Imaging perylene derivatives on rutile TiO2(110) by noncontact atomic force microscopy

The adsorption of 3,4,9,10-perylene tetracarboxylic diimide derivative molecules on the rutile TiO2(110) surface was investigated by noncontact atomic force microscopy and density-functional theory (DFT) calculations. After submonolayer deposition, individual molecules are observed to adsorb with their main axis aligned along the [001] direction and centered on top of the bridging oxygen rows. Depending on the tip termination, two distinctly different molecular contrasts are achieved. In the first mode, the molecules are imaged as bright elongated features, while in another mode the molecules appear with a bright rim and a dark bow-shaped center. Comparison with the defect density on the bare TiO2(110) surface suggests that the molecules preferentially anchor to surface defects. Our DFT calculations reveal details of the molecular adsorption position, confirming the experimentally observed adsorption on top of the bridging oxygen rows. The DFT results indicate that diffusion along the rows should be quite easily possible, while diffusion perpendicular to the rows seems to be hindered by a significant energy barrier.