Nanoscale oxide growth on Al single crystals at low temperatures: Variable charge molecular dynamics simulations

We investigate the oxidation of aluminum low-index surfaces [(100), (110), and (111)] at low temperatures (300-600 K) and three different gas pressure values. We use molecular dynamics (MD) simulations with dynamic charge transfer between atoms where the interaction between atoms is described by the Es+ potential composed of the embedded atom method (EAM) potential and an electrostatic contribution. In the considered temperature range and under different gas pressure conditions, the growth kinetics follow a direct logarithmic law where the oxide thickness is limited to a value of similar to 3 nm. The fitted curves allow us to determine the temperature and the pressure dependencies of the parameters involved in the growth law. During the adsorption stage, we observe a rotation of the oxygen pair as a precursor process to its dissociation. In most cases, the rotation aligns the molecule vertically to the Al surface. The separation distance after dissociation ranges from 3 to 9 A. Atomistic observations revealed that the oxide presents a dominant tetrahedral (AlO4) environment in the inner layer and mixed tetrahedral and octahedral (AlO6) environments in the outer oxide region when the oxide thickness reaches values beyond similar to 2 nm.