Photoresponse and morphology of pentacene thin films modified by oxidized and reduced diamond surfaces

Because of its large band gap and variety of stable surface terminations, diamond is a suitable material to study the optical and electronic properties of organic films. Optical absorption and photocurrent experiments with pentacene on hydrogen- and oxygen-terminated diamond surfaces reveal a strong, polarization-dependent photoresponse of pentacene films. The diamond surface reconstruction as well as the molecule-surface interactions influence the morphology and the molecular structure of the films, causing the associated polarization dependence. On oxygen-terminated diamond, the pentacene thin-film phase typical for electronically inert substrates such as SiO2 is formed. On hydrogen-terminated diamond, on the other hand, a three-dimensional growth mode of a filamentlike pentacene morphology is observed by atomic force microscopy, with pentacene molecules arranged with their long molecular axis oriented along the hydrogen-terminated diamond surface, as confirmed by x-ray diffraction. Furthermore, on hydrogen-terminated single crystalline diamond, the b axis of the pentacene unit cell is found to orient preferentially perpendicular to the surface, in agreement with photocurrent and optical-absorption experiments.