Patrick Soukiassian

In an effort to extend the reach of current ab initio calculations to simulations requiring millions of configurations for complex systems such as heterostructures, we have parameterized the third-generation Charge Optimized Many-Body (COMB3) potential using solely ab initio total energies, forces, and stress tensors as input. The quality and the predictive power of the new forcefield are assessed by computing properties including the cohesive energy and density of SiO₂ polymorphs, surface energies of alpha-quartz, and phonon densities of states of crystalline and amorphous phases of SiO₂. Comparison with data from experiments, ab initio calculations, and molecular dynamics simulations using published forcefields including BKS (van Beest, Kramer, and van Santen), ReaxFF, and COMB2 demonstrates an overall improvement of the new parameterization. The computed temperature dependence of the thermal conductivity of crystalline alpha-quartz and the Kapitza resistance of the interface between crystalline Si(001) and amorphous silica is in excellent agreement with experiment, setting the stage for simulations of complex nanoscale heterostructures.

We investigate the initial Na-promoted oxidation of an Al surface at room temperature by core-level photoemission spectroscopy using synchrotron radiation. We bring the first evidence that, in the initial state, adsorbed oxygen atoms break the Na-Al bonds. This leads to the formation of an intermediate oxidation state involving both Na and Al sites with oxygen atom in a bridge-type site. This bonding configuration is shown to result from the Na/Al intermixing and appears as a precursor step playing a key role in the observed promoted oxidation. It is also likely to explain the formation, upon annealing, of Al oxides having a higher oxygen content than Al ₂ O ₃.