EXELFS revisited

Abstract

EXELFS (Extended Electron Energy Loss Fine Structure) spectroscopy contains the same local atomic structural information as XAFS (i.e. short-range order), but also it has good low-Z elemental sensitivity, much higher spatial resolution (nanoscale) and the capacity of combining other high spatial resolution TEM measurements together with EXELFS. Until recently, due to poor quality of the EELS data, however, the EXELFS technique has not been developed to its full advantage. Various new methods to improve the data acquisition technique have been introduced, which include on-line removal of channel-to-channel gain variation and correction of dark-current background under real acquisition conditions; also aligning and accumulating a virtually unlimited number of spectra while monitoring the thickness change (due to sample drift), radiation damage and change in energy resolution during measurements. A systematic data analysis procedure has been developed which includes removing edge overlapping, χ-data normalization and adopting the UWXAFS data analysis software package to perform EXELFS data analysis at the same level of sophistication. Pure aluminium and silicon carbide were used as calibration materials. A complex carbonitride material with unknown (but theoretically predicted) structure and thin-film nickel oxide samples were used for further demonstration of the technique's current capabilities as a tool for structural characterization in a wide range of materials applications. The K-edges of Al (at 1560 eV), Si (1839 eV), O (532 eV), N (402 eV), C (284 eV) were used for EXELFS analysis; for Ni, the L 2,3 edge (at 855 eV) was used. EXELFS spectra were measured and analyzed using theoretical calculations and r-space non-linear least-square fits to determine the structural parameters. Good agreements with the known and predicted structures were obtained.