Nitrogen Adsorption on Cu(001): Mechanisms of Stress Relief and Coexistence of Two Domains

We have studied nanometer-scale features observed with scanning tunneling microscopy on nitrogen (N)-adsorbed Cu(001) surfaces. Grid-like nanopattern aligned in [100] directions at a medium N-coverage and trenches along [110] appearing at a higher N-coverage are frequently reported. In addition, we observed “diagonal lines” along [110] on a grid-patterned surface and “dark curves” growing from the trench ends. All features above are responsible for relief of N-induced compressive stress. We propose structural models for the five features, based on a hard-sphere model, and estimate how much strain is absorbed by the feature. Nitrogen atoms are adsorbed on Cu(001) in c(2 × 2) arrangement. Since two energetically-equivalent c(2 × 2) arrangements are possible, two N-adsorbed domains can be “in-phase” or “antiphase” in a [100] direction at the domain boundary. We show also that each strain-relief feature has specific preference in phases. Looking out over the five mechanisms, we note that, the higher the N coverage is, the more effective relief mechanism is chosen, and that “monoatomic line” (a component of the grid) is very unique since it is formed between in-phase domains.