Observation of epitaxial growth of pentacene thin films on KCl substrate by X-ray diffractometry

The epitaxial relationships of vapor-deposited thin films of pentacene polymorphs on KCl (1 0 0) substrates were investigated by X-ray diffractometry. The in-plane and out-of-plane structures of the thin films were investigated using a four-axis X-ray diffractometer. The presence of two polymorphs, bulk and thin-film phases, depended on the substrate temperature, and the epitaxial relationships were determined for each polymorph. Three epitaxial patterns were observed for each polymorph, and the ratio of in-plane orientations changed as a function of substrate temperature. The potential energies calculated for a cluster including four pentacene molecules as a function of azimuth angles were well consistent with the experimental results. Based on these findings, the temperature dependence of the in-plane orientations could be attributed to differences in cluster size during nucleation processes.