First-principles study on the strain effect of the Cu(0 0 1)-c(2×2)N self-organized structure

Nitrogen atoms adsorbed on Cu(0 0 1) surface are known to form a self-organized structure, in which islands of nitrogen-adsorbed region are arranged into a square lattice. To clarify the mechanism of the self-organization, the strain effect in this surface is investigated by first-principles theoretical calculations. The difference between the calculated surface stress of Cu(0 0 1)- c ( 2 × 2 ) N surface and that of clean Cu(0 0 1) surface is in good agreement with the value estimated from experiments. In the stripe model of the self-organized surface, the top-most Cu atoms are largely displaced in lateral directions, while the nitrogen atoms at the edge of its nitrogen-adsorbed region slightly protrude in the surface normal. These results are consistent with observations. Spontaneous formation of the clean Cu region is also confirmed by calculating the formation energy within the stripe model. The formation energy is fitted well by a function deduced from the theory of elasticity. Nevertheless, the parameter of the fitting cannot be explained only by the difference of the surface stresses.