Petra Reinke: Materials Science at Surfaces: From Oxidation of Alloys to Silicene Synthesis

This seminar illustrates how the integration of surface science studies and material synthesis can be applied to understand mechanisms of surface reactions and degradation, and is at the same time a powerful took in the discovery of nano- and 2D materials. I will showcase some of the recent results in my research group and focus on unraveling atomic scale mechanisms of alloy oxidation (NiCr, W/Mo) and the discovery of a new pathway to make silicene, which is suitable for device integration.

In Ni-based superalloys superb corrosion resistance is combined with excellent mechanical properties, and the oxidation process is characterized by the competition between the formation of chromia, and the rapid nucleation and growth of NiO. We target in our work a temperature regime of where the competition between chemically distinct oxides (NiO, Cr2O3, NiCr2O4 , non-stoichiometric) offers critical information to develop our understanding of NiCr alloy oxidation. Of particular interest is the influence of minor alloying elements, Mo and W with less than 5wt% in the alloy, which are both known to improve oxide passivity. We will focus on the impact of alloy composition on the oxidation process, and use in-situ deposition of alloy thin films on MgO(100) for the STM studies, and cast alloys of Ni 5wt%Cr, Ni 15wt%Cr and Ni 30wt%Cr, Ni 15wt%Cr 6wt%Mo, and Ni 15wt%Cr 6wt%W for the ambient pressure XPS experiments (AP-XPS). Most recent work shows the differential reactivity as a function of crystallographic orientation.

I will present the results of two complementary experimental studies: (1) an atomic scale STM study which isolates the transition from alloy to the first few oxide layers, and (2) an AP-XPS study which extends from the clean alloy to oxide layers with oxide thicknesses significantly exceeding the information depth of XPS. Particularly striking is the geometric and electronic heterogeneity, the latter is seen clearly in STS (scanning tunneling spectroscopy) attributed this to local doping and variable defect concentrations. The AP-XPS results demonstrate that the role of Mo/W is also critical to promote chromia nucleation in the initial stages of oxidation thus leading immediately to a more protective oxide. We will integrate these results and present a model for alloy oxidation which hopefully inspires future work.

I will discuss in the second part of this seminar a new, and promising pathway to the synthesis of silicene, based on the Si-terminated h-MoSi2 (0001) surface. In contrast to graphene, the silicene properties are strongly influenced by sheet buckling due to a partial sp3 bonding within the layer. Control of buckling is generally seen as a pathway to manipulate silicene properties and could lead to the expression of rather exotic quantum phases. The growth of silicene on MoSi2 opens new possibilities of monolithic integration in future devices. An array of geometrically and electronically distinct reconstructions, silicene layers and silicene nanoribbons present on this surface, and challenge our understanding of the silicene formation, and, ultimately, its integration for electronic applications and the study of complex quantum phases.