Low-Temperature Growth of C60 Monolayers on Au(111): Island

Mar 24, 2010 - At 180−190 K, C60 molecules are found to preferentially attach to the bulged elbow site and island nucleation occurs exclusively on t...
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J. Phys. Chem. C 2010, 114, 6433–6439

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Low-Temperature Growth of C60 Monolayers on Au(111): Island Orientation Control with Site-Selective Nucleation Xin Zhang, Lin Tang, and Quanmin Guo* Nanoscale Physics Research Laboratory, School of Physics and Astronomy, UniVersity of Birmingham, Birmingham, B15 2TT, United Kingdom ReceiVed: January 22, 2010; ReVised Manuscript ReceiVed: March 17, 2010

We report a low-temperature scanning tunnelling microscopy (STM) study of the adsorption of C60 molecules on the (111) surface of gold. At 46 K, occupation of the elbow sites on Au(111) by individual C60 molecules as well as small molecular clusters is observed at a coverage of 0.01monolayer (ML). The molecular clusters are mostly found in the FCC regions adjacent to the bulged elbows while most of the pinched elbows remain decorated by single molecules. At a higher coverage of 0.05 ML, the islands nucleated at the bulged elbow sites increase in size, and at the same time, a delayed nucleation process starts around the pinched elbow sites. The C60 islands show different azimuthal orientations similar to that found for larger islands formed at room temperature. The islands in the FCC region near the bulged elbows grow with coverage, but are confined by the discommensuration lines. At 180-190 K, C60 molecules are found to preferentially attach to the bulged elbow site and island nucleation occurs exclusively on the bulged site. This single-site-nucleation phenomenon leads to the formation of C60 islands with a single orientation. 1. Introduction Molecular self-assembly on surfaces has attracted a great deal of attention during the past decade, because of its potential in molecular electronic device fabrication.1 Many surface assembly processes employ regularly patterned substrates as templates for guided growth of molecular structures.2-4 Substrate patterning can be achieved by using lithography-based techniques, although there are plenty of naturally existing reconstructed surfaces with regular patterns that can be used to organize molecules at the nanometer scale.5-8 One of such reconstructed surfaces that has been widely used in the research community is the unique 22 × 3-Au(111) surface, which has a herringbone-patterned dislocation network consisting of alternating FCC-stacking and HCP-stacking regions.9-11 The reconstructed Au(111) surface has been used for template-directed growth of two-dimensional atomic and molecular structures. For instance, Ag atoms have been reported to diffuse to step edges to form finger-like structures,12,13 while Fe,14 Mo,15 and Ni16 atoms adsorb preferentially at the elbow sites of the herringbone pattern, involving an atom exchange process with the substrate. For organic molecules, the FCC and HCP regions exhibit different affinities to the molecules and the FCC region is usually found to be able to capture molecules more effectively due to its stronger interaction with the molecules.17-19 Attachment of individual molecules by the elbow sites is mostly found at low temperatures, and at room temperature many molecules are free to diffuse on the surface.3,7,8,20 In the case of the C60 monolayer on the Au(111) surface or similar (111) surfaces of other FCC metals, interesting morphology21 and electronic structure22 have been observed at low temperatures. A previous report by Fujita23 shows the adsorption of individual C60 molecules at the herringbone elbow sites at 30 K. But as far as we know there is no detailed study on the nucleation and growth of C60 islands around the elbow sites. In this paper, we report a systematic * To whom correspondence should be addressed. E-mail: q.guo@ bham.ac.uk.

investigation of the adsorption process of C60 molecules on the Au(111) surface and the subsequent growth of C60 clusters at 46 and 190 K. By increasing the surface coverage step by step, we found different nucleation behavior at the two types of elbow sites. The discommensuration lines are found to play an interesting role in confining the growth fronts. Moreover, for the small two-dimensional C60 clusters formed at 46 K, three different orientations have been observed and the orientation is clearly identified even for dimers. At 190 K, nucleation of C60 islands occurs exclusively at the bulged elbow site, and all the 2-dimensional molecular islands formed at this temperature adopt a single orientation. 2. Experimental Methods The Au(111) sample was prepared by thermal evaporation of gold onto a highly oriented pyrolytic graphite (HOPG) substrate inside a high-vacuum chamber operating with a base pressure of 5 × 10-7 mbar. The substrate was preheated to 650 K with the shutter closed, and maintained at this temperature during deposition with the shutter open. The deposition rate as monitored with a quartz crystal oscillator was 2 nm/min, and the total thickness of the gold film is 250 nm. At the end of deposition, the shutter was closed while the sample was kept at 650 K for a further 30 min before cooling gradually to room temperature. The sample was then transferred to an ultrahighvacuum system (base pressure 1 × 10-10 mbar), which houses a variable-temperature scanning tunnelling microscope (VTSTM, Omicron), and treated with several cycles of Ar ion bombardment (1 keV energy, 10 µA/cm2 ion current density) and thermal annealing (45 min at 1000 K). The sample surface prepared in such a way has large terraces with the (111) orientation, and shows the typical 22 × 3 surface reconstruction. STM imaging was carried out with the Omicron VT-STM, using chemically etched tungsten STM tips under constant current mode. The sample temperature can be reduced to 40 K on the STM stage with liquid helium, and with the help of a Lakeshore temperature controller, the sample temperature can

10.1021/jp1006318  2010 American Chemical Society Published on Web 03/24/2010

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Figure 1. The reconstructed Au(111) surface. (A) STM image, 40 nm × 25 nm, showing alternating HCP and FCC regions separated by two kinds of discommensuration lines (DLs): type-x with pointed elbows as marked by the green dashed line, and type-y with rounded elbows as marked by the blue dashed line. Rows of bulged elbows and pinched elbows are marked by green and blue bands, respectively. (B) An atomic resolved image, 19 nm × 20 nm, obtained using It ) 12 nA and Vb ) 4 mV shows the presence of a group of “bright” atoms around each elbow. (C) Magnified view around a bulged elbow, with blue lines linking atoms in close packed rows to identify the extra row of atoms. (D) Magnified view around a pinched elbow, where the extra row joins the elbow from the opposite direction.

be kept stable at any temperature from 40 K to room temperature. C60 molecules were deposited in situ onto the sample from a homemade Knudsen cell, which was resistive heated. The operating temperature of the Knudsen cell was kept at 610 K to give a deposition flux of ∼0.02 ML per minute. 3. Results and Discussion Figure 1 shows STM images of the clean Au (111) surface with its typical herringbone type of reconstruction. A 4.4% contraction within the top atomic layer, along the [11j0] direction, leads to atomic stacking-fault. Consequently, the surface presents alternating FCC (wider, ∼3.5 nm) and HCP (narrower, ∼2.4 nm) regions as indicated in the figure. The two regions are separated by discommensuration lines (DLs), which bend systematically into the herringbone pattern. The atoms on the discommensuration lines occupy bridge sites and are ∼0.02 nm higher than the surface plane containing atoms in the FCC region. There are two kinds of discommensuration lines that can be distinguished by the different ways they bend at the elbows: type-x discommensuration lines have pointed elbows while the elbows of the type-y discommensuration lines are rounded. Furthermore, at an elbow site, if the type-x discommensuration line points into the HCP region, it is called a pinched elbow, otherwise, if it points into the FCC region, it is called a bulged elbow. In this manner, the herringbone pattern presents alternating rows of pinched and bulged elbows, as marked in Figure 1A. The structure of the clean Au(111) surface has been characterized rather well over the years and here we use the images to emphasize fine details of the atomic structure around the elbow sites, as we will use them later on to discuss the different adsorption behavior of C60 molecules at the two types of elbow sites. Figure 1B is an atomic resolution image of a region containing two pinched elbows and a bulged elbow. There are several rows of “bright atoms” at each elbow site (both bulged and pinched elbows) of a type-x discommensuration line, indicating a higher surface charge density. Magnified views around a pinched elbow and a bulged elbow are shown

in Figure 1, parts C and D, respectively. By linking atoms within close-packed atomic rows across the elbow, it can be seen clearly that there is an extra row of atoms terminating at each elbow site. At the bulged elbow site, the extra atom row comes from the HCP region, Figure 1C, while at the pinched elbow, the extra row comes from the FCC region, Figure 1D. The termination of an entire atom row at the elbow site gives rise to an “edge dislocation” at the elbow and it could therefore make the elbow site much more reactive in terms of interaction with deposited atoms14-16 and molecules.3,7,8,20 Figure 2 shows an STM image of the Au(111) surface after deposition of 0.01 ML of C60 molecules at 46 K. At this temperature, the thermal energy of the molecules is low in comparison to the depth of the potential well presented by the elbow sites, so molecules get trapped at the elbows. Because of the efficient capturing of C60 molecules by the elbow sites, no aggregation of molecules is found at Au(111) step edges. This is in contrast to room temperature adsorption, where rapid surface diffusion leads to the accumulation of molecules predominantly at step sites.24 The trapping of individual C60 molecules by the elbows shown in Figure 2 is similar to that found previously23 at 30 K, but here, in addition to the individually trapped molecules, we have also found the coexisting small molecular clusters. We believe that in a previous study23 the coverage was lower than that shown in Figure 2. The nucleation of molecular clusters is strongly dependent on the elbow types as will be described later in detail. Before we do that, we will describe the various features in Figure 2 in more detail. From the image, it is found that 91.2% of all the elbow sites are occupied by either single molecules or small molecular clusters. Among the 56 occupied sites, 49.4% are occupied by individual molecules, 9.0% by C60 dimers, 5.1% by C60 trimers, 5.1% by tetramers, and the remaining sites by clusters with five molecules or more. Two thirds of the molecules found in the image are at the bulged elbow sites, and this excess number of molecules at the bulged elbows arises from the fact that ∼76% of the molecular clusters of five

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Figure 2. STM image, 80 nm × 80 nm, from a Au(111) surface with 0.01 ML of adsorbed C60 molecules. Tunnelling parameters: It ) 0.03 nA, Vb ) -3.00 V. C60 molecules and clusters on each pointed elbow site form ordered arrays in the [21j1j] direction. The close packing directions of the molecular clusters are found by using the [011j] direction of Au(111) as a reference. Lines marking the close packing directions of various molecular clusters are drawn to assist the eyes. Molecules were deposited at a sample temperature of 46 K and the image was acquired at the same temperature. Arrows indicate the rows of bulged elbows.

molecules or larger are found at the bulged elbow site. The clusters with 5 or more C60 molecules do not have a uniform shape. This is because the edge diffusion of the molecules at this temperature is restricted and the structures observed do not yet represent the thermodynamic equilibrium configuration. All the clusters are much smaller than the compact islands usually found at room temperature.25 Even for these relatively small clusters, it is possible to identify their azimuthal orientations by comparing the direction of close packing to the principal axes of the Au(111) substrate. By drawing lines linking closepacked C60 molecules, three orientations are found: R0° (or inphase) where the close-packing direction follows one of the 〈110〉 directions; R14° and R30° where the close-packing directions rotate by 14 and 30 deg, respectively, from the relevant 〈110〉 directions. All three orientations have been found for large compact C60 islands formed at room temperature and structural models are provided by previous studies.24,25 Our data show that specified orientations occur for molecular clusters as small as dimers. It is noted that the small molecular clusters are not stable and can switch between the three possible orientations under STM scan. As mentioned earlier, the two types of elbows have different local structures, Figure 1, and in Figure 2 the higher capability of the bulged elbow for attracting C60 molecules has been demonstrated. Another example is shown in Figure 3. Figure 3A shows an STM image of the surface for an overall surface coverage of ∼0.015 ML. A close inspection of this image reveals that nearly all the dimers, trimers, and larger islands appear in every other row, alternated by rows of exclusively single C60 molecules. Figure 3B shows the same area as that in Figure 3A, but the image contrast is adjusted to reveal the

herringbone pattern of the substrate. This allows us to identify that the rows of individual molecules are on the pinched elbows and the molecular clusters are almost exclusively nucleated from the bulged elbow sites. The location of individually adsorbed C60 molecules at the bulged elbows is illustrated with the help of the schematic diagram in Figure 3C. Within each molecular cluster, we can identify a “special” molecule, which is the first molecule captured by the elbow site, highlighted with a red sphere in Figure 3B. Once this special molecule is joined by a second molecule, a dimer is formed. This is then followed by the formation of a trimer, tetramer, and so on. It can be seen from Figure 3B that the special molecule is located at the edge of the cluster, with the center of gravity of the cluster found in the FCC region. By using a simple counting method, we find that 81% of the C60 molecules in the image, Figure 3, are in the FCC region of the surface. The individual molecules at the pinched elbow sites are the only ones located inside the HCP region. Since the molecules land on the surface randomly, one expects the number of molecules falling into the FCC/HCP regions proportionate to their area ratio. In such a case we expect to see ∼60% of molecules in the FCC region and 40% in the HCP region, assuming no stable adsorption on the DLs. However, our finding of 80% molecules in the FCC region clearly show that preferential segregation of molecules has occurred on the surface, as also found previously for other organic molecules on Au(111).3,7,8,19 Previous investigations discovered that certain organic molecules have the strong tendency to move from the HCP region over the DLs to the FCC region, and this seems to occur very shortly after landing on the surface. Once in the FCC region, the molecules are found to diffuse along the direction

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Figure 4. STM image, 60 nm × 110 nm, of the surface with 0.05 ML of C60 molecules. Tunnelling parameters: It ) 0.03 nA, Vb ) -2.50 V. C60 dimers, trimers, and tetramers appear at pinched elbow sites at this coverage. At some pinched elbow sites, C60 clusters have grown to bigger islands extending into neighboring FCC regions across the discommensuration lines. Both deposition and imaging were conducted at 46 K sample temperature.

Figure 3. (A) STM image, 60 nm × 100 nm, of the Au(111) surface with 0.015 ML of adsorbed C60 molecules. The image was obtained by using It ) 0.03 nA and Vb ) -3.00 V. (B) The same image as in part A, but with enhanced contrast to show the herringbone pattern of the substrate. Blue spheres are drawn into the image to represent the exact locations of C60 molecules found in part A. Within each molecular cluster, there is a “special” molecule that is the very first molecule to be found at the elbow site. The red spheres in part B, as well as the arrows in part A, highlight such molecules. At bulged elbow sites, the initial adsorbed individual C60 molecules are found on the FCC side of DLs, as sketched in part C. Both deposition and imaging were conducted at 46 K sample temperature.

of the DLs but unable to move back adjacent to the HCP region. Our observation with C60 here is consistent with previous findings with other molecules. When C60 molecules diffuse along the length of the FCC domain, nucleation is facilitated by the single molecule already pinned at the bulged elbow site. Subsequent growth of the molecular cluster is achieved by the addition of more molecules. The molecules pinned at the pinched elbows sit in the HCP region due to the intrinsic locations of the surface defects at pinched elbows on Au(111). Because the HCP region is depleted of diffusing molecules, nucleation occurs less frequently at the pinched elbow sites. To promote nucleation at the pinched elbows, one can use either a higher deposition flux or lower the sample temperature even further. Figure 4 shows an STM image from a surface with 0.05 ML of C60 molecules. It can be seen that at this higher coverage the average size of the molecular clusters at the bulged elbows has increased. At the same time, molecular clusters including dimers, trimers, and tetramers start to appear at pinched elbows. The growth of the clusters from the bulged elbow sites is clearly confined by the DLs. A number of clusters, such as the ones marked with blue arrows, are just two molecules wide, but 7 to 8 molecules long. Because the growth follows the trend of DLs, some clusters have developed a characteristic chevron shape. Several clusters are three molecules wide. Considering that the

Figure 5. STM image (220 nm × 220 nm), obtained with It ) 0.01 nA, Vb) -1.2 V, from a Au(111) surface covered by 0.006 ML of C60 at 180 K. Shadowing has been applied to the original image to enhance the DLs. The image was acquired at 180 K.

width of the FCC domain is about 3.5 nm, it can accommodate no more than three molecules in the direction perpendicular to the DLs. The nucleation at the pinched elbow sites is less well-defined. A number of clusters around the pinched elbows, such as those pointed out by green arrows, also have a large aspect ratio, but the long direction of these clusters is not parallel to the local DLs. Instead, the clusters cover both the FCC and HCP regions. It seems that once a small cluster is nucleated at the pinched elbow site, it can capture diffusing molecules from both the HCP and the FCC region. Because there are many more molecules in the FCC region, such a cluster can expand into the neighboring FCC regions giving rise to the observed cluster shape. In Figure 4, the red arrows indicate a situation where two C60 clusters, one at the bulged site and the other at the pinched site, sit next to each other. Suppose a molecule is now deposited onto the surface so it lands in the FCC region between two such clusters, it has an equal probability of diffusing in one direction and joining the cluster at the bulged elbow site or

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Figure 6. (A) STM image, 117 nm × 113 nm, obtained from a Au(111) surface with 0.15 ML adsorbed C60 molecules at 190 K. It ) 0.03 nA, Vb ) 5.0 V. The shape of the islands is distorted slightly due to thermal drift. (B) STM image (60 nm × 40 nm, Vb ) -5.01 V, It ) 0.03 nA) showing one C60 island with the R30° orientation. The observation of the discommensuration lines through the molecular layer confirms that the island rests on a herringbone-reconstructed surface. (C) Islands are nucleated from the bulged elbow sites. Pinched elbow sites are free from molecules. Black arrows drawn into the image point to two rows of pinched elbows. High-pass filter has been applied to the image to bring out the herringbone pattern and thus the artificial brightness of the island edges. Vb ) -5.01 V, It ) 0.03 nA. Molecules were deposited at 190 K sample temperature, and STM imaging was conducted at 170 K.

in the opposite direction and thus joining the cluster at the pinched elbow site. Under such situations, the cluster at the pinched elbow site has a similar probability to grow as the cluster at the bulged site. Thus, the initial nucleation rate is higher at the bulged site, but the growth rate at a later stage is not dramatically different for clusters at the two types of elbows. Experiments conducted at 46 K, as described above, show that C60 molecules can attach to both types of elbow sites. Nucleation of molecular clusters, however, strongly favors the bulged elbow site, and this is due to the preferential segregation of molecules into the FCC regions. In the following, we analyze the experimental findings for molecular deposition performed at 180 K. Figure 5 shows an STM image of Au(111) with 0.006 ML of C60 molecules. At this low coverage, there are only individually adsorbed molecules at the elbow sites, with no molecular clusters. Even at such a low coverage, preferential occupation of the bulged sites is evident. An area marked inside a rectangle is highlighted in Figure 5, where it is found that 93% of the bulged sites are occupied whereas only 25% of the pinched sites are occupied by C60 molecules. The enhanced trapping probability at the bulged elbow site at 180 K will thus amplify the nucleation rate difference at the two types of elbows as demonstrated in Figure 6, which was obtained from a surface with 0.15 ML of C60 deposited at 190 K. Large islands resembling those seen at room temperature25 are observed on the flat terrace. At room temperature, the islands are formed from step edges of Au(111), here at 190 K they are formed far

away from steps. The site-specific aspect of adsorption is even more clearly demonstrated by the image in Figure 6 than that in previous images. Here it can be seen that nearly all the bulged elbows are occupied by C60: either as individual molecules or in the form of molecular islands. The pinched elbows, however, are almost free from molecular adsorption as shown in Figure 6C where two black arrows are drawn to highlight two rows of empty pinched elbows. All the islands in Figure 6A are nucleated from bulged elbow sites and they are exclusively R30° orientated. Furthermore, the corrugation of the gold substrate from the DLs is projected onto the C60 island, which can be see in the Figure 6B. A similar phenomenon was found and discussed in our earlier study of C60 adsorption at room temperature.25 Therefore, the large C60 islands shown in Figure 6 are sitting on top of the herringbone reconstructed Au(111) at 190 K. Previously, it was suggested that the R30° structure formed at room temperature is associated with closed packed molecules sitting on a (1 × 1) Au(111),25 hence the structural model proposed. Here it can be seen that at 190 K, the almost defectfree R30° structure is actually placed over a herringbone reconstructed Au(111). As shown in Figure 1, the herringbone reconstructed Au(111) consists of elbows and extra rows of atoms terminating at the elbows. Thus, the R30° structure cannot be commensurate with the substrate over long distances. At the moment there is no structural model available for this kind of C60 islands. Our explanation for the ordered R30° islands on

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Figure 7. STM image, 220 nm × 220 nm, obtained from a Au(111) surface with adsorbed C60 molecules at 180 K. It ) 0.03 nA, Vb ) -1.2 V. The arrows point to the rows of bulged elbow sites which are populated by adsorbed C60 molecules. Circles highlight a few islands which are nucleated from the elbow sites. The islands do not touch any pinched elbow sites, and hence they are clearly nucleated from the bulged sites.

top of a reconstructed Au(111) is as follows. The azimuthal orientation of an island is determined at the very early stage of growth as demonstrated by images in Figures 2 and 3. As the island grows, the intermolecular interaction controls the growing process while the molecular substrate interaction is not strong enough to lift the reconstruction. The role of the substrate for the relatively large islands is therefore mostly guiding the diffusion of free molecules as they approach the edges of the islands. Since the islands shown in Figure 6 are quite large and they cover both bulged and pinched elbows, it is not so obvious that the islands are nucleated from the bulged sites. In Figure 7, we show an STM image after C60 deposition at 180 K. In this image, we circled a number of molecular islands. The edges of these islands do not touch any pinched elbow site. Therefore, they must be nucleated from bulged sites. It is very rare to find islands that do not touch a bulged site. Combining our findings at low temperatures with those reported for room temperature adsorption, we can classify the adsorption of C60 molecules on Au(111) into three categories according to the sample temperature: (i) step controlled process at room temperature; (ii) nucleation at both types of elbow sites at 46 K; and (iii) single orientation growth due to selective nucleation at the bulged elbow sites at 190 K. At room temperature, a C60 molecule, upon landing, can diffuse through a long distance and it stops only at step edges of Au(111). Subsequent nucleation and growth of C60 islands are purely stepmediated. The orientation of a resulting C60 island depends on the orientation of the step to which the molecular island is attached. At 46 K, the landing C60 molecule rapidly loses its energy to the substrate as it diffuses on the surface. If the molecule lands inside a HCP region, it has a certain probability of moving over a DL line into the nearby FCC region during

its diffusion lifetime. The same molecule also has a probability of being captured by a pinched elbow. Molecules that land inside the FCC domain will stay in the FCC domain. This leads to an enhanced molecular density in the FCC region of the surface, and thus a higher nucleation rate at the bulged elbow sites. At 180-190 K, the pinched elbows no longer retain molecules. There are two possible reasons for this. One is that the binding energy for C60 at the pinched elbow site is lower than that at the bulged elbow site. Thus at 190 K, the potential well at pinched elbows is not deep enough to trap molecules. The other reason can be traced back to the efficient “one-way” transport of molecules from the HCP to the FCC domain. This transport process needs to overcome the barrier presented by the DLs, and it is thus thermally activated. Hence at high enough temperatures, the rate of molecular transfer is so fast that almost all molecules landing in the HCP domain reach the FCC domain before they have the chance of being captured by pinched elbows. A recent study shows that if the substrate temperature is higher than room temperature, nanopits can form at the C60-Au(111) interface26 demonstrating the complex nature of this seemingly simple adsorption system. 4. Conclusions In conclusion, we studied the initial adsorption and island formation processes of C60 molecules on Au(111) at low temperatures. It is found that the atomic structure of gold surface has a strong influence in the formation of two-dimensional islands of C60 molecules. On the bare terraces of Au(111) a C60 island can nucleate only at either a bulged or a pinched elbow site at 46 K. The molecular clusters of C60 can take any one of the following orientations at 46 K: 0°, 14°, or 30° rotated from the close packing directions of gold atoms on Au(111).

Low-Temperature Growth of C60 Monolayers on Au(111) Nucleation rate is much higher at the bulged elbows due to the segregation of molecules from the HCP to the FCC domain. At 180-190 K, large molecular islands are formed due to enhanced surface diffusion. Site-specific attachment of individual C60 molecules at this temperature gives rise to almost 100% selectivity of C60 molecules for the bulged elbows, leading to the formation of C60 islands with a single orientation. Acknowledgment. We thank the Engineering and Physical Science Research Council (EPSRC) of the United Kingdom for financial support. References and Notes (1) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671. (2) Weckesser, J.; De Vita, A.; Barth, J. V.; Cai, C.; Kern, K. Phys. ReV. Lett. 2001, 87, 096101. (3) Yokoyama, T.; Yokoyama, S.; Kamikado, T.; Okuno, Y.; Mashiko, S. Nature 2001, 413, 619. (4) Temirov, R.; Soubatch, S.; Luican, A.; Tautz, F. S. Nature 2006, 444, 350. (5) Aı¨t-Mansour, K.; Ruffieux, P.; Groning, P.; Fasel, R.; Gro¨ning, O. J. Phys. Chem. C 2009, 113, 5292. (6) Aı¨t-Mansour, K.; Ruffieux, P.; Xiao, W.; Gro¨ning, P.; Fasel, R.; Gro¨ning, O. Phys. ReV. B 2006, 74, 195418. (7) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Berndt, R.; Mauri, F.; De Vita, A.; Car, R. Phys. ReV. Lett. 1999, 83, 324. (8) Bo¨hringer, M.; Morgenstern, K.; Schneider, W.-D.; Wu¨hn, M.; Woll, C.; Berndt, R. Surf. Sci. 2000, 444, 199.

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