Surface Dynamics of Cu Oxidation
Authors: Yang Judith , Zhu Qing , Zou Lianfeng , Andolina Christopher , Xiao Penghao , Stach Eric , Henkelman Graeme , Zhou Guangwen , Saidi Wissam Abdo 
Published: December 2016

Much is known about oxygen interaction with metal surfaces and about the macroscopic growth of thermodynamically stable oxides. At present, however, the nanoscale stages of oxidation – from nucleation of the metal oxide to formation of the thermodynamically stable oxide – represent a scientifically challenging and technologically important
terra incognito
. As engineered materials approach the nanometer regime, control of their environmental stability at this scale becomes crucial. As environmental stability is an essential property of most engineered materials, many oxidation theories exist to explain its mechanisms. However, most classical oxidation theories assume a uniform growing film, where structural changes are not considered due to the lack of traditional experimental procedure to visualize this non‐uniform growth under conditions that allow highly controlled surfaces and impurities. Yet, in situ transmission electron microscopy studies reveal that the initial stages of Cu oxidation are due to surface diffusion of oxygen followed by nucleation and growth of oxide islands, and thereby challenge the common assumption of a uniform oxide formation [1]. Understanding this initial oxidation of the metal surface, from the atomic to mesoscale, is the fundamental challenge.

We have previously demonstrated that the formation of epitaxial Cu
2
O islands during the transient oxidation of Cu(100), (110) and (111) films bear a striking resemblance to heteroepitaxy, where the initial stages of growth are dominated by oxygen surface diffusion and strain impacts the evolution of the oxide morphologies. We are developing a kinetic Monte Carlo code, Thin Film Oxidation (TFOx), to simulate the nucleation and growth of Cu oxides (see Figure). We are currently investigating oxidation of stepped Cu surfaces as realistic surfaces have many defects such as step edges that can dictate the oxide nucleation and growth dynamics, and result in novel oxide nanostructures. In situ ETEM studies are complemented with a systematic multiscale theoretical approach. This approach includes density functional theory (DFT) of (100), (110) and (111) Cu surfaces to illustrate the thermodynamic and kinetic factors of initial oxygen‐metal interactions, and molecular dynamics (MD) simulations to validate the DFT predictions and the oxidation process. Our DFT results show that the Ehrlich‐Schwöbel (ES) barrier can favor either oxygen ascending or descending diffusion directions, or limited interlayer diffusion depending on the surface step morphology. These simulations are compared to ETEM investigations of stepped Cu surfaces in situ. The potential and limitations of recent developments in theoretical simulations such as bridging the spatial and temporal gap to in situ ETEM results will be discussed. Correlating the experimental results with theoretical predictions is needed for rational design of oxidation‐resistant coating materials and may lead to a paradigm shift in the fundamental understanding of oxidation where surfaces and defects control the early stages of oxidation.