Secondary electron microscopy contrast

Near surface techniques, in contrast to true surface techniques, do not require an ultrahigh vacuum. Secondary Electron Microscopy-Eneigy Dispersive X-ray Spectroscopy (SEM-EDS) is an example of a particularly useful near surface technique that will be discussed in a later section. Micro surface electrochemical techniques, such as the Scaiming Kelvin probe, overcome these limitations and do give detailed electrochemical (rather than elemental) information about the system in question. 

A completely new method of determining siufaces arises from the enormous developments in electron microscopy. In contrast to the above-mentioned methods where the surfaces were calculated, molecular surfaces can be determined experimentally through new technologies such as electron cryomicroscopy [188]. Here, the molecular surface is limited by the resolution of the experimental instruments. Current methods can reach resolutions down to about 10 A, which allows the visualization of protein structures and secondary structure elements [189]. The advantage of this method is that it can be apphed to derive molecular structures of maaomolecules in the native state. 

When secondary electrons are emitted from a magnetic material they become polarised and so by using a polarisation sensitive detector such as a Mott detector to collect the secondary electrons an image can be obtained that has magnetic contrast, allowing magnetic domain structures to be studied. This technique is known as scanning electron microscopy with polarization analysis (SEMPA). 

Cazaux, J. From the physics of secondary electron emission to image contrast in scanning electron microscopy. J. of Elec. Micms. 61, 261-284 (2012). 

Optical microscopy (OM), polarized light microscopy (PLM), phase contrast microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM) are the methods normally used for identification and quantification of the trace amounts of asbestos fibers that are encountered in the environment and lung tissue. Energy-dispersive X-ray spectrometry (EDXS) is used in both SEM and TEM for chemical analysis of individual particles, while selected-area electron diffraction (SAED) pattern analysis in TEM can provide details of the cell unit of individual particles of mass down to 10 g. It helps to differentiate between antigorite and chrysotile. Secondary ion mass spectrometry, laser microprobe mass spectrometry (EMMS), electron probe X-ray microanalysis (EPXMA), and X-ray photoelectron spectroscopy (XPS) are also analytical techniques used for asbestos chemical characterization. 

A third disadvantage of studying polymers with electron microscopy is the radiation damage due to the electron beam. There are several primary and secondary irradiation effects, which can, on one side, damage the polymer, but on the other hand, contribute to a contrast enhancement, for example, if in polymer blends the thickness or the density in one part of the specimen is reduced by evaporation of volatile fractions of polymer chains, as in PVC/SAN blends (see Fig. 3.15) or if secondary cross-linking effects in semicrystalline polymers are stronger in the amorphous regions than in the crystalline ones, as in PEs [1,15,16]. 

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