Post-deposition reorganization of pentacene films deposited on low-energy surfaces

We demonstrate that small-molecule organic thin films of pentacene deposited from thermal and supersonic molecular beam sources can undergo significant reorganization under vacuum or in N(2) atmosphere, beginning immediately after deposition of thin films onto SiO(2) gate dielectric treated with hexamethyldisilazane (HMDS) and fluorinated octy trichlorosilane (FOTS). Films deposited on bare SiO(2) remain unchanged even after extended aging in vacuum. The changes observed on low-energy surfaces include the depletion of molecules in the inerfacial monolayer resulting in the population of upper layers via upward interlayer transport of molecules, indicating a dewetting-like behavior. The morphology of pristine, as-deposited thin films was determined during growth by in situ real-time synchrotron X-ray reflectivity and was measured again, ex situ, by atomic force microscopy (AFM) following aging at room temperature in vacuum, in N(2) atmosphere, and in ambient air. Important morphological changes are observed in ultra-thin films (coverage < 5 ML) kept in vacuum or in N(2) atmosphere, but not in ambient air. AFM measurements conducted for a series of time intervals reveal that the rate of dewetting increases with decreasing surface energy of the gate dielectric. Films thicker than 65 ML remain stable under all conditions; this attributed to the fact that the interfacial layer is buried completely for films thicker than 65 ML. This work highlights the propensity of small-molecule thin films to undergo significant molecular-scale reorganization at room temperature when kept in vacuum or in N(2) atmosphere after the end of deposition; it should serve as a cautionary note to anyone investigating the behavior of organic electronic devices and its relationship with the initial growth of ultra-thin molecular films on low-energy surfaces.