Formation stability and electronic structure of surfaces and interfaces from first principles

Doktorsavhandling, 2010

This thesis deals with two closely interwoven aspects of first-principle (density functional theory) materials modeling: (1)~prediction of atomic structure & chemical composition, and (2)~prediction of electronic properties. In the first part, we focus on atomic structure (AS) and chemical composition (CC) of surface and interface systems. These systems have a large range of technical applications, building on both mechanical strength and electron behavior. Surface and interface systems are often fabricated in complex gas-phase deposition environments. Characterizing and predicting AS and CC is an important challenge and understanding of how these result in a growth environment is of particular interest. We formulate a novel nonequilibrium thermodynamic method to predict AS and CC as a function of the deposition environment. The method combines first-principle calculations with chemical reaction theory and rate-equation modeling. We implement this method and use it to illustrate its predictive power for characterizing AS and CC at industrially relevant interfaces between alumina and titanium carbide, grown by chemical vapor deposition. Our predictions of AS and CC result in adhesion properties that agree with the wear-resistant nature of TiC/alumina multilayers; equilibrium predictions do not. This result suggests that our method is a useful theoretical tool for characterizing materials whose AS and CC is determined by the specific deposition conditions. In the second part, we investigate the relevance of van der Waals (vdW) interactions for electronic properties. We focus on vdW binding in graphene overlayers at silicon carbide surfaces and in multilayers of graphane (a fully hydrogenated derivative of graphene). These materials are promising candidates for future electronic devices. Performing band-structure calculations and wave-function analysis, we find that vdW binding to a neighboring layer or substrate can significantly alter the electronic behavior and in particular the band struc ture. Our calculations predict strong local band-gap modifications in insulating graphane multilayers due to vdW interactions. We also document that vdW binding effectively amounts to a doping of graphene overlayers at SiC surfaces.