Organic epitaxy as a tool for controlling crystallinity, orientation, and properties of organic thin films

In the research field of functional organic materials interesting for technological applications[1], the possibility of growing thin films with controlled properties is a main goal. Indeed, the architecture of all devices requires integration of different materials in the form of thin films, which have to display the intrinsic physical properties of the corresponding bulk materials, free from growth-dependent defects, impurities, or structural characteristics related to non-equilibrium states. Among the different techniques for thin film growth, organic molecular beam epitaxy (OMBE)[2] offers the possibility to reach not only a high control on material purity, on film thickness and morphology, on surface and interface quality, but also on film crystallinity and orientation, thanks to organic epitaxy (OE)[3]. Here, after reviewing the concept itself of OE and its evolution, some examples are given where different organic molecules may form epitaxial interfaces on properly selected organic crystalline substrates. In some cases, crystalline thin films displaying the crystal structure and properties of the bulk single crystal are grown, while in other cases nanostructures with precise crystal structure and orientation are obtained and deeply characterized. [1] W. Brutting (ed), 'Physics of organic semiconductors', Wiley-VCH, Weinheim (2005). [2] S.R. Forrest, Chem. Rev. 97 1793 (1997); A. Sassella et al., Riv. Nuovo Cimento 31 457 (2008). [3] see, e.g.: M. Oehzelt et al., Adv. Mater. 18 2466 (2006); C. Wagner et al., J. Phys. Chem. Lett. 3 419 (2012); A. Sassella, Cryst. Res. Technol. 48 840 (2013); M. Campione et al., J. Phys. Chem. C 119 18210 (2015); L. Raimondo et al., Adv. Mater. Interfaces 4 1700670 (2017).