The growth of thin NiO films on Ag(001) studied by scanning tunneling microscopy and spectroscopy

We have used scanning tunneling microscopy and spectroscopy to study the growth of NiO films on Ag(001) at a coverage of 1.5 monolayers. The films have been obtained by reactive deposition of Ni in O-2 atmosphere. Particular attention has been paid to the development of the growth morphology of the deposit as a function of substrate and annealing temperature. For room temperature deposition, the Ag(001) surface is completely covered with the (2 x 1) NiO precursor structure with a thickness of one monolayer and small double-layer NiO islands. The Ag(001) substrate surface is nearly unaffected. Subsequent annealing leads to drastic changes of the morphology of the film. The (2 x 1) precursor structure gradually disappears with annealing time and transforms to double-layer NiO islands. One part of these NiO islands is embedded in the initially flat Ag substrate, and additional Ag islands appear on the surface. Local I/U characteristics (1: tunneling current, U: tunneling bias) have been measured on each kind of island and we found characteristic differences for the various parts of the surface, e.g. for Ag, the (2 x 1) precursor, double-layer and trilayer NiO. These differences can be used to directly map the individual species on the surface.