Charged specimen, scanning electron microscopy

For scanning electron microscopy of electrically insulating materials, the surface of the specimen may be electrically isolated when bombarded with electrons. This leads to charge buildup on the specimens that makes imaging or other analysis difficult. To address this issue, special sample coating steps are often required and have been discussed in Section 9.2.3. 

When smrface coating of ceramic materials is not appropriate environmental scanning electron microscope (ESEM) can be used. Whereas conventional scanning electron microscopy requires a relatively high vacuum in the specimen chamber to prevent atmospheric interference with primary or secondary electrons, an ESEM instrument uses a thin membrane to separate the specimen chamber from the vacuum system. Thus, nonconducting samples can be analyzed without charging, and hydrated specimens can be analyzed without fear of dehydration in the vacuum. 

Figure 4.33. Scanning electron microscopy micrographs show a few of the spurious imaging effects that result from specimen charging. The banding shown in (A) and what appears as movement of the specimens are also due to charge buildup. The edges of the fiber are bright due to a bright edge effect (B).

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