Abstract:

Atomic pair distribution function (PDF) analysis has evolved as an ever more widely used technique in characterizing the short and long range order in nanostructured and complex materials. In this regard, the increasing availability of high-flux high-energy x-ray sources and exponentially growing computing power were the main driving forces. The standard PDF experiment is based on an x-ray diffraction measurement out to high momentum transfer of an isotropic sample, e.g. powder, nanoparticle dispersion, glass or solution. In such structurally homogeneous materials, the data is averaged over all three dimensions in reciprocal space and Fourier transformed into real space to obtain the PDF. However, these established PDF analysis schemes fail when the samples are strongly anisotropic in structure and/or shape such as single crystals and thin films. New modeling approaches based on 2D and 3D PDF are being developed in order to determine the local order in these kinds of samples. What is more, in the case of thin layers, the actual acquisition of high-quality data suitable for PDF evaluation has to be addressed in a different way than for bulk samples. Usually, the films are deposited onto substrates whose thickness exceeds that of the film easily by three orders of magnitude, e.g. 100 nm film on a 100 μm thick substrate. In the commonly applied transmission geometry, the beam passes through both the layer and the substrate, yielding a largely unfavorable signal-to-background ratio of the obtained scattering. This presentation illustrates our recently performed PDF analysis of thin films measured in grazing incidence geometry on the surface diffractometer at beamline P07 at PETRA III. Applying this surface-sensitive geometry enables us to investigate the local structure of thin films of metals and oxides down to <30 nm. Examples of polycrystalline and amorphous layers and the effect of texture on the analysis are shown.