An important precondition for constructing functional molecular assemblies at the surface is the selective positioning of molecular building blocks on predefined adsorption sites. This is required for the sequential building of supramolecular structures where a first molecule is anchored and defines the base unit for further molecules that will be attached via attractive intermolecular interactions. This strategy requires the preparation of substrate surfaces acting as appropriate templates for supramolecular structures, and the design of appropriate molecular building blocks. The molecular core in this case enforces the adsorption at predefined sites of the surface, while the functional groups remain accessible for bonding with post-deposited molecules.
We investigate the adsorption behavior of large polycyclic aromatic hydrocarbons (PAHs) on various template surfaces [1]. Here, we present the successful site-selective anchoring of PAHs on two monolayers of Ag on Pt(111) forming a two-dimensional strain-relief network [2,3]. Molecules preferably adsorb individually on the corners of the discommensuration triangle around the hcp1 region, indicating that the local increase of the binding energy exceeds attractive intermolecular interactions.
In order to explain this selective behavior we have investigated the local electronic properties of the strain-relief pattern with scanning tunneling spectroscopy. With respect to the surface state the onset varies by 170 meV on the different stacking regions and the unoccupied density of states is largely increased on the discommensuration triangle. Furthermore, we find an energy shift of the image potential states (IPS) of up to 230 mV for the different stacking regions indicating a local variation of the surface potential. Modelling the IPS for the local Coulomb potential allows the assignment of the local work function to the different stacking areas.
[1] P. Ruffieux, K. Palotás, O. Gröning, D. Wasserfallen, K. Müllen, W. A. Hofer, P. Gröning and R. Fasel, JACS (in press, 2007)
[2] H. Brune, M. Giovannini, K. Bromann, and K. Kern, Nature 394 451 (1998).
[3] K. Aït-Mansour, P. Ruffieux, W. Xiao, P. Gröning, R. Fasel, and O. Gröning. Phys. Rev. B 74, 195418 (2006).
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