Morphological Instability Of The Cu(110)-(2 X 1)-O Surface Under Thermal Annealing

We present a scanning tunneling microscope study on reactivity of chemisorbed oxygen on the Cu(110)-(2 x 1)-O surface. We have found that the Cu(110)-(2 x 1)-O surface is intrinsically unstable under thermal annealing in the 400-900 K range. In the 455-570 K range, the surface undergoes faceting. The orientational transition of the adsorbed oxygen phase displays wide [110] terraces, covered by (2 x 1)-O bands self-assembled into a superstructure, as well as bunches of oxygen-free narrow terraces. We found that the wide [110] terraces are intrinsically unstable against further restructuring at their edges. The restructuration is driven by reversible thermal dissociation of the (2 x 1)-O bands. The slightly uneven oxygen band density between terraces, consequently differing in reactivity with respect to Cu-O fragments, induces Cu atom transport between their edges. The interplay between thermal dissociation of the (2 x 1)-O bands and long-range elastic relaxation of the strained surface is suggested to be the origin of the observed inhomogeneous oxygen distribution. In the 570-810 K range the Cu atom transport reveals continuous growth of the oxygenated [110] terraces. We discuss in detail the mechanism of the Cu transport, which results in a rapid propagation of the oxygenated terraces as well as a strain development on the surface.