Epitaxial Growth of Organic Semiconductor Polymorphs on Natural Amino Acid Single Crystals

Single crystals of a well-known amino acid, namely β-alanine, were exploited as a novel substrate for the deposition of thin films of organic molecules suitable for optoelectronic device applications. Quaterthiophene and hexathiophene, two of the most studied organic semiconductors, were selected as a paradigmatic case to investigate the β-alanine surface properties in terms of possible polymorph selectivity over the deposited overlayers. They are known to grow as crystalline films on organic crystal substrates, preferentially in the low temperature polymorphs, although the high temperature ones are identified. The morphology, the crystal structure, and the orientation of the deposited films were analyzed by atomic force microscopy, X-ray diffraction, and UV–vis spectroscopy. The experimental results, also compared to computational simulations of the oligothiophene/β-alanine interface, demonstrated that β-alanine induces the growth of the metastable polymorphs of both the oligothiophenes deposited as thin films, by organic molecular beam deposition and by physical vapor transport. Thanks to its high water solubility, β-alanine, a low-cost and biocompatible material, can then be employed to grow even metastable polymorph overlayers of technological interest, wet-transferable easily on other substrates.