Study of metals by transmission electron microscopy (TEM) - Microscope, samples and diffraction

Electron microscopy was invented by Knoll and Ruska in 1931. Since then, the technique has evolved considerably, becoming indispensable for the study of nanoscale materials and nanomaterials themselves.

There are two types of transmission electron microscope: the transmission microscope, which gives a global image of the thin object (MET – transmission electron microscope), and the scanning mode, where a small probe explores the object (STEM – scanning transmission electron microscope). Modern microscopes increasingly allow both modes of use. This dossier briefly describes both approaches in practical terms.

This technique only exists because electrons are charged particles whose trajectories can be altered by the action of magnetic and electrostatic fields. Lenses, a kind of solenoid, can be used to focus the electron beam at will, thanks to their high but adjustable magnetic field.

The great advantage of the TEM is that it can produce an image of a thin object that can now be resolved to better than a tenth of a nanometer. In a fraction of a second, we can go from an image of the object to a diffraction pattern of the same region. This can be obtained in several ways. By comparing the different modes, we can gain a more complete understanding of the structure of the material under study.

The quality of modern electron microscopes is linked to recent improvements in electron sources (field emission), computer control and magnetic lenses. We will see that aberrations, notably spherical aberration of the objective, can now be corrected.

Different techniques for recording images or diffraction patterns are discussed in this text.

We also describe the main methods of sample preparation.

This file introduces general principles and orders of magnitude. A more detailed study of the theoretical bases of electron-atom, electron-sample and diffraction interactions can be found in [M 4 125] "Diffraction of metals and alloys. Particle-matter interactions", [M 4 126] "Diffraction in metals and alloys: diffraction conditions",