Standing Waves technique, nano-EXAFS
X-rays are highly penetrating radiations and, therefore, any information obtained through X-ray-based measurements is averaged over a depth of several microns.
However, X-ray based techniques can be made depth-selective by generating standing waves inside the nanostructure of interest by making use of the phenomenon of total reflection. X-ray intensity is localized in the anti-nodal regions, the position of which inside the nanostructure can be varied by varying the angle of incidence. Use of such X-ray standing waves (XSW) in elemental depth profiling or XANES measurements with nanometer depth resolution has been demonstrated. The angular dependences of the fluorescence yield from a single organic monolayer on a solid substrate modulated by a standing wave in a total external reflection conditions has been measured experimentally. Scanning by standing wave field of a single organic molecule has been done; the depth positions of particular ions in the molecule structure have been determined. In this experiment, a depth resolution on the order of 1–5 nm has been achieved. Depth selectivity can further be enhanced by making use of waveguide structures.
For the first time at BESSY, depth-selective EXAFS studies have been performed. The absorption spectra of Fe and W nano-layers were recorded with in-depth resolution on the order of 1nm. This method is combining total external reflection standing waveguide mode and EXAFS measurements.