Homoepitaxial growth modes in textured, polycrystalline ultrathin pentacene films on dielectrics

Detailed knowledge of homoepitaxial growth modes in organic semiconductor thin films is critically important for understanding the interlayer electrostatic coupling and the surface energetics of device-active organic films, yet it is still largely missing due to the lack of effective characterization tools. This paper presents a convenient approach to characterize explicitly the homoepitaxial growth modes of pentacene bilayers thermally deposited on different dielectrics by combining two scanning probe microscopy (SPM) imaging modes, i.e., friction force microscopy (FFM) and transverse shear microscopy (TSM). It is found that pentacene second layer grains consistently exhibit a mosaic of homoepitaxial modes (commensurism, coincidence, and non-epitaxy) regardless of the substrate type and deposition condition. Among different homoepitaxial modes, a coincident twist epitaxy is more frequently observed. This combined FFM/TSM technique offers a feasible way to identify complex microstructural motifs such that a deeper understanding of growth and structure-property relationships in organic semiconductor thin films is possible.