Origins of texture in TTF-TCNQ thin films grown by organic CVD from TTF and TCNQ precursors

The preparation and characterization of thin films of the organic molecular conductor TTF-TCNQ formed by the donor TTF (tetrathiafulvalene) and the acceptor TCNQ (7,7,8,8-tetracyano-p-quinodimethane) is presented. The layers were grown on KCl (001) substrates by OCVD (organic chemical vapor deposition). The experiments were carried out on a horizontal hotwall reactor using TTF and TCNQ as precursors and argon as carrier gas. The obtained films are polycrystalline, showing a different in-plane texture depending on the substrate temperature. The existence, under very well defined experimental conditions, of new in-plane textures in addition to the energetically most favorable has been interpreted in terms of a local minimum in the interaction energy between the first monolayer of a c-oriented crystalline unit and the substrate. This interaction energy was calculated taking into account the van der Waals and electrostatic forces existing between the organic layer and the alkali halide substrate.