Electron Spectroscopic Imaging (ESI)

An energy filter (Gatan imaging filter, GIF) installed at the bottom of a TEM allows to record electron energy loss spectra (EELS) and element specific images (elemental maps) by means of the electron spectroscopic imaging (ESI) technique. Mostly, the so-called three-window method is applied for mappings (see figure): an image is taken after a suitable ionisation edge of the corresponding element (post-edge image, ΔE3), two additional images (pre-edge 1, ΔE1, and pre-edge 2, ΔE2) are recorded at energy losses smaller than the ionisation edge. Only the electrons passing through the selected energy slit contribute to these images. Specimen drift during recording of these images is compensated for by a cross-correlation algorithm implemented in the software of the GIF. The pre-edge images are used for an approximate determination of the unspecific background which is then subtracted from the post-edge image leading to an elemental map with enhanced contrast. Alternatively, the post-edge image can be divided pixel by pixel by a pre-edge image (ratio method).

Example

Recording and calculating an elemental map by the three-window method is demonstrated by the corresponding images obtained at the K edge of carbon in a vanadium oxide nanotube. The TEM image shows the cross-section of a single VO_x-NT. In the post-edge image, the carbon distribution is clearly visvible (bright contrast at the C-containing sites): C is present around the tube due to the organic resins used during the cross-sectional preparation, in the center of the tubes and between the VO_x layers. Due to the absence of C there, the VO_x layers appear with dark contrast in the ESI and post-edge image, whereas they are bright in the two pre-edge images since these in fact are post-edge images of the V_M edge.

The Cross-Sectional Structure of Vanadium Oxide Nanotubes Studied by Transmission Electron Microscopy and Electron Spectroscopic Imaging
F. Krumeich, H.-J. Muhr, M. Niederberger, F. Bieri, and R. Nesper
Z. anorg. allg. Chem. 626 (2000) 2208-2216 DOI

EELS