Nanostructure by high-energy X-ray diffraction

X-Ray Diffraction (XRD) has long been used to determine the atomic-scale structure of materials. This technique is based on the fact that the wavelength of X-rays is comparable to the distances between atoms in condensed matter. When a material exhibiting a long-range (i.e. at least micrometers), periodic atomic order, such as a crystal, is irradiated with X-rays it acts as an extended, well-defined grating and produces a diffraction pattern showing numerous sharp spots, called Bragg diffraction peaks. By measuring and analyzing the positions and intensities of these peaks it is possible to determine the spatial characteristics of the grating – i.e. to determine the three-dimensional (3-D) arrangement of atoms in the crystalline material being studied. This is the essence of ‘crystal structure’ determination by XRD1. Over the years the technique has been refined and applied successfully to a variety of crystalline materials – from simple solids to complex proteins. XRD has also been successfully applied to study the structure of materials where atoms are ordered only at short distances (i.e. less than a nanometer), such as glasses and liquids. When irradiated with X-rays these materials act as very imperfect gratings and produce XRD patterns that are highly diffuse. A specialized approach, known as the atomic Pair Distribution Function (PDF) technique, has been used to analyze diffuse (i.e. non-Bragg type) XRD patterns and obtain important structural information such as nearest neighbor atomic distances and coordination numbers for noncrystalline materials2. Thus XRD has proven to be a valuable research tool for both regular crystals and noncrystals. With current technology moving rapidly towards smaller scales, nanocrystalline materials are being produced in increasing numbers. As their name Detailed knowledge of the atomic-scale structure is needed to understand and predict properties of materials. For ordinary crystals, this information is obtained by traditional (Bragg) X-ray diffraction. It is difficult to use this approach on materials structured at the nanoscale because their diffraction patterns show few, if any, Bragg peaks, and have a pronounced diffuse component. A non traditional approach based on high-energy X-ray diffraction and atomic pair distribution function data analysis may be used instead. This article describes the essentials of this approach and its great potential. The purpose is to encourage the nanoscience community go beyond traditional X-ray diffraction.