Determination of accurate microstructures using x-rays

Materials have specific intrinsic properties, such as conductivity, magnetization, chemical potential, etc. These physical and chemical properties are mainly determined by the constituent elements of the material and the nature of their bonding. For example, diamond and graphite both consist of the same element (carbon), but their properties are quite distinct. Hence, to understand the properties of matter, structural determination is required.

X-ray diffraction is a canonical method used to determine crystal structures. However, it has limitations. For example, it cannot be used to determine the structure of materials that do not crystallize well, such as amorphous materials and nanoparticles. Extended x-ray absorption fine structure (EXAFS), however, is a structural determination method that has the advantage that it can describe the atomic species, bonding lengths, and coordination number around a reference atom.¹

As x-rays penetrate a material, they are partially scattered and absorbed by interaction with bonded electrons, and partially transmitted through the material. An electron that jumps out of an atom absorbing an x-ray (a photoelectron) then moves around the atom and is scattered by its neighbors. This illustrated in Figure 1(a) for the case of a ZnO nanorod array subjected to x-ray absorption: shown in Figure 1(b). Photoelectrons are then partially backscattered to their original atom, thus affecting the x-ray absorption of the original atom. Figure 1(c) and (d) shows the x-ray absorption coefficients (y-axis) measured for the ZnO nanorods in two different polarization geometries.

The oscillations above the x-ray absorption edge are the EXAFS due to the interference of the outgoing and backscattered photoelectrons. Data analysis yields local structural information, such as the coordination environment of the absorbing atom.²

The EXAFS technique was developed in the early 1970s,¹ and widely applied to microstructural studies. Using it, we have investigated high T_c superconductors,^2,3 geometrically frustrated systems,⁴ Kondo nanoparticles,⁵ T-rays semiconductors,⁶ ZnO nanorods,⁷ GaN nanotubes,⁸ textured magnetic nanoparticles,⁹ and other materials.

EXAFS can also detect the direction of atomic bonds. Since x-rays are linearly polarized, the photoelectrons have a momentum in the polarization plane. This explains why the spectra shown in Figure 1(c) and (d) are different. In our ZnO nanorods, the orientation-dependent EXAFS revealed that the bond lengths of atomic pairs along the rod length direction were elongated by ∼0.1Å while smaller in the perpendicular direction. The EXAFS also showed that the terminating atoms at the lateral surface of the ZnO nanorods were oxygen.

Knowledge of structural properties is a required starting point for understanding the properties of materials. Many techniques including both micro and macro probes have been developed to detect structural properties, and each technique has its advantages and disadvantages. As a micro structural probe that does not destroy the sample, EXAFS has many merits, although it can not provide accurate local structure determination beyond ∼7Å. Since it can be used irrespective of sample conditions, such as types (film, bulk, powder, nanoparticles), crystallinity (crystal, amorphous material) and density, EXAFS is gaining increased recognition in various research fields.