Langmuir 2008, 24, 11611-11615
11611
Highly Ordered C60 Monolayer Self-Assembled by Using an Iodine Template on an Au(111) Surface in Solution Yaw-Chia Yang,† Chi-Yun Chen,‡ and Yuh-Lang Lee*,† Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan, and Nan Jeon Institute of Technology, Tainan 73746, Taiwan ReceiVed June 3, 2008. ReVised Manuscript ReceiVed July 27, 2008 An iodine-modified Au(111) surface, (I/Au(111)), was used as a substrate to prepare a C60 adlayer by self-organization in a benzene solution. A highly ordered C60 adlayer was successfully prepared due to the moderate C60-I/Au(111) interaction. Two lattice structures, (23 × 23)R30° and p(2 × 2), were imaged for this C60 adlayer. For the first structure, a featureless ball-like molecular shape was imaged, ascribed to the molecular rotation resulting from a symmetrical location between C60 and iodine atoms. For the p(2 × 2) structure, the asymmetrical location of C60 with respect to the iodine atoms freezes the C60 molecules on the substrate, leading to a clear image of intramolecular structure. The intermediate iodine atoms in the C60/I/Au(111) adlayer can be desorbed by electrochemically reduction without significantly affecting the ordering of the C60 adlayer. However, the internal pattern of C60 disappears in the absence of iodine.
Introduction Recently, ordered adlayers of fullerenes on semiconductors (Si, Ge, GaAs) and metals (Au, Ag, Cu) have attracted high amounts of interest due to their unique physical and chemical properties for fabrication molecular electronic devices.1-8 Various methods have been used to prepare fullerene adlayers, including sublimation in an ultrahigh vacuum (UHV) system, self-assembly in liquid solutions,9 and deposition by the Langmuir-Blodgett technique.10 By using an appropriate assembling method, ordered adlayers of fullerene have been prepared.11 By using a sublimation process performed in an UHV system, long-range ordered fullerene domains with size above 30 nm were reported in the literature.12 However, for the assembling process performed in mild conditions such as in a solution system, the domains of * Corresponding author. Telephone: 886-6-2757575 ext 62693. Fax: 8866-2344496. E-mail:
[email protected]. † National Cheng Kung University. ‡ Nan Jeon Institute of Technology. (1) Shi, B. R.; Wang, X. S.; Huang, H. J.; Yang, S. H.; Heil, W.; Cue, N. J. Phys. Chem. B 2001, 105, 11414. (2) Taninaka, A.; Shino, K.; Sugai, T.; Heike, S.; Terada, Y.; Hashizume, T.; Shinohara, H. Nano Lett. 2003, 3, 337. (3) Wang, K. D.; Zhao, J.; Yang, S. F.; Chen, L.; Li, Q. X.; Wang, B.; Yang, S. H.; Yang, J. L.; Hou, J. G.; Zhu, Q. S. Phys. ReV. Lett. 2003, 91, 185504. (4) Sakurai, T.; Wang, X. D.; Xue, Q. K.; Hasegawa, Y.; Hashizume, T.; Shinohara, H. Prog. Surf. Sci. 1996, 51, 263. (5) Guo, S.; Fogarty, D. P.; Nagel, P. M.; Kandel, S. A. J. Phys. Chem. B 2004, 108, 14074. (6) Shinohara, H.; Inakuma, M.; Kishida, M.; Yamazaki, S.; Hashizume, T.; Sakurai, T. J. Phys. Chem. 1995, 99, 13769. (7) Hasegawa, Y.; Ling, Y.; Yamazaki, S.; Hashizume, T.; Shinohara, H.; Sakai, A.; Pickering, H. W.; Sakurai, T. Phys. ReV. B 1997, 56, 6470. (8) Murray, P. W.; Pedersen, M. O.; Laegsgaard, E.; Stensgaard, I.; Besenbacher, F. Phys. ReV. B 1997, 55, 9360. (9) (a) Yoshimoto, S.; Narita, R.; Tsutsumi, E.; Matsumoto, M.; Itaya, K.; Ito, O.; Fujiwara, K.; Murata, Y.; Komatsu, K. Langmuir 2002, 18, 8518. (b) Marchenko, A.; Cousty, J. Surf. Sci. 2002, 513, 233. (10) (a) Uemura, S.; Sakata, M.; Taniguchi, I.; Kunitake, M.; Hirayama, C. Langmuir 2001, 17, 5. (b) Uemura, S.; Sakata, M.; Hirayama, C.; Kunitake, M. Langmuir 2004, 20, 9198. (11) (a) Altman, E. I.; Colton, R. J. Surf. Sci. 1992, 279, 49. (b) Sakurai, T.; Wang, X. D.; Xue, Q. K.; Hasegawa, Y.; Hashizume, T.; Shinohara, H. Prog. Surf. Sci. 1996, 51, 263, and references therein. (12) (a) 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. (b) Wachowiak, A.; Yamachika, R.; Khoo, K. H.; Wang, Y.; Grobis, M.; Lee, D.-H.; Louie, S. G.; Crommie, M. F. Science 2005, 310, 468. (c) Schull, G.; Berndt, R. Phys. ReV. Lett. 2007, 99, 226105.
adlayers are always short-range (less than 10 nm in size) and it is still difficult to prepare a highly ordered fullerene adlayer with controlled structure and long-ranged ordered domains. The preparation of an ordered adlayer on a solid substrate is a process involving two-dimensional (2D) nucleation and growth of a 2D crystalline monolayer. This process is governed by both molecule-molecule and molecule-substrate interactions, which directly affect the quality and properties of an adlayer. To adjust these interactions, substrate temperature and deposition rate were controlled for sublimation in an UHV system.13 For assembling in a liquid system, template layers were used to modify the molecular mobility on a substrate. Organic materials such as porphyins,14 calixarenes,15,16 cyclothiophene,17 coronene,18 and pentacene19 were commonly used as template monolayers to incorporate C60 using donor-acceptor interaction. As an alternative, iodine-modified Au(111) electrodes (I/Au(111)) were used to improve the ordering of a C60 adlayer transferred from a Langmuir monolayer at the air/water interface.9,10 In that work, the C60 adlayer on I/Au(111) cannot be imaged by scanning tunneling microscopy (STM); however, after an electrochemical removal and replacement of an iodine adlayer, a highly ordered C60 adlayer was reported. Iodine-modified metal surfaces had been used to prepare ordered arrays of porphyrin monolayers by self-organization in electrolyte solutions.20 In this work, I/Au(111) is used as a substrate to prepare a highly ordered C60 adlattice. Because (13) (a) Hsu, C. L.; Pai, W. W. Phys. ReV. B 2003, 68, 245414. (b) Pai, W. W.; Hsu, C. L. Phys. ReV. B 2003, 68, 121403R. (c) Pai, W. W.; Hsu, C. L.; Chiang, C. R.; Chang, Y.; Lin, K. C. Surf. Sci. 2002, 519, 605. (d) Pai, W. W.; Hsu, C. L.; Lin, M. C.; Lin, K. C.; Tang, T. B. Phys. ReV. B 2004, 69, 125405. (e) Pai, W. W.; Hsu, C. L.; Lin, M. C.; Lin, K. C.; Tang, T. B. Appl. Surf. Sci. 2005, 241, 194. (14) Yoshimoto, S.; Saito, A.; Tsutsumi, E.; D’Souza, F.; Ito, O.; Itaya, K. Langmuir 2004, 20, 11046. (15) Wan, L.-J. Acc. Chem. Res. 2006, 39, 334. (16) (a) Pan, G.-B.; Liu, J.-M.; Zhang, H.-M.; Wan, L.-J.; Zheng, Q.-Y.; Bai, C.-L. Angew. Chem., Int. Ed. 2003, 115, 2853. (b) Pan, G.-B.; Cheng, X.-H.; Ho¨ger, S.; Freyland, W. J. Am. Chem. Soc. 2006, 128, 4218. (17) Mena-Osteritz, E.; Ba¨uerle, P. AdV. Mater. 2006, 18, 447. (18) Yoshimoto, S.; Tsutsumi, E.; Fujii, O.; Narita, R.; Itaya, K. Chem. Commun. 2005, 1188. (19) Yang, Y. C.; Chang, C. H.; Lee, Y. L. Chem. Mater. 2007, 19, 6126. (20) (a) Kunitake, M.; Batina, N.; Itaya, K. Langmuir 1995, 11, 2337. (b) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (c) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607.
10.1021/la801704n CCC: $40.75 2008 American Chemical Society Published on Web 09/04/2008
11612 Langmuir, Vol. 24, No. 20, 2008
Yang et al.
I/Au(111) was known to be more inert than bare Au(111), a weaker C60-substrate interaction is expected on the I/Au(111) surface.10,21 This effect is supposed to cause a higher mobility of C60 on the substrate, triggering a lower nucleation rate and larger domain of 2D crystal. This inference is confirmed in this work, and a facile method for preparing highly ordered C60 adlayers is reported in the present study.
Experimental Section Fullerene-C60 was purchased from Aldrich (99.5%). An aqueous HClO4 solution was prepared with HClO4 (Cica-Merk) and ultrapure water (Milli-Q SP-TOC; g18.2 MΩ cm). Benzene was obtained from Riedel-de Hae¨n (Germany) Chemical Co. Potassium iodine (KI) was obtained from SHOWA (Japan) Chemical Co. A C60 adlayer on an iodine array was formed by immersing an Au(111) electrode successively into a 0.1 M HClO4 solution containing 1 mM KI for about 1 min and then into a 10 µM C60-benzene solution for 2 min. The C60/I/Au electrode was then transferred into an electrochemical cell for both voltammetric and STM measurements. Au electrodes were prepared according to a procedure described elsewhere.22,23 The scanning tunneling microscope used is a Nanoscope E (Digital Instruments) equipped with an electrochemical system. Cyclic voltammetry (CV) was carried out at 25 °C using a potentiostat (CHI-703, Austin, TX). The tip was made of tungsten (diameter, 0.25 mm) prepared by electrochemical etching in 2 M KOH. A reversible hydrogen electrode (RHE) was used in the electrochemical and STM measurements, and all the potentials in this work refer to a RHE scale.
Results and Discussion An iodine-modified Au(111) substrate (I/Au(111)) was prepared by immersing an Au(111) electrode in a 0.1 M HClO4 solution containing 1 mM KI for about 1 min. An ordered array of iodine adlayer with a structure of (3 × 3)R30° was prepared, as determined by STM (Nanoscope E, Digital Instruments). This structure is consistent with that reported in the literature.24 A C60 adlayer was self-assembled on the I/Au(111) or Au(111) surface by immersing the electrode in a 10 µM C60benzene solution for 2 min. Figure 1 shows typical cyclic voltammograms of a clean Au(111) electrode, as well as the electrodes modified by iodine, C60, and C60/iodine. The nearly featureless CV curve observed for the bare Au(111) electrode is identical to that reported for a well-defined Au(111) electrode.22 For the Au(111) electrode modified by iodine, C60, or C60/iodine, a negative scanning was initiated at the open-circuit potential (OCP, about 0.8 V) using a scan rate of 50 mV/s. A decrease of double-layer charging current was observed for these modified electrodes. These adlayers are found to be stable in the featureless double-layer charging region between 0.4 and 0.9 V, and a small reduction current commences at 0.4 V, attributable to the partially reductive desorption of the admolecules. The reductive curves corresponding to the I/Au, C60/Au, and C60/I/Au electrodes are very similar except that the I/Au electrode has a higher reductive current at potentials more negative than 0.2 V, indicating a more significant desorption of iodine in the region.24 For the three modified Au(111) electrodes, the apparent difference was observed for C60/I/Au in the positive sweeping. The reductive (21) (a) Wang, D.; Wan, L.-J.; Wang, C.; Bai, C.-L. J. Phys. Chem. B 2002, 106, 4223. (b) Su, G.-J.; Gan, L.-H.; Yang, Z.-Y.; Pan, G.-B.; Wan, L.-J.; Wang, C.-R. J. Phys. Chem. B 2006, 110, 5559. (22) (a) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1. (b) Kolb, D. M. Prog. Surf. Sci. 1996, 51, 109. (23) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (24) (a) Gao, X.; Weaver, M. J. J. Am. Chem. Soc. 1992, 114, 8544. (b) Sugita, S.; Abe, T.; Itaya, K. J. Phys. Chem. 1993, 97, 8780. (c) Batina, N.; Yamada, T.; Itaya, K. Langmuir 1995, 11, 4568.
Figure 1. Cyclic voltammograms recorded at 50 mV/s in 0.1 M HClO4 for a bare Au(111) electrode and electrodes modified by iodine, C60, or C60/iodine.
currents of I/Au and C60/Au decrease slowly with an increase of potential during the anodic scan, indicating that the desorbed molecules gradually readsorb. For the C60/I/Au, the current decreases abruptly just when the scan was reversed and quickly approaches a negligible value in the following positive scan. The sharp decrease of the current density to an approximately zero value implies that the Au(111) surface was quickly covered by a C60/I adlayer in the initial period of the positive sweeping. The different behaviors between C60/Au and C60/I/Au electrodes suggest that the desorbed molecules from an electrode can readsorb very quickly for C60/I/Au, while a much slower rate is expected on C60/Au. The different interactions of C60 on Au(111) and I/Au(111) electrodes are supposed to give different properties of the C60 adlayer on the two electrodes. STM images of C60 adlayers self-assembled on Au(111) and I/Au(111) surfaces are shown in Figure 2. On a bare gold surface (Figure 2a), the patchy appearance of the C60 adlayer indicates that the ordered domains are short-range, containing a lot of defects. Two various structures, (23 × 23)R30° and “inphase”, were identified for the ordered C60 lattices, consistent with the results reported in the literature.9,11 When the Au(111) surface was modified by iodine, the C60 adlayer became more regular and larger-scale ordered domains were imaged (Figure 2b). The ordered domains shown in Figure 2b have an average scale of ca. 20-30 nm, but several domains can extend to a length over 60 nm (phase II in Figure 2b) with only few defects distributing among a domain. The coverage of the C60 adlayer on the substrate is high, as revealed by the 150 × 150 nm2 scale STM image shown in Figure 2c. For the C60 adlayer shown in Figure 2, the presence of an iodine adlayer below C60 is confirmed by detaching C60 molecules using tip scraping.25 Apparently, the iodine interlayer plays an important role in modifying the C60-substrate interaction, leading to a highly ordered arrangement of C60 molecules. The interaction of C60 with the substrate was evaluated by the tunneling current (set point) required to detach a C60 adlayer. For the I/Au(111) electrode, a tunneling current of about 3 nA should be applied, while a much higher value (about 10 nA) is required for the bare Au(111) surface. This result indicates that C60 has a weaker interaction with the I/Au(111). A weak C60-substrate interaction is known to give a higher mobility of C60 on the substrate, which is responsible for the formation of a highly ordered C60 adlayer. (25) Kolb, D. M.; Simeone, F. C. Electrochim. Acta 2005, 50, 2989.
C60 Monolayer Assembled on an Iodine Template
Langmuir, Vol. 24, No. 20, 2008 11613
Figure 2. STM images of C60 adlayers self-assembled on Au(111) (a) and I/Au(111) (b, c) surfaces in benzene solution. The imaging was performed in a 0.1 M HClO4 solution at 0.75 V. The bias voltage and tip current were -135 to 250 mV and 500-800 pA, respectively.
Figure 3. Molecular-resolution STM image (5 × 5 nm2) of a C60 adlayer on I/Au(111) with a structure of (23 × 23)R30° (a). The imaging was obtained in 0.1 M HClO4 at 0.75 V. The bias voltage and tip current were -250 mV and 1.3 nA, respectively. The corresponding molecular structures of C60 and iodine on a Au(111) surface are shown in part b.
For the C60 adlayer on I/Au(111), two various structures (phases I and II) were clearly exhibited in Figure 2b. The two phases appear almost simultaneously in every STM image with a scale larger than 100 × 100 nm2 (Figure S1). However, it is difficult to estimate the precise fractional area covered by a specific phase. According to the typical STM images shown in Figure S1, the fractional area covered by structure I ranges between 0.2 and 0.8. Figure 3 shows high resolution STM images corresponding to phase I. For phase I shown in Figure 3a, all C60 molecules were demonstrated as spots of similar brightness and ball-like shape, arranged in a hexagonal lattice. The distance between neighboring molecules was estimated to be 1.0 ( 0.1 nm, which is consistent with the van der Waals diameter of C60 reported in the literature.9,11 Because the molecular image of structure I resembles that on a bare Au(111) surface, it is possible that these C60 molecules were assembled on Au(111) rather than on a I/Au(111) surface. To decipher this possibility, the tip scraping experiment was performed only on structure I and the result is shown in Figure S2. A C60-free region, obtained by tip scraping, appears in the central region of Figure S2a. After zooming in this on region, an iodine adlayer is observed, confirming the presence of an iodine adlayer below C60. This structure is identified as (23 × 23)R30° with respect to the Au(111) substrate. This structure, as well as the relative position of C60 to the underlying iodine atoms, are derived from STM images containing both C60 and iodine adlayers, as shown in Figure S2. The detailed location of C60 and iodine on a Au(111) lattice is schematically shown in Figure 3b. Each C60 is located just above an iodine atom at the 3-fold site of the Au(111) lattice and surrounded by six iodine atoms symmetrically located around the periphery of a C60. According to this model, every C60 in this structure has an equivalent environment with respect to iodine, gold atoms, and
other C60, leading to the identical molecular contrast shown in Figure 3a. A higher resolution image shown for structure II exhibits an ordered array with a modulation in intensity (Figure 4a). The moire´ pattern, as well as all spots contained in the adlattice, exhibits hexagonal order. The molecular row of C60 aligns along the atomic row of Au, the 〈110〉 direction. According to the unit cell shown in Figure 4c, a p(2 × 2) suprastructure was determined for this C60 structure, which is equivalent to a (7 × 7) hexagonal lattice reported for C60/Au(111).5,13d,26 The molecular resolution STM image shown in Figure 4b demonstrates not only the contrast between C60 spots but also detailed information regarding the intramolecular structure of C60 molecules. Apparently, the C60 molecules present in structure II do not have an identical state as that in structure I. One of the reasons leading to the distinct properties of the two structures is the difference of iodine structure below C60. Various iodine structures, depending on the applied potential, were reported in the literature.24 However, this possibility is excluded because only one structure, (3 × 3)R30°, was obtained at the potential range of the present study (0.75-0.8 V). Therefore, the difference between the two C60 structures is attributed to different locations of C60 molecules on an iodine adlayer. The nonspherical and asymmetric molecular shape shown in Figure 4b implies an asymmetrical location of C60 with respect to the underlying iodine adlayer. A proposed arrangement of C60 molecules for structure II is shown in Figure 4c. According to the brightness of spots, three kinds of spots (B, M, and D indicated in Figure 4b) can be classified for the C60 molecules. Such bright-dim contrast between molecules of a C60 adlayer on single crystal surfaces had been reported in the literature using an UHV-STM.13 In those papers, this brightness difference was ascribed to the electronic difference due to
11614 Langmuir, Vol. 24, No. 20, 2008
Yang et al.
Figure 4. High resolution STM images show a moire´ pattern (a) and a molecular resolution (b) of the C60 adlayer on I/Au(111) with a structure of p(2 × 2). The imaging was obtained in 0.1 M HClO4 at 0.75 V. The bias voltage and tip current were -250 mV and 0.65 ∼ 2 nA, respectively. Proposed molecular structures of C60 and iodine on a Au(111) surface are shown in part c.
nonequivalent orientation of C60 rather than a geometric feature. Furthermore, surface reconstruction of substrate was also reported to induce bright-dim molecular images of a C60 adlayer.27 For the present result, the molecule with the highest brightness (B) is supposed to locate on the top position of an Au atom, which is known to have a higher brightness compared to that located on the bridge or 3-fold sites. On the corner of the unit cell shown in Figure 4c, four molecules of spot B are positioned. The other sites in the unit cell were filled by molecules of medium (M) or dim (D) contrast, both located on the bridge site of the Au lattice. The different brightnesses between molecules at M and D sites are attributable to the different interactions between neighboring molecules and/or to the orientation difference.26,28 Comparing between molecular shapes obtained for structures I and II, a symmetrical and spherelike feature was imaged for structure I, but not for structure II. Since a STM image reveals the distribution of electronic density on a molecule, the uniform molecular image of structure I implies that the electrons are not localized at specific positions. Due to the weak interaction between C60 and iodine, as well as the symmetric location of iodine around a C60, it is inferred that the C60 can rotate on its adsorption site, leading to a uniform distribution of electron density on a C60.26-30 In Figure 4b, four holes corresponding to the internal pattern of C60 were imaged for structure II. This internal pattern is especially clear for the brightest spots (B) but appears only occasionally for the medium and dim spots, probably due to their lower contrast. The visualization of intramolecular structure of C60 was only achieved by a few researchers using a UHV system at an extremely low temperature where the C60 is frozen from rotating.13,28,31-33 At higher temperatures, such as room temperature, only a featureless molecular image can be observed, attributed to the molecular rotation on the surface. For the present study, the electronic contribution of an iodine adlayer (that is, the distribution of underlying iodine atoms on the variation of conductivity) to the appearance of the intramolecular image is excluded due to the following reasons: (a) Because an iodine adlayer exists in both structures I and II, if the conductivity variation leads to the intramolecular image of structure II, it should also trigger the intramolecular image of structure I. (b) If C60 is not frozen and there is a significant effect (26) Rogero, C.; Pascual, J. I.; Go´mez-Herrero, J.; Baro´, A. M. J. Chem. Phys. 2002, 116, 832. (27) Grobis, M.; Lu, X.; Crommie, M. F. Phys. ReV. B 2002, 66, 161408(R) (28) (a) Giudice, E.; Magnano, E.; Rusponi, S.; Boragno, C.; Valbusa, U. Surf. Sci. Lett. 1998, 405, L561. (b) Costantini, G.; Rusponi, S.; Giudice, E.; Boragno, C.; Valbusa, U. Carbon 1999, 37, 727. (29) (a) Hou, J. G.; Jinlong, Y.; Haiqian, W.; Qunxiang, L.; Changgan, Z.; Hai, L.; Wang, B.; Chen, D. M.; Qingshi, Z. Phys. ReV. Lett. 1999, 83, 3001. (b) Cepek, C.; Fasel, R.; Sancrotti, M.; Greber, T.; Osterwalder, J. Phys. ReV. B 2001, 63, 125406. (30) Altman, E. I.; Colton, R. J. Phys. ReV. B 1993, 48, 18244.
Figure 5. STM image of a C60 adlayer acquired after electrochemical desorption of iodine from a C60/I/Au(111) electrode. The ordered C60 array was preserved, but the moire´ pattern and intramolecular structure disappear due to the absence of the iodine layer. The bias voltage and tip current were -234 mV and 8 nA, respectively.
of the conductivity variation (of the iodine adlayer) on the imaging process, the image should reflect the conductivity distribution of the iodine layer but not the C60-like image shown in Figure 4b. (c) The molecular image of structure II is similar to that reported in the literature for the UHV-STM system at very low temperature.13,28,31-33 Therefore, the C60 molecules are supposed to be frozen from rotating, as proposed in those papers. The appearance of the internal pattern in the present work indicates that these C60 molecules are frozen at the adsorption sites even at room temperature. Compared with the symmetrical location of structure I, it is inferred that the asymmetrical location of C60 compared to the underlying iodine atoms prevents the free rotation of C60, and therefore, the intramolecular structure can be imaged. The different molecular features obtained for structures I and II also tell that a molecular rotation on a solid surface is not only controlled by the molecule-substrate interaction but also by the symmetrization of an admolecule to the underlying template layer. It is noteworthy that this is the first one clearly imaging the internal pattern of C60 on a gold substrate in a solution system. The CV result shown in Figure 1 indicates that both iodine and C60 display cathodic desorptions at negative potentials and readsorption in the anodic scan. After observation of the highly ordered C60 adlayer and confirming the presence of an underlying
C60 Monolayer Assembled on an Iodine Template
iodine layer, the reductive desorption behavior of the C60/I/ Au(111) adlayer was examined under the in situ STM cell by applying a reductive potential of 0.35 V. After about 30 min, the potential was switched back to the imaging conditions. An ordered array of C60 adlayers can also be observed (as shown in Figure 5) but with slight decrease of the ordering. According to a tip scraping experiment performed on this adlayer, the iodine adlayer no longer presents below the C60. However, the C60/Au(111) adlayer obtained by reductive desorption of iodine has a much higher ordering in comparison with that prepared by direct assembling of C60 on Au(111) (Figure 2a). Repeated experiments have been made to confirm the reproducibility of this phenomenon. A further examination of the C60 structure across the surface, after this reductive reaction, did not find either structure II or the appearance of the internal pattern of C60. The lattice structures determined for this adlayer are (23 × 23)R30° and “inphase”, consistent with the result of C60 adsorbed on bare Au(111).9,11,30 This result indicates that iodine atom, although present in the inner layer of the adsorbed film, is more easily detached than C60. It is inferred that both iodine and C60 do not desorb far away from the Au(111) surface but are present in the double layer. When the potential is positively swept, C60 readsorbs more quickly than iodine does and replaces the iodine. It is amazing to find that the C60 adlayer can still preserve the high ordering after readsorption without incorporation of iodine.10a (31) (a) Silien, C.; Pradhan, N. A.; Ho, W. Phys. ReV. B 2004, 69, 115434. (b) Ne´el, N.; Kro¨ger, J.; Limot, L.; Frederiksen, T.; Brandbyge, M.; Berndt, R. Phys. ReV. Lett. 2007, 98, 065502. (c) Andreas Larsson, J.; Elliott, S. D.; Greer, J. C.; Repp, J.; Meyer, G.; Allenspach, R. Phys. ReV. B 2008, 77, 115434.
Langmuir, Vol. 24, No. 20, 2008 11615
Conclusion By using an iodine-modified Au(111) surface to adjust the molecule-substrate interaction, a highly ordered C60 adlayer was prepared by self-organization in a solution system. Two lattice structures were obtained for a C60 adlayer on a I/Au(111) surface. For structure I, C60 molecules are located at a site symmetrical to the iodine atoms. A featureless molecular shape was imaged for this structure, ascribed to the rotating of C60 at the adsorption site. For structure II, C60 molecules are frozen on the substrate due to the asymmetrical interaction with the neighboring iodine atoms. Therefore, the intramolecular structure of C60 was clearly imaged by STM. The iodine template layer in the C60/I/Au(111) electrode can be detached by electrochemical desorption without affecting the ordering of the C60 adlayer. Acknowledgment. This work was sponsored by the National Science Council of Taiwan under Contract Nos. NSC 96-ET7-006-001-ET and NSC 96-2221-E-006-058. Supporting Information Available: STM images of C60 adlayers on I/Au(111) surfaces. This material is available free of charge via the Internet at http://pubs.acs.org. LA801704N (32) Wang, H.; Zeng, C.; Wang, B.; Hou, J. G. Phys. ReV. B 2001, 63, 085417. (33) Lu, X.; Grobis, M.; Khoo, K. H.; Louie, S. G.; Crommie, M. F. Phys. ReV. Lett. 2003, 90, 096802.
