Strain-Relief Pattern - American Chemical Society

Mar 11, 2009 - We report on the site-specific nucleation of small C60 molecular clusters on the strain-relief pattern formed by two monolayers of Ag o...
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J. Phys. Chem. C 2009, 113, 5292–5299

Positional and Orientational Templating of C60 Molecules on the Ag/Pt(111) Strain-Relief Pattern Kamel Aı¨t-Mansour,* Pascal Ruffieux, Pierangelo Gro¨ning, Roman Fasel, and Oliver Gro¨ning Empa, Swiss Federal Laboratories for Materials Testing and Research, nanotech@surfaces Laboratory, Feuerwerkerstrasse 39, CH-3602 Thun, Switzerland ReceiVed: NoVember 19, 2008; ReVised Manuscript ReceiVed: January 26, 2009

We report on the site-specific nucleation of small C60 molecular clusters on the strain-relief pattern formed by two monolayers of Ag on Pt(111) as well as on the orientational ordering within a molecular monolayer formed on this same template surface. Small triangular patches, which are characteristic of the strain-relief pattern, are found to be the most favorable sites for trapping single molecules. The reason for this site specificity seems to reside in a higher adsorption energy of the C60 molecules in these patches. This is different from the previously reported case of metal (Ag) deposition where diffusion barriers and therefore kinetic reasons play the major role in the confinement of the metal adatoms in hexagonal patches of the strain-relief pattern, where they form nanoislands [Brune, H., et al. Nature 1998, 394, 451]. Here, for organic molecules, the small triangular patches are definitely the nucleation centers of two-dimensional islands covering the surface up to the full monolayer coverage. The particular pinning of the molecular monolayer in these patches imposes specific orientations for the molecules, whereas anywhere else on the surface the orientational order is strongly perturbed due to intrinsic irregularities of the strain-relief pattern. Introduction Nanopatterned surfaces offer a real opportunity to trap adsorbates in a controlled fashion and to assemble them into periodic two-dimensional (2D) nanostructure arrays in view of possible applications in nanotechnology.1 On the level of single molecules or very small molecular clusters, the required template structures can, in general, no longer be fabricated by classical lithographic methods. Instead, one can turn to self-organized nanopatterned surfaces which have been reported in various systems including the herringbone reconstruction of Au(111)2 and its vicinal surfaces,3 dislocation networks in heteroepitaxial systems such as Ag/Pt(111),4 and molecular networks on singlecrystal substrates,5-7 on which site-specific adsorption or selforganized growth of foreign species has been well-demonstrated.5-15 The stain-relief pattern observed for two monolayers (ML) of silver on the (111) surface of platinum stems from the formation of a dislocation network4 in order to relax the strain induced by the 4.3% Ag/Pt lattice mismatch. It is a robust,14 laterally anisotropic template surface which strongly influences the nucleation of adsorbed metals (Ag)10,11 as well as molecules (fullerene C60).12,13 In the case of the Ag metal deposited at low temperature (LT, 110 K), a self-organized growth of 2D nanoislands has been achieved on specific hexagonal domains of the strain-relief pattern superstructure.10,11 This site-specific island growth has been explained in terms of repulsive Ag adatom surface diffusion barriers represented by the boundaries of the hexagonal domains attributed to partial dislocations, which results in the confinement of the adatoms inside the hexagons where their binding to the surface is preferred with respect to the other surface regions.10,11 On the other hand, we have previously shown that the orientation of large 2D islands (covering several unit cells of the strain-relief superstructure) of C60 molecules adsorbed at room temperature (RT) is not driven by the hexagonal domains of the template surface but is * Corresponding author. E-mail: [email protected].

forced by other regions, namely, small triangular domains.12,13 The edges of these triangular domains seem to be pinning centers for C60 molecules12 whereas for Ag adatoms they are of repulsive nature.10,11 This might suggest that the cause of nucleation in these two cases is different; that is, the diffusion barriers and therefore kinetic reasons do not play the major role in the C60 nucleation. In this paper, we investigate in detail the adsorption landscape of C60 molecules on the 2 ML Ag/Pt(111) strain-relief pattern by means of scanning tunneling microscopy (STM). More precisely, we elucidate the most favorable sites for C60 nucleation at low C60 coverages (0.05-0.1 ML) adsorbed at LT (150 K). Further, with intramolecular resolution STM images supported by extended Hu¨ckel theory simulations, we address some aspects of the molecular orientational ordering caused by the template superstructure in the C60 ML coverage range formed at RT. The present study confirms our precedent findings12 and significantly deepens the insight into the mechanism of C60 cluster and island nucleation. The small triangular domains of the strain-relief pattern are found to be the most favorable sites for trapping the molecules, which results in self-organized growth of C60 clusters. Based on a recent study on spatial work function mapping of this strain-relief pattern, one can link the stronger adsorption to a lower work function in these domains.15 In the case of extended C60 islands, covering several strainrelief unit cells, up to the formation of a full C60 ML, the triangular domains play an important role as nucleation and pinning centers. This is exemplified by the fact that specific orientations are imposed to the molecules adsorbed on the small triangular domains through which the molecular film is mainly pinned to the strain-relief pattern. Experimental Procedures Experiments were carried out in an ultrahigh vacuum system containing two connected chambers, one for sample preparation

10.1021/jp8101749 CCC: $40.75  2009 American Chemical Society Published on Web 03/11/2009

C60 Molecules on a Ag/Pt(111) Strain-Relief Pattern

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Figure 1. (a) STM image of a 2 ML Ag/Pt(111) strain-relief pattern showing long-range order as illustrated in the FFT image (inset) (-2 V, 1 nA). (b) Close-up STM image of the strain-relief pattern showing the three types of domains (fcc, hcp1, and hcp2; see text) present in the superstructure unit cell (-1 V, 2 nA). (c) Atomically resolved STM image demonstrating that the topmost Ag layer is free of cut partial dislocations between the three types of domains (-10 mV, 40 nA). (d) STM image from a sample of about 1.5 ML Ag/Pt(111) showing a large island of 2 ML Ag (exhibiting the strain-relief trigonal network) grown from a Pt step together with domains of 1 ML Ag pseudomorphic with Pt(111) and appearing here smooth (-1 V, 2 nA).

and the other housing a LT STM12-15 (from Omicron) operated here at 77 K. All STM images were measured in the constantcurrent mode; the stated voltage refers to the electric potential of the sample with respect to the tip which has been mechanically cut from a Pt/Ir wire. The STM images have been processed with the WSxM software.16 The Pt(111) single-crystal substrate was cleaned by several cycles of Ar-ion sputtering (1 keV) at RT followed by sputtering at 850 °C, resulting in large terraces with widths sometimes exceeding 200 nm, separated by monatomic steps, free of impurities, as checked by X-ray and ultraviolet photoelectron spectroscopy. Ag (purity ) 99.99%) was evaporated (in the pressure range of 7 × 10-10 mbar) from a home-built evaporator using electron-bombardment heating, with a rate of about 0.5 ML per minute, as monitored by a quartz microbalance. One ML Ag refers to the completion of a closed atomic layer on Pt(111), as calibrated by STM. The Ag/Pt(111) template surfaces prior to C60 adsorption were prepared by depositing either 2 or 1.5 ML Ag on the Pt substrate held at RT and subsequently annealed to 800 K (during 5 min), as reported elsewhere.4,10-14 C60 molecules were sublimated from a resistively heated crucible and deposited on Ag/Pt(111) either at 150 K or at RT. Results and Discussion Figure 1a shows an overview STM image of the 2 ML Ag/ Pt(111) strain-relief pattern formed on a single Pt(111) terrace

and having a periodic character which is illustrated in the corresponding fast Fourier transform (FFT) image (inset in Figure 1a) with first- and second-order spots. With an average period of about 7 nm, the strain-relief pattern (also referred to as “trigonal network” in ref 4) can be regarded as a (25 × 25) superstructure with respect to the underlying Pt(111) atomic lattice. The crossing dark lines in the STM images of Figure 1a,b (corresponding to topographic depressions) aligned along the Ag (and Pt ) close-packed directions were previously attributed to Shockley partial dislocations expected to occur in the topmost Ag layer in order to relieve the 4.3% Ag/Pt lattice mismatch induced strain. As can be seen in Figure 1b, the dark lines separate three types of domains present in the superstructure unit cell: a hexagonal domain where the toplayer Ag atoms are expected to be in face-centered cubic (fcc) stacking and two differently sized triangular domains where the top-layer Ag atoms are expected to be in hexagonal close-packed (hcp) stacking.4 This proposed atomic stacking was intuitively inspired by the one of the herringbone reconstruction of Au(111), but, so far, it has never been elucidated in detail, even though X-ray photoelectron diffraction measurements showed that the top-layer Ag atoms adopt, in majority, an fcc stacking.17 In analogy to the proposed top-layer atomic stacking structure,4 we call “fcc” the hexagonal domain and “hcp1” and “hcp2” the small and big triangular domains, respectively. We note that the lines surrounding the hcp1 domains appear markedly deeper

5294 J. Phys. Chem. C, Vol. 113, No. 13, 2009 (by about 0.3 Å with respect to the fcc regions), in particular, the hcp1 corners where the maximum depression of about 0.4 Å occurs. In contrast to the originally proposed model suggesting partial dislocations in the topmost Ag layer,4 the atomically resolved STM image of the trigonal network displayed in Figure 1c shows that the atomic structure of this layer is close to hexagonal and free of cut partial dislocations. This suggests that the anticipated partial dislocations are buried in the deeper layers,14 and therefore the exact atomic structure of this system still needs to be solved. Still there is an important conclusion to draw from the observation in Figure 1c. The fact that there are no apparent partial dislocations present in the topmost Ag layer means that an adsorbate will not experience a high spatial anisotropy regarding the coordination to the substrate, as would be the case if partial dislocations were present. Figure 1d shows a Pt(111) sample covered by a nominal thickness of 1.5 ML Ag where one can observe the coexistence of two types of large domains with different structure: a large Ag island of 2 ML grown from a Pt step edge (indicated by an arrow) and exhibiting the trigonal network, together with domains of 1 ML Ag which are pseudomorphic4 with the Pt(111) substrate and then appear flat in the STM image. To investigate the very early stages of C60 adsorption, we have deposited 0.05 ML C60 at 150 K on 1.5 ML Ag/Pt(111), the structure of which is shown in Figure 1d. As can be readily seen in the overview STM image of Figure 2a, self-organized growth of small C60 clusters (see also FFT inset in Figure 2a) occurs only on the 2 ML Ag/Pt(111) strain-relief trigonal network, whereas on the pseudomorphic 1 ML Ag/Pt(111) domains the C60 coverage is significantly lower and the nucleation sites are randomly distributed. The random distribution of C60 clusters on 1 ML Ag/Pt(111) can be attributed to a relatively high mobility of the molecules resulting from a rather flat adsorption potential landscape on these smooth regions. For instance, for Ag adatoms, it has been shown that the diffusion barrier on this compressively strained 1 ML Ag/Pt(111) is significantly lower than on the Ag(111) surface (60 versus 97 meV).11 In contrast, on the 2 ML Ag/Pt(111) strain-relief pattern, the electronic surface potential varies as much as 0.35 eV for the different domains within the unit cell; it is lowest in center of the hcp1 patches and highest in the center of the fcc patches.15 This spatial anisotropy in the electronic surface potential will translate in a corrugated adsorption potential landscape for a charged or polarizable adsorbate. The preferential adsorption of C60 in the hcp1 regions as shown in the STM image of Figure 2b is attributed to the partially ionic character of the molecule-substrate interaction, which in the case of a molecule with a high electronegativity is expected to result in a higher binding energy in regions with a low work function.15 We have conducted adsorption experiments with numerous polyaromatic molecules, which always resulted in a preferential adsorption at the hcp1 region, so this behavior is not exclusive to C60. It is worth mentioning that, similarly, step edges which are also characterized by a locally reduced work function18 due to the Smoluchowski effect19 are well-known to be favorable sites for immobilizing molecules,9,13,14 even though here other effects such as higher coordination can also contribute to the molecular immobilization. We like to note here that the present results of C60 molecules on the strain-relief pattern are different from those previously reported for Ag metal on the same template surface, where Ag nanoislands form not on the hcp1 but on the fcc patches.10,11 Ag island nucleation is driven by the fcc domain walls which constitute repulsive barriers for Ag adatoms to

Aı¨t-Mansour et al.

Figure 2. (a) STM image of 0.05 ML C60 adsorbed at 150 K on 1.5 ML Ag/Pt(111) containing trigonal network domains of 2 ML Ag/ Pt(111) together with pseudomorphic domains of 1 ML Ag/Pt(111) (-7 V, 5 pA). (b) Close-up STM image of C60 molecules adsorbed on a 2 ML Ag/Pt(111) domain (-5 V, 2 pA). (c) Models of different C60 nanoclusters observed in (b): Compact trimer (left), large trimer (middle), and pentamer (right).

cross, and which then imposes the confinement of the adatoms in the fcc regions where their binding is energetically preferred.10,11,20 Among the C60 nanoclusters adsorbed on the hcp1 regions of the strain-relief pattern, numerous molecules are not stable during the STM measurements and can be easily moved with the STM tip; that is why high tunneling resistance (leading to a relatively large tip-sample separation) has been applied to image the molecules in Figure 2. Hence, the molecules in Figure 2b are not highly resolved. However, different C60 nanocluster configurations highlighted by circles can be identified, as

C60 Molecules on a Ag/Pt(111) Strain-Relief Pattern

Figure 3. (a) STM image of 0.1 ML C60 adsorbed at 150 K on 1.5 ML Ag/Pt(111) where domains of 2 ML Ag/Pt(111) and 1 ML Ag/ Pt(111) coexist (-0.5 V, 50 pA). (b) Close-up STM image of C60 nanoclusters adsorbed on a 2 ML Ag/Pt(111) domain (-3 V, 20 pA).

schematized in Figure 2c. One can see here that the small compact C60 trimers (green circles) are imaged in a stable manner, whereas the large C60 trimers (red circles) are imaged with instabilities. This can be understood from the intermolecular interactions exerting a certain cohesive energy of the order of 0.44 eV per C60 molecule,21 which is sufficient to stabilize the molecules in the small trimer configuration. This is further corroborated by the fact that a large trimer with two additional molecules (pentamer of Figure 2c, orange circle in Figure 2b) is found to be more stable than the large C60 trimer. Because single molecules as well as the molecules of large trimers (where each C60 can be considered as completely isolated from the two others) are found only on the hcp1 corners, one can reasonably assume that these corners represent the points of the highest adsorption energy for the free molecule on the bare terraces of the strain-relief pattern. Further investigations are needed to fully understand the mechanisms involved in the molecular anchoring at the hcp1 corners. Again this behavior is not exclusive to C60, but we have observed it for other polyaromatic molecules. The STM images of Figure 3 depict the situation where 0.1 instead of 0.05 ML C60 has been adsorbed on the Ag/Pt(111) template. At large scale (see Figure 3a), the surface morphology

J. Phys. Chem. C, Vol. 113, No. 13, 2009 5295 is very similar to the one shown in Figure 2a; that is, self-organized growth of C60 clusters as highlighted in the FFT (inset in Figure 3a) occurs only on domains of 2 ML Ag/Pt(111). Further, one can see that on the 2 ML Ag/Pt(111) domains the cluster density is practically the same for the 0.05 ML and the 0.1 ML situations. Zooming in one of these domains (Figure 3b) reveals that the molecules decorate the hcp1 patches. Moreover, two clusters highlighted by circles perfectly fit the hcp1 regions; each one contains exactly 10 molecules, 9 at the edges and 1 in the center. It is seen in Figure 3b that numerous hcp1 patches are completely free of molecules, whereas in Figure 3a almost all hcp1 patches contain molecules as in the case of the lower C60 coverage shown in Figure 2a. The absence of molecules in numerous hcp1 regions in the STM image of Figure 3b is due to the fact that this image was recorded after scanning this surface area several times and unintentionally moving with the STM tip C60 molecules from hcp1 regions to other hcp1 regions. In any case, this proves again that these regions are the energetically most favorable for the nucleation of C60 clusters on the bare terraces of the strain-relief pattern. The C60 clusters seen in Figure 3b contain, in majority, more molecules than those of Figure 2b, which makes them more stable when measuring with usual tunneling parameters, because of additional intermolecular interactions resulting in a higher cohesive energy.21 In view of the preceding results obtained from low C60 coverages adsorbed at LT on the strain-relief pattern, in the following paragraphs, we want to discuss the case of higher C60 coverages adsorbed at RT. It is well-known that C60 molecules are highly mobile at RT on close-packed noble metal single crystal surfaces like Ag(111),22 Au(111),22 Al(111),23 and Cu(111)24 and initial adsorption takes place nearly exclusively at step edges which are the sites of nucleation of 2D islands covering the bare terraces and eventually forming a full ML. On 2 ML Ag/Pt(111), C60 2D islands also form in the middle of the bare terraces and far from the step edges12,13 because of pronounced lateral anisotropy in the molecule-substrate interaction within the unit cell of the strain-relief pattern, which leads to an efficient trapping of the molecules and therefore to island formation. Figure 4 displays STM images of a C60 coverage of 0.7 ML deposited at RT on the strain-relief pattern. Closer inspection of the images reveals two main orientations of the hexagonally closed-packed C60 islands, the first where the closed-packed C60 rows are oriented along the substrate direction and the second which is rotated by 30° with respect to the first one, where the closed-packed C60 rows are oriented along the substrate direction. The oriented C60 domains are shaded in yellow in Figure 4a,b, and as previously reported,12 they are significantly less abundant on the 2 ML Ag/Pt(111) strain-relief pattern. The surface ratio A/A between the two C60 domains in Figure 4a is roughly 5. In the case of C60 on the Ag(111) surface, the situation is such that the ML adopts a (23 × 23)R30° structure22,25,26 which corresponds to the less frequent oriented domains on 2 ML Ag/Pt(111). In addition to the large C60 islands, some C60 clusters can be seen in Figure 4 and are adsorbed only on the hcp1 regions. One of the two C60 clusters in Figure 4b contains exactly 10 molecules here for RT adsorption, like what was discussed above in the case of LT adsorption (Figure 3b). It is clear that such C60 clusters can not form at RT on defect-free bare terraces of Ag(111) as well as Au(111), Al(111) and Cu(111) because of the high molecular mobility.22-25 The C60 clusters observed on the hcp1 regions in Figure 4 (as well as in Figure 2b and Figure 3b) show that these

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Figure 4. (a) STM image of 0.7 ML C60 deposited at RT on the 2 ML Ag/Pt(111) strain-relief pattern (-1.2 V, 0.1 nA). (b) Close-up STM image of the upper region of (a) (-1.2 V, 0.1 nA). Two C60 domains can be seen where the close-packed C60 rows are oriented along the and substrate directions with a surface ratio A/A of about 5. The C60 oriented domains are shaded in yellow. The height scale is for both images.

regions are important for the nucleation and orientation of the large C60 islands grown in the middle of the bare terraces. This can also be seen in the fact that the island edges have the tendency to avoid partial crossing of the hcp1 triangles; that is, the island edges are arranged in a way to either fully fill or fully avoid the hcp1 triangles. This is highlighted in the C60 island of the lower part of Figure 4b where the hcp1 regions at the island edges are shaded in red. This rule, which is respected in general, is however not rigorously strict as, for example, one C60 molecule in the second shaded triangle from the right is missing. Figure 5a shows an STM image of a 1 ML C60 coverage deposited at RT on the 2 ML Ag/Pt(111) strain-relief pattern. Large scale STM images reveal that the molecular film is almost closed, which shows that C60 molecules initially grow layerby-layer.13 In contrast to the occupied states STM image at negative sample bias (-1.2 V) shown in Figure 4b, the unoccupied states STM image at positive sample bias (+2 V) of Figure 5a clearly reveals intramolecular structure of the C60 molecules reflecting the orientation of the molecular orbitals and therefore the orientation of the molecules themselves.23-31 The C60 shape strongly depends on the sample bias and the

Aı¨t-Mansour et al.

Figure 5. (a) STM image of 1 ML C60 coverage on the 2 ML Ag/ Pt(111) strain-relief pattern exhibiting internal structures for the C60 molecules in particular through 3 or 2 lobe shapes (+2 V, 50 pA). (b) Identification of the 3 or 2 lobe shapes as molecules facing up (and down) with a 6-ring and a 6-6 bond, respectively: Top-view schematic representations (green color) and their corresponding simulations based on the extended Hu¨ckel theory LUMO of the free C60 molecule (red color). (c) STM image showing an island characteristic of C60 on the bare Ag(111) surface grown at 150 K (+0.5 V, 20 pA). (d) STM simulation of a C60 island where all molecules face up (and down) with a 6-ring.

molecular orientation with respect to the surface, which in turn is closely related to the adsorption site on the substrate surface.24-26,30,31 In Figure 5a, one can see that the C60 molecules exhibit two principal shapes, namely, 3 or 2 lobes. As the STM image of Figure 5a has been recorded with a positive sample bias (+2 V), the intramolecular contrast stems from the lowest unoccupied molecular orbital (LUMO) of the C60 molecule, for which the charge density is mainly concentrated on the pentagonal C rings, as shown by charge density simulations in Figure 5b based on the extended Hu¨ckel theory LUMO of the free C60 molecule. These charge density calculations clearly allow one to identify the 3 and 2 lobe molecules seen in STM as molecules facing up (and also facing the surface due to the molecule symmetry) with a hexagonal C ring (6-ring) and a double bond between two adjacent hexagonal rings (6-6 bond), respectively. Table 1 summarizes the analysis of the 1500 C60 molecules seen in the STM image of Figure 5a with respect to their apparent shape and thus orientation on the surface and with respect to their location on the surface: (i) outside the hcp1

C60 Molecules on a Ag/Pt(111) Strain-Relief Pattern

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TABLE 1: Statistical Analysis of the 1500 C60 Molecules Seen in Figure 5a with Respect to Their Apparent Shape (Orientation) and with Respect to the Three Types of Surface Positions: Outside the hcp1 Domain, on the hcp1 Edges and in the Center of the hcp1 Domain C60 molecules 3 lobes 2 lobes Other or unknown

total surface outside hcp1 hcp1 edges hcp1 centers 56% 36% 8%

60% 32% 8%

32% 60% 8%

83% 7% 10%

domains, (ii) on the hcp1 edges, and (iii) in the centers of the hcp1 domains. Although the orientational pattern of the C60 molecules is complex and to some extent irregular, the regions of the hcp1 triangle can still be distinguished as they adopt an orientation of the C60 molecules which is markedly different from the rest of the surface, as evidenced from the statistical data of Table 1. This fact further points to the particular characteristics of the hcp1 regions regarding the molecule adsorption properties. We like to mention that, in the classification of Table 1, all molecules identified as 3 (or 2) lobes do not rigorously sit on the surface on a 6-ring (or 6-6 bond) but certain of them present a slight polar misorientation that we do not consider here. Figure 5a together with Table 1 shows that the C60 molecules are not fully regularly oriented on the strain-relief pattern, but their orientation is neither fully random. Before discussing the orientational ordering of the molecules within the full ML on 2 ML Ag/Pt(111), we first want to discuss the simpler case of C60 orientation on the ideally unreconstructed Ag(111) surface. Figure 5c shows an STM image of a small island characteristic of C60 on Ag(111) which is grown at 150 K and adopts the (23 × 23)R30° film orientation. One can clearly see here that all molecules exhibit 3 lobes, which means that all molecules face up (and down) with a 6-ring as confirmed by the STM simulation of Figure 5d. Because of the fact that in this case the C60 lattice is commensurate with the Ag(111) lattice (the C60 nearest-neighbor distance is exactly 23 times the nearest-neighbor distance of Ag), here, all molecules are expected to occupy equivalent sites on the Ag(111) surface. Density functional theory calculations have shown that the energetically most favorable configuration for C60 on Ag(111) corresponds to a 6-ring centered on a 3-fold hollow site.26 This specific C60 orientation has also been found favorable on Au(111),26 Al(111),32,33 and Cu(111).24,31,32,34 In contrast to these close-packed, unreconstructed single crystal surfaces where the atomic lattice is very regular, the 2 ML Ag/Pt(111) strain-relief pattern shows strong, long-range anisotropies (depressions reaching 0.4 Å at the hcp1 corners) and intrinsic irregularities like the unit-cell parameter which, as can be seen in Figure 1, clearly varies from region to region between 6 and 8 nm. Moreover, from different atomic resolution STM images like Figure 1c, we find that inside the superstructure unit cell the average Ag lattice parameter varies locally from domain to domain: 2.80 ( 0.04 Å in fcc, 2.89 ( 0.04 Å in hcp2, and 2.99 ( 0.05 Å in hcp1 domains. These long-range anisotropies and intrinsic irregularities (with respect to the nearest-neighbor atom distance) of the strain-relief pattern result in the situation that the C60 molecules in the close-packed lattice cannot all adopt equivalent adsorption sites, and therefore result in a mixture of C60 orientations on the surface as seen in Figure 5a and Figure 6a. Nevertheless, triangular features corresponding to the hcp1 regions of the substrate, seen through the molecular film, show that the molecules adsorbed on the hcp1 regions are

Figure 6. (a) Zoom-in to the STM image of Figure 5a. (b) Schematic representation of the molecular tetramer highlighted in (a) where the central molecule faces up (and down) with a 6-ring and the peripheral molecules with a 6-6 bond. (c) STM simulation of (b) reproducing the molecular tetramer outlined in (a).

rather specifically oriented (see Table 1). For instance, on the hcp1 edges, the molecules preferentially sit on a 6-6 bond. We like to mention that the 6-6 bond C60 orientation has also been found (in equal proportion with the C60 6-ring orientation) in X-ray photoelectron diffraction measurements from a C60 ML on Ag(111) annealed to 300 °C,35 where it is known that the substrate topmost layer reconstructs by losing its 2D character.28 In analogy, one might consider the edges of the hcp1 triangles as the sites where the strain-relief reconstruction is strongest and where major effects on the molecule orientation can be expected. For the C60 molecules adsorbed in the central positions of the hcp1 domains, one can notice that, in general (in more than 80% of the cases), they exhibit 3 lobes, which means that they adsorb preferentially with a 6-ring as is the case on Ag(111).26 The specific orientation of C60 molecules in these positions will depend on both (i) the molecule-molecule interaction and (ii) the substrate-molecule interaction. For the molecule-molecule interaction (i), one has to look at how the orientation of a given C60 can influence the orientation of an adjacent C60. In the C60 tetramer configuration highlighted in Figure 6a, which is schematized in Figure 6b and simulated in Figure 6c, one can observe that the three peripheral molecules sitting on a 6-6 bond expose a pentagonal face toward the central molecule which faces up (and down) with a 6-ring and which, in turn, exposes a 6-6 bond toward the peripheral molecules. Such a pentagon versus 6-6 bond configuration is favored for electrostatic reasons, because the 6-6 bond is electron-rich and the pentagonal face is electron-poor.36 The configuration of Figure 6b is often adopted, at least partially, by the molecules on the hcp1 regions (Figure 5a and Figure 6a). We note that orienta-

5298 J. Phys. Chem. C, Vol. 113, No. 13, 2009 tional order similar to that of Figure 6b,c has been reported elsewhere for a C60 multilayer island27 and a C60 ML island on Au(111).30 To discuss the role of the substrate-molecule interaction (ii), one needs to know the atomic structure of the topmost layer of the strain-relief pattern, which is displayed in the atomically resolved STM image of Figure 1c. From different atomic resolution STM images like Figure 1c, we find that the edges of the hcp1 equilateral triangles are, in general, 10 or 11 Ag-Ag spacings long, and their centers-of-mass are thus 3-fold hollow sites. As discussed above, this site is known to favor the adsorption of the C60 molecule with a 6-ring on various (111) noble metal surfaces.24,26,33,34 Keeping in mind the preferred filling of the hcp1 domains by 10 C60 molecules, we can understand that the 6 molecules sitting on the edges and the 3 sitting on the corners of the equilateral triangle push the middle 1 “automatically” on the 3-fold hollow site where it adopts the 6-ring adsorption configuration. The configuration of the middle C60 will in turn also influence to some degree the orientation of its nearest neighbors on the hcp1 edges. In contrast to the hcp1 edges where laterally the strongest surface reconstruction of the strain-relief pattern occurs in form of vertical depressions, the regions outside the hcp1 domains show much more isotropic properties (Figure 1). The consequence is that the proportions of the molecular orientations in these regions are completely inverted with respect to those on the hcp1 edges (Table 1). Outside the hcp1 domains, the major part of the molecules show 3 lobes, which means that the molecules prefer to face the surface with a 6-ring, which is the usual C60 orientation on the unreconstructed Ag(111) surface.26 However, whereas the hcp1 domains clearly impose a specific orientation for the molecules in their centers, in the other regions of the strain-relief pattern, the orientational ordering of the molecules is much less stringent. This can be explained by the fact that, within the full C60 ML, the molecules on the hcp1 regions are those which are most strongly pinned to the strainrelief pattern due the preferential binding on these regions. On the other surface regions, the molecules simply have to fill a hexagonally close-packed lattice which, in turn, will adopt in order to match the strain-relief superstructure. Because of the varying hcp1-hcp1 distances, outside the hcp1 regions, different C60 nearest-neighbor distances are found,12,13 and at the same time a mixture of C60 orientations without significant orientational ordering. Conclusions Choosing C60 as a probe, we have examined the molecular adsorption landscape on the 2 ML Ag/Pt(111) strain-relief pattern by means of STM. We have shown that the hcp1 patches of this template surface have special character in this respect. The hcp1 domains are the most favorable sites for trapping single molecules at low coverages (below 0.1 ML) and assembling them into ordered nanoclusters replicating the periodicity of the strain-relief pattern. The corners of the hcp1 domains act as preferred sites for single molecules adsorbed at 150 K. Such single molecules are however rather unstable and have a strong tendency to coalesce to small clusters of three or more molecules located at the hcp1 triangles and stabilized by the cohesive energy of the cluster. The adsorption behavior indicates a higher adsorption energy of the molecules on the hcp1 triangles. This result is different from the case of metal (Ag) deposition reported elsewhere where diffusion barriers are the driving force of the selforganized growth of nanostructures which form not on the hcp1 but on the fcc regions.10,11 In the case of molecules,

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