AVS 65th International Symposium & Exhibition
    Surface Science Division Thursday Sessions
       Session SS+EM+PS+TF-ThA

Paper SS+EM+PS+TF-ThA8
Early Stage Oxidation and Evolution of Surface Oxides in Ni(100) and Ni-Cr(100) Thin Films

Thursday, October 25, 2018, 4:40 pm, Room 102A

Session: Deposition, Etching and Growth at Surfaces
Presenter: William H. Blades, University of Virginia
Authors: W.H. Blades, University of Virginia
P. Reinke, University of Virginia
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The interaction of molecular oxygen with Ni(100) and Ni-Cr(100) thin films has been studied through a synergetic experimental and computational effort. The physical and chemical processes behind the initial stages of oxidation prior to the formation of a full oxide layer are not well understood. By oxidizing Ni(100) and Ni-Cr(100) thin films and studying the growth of the surface oxides with Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy, the evolution of oxides grown within the pre-Cabrera-Mott regime can be captured. The data collected are combined with Bandgap and Density of States maps and statistical distributions of the surface’s electronic structure are generated. Pure Ni(100) and Ni-(8-12)wt.%Cr(100) thin films were prepared on MgO(100) in UHV and exposed to oxygen up to 400 L. Under these oxidation conditions the Ni(100) prefers the Ni(100)-c(2x2)O reconstruction, which drives step faceting into {100} segments subsequently limiting the growth of NiO at elevated temperatures. Our experiments demonstrate that once a nominal amount of Cr is added to Ni(100), the Ni-Cr(100) surface will undergo a different oxidation reaction pathway. After just 14 L of O2 exposure we observe NiO growth across the surface, the presence of large oxide nodules and three distinct chemisorbed phases. Initial NiO nucleation and growth occurs along the step edges of the Ni-Cr surface. The superlattice and modulation of tip bias has revealed a NiO-Ni(6x7) cube-on-cube interfacial relationship. NiO wedge-like features are also observed and have characteristically different superlattice spacing, which offers insight into the manner in which metal-oxides mitigate strain within the pre-Cabrera-Mott regime. One of the chemisorbed phases we observe has been identified as Cr(100)-c(2x4)O, suggesting surface segregation and subsequent phase separation of BCC Cr. Two other chemisorbed domains are present and possess distinct electronic signatures that have been captured by STS. Density Functional Theory is used to develop an understanding of the effect of Ni and Cr on the local bonding environment of the prevalent chemisorbed phases of Ni-Cr(100). This combined experimental and theoretical approach has offered greater insight into alloy-oxide interface structure, and the role of transition metal dopants in the oxidation process in the pre-Cabrera-Mott regime. This work is supported by the Office of Navel Research MURI “Understanding Corrosion in Four Dimensions,” Grant N00014-14-1-0675.