Elemental Mapping Using EELS

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

Table 5.28 shows the main features of STEM. The technique provides a variety of facilities for bright field (BE) and dark field (DF) imaging, electron imaging (ESI, SE, BSE), elemental mapping (EDS), structural analysis (EBSD, SAD, CBED), and spectroscopy (EELS, EXELFS). A modern electron microscope uses all the signals that are generated during interaction between electron beam and... 

EEL spectra can also provide information on electronic band structures and core losses can be used to chemically identify different atomic species. The high current and small probe size obtainable with aberration corrected STEM instrumentation has enabled atomic resolution elemental mapping of a wide range of materials. The sensitivity of the technique has allowed the chemical identification of single atoms including dopants in semiconductor quantum dots (Fig. 4). ... 

Local chemical composition from areas less than 1 nm in diameter can be measured by energy dispersive X-ray spectroscopy (EDS) or electron energy loss spectroscopy (EELS). Such spectroscopic information may be presented in 2D maps showing the spatial element distribution in the specimen (13). Furthermore, information about the local density of unoccupied electron states of a specific element can be extracted from EELS data and used to estimate the oxidation state and the local coordination geometry of the excited atoms (14). In some favorable cases, electronic structure information with a resolution of about 1 eV from individual atomic columns has been attained (15,16). Recent developments of monochromators and spectrometers have brought the resolution down to 0.1 eV (17,18), and this capability may offer new opportunities to determine relationships between electronic structure information, the atomic arrangements and the catalytic activities of solids. 

---