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-8wt.%Cr(100) and Ni-12wt.%Cr(100), which were grown on MgO(100) in-situ. These surfaces were exposed to between one and 150 L O2 at 500 ºC, and additional annealing steps were performed at 500 ºC and 600 ºC. Each oxidation and annealing step was studied with STM and STS, and differential conductance maps delivered spatially resolved information on doping and bandgap 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 step edge decoration of a NiO-Ni (78) coincidence lattice, which is expressed in moiré patterns in the STM images. Small patches of NiO are susceptible to reduction during annealing, but additional oxidation steps stabilize the NiO, which has a cube-on-cube epitaxial interface and a NiO-Ni (67) coincidence lattice grows into the terrace. 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 illustrate 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.