Highly Ordered Anthracene Adlayers on Ag Single-Crystal Surfaces in

The structures of anthracene adlayers on Ag(111), Ag(100), and Ag(110) electrode surfaces in dilute perchloric acid .... 0.15 V in the electrochemical...
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Highly Ordered Anthracene Adlayers on Ag Single-Crystal Surfaces in Perchloric Acid Solution: In Situ STM Study Taishin Shimooka,† Soichiro Yoshimoto,† Mitsuru Wakisaka,† Junji Inukai,†,‡ and Kingo Itaya*,† Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba-yama 04, Sendai 980-8579, Japan, and PRESTO, JST, Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi City, Saitama Pref. 332-0012, Japan Received June 4, 2001 The structures of anthracene adlayers on Ag(111), Ag(100), and Ag(110) electrode surfaces in dilute perchloric acid have been investigated in detail by using in situ scanning tunneling microscopy (STM). Anthracene was found to form highly ordered molecular layers with a flat-lying orientation on the Ag single-crystal electrode surfaces in the double-layer region. High-resolution STM images allowed us to reveal packing arrangements of anthracene. Highly ordered anthracene adlayers with (2x3 × 2x3)R30° (θ ) 0.08), c(4 × 6) (θ ) 0.08), and c(4 × 4) (θ ) 0.125) symmetries were observed on Ag(111), Ag(100), and Ag(110) surfaces, respectively.

Introduction In interfacial electrochemistry, scanning tunneling microscopy (STM) has been widely accepted as a technique for understanding the structure of adsorbed layers of molecules on metal surfaces at the atomic scale.1,2 Highresolution STM has made it possible to directly determine packing arrangements and even internal structures of organic molecules adsorbed at electrode-electrolyte interfaces. We have already reported adlayer structures of typical aromatic molecules such as benzene,1,3,4 pyrazine,5 benzoic acid,5 terephthalic acid,5 naphthalene,4,6,7 naphthoquinone,6 anthracene,4,6 and anthraquinone6 on various well-defined single-crystal electrodes. Especially, the adlayer structure of benzene on the surface of metals such as Pt, Rh, and Cu has extensively been studied both in an ultrahigh vacuum (UHV) environment8,9 and in solutions.3-7 It has been reported that benzene adlayers with c(2x3 × 3)rect and (3 × 3) symmetries were identified on a Rh(111) surface not only in UHV but also in hydrofluoric acid (HF) solution.3 On the other hand, benzene forms a well-ordered structure of (x21 × x21)R10.9° at negative potentials on Pt(111) in HF solution, whereas it does not form an ordered structure on Pt(111) in UHV,10,11 suggesting that the nature of electrified interfaces plays an important role in the ordering process of adsorbed organic * To whom correspondence should be addressed. Phone: +8122-217-5868. Fax: +81-22-214-5380. E-mail: [email protected]. tohoku.ac.jp. † Tohoku University. ‡ JST. (1) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (2) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129. (3) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (4) Wan, L.-J.; Itaya, K. Langmuir 1997, 13, 7173. (5) Kim, Y.-G.; Yau, S.-L.; Itaya, K. Langmuir 1999, 15, 7810. (6) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. B 1997, 101, 3547. (7) Yau, S.-L.; Itaya, K. Colloids Surf., A 1998, 134, 21. (8) Ohtani, H.; Wilson, R. J.; Chiang, S.; Mate, C. M. Phys. Rev. Lett. 1988, 60, 2398. (9) Weiss, P. S.; Eigler, D. M. Phys. Rev. Lett. 1993, 71, 3139. (10) Wander, A.; Held, G.; Hwang, R. Q.; Blackman, G. S.; Xu, M. L.; de Anders, P.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1991, 249, 21. (11) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons Inc.: New York, 1994.

molecules. Naphthalene was found to form a well-ordered structure of (3x3 × 3x3)R30° on Rh(111)6 and (4 × 5) on Cu(111),4 while only a disordered structure was observed on Pt(111).6 Anthracene adlayers have been investigated on singlecrystal electrodes such as Pt(111), Rh(111), and Cu(111) in solutions as described in our previous papers.1,3-7 Different structures of anthracene adlayers on Pt(111), Rh(111), and Cu(111) unveiled the important role of substrates in the process of organizing molecular adlayers. A highly ordered (4 × 5) structure was formed on Cu(111),4 while only local ordering was found on both Pt(111) and Rh(111) surfaces.3 This substrate effect was interpreted by assuming that anthracene molecules are more strongly adsorbed on Pt(111) and Rh(111) than on Cu(111), resulting in a lower surface diffusion rate and hence a disordered phase on the former two substrates. We have also found that highly ordered molecular arrays of tetrakis(N-methylpyridinium-4-yl)-21H,23H-porphine (TMPyP) and crystal violet molecules were easily formed on iodine-modified Au(111),12,13 Ag(111),14 and Pt(100)15 surfaces, while they formed a disordered adlayer on bare Au(111) in the absence of an iodine monolayer.13 The results described above clearly show that the interaction between molecules and substrates is an important factor in the process of ordering of molecular adsorbates on metal substrates. On the other hand, Bo¨hringer et al. recently investigated the adsorption of aromatic molecules such as anthracene and perylene derivatives (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and N,N′-dimethylperylene-3,4,9,10-bis(dicarboximide) (DM-PBDCI)) on Ag single-crystal surfaces under UHV.16,17 They reported on (12) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245. (13) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607. (14) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (15) Sashikata, K.; Sugata, T.; Sugimasa, M.; Itaya, K. Langmuir 1998, 14, 2896. (16) Glo¨ckler, K.; Seidel, C.; Soukopp, A.; Sokolowski, M.; Umbach, E.; Bo¨hringer, M.; Schneider, W.-D.; Berndt, R. Surf. Sci. 1998, 405, 1. (17) Bo¨hringer, M.; Schneider, W.-D.; Berndt, R. Surf. Sci. 1998, 408, 72.

10.1021/la0108196 CCC: $20.00 © 2001 American Chemical Society Published on Web 09/06/2001

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the formation of a well-packed adlayer of these molecules on Ag(110) surfaces in UHV at 50 K.17 These results obtained in UHV strongly encouraged us to investigate adlayer structures on Ag single-crystal electrodes in solution. To our knowledge, only a few compounds have been investigated in the past on Ag(111) by in situ STM, for example, 1,8-octanedithiol18 and uracil19 in NaF solutions. In this paper, we report the results of our successful in situ STM imaging of highly ordered adlayers of anthracene directly attached to well-defined Ag single-crystal surfaces in 0.1 M HClO4. High-resolution STM images allowed us to determine packing arrangements of anthracene on Ag(111), Ag(100), and Ag(110) single-crystal electrodes in 0.1 M HClO4. Experimental Section Ag single-crystal disks with a diameter of 10 mm and a thickness of 2 mm were obtained from MaTeck Inc., Germany. Before electrochemical and STM measurements, an Ag single crystal was metallographically polished by diamond paste and sonicated successively in acetone, methanol, and pure water. After annealing at 1000 K for 2 h in a stream of hydrogen, the sample was cooled to room temperature in the H2 stream and quickly brought into contact with ultrapure water as described in the previous paper.20 The electrode was left covered with a droplet of pure water to prevent surface contamination, and it was transferred into an electrochemical cell for both voltammetric and STM measurements. The solution was prepared with HClO4 (Cica-Merck) and ultrapure water (Milli-Q SP-TOC; g18.2 MΩ cm-1). Zone-refined anthracene supplied from Aldrich was used. An anthracene saturated aqueous solution was made by ultrasonicating in pure water for 20 min. The cyclic voltammetry was carried out at 20 °C using a potentiostat (HOKUTO HAB-151) and the hanging meniscus method20 in a three-compartment electrochemical cell in a N2 atmosphere. Electrochemical STM measurements were performed by using a Nanoscope E with a commercially available Pt-Ir (80:20) tip (Digital Instruments Inc.). To minimize residual faradic currents, tips were coated with nail polish. Platinum wires were used as quasi-reference and counter electrodes for STM measurements. All potential values are referred to the saturated calomel electrode (SCE).

Results and Discussion Voltammetry. Cyclic voltammograms of Ag singlecrystal electrodes were obtained in 0.1 M HClO4. Figure 1a shows a steady-state cyclic voltammogram of a bare Ag(111) single-crystal electrode in 0.1 M HClO4. The open circuit potential (OCP) of the Ag(111) single-crystal electrode was ca. 0.15 V versus SCE. The electrode potential was held at OCP for at least 30 min. During this period of time, the Ag surface was expected to become atomically flat as a result of surface annealing.20 The first potential scan was started in the negative direction from the OCP, and repetitive potential cycling between -0.7 and 0.3 V was continued several times. At 0.2 V, the anodic dissolution of Ag started, and the anodic current increased abruptly at potentials more positive than 0.2 V. The cathodic peak at 0.28 V observed upon reversal of the potential scan is due to the electrochemical deposition of Ag metal from Ag+ ions formed during the anodic scan. The cathodic current commencing at -0.6 V is due to the hydrogen evolution reaction. (18) Cavallini, M.; Bracali, M.; Aloisi, G.; Guidelli, R. Langmuir 1999, 15, 3003. (19) Cavallini, M.; Aloisi, G.; Bracali, M.; Guidelli, R. J. Electroanal. Chem. 1998, 444, 75. (20) Teshima, T.; Ogaki, K.; Itaya, K. J. Phys. Chem. B 1997, 101, 2046.

Figure 1. Typical cyclic voltammograms of the Ag(111) singlecrystal electrode in 0.1 M HClO4 in the absence (a) and the presence (b) of anthracene. The scan rate was 0.05 V s-1.

After the potential cycling experiment described above, the Ag electrode was transferred into an anthracenesaturated 0.1 M HClO4. The electrode potential was held at -0.15 V for more than 10 min to achieve equilibrium. The cyclic voltammogram obtained in the presence of anthracene is given in Figure 1b. The overall profile of the cyclic voltammogram is almost the same as that of Figure 1a, which was obtained in the absence of anthracene. The double-layer charging current usually becomes smaller than that of bare metal surfaces upon adsorption of organic molecules.3-7 However, in the case of the Ag(111) single-crystal electrode, such a decrease in double-layer charging current was not clearly seen upon addition of anthracene. Only the potentials of the hydrogen evolution reaction and the anodic dissolution of Ag were slightly shifted in the negative and positive directions, respectively. Similar voltammograms for Ag(100) and Ag(110) electrodes were obtained in 0.1 M HClO4 solutions both with and without anthracene. In Situ STM. (1) Ag(111)-(1 × 1) Surface. Figure 2a shows a large-scale STM image of a clean surface of Ag(111) obtained at -0.2 V in 0.1 M HClO4. As described in our previous paper,20 the well-defined Ag surface was easily obtained by holding the electrode potential at ca. 0.15 V in the electrochemical STM cell for about 30 min. During this period of time, the Ag(111) surface became atomically flat, suggesting that surface annealing took place on the Ag single-crystal electrode as described by Kolb et al.20,21 Eventually, atomically flat terraces extended over a few hundred nanometers without the formation of pits and islands, which were frequently found on Pt and Rh.1 Monatomic steps characterized by arc-shaped edges were found, and the terraces were atomically flat with a low defect density, suggesting that the Ag(111) surface had a well-defined structure in the potential range between -0.5 and 0.2 V. Figure 2b is a typical atomic-scale STM image of the Ag(111)-(1 × 1) surface. A hexagonal closepacked structure can be seen with an interatomic distance of 0.29 ( 0.01 nm, indicating that the Ag(111) surface has a (1 × 1) structure. The corrugation amplitude for each Ag atom was found to be ca. 0.02 nm. The identical (1 × (21) Dietterle, M.; Will, T.; Kolb, D. M. Surf. Sci. 1995, 327, L495.

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Figure 2. Large-scale (100 × 100 nm2) (a) and atomically resolved (b) STM images of the Ag(111) surface in 0.1 M HClO4. Substrate and tip potentials were held at -0.15 and 0.61 V, respectively. The tunneling current was 8 nA. Three sets of arrows indicate the close-packed directions of the Ag(111) substrate.

1) structure was consistently observed in the potential range between -0.5 and 0.2 V. After achieving the atomic resolution of Ag(111)-(1 × 1), an anthracene saturated aqueous solution was carefully injected into the electrochemical STM cell at -0.15 V. Within several minutes after the injection of anthracene, a highly ordered adlayer appeared on the Ag(111)-(1 × 1) surface as shown in Figure 3a. In the image acquired in the relatively large area of 20 × 20 nm2, the highly ordered molecular array appears as a single domain consisting of well-ordered spots with almost no defects. Each molecule appears as an elongated spot. The molecular rows cross each other at an angle of either 60° or 120° within an experimental error of (3°. To obtain structural details of the adsorbed molecule, molecular resolution STM images were also acquired at -0.15 V. A typical highresolution STM image acquired in a small area of 10 × 10 nm2 is shown in Figure 3b. From this image, it was concluded that anthracene molecules were adsorbed parallel to the surface and that the length of the longer molecular axis (C2) was ca. 1.0 nm, which corresponds to the length of an anthracene molecule. The cross-sectional profile along the x3 direction indicated that the corrugation height of each molecule is ca. 0.08 nm. It is comparable to the value obtained on a Rh(111) surface.6 Therefore, one elliptical spot is attributable to one anthracene molecule with a flat-lying orientation on the Ag(111) surface. The longer C2 axis of the anthracene molecule was aligned in the 〈110〉 direction, the so-called atomic direction. A precise comparison between this image and that of Ag(111)-(1 × 1) revealed that the molecular row marked by the arrow A is rotated by ca. 30° (x3 directions) from the lattice directions of Ag(111)-(1 × 1). The nearest intermolecular distance was 1.0 nm, which corresponded to 2x3 times the Ag lattice parameter. The unit cell of

Figure 3. High-resolution STM images of a highly ordered anthracene adlayer on Ag(111) in 0.1 M HClO4 saturated with anthracene (a and b). Potentials of the substrate and the tip were held at -0.15 and 0.61 V, respectively. The tunneling current was 8 nA. Schematic representation for the anthracene adlayer on Ag(111) with a (2x3 × 2x3)R30° unit cell (c). Three sets of arrows indicate the close-packed directions of the Ag(111) substrate. The arrow marked A indicates the x3 direction.

anthracene adsorbed onto the Ag(111) surface can be assigned a (2x3 × 2x3)R30° structure with a surface coverage of 0.08. The unit cell is superimposed in Figure 3b. A proposed structural model for the packing arrangement of anthracene on Ag(111)-(1 × 1) is shown in Figure 3c. The central benzene ring of anthracene is assumed to be located on the 2-fold bridge site, the same as for the anthracene adlayer on a Cu(111) surface.4 Interestingly, there is a clear difference between the adlayer structures on Ag(111) and Cu(111) substrates. As shown in our previous work, the adlayer of anthracene on a Cu(111) single-crystal surface has a (4 × 5) structure with a surface coverage of 0.05, in which the two directions in the unit cell are parallel to the Cu(111) lattice for all adsorbed anthracene molecules.4 Many factors must be taken into account to explain the difference between the adlayer structures on Ag(111) and Cu(111). The surface coverage of anthracene (0.08) on Ag(111) is higher than

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Figure 4. Large-scale (60 × 60 nm2) (a) and atomically resolved (b) STM images of the Ag(100)-(1 × 1) surface in 0.1 M HClO4. The Ag(100) electrode and tip potentials were held at -0.4 and 0.56 V, respectively. The tunneling current was 10 nA.

that (0.05) on Cu(111). This difference might primarily be due to the difference in lattice parameters between Ag (0.289 nm) and Cu (0.256 nm). The anthracene adlayer with a (2x3 × 2x3)R30° structure on Ag(111)-(1 × 1) was observed only in a limited potential range between -0.3 and 0 V. When the electrode potential was held at a more positive potential than 0 V, STM images became unclear, but the adlayer of anthracene could still be recognized. These results suggest that either desorption or reorientation of anthracene took place at positive potentials. The featureless cyclic voltammogram profile in Figure 1b shows no indication of any phase transitions. However, because the interaction energy between an aromatic molecule and an Ag electrode should be dependent on the charge at the electrode surface, it is expected that the positive charge at the Ag(111) electrode should decrease the interaction between anthracene molecules and Ag to form a highly ordered adlayer. On the other hand, at potentials lower than -0.3 V, it is assumed that anthracene molecules are either desorbed or highly mobile on the surface. These results might suggest that the charge on Ag(111) plays an important role in determining the adlayer structure. However, the resolution of STM usually depends on tunneling conditions, such as potentials of the substrate and the tip, and tunneling current. Therefore, it is not clear in the present study whether the structural change was caused by a change in the electrode potential or in the tunneling current, which resulted in the unclear STM images. Further work is needed to understand the potential dependence of the adlayer structure. (2) Ag(100)-(1 × 1) Surface. Figure 4a shows an STM image of the clean surface of Ag(100)-(1 × 1) acquired at -0.4 V in 0.1 M HClO4. Atomically flat terraces with a few monatomic step lines were found, which were similar to those observed on Ag(111)-(1 × 1). It can be clearly seen that the Ag(100)-(1 × 1) surface has a square lattice

Figure 5. High-resolution STM images of a highly ordered anthracene adlayer on Ag(100)-(1 × 1) in 0.1 M HClO4 saturated with anthracene (a and b). Potentials of the substrate and the tip were held at -0.4 and 0.56 V, respectively. The tunneling current was 10 nA. Schematic representation for an anthracene adlayer on Ag(100)-(1 × 1) with a c(4 × 6) adlattice (c).

with an interatomic distance of 0.29 ( 0.01 nm along both the [110] and [11 h 0] atomic directions. The close-packed directions, 〈110〉 , of the Ag(100) substrate are indicated by the arrows in Figure 4. The atomic rows of Ag cross each other at 90° within an experimental error of (3°. The corrugation amplitude of each Ag atom is about 0.04 nm. The unreconstructed (1 × 1) structure of Ag(100) was retained in the potential range from -0.5 to 0.1 V, suggesting that potential-induced reconstruction does not occur on the Ag(100) surface under the present condition.20 After atomic resolution of Ag(100)-(1 × 1) was achieved, 1 drop of a saturated anthracene aqueous solution was added into 0.1 M HClO4 at -0.4 V. Completely different patterns appeared in STM images within 20 min after the injection of anthracene. Figure 5a shows a typical example of the STM images obtained, revealing a long-range ordered anthracene adlayer. In the image of 40 × 40 nm2,

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Figure 6. Large-scale (100 × 100 nm2) (a) and atomically resolved (b) STM images of the Ag(110)-(1 × 1) surface in 0.1 M HClO4. Potentials of the Ag(110) electrode and the tip were held at -0.5 and 0.64 V, respectively. The tunneling current was 8 nA.

only a single domain with well-ordered elongated spots was seen over the scanned areas as shown in Figure 5a. A typical high-resolution STM image of anthracene molecules acquired in an ordered domain is shown in Figure 5b. It is clearly seen that the longer C2 axis of anthracene molecules is aligned along the [110] atomic direction of the Ag substrate indicated by the arrows. In this image, anthracene molecules are adsorbed in parallel to the surface, and the spot length of the longer C2 axis is ca. 1.0 nm, which corresponds to the length of an anthracene molecule. The intermolecular distance was 1.16 and 1.74 nm along the [11 h 0] and [110] atomic directions, respectively, which are nearly equal to 4 and 6 times the Ag lattice parameter. The adlattice of anthracene adsorbed onto the Ag(100)-(1 × 1) surface was determined to be a commensurate c(4 × 6) structure with the coverage of 0.08. A possible model is presented in Figure 5c. The center portion of an anthracene molecule is expected to be located on a 2-fold coordinated bridge site of the square lattice. The commensurate c(4 × 6) anthracene adlayer on Ag(100)-(1 × 1) was clearly observed in the potential range between -0.5 and -0.3 V. (3) Ag(110)-(1 × 1) Surface. Recently, the structure of an anthracene adlayer on Ag(110) has been scrutinized by low-temperature STM in UHV.17 A c(4 × 4) structure was revealed as the packing arrangement in UHV at 50 K, and an extraordinary high-resolution STM image of each anthracene molecule was shown. Those results strongly encouraged us to investigate the adlayer structure of anthracene on Ag(110) in solution at room temperature. STM images of an Ag(110)-(1 × 1) surface in 0.1 M HClO4 are shown in Figure 6. When the electrode potential was held at OCP, wide atomically flat terraces were easily observed as shown in Figure 6a. Figure 6b shows a highresolution STM image acquired at -0.5 V, revealing that

Figure 7. High-resolution STM images of a highly ordered anthracene adlayer on Ag(110)-(1 × 1) in 0.1 M HClO4 saturated with anthracene (a and b). Potentials of the substrate and the tip were held at -0.5 and 0.64 V, respectively. The tunneling current was 8 nA. The cross-sectional profile in (c) reveals the corrugation along the arrow in (b). Schematic representation for an anthracene adlayer on Ag(110) with a rectangular c(4 × 4) adlattice (d).

the Ag(110) surface has a (1 × 1) structure. The interatomic distances for the close-packed atomic row directions of [11 h 0] and [001] were 0.29 and 0.41 nm, respectively. The corrugation amplitude of each Ag atom was found to be ca. 0.05 nm. Under the present condition, an unreconstructed (1 × 1) structure was clearly observed in the

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potential range between -0.5 and 0.2 V without potentialinduced reconstruction. After the lattice direction was confirmed, STM images were acquired at -0.5 V in an anthracene-saturated 0.1 M HClO4 solution. A long-range ordered domain with almost no defects was consistently observed over the wide terraces as shown in Figure 7a. The molecular rows consisted of elongated bright spots. From a comparison with the [001] direction determined by the Ag(110)-(1 × 1) atomic image as indicated by the arrows, it can be seen that all molecular rows are almost perfectly parallel to the [001] direction. Figure 7b shows a typical high-resolution STM image, in which the image of each molecule shows details of the internal structure of anthracene. It is also clear that the longer C2 axis of each anthracene molecule is almost aligned along the [001] direction. The cross-sectional profile along the arrow sign in Figure 7b is shown in Figure 7c. The length of the bright spot was 0.9-1.0 nm, which corresponds to the length of the longer C2 axis of a single anthracene molecule. The averaged corrugation height of anthracene was found to be ca. 0.08 nm, which is comparable to the values for benzene and naphthalene imaged in UHV8 and in solution.3,4 The intermolecular distances along the [001] and [11 h 0] directions were 1.64 and 1.16 nm, which correspond to 4 times the Ag(110) lattice parameter for the [001] and [11h 0] directions, respectively. From the results described above, the adlattice of anthracene on the Ag(110)-(1 × 1) surface can be defined as a c(4 × 4) structure shown by the adlattice superimposed in Figure 7b. In the high-resolution STM image of Figure 7b, each anthracene molecule appears as a set of two parts with similar intensities. Although eight separated spots were observed for a single anthracene molecule at Ag(110) in UHV at 50 K,17 we did not find such separated spots under the present experimental conditions. The center ring and two side rings of anthracene appear as a darker spot and two brighter spots, respectively. In our previous study, the packing arrangement and internal structure of the anthracene adlayer on Rh(111) was investigated in a hydrofluoric acid solution by in situ STM.6 Despite the lack of ordering, the STM image unambiguously disclosed the internal structure of anthracene with clear identification of three craters in each molecule. These results strongly suggest that the STM images of anthracene depend on the substrates as well as tunneling conditions such as the magnitudes of tunneling current and the electrode potentials of both the substrate and the tip. Figure 7d displays a model of a c(4 × 4) adlayer on Ag(110)-(1 × 1), which is identical to that proposed previously for anthracene on Ag(110) in UHV at 50 K.17 In this model, the longer C2 axis of the anthracene molecule

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is well aligned along the [001] direction and the perpendicular [11 h 0] direction, and the central benzene ring of anthracene is assumed to be located on a 2-fold coordinated bridge site. It is surprising that the packing arrangement of anthracene adsorbed on Ag(110) in solution is the same as that observed in UHV at 50 K, suggesting that water molecules play a minor role in stabilizing the adlayer of anthracene. We believe that the two-dimensional arrangement of anthracene molecules is determined mainly by adsorbate-adsorbate interaction. The commensurate c(4 × 4) anthracene adlayer was seen in the potential range between -0.55 and -0.4 V. Finally, the present results encourage us to explore potential applications of in situ STM in solutions for investigating the adsorption of various organic molecules on the low-index Ag singlecrystal electrodes. Conclusions In the present study, highly ordered adlayers of anthracene were observed on well-defined Ag single-crystal surfaces, and the packing arrangement and internal molecular structure of anthracene were demonstrated in an aqueous HClO4 solution containing saturated anthracene by high-resolution STM images. Anthracene molecules formed a long-range ordered adlayer on all three Ag single-crystal surfaces with a flat-lying orientation. The in situ STM image of the adlayer of anthracene on Ag(111) revealed a (2x3 × 2x3)R30° structure with a coverage of 0.08. Each molecule appeared with its longer C2 axis along the 〈110〉 atomic direction of the Ag(111)-(1 × 1) lattice. On Ag(100)-(1 × 1), a c(4 × 6) structure with a coverage of 0.08 was observed with the elongated spots of the longer C2 axis of the anthracene molecule aligned along the Ag(100)-(1 × 1) lattice direction. A c(4 × 4) structure with a coverage of 0.125 was found on Ag(110)(1 × 1) surfaces. The present results show that in situ STM imaging can supply direct information on the adsorption of anthracene on low-index Ag single-crystal electrodes. Internal structures of anthracene adlayers on Ag(111), Ag(100), and Ag(110) surfaces were successfully probed in solution. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research for Science Research (A) (No. 12305055) from the Ministry of Education, Science, Sports and Culture, Japan, and partially by Precursory Research for Embryonic Science and Technology (PRESTO) organized by the Japan Science and Technology Corporation (JST). The authors acknowledge Dr. Y. Okinaka for his useful discussion in writing this manuscript. LA0108196