Langmuir 2001, 17, 5-7
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Novel “Wet Process” Technique Based on Electrochemical Replacement for the Preparation of Fullerene Epitaxial Adlayers Shinobu Uemura, Masayo Sakata, Isao Taniguchi, Masashi Kunitake,* and Chuichi Hirayama Department of Applied Chemistry & Biochemistry, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan Received June 12, 2000. In Final Form: September 13, 2000 The electrochemical replacement method to form epitaxial adlayers of fullerene on Au(111) surfaces was proposed and demonstrated by in situ electrochemical STM. The new “wet process” method consists of the transfer of Lanmuir films of fullerene onto iodine-modified Au(111) surfaces at an air-water interface followed by the electrochemical removal and replacement of iodine adlayers with fullerene adlayers in solution. The fullerene adlayers prepared by this method showed excellent quality and uniformity, and they were essentially the same as epitaxial adlayers prepared by sublimation.
Various “wet process” techniques such as selfassembly1-7 and adsorption-induced self-organization8-12 have been proposed from several fields for the preparation of highly ordered adlayers on well-defined surfaces. Such techniques have great advantages in facility and convenience compared with “dry process” techniques such as sublimation. In particular, the self-assembly method using the binding of a thiol group to gold has become popular as a very simple technique.1-7 Recently, we introduced the technique of adsorption-induced self-organization for the formation of highly ordered molecular layers by means of spontaneous adsorption and two-dimensional selforganization on the surface. In-situ STM imaging of highly ordered molecular layers of various organic molecules has been reported.8-14 We have also shown the simple transfer of Langmuir films (L films) on air-water interfaces in the preparation of epitaxial films of water-insoluble molecules on singlecrystal metal surfaces (the direct transfer method). Previously, we reported that the simple transfer of L films at the air-water interface allowed us to prepare epitaxial adlayers of fullerenes (C60 and C60/C70) on Au(111) surfaces, which were essentially the same as the epitaxial adlayers * Corresponding author. Telephone: +81-96-342-3675. Fax: +81-96-342-3679. E-mail:
[email protected]. (1) Ulman, A. Chem. Rev. 1996, 96, 1533. (2) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (3) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyor-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (4) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Langmuir 1998, 14, 3565. (5) Wan, L.-J.; Hara, Y.; Noda, H.; Osawa, M. J. Phys. Chem. B 1998, 102, 5943. (6) Jin, Q.; Rodriguez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101. (7) Yoshimoto, S.; Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Electrochem. Commun. 2000, 2, 39. (8) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (9) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. Soc. 1996, 405, 245. (10) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (11) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607. (12) Kunitake, M.; Miyano. S.; Itaya, K. Proceedings of International Conference on Electrochemistry of Ordered Interfaces, Sapporo, Japan, 1998, PB14. (13) Ulman, A. Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (14) Honda, K., Ed. Functionality of Molecular Systems; SpringerVerlag: New York, 1998.
prepared by sublimation in an ultrahigh vacuum.15 The adlayers are stable and show no reaction in the entire double-layer potential region in 0.1 M perchloric acid. Although the direct transfer method can be used to prepare epitaxial films of fullerenes by the same sublimation techniques in a vacuum, the quality of epitaxial adlayers is worth discussing. The films prepared by the transfer of L films may not be comparable to those prepared by a sophisticated “dry process” technique like molecular beam epitaxy (MBE) in terms of quality, that is, having few defects, a low phase boundary, and uniformity. In most cases of sublimation, substrate temperature and deposition speed are the key to obtaining high-quality epitaxial films. High surface mobility and slow deposition (adsorption) are crucial when preparing films not only for “dry process” but also for “wet process” techniques. To meet this demand using the “wet process”, we established a new technique based on the electrochemically finecontrolled replacement with fullerenes after the transfer of L films, as shown in Figure 1. Iodine-modified Au(111) surfaces (I/Au(111)) were selected as an electrochemically removable substrate. The preparation and characteristics of I/Au(111) have been investigated by various techniques.16-18 Chemically inert I/Au(111) shows relatively weak adsorption of organic adsorbates compared to bare Au(111) surfaces.8,11 We have already reported that I/Au(111) surfaces are one of the most suitable substrates for the formation of highly ordered molecular layers of various organic molecules by means of adsorption-induced self-organization.8-12 L films of C60 (MER Co. Ltd., 99.95%) on pure water15,19-24 at ∼10 mN/m were transferred onto (15) Uemura, S.; Ohira, A.; Ishizaki, T.; Sakata, M.; Kunitake, M.; Taniguchi, I.; Hirayama, C. Chem. Lett. 1999, 279; Correction 1999, 535. (16) Yamada, T.; Batina, N.; Itaya, K. J. Phys. Chem. 1995, 99 (21), 8817. (17) Batina, N.; Yamada, T.; Itaya, K. Langmuir 1995, 11 (11), 4568. (18) Yamada, T.; Batina, N.; Itaya, K. Surf. Sci. 1995, 335, 204. (19) Ocko, B. M.; Watson, G. M.; Wang, J. J. Phys. Chem. 1994, 98, 897. (20) Obeng, Y. S.; Bard, A. J. J. Am. Chem. Soc. 1991, 113, 6279. (21) Jehoulet, C.; Obeng, Y. S.; Kim, Y.-T.; Zhou, F.; Bard, A. J. J. Am. Chem. Soc. 1992, 114, 4237. (22) Bulhoes, L. O. S.; Obeng, Y. S.; Bard, A. J. Chem. Mater. 1993, 5, 110. (23) Tomioka, Y.; Ishibashi, M.; Kajiyama, H.; Taniguchi, Y. Langmuir 1993, 9, 32.
10.1021/la000816d CCC: $20.00 © 2001 American Chemical Society Published on Web 12/12/2000
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Figure 1. Schematic representation of the direct transfer method and the electrochemical replacement method.
I/Au(111) 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. Interestingly, C60 molecules could not be observed by in-situ STM at all, and only iodine lattices were visible. Needless to say, C60 molecules should be located on the surface because C60 is not soluble in water. The reason C60 molecules are invisible on I/Au(111) surfaces might be due to the weak interaction between C60 and the iodine adlayer. This would cause high mobility of molecules on the surface or sweeping by the tip. Such rapid motions cannot be visualized by scanning microscopy. Next, the electrochemical removal of iodine adlayers was conducted to replace the iodine with C60 adlayers (Figure 1). It is well-known that an iodine adlayer on Au(111) surfaces displays anodic electrocompression and cathodic desorption properties.16-19 When the potential was set at less than -180 mV vs SCE, at which point the iodine adlayers started to detach, the iodine lattice became unclear and gradually C60 lattices began to replace it. We reset the potential to that at which the reduction of the iodine adlayer starts, to initiate slow replacement. The desorption of iodine adlayers and the adsorption of C60 onto Au(111) surfaces occurred simultaneously. The array of C60 adlayers expanded with time. Parts A and B of Figure 2 show typical in situ STM images of C60 adlayers on Au(111) surfaces prepared by the direct transfer method and the electrochemical replacement method, respectively. The image in Figure 2B was taken approximately 1 h after the potential was set to -180 mV. The highly ordered molecular layers of C60 on Au(111) surfaces can also be prepared by the electrochemical replacement method. Both images in Figure 2 showed essentially the same hexagonal lattices, which are assigned to 2x3×2x3R30° or the so-called “inphase” domain.25-27 The C60 adlayers after the removal of (24) Yanagida, M.; Kuri, T.; Kajiyama, T. Chem. Lett. 1997, 911. (25) Williams, G.; Pearson, C.; Bryce, M. R.; Petty, M. C. Thin Solid Films 1992, 209, 150. (26) Sakurai, T.; Wang, X.-D.; Xue, Q. K.; Hasegawa, Y.; Hashizume, T.; Shinohara, H. Prog. Surf. Sci. 1996, 51, 263.
Figure 2. In situ STM images (60 × 60 nm2) of C60 adlayers on Au(111) surfaces in 0.1 M perchloric acid prepared by the direct transfer method (A) and the electrochemical replacement method (B). The conditions in image A were +0.10 V, 0.00 V, and 1.0 nA as an electrode potential (Es), a tip potential (Et), and a tunneling current (Itip), respectively. The conditions in image B were -0.18 V, -0.12 V, and 0.5 nA as Es, Et, and Itip, respectively.
iodine are very stable in the potential region +650 to -350 mV, as are those prepared by the direct transfer method.15 The adsorption of C60 onto Au(111) surfaces is stronger than that of iodine, and iodine atoms cannot displace the adsorbed C60 molecules. Thus, in terms of the quality and uniformity of the C60 adlayers, the electrochemical replacement method is (27) Zhang, Y.; Gao, X.; Weaver, M. J. J. Phys. Chem. 1992, 96, 510. (28) Wilson, R. J.; Meijer, G.; Bethune, D. S.; Johnson, R. D.; Chambliss, D. D.; de Veries, M. S.; Hunziker, H. E.; Wendt, H. R. Nature 1990, 348, 621.
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Figure 3. In situ STM images (58 × 58 nm2) of C70 adlayers on Au(111) surfaces in 0.1 M perchloric acid prepared by the “direct transfer” method (A) and the “electrochemical replacement” method (B). The conditions in image A were +0.60 V, -0.03 V, and 3.0 nA as Es, Et, and Itip, respectively. The conditions in image B were -0.28 V, -0.03 V, and 0.8 nA as Es, Et, and Itip, respectively.
obviously superior to the direct transfer method. The adlayers prepared by the direct transfer method frequently showed large disordered or unmodified regions, as seen
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in Figure 2A. On the other hand, the highly ordered layers prepared by the electrochemical replacement method extended onto the terrace, and there were few local defects such as point defects and phase boundaries, as shown in Figure 2B. The adlayers prepared by the electrochemical replacement method are comparable to those prepared by sophisticated sublimation techniques. The preparation of C70 (MER Co., Ltd., 98%) epitaxial adlayers on Au(111) surfaces was also performed, and the two procedures were compared. Compared with C60, C70 is ellipsoidal in shape with two molecular orientations on the surface, “standing“ and “lying-down”. It is difficult to obtain uniform ordered layers. Therefore, the difference in film quality between the two methods was clear. C60 and C70 mixed adlayers show the same hexagonal lattices, and “standing” C70 molecules can be easily identified in the array as brightened spots.15 Parts A and B of Figure 3 show typical in situ STM images of C70 adlayers on Au(111) surfaces prepared by the direct transfer method and the electrochemical replacement method, respectively. C70 homogeneous adlayers have hexagonal lattices that are the same as C60 adlayers with a “standing” orientation. However, C70 adlayers prepared by the direct transfer method showed small hexagonal arrays and large disordered or unmodified regions. The C70 adlayer prepared by the electrochemical replacement method showed much greater coverage and uniformity. Furthermore, distinguishable dark and bright protrusions can be seen in the array. The dark spots in Figure 3B are attributed to “standing” C70 molecules, and the bright ones to “lyingdown” C70 and/or C60 as impurities. The successive preparation of high-quality epitaxial films by the electrochemical replacement method is attributed to the management of electrochemical potential for the slow replacement and high mobility of fullerenes on I/Au(111) surfaces. Note that the setting and management of the potential enable the control of deposition speed. When the electrode potential was abruptly changed to negative (less than -350 mV) for rapid replacement, the quality and uniformity of the adlayers decreased. Many defects and disordered regions were observed, as in the adlayers prepared by the direct transfer method. This clearly proved that the slow replacement is crucial and that the deposition rate can be controlled electrochemically. In addition, the high mobility of fullerenes on I/Au(111) surfaces is also an important factor. The surface morphology of L films at the air-water interface was minimally affected by the structure and uniformity of the adlayers on the substrates. These findings are encouraging for applying the novel “wet process” technique to various insoluble molecules in the preparation of high-quality epitaxial adlayers and self-organization films. 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. LA000816D
