NITROGEN INDUCED RESTRUCTURING OF CU(111) AND EXPLOSIVE DESORPTION OFN-2

Thermal N atoms from an atomic beam adsorb on Cu(lll) at 300 K to prod uce a disordered surface with a N coverage of up to 2 ML. Annealing ab ove 500 K produced an ordered surface showing three domains of a Cu(10 0)-c(2 x 2)N overlayer, with a structure similar to that of bulk Cu,N. Parallel to the Cu(lll) and the Cu(100) close packed directions, the overlayer is expanded by similar to 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 approxima tely parallel to the close packed direction. This corrugation is cause d by buckling of the Cu(100) overlayer to obtain local registry with t he Cu(lll) close packed rows and relieve stress in the Cu,N overlayer. N from the ordered Cu(100)-c(2 x 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 theta(N)>0.42 ML a broad desorption feature appeared above 500 K with an activation energy greater than o r equal to 88 kJ mol(-1). This peak is associated with desorption from a disordered N/Cu(lll) surface, which can accommodate in excess of 2 ML of N with considerable penetration into the Cu surface. Desorption from surfaces with theta(N)>0.42 ML forms the stable Cu(100)-c(2 x 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 int ense for N+/N-2(+) sputtered surfaces and is attributed to a subsurfac e site. At high coverage and heating rates the presence of excess N st abilises desorption from the Cu(100)-c(2 x 2)N overlayer and N, desorp tion becomes explosive. The desorption behaviour can be modelled by as suming desorption occurs preferentially from a dilute phase on Cu(lll) terraces, with Cu(100)-c(2 x 2)N islands acting as a reservoir for N. We discuss evidence for this and other possible models using informat ion from STM images of the surface and speculate on the N, desorption site. (C) 1998 Elsevier Science B.V. All rights reserved.