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Fullerene Adlayers on Various Single-Crystal Metal Surfaces Prepared by Transfer from L Films Shinobu Uemura,† Masayo Sakata, Chuichi Hirayama, and Masashi Kunitake* Department of Applied Chemistry & Biochemistry, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan Received April 22, 2004. In Final Form: July 28, 2004 Fullerene adlayers prepared by the simple Langmuir-Blodgett (LB) method onto various well-defined single-crystal metal surfaces were investigated by in situ scanning tunneling microscopy (STM). The surface morphologies of fullerene adsorbed onto metal surfaces depended largely on the adsorbate-substrate interactions, which are governed by the types of surfaces. Too weak adsorption of C60 molecules onto iodine-modified Au(111) (I/Au(111)) allows surface migration of the molecules, and then, STM cannot visualize the C60 molecules. Stronger and appropriate adsorption onto bare Au(111) leads to highly ordered arrays relatively easily due to the limited surface migration of C60. On iodine-modified Pt(111) (I/Pt(111)) and bare Pt(111) surfaces, which have stronger adsorption, randomly adsorbed molecular adlayers were observed. Although C60 molecules on Au(111) were visualized as a featureless ball due to the maintenance of the rapid rotational motion (perturbation) of C60 on the surface at room temperature, those on I/Pt(111) revealed the intramolecular structures, thus indicating that the perturbation motion of molecules on the surface was prohibited.
Introduction Recently, fullerenes, carbon nanotubes, and their derivatives have attracted considerable interest from both the scientific and technological points of view. Highly ordered adlayers of fullerene derivatives on various substrates are of particular interest because of their potential usefulness as molecular devices and in the fields of optoelectronics and nanotechnology. In 1990, the first scanning tunneling microscopy (STM) image of fullerene clusters on Au(111) and Au(110) was reported by Bethune and co-workers.1 Since then, fullerene adlayers on singlecrystal metal surfaces and semiconductor surfaces prepared by sublimation, and their STM observation in an ultrahigh vacuum (UHV), have been investigated in detail.2 Weaver and co-workers succeeded in the STM visualization of fullerene on Au(111) and Au(110) in an aqueous solution.3 Furthermore, Bard and co-workers reported fullerene films prepared by the LangmuirBlodgett (LB) method onto iodine-modified Pt(111) (I/Pt(111)) for the first time.4 After that report, we also investigated the fullerene LB films on Au(111).5 We observed that fullerenes formed epitaxial adlayers on Au(111) when prepared by this simple method. The LB method has also been applied to the sample preparation of all-carbon C180 fullerene trimers, which were too fragile * To whom correspondence should be addressed. Phone: +8196-342-3675. Fax: +81-96-342-3679. E-mail: kunitake@chem. kumamoto-u.ac.jp. † Present address: Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan. (1) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; de Vries, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990, 348, 621-622. (2) For example, see: Sakurai, T.; Wang, X.-D.; Xue, Q. K.; Hasegawa, Y.; Hashizume, T.; Shinohara, H. Prog. Surf. Sci. 1996, 51, 263-408. (3) Zhang, Y.; Gao, X.; Weaver, M. J. J. Phys. Chem. 1992, 96, 510513. (4) Jehoulet, C.; Obeng, Y. S.; Kim, Y.-T.; Zhou, F.; Bard, A. J. J. Am. Chem. Soc. 1992, 114, 4237-4247. (5) Uemura, S.; Ohira, A.; Ishizaki, T.; Sakata, M.; Kunitake, M.; Taniguchi, I.; Hirayama, C. Chem. Lett. 1999, 279-280; 1999, 535537 (correction).
to apply by vapor deposition, and enabled the structural identification of C180 isomers by STM.6 Furthermore, the electrochemical replacement method has been proposed as a drastic improvement of the simple LB method to provide a high-quality epitaxial film with excellent uniformity.7 In this method, fullerene adlayers on Au(111) are prepared by transferring them onto iodine-modified Au(111) (I/Au(111)), following the electrochemical replacement between the fullerene and iodine adlayers by the reductive desorption of iodine. These methodologies based on the LB method have been applied not only to fullerenes but also to various water-insoluble molecules.8 Very recently, the adsorption of fullerenes from nonaqueous solvents such as tetradecane,9 1,2,4-trichlorobenzene (TCB),10 and benzene11 has independently come into use to prepare ordered adlayers of fullerenes on Au(111) in an epitaxial fashion. The preparation and STM observation of fullerene adlayers on Au(111) has been successfully conducted in tetradecane and TCB. Itaya and co-workers reported an ordered adlayer of C120 (fullerene dimer) prepared by adsorption from benzene solution, which was observed by STM in aqueous solution. It should be noted that these various methods, including sublimation, the LB method, and adsorption from organic solvents, all provide essentially the same epitaxial latticessthe socalled (2x3 × 2x3)R30° or “in-phase” latticessfor C60 adlayers on Au(111). Here, we describe adlayers of fullerenes on various single-crystal metal surfaces prepared by the simple LB method. The variety of surface morphologies of the (6) Kunitake, M.; Uemura, S.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu, K. Angew. Chem., Int. Ed. 2002, 41, 969-972. (7) Uemura, S.; Sakata, M.; Taniguchi, I.; Kunitake, M.; Hirayama, C. Langmuir 2002, 17, 5-7. (8) Uemura, S.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. Thin Solid Films 2002, 409, 206-210. (9) Marchenko, A.; Cousty, J. Surf. Sci. 2002, 513, 233-237. (10) Uemura, S.; Samorı´, P.; Kunitake, M.; Hirayama, C.; Rabe, J. P. J. Mater. Chem. 2002, 12, 3366-3367. (11) Yoshimoto, S.; Narita, R.; Tsutsumi, E.; Matsumoto, M.; Itaya, K.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu, K. Langmuir 2002, 18, 8518-8522.
10.1021/la048982z CCC: $27.50 © 2004 American Chemical Society Published on Web 09/04/2004
Fullerene Adlayers on Single-Crystal Metal Surfaces
Figure 1. Typical in situ STM images (A, 20 × 20 nm2; B, 12 × 12 nm2) of C60 adlayers on Au(111) in 0.1 M perchloric acid prepared by the direct transfer method. The conditions in image A were +0.10 V, -0.05 V, and 0.8 nA for the electrode potential (Es), the tip potential (Et), and the tunneling current (Itip), respectively. The conditions in image B were +0.10 V, -0.05 V, and 1.0 nA for Es, Et, and Itip, respectively.
fullerene adlayers, which largely depends on the adsorbate-substrate interactions, was visualized by in situ STM. Experimental Section C60 (MER Co. Ltd, 99.95%) was purchased and used without further purification. The L films were set at ∼10 mN/m on pure water with a benzene solution and then transferred onto welldefined single-crystal metal surfaces by the simple crossing of the air-water interface in a single step. After the transfer, the sample was placed in an in situ STM cell filled with 0.1 M perchloric acid. The preparation of C60 adlayers on Au(111) by the direct transfer method and the electrochemical replacement method and the details of the in situ STM observation method have all been previously described.5-8 A well-defined bare Pt(111) substrate was also prepared in essentially the same manner on the basis of flaming and quenching techniques, and I/Pt(111) was prepared by simple immersion into 10 mM KI aqueous solution and rinsing. The preparation of C60 adlayers on bare Pt(111) and I/Pt(111) surfaces was essentially the same as the transfer method reported for fullerene adlayers on Au(111).5-8 The in situ STM measurements were performed with a Nanoscope E instrument (Digital Instruments, Santa Barbara, CA), with an electrochemically etched W tunneling tip which was sealed with transparent nail polish. All images were collected in the constant current mode. Two Pt wires were used as the quasireference electrode (RE) and the counter electrode (CE) in the STM cell, respectively. All potentials are reported versus a saturated calomel electrode (SCE) in 0.1 M perchloric acid.
Results and Discussions Figure 1 shows typical in situ STM images of C60 adlayers on Au(111) prepared by the simple transfer technique (the direct transfer method). In Figure 1A, two domains of the ordered hexagonal lattices, which are 30° rotationally shifted against each other, can be observed: they were assigned to (2x3 × 2x3)R30° and the in-phase domain. The formation of ordered adlayers by the simple transfer of L films might indicate a certain surface mobility of the C60 molecules, although many defects and uncovered portions might also indicate limited diffusion on the surface. Marchenko and Cousty have reported mobile C60 islands on Au(111) in tetradecane.10 The application of I/Au(111) as a substrate was one of the most excellent solutions for forming highly ordered adlayers of relatively large organic molecules, such as porphyrins and organic dyes,12 when the interaction between the adsorbate and the Au(111) surface was too (12) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 23372340. Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245-250.
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Figure 2. In situ STM images (A, 64.5 × 64.5 nm2; B, 6.5 × 6.5 nm2; C, 1.4 × 1.4 nm2; D, 1.6 × 1.6 nm2) of C60 on I/Pt(111). The conditions in image A were -0.30 V, -0.21 V, and 3.0 nA for Es, Et, and Itip, respectively. The conditions in images B, C, and D were -0.30 V, -0.21 V, and 4.0 nA for Es, Et, and Itip, respectively.
strong. However, in the case of fullerenes, even for the C180 trimer, the interaction between the fullerenes and the I/Au(111) was too weak to immobilize under ambient conditions.6 As we mentioned in a previous article,7 C60 molecules transferred onto I/Au(111) could not be observed by in situ STM at all, and only the iodine lattices were visible. This was due to the too weak interactions between C60 and the iodine adlayer. This would cause high mobility of C60 on this surface or sweeping by the tip. These invisible C60 molecules should be located on the surface, because C60 has an extremely low solubility in aqueous solutions. This fact has been proven by the appearance of C60 adlayers on Au(111) following the cathodic desorption of the iodine adlayer. This phenomenon has been utilized to improve the preparation of C60 adlayers on Au(111), which has been called the electrochemical replacement technique. This methodology allows us to provide high-quality adlayers with few defects and high uniformity by controlling the deposition process in terms of “slow supply” and “high surface diffusion”. I/Pt(111) surfaces were also used as a substrate for C60, and it was expected to result in a stronger adsorption than adsorption on Au(111) from our limited knowledge. As we mentioned before, STM images of C60 on I/Pt(111) were reported by Bard and co-workers in their pioneer work.4 The surface structure and potential dependence of the iodine adlayers on a Pt(111) surface have been carefully examined and reported.13 Figure 2A shows a typical in situ STM image of C60 on I/Pt(111). Randomly adsorbed fullerene molecules covered on whole terraces are seen with a triangular atomic step; in contrast, the hexagonal lattice of C60 on Au(111) and each C60 molecule are clearly recognized in the image. Amazingly, the high-resolution STM images of the fullerene adlayers on I/Pt(111) clearly visualized the intramolecular structure of the fullerenes, in contrast to the high-resolution images which were visualized on Au(111) as a featureless ball. Figures 2B shows a zoomed high-resolution STM image of fullerene adlayers on I/Pt(111). Each C60 molecule can be seen as a spot with soccer-ball-like honeycomb features. The honeycomb (13) (a) Inukai, J.; Osawa, Y.; Wakisaka, M.; Sashikata, K.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1998, 102, 3498-3505. (b) Singh, S.; Robertson, D. H.; Peng, Q.; Breen, J. J. Langmuir 1997, 13, 51975203. (c) Chang, S.-C.; Yau, S.-L.; Schardt, B. C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 4787-4794.
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features might be attributed to the five- and six-membered rings in the chemical structures of fullerenes. Furthermore, C70 molecules, which comprised a few percent of the total, can also be seen in Figure 2D. Those were obviously consistent with the chemical structures of C60 and C70, respectively. The patterns of the molecular images corresponded with the arrangement of five- and sixmembered rings in the chemical structures of the fullerenes. Typical molecular STM images for C60 and C70, in which inner structures can be visualized, were picked up with the corresponding chemical model of the fullerenes in Figure 2C and D. Incidentally, no motion of molecules on I/Pt(111) surfaces such as changing of location by the STM tip14 and “ratcheting” rotation by the neighboring molecules15 was observed at all. The visualization of the intramolecular structure of fullerenes has been frequently achieved by a low-temperature UHV system.16 However, C60 molecules on Au(111) were regularly visualized by in situ and UHV STM as featureless protrusions under room-temperature conditions, as shown in Figure 1B.17 The featureless molecular images are due to the perturbation motion of the fullerenes onto Au(111) at room temperature. Behler and co-workers have reported the temperature dependence of STM images of C60 on Au(111).16a The intramolecular structure was clearly observed at an extremely low temperature, 4.5 K, but featureless images have been seen at higher temperatures such as room temperature. They concluded that this was due to molecular rotation. The rotation of C60 is not fully stopped, even in the bulk crystalline state at room temperature. Only under conditions when the molecular rotation is frozen on the surface, the intramolecular structures of C60 molecules can be observed. Therefore, the successful visualization on I/Pt(111) indicates that the perturbation motion of fullerene molecules was frozen on I/Pt(111) even at room temperature. This is consistent with the stronger adsorption onto I/Pt(111) rather than onto bare Au(111). In addition, Weaver reported an in situ STM observation of C60 on Au(110) in aqueous solution.3 C60 molecules were adsorbed into hollow sites on Au(110), and then, the inner features were observed as a spot with several separated lines, probably due to the restricted rotation. Recently, Itaya and co-workers reported a similar strip pattern for a C120 fullerene dimer on Au(111) as an inner feature from an in situ STM image taken at room temperature.11 It is interesting that a multiplied interaction between C120 and surfaces and a restricted perturbation motion on the C60 moiety of C120 allowed us to visualize an inner structure even on Au(111) at room temperature. The C60 adlayer onto Pt(111) prepared by the LB method was also investigated. It was easily expected that organic molecules adsorb much more strongly onto Pt(111) or Rh(111) rather than onto Au(111) and I/Pt(111).18 Certainly, C60 molecules adsorbed onto Pt(111) randomly, as shown in Figure 3. However, the STM images were (14) Cuberes, M. T.; Schlittler R. R.; Gimzewski, J. K. Appl. Phys. Lett. 1996, 69, 3016-3018. (15) Gimzewski, J. K.; Joachim, C.; Schlittler, R. R.; Langlais, V.; Tang, H.; Johannsen, I. Science 1998, 281, 531-533. (16) (a) Behler, S.; Lang, H. P.; Pan, S. H.; Thommen-Geiser, V.; Hofer, R.; Bernasconi, M.; Guentherodt, H.-J. In Electronic Properties of Fullerenes; Kuzmany, H., Fink, J., Mehring, M., Roth S., Eds.; Springer Series in Solid-State Sciences; Springer-Verlag: Berlin, 1993; Vol. 117, pp 232-235. (b) Hou, J. G. Nature 2001, 409, 304-305. (17) Lang, H. P.; Thommen-Geiser, V.; Bolm, C.; Felder, M.; Frommer, J.; Wiesendanger, R.; Werner, H.; Schlo¨gl, R.; Zahab, A.; Bernier, P.; Gerth, G.; Anselmetti, D.; Gu¨ntherodt, H.-J. Appl. Phys. A 1993, 56, 197-205. (18) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1997, 101, 3547-3553.
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Figure 3. Typical in situ STM image (24 × 24 nm2) of C60 adlayers randomly adsorbed on Pt(111). The conditions in the image were -0.08 V, -0.33 V, and 10.0 nA for Es, Et, and Itip, respectively.
Figure 4. Schematic representations of the surface morphologies on I/Au(111), Au(111), I/Pt(111), and Pt(111) surfaces.
regularlyquite obscure and each C60 molecule could be difficult to be recognized, although several round spots ∼1 nm in diameter might be attributed to C60 molecules. The C60 adlayer on Pt(111) seems to possess not monolayers but rather multilayers.8 Certainly, the corrugation of observed spots on Pt(111) was not constant, in contrast with the disordered but relatively uniform adlayer on I/Pt(111). In the case of the most strong adsorption on bare Pt(111), the strongest adsorption could lead to the secondary adsorption of molecules on the first adlayer, which connected onto the Pt surfaces. An obscure image of C60 onto Pt(111) could be caused by overlayered C60 molecules or otherwise a high sensitivity for impurities introduced by the transfer process. It might also indicate the stronger interaction from the Pt(111) surface rather than that from I/Pt(111). As we mentioned above, the differences of the morphologies of C60 adlayers on various substrates can be explained by adsorbate-substrate interactions. All of the results indicate that adsorbate-substrate interactions dominantly govern the morphologies of C60 molecules on single-crystal metal surfaces. It should be emphasized that the procedures we used to observe fullerene adlayers were entirely conducted at room temperature under ambient conditions without thermal treatment. Using thermal treatment to increase surface diffusion could change the morphology drastically. Figure 4 shows a schematic representation of the relationship between the surface morphology and the substrates. The order of adsorption strength for C60 is as follows: I/Au(111) < Au(111) < I/Pt(111) < Pt(111). This order would probably be consistent even for other molecules. Conclusions In conclusion, the surface morphology of LB films at the air-water interface was minimally affected by the
Fullerene Adlayers on Single-Crystal Metal Surfaces
structure and uniformity of the adlayers on the substrates and the adsorbate-substrate interactions dominantly determine the morphologies of C60 molecules on singlecrystal metal surfaces. These results proved that surface diffusion governed by adsorbate-substrate interactions is crucial for forming ordered epitaxial adlayers. Furthermore, the molecular images visualized by STM, such as an inner molecular structure, are strongly reflected by the perturbation motion of molecules on the surface, which is also governed by adsorbate-substrate interactions. Successful STM imaging on I/Pt(111) in aqueous solution would be encouraging for applying the methodology to
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various fullerene analogue molecules to visualize and compare their inner structures. Acknowledgment. This work was supported, in part, by Grants-in-Aid from the Ministry of Education, Science, Sports and Culture, Japan for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces and JST-CREST, Japan. S.U. greatly acknowledges the JSPS Research Fellowship for Young Scientists. LA048982Z
