Structure Prediction of Epitaxial Organic and Inorganic Interfaces

Interface structures are a decisive element of device operation and performance for a wide range of modern electronic devices from organic light-emitting diodes (OLEDs) and organic photovoltaic (OPVs) to semiconductors and spintronic devices. With the need to integrate new materials into devices and applications ranging from quantum computing to medicine, the development of high-quality interfaces with precisely controlled structures is vital to the development of new technologies to meet today’s challenges. Although the phenomenon of organic epitaxy has been known for decades, computational methods for structure prediction of epitaxial organic interfaces have lagged far behind the existing methods for their inorganic counterparts. In this thesis, I developed a new version of the Ogre open source Python package with the capability to perform structure prediction of organic and inorganic epitaxial interfaces by lattice and surface matching. In the lattice matching step, a scan over combinations of substrate and film Miller indices is performed to identify the domain-matched interfaces with the lowest mismatch. Subsequently, surface matching is conducted by Bayesian optimization to find the optimal interfacial distance and in-plane registry between the substrate and film. In the case of inorganic systems, the objective function is a geometric score function based on the overlap and empty space between atomic spheres at the interface and for organic systems it is based on dispersion corrected deep neural network interatomic potentials, shown to be in excellent agreement with density functional theory (DFT). The optimized interfaces are pre-ranked using a score function and the optimized structures of the best candidates interfaces are exported to input geometry files for DFT calculations for further analysis. The application of Ogre is demonstrated for two inorganic interfaces of interest for quantum computing and spintronics, Al/InAs and Fe/InSb and also an epitaxial organic interface of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on tetrathiafulvalene (TTF), whose electronic structure has been probed by ultraviolet photoemission spectroscopy (UPS), but whose structure had been hitherto unknown.