Structure and Property Prediction of Hybrid Inorganic-Organic Systems: Tetracene and Pentacene at H:Si(111)*

Hybrid organic inorganic systems allow to combine the properties of organic and inorganic substances and hence open up a wide field for the construction of new materials with tunable properties. In particular the organic compound offers a large range of functionalities and can be designed to be environmentally benign. As a key example of aromatic molecule-interface systems, we here present a first-principle based structure search and electronic property investigation of pentacene and tetracene adsorbed at the hydrogenated Si(111) surface. The adsorption of single molecules is explored at different thin film coverage conditions with a structure search routine that includes grid- and random-structure search elements. This enables us to identify the most favorable orientation of the acene molecules and, using hybrid functionals, to investigate the level alignment between the organic and inorganic compounds with respect to the possibility of charge transfer: Single tetracene and pentacene molecules prefer to lie flat on the surface. For both organic molecules, the HOMO lies at the top of the valence band maximum allowing at best little charge transfer. For increasing coverage, we expect a transition to a dense geometry of standing molecules.