Observation of Unusual Homoepitaxy in Ultrathin Pentacene Films and Correlation with Surface Electrostatic Potential
Vivek Kalihari,1 D. J. Ellison,1 Greg Haugstad2 and C. Daniel Frisbie1
1Department of Chemical Engineering and Materials
Science, University of Minnesota, 421 Washington Avenue SE and 2Characterization
Facility, Institute of Technology, University of Minnesota, 100 Union Street SE,
Minneapolis, Minnesota 55455
Identifying specific microstructure-property relationships in polycrystalline organic semiconductor films is a key goal for the field of organic electronics. Using transverse shear microscopy, we have established for the first time the presence of both epitaxial and non-epitaxial domains in ultrathin polycrystalline layers of pentacene grown on SiO2. The microstructure of pentacene films is particularly important, as pentacene is a benchmark semiconductor for organic field effect transistors (OFETs). Epitaxial domains in the second pentacene molecular layer exhibit unusual type-II coincidence with respect to the first pentacene monolayer, while the third and subsequent layers show commensurism with their respective underlayers. In addition, Kelvin probe force microscopy (KFM) reveals that the epitaxial domains have significantly more positive surface potential than the non-epitaxial domains. Collectively, these findings establish a previously unrecognized link between epitaxial order in organic semiconductor films and a well-defined electrical property, the surface potential, which is known to influence charge carrier transport parallel to the pentacene/SiO2 interface in OFETs.