Transmission Electron Microscope

Transmission electron microscopy “TEM”

This microscope was first built electron microscope in the world, which has been developed its structure in 1931 by Max knoll and Ernst Ruska in Germany. These microscopes are built based on the structure of the optical microscope, and it was used electron beam instead of light at it.
In this microscope an electron beam, like light, passes through the sample and lose their energy and out from the other side of samples. Passing electrons have a particular distribution of energy, which their amounts is characteristic of the producing element or elements of the passing element.
Figure 1 shows the schematic of a TEM. Electron production is usually of the type thermoionic emission. Electron beams are focused by two or more focusing electromagnetic lens and as a result, the electron beam diameter become small, so much that when collide to the sample, its diameter be approximately 2 to 10 nm. The electron beam is controlled by the coils, and thus, both intensity and location of radiation collision with sample is adjustable by the operator. Electron beam passes through the sample and is focused on a phosphor plate, and then sent to a computer for processing.

Figure1: Schematic view from a TEM system

In transmission electron microscopes, because, they use of passing electrons through the sample for imaging, it can be obtained some information from the sample, such as the thickness of the layer. System is located in a vacuum at least 4-10 torr, to create a long free path for electrons.
This type of microscope is used to observe the boundaries, determine the structure and crystalline orientation, the crystalline plate photography, imaging of microstructures, the magnifications up to 1000,000 times, to detect structural details with a resolution of less than 1 nm Microstructural characterization of biological material, identification of chemical composition and crystalline structure of inorganic phases, sediments and pollutants, quality control and evaluation of semiconductor defects.

Figure 2 shows an example of an advanced system of transmission electron microscopy, manufactured by JEOL company.

Sample preparation:
In TEM electron microscope, because it used the passing electrons to create the image, it is necessary to product very thin samples. In addition, the sample should be small and dry. For make thin the TEM samples, is used different methods, such as micro, mechanical, chemical (use of corrosive solution) cutting.

Todays, it is used also the ion bombardment method. In this technique, known as FIB: Focused Ion Beam, the thin slicing of samples is done by an ion beam. In the TEM electron microscope such as scanning electron microscopy, non-conductive surfaces must be used from thin metal coating, such as gold or carbon. The electrons passing ability from the sample, except sample thickness, depends on the accelerator voltage of the electron beams, as well as density and atomic number of the sample. In Figure 3, is shown, the structural difference between electron and optical microscopes, as schematic.

Figure 3: Schematic view of the structure of electron and optical microscopes

Transmission Electron Microscope

Transmission Electron Microscope

Movie about TEM