Transmission electron microscopes usually have thermionic emission guns and electrons are accelerated anywhere between 40 – 200 kV potential. However, TEM with >1000 kV acceleration potentials have been developed for obtaining higher resolutions. Owing to their brightness and very fine electron beams, field emission guns are becoming more popular as the electron guns.

Sample preparation for TEM: The very first requirement of TEM is that the specimens have to be very thin. As for SEM, the specimens to be used for TEM also need to be fixed and dried. Preparation of specimens for TEM can be a fairly tedious process: The samples are usually fixed using a combination of glutaraldehyde and paraformaldehyde. A secondary staining can be done with OsO₄ (Osmium tetroxide). OsO₄ fixes the unsaturated lipids and being a heavy metal acts as an electron stain too. The samples are then dehydrated exactly as done for SEM analysis. The dried samples are then sectioned to obtain ultrathin (<100 nm thickness) sections. This is typically achieved by embedding the sample in a plastic mold and cutting the sections. Epoxy and acrylic resins are also used for embedding the samples for sectioning. The sections are then stained with a heavy metal stain such as uranyl acetate and phosphotungstic acid. The stained sample is then deposited on a carbon coated grid and analyzed by TEM. Figure 18.6 shows a TEM image recorded for a peptide that self-assembled into spherical structures.

Figure 18.6 A transmission electron micrograph of a self-assembled peptide.

Scanning transmission electron microscopy: A scanning transmission electron microscope or STEM is a transmission electron microscope that works in the scanning mode like a SEM. An electron beam is focused to a small spot and scanned across the specimen exactly as done in SEM. A STEM allows detecting the transmitted as well as secondary and backscattered electrons. This mode of electron microscope provides spatially resolved information about the specimen.

All other types of electron microscopes are the modifications of SEM or TEM.