EXELFS REVISITED

EXELFS (Extended Electron Energy Loss Fine Structure) spectroscopy con tains 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 ot her high spatial resolution TEM measurements together with EXELFS. Unt il recently, due to poor quality of the EELS data, however, the EXELFS technique has not been developed to its full advantage. Various new m ethods to improve the data acquisition technique have been introduced, which include on-line removal of channel-to-channel gain variation an d correction of dark-current background under real acquisition conditi ons; also aligning and accumulating a virtually unlimited number of sp ectra while monitoring the thickness change (due to sample drift), rad iation damage and change in energy resolution during measurements. A s ystematic data analysis procedure has been developed which includes re moving edge overlapping, chi-data normalization and adopting the UWXAF S data analysis software package to perform EXELFS data analysis at th e same level of sophistication. Pure aluminium and silicon carbide wer e used as calibration materials. A complex carbonitride material with unknown (but theoretically predicted) structure and thin-film nickel o xide samples were used for further demonstration of the technique's cu rrent 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 anal ysis; for Ni, the L-2,L-3 edge (at 855 eV) was used. EXELFS spectra we re measured and analyzed using theoretical calculations and r-space no n-linear least-square fits to determine the structural parameters. Goo d agreements with the known and predicted structures were obtained. (C ) 1997 Elsevier Science Ltd.