Scanning Electron Microscopy


The inelastic scattering of fast electrons provides a detailed means of characterizing the chemical composition and electronic properties of thin samples in an electron microscope. Collective and single-electron excitations occuring in the low energy region of the spectrum can be described in terms of the generalized dielectric formulation. Important information is contained in this part of the spectrum but some prior detailed knowledge of the sample is usually required for proper interpretation. The core excitations allow microanalytical information to be obtained and quantitative procedures are now quite well developed at least for K and L edges. Sample thickness is one factor that limits the quality of data in energy loss spectra and it is now possible to remove the effects of plural scattering from core edges as well as from the low loss spectrum. Several advances in instrumentation have been made recently both in electron optics and recording devices. It appears that the detection limits are very low, possibly 1 to 10 atoms in an optimized system. Measurements also show that the core edges offer a sensitive method for probing the chemical bonding and electronic structure, provided the energy resolution is sufficient (≤ 1 eV). Of particular interest is the momentum transfer and orientation dependence of the fine structure for crystalline materials. The transition elements exhibit very sharp features near the L23 threshold due to transitions to unoccupied d states and reasonable agreement is found with theory here. Another type of information can be obtained from the extended fine structure above the core edges (EXELFS). This is capable of yielding the local atomic environment around the different atomic species.

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