With a significant role in material sciences, physics, (soft matter) chemistry, and biology, the transmission electron microscope is one of the most widely applied structural analysis tools to date. It has the power to visualize almost everything from the micrometer to the angstrom scale. Technical developments keep opening doors to new fields of research by improving aspects such as sample preservation, detector performance, computational power, and workflow automation. For more than half a century, and continuing into the future, electron microscopy has been, and is, a cornerstone methodology in science.
Transmission Electron Microscopy (TEM) has long been used in materials science as a powerful analytical tool. In transmission electron microscopy (TEM), a thin sample, less than 200 nm thick, is bombarded by a highly focused beam of single-energy electrons. The beam has enough energy for the electrons to be transmitted through the sample, and the transmitted electron signal is greatly magnified by a series of electromagnetic lenses. Transmission Electron Microscope (TEM) combined with precession 3D electron diffraction tomography technique has produced very promising results in the field of crystal structure determination and has the great advantage of requiring very small single crystals (from 25-500 nm) and very small quantity of material.
How does TEM work?
Transmission Electron Microscope (TEM) uses an electron gun to fire a beam of electrons. The gun accelerates the electrons to extremely high speeds using electromagnetic coils and voltages of up to several million volts. The electron beam is focused into a thin, small beam by a condenser lens, which has a high aperture that eliminates high angle electrons. Having reached their highest speed, the electrons zoom through the ultra-thin specimen, and parts of the beam are transmitted depending on how transparent the sample is to electrons. The objective lens focuses the portion of the beam that is emitted from the sample into an image.
Another component of the TEM is the vacuum system, which is essential to ensure electrons do not collide with gas atoms. A low vacuum is first achieved using either a rotary pump or diaphragm pumps which enable a low enough pressure for the operation of a diffusion pump, which then achieves a vacuum level that is high enough for operations. High voltage TEMs require particularly high vacuum levels and a third vacuum system may be used. The image produced by the TEM, called a micrograph, is seen through projection onto a screen that is phosphorescent. When irradiated by the electron beam, this screen emits photons. A film camera positioned underneath the screen can be used to capture the image with a charge-coupled device (CCD) camera.
This technology can tell us about the structure, crystallization, morphology and stress of a substance whereas scanning electron microscopy (SEM) can only provide information about the morphology of a specimen. However, TEM requires very thin specimens that are semi-transparent to electrons, which can mean sample preparation takes longer.
Where can TEM help?
1. The transmission electron microscope (TEM) is used to examine the structure, composition, and properties of specimens in sub-micron detail. Aside from using it to study general biological and medical materials, TEM has a significant impact on fields such as:
materials science
geology
environmental science
2. The investigation of the morphology, structure, and local chemistry of metals, ceramics, and minerals is an important aspect of contemporary material science. It also enables the investigation of crystal structures, orientations, and chemical compositions of phases, precipitates, and contaminants through diffraction pattern, characteristic X-ray, and electron energy loss analysis.
Transmission electron microscopy can :
Image morphology of samples, e.g. view sections of material, fine powders suspended on a thin film, small whole organisms such as viruses or bacteria, and frozen solutions.
Tilt a sample and collect a series of images to construct a 3-dimensional image.
Analyze the composition and some bonding differences (through contrast and by using spectroscopy techniques: microanalysis and electron energy loss).
Physically manipulate samples while viewing them, such as indent or compress them to measure mechanical properties (only when holders specialized for these techniques are available).
View frozen material (in a TEM with a cryostage).
Generate characteristic X-rays from samples for microanalysis.
Acquire electron diffraction patterns (using the physics of Bragg Diffraction).
Perform electron energy loss spectroscopy of the beam passing through a sample to determine sample composition or the bonding states of atoms in the sample.
3. Environmental forensic microscopy identifies sources of indoor and outdoor contaminants and improves estimates of total human exposure in residential and office settings. Samples of particles (dust, dirt, soil or suspensions in liquid) are collected and analyzed by transmission electron microscopes to identify the particle size and shape, and determine possible sources. Building materials including floor tiles, roofing tars and dust samples are analyzed by transmission electron microscopes to determine surface contamination resulting from settling asbestos and fiberglass dust.
4. TEM is used as both a resistivity sounding tool and a deep metal detector, and the decay time constants may eventually prove useful in target characterization. It is now used extensively by an increasing number of earth scientists for direct observation of defective microstructures in minerals and rocks.
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