Transmission electron microscopy

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Transmission Electron Microscopy (TEM) is a microscopy technique in which a beam of electrons is transmitted through a specimen to form an image. The specimen is most often an ultrathin section less than 100 nanometers thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted through the specimen. The image is then magnified and focused onto an imaging device, such as a fluorescent screen, on a layer of photographic film, or detected by a sensor such as a CCD camera.

Principles of Operation

The basic principle behind TEM is similar to that of light microscopy (LM), but instead of using light to illuminate the specimen, TEM uses electrons. Since electrons have much shorter wavelengths than visible light, TEMs have a much higher resolving power than light microscopes, allowing them to see much smaller objects in finer detail. The maximum resolution of a TEM is around 0.1 nanometers, more than 1000 times better than a light microscope.

Electrons are generated in an electron gun, typically by heating a tungsten filament to produce free electrons. These electrons are then accelerated towards the specimen using an electric potential. The accelerated electrons interact with the specimen, either being scattered by the atomic nuclei or electrons within the sample. The unscattered electrons, and those scattered at small angles, pass through the specimen to form an image, while those scattered at high angles are blocked by an aperture.

Sample Preparation

Sample preparation for TEM is a critical step and can be quite complex, depending on the nature of the specimen. Biological specimens, for example, must be fixed to preserve their structure, dehydrated, and embedded in a medium that provides support during sectioning. The samples are then ultrathin sectioned using a microtome to produce slices thin enough to be electron transparent. Materials science samples may require less preparation but often need to be thinned to electron transparency through processes such as ion milling.

Imaging Modes

TEM offers several imaging modes, including bright field, dark field, and electron diffraction patterns. In bright field TEM, the image is formed from electrons that pass directly through the specimen. In dark field TEM, the image is formed from electrons that have been scattered at small angles by the specimen, with the unscattered beam blocked. This mode is useful for highlighting features not visible in bright field. Electron diffraction patterns can be used to determine the crystal structure of the material being examined.

Applications

TEM is a powerful tool used in a wide range of scientific fields, including biology, materials science, chemistry, and physics. In biology, it is used to study the ultrastructure of cells, viruses, and other microscopic entities. In materials science, it helps in understanding the microstructure of metals, ceramics, and polymers. TEM is also used in nanotechnology for imaging and analyzing nanoparticles and nanostructures.

Advantages and Limitations

The main advantage of TEM is its high resolution, which allows for the detailed visualization of structures at the nanoscale. However, TEM also has several limitations. The preparation of samples is time-consuming and can introduce artifacts. The requirement for thin samples means that it is difficult to obtain information about the three-dimensional structure of the specimen. Additionally, TEMs are expensive to purchase and maintain, and require specialized training to operate.

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