Physical vapor growth of centimeter-sized crystals of α-hexathiophene

The processes in physical vapor transport: vaporization, transport and crystal growth, the regimes for transport: molecular flow path limited, diffusion-limited, convection-limited and forced-convection-limited are analyzed and the results are used to guide a systematic investigation of physical vapor transport and crystal growth of alpha-hexathiophene (alpha 6T), a promising thin-film transistor organic material. Successful growth occurred when the gas pressure was such that the regime was convective and when deliberate inert-gas flow (forced convection) improved volatilization. Plate-like growth morphology and thickness differences between the high-temperature and low-temperature polymorphs is explained on the basis of differing atomic structure. Conditions for the reproducible growth of crystals of up to 1 cm in size are reported. We feel that the analyses and procedures reported here can be used to grow crystals of other organic materials.