AVS2012 Session OX-TuP: Oxide Interfaces and Heterostructures Poster Session

Heterogeneous systems of Cu clusters on ZnO surfaces are used as catalysts and it is a challenge to maintain the stability and activity of the catalyst under reaction conditions. Processes such as sintering, alloying, and encapsulation may play an important role in the activity of the catalyst but are difficult to model directly with electronic structure calculations. In this work, we will report on the development and use of charge-optimized many-body (COMB) potentials to model the Cu/ZnO system. In particular, the diffusion of Cu atoms and the ripening of Cu clusters on Cu and ZnO surfaces are modeled using kinetic Monte Carlo simulations, which is used in conjugation with the dimer method to find possible transition paths for Cu migration. Simulations allow for a comparison of transport mechanisms on the two different surfaces (Cu and ZnO) and the predictions are compared to the results of density functional theory calculations and published experimental data. This work was supported as part of the Center for Atomic Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award number DE-SC0001058.

An empirical, variable charge potential for Nitrogen hydride and Nitrogen oxide molecular systems based on the charge optimized many body (COMB) potential framework is proposed. The potential is fitted to structure and energetic properties of certain key molecules such as NH₃ and NO₂. This potential is used in conjunction with TiN/TiO₂ and the AlN/Al₂O₃ variable charge potentials based in the same COMB framework to study the structure, energetics, and mechanics of oxide growth on TiN and AlN under varying conditions.

In this study, we have grown (i) multilayers of high quality samaria doped ceria (SDC) and scandia stabilized zirconia (ScSZ) epitaxial thin films, and (ii) samaria and gadolinia co-doped high quality ceria thin films using oxygen plasma-assisted molecular beam epitaxy and characterized using various capabilities. The number of layers in the SDC/ScSZ multi-layer thin films was varied from 2 to 20 by keeping the total film thickness constant at 140 nm to understand the effect of nano-scale interfaces on the ionic conductivity. To understand the effect of co-doping on the ionic conductivity of optimized SDC thin films, Ce_0.85Sm_0.15-xGd_xO_2-δ (SGDC) thin films were deposited by varying the Gd concentration. The film growth was monitored using in-situ reflection high energy electron diffraction (RHEED). Structural properties of these films were studied using x-ray diffraction (XRD) and the XRD patterns confirmed the growth of epitaxial thin films. The film and layer thicknesses were determined by x-ray reflectivity. X-ray photoelectron spectroscopy was used to find the composition, depth profile and chemical state of elements of the films. The SDC/ScSZ multi-layer and SGDC thin films were carefully characterized using Rutherford backscattering spectrometry, coupled scanning transmission electron microscopy and atom probe tomography to study the oxygen vacancy and dopant distributions along with the inter-diffusion and dopant segregation at the interfaces. Oxygen ionic conductivity measurements were carried out as a function of temperature on well characterized samples using four probe surface impedance spectroscopy. Detailed analysis of oxygen ionic conductivity as a function of individual layer thickness, dopant concentration, and crystalline quality of the films will be discussed.