Influence of substrate morphology on growth mode of thin organic films: An atomic force microscopy study

Morphology of high-vacuum deposited thin films of pentacene and rubrene on annealed vicinal (0001) sapphire surfaces was studied by atomic force microscopy in non-contact mode. Pentacene molecules in the first monolayer form two-dimensional islands with an average height of 1.5 nm. The majority of islands nucleate at the steps on the sapphire surface. Surface steps also act as nucleation sites for subsequent monolayers of pentacene. The terrace-and-step morphology of the substrate surface has a larger impact in the case of rubrene thin film growth. Rubrene grows in wire-like structures that follow the direction of the steps. In the case of rubrene films with a nominal thickness of 10 nm, the typical widths and heights of the wires are 150 and 5 nm, respectively. The separation between the wires is determined by the terrace width on the substrate surface. The difference in the morphology of pentacene and rubrene films may be explained by taking into account different geometries of the two molecules. (c) 2007 American Vacuum Society.