - -), 1:lmixture with icosanoic acid (-), and 1:l mixture with DODMAP (- -).
-
I
I
A
Wavelength / nm
Figure 2. Absorption spectra of CW: (-) 42-layered sample of 1:lmixed PB film of CWand icosanoicacid; (- - -) hexane solution.
LAUDA film balance at a barrier speed of 2 cm/min at 17 "C. Benzene was used as a spreading solvent. The deposition was performed at 25 mN/m onto quartz (1)Krhtschmer, W.; Fostiropoulos, K.; Huffman, D. R. Chem. Phys. substrates hydrophobized with hexamethyldieilazane.The Lett. 1990, 170, 167. absorption spectra were measured by a Shimadzu UV(2) Hebard, A. F.; Rosseinsky, M. J.; Haddon, R. C.; Murphy, D. W.; 3100 spectrophotometer. An atomic-force microscope Glarum, S. H.; Palstra, T. T. M.; Ramirez, A. P.; Kortan, A. R. Nature 1991,350,600. (AFM) image was obtained by a Nanoscope 11, Digital (3) (a)Holczer,K.;Klein,O.;Huang,S.;Kaner,R.B.;Fu,K.;Whetten, Instrument, using graphite (HOPG) substrates. R.L.;Diederich,F.Science 1991,252,1154. (b) Roaseinsky,M. J.;Ramirez, Figure 1shows the r-A isotherms of CSOwith and without A. P.; Galarum, S. H.; Murphy, D. W.; Haddon, R. C.; Herbard, A. F.; matrix molecules. The abscissa is the mean area per Palstra, T. T. M.; Kortan, A. R.; Zahurak, S. M.; Makhija, A. V. Phys. Reu. Lett. 1991, 66, 2830. (c) Tanigaki, K.; Ebbesen, T. W.; Saito, S.; molecule. Pure c 6 0 exhibited an area per molecule of 0.17 Mizuki, J.; Tsai, J. S.; Kubo, Y.; Kuroshima, S. Nature 1991, 352, 222. nm2 at 25 mN/m.g c 6 0 molecules closely packed in two (d) Kelty, S. P.; Chen, C.; Lieber, C. M. Nature 1991, 352, 223. dimensions have an area of ca. 1 nm2 calculated from the (4) (a) Kovacs, G. J.; Vincett, P. S.; Sharp, J. H. Can. J. Phys. 1985, 63,346. (b) Snow, A. W.; Barger, W. R.; Klusty, M.; Wohltjen, H.; Jarvis, cell parameters of the crystals Of C60.l~Consequently, C ~ O N. L. Langmuir 1986,2,513. molecules exist in the form of a multilayer at the air( 5 ) Nakamura, T.; Kojima, K.; Matsumoto, M.; Tanaka, H.; Tanaka, M.; Manda, E.; Kawabata, Y. Chem. Lett. 1989,367. (6) Obeng, Y. S.; Bard, A. J. J. Am. Chem. SOC.1991, 113, 6279. (7) Haufler, R. E.; Conceicao, J.; Chibante, L. P. F.; Chai, Y.; Byrne, 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. (8) 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.1991,113,1050.
0743-7463/92/2408-0004$03.00/0
(9) Obeng and Bard reported that the monolayer film of Cw was formed when arelativelysmallamountwasspreadona watersurface. Thelimiting area of this film was 0.96 nm2 (see ref 6). (IO)KrHtachmer, W.; Lamb, L. D.; Fostiropoulos, K.; Haffman, D. R. Nature 1990, 347, 354. (11) Hare, J. P.; Kroto, H. W.; Taylor, R. Chem. Phys. Lett. 1991,177, 394.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 1, 1992 5
Letters a
b
Figure 3. AFM image of a one-layered sample on the HOPG substrate: (a) Cm and (b) 1:l mixture of Cm and icosanoic acid.
~
water interface. The floating layer at the water surface could be transferred onto substrates by the horizontal lifting method. However, the surface of the film was wet due to disorders present in the film. The 1:l mixture with icosanoic acid exhibited a reproducible T-A isotherm. Taking the area of icosanoic acid (0.2 nm2at 25 mN/m) into account, the area per molecule of c 6 0 is 0.07 nm2 at 25 mN/m. The mixed film with icosanoic acid was transferred onto solid substrates by the horizontal lifting method, and the surface of the resultant LB films was dry in contrast to the case of pure c 6 0 film. The 1:l mixture with dioctadecyldimethylamonium perchlorate (DODMAP) also showed a reproducible T-A isotherm, and the calculated area per c 6 0 was 0.06 nm2, similar to that of the case in the mixture with icosanoic acid. The floating layer was transferred by usual LB method (vertical dipping method) through Z-type deposition. The transfer ratio was 0-0.4 for the downstroke and almost unity for the upstroke. The absorption spectra of both of the 1:l mixed LB films resembled that of an evaporated film, reported previously.1° Figure 2 shows the UV-visible spectra of a
1:l mixed LB film of
c60
and icosanoic acid and that of
CSOin hexane solution. The spectrum of the LB film exhibits three bands in the UV region at 340,265, and 220 nm, characteristic of C60. All of these peaks are red-shifted by ca. 10nm compared with those of the solution sample.ll The onset of the absorption occurs around 700 nm, which agrees with the results of evaporated film on CaF2.12 The relationship between the layer number and the absorbance maxima of the three peaks in the UV region was examined to clarify the quality of the 1:l mixed LB films of c 6 0 and icosanoic acid. All the peaks are proportional to the layer number, showing that the deposition of the film was carried out reproducibly and that the microscopic environment of the c 6 0 molecules in the LB film remains unchanged with increasing layer number. Figure 3 shows the AFM imagesofthe transferred layers of C60 with and without icosanoic acid onto HOPG. In the case of pure c 6 0 film, large crystallites and naked graphite surfaces are seen (Figure 3a). The shape of crystallites resembles that of the crystals obtained from benzene (12) Reber, C.; Yee, L.; McKiernen, J.; Zink, J. I.; Williams, R. S.; Tong, W. M.; Ohlberg, D. A. A.; Whetten, R. L.; Diederich, F. J. Phys. Chem. 1991,95,2127.
Letters
6 Langmuir, Vol. 8, No. 1, 1992
Figure 4. The same AFM image of Figure 3b enlarged 10 times in height.
solutions observed by AFM. The height of the crystallites was ca. 100nm. The shape of crystallitesof Cm seems to be maintained during the transfer process. The large area of mixed graphite, however, suggests that not all of the floating layer was transferred from the water surface. The 1:1mixed film gave a flat surface as is seen in Figure 3b compared with pure c60 film. The image enlarged 10 times in height is shown in Figure 4. The naked graphite surface could not be observed. The bright part seen in the figure should be crystallites of c60. The height of the crystallites, however, is at most 5 nm, comparable to that of the monolayer thickness of icosanoic acid and much smaller than that in the films of pure c60. Taking the size of c 6 0 into account (ca. 1nm in diameter), only a part of the c 6 0 molecules in the film are attributed to the crys-
tallites of this type: the other Cm molecules should be present under the flat surface. The studies on the detailed structure of the film are now in progress. We have developed the LB films of a fullerene molecule by mixing with matrix molecules. Further efforts are also needed to construct LB films of c 6 0 anions. The c60 molecules in the mixed LB films with alkylammonium salts can be reduced chemically or ele~trochemicallyl~ to form the radical salts with alkylammonium cation, providing electrically and magnetically functionalized films. Registry No. Cw,99685-96-8; eicosanoic acid, 506-30-9. (13)Jehoulet, C.;Bard, A. J.; Wudl, F.J. Am. Chem. SOC.1991,113, 5456.
