X-Ray Standing Wave Imaging of Atoms at Interfaces

Abstract

Format

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Abstract

We have developed an x-ray synchrotron based strategy for determining the element-specific atomic-scale structure of crystalline interfaces. When combined with XPS and AFM we gain chemical sensitivity and nano-scale morphology. Using conventional X-ray standing wave (XSW) analysis (based on single-crystal Bragg diffraction), the hkl Fourier component for a x-ray fluorescence-selected atomic species is measured. By summing together several such hkl Fourier components, it is possible to directly generate a 3D, direct-space, 0.5 Å resolution, image of the atomic distribution with respect to the bulk crystal primitive unit cell. We have recently demonstrated this for the cases of bulk impurity atoms [1], cations adsorbed at the aqueous / oxide interface [2], metallic atoms at semiconductor surfaces [3], and oxide supported catalysts [4]. This new model-independent XSW imaging approach proves to be very insightful for complex cases in which atoms occupy unknown multiple crystallographic sites. In comparison to direct-methods based on conventional diffraction, the Fourier inversion process for generating an XSW image is much simpler, since the hkl phase (as well as amplitude) of each Fourier component is directly measured. Based on these model-independent XSW atomic images, we then develop models to refine the data analysis into 0.05 Å resolved atomic lattice positions that are used to measure effects such as strain. As part of our procedure, we calibrate the XRF yields to achieve a quantitative measure of the occupation fraction (stoichiometry) as well as the occupation lattice site for each XRF detectable species. In separate XPS measurements, we correlate this structural information with the chemical state of the adsorbed species. We are now applying this method to ALD and MBE grown oxide/oxide, metal/oxide and oxide/semiconductor heteroepitaxial structures and observing how the atoms at the interface redistribute after oxidation and reduction processes. In combination with AFM we are also correlating the atomic-scale and nano-scale structure of metal nanocrystals grown on oxide surfaces. Future directions include microbeam in situ real-time studies of growth and ferroelectric polarity switching.[1] L. Cheng, P. Fenter, M. J. Bedzyk, N. C. Sturchio, Phys. Rev. Lett. 90, 255503-1 (2003).[2] Z. Zhang, P. Fenter, L. Cheng, N. C. Sturchio, M. J. Bedzyk, M. L. Machesky, D. J. Wesolowski, Surf. Sci. Lett., 554(2-3) L95 (2004).[3] A.A. Escuadro, D.M. Goodner, J.S. Okasinski, M.J. Bedzyk, Phys. Rev. B, 70 235416-1-7 (2004).[4]. C.-Y. Kim, J.W. Elam, M. J. Pellin, D.K. Goswami, S. T. Christensen, M. C. Hersam, P. C. Stair, M. J. Bedzyk, J. Phys Chem. B (in press) (2006).