Deposition of fullerenes on the CaF(2)(111) surface yields peculiar island morphologies with close similarities to previous findings for (100) surfaces of other ionic crystals. By means of noncontact atomic force microscopy we find a smooth transition from compact, triangular islands to branched hexagonal islands upon lowering the temperature. While triangular islands are two monolayers high, hexagonal islands have a base of one monolayer and exhibit a complicated structure with a second-layer outer rim and trenches oriented towards the interior. By developing a kinetic growth model we unravel the microscopic mechanisms of the structure formation.

Non-contact atomic force microscopy is used to study C(60) molecules deposited on the rutile TiO(2)(110) surface in situ at room temperature. At submonolayer coverages, molecules adsorb preferentially at substrate step edges. Upon increasing coverage, ordered islands grow from the decorated step edges onto the lower terraces. Simultaneous imaging of bridging oxygen rows of the substrate and the C(60) island structure reveals that the C(60) molecules arrange themselves in a centered rectangular superstructure, with the molecules lying centered in the troughs formed by the bridging oxygen rows. Although the TiO(2)(110) surface exhibits a high density of surface defects, the observed C(60) islands are of high order. This indicates that the C(60) intermolecular interaction dominates over the molecule-substrate interactions that may cause structural perturbations on a defective surface. Slightly protruding C(60) strands on the islands are attributed to anti-phase boundaries due to stacking faults resulting from two islands growing together.

Non-contact atomic force microscopy (NC-AFM) was applied to study C(60) molecules on rutile TiO(2)(110). Depending on the tip-sample distance, distinctly different molecular contrasts are observed. Systematically decreasing the tip-sample distance results in contrast inversion that is obtained reproducibly on the C(60) islands. This change in contrast can be related to frequency shift versus distance (df(z)) curves at different sample sites, unraveling crossing points in the df(z) curves in the attractive regime. We have performed simulations based on a simple Morse potential, which reproduce the experimental results. This combined experimental and simulation study provides insight into the mechanisms responsible for molecular contrast in NC-AFM imaging. Moreover, this work demonstrates the importance of distance-dependent measurements for unambiguously identifying molecular positions within a molecular island using NC-AFM.

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