Role of Graphene Topography in the Initial Stages of Pentacene Layer Growth

Using atomic force microscopy, we probed the growth of pentacene molecules on graphene that was fabricated by chemical vapor deposition and transferred onto 300 nm-thick SiO₂ substrates. The topography of such graphene has two important properties. First, its surface is comprised of folds that have different orientations, and second, it has several multilayer-graphene regions distributed over the monolayer-graphene surface. On such folded graphene features, we vacuum evaporated pentacene and observed three-dimensional islands with an average height of ∼15 nm. They are oriented either parallel or perpendicular to the folds, and they are also predominantly oriented along the symmetry axes of graphene. Orientation of pentacene islands on graphene evaporated at room temperature has a wide distribution. On the contrary, most of the pentacene islands evaporated at 60 °C are oriented at 30° with respect to the fold direction. We observed that the folds act as a potential barrier for the surface transport of pentacene molecules. In addition, we interpret the 3D growth of pentacene islands on graphene in terms of reduced polar components of the surface energy on graphene investigated with contact angle measurements.