SEM: Imaging with Secondary Electrons
Voltage-Dependence of Contrast

For the investigation of the morphology of small structures like nanotubes, it is advantageous to use rather low acceleration voltages for the electron beam (low energy electrons). The penetration depth of electrons increases strongly with increasing energy. Thus, low energy electrons interact only with the surface of the sample, leading to detailed images of the sample, whereas high energy electrons pass through thin samples, which consequently look transparent.

The development of scanning electron microscopes that provide high resolution at low voltages constitutes a major achievement for materials science.