How does tem

Rather than having a glass lens focusing the light as in the case of light microscopes , the TEM employs an electromagnetic lens which focuses the electrons into a very fine beam. This beam then passes through the specimen, which is very thin, and the electrons either scatter or hit a fluorescent screen at the bottom of the microscope.

An image of the specimen with its assorted parts shown in different shades according to its density appears on the screen.

This image can be then studied directly within the TEM or photographed. Figure 1 shows a diagram of a TEM and its basic parts. Drawing by Graham Colm, courtesy of Wikimedia Commons. Although TEMs and light microscopes operate on the same basic principles, there are several differences between the two. The main difference is that TEMs use electrons rather than light in order to magnify images. The power of the light microscope is limited by the wavelength of light and can magnify something up to 2, times.

Electron microscopes, on the other hand, can produce much more highly magnified images because the beam of electrons has a smaller wavelength which creates images of higher resolution. Resolution is the degree of sharpness of an image. Figure 2 compares the magnification of a light microscope to that of a TEM. Light microscope x The transmission electron microscope is a very powerful tool for material science. A high energy beam of electrons is shone through a very thin sample, and the interactions between the electrons and the atoms can be used to observe features such as the crystal structure and features in the structure like dislocations and grain boundaries.

Chemical analysis can also be performed. TEM can be used to study the growth of layers, their composition and defects in semiconductors. High resolution can be used to analyze the quality, shape, size and density of quantum wells, wires and dots. The TEM operates on the same basic principles as the light microscope but uses electrons instead of light. Because the wavelength of electrons is much smaller than that of light, the optimal resolution attainable for TEM images is many orders of magnitude better than that from a light microscope.

Thus, TEMs can reveal the finest details of internal structure - in some cases as small as individual atoms. The beam of electrons from the electron gun is focused into a small, thin, coherent beam by the use of the condenser lens.

This beam is restricted by the condenser aperture, which excludes high angle electrons. The beam then strikes the specimen and parts of it are transmitted depending upon the thickness and electron transparency of the specimen. This transmitted portion is focused by the objective lens into an image on phosphor screen or charge coupled device CCD camera.

Electromagnetic lenses are used to focus the electrons into a very thin beam and this is then directed through the specimen of interest. The electrons passing through the specimen then impact on a detector. Traditional bright field imaging relies on incident electrons being scattered and disappearing from the beam depending upon the compositional density and crystal orientation of the sample.

The intensity of un-scattered electrons gives rise to a "shadow image" of the specimen, with different parts of a specimen displayed in varied darkness according to density. By rotating a sample, and taking multiple images at each rotation, it is also possible to build a 3D representation of the specimen tomography.

The crystal structure of samples with regular atomic structure crystalline material may also be analysed via electron diffraction. Positive interference in the back focal plane leads to discreet spots of electron localisation, which can then be visualised by mapping the back focal plane to the imaging apparatus.

The diffraction patterns can then be used to analyse to the crystal structure of the specimen. X-ray emission consequent to the interaction of the primary electron beam with the sample, can also be detected by an energy-dispersive spectrometer EDS within the TEM. As the resulting X-ray energies are characteristic of the atomic structure of the element they originated from, the spectra generated can be used to identify the constituent elements.

It is also possible to measure the loss of energy from the inelastic scattering of electrons in specimen transmission EELS. This information can be used to infer elemental composition, chemical bonding, valence and conduction band electronic properties. Range of specialised sample rods including heating and cryogenic stages for variable temperature work and a analytical stage for tomographic and high contrast chemical analysis.