Initial stages of growth of organic semiconductors on vicinal (0Â 0Â 0Â 1) sapphire surfaces

Initial stages of growth of pentacene, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), rubrene, and N,N′-bis(3-methylphenyl)- N,N′-diphenylbenzidine (TPD) on vicinal (0 0 0 1) sapphire surface were examined by non-contact atomic force microscopy. Pentacene nucleates at the nm-size steps, however island growth proceeds on the terraces in a layer-plus-island growth mode. PTCDA nucleates randomly at room temperature, while at 135 °C and low coverage the molecules aggregate at the steps. At increased coverage the island growth proceeds in the directions determined by the intermolecular interactions and not along the steps. Rubrene and TPD nucleate in 3D islands that evolve over time by ripening. TPD nucleates along the steps and also ripening proceeds along the steps. The rate of ripening of rubrene islands is one order of magnitude slower than the rate of ripening of TPD islands. We associate this difference to the wealth of rotational degrees of freedom in TPD molecules as opposed to only twisting degree of the tetracene backbone in rubrene.