Self-assembled submicron-height terrace structures of pentacene single crystals formed in liquid crystal cells

A flat-surface single crystal structure of pentacene organic semiconductor was formed, with a submicron-height terrace structure, in liquid crystal solvent cells; the formation mechanism is discussed. By cooling the pentacene solution in a heated cell until supersaturated, a variety of segregated crystal morphology was observed, including dendrite, lozenge and needle-like crystals. Segregation of lozenge crystals was promoted by the appropriate pentacene concentration combined with the rubbing process of polyimide alignment layers, and the crystal morphology was examined in detailed. As a result, based on terrace-structural growth, low-profile flat-surface crystal morphology was found in addition to conventional pyramidal morphology. The molecular alignment of the flat pentacene crystal was confirmed by anisotropy of the microscopic Raman scattering intensity of the polarized incident light used for excitation. The self-assembly of flat thin single crystal plates, whose maximum size reach 150 μm approximately, may be applicable to practical electronic devices such as organic transistors.