Observation of a new ridge structure along steps on the MgO(100) surface by non-contact atomic force microscopy

The surface topography of an MgO(100) surface after annealing at high temperatures followed by Ar+-ion sputtering was examined by non-contact atomic force microscopy (NC-AFM) and X- ray photoelectron spectroscopy (XPS). The calcium impurity (210 ppm in the bulk) was segregated to the surface by annealing above 1200 K and agglomerated as CaOx particles at monolayer or bilayer steps along the [011] direction after annealing at similar to 1600 Ii for 1 h. The step was always accompanied by a band-like structure with a protrusive line running parallel to the step edge with a separation longer than 5 nm. The structure was stable on the surface after removal of the CaOx particles by Ar+-ion sputtering at similar to 1600 K, but eventually disappeared after repeated Ar+-ion sputtering and annealing cycles. This unusual ridge structure is probably due to long-range reconstruction induced by segregated calcium of less than 3% coverage. The calcium-induced structure on MgO(100) is also discussed in relation to the chemical reactivity for methane activation. (C) 1999 Published by Elsevier Science B.V. All rights reserved.