Mass-Thickness Contrast in STEM Images - Practical Electron Microscopy and Database - - An Online Book - |
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Microanalysis | EM Book https://www.globalsino.com/EM/ | ||||||||
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STEM image contrast C can be given by (see page1226), Since the scattering cross section σ relates
to the atomic number Z with a power-law dependence Zx, a local contrast in STEM images can also be written in the empirical form below,
Figure 1225 shows the atomic number (zx) dependence on scattering angle β at high voltages. At large scattering angle, electron scattering is proportional to Z2 according to the Rutherford approximation but the exponent falls to zero with the decrease of scattering angle. If ρ/A is equal for all the elements in the TEM specimen, then the contrast depends only on zx (e.g. Z contrast in HAADF STEM imaging).
Since k·log10e is a material-independent factor, then the local contrast depends on the mass and thickness of the specimen as shown in Equation [1225e], and thus it is called mass-thickness contrast. Note that the relations described in Equations 1225d - 1225f will break down for thick specimens above a certain thickness, due to multiple scattering of electrons, which depends on the specimen material, accelerating voltage and collection semi-angle. Table 1225. Thickness dependence of validation of Equations 1225d - 1225f.
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