From Alloy to Oxide: Capturing the Early Stages of Oxidation on Ni-Cr(100) Alloys

The interaction of oxygen with Ni-Cr(100) alloy surfaces is studied using scanning tunneling microscopy (STM) and spectroscopy (STS) to observe the initial steps of oxidation and formation of the alloy-oxide interface. The progression of oxidation was observed for Ni(100) and Ni-Cr(100) thin films including Ni-8 wt % Cr(100) and Ni-12 wt % Cr(100), which were grown on MgO(100) in situ. These surfaces were exposed to between 1 and 150 L O-2 at 500 degrees C, and additional annealing steps were performed at 500 and 600 degrees C. Each oxidation and annealing step was studied with STM and STS, and differential conductance maps delivered spatially resolved information on doping and band gap distributions. Initial NiO nucleation and growth begins along the step edges of the Ni-Cr alloy accompanied by the formation of small oxide particles on the terraces. The incubation period known in oxidation of Ni(100) is absent on Ni-Cr alloy surfaces illustrating the significant changes in surface chemistry triggered by Cr-alloying. Step edge faceting is initiated by oxide decoration along the step edges and is expressed as moire patterns in the STM images. The surface oxide can be characterized by NiONi(6 x 7) and NiO-Ni(7 x 8) coincidence lattices, which have a cube-on-cube epitaxial relationship. Small patches of NiO are susceptible to reduction during annealing; however, additional oxide coverage stabilizes the NiO. NiO regions are interspersed with areas covered predominantly with a novel cross-type reconstruction, which is interpreted tentatively as a Cr-rich, phase-separated region. Statistical analysis of the geometric features of the surface oxide including step edge heights, and NiO wedge angles illustrates the layer-by-layer growth mode of NiO in this pre-Cabrera Mott regime, and the restructuring of the alloy oxide interface during the oxidation process. This experimental approach has offered greater insight into the progression of oxide growth in Ni-Cr thin films and underscores the dramatic impact of alloying on oxidation process in the pre-Cabrera Mott regime.