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Adlayers of Fullerene Monomer and [2 + 2]-Type Dimer on Au(111) in Aqueous Solution Studied by in Situ Scanning Tunneling Microscopy Soichiro Yoshimoto,† Ryuji Narita,† Eishi Tsutsumi,† Masashi Matsumoto,† and Kingo Itaya*,† Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 04, Sendai 980-8579, Japan
Osamu Ito‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan
Koichi Fujiwara,§ Yasujiro Murata,§ and Koichi Komatsu§ Institute of Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Received June 3, 2002. In Final Form: August 5, 2002 In situ scanning tunneling microscopy (STM) revealed that highly ordered adlayers of fullerene monomer (C60) and [2 + 2]-type dimer (C120) were formed on Au(111) by immersing the surface in benzene solution containing C60 or C120 molecules. High-resolution STM images allowed us to determine packing arrangements of C60 and C120 on a Au(111) surface in an aqueous HClO4 solution. The (2x3 × 2x3)R30° and the so-called in-phase structures of the C60 adlayer were found on the Au(111) surface, whereas the (2x3 × 4x3)R30° and (2x3 × 5x3)R30° structures were observed for the C120 dimer. Each C120 dimer was imaged in a dumbbell shape by in situ STM.
Introduction Fullerenes have been studied extensively because of their unique physical and chemical properties.1,2 The packing arrangements and the electronic states of adlayers of fullerenes, such as buckyball C60 and C70, have been investigated on semiconductors (Si, GaAs, Ge) and metals (Au, Ag, Cu) using scanning tunneling microscopy (STM)3-10 and atomic force microscopy (AFM).10 Especially, STM is now well-recognized as an important method for * To whom correspondence should be addressed. Phone/Fax: +81-22-214-5380. E-mail:
[email protected]. † Department of Applied Chemistry, Graduate School of Engineering, Tohoku University. ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University. § Kyoto University. (1) Billups, W. E., Ciufolini, M. A., Eds.; Buckminsterfullerenes; VCH: New York, 1993. (2) Fujitsuka, M.; Luo, C.; Ito, O.; Murata, Y.; Komatsu, K. J. Phys. Chem. A 1999, 103, 7155. (3) Sakurai, T.; Wang, X. D.; Xue, Q. K.; Hasegawa, Y.; Hashizume, T.; Shinohara, H. Prog. Surf. Sci. 1996, 51, 263 and references therein. (4) (a) Altman, E. I.; Colton, R. J. Surf. Sci. 1992, 279, 49. (b) Altman, E. I.; Colton, R. J. Surf. Sci. 1993, 295, 13. (c) Altman, E. I.; Colton, R. J. Phys. Rev. B 1993, 48, 18244. (d) Altman, E. I.; Colton, R. J. J. Vac. Sci. Technol., B 1994, 12, 1906. (5) Gimzewski, J. K.; Modesti, S.; David, T.; Schlittler, R. R. J. Vac. Sci. Technol., B 1994, 12, 1942. (6) Motai, K.; Hashizume, T.; Shinohara, H.; Saito, Y., Pickering, H. W.; Nishina, Y.; Sakurai, T. Jpn. J. Appl. Phys. 1993, 32, L450. (7) Hashizume, T.; Motai, K.; Wang, X. D.; Shinohara, H.; Saito, Y., Maruyama, Y.; Ohno, K.; Kawazoe, Y.; Nishina, Y.; Pickering, H. W.; Kuk, Y.; Sakurai, T. Phys. Rev. Lett.1993, 71, 2959. (8) Zhang, Y.; Gao, X.; Weaver, M. J. J. Phys. Chem. 1992, 96, 510. (9) (a) Uemura, S.; Ohira, A.; Ishizaki, T.; Sakata, M.; Kunitake, M.; Taniguchi, I.; Kunitake, M.; Hirayama, C. Chem. Lett. 1999, 535. (b) Uemura, S.; Sakata, M.; Taniguchi, I.; Kunitake, M.; Hirayama, C. Langmuir 2001, 17, 5.
structural investigation of adsorbed layers of molecules on well-defined surfaces in both ultrahigh vacuum (UHV)3 and electrolyte solutions.11,12 High-resolution STM made it possible to determine the packing arrangements and even electronic structures of fullerene molecules adsorbed on various surfaces.3 For example, the epitaxial thin film of C60 on Au(111) was found to take two different structures, (2x3 × 2x3)R30° and the so-called “in-phase” (38 × 38), in UHV.4,5 On Ag(111)3,4b,c and Cu(111),3,5-7 the C60 monolayers formed (2x3 × 2x3)R30° and (4 × 4) structures in UHV, respectively. Zhang et al. investigated C60 and C70 films on Au(111) and Au(110) surfaces in air and in aqueous HClO4.8 To prepare the C60 and C70 films, they covered the Au surfaces with a dichloromethane solution containing fullerene and evaporated the solvent off.8 Uemura et al. succeeded in preparing a well-ordered C60 adlayer on a Au(111) surface in aqueous HClO4 by the transfer of Langmuir (L) films of C60 and C70 on bare and iodine-modified Au(111) electrodes.9 They found hexagonal lattices in HClO4, to which the structures of (2x3 × 2x3)R30° and (7 × 7) were assigned.9a Recently, Kunitake et al. succeeded in observing five structural isomers of C180 (C60 trimers) on Au(111) in solution using in situ STM.13 As for fullerene dimers, photoinduced dimerization of C60 molecules has been studied on a mica surface and on a Si(111) surface covered with Ag atoms (Si(111)-Ag) by (10) Sarid, D.; Chen, T.; Howells, S.; Gallagher, M.; Yi, L.; Lichtenberger, D. L.; Nebesney, K. W.; Ray, C. D.; Huffman, D. R.; Lamb, L. D. Ultramicroscopy 1992, 42-44, 610. (11) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (12) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (13) Kunitake, M.; Uemura, S.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu, K. Angew. Chem., Int. Ed. 2002, 41, 969.
10.1021/la0205147 CCC: $22.00 © 2002 American Chemical Society Published on Web 10/02/2002
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Chart 1. Illustration of a [2+2]-Type Fullerene Dimer (C120)
using AFM14 and STM15 in UHV, respectively. On Si(111)Ag, it was reported that some couples of C60 reacted with each other to form two types of C120 isomers, single-bonded and double-bonded dimers, in the photoinduced polymerization (Chart 1).15 Meanwhile, three coauthors of this paper discovered a unique method to prepare [2 + 2]-type fullerene dimers (C120) linked by two C60 cages sharing a cyclobutane ring (see scheme), by a solid-state mechanochemical reaction of C60 with KCN.16,17 It was reported that C120 dimer completely decomposes to form two molecules of C60 at ∼175 °C in 15 min.16 Therefore, the preparation of C120 thin films on substrates should be carried out at a more moderate temperature to prevent the decomposition. The evaporation technique used in UHV cannot be applied for the preparation of C120 adlayers, because it requires a relatively high temperature. Recently, we proposed a simple method for the preparation of an ordered structure of water insoluble organic molecules such as coronene on Au(111) by immersing the substrate into its benzene solution at room temperature.18 This method for the preparation of ordered adlayers using benzene solutions is expected to be applicable to the investigation of adlayers of C60 and C120 on Au(111). In the present study, we prepared adlayers of C60 and C120 on Au(111) by immersing the Au(111) substrate in benzene solutions containing those fullerene molecules. Adlayers of C60 and C120 formed ordered structures on a Au(111) surface, and the packing arrangements of C60 and C120 were determined by in situ STM in an aqueous solution of HClO4. The adlayer structure of C120 on the Au(111) surface was revealed for the first time in solution. Experimental Section C60 was purchased from Aldrich (99.5%). C120 was synthesized by the use of a high-speed vibration milling technique described in the previous reports.16,17 An aqueous HClO4 solution was prepared with HClO4 (Cica-Merck) and ultrapure water (Milli-Q SP-TOC; g18.2 MΩ cm). Spectroscopy grade benzene (Kanto Chemical Co.) was used without further purification. Au(111) single-crystal electrodes were prepared by the Clavilier method.19 They were immersed into Milli-Q water saturated with hydrogen quickly after annealing in a hydrogen-flame and cooling in the stream of hydrogen.18 The adlayers of C60 were prepared by immersing Au(111) electrode into ∼10 µM C60-benzene solution for 5-10 s, whereas C120 adlayers were formed by immersion in a saturated C120-benzene solution for 20-120 s. The fullerene-adsorbed Au(111) electrode was then transferred (14) Hassanien, A.; Gasperic, J.; Demsar, J.; Musevic, I.; Mihailovic, D. Appl. Phys. Lett. 1997, 70, 27. (15) Nakayama, T.; Onoe, J.; Nakatsuji, K.; Nakamura, J.; Takeuchi, K.; Aono, M. Surf. Rev. Lett. 1999, 6, 1073. (16) Wang, G.-W.; Komatsu, K.; Murata, Y.; Shiro, M. Nature 1997, 387, 583. (17) Komatsu, K.; Wang, G.-W.; Murata, Y.; Tanaka, T.; Fujiwara, K.; Yamamoto, K.; Saunders, M. J. Org. Chem. 1998, 63, 9358. (18) (a) Yoshimoto, S.; Narita, R.; Itaya, K. Chem. Lett. 2002, 356. (b) Yoshimoto, S.; Narita, R.; Wakisaka, M.; Itaya, K. J. Electroanal. Chem., in press. (19) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205.
Figure 1. Typical cyclic voltammogram of a C120-adsorbed Au(111) electrode in 0.1 M HClO4. The dotted line shows the CV of a bare Au(111) electrode obtained in the double-layer region. The potential scan rate was 50 mV s-1. into pure benzene to remove excess fullerene molecules and finally rinsed with ultrapure water. Both voltammetric and STM measurements were carried out in pure 0.1 M HClO4. In situ STM measurements were performed by using a Nanoscope E (Digital Instruments, Santa Barbara, CA) with a tungsten tip etched in 1 M KOH. To minimize residual faradic current, tips were coated with nail polish. STM images were taken in the constant-current mode. All potential values are referred to a reversible hydrogen electrode (RHE) in 0.1 M HClO4.
Results and Discussion Voltammetry. Cyclic voltammetry of C60- and C120adsorbed Au(111) electrodes was performed in 0.1 M HClO4. Figure 1 shows cyclic voltammograms (CVs) of bare and C120-adsorbed Au(111) electrodes in 0.1 M HClO4 recorded at the scan rate 50 mV s-1. The voltammogram for bare Au(111) (dotted line) in the double-layer potential region is the same as that reported previously,18b which shows that a well-defined Au(111) surface was exposed to the solution. The CV profile of an Au(111) electrode immersed in pure benzene for 10-60 s and rinsed by ultrapure water was essentially the same as that observed on a clean Au(111) electrode.18b This result indicates that benzene does not adsorb on Au(111) under the present conditions. The solid line shown in Figure 1 is the CV obtained for a C120-adsorbed Au(111) electrode. The open circuit potential (OCP) of the C120-adsorbed Au(111) electrode was around 0.8-0.85 V versus RHE, and the potential scan was started in the negative direction from the OCP. The effect of the adsorption of C120 was clearly observed in the double-layer charging current. The decrease in the double-layer charging suggests that the Au(111) surface is covered with hydrophobic C120 molecules. The cathodic current commencing at -0.05 V is due to the H2 evolution reaction. The anodic current beginning at 1.0 V is probably due to the oxidative desorption of C120 molecules from the Au(111) surface. Note that the oxidation of Au(111) does not occur in this potential region. Repetitive potential cycles between 0 and 0.8 V caused no change in the CV profile. A similar voltammogram was also obtained at the C60-adsorbed Au(111) electrode in the present study. It was noted that the CV profile for the C60-adsorbed Au(111) reported previously by Uemura et al.9 was almost the same as that for the C120-adsorbed Au(111) shown in Figure 1. In Situ STM. (1) C60 Adlayer. Figure 2 shows a typical STM image of a C60 adlayer observed at 0.85 V (near the OCP) on Au(111) in 0.1 M HClO4. Atomically flat terraces were almost completely covered with a well-ordered adlayer of C60. Each molecule of C60 is clearly recognized
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Figure 2. STM image of a C60 adlayer on a Au(111) surface in 0.1 M HClO4 acquired at 0.85 V versus RHE. The potential of the tip and the tunneling current were 0.35 V and 1.5 nA, respectively. Two different domains are labeled A and B, and the domain boundary is marked by the white dashed line. The set of three arrows indicates the 〈110〉 directions of the Au(111) substrate. Dark spots pointed at by arrow signs are defects in the adlayer.
as a round bright spot. In Figure 2, two different domains, A and B, are seen. The domain boundary between A and B is indicated by the dashed line. The molecular rows in domains A and B cross each other at the boundary at an angle of either 30° or 90° within an experimental error of (3°. In the ordered domains, some C60 molecules are seen to be missing, as indicated by the white arrows in Figure 2. To obtain structural details of the C60 adlayer, a highresolution STM image was acquired in domain A at 0.8 V, which is shown in Figure 3a. From the cross-sectional profile, the diameter of one bright spot was found to be ∼0.7 nm, which is in good agreement with the molecular size of C60. The average corrugation height was found to be ∼0.2 nm. The C60 molecular rows in the domain cross each other at an angle of either 60° or 120° within an experimental error of (3°. A precise comparison between this image and that of the underlying Au(111)-(1 × 1) lattice revealed that the molecular rows are in parallel with 〈112〉, the so-called x3 direction of Au(111). The intermolecular spacing along the x3 direction was found to be 1.0 ( 0.03 nm, which corresponds to 2x3 times the Au lattice constant (0.289 nm). Therefore, we can assign (2x3 × 2x3)R30° to the unit cell, the outline of which is superimposed in Figure 3a. The surface coverage is calculated to be 0.08. The high-resolution STM image obtained in domain B is shown in Figure 3b. The molecular rows in domain B were found to run parallel to the 〈110〉 direction. The intermolecular spacing along the 〈110〉 direction was estimated to be 1.03 ( 0.03 nm. This structure was called “in-phase” in the previous papers4,5 because the molecules of C60 are aligned with the atomic direction of the Au lattice. Both the (2x3 × 2x3)R30° and the “in-phase” structures obtained in the present study are essentially the same as those of the adlayers prepared by the sublimation of C60 in UHV3-5 and by the transfer of L films.9 In the present study, the (2x3 × 2x3)R30° symmetry seemed to be the dominant structure. As reported by Altman et al.,4a the formation of the (2x3 × 2x3)R30° structure is thermodynamically favored compared with the “in-phase” structure. (2) C120 Adlayer. A further investigation was carried out for the adlayer structure of C120 on Au(111) using the same technique described above. Figure 4 shows a typical
Figure 3. High-resolution STM images of a C60 adlayer on a Au(111) surface obtained in region A (a) and region B (b) in 0.1 M HClO4 at 0.85 V versus RHE. The potential of the tip and the tunneling current were 0.35 V and 1.5 nA, respectively. The set of three arrows indicates the 〈110〉 directions of the Au(111) substrate.
Figure 4. Typical large-scale (30 × 30 nm2) STM image of a C120 adlayer on a Au(111) surface in 0.1 M HClO4 acquired at 0.8 V versus RHE. The potential of the tip and the tunneling current were 0.35 V and 1.0 nA, respectively. The set of three arrows indicates the 〈110〉 directions of the Au(111) substrate.
large-scale STM image of a C120 adlayer observed at 0.8 V (near OCP) on Au(111) in 0.1 M HClO4. In the image acquired in a relatively large area of 30 × 30 nm2, individual molecules can be clearly recognized on an atomically flat terrace. Ordered molecular arrays were observed on the terrace, which were quite different from those of C60. Briefly, domains were composed of three regions marked I, II, and III in Figure 4. A precise comparison between this STM image and that of the
Adlayers of Fullerene Monomer and [2 + 2]-Type Dimer
underlying Au(111)-(1 × 1) lattice revealed that the molecular rows in regions I and II are in parallel with the x3 directions of Au(111) lattice. In region I, molecular rows consisting of a set of two bright spots ran parallel to the x3 direction, as indicated by arrow A. Each set of two bright spots should be assigned to a C120 molecule. Between molecular rows of C120, dark, straight lines were also found in region I. It is also seen that each C120 molecule was rotated at an angle of 30° with respect to the lattice direction of Au(111). Therefore, the long axis of the C120 molecule is situated in the x3 direction. In region II, the molecular rows were also composed of sets of two spots, and they were aligned along the x3 direction, as indicated by arrow B. Although the rows of C120 molecules in both regions I and II are running in the same direction, the packing density of the C120 molecule is clearly different. In region II, the packing density of C120 molecules is apparently larger than that in region I. In region III, a disordered phase was found on the terrace. Although some spots were observed on the terrace, C120 molecules seem to be located randomly in this region. The surface coverage seemed to be lower than the coverages in regions I and II. Adlayers of C120 were consistently observed in the potential range between 0.8 and 0 V. At potentials more negative than 0 V, C120 adlayers on the terraces disappeared and the reconstructed Au(111) surface was observed. It indicates that the C120 molecules are highly mobile on the surface or desorbed from the Au surface. To understand the structural details of the adlayer of C120, high-resolution STM images were obtained. Figure 5a shows an STM image containing both regions I and II. Each C120 molecule is seen as a set of two bright spots separated by a distance of 0.91 ( 0.02 nm, which is in good agreement with the X-ray crystal structure of the C120 molecule20 and the theoretical calculation.21,22 In region I, the molecular rows in which each molecule consists of a set of two bright spots are clearly seen in the lower part of Figure 5a. From the cross-sectional profile, the intermolecular spacing along the [11h 2 h ] direction was found to be 1.01 ( 0.03 nm, which corresponds to 2x3 times the Au lattice constant. In the [12h 1] direction, the next neighbor distance between C120 molecules was found to be 2.52 ( 0.04 nm, or 5x3 times the Au(111) lattice constant. The unit cell is assumed to be in a (2x3 × 5x3)R30° symmetry with a surface coverage of 0.033. The corrugation height of the two spots with respect to the Au(111) surface was found to be ∼0.5 nm. A structural model of the C120 molecule is superimposed in Figure 5a. In region II, the intermolecular spacings along the [11 h2 h] and [12 h1 h ] directions were found to be 1.01 ( 0.03 and 2.01 ( 0.03 nm, respectively, which correspond to 2x3 and 4x3 times the Au lattice constant. In region II, we assign (2x3 × 4x3)R30° to the unit cell, as with a surface coverage of 0.04. To display the structural details of the C120 adlayer, a height-shaded view of the high-resolution STM image in Figure 5a is shown in Figure 5b. Each C120 molecule is clearly seen to have a dumbbell shape. C60 cages in a C120 molecule were not simply round but showed an internal structure, although C60 molecules are seen as round circles, as shown in Figures 2 and 3. It was reported that when C60 molecules are adsorbed on surfaces such as Si(100)(2 × 1), the rotational movement of C60 is hindered and (20) See the supplementary information of ref 16. (21) Ozaki, T.; Iwasa, Y.; Mitani, T. Chem. Phys. Lett. 1998, 285, 289. (22) Nakamura, J.; Nakayama, T.; Watanabe, S.; Aono, M. Phys. Rev. Lett. 2001, 87, 048301.
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Figure 5. Typical high-resolution STM images of a C120 adlayer on a Au(111) surface (a) and a height-shaded plot (b) in 0.1 M HClO4 acquired at 0.8 V versus RHE. The potential of the tip and the tunneling current were 0.35 V and 1.0 nA, respectively.
the electronic structure can be observed by STM.3,23 For the C120 molecule, the rotational motion may also be prohibited on the Au surface. However, a further study is needed to reveal the intramolecular structure of C120. Finally, it is noteworthy that the open-spaced (2x3 × 5x3)R30° structure was predominantly observed when the surface coverage was decreased. On the other hand, the domain of the more close-packed (2x3 × 4x3)R30° structure was increased with increasing surface coverage. The surface coverage could be controlled by changing the immersion time. Models of the C120 adlayers in (2x3 × 4x3)R30° and (2x3 × 5x3)R30° symmetries on the Au(111)-(1 × 1) surface are presented in Figure 6. The structural models for regions I and II in Figure 4 are given in the lower and upper parts in Figure 6, respectively. In Figure 6, the long axis of each C120 molecule is aligned along the x3 direction of the Au(111) substrate. We assume the center of C120 to be located at the 2-fold bridge site, because, in this orientation, two C60 cages in one C120 molecule are expected to be situated on the same geometric site (near 3-fold site) on the Au(111) surface. As shown in the lower part of Figure 6 (region I, (2x3 × 5x3)R30° structure), (23) Wang, X. D.; Hashizume, T.; Shinohara, H.; Saito, Y.; Nishina, Y.; Sakurai, T. Phys. Rev. B 1993, 47, 15 923.
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molecules are closely packed in the upper part of the model in Figure 6 (region II, (2x3 × 4x3)R30° structure). Correspondingly, in the STM image in Figure 4, no gap is seen between the molecular rows in region II. Conclusions An adlayer of C60 and C120 molecules was formed on the Au(111) surface in benzene solutions and investigated in aqueous HClO4 by in situ STM. High-resolution STM images allowed us to determine packing arrangements of individual C60 and C120 molecules adsorbed on Au(111) in 0.1 M HClO4 at room temperature. A high-resolution STM image exhibited C60 molecules as circles and C120 molecules as dumbbells. Adlayers of C60 molecules formed on Au(111) had either (2x3 × 2x3)R30° or “in-phase” structure. Adlayers of C120 molecules on the Au(111) surface were found to form (2x3 × 4x3)R30° (θ ) 0.04) and (2x3 × 5x3)R30° (θ ) 0.033) structures depending on the surface coverage. Figure 6. Models of C120 adlayers on a Au(111)-(1 × 1) surface with (2x3 × 4x3)R30° and (2x3 × 5x3)R30° superimposed unit cells drawn in the upper left and the lower right sections, respectively.
there is an open space between two molecular rows. This open space corresponds to the dark straight line in the STM image presented in Figure 4. On the other hand,
Acknowledgment. This work was supported in part by a Grant-in-Aid for Science Research (A) (No. 12305055) from the Ministry of Education, Science, Sports and Culture, Japan. The authors acknowledge Dr. Y. Okinaka for his assistance in writing this manuscript. LA0205147
