Laterally resolved EELS for ELNES mapping of the Fe L 2,3- and O K-edge

Abstract

Nowadays fingerprinting techniques are well established for phase analysis. One of the common problems is the accurate calibration of the energy scale to compare the electron energy loss (ELNES) and to determine the energy shift precisely. One solution to this problem is laterally resolved electron energy loss spectroscopy (EELS), which involves orienting the specimen area or structure of interest, parallel to the energy dispersive direction and dispersing the intensity across the interface as a function of energy. This ELNES information can now be used to quantify and map changes in the electronic environment. The most critical instrumental performance for ELNES investigations is the available energy resolution, which for our instrument was estimated using the 0.5 eV splitting of the Mn L 3-edge of the mineral bixbyite. An ideal test sample for the ELNES investigations is a titanohematite, a solid solution between ilmenite (FeTiO 3), with Fe in a divalent oxidation state, and hematite (Fe 2O 3) with Fe in a trivalent oxidation state. Using energy filtered imaging with a slit width of 4 eV it is possible to map the Fe 2+/Fe 3+ ratio as well as the near-edge structure of the O K signal and correlate these ELNES maps with a spatial resolution of a few nanometres. Quantitative compositional mapping on a nanometre scale was obtained by electron spectroscopic imaging. Quantitative point analyses also yield the chemical composition and the valence states. The precise knowledge of the energy shift and near edge structure enables us to select the characteristic ELNES structure and calculate jump ratio images. This yields quantitative valence state maps by using the Fe L 2,3-edge, as well as phase maps by using the O K-edge.