Segregation effects and chemical properties of nickel monolayers on Cu(111)

Temperature-dependent structural changes of Ni monolayers on Cu(1 1 1) and the adsorption of benzene on these layers were investigated using photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Ni films evaporated at 120 K show a sharp (1 x 1) LEED pattern after annealing to 300 K. Further annealing to 600 K leads to the segregation of Cu atoms on top of the Ni film. This is seen from the appearance of a surface core level shift in the Cu2P(3/2) XPS signal. The Ni2p(3/2)= signal does not show any shifts in this temperature range: only annealing to 800 K leads to a change in the Ni2P(3/2) peak energy which indicates the formation of a Cu-Ni bulk alloy. Adsorption of benzene on the Ni-adlayer (annealed to 300 K) causes a small peak shift in the Ni2p(3/2) signal. The C 1s signal indicates molecularly chemisorbed benzene up to 300 K. Further annealing to 500 K leads to the decomposition of benzene into hydrogen and carbon which remains on the surface and forms a stable bond with the Ni-adlayer, preventing Cu segregation up to 500 K. (C) 2001 Elsevier Science B.V. All rights reserved.