Transmission electron microscopy contrast problem

Despite these rapid developments, transmission electron microscopy could not possibly contribute significantly to coatings and polymer research until commercial machines became available. The first Siemens microscope was marketed in 1938 (35). followed the next year by a more advanced unit. In this country, RCA marketed its first unit in 1941 (36). Even with commercial equipment available, many other problems had to be overcome. The main difficulty was that of sample preparation. Sections had to be extremely thin to be penetrated sufficiently by the electron beam. Often contrast was not sufficient to form a suitable image. In 1939, the shadowing technique was developed to enhance contrast... 

Transmission electron microscopy (TEM) is used to analyze very thin samples (less than 100 nm thick), provided that the specimen has structural features that scatter electrons in difframt amounts. One of the main problems with using TEM for polymers is that these arc made up of low-atomic-number elements, which are low scatterCTs of electrons. Low scattoing and the fact that there is little spatial variation in electron density in a polymer sample leads to poor contrast. One way to overcome this problem is to stain the specimen with a heavy metal, such as uranium or osmium, that preferentially attaches itself to certain regions of the sample. 

FIGURE 19 (a) The corrector of Fig. 18 incorporated in a transmission electron microscope, (b) The phase contrast transfer function of the corrected microscope. Dashed line no correction. Full line corrector switched on, energy width (a measure of the temporal coherence) 0.7 eV. Dotted line energy width 0.2 eV. Chromatic aberration remains a problem, and the full benefit of the corrector is obtained only if the energy width is very narrow. [From Haider, M., et al. (1998). J. Electron Microsc. 47,395. Copyright Japanese Society of Electron Microscopy.]... 

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