Epitaxial Tetrathiafulvalene–Tetracyanoquinodimethane Thin Films on KCl(100): New Preparation Methods and Observation of Interface-Mediated Thin Film Polymorph

Combining organic compounds of complementary ionization potential and electron affinity allows fabrication of charge-transfer complexes that exhibit remarkable properties, resulting, for example, in very high conductivity. Though the bulk properties of the prototypical organic conductor tetrathiafulvalene–tetracyanoquinodimethane (TTF-TCNQ) have been studied in detail, the influence of defects and crystallite size on the resulting electronic properties, as well as an integration of these materials in organic thin film devices, is barely explored. One important requirement for such a comprehension is the precise control over crystallite size and quality. In this study, we report on different strategies to prepare crystalline TTF-TCNQ thin films and compare their structural quality. While conventional organic molecular beam deposition of TTF-TCNQ onto KCl(100) substrates enables the growth of epitaxial thin films with grain dimensions of up to 2 μm, further enhancement of the crystallite dimensions by raising the growth temperature is thermally limited by vanishing sticking and onset of vaporization. Using more sophisticated methods like hot wall evaporation, however, allows one to overcome these limitations and yields crystalline islands with extensions enhanced by 2 orders of magnitude. Furthermore, we identify and provide a full structure solution of a yet unknown interface-mediated thin film polymorph of TTF-TCNQ, which is adopted in films of thicknesses below 1 μm.