Scanning electron microscopy imaging signals used

Scanning Electron Microscopy (SEM) is a powerful tool for the surface observation of samples. The scanning of a small electron probe on the surface of the sample induces the emission of several signals. The main imaging mode used in SEM is based on the detection of the secondary electrons (called SEI for Secondary Electrons... 

The scanning electron microscope (SEM) forms an image by scanning a probe across the specimen, and in the SEM the probe is a focused electron beam. The probe interacts with a thin surface layer of the specimen, a few micrometers thick at most. The detected signal commonly used to form the TV-type image is the number of low energy secondary electrons emitted from the sample surface. Scanning electron microscopy is fully described in several texts [22-26], and its use with polymers has been reviewed by White and Thomas [27]. 

Fig. 12. In scanning tunneling microscopy, STM, the distance between a metal tip and a conductive substrate with a small potential difference (typ. 0.1 eV - 2 eV) is reduced until quantum mechanical tunneling leads to the flow of a small current. When the tip is scanned across the surface a feedback electronic readjusts the distance so as to keep the current constant. The corresponding feedback signal can then be used to generate a topographical image of the substrate.

---