Nitrogen induced restructuring of Cu(111) and explosive desorption of N 2

Abstract

Thermal N atoms from an atomic beam adsorb on Cu(111) at 300 K to produce a disordered surface with a N coverage of up to 2 ML. Annealing above 500 K produced an ordered surface showing three domains of a Cu(100)-c(2×2)N overlayer, with a structure similar to that of bulk Cu 3N. Parallel to the Cu(111) and the Cu(100) close packed directions, the overlayer is expanded by ∼3% compared to the Cu close packed distance, but by less than 1% perpendicular to this. STM images show the overlayer has an irregular corrugation with rows running approximately parallel to the close packed direction. This corrugation is caused by buckling of the Cu(100) overlayer to obtain local registry with the Cu(111) close packed rows and relieve stress in the Cu 3N overlayer. N from the ordered Cu(100)-c(2×2)N overlayer, formed by atom dosing, desorbed in a zero order peak near 700 K with an activation barrier of 143 kJ mol −1. For N coverages θ N>0.42 ML a broad desorption feature appeared above 500 K with an activation energy ≥88 kJ mol −1. This peak is associated with desorption from a disordered N/Cu(111) surface, which can accommodate in excess of 2 ML of N with considerable penetration into the Cu surface. Desorption from surfaces with θ N>0.42 ML forms the stable Cu(100)-c(2×2)N overlayer and also populates a new desorption peak, near 780 K, which is not seen for initial coverages less than 0.42 ML. This peak is intense for N +/N + 2 sputtered surfaces and is attributed to a subsurface site. At high coverage and heating rates the presence of excess N stabilises desorption from the Cu(100)-c(2×2)N overlayer and N 2 desorption becomes explosive. The desorption behaviour can be modelled by assuming desorption occurs preferentially from a dilute phase on Cu(111) terraces, with Cu(100)-c(2×2)N islands acting as a reservoir for N. We discuss evidence for this and other possible models using information from STM images of the surface and speculate on the N 2 desorption site.