Surface relaxation and near-surface atomic displacements in the N/Cu(1 0 0) self-ordered system

The atomic displacements of Cu atoms induced by nitrogen adsorption on Cu(1 0 0) have been studied by channelling–blocking of swift 4He ions. This study has been performed at two adsorption stages. The first one corresponds to the formation of a dense, two-dimensional, self-ordered array of square-shaped islands covered by nitrogen. The second one corresponds to uniform coverage at saturation. We have determined by nuclear reaction analysis the absolute quantity of nitrogen adsorbed at these two stages. The values obtained, when confronted to previous observations of these stages by low energy electron diffraction and by scanning tunnelling microscopy, demonstrate that nitrogen remains mostly at the sample surface and that the N concentration in bulk Cu could not exceed 1%. However, channelling measurements show that this surface adsorption generates atomic displacements of Cu atoms down to depths of a few ten (1 0 0) interplanar distances. In the mean time, blocking measurements reveal that nitrogen adsorption induces a strong surface expansion: the interplanar distance between the first two (1 0 0) planes increases of about 0.2 Å, in contrast with the weak contraction observed on bare Cu(1 0 0) surfaces. This observation supports the hypothesis that, when nitrogen is adsorbed, the surface is submitted to stress variations, from tensile to compressive stress for, respectively, bare and nitrogen-covered surface regions. The surface forces corresponding to such variations have been introduced in molecular dynamics simulations. For coverage leading to self-ordering, these simulations do indeed predict displacements of subsurface Cu atoms. The adjustment of these displacements to those measured by channelling gives the amplitude of the stress variation.