Nano-Materials Seen from Inside
Though unnoticed, nanomaterials are crucial to modern technology. Aarhus University develops state-of-the-art methods to analyse nano-structures “bottom up”.
Aarhus University has long used X-ray scattering techniques to probe positions of individual atoms in functional materials. The only prerequisite to these methods is that the atoms must be organized with some degree of order. Such repeated ordering is also called “crystalline order”. Nonetheless, the deviation from crystalline order is often what accounts for the desirable properties found in functional materials. For the technique of Total Scattering, no atomic ordering is required, and the local chemistry giving rise to specific functionalities can be understood. Getting this information in the same “package” as the crystalline ordering data is groundbreaking. Why? Because the local, atomic environments are at the roots of all chemistry – and hence our ability to alter and improve materials. Put shortly, total scattering can reveal the chemical abilities of materials in unprecedented depth.
Techniques and Methods
In principle, every crystalline material can be thought of as composed of an average crystal structure along with a deviation therefrom. The scattering from the average crystal structure will be concentrated in directions of constructive interference, the so-called Bragg-peaks, which is what conventional crystallography revolves around. The scattering from the part of the crystal deviating from the average structure will be more diffusely distributed, and is omitted in the crystallographic Rietveld analysis. In the technique of Total Scattering, all the scattering from the sample is considered, which means that non-crystalline components can also be detected. This specific advantage has aided the understanding of a thermoelectric system, where the presence of an amorphous component was speculated.1
To obtain the short-range information on the atomic structure, a wide scattering detection range has to be covered. This is realized using a high-energy X-ray beam, which is only viable at synchrotron facilities. The scattering data is mathematically transformed into the Pair Distribution Function, which is a function that contains information on the distribution of atomic pairs within a sample, regardless of crystallinity.
Total scattering has particular advantages on structural-chemical analysis of nanomaterials, where even small defects and deviation from the average atomic structure can be decisive for the properties. One field is catalysis; here particular, individual atoms must often be present at very exact structural positions in order for a catalytic material to work. Another example is rechargeable ion-batteries, where lifetime is strongly determined by the formation of structural defects – tiny displacement of individual atoms – which may occur spontaneously during charging and discharging, and which do not disappear again. As a result, the battery gradually dies.
Total scattering can “see” all of this. It is, however, a relatively new technique, which has only been adopted recently by a wider set of research groups around the world. Thus, the potential for improvement is considerable, and AU has – within LINX – an ambition of method development to the point where a wide range of companies are likely to benefit from total scattering for innovation- and analysis purposes.
 L. R. Jørgensen, J. Zhang, C. B. Zeuthen and B. B. Iversen, J. Mater. Chem. A
, 2018, 6
Participants: Aarhus University.
Title: Combining Total Scattering and Conventional X-ray Diffraction (AU GDP).