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2007, 111, 12139-12144 Published on Web 08/01/2007
Structures and Stability of Ferrocene Derivative Monolayers on Ag(110): Scanning Tunneling Microscopy Study R. F. Dou,† D. Y. Zhong,† W. C. Wang,† K. Wedeking,‡ G. Erker,‡ L. Chi,*,† and H. Fuchs† Physikalisches Institut, Wilhelm-Klemm-Strasse 10 & Center for Nanotechnology (CeNTech), GieVenbecker Weg 11, UniVersita¨t Mu¨nster, 48149 Mu¨nster, Germany, and Organisch-Chemisches Institut, UniVersita¨t Mu¨nster, Corrensstrasse 40, 48149 Mu¨nster, Germany ReceiVed: March 14, 2007; In Final Form: July 2, 2007
A scanning tunneling microscopy (STM) study of an unsaturated long-chain-bridged diferrocene, Fc(CH2)6HCdCH(CH2)6Fc (u-diFc, Fc ) ferrocenyl), on Ag(110) reveals three ordered molecular phases, R, β, and β′. It is found that the R and the β phase are predominant on the ordered structures with the Fc group neighboring to the center of the hydrocarbon chain in the former phase but with the Fc group arranging into a row-like structure in the latter phase. High-resolution STM images also show that the β′ structure is analogous to the β phase but is not well-defined and exhibits the zigzag shape because of the variable distance between two neighboring molecular rows and molecular arrangement shifted along the molecular row. Interestingly, the irreversible phase transition from R to β′ has been observed in the recorded regions by using the real-time STM. On the basis of the experimental results, three molecular models are provided, and the stability of these three structures has been investigated as well.
1. Introduction Ferrocene, a “sandwich” compound of two cylopentadienyl (Cp) rings sitting above and below the Fe2+ ion, has been widely investigated because of its diverse properties in redox reaction, chemical catalysis, and easy decomposition of ligands. These properties are of interest in various applications, such as integrating molecular diodes and transistors which can be further used to build computational logical gates,1 utilization as an asymmetric catalysis in the synthesis of chiral ligands,2 utility of the property of readily decomposing ligands under electron or photon irradiation, and retaining the metal, which makes ferrocene an alternative for selective area deposition of ironcontaining ultrathin films.3,4 By using surface sensitive facilities, e.g., high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and angle resolved photoemission spectroscopy (ARPES), surface chemistry and adsorption features of ferrocene on different surfaces have been extensively investigated because they could provide the theoretical foundation for the potential applications of ferrocene.5-9 Previous studies reported that the ferrocene molecules are bonded on metals and graphite surfaces with preferential bonding orientation by the weak adsorbate-substrate interactions, whereas the molecular orientation is strongly dependent upon substrates.6,8 Up to date, however, little is addressed on ordered structures of ferrocene adsorbed on the supported surfaces. Although a recent paper declared that ferrocene molecules can adsorb dissociatively and form thus ordered two-layer structures on Au(111) surface,10 it is found that no * Corresponding author. E-mail:
[email protected]. † Physikalisches Institut. ‡ Organisch-Chemisches Institut.
10.1021/jp0720643 CCC: $37.00
well-ordered ferrocene structures form on the underlying substrates.6-8 Generally, it is well accepted that physical and chemical properties of various low-dimensional materials predominantly rely on the formation of ordered structures. To improve the ordering of ferrocene on surfaces and obtain the controllable and expectable structures, a serial of long hydrocarbon-chain-bridged diferrocenes have been synthesized, which were successfully utilized for the formation of two-dimensionally ordered structures in the organic systems.11,12 Previous STM studies showed that two kinds of diferrocenes bridged by saturated and unsaturated long chains can form highly ordered two-dimensional monolayers on metal and graphite surfaces.11-13 However, the details about molecular packing structures and adsorption characteristics of unsaturated long-chain-bridged diFc are limited in the former papers. Considering that unsaturated molecules are more chemically reactive than the saturated ones and thus can be used as the precursor for chemical reactions to form new organic materials, in the present work, we choose a diferrocene derivative bridged by an unsaturated alkyl chain, Fcs(CH2)6HCdCH(CH2)6sFc (u-diFc, see Figure 1, inset) to intensively investigate the adsorption features and structures as well as the stability of self-assembly of this molecule on the Ag(110) surface. 2. Experimental Results All experiments were performed with a commercial variabletemperature STM from Omicron. The growth of diferrocene monolayers on Ag(110) was carried out by means of organic molecular beam deposition (OMBD) in an ultrahigh vacuum (UHV) chamber with a base vacuum better than 10-9 mbar. Before deposition of the molecules, clean crystalline Ag(110) substrates were prepared by consecutive sputtering and annealing © 2007 American Chemical Society
12140 J. Phys. Chem. C, Vol. 111, No. 33, 2007
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Figure 1. (a) Large scale STM image of the ordered u-diFc monolayer on Ag(110) shows that two ordered structures, R and β, coexist on the surface. The inset is the molecular structure of the u-diFc molecule. The image size is 100 nm × 100 nm. Scanning voltage and current are 1.7 V and 0.07 nA, respectively. (b) and (c) High-resolution STM images in the left panel and the schematically structural models of the R and the β phase in the right panel, respectively. Two kinds of unit cells are outlined by basic vectors in the right panel of b and c and the [110] direction of the Ag(110) surface are also indicated by the arrows. The sizes of the STM image in b and c are both 20 nm × 20 nm. The scanning parameters are -0.9 V and 0.2 nA in b; -0.8 V and 0.2 nA in c.
to about 800 K for several times. The molecules were deposited onto the clean substrates at room temperature (RT) at a fixed deposition rate. The details about sample preparation have been reported previosly.13-15 After deposition, the samples were transferred to another chamber for STM measurement. All STM images shown here were recorded at RT with constant current mode and a tungsten tip prepared by direct-current chemical etching. The tunneling current was set between 70 and 200 pA, and the tip bias voltage was varied from -1.5 V to +2.5 V, with the sample grounded. 3. Results and Discussion 3.1. Monolayer Structures. Figure 1a presents the structures obtained right after deposition of u-diFc on Ag(110) without any postgrowth treatment. This image shows two types of ordered structures of diferrocene molecules, R and β, coexisting on the Ag(110) surface with monolayer coverage. A careful inspection of Figure 1a shows that in both phases the molecules can be resolved into bright spots, as shown by the submolecular resolution STM images in the left panel of Figure 1b,c. Considering the molecular length, that is, 2.09 nm, and the van der Waals (vdWs) radii of this molecule, we infer that these bright spots stem from the ferrocenes parts terminating the unsaturated long chain. According to previous studies, the ferrocene molecules adsorbed on Ag(100), and graphite surfaces are oriented with their molecular axis perpendicular to the surfaces, whereas they are oriented with their axis parallel to the surface in the case of ferrocene adsorbed on the Cu(100) surface.7,8 In our case, we also assume that, similar to the individual ferrocene, long-chain-bridged diferrocenes are adsorbed on Ag(110) with the axis of ferrocene perpendicular to the underlying substrate surface. Accordingly, the ferrocenes are more readily discerned into bright spots than the hydrocarbon chains by STM because of the higher topography and tunneling contrast of ferrocene than hydrocarbon chain. The similar Fc orientation was observed in studies on other long-chain-bridged diferrocene molecules on Ag(110).12,13 On the basis of the above consideration, the schematic models representing the molecular structures of the R and the β phase are overlaid on the STM images in the left panel of Figure 1b,c, respectively. Here, one circle represents the vdWs size of a
staggered Fc group, and one line represents a hydrocarbon chain. Combined with the Ag(110) substrate surface, the structural models of the R and the β phase are schematized in the right panel of Figure 1b,c. In the R phase, all u-diFc molecules are oriented along (1 7) direction of the Ag(110) surface with the Fc group neighboring to the hydrocarbon chain. Two arrows indicate a unit cell, showing a quadrilateral structure with an angel of 138.5° and two vectors of 2.18 and 1.22 nm. One unit cell contains one molecule and the average area per molecule is about 1.69 nm2. In the model of the β phase, the u-diFc molecules align into the straight rows and are oriented along (2 6) direction of the Ag(110) surface. A unit cell consisting of one molecule is shown in the right panel of Figure 1c. The lattice constant of a unit cell is 2.62 and 0.70 nm with a 63.2° angle. In this case, the average area for single molecule is about 1.35 nm2. This means that the molecular packing is much denser in the β phase than that in the R phase. The red dots highlighting the molecular centers in both phases further indicate that the Fc groups prefer to be near the center of the long chain in the R phase, while the Fc groups are arranged into straight rows in the β phase. This means that in the R phase the Fc groups prefer to be close to the CdC double bond. The particular molecular orientation might be caused by the preferred π-π interaction between the Cp ring and CdC bond. However, this π-π interaction is not so strong because the distance between the Cp ring and CdC bond is larger than the optimal distance of the π-π interaction, being 3.5 Å.16 After carefully exploring different regions of the sample surface, on one hand, the R and the β phases are found to be commonly dominated phases in the ordered u-diFc monolayers. On the other hand, the zigzag row structure designated as the β′ phase is observed simultaneously coexisting with the R phase even though the β′ phase is observed less frequently. Obviously, this structure is a minority phase compared with the above two. The STM image in Figure 2a also displays that the image quality is not pretty nice. From the obscure step edge, it seems to be that the molecules are mobile on the Ag(110) surface owing to disturbance caused by the STM tip, which can result in the bad STM resolution on the recorded areas. When zooming into the region denoted by a square in Figure 2a, an evidently structural feature of the β′ phase is seen (Figure 2b) that the molecules in
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Figure 2. (a) Large scale STM image of the ordered monolayers consisted of u-diFc on Ag(110) shows the coexisting structures of R and β′. The image size is 200 nm × 200 nm. (b) Submolecular resolution STM image zoomed from the area donated by the color square in a. The image size is 20 nm × 20 nm. The scanning current and voltage are 0.06 nA and -0.9 V in both STM images.
Figure 3. (a) STM image of the β′ phase. A, B, C, and D show that molecular packing structures are various in the different regions. The image size is 20 nm × 20 nm. The scanning current and voltage are 0.06 nA and -0.6 V. (b) Schematically, structural models represent molecular packing features in the region A, B, C, and D. The average area of single molecule on Ag(110) are provided in the corresponding molecular model.
the β′ phase are packed into the row-like structure, analogous to the β phase, but not well-defined with some defects due to the molecular shift along the molecular row and the changeable distance between two neighboring molecular rows. Another feature of Figure 2b is that the nebulous traces (labeled by the arrows) along the STM scanning direction which further proves the fact that molecules can easily diffuse and transfer on the surface due to the course of scanning. Details will be discussed in the following sections. Figure 3a is a close-up of the β′ phase. It is evident that the molecular packing characteristics vary in the different regions indicated by letters of A, B, C, and D. This indicates that the β′ structure is not well-defined compared with those of R and β. Structural models of the β′ phase are proposed and shown in Figure 3b. In the small region A, molecules align into the straight rows, and the molecular packing is analogous to that in the β phase. In this case, the single molecular area of 1.35 nm2 is the same as that in the β phase. Notably, the structural difference between regions A and B is that the distance between two neighboring molecular rows, that is, the distance between the head and the tail of the neighboring molecules, is different. In
region A, the nearest distance of ferrocenes located in the neighboring rows is about 0.69 nm, whereas it is 0.83 nm in region B. Thus the average area of individual molecule is increased from 1.35 to 1.47 nm2. As for the molecular structures in regions C and D, even though the molecular orientation is quite similar to that in regions A and B, the distances of Fc groups in the neighboring rows are increased to 1.09 and 1.36 nm, respectively. Furthermore, by closely inspecting the STM image, some bright spots can be found between two neighboring molecular rows. The distance between two bright spots (labeled by the arrows in Figure 3a) is about 2.10 nm, which is approximately equal to the molecular length of a u-diFc molecule. Thus, such two spots could be corresponding to two Fc groups of one u-diFc molecule. The average area of individual molecule in regions C and D are still increased up to 1.59 and 1.71 nm2. The structure models representing molecular structures in regions C and D are schematized in the left and right lower panel of Figure 3b. To further dig into these different molecular packing characteristics on Ag(110) and compare these three phases with the highly packed (-2 1 1) plane cleaved from the u-diFc crystalline
12142 J. Phys. Chem. C, Vol. 111, No. 33, 2007 TABLE 1: Crystallographic Data for the Ordered u-diFc Molecular Phases of r and β Formed on Ag(110) as well as the Highly Closed Cleaved (-2 1 1) Plane in the Crystal R
β
orientation
|2 7| |1 -4| b1 ) 2.18 b2 ) 1.22 θ ) 138.3° 1 mole. (1 7)
|-3 8| |1 2| b1 ) 2.62 b2 ) 0.70 θ ) 63.2° 1 mole. (5 2)
Fc-Fc
0.86 nm
alkyl-alkyl area/mole.
0.64 nm 1.69 nm2
0.71 nm 0.81 nm 0.58 nm 1.35 nm2
structure matrix unit cell (nm)
β′ (not well-ordered) (-211)
b1 ) 1.76 b2 ) 1.53 θ ) 99.5° 2 mole. 0.57 nm 0.64 nm 0.67 nm 0.46 nm 1.33 nm2
structure,11 the geometrical data, including the relationship matrix and the base vectors (excluding the case of the β′ phase due to its not well-defined structure), the orientation of molecules related to the substrate, the distance between the neighboring Fc groups and between the parallel oligoethylene chains, as well as the average area per molecule are summarized in Table 1. In the highly packed (-2 1 1) cleaved plane (see Supporting Information), all the molecules are arranged into the row-like structure with molecules parallel to each other. Half of the u-diFc molecules are orientated with the axis of the Fc group perpendicular to the cleaved plane, while the left half numbers are parallel. Such molecular packing feature efficiently reduces the distance between the hydrocarbon chains (see Table 1). Nevertheless, because of the effect of an unsaturated bond, this row-like structure is not perfectly straight, which simultaneously produces each Fc group containing three neighboring coordination Fc groups with one nearest to it and two a bit far from it. This packing structure facilitates strengthening the chain-chain interaction and the Fc-Fc interaction. Compared with the cleaved plane, in the R phase, every Fc group has one nearest group, and the distance of hydrocarbon chains is much larger than that in the (-2 1 1) plane. In the β phase, the molecules are densely aligned with their hydrocarbon chains parallel to each other. This molecular packing mode is very similar to that in the cleaved plane from the crystal. Moreover, the nearest coordinated number per Fc group is increased to three, and the single molecular area on Ag(110) is very close to that in the cleaved (-2 1 1) surface. Accordingly, we maintain that the β phase is an energetically favored structure ascribed to the strong intermolecular interactions including the chain-chain interaction and Fc-Fc interaction. However, the fact of the β phase coexisting with R and β′ implies that the adsorbate-substrate interaction plays an important role in molecular packing on surfaces, which hinders the molecules exclusively adopt a highly packed structure similar to its crystal structure. Especially, the β′ phase is not welldefined, even if u-diFc molecules are packed into the row-like structure similar to the β. Thus we assume that the β′ phase might be a high-energy phase like this phase is a minority one. 3.2 Stability of the Monolayers. Figure 4a shows a large size STM image of the two-phase R and β′ coexisting structure. After consecutively scanning this area up to 24 min, we observed that the area covered by the β′ phase is increased at the expense of the R phase, as designated by the arrows in Figure 4b,c. As discussed above, from the molecular packing point of the view, the molecular packing features in the R phase determines that the intermolecular interactions in the R are weaker than those in the crystal and in the β phase. This means the R phase is an energetically unfavored structure. If the additive activation
Letters energies are provided, such as changing the internal energy (heating the samples)17 or applying external forces (the scanning force),18 the new energetically favored phase can be generated at the expense of the metastable one. Therefore, the u-diFc molecules tend to rearrange themselves into a structure similar to that in the u-diFc crystal, in which the Fc-Fc distance and the chain-chain distance could be optimized to further increase the intermolecular interactions. Going back to observation of the detailed molecular structures in the β′ phase, the molecular packing mode is analogous to that in the β phase, but the structure is not well-defined. It is plausible that the phase transition from R to β′ needs less activation energy than that from R to β. Thus, the phase transition from R to β′ can happen during STM scanning. In our case, the additive activation energy that causes the phase transition from R to β′ could be related to the external force induced mostly by the STM probe.18 So the mechanism of the phase transition is based on the fact that the STM tip induces molecular rearrangement. However, another possible reason that can also cause molecular migration and rearrangement could be the electric field engendered by the sample bias voltage.19,20 It is known that there is an Fe2+ ion in one Fc group; it is thus possible that the u-diFc molecules can move directionally on the surfaces under the appropriate electric field when the applied electric field is larger than the critical value. Here, after ensuring a constant scanning tunneling current, we have found that the phase transition is independent of the sample bias voltages. Therefore, the effect of the electric filed on the molecular rearrangement can be excluded. To examine the stability of the β phase and whether the phase transition can happen as the R phase did, we deliberately pick up one region mostly covered with the ordered β structure and continue scanning this same area for a long time. Figure 4d-f shows a series of STM images obtained in the same scanned region with the scanning time longer than 36 min. It is seen that after scanning for 15 min, the ordered β phase near to the terrace step-edge is damaged into disorder directed by the arrows in Figure 4e, while in the other areas the ordered structure remains. After continuously scanning at the same area for as long as 36 min, the areas occupied by the disordered phase somewhat expand but not too much as shown in Figure 4f. Moreover, the phase transition is not observed except that the zigzag shape structure appears as indicated by the arrow in Figure 4f. This means the molecular rearrangement can still take place during scanning for a longer interval time. Actually, it is very normal to observe the damage of ordered molecular structures by the scanning force in other molecular structures systems because of the tip disruption of the molecules.21 Compared with the phase transition from R to β′, it seems that it is hard to induce the structural evolution under the same scanning conditions from the energetically stable structure (the β phase) to another ordered phase, although molecules can rearrange in small areas driven by the STM probe. This hints that the β phase is indeed the most stable structure compared with the R and the β′, which is not easily changed by the external force. In the former studies on self-assembly of oligoethylene-chainbridged diferrocenes (i.e., Fc(CH2)14Fc) on Ag(110), the phase transition is not observed.13 In the present work, however, we have disclosed that the u-diFc molecule containing an unsaturated hydrocarbon chain is mobile and easily migrates on the surface. The molecular mobility and the essential reason that allows the phase transition are related to the unsaturated carbon
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Figure 4. (a-c) In situ STM images display the phase transition from R to β′ when scanning the same area for a long time. The scanning current and voltage of a-c are 0.06 nA and -0.6 V. (d-f) In situ STM images show the structural change of the β phase when scanning this same area for a long time. The scanning current and voltage of d-f are 0.07 nA and -1.7 V. The image size of a-f is 100 nm × 100 nm. The scanning interval time is shown on every STM image.
bond, which makes the u-diFc molecule more labile than the analogue molecules with a saturated hydrocarbon chain on the surface. The lability of u-diFc on the Ag(110) surface could be ascribed to a direct reason that there are no preferential adsorption sites of u-diFc on Ag(110). Compared with the case of diFc adsorbed on Ag(110) (see ref 13), it is found that in three different phases, R, β, and γ composed of diFc, the diFc molecules are located almost along the groove of silver atoms in the (110) surface. However, the u-diFc molecules in the different phases are arranged across the silver atomic groove in the (110) surface. Because of the excellent structural stability of diFc on Ag(110) compared with that of u-diFc, we deduce that the adsorption site along the atomic groove might be the
preferential site with low adsorption energy for ferrocene derivatives on Ag(110). It has been reported that the molecules with unsaturated hydrocarbons (i.e., alkenes) exhibit stronger binding interactions with metal surfaces than those with saturated bonds (alkanes) on metals.23 In our case, however, the adsorption energy of u-diFc on Ag(110) might be weakened because there is no the preferential adsorption site of u-diFc. Thus, this would be one reason causing the u-diFc molecules on Ag(110) to be labile on Ag(110). In addition, from the above discussion, we have known that even in the energetically favored β phase the intermolecular interaction is dominated rather than the substrate-molecular interaction. This means that the molecular-substrate interaction is indeed compromised with the intermolecular interactions,
12144 J. Phys. Chem. C, Vol. 111, No. 33, 2007 which further decreases the adsorption energy for u-diFc on Ag(110). Thus, this kind of molecular packing feature of u-diFc on the silver surface makes the molecules diffusing and migrating under the external force derived from the scanning probe. 4. Conclusion In summary, we have investigated the structures and the stability of self-assembly of ferrocene derivative, u-diFc molecule on Ag(110), by using STM. High-resolution STM results reveal that this molecule can form three ordered phases R, β, and β′. The R and the β phases are predominant on the ordered monolayers. The β′ structure analogous to the β phase is a minority phase and not well-defined. The β′ phase can evolve from the R under the driving force from the STM tip. The mechanism of the phase transition can be explained by the fact that molecules may readily migrate and rearrange on the surface driven by the external force due to the weak intermolecular and adsorbate-substrate interactions in the R phase. In contrast, the molecular packing is optimized in the β phase, which increases the intermolecular interactions and stability. Thus, the β is proved to be the energetically favored structure and more stable than the others. Acknowledgment. Authors thank Professor W. S. Yang (Peking University, China) for helpful discussion. Financial support from the Sonderforschungsbereich (SFB) 424 of Deutsche Forschungsgemeinschaft (DFG) and Helmholtz Gemeinschaft is gratefully acknowledged. Supporting Information Available: The highly closed (-2 1 1) plane cleaved from the u-diFc crystal (PDF). This material is available free of charge via the Internet at http://pubs. acs.org.
Letters References and Notes (1) Chaiwat, E.; Sita, L. R. Nano Lett. 2001, 1, 541-549. (2) Sawamura, M.; Hamashima, H.; Ito, Y. J. Am. Chem. Soc. 1992, 114, 8295-8296. (3) Welipitiya, D.; Green, A.; Woods, J. P.; Dowben, A. J. Appl. Phys. 1996, 79, 8730-8724. (4) Thibaudau, F.; Roche, J. R.; Salvan, F. Appl. Phys. Lett. 1994, 64, 523-525. (5) Waldfried, C.; Welipitiya, D.; Hutchings, C. W.; de Silva, H. S. V.; Gallup, G. A.; Dowben, P. A.; Pai, W. W.; Zhang, J. D.; Wendelken, J. F.; Boag, N. M. J. Phys. Chem. B 1997, 101, 9782-9789. (6) Welipitiya, D.; Dowben, P. A.; Zhang, J.; Pai, W. W.; Wendelken, J. F. Surf. Sci. 1996, 367, 20-32. (7) Woodbridge, C. M.; Pugmire, D. L.; Johnson, R. C.; Boag, N. M.; Langell, M. A. J. Phys. Chem. B 2000, 104, 3085-3093. (8) Durston, P. J.; Palmer, R. E. Surf. Sci. 1998, 400, 277-280. (9) Dowben, P. A.; Waldfried, C.; Komesu, T.; Welipitiya, D.; McAvoy, T.; Vescovo, E. Chem. Phys. Lett. 1998, 283, 44-50. (10) Braun, K.-F.; Iancu, V.; Pertaya, N.; Rieder, K.-H.; Hla, S.-W. Phys. ReV. Lett. 2006, 96, 246102 (1-4). (11) Wedeking, K.; Mu, Z. C.; Kehr, G.; Sierra, J. C.; Lichtenfeld, C. M.; Grimme, S.; Erker, G.; Froehlich, R.; Chi, L. F.; Wang, W. C.; Zhong, D. Y.; Fuchs, H. Chem. Euro. J. 2006, 12, 1618-1628. (12) Wedeking, K.; Mu, Z. H.; Kehr, G.; Fro¨lich, R.; Erker, G.; Chi, L. F.; Fuchs, H. Langmuir 2006, 22, 3161-3165. (13) Zhong, D. Y.; Wang, W. C.; Dou, R. F.; Wedeking, K.; Erker, G.; Chi, L. F.; Fuchs, H. Phys. ReV. B, reviewed. (14) Zhong, D. Y.; Lin, F.; Chi, L. F.; Fuchs, H. Phys. ReV. B 2005, 71, 125336 (1-8). (15) Lin, F.; Zhong, D. Y.; Chi, L. F.; Ye, K.; Wang, Y.; Fuchs, H. Phys. ReV. B 2006, 73, 235420 (1-8). (16) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-5534. (17) Pi, U. H.; Kim, J. H.; Yu, H. Y.; Park, C. W.; Choi, S. Y.; Kin, Y. K.; Ha, J. S. Surf. Sci. 2006, 600, 625-631. (18) Munuera, C.; Ocal, C. J. Chem. Phys. 2006, 124, 206102 (1-5). (19) Yang, G. H.; Liu, G. Y. J. Phys. Chem. B 2003, 107, 8746-8759. (20) Liu, G. Y.; Xu, S.; Qian, Y. L. Acc. Chem. Res. 2000, 33, 457466. (21) Schoer, J. K.; Crooks, R. M. Langmuir 1997, 13, 2323-2332. (22) Pawela-Crew, J.; Madix, R. J. Surf. Sci. 1995, 339, 8-22.
