Polymorphism in pentacene thin films on SiO2 substrate

The polymorphism of pentacene thin films on SiO2 substrate was investigated by grazing incidence x-ray diffractometry (GIXD). The in-plane GIXD patterns were obtained from vacuum deposited ultrathin films. By comparing the in-plane structures with thicker films, it was elucidated that ultrathin films have the same lattice constants as that of films of 100 nm thick. Consistency of determined interplanar d(11) spacings of the two polymorphs suggests the epitaxial growth that bulk phase grows onto thin-film phase. Considering the obtained unit cell parameters, the mechanism of the transformation between polymorphs was discussed in terms of equilibrium shapes during nucleation process. (c) 2007 American Institute of Physics.