Langmuir 1992,8,2980-2984
2980
Morphological Study of Thin-Film Systems of Pure Fullerene (c60) and Some Other Amphiphilic Compounds on the Electron Microscopic Scale Makio Iwahashi' Department of Chemistry, School of Hygienic Sciences, Kitasato University, Sagamihara 228, Japan
Koichi Kikuchi, Yohji Achiba, and Isao Ikemoto Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan
Toshinari Araki, Tadashi Mochida, and Sei-ichi Yokoi Frontier Technology Research Institute, Tokyo Gas Co. Ltd., Tsurumi-ku, Yokohama 230, Japan
Akira Tanaka and Keiji Iriyama Institute of Medical Science, The Jikei University School of Medicine, Minato-ku, Tokyo 105, Japan Received April 1, 1992. In Final Form: July 8, 1992 Morphologicalstudy of thin-film systemsof pure fullerene (Cdand some other amphiphilic compounds adsorbed on glass plates was conducted on the electron microscopic scale. Each component in benzene was placed on the aqueous subphaee of distilled water (pH 5.8) according to the Langmuir technique to form ita thin film; the thin film was transferred onto one side of a glass-plate surface by the subphaselowering method. Each film-adsorbedsite was replicated with a plasma-inducedpolymerized naphthalene film. The glass plates thus surface-treated were gradually dipped into 5% fluoric acid solution in order to obtain their respective single replica stage f i e for transmission electron microscopic observation. For O was able to show numerous necklace-like patterns forming example, the replica image of the C ~thin-film a two-dimensional network on the electron microscopic scale. This network structureprovides the possibility of applicational utilities such as a superconductive thin-film.
Introduction The structures of mono- and multilayer systems of amphiphilic compounds have been the subject of continuous studies since the discoveries by Langmuirl and Blodgett.2 As pointed out by K a h 3there was a question for a long time, for an example, as to why some amphiphilic materials could exist as rather stable insoluble monolayers above their equilibrium-spreading pressure (ESP), as exemplified by the solid state of the long-chain acid monolayers. In general, film-formingmaterials spread on a water surface doea not always take an uniform monolayer geometry at a surface pressure above their ESP: Namely it is important to know how the prepared film exista on an aqueous surface. An old saying, seeing is believing, seems to be still valid in science. As pointed out elsewhere:+ electron microscopists have therefore developed a number of techniques to identify, localize, quantify, and understand the dynamica a t the ultrastructural level or molecular level. For example, the plasma-polymerization replica (PPR) technique coupled with transmission electron microscopy (TEM), which Author to whom all correspondence should be addressed. (1) Lan uir, 1. J. Am. Chem. SOC.1916,98,2221. (2) B I Zett, K. B. J. Am. Chem. SOC.1985,60,1007. (3) &to, T. hngmuir ISSO,6,870. (4) T d a , A.; Yamaguchi, M.; Iwaanhi, T.;Iriyama K. Chem. Lett. 1989,1219.
(6) Iriyama, K.; Araki, T.;Shimada, N.; Iwamato, M.; Sasaki, T.; Atuzawa, M. Thin Solid F i l m 1991,201,176. (6) Araki,T.;Oinuma, 5.;Iriyama, K. Langmuir 1991,7, 738.
was originally developed by Tanaka et al.' for the electron microscopic observation of biomaterial microsurface, has been also found to visualize the surface microstructures of various other materials such as Langmuir-Blodgett (LB) films4-' and trenched silicon wafers.' Recently, we used the subphase-loweringmethod (SL method) for the deposition of an ultrathin (not a monolayer, but monolayer-like or very thin) film on an aqueous subphase even at a surface pressure of 0 mN/m onto a solid substrate. The SL method coupled with the PPR technique favorably allow us to study the so-called marbling with amphiphilic molecules on the electron microscopic scale.8 We call here L-film as a so-called thin film which does not always mean a true Langmuir monolayer (L-monolayer). Of interest is also to study the behavior of componente without any long alkyl chains placed on an aqueous subphase. Recently, Obeng and Bards reported for the first time an L-film of fullerene (Cm) without any long alkyl chain as a hydrophobic moiety. Their experimental resulte suggested that bi- or multilayers of Cw were formed instead of the ideal Langmuir monolayer (L-monolayer) on an aqueous subphase and also that deposition of an L-film on a variety of solid substrates according to the (7) Tanaka,A. Denaikenbikyou (Electron Microscopy) 1992,26,141 (in Japanese). (8) Iriyama, K.; Araki, T.; Tanaka, A.; Iwaeaki, T. Chem. Lett. 1990, 866. (9) Obeng, Y. S.;
Bard,A. J. J. Am. Chem. SOC.1991,113,6279.
0743-7463/92/2408-2980$03.00/0 Ca 1992 American Chemical Society
Langmuir, Vol. 8, No.12,1992 2981
A Monolayer of Pure Fullerene
vertically dipping (VD)method was unable to be carried out. Nakamura e t al.l0also missed to transfer a monolayer (probably monolayer-like film) of pure Cm onto a solid subtrate by the VD and horizontally lifting (HL) methods. In the present study, we were able to transfer an Efilm (not an Emonolayer) of pure Cm onto a glass plate surface according to the SL technique. As a result, the surface microstructure of the Cm was electron microscopically visualized by using the PPR method coupled with TEM. In addition two other film-deposition techniques such as the HL method, originally proposed by Langmuir and Shaefer" to transfer an L-monolayer12 or two L-monolayers13onto a solid substrate for a onetime lifting process, and the VD method were examined for the preparation of multilayers of some other film components as a preliminary study.
Experimental Section
Figure 1. A replica image of one of the glass plates which are commercially available and were used through this study (X200000).
50h
Materials. Cm was prepared according to the procedure recently developed by Kikuchi et al.14 Briefly, fullerenes were separated by high-performance liquid chromatography (HPLC) with benzene from carbon soot. The mass analysis confirmed the complete separation and isolation of CSOon a column for HPLC. CSOwas passed through a column for the HPLC purificationjust before ita film study. L-a-dioleoyllecithin(DOL) purchased from Sigma was further purified on an HPLC column before use according to the procedure previously employed.ls Tetrakis(dimethyldioctadecy1a"onium)nickel phthalocyaninetetrasulfonate (Pc4) was originally synthesized and was then purified on an HPLC column. Apparatus for the Film Study. All the film studies were conducted by using a temperature-programmable Langmuir trough originally proposed by Kato and Akiyama.lg The sensitivity of the temperature-sensing system was found to be less than 0.01 "C. The fluctuation of the temperature of the water surface and the air above the aqueous surface was found to be within 0.02 "C. The sensitivity of the surface balance was less than 0.01 mN/m. Film Preparation. An appropriate volume of each spreading solution was delivered onto an aqueous subphase a t 22 f 0.02 "C, unless otherwise stated. The ultrathin (monomolecular, monolayer-like or very thin) film deposition onto a glass plate was conducted for 15 min after the L-film preparation. For the the SL method, an outlet of aqueous subphase with a faucet was equipped on the bottom of the Langmuir trough as schematically illustrated elsewhere.s Electron Microscopic ObservationTechnique. In order to replicate the surface microstructures of the respective samples, the plasma-initiated polymerized naphthalene (PPN) films were prepared on the sample surfacesaccordingto the same procedures as employed in our previous As a preliminary study, the transmission electron microscopic examination of a PPN film prepared on one of the glass plates'' used through this study was conducted (see Figure 1). As exemplified in Figure 1, numerous fiberlike patterns without any structural defects such as micropore formation and/or irregular granular-texture formation were visualized on the expanded scale (X200000), and ~~~
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(10)Nakamura. T.; Tachibana, H.; Yumura, M.; Mataumoto, M.; Azumi, R; Tanaka, M.; Kawabata, Y. Lanlpmuir 1992,8,4. (11) Langmuir, I.; Shaefer, V. J. J . Am. Chem. SOC.1938, 60, 1351. (12) Fukuda, K.; Nakahara, H.; Kato, T.J. CoffoidInterface Sci. 1976, 54,430. (13) Iwahashi, M.; Naito, N.; Watanabe, N.; Seimiya, T. Chem. Lett. 1985,187. Iwahashi,M.; Naito, N.; Watanabe, N.; Seimiya,T.; Morikawa, N.; Ohahima, M. Bull. Chem. Soc. Jpn. 1985,58, 2093. (14) Kikuchi, K.; Nakanara, N.; Honda, M.; Suzuki, S.; Saito, K.; Shiromaru, H.; Yamaguchi, K.; Ikemoto, I.; Kuramochi, T.; Hino, S.; Achiba, Y. Chem. Lett. 1991, 1607. (15) Iriyama, K. J . Membr. Biol. 1980,52, 115. (16) Kato, T.; Akiyama, H. Nippon Kagaku Kakhi (J. Chem. Soc. Jpn. Chem. Indust. Chem.) 1991,1027 (in Japanese). (17) Iriyama, K.; Yoshiura, M.; Ozaki, Y.; Ishii, T.; Tasui, S. Thin Solid Films 1985, 132, 229.
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Figure2. Surface pressure-area (FA) relationship for pure Cgo film on the surface of distilled water a t 22 "C. Arrows (a, b, and c) indicate the positionswhere the Cafilm depoetionswere carried out. the mean width of the patterns was estimated to be 0.2 nm. The micrograph was overfocused in order to strengthen the fiberlike patterns.
Results and Discussion Chemical Stability of Cw. Chemical stability of Cm molecules in Efilm systems was checked before and after their film studies according to the same precedures as employed e1~ewhere.l~ It was found that the Cmmolecules were not chemically decomposed under the experimental conditions employed in this study, keeping the original fast-atom-bombardment mass spectrum of pure CM. Surface-Pressure Area Curve of a C w L-Film, A 2400-pL portion of 0.05 mM CM solution in benzene was gradually introduced to about 900 cm2of deionized doubly distilled water (pH 5.8) a t 22 f 0.02 "Cto prepare a L-film of Cm. After the film preparation for 15 min, its surface pressurearea (FA) curve was recorded as demonstrated in Figure 2. The obtained 7r-A isotherm was almost identical with those reported by Nakamura e t al.l0and by Williams e t al.18 We were unable to find any remarkable temperature-dependence of the F A characteristics, over the temperature range 5-40 "C. The limiting area obtained from the F A curve is ca. 0.27 nm2/molecule. The average (18) Williams, G.; Pearson, C.; Bryce,M. R.; Petty, M. C. Thin Solid Films 1992,209,150.
2982 Langmuir, Vol. 8, No. 12, 1992
molecular radius roughly calculated for the value of the limiting area is 0.28 nm. Obeng and Bardg found that, when an ultra-thin film of Cm was prepared by spreading 100 pL of 0.05-0.1 mM solution on an original aqueous surface area of 400 cm2,the limiting area per molecule was 0.96 nm2;the molecular radius calculated from the value of the limiting area was 0.56 nm. The molecular radius of Cm is near that obtained by STM and X-ray powder diffraction.'* In addition, Obeng and Bardg found that, when L-films of Cm were prepared from "larger sample sizes" (presumably more concentrated solutions or large amount of solutions), the molecular radius of Cm in the films was always 0.35 nm smaller that that found by STM and X-ray powder diffraction studies, probably due to the formation of bi- or multilayers of Cm instead of an L-monolayer. In our study, each mother L-film was also prepared by spreading a larger amount of Cm solution on an aqueous surface. Namely the spreading amount or concentration of solution seems to influence the limiting area of Cm;a large amount of the spreading material (Cm) results in progressing the multilayer formation. In fact Williams et al.l* reported that a diluted spreading solution produces a more expanded isotherm with an area per molecule of approximately 0.60 nm2 than the original spreading solution (200 pL of -0.7 mM solution) does. They also reported that, after a number (-6)of successive compressions for the film prepared from the original spreading solution, a repeatable isotherm was recorded; the limiting area for the isotherm was 0.28 nm2, which is almost equal to our value of 0.27 nm2. Long e t aL20 also obtained the limiting area of ca. 0.31 nm2 by using 200 pL of 0.99 mM spreading solution. From this information and evidence it can be concluded that the thickness of the multilayer of Cm prepared on a water surface, especially by the spreading of a large amount of sample, is presumably constant. Deposition of an Ultrathin Film of Cm on a Solid Substrate. Obeng and Bardg were unable to transfer a L-film of pure Cm onto any solid substrates by using the VD method. On the basis of findings by Obeng and Bard? Nakamura e t al.l0 tried to transfer a L-film of pure Ca onto a graphite film according to the procedure (HL method) improved by Fukuda e t al.12 and pointed out that the film-adsorbed graphite surface was wet due to disorders present in the film on the basis of the atomic force microscopic image of the film surface. Williams et al.I9 also reported that the transfer of pure C a by the VD method was poor and that &type deposition (i.e., film transfer on the upstroke only) with transfer ratios of less than 0.6 was typical. Long e t aL20 reported that the Cm film transferred onto an electron microscope copper grid coated with Formvar film showed straight lattice fringes, which were uniformly spaced, defects, dislocations, and disordered arrangement of C a molecules. Such morphological appearances were dependent on the observed position in the transferred film. As described above, Ca molecules placed on an aqueous subphase tend to form their bi- or multilayer structures in fairly condensed states. We aimed to study a L-film of Ca a t a surface pressure of 0 mN/m on the electron microscopic scale in order to understand the behavior of Cm molecules before the decrease of a film-covering aqueous surface. A t first, we conducted the selection of film preparation methods for the transfer of a Cm film on an aqueous (19) Kratschmer, W.; Lamb, L. D.; Fostiropoulos,K.; Haffman, D. R Nature 1990,347,354. (20) Cheng-Fen Long Yu Xu; Fan-Xiu Guo; Yu-Liang Li; Duan-Fu Xu; You-Xin Yao; Dao-Ben Zhu Solid State Commun. 1992,82,381.
Iwahashi et al. 4
.* . I
Figure3. A typical replica imageof the DOL monolayerprepared by the HL method (XlooOO).
Figure4. A typical replicaimageoftheDOLmonolayerprepared by the SL method (XlooOO).
subphase onto a solid substrate. The VD method does not allow the deposition of a L-film or L-monolayer at a surface pressure of 0 mN/m. The HL method~ll-'~ seem to allow principally the deposition of a loosely packed L-film, even at a surface pressure of 0 mN/m, onto a solid substrate. However, it has generally been recognized that the deposition of a DOL L-monolayer onto a solid substrate at a low surface pressure is a difficult task with the values for deposition ratio less than unity. For a better understanding of the limitation of the HL methods, we demonstrate here a replica image of the DOL monolayer transferred on the glass plate, which was previously covered with three monolayers of cadmium arachidate, as an example (see Figure 3). Aa shown in Figure 3, numerous fragmentlike patterns with alopecia areata-like one8were visualized on the electron microgcopic scale. The patterns might result from the repulsion between the solid substrate and the DOL monolayer. Figure 4 shows a replica image of a DOL monolayer, when the film was adsorbed directly on a glass plate under the surface chemical conditions as used in Figure 3 according to the SL method. As electron microscopicallyexemplified in Figure 4, the SL method allows the deposition of a DOL L-monolayer without any structural defects. Comparison of Figures 3 and 4 suggests that the SL method is an appropriate technique for the transfer of a loosely packed L-film or L-monolayer onto a solid by its nature. This conclusion was also confirmed for a L-monolayer of P c 4 The rigid L-monolayer of Pc4 deposited onto a glass plate by the VD method collapsed during the course of dipping and lifting cycles for the monolayer deposition, while the
A Monolayer of Pure Fullerene
Figure 5. A typical replica image of a CWL-film deposited on a glass plate at 1 = 1.0mN/m by the SL method (seethe position pointed out by arrow b in Figure 2) (X3000).
SL technique did not allow any introduction of film structural defect2*in the obtained LB film. On the basis of the information described above, we conducted the Cm L-film transfer onto a glass plate by the SL technique. Transfer ratio of Cm onto the glass plate was almost unity, which was estimated from the fact that the total film area, including the surface area of the substrate glass plate, did not change under a constant surface pressure even after the appearance of the substrate glass plate on the water surface and the draining-off of water from the glass plate which was set with a slight inclination; the slight inclination was for the draining of water. A separate experiment indicated that the surface pressure change due to the water-level loweringwas almost negligible in this experiment. Surface Microstructure of a Cm L-Film on the Electron Microscopic Scale. Figure 5 shows a replica image of a Cm L-film on a glass plate. The adsorption of the film onto the solid substrate was conducted under the constant surface pressure automatically well-controlled a t 1.0 mN/m (see the position pointed out by an arrow b in Figure 2). In Figure 5,numerous network-like patterns are visualized indicating that when Ca molecules were placed on the surface of highly purified water, they had a tendency to form not an L-monolayer, but an L-film. The replica image reveals that there was the aqueous surface area without Cm molecules after the Efilm preparation on it. On the basis of the information that the limiting area of a Cm L-film was smaller than that obtained by STM and X-ray powder diffraction studies, together with the information from Figure 5, we assume that the electron microscopically visualized network patterns should have their three-dimensional structures. Figure 5 was further expanded as demonstrated in Figure 6 to visualize the more detailed structure of the network patterns. Figure 6 reveals the fact that the network patterns consist of linearly combined particle-like clusters. Further decrease of a film-covering area by an automatically movable barrier did not induce any remarkable change of replica images. Figure 7 exemplifies a replica image of a single ultrathin film of Cm on a glass plate which was prepared a t a surface pressure of 17 mN/m. Kumaki22has reported TEM observation of polystyrene monomolecular particles on water surface. He observed that a network pattern of polystyrene monomolecular particles resembles that for Cm. The situation on the polystyrene system is very similar to the Cm system except (21) Iriyama, K.; Araki, T.; Shimada, N.; Yokoi, S.;Ozaki, Y.; Iwasaki,
T.Thin Solid Films, in press.
(22) Kumaki,J. Macromolecules 1986,19,2258.
Langmuir, Vol. 8, No. 12, 1992 2983
Figure 6. An expanded replica image of Figure 5 (X4oooO). -
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Figure 7. A typical replica image of a CWL-film deposited on a glass plate at 1 = 17 mN/m by the SL method (see the position pointed out by arrow a in Figure 2) (X2oooO).
Figure 8. A typical replica image of a CWEfilm deposited on a glass plate at 7r = 0 mN/m by the SL method (see the position pointed out by arrow c in Figure 2) (X2oooO).
for the size of the particles; Le., particles on the water surface are molecular spheres having no hydrophilic part. A preliminary study was conducted to understand the mechanism for the formation of the network-like patterns. A Cm L-film was adsorbed on a glass plate a t a surface pressure of 0 mN/m (see the position pointed out by an arrow c in Figure 2) and the obtained film-coated plate was replicated with a PPN film for the electron microscopic observation (see Figure 8). Numerous isolated clusters existed in Figure 8, although the initial formation of the fiberlike patterns was also visualized. The information from Figures 6 and 8 suggests that the necklace-like
2984 hngmuir, Vol. 8, No. 12, 1992
Figure 9. A typical expanded replica image of Figure 7 (X200000).
structures formed after the initial fomation of numerous particle-likeclusters were further networked. In this study, we missed visualizing a single molecule of Cm on a glass plate, although the resolution of the PPR method used in this study was found to be 0.6 nm. A typical expanded replica image of Figure 6 is shown in Figure 9. We aimed to estimate conventionally the numbers of Cw moleculespiled up perpendicularly against the aqueous surface. A t first, the mean number of Cm particle-like clusters per 1 x IO6 nm2 in several areas of a Cm L-film adsorbed on a glass plate, whose replica image is shown in Figure 6,was counted to be 273 particles. The average diameter of one Cm cluster is estimated to be 52.2 nm; the average two-dimensional area per one Cm cluster is
Zwahcrshi et al. calculated to be 2140nm2. Thus,the mean area occupied with Cm clusters in 1 X 106 nm2of the replica image shown in Figure 6 is obtained to be 2140 nm2 X 273 (=584 OOO nm2). Hence, the Cm cluster-coveringarea on a glass plate is roughly estimated to be 58.4%. We w u m e here that the two-dimensional shape of each cluster is round and also that Cw molecules are closely packed in each cluster. Obeng and Bardg reported that the limiting area per C a molecule calculated from its F A curve yielded a radius 0.56 f 0.07 nm for a Cm molecule in a Emonolayer-like film. On the basis of the assumption and information, the two-dimensional area occupied with a Cm molecule is estimated to be 0.56 X 0.65 X l/2 X 6 (~1.08 nm2).18 As shown in Figure 2, the area per Cw molecule at which d e w i t i o n was carried out (seearrow b) is 0.3nm2. Hence, the number of Cm molecules in the film perpendicularly attached against the glass surface is estimated to be 1.08 nm2/(0.584 X 0.3 nm2) (=6.1). If this value (6.1)is valid, each cluster visualized in Figure 6 should have its mean thickness corresponding to ca. six linearly chained C a molecules in the film, prepared from its mother L-film, a t a surface pressure of 1.0 mN/m. Our finding described above seems to be consistent with the assumption by Obeng and Bardg that the formation is a bi- or multilayer of C a instead of a single monolayer at an air-water interface. In addition, the network structure constructed with pure Cm clusters without any matrix molecules would provide the possibility of applicational utilities, such as superconductive film. R8gietW NO. DOL,4235-95-4; P c ~139096-16-5; , C a 9968596-8.
