Why STEM Not TEM?

The TEM The TEM has a similar optical configuration to an optical microscope. A flood beam of electrons illuminates a thin sample. The electron transmitted through the sample are projected onto a viewing screen or camera for observation (Figure 1). Samples must be thin (around 100 nm) and the beam energies must be high. Electrons may either pass through the sample without being scattered or may be diffracted off axis by interaction with the sample. Electrons may also be backscattered in the sample to re-emerge from the top surface. The primary beam also interacts with the sample to produce characteristic X-rays. With crystalline samples, most of the detail in the image is a result of Bragg diffraction. By choosing the position of the aperture, either the diffracted beam (dark field) or the unscattered electrons (bright field) can be used to form the image. The TEM is used extensively in the life sciences, where its similarity to the light microscope is self evident, and also in materials science. In materials science, the combination of diffraction and imaging provides a unique capability for understanding the properties of crystals and defects in crystalline materials, which can be interpreted in detail. Imaging is possible to the nanometer scale and the spatial resolution can now extend to approach the atomic level.