Nitrogen adsorption structures on Cu(1 0 0) and the role of a symmetry-lowering surface reconstruction in the c(2×2)-N phase

Scanning tunnelling microscopy (STM) has been used to study the adsorption structures formed by atomic nitrogen on Cu(1 0 0), produced by low energy nitrogen ion bombardment and annealing. Atomic resolution imaging under varying tip conditions shows that in the c(2×2) phase STM appears to always image as asperities the Cu atoms, and not the N atoms as has been previously supposed. These images also indicate that in this phase the N induces a novel large-amplitude symmetry-lowering rumpling reconstruction of the outermost Cu layer. This conclusion allows one to understand many aspects of the mesoscopic c(2×2) island structures observed in this system and also reported in earlier STM investigations, and specifically the systematic behaviour of inter-island boundary widths which provide an indirect indication of the symmetry reduction. The possible role of adsorbate-induced surface stress changes and strain perpendicular to the surface at both local and longer ranges in the formation and self-organisation of the islands is discussed. Evidence for local strain and information on the anisotropy of step energies in the saturation coverage `trenchʹ phase is also discussed.