X-ray atomic-scale structural studies of interfaces with chemical-state sensitivity

Michael Bedzyk, Materials Science and Engineering, Northwestern University

Interfacial science by its very nature brings together diverse interests in areas such as: electronic materials, oxide film growth, nano-science, biomembranes, geochemistry, surface physics, catalysis, and electrical-energy storage. Sophisticated in situ X-ray methods are now being developed to understand the assembly of atoms, molecules and supported nanoparticles at well-defined interfaces in complex environments. The talk will introduce the use of synchrotron X-ray methods for atomic-scale studies of interfaces. Examples will include the use of X-ray reflectivity (XRR), X-ray standing waves (XSW) and X-ray photoelectron spectroscopy (XPS) for studying interfaces formed by the growth of graphene on silicon-carbide and metallic monolayers on oxide single crystal surfaces. For epitaxial graphene grown on the Si-face of SiC(0001), XPS shows a number of distinct chemically-shifted C 1s peaks that when monitored individually under the XSW condition are shown to correspond to C atoms located in different sites including the controversial interface layer that lies between the substrate surface and first graphene layer. For the case of an oxide supported monolayer catalyst, VO_X on a-TiO₂(110), we are able to correlate the redox-induced interfacial chemical-state changes of the V ions (observed by XPS) with their atomic-scale structural changes (observed by XSW 3D atomic imaging). In both cases the chemically-specific structural measurements are well-suited for testing modeled theoretical predictions and for complimenting local-probe imaging methods. The measurements were made at the Advanced Photon Source and the European Synchrotron Radiation Facility.

[loc]PISB 108[/loc]