High-temperature scanning tunnelling microscopy was used to obtain atomic-resolution images of the (001) surfaces of single crystals. Empty-state images of the (1 x 1) surface revealed Ni-atom sites, and filled-state images revealed O ions; in agreement with theoretical studies of the electronic structure of this oxide. The empty-state and filled-state images of point defects, and patterning effects which were observed on the upper terraces of (100) step edges, were explained in terms of directional 2p-3d covalent bonding between the O and Ni sub-lattices.

M.R.Castell, P.L.Wincott, N.G.Condon, C.Muggelberg, G.Thornton, S.L.Dudarev, A.P.Sutton, G.A.D.Briggs: Physical Review B, 1997, 55[12], 7859-63