1 of 5 10/24/2006 12:55 PM Transmission electron microscopy From Wikipedia, the free encyclopedia (Redirected from Transmission electron microscope) It has been suggested that Selected area diffraction be merged into this article or section. (Discuss) Transmission electron microscopy (TEM) is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film (see electron microscope), or to be detected by a CCD camera. The first practical transmission electron microscope was built by Albert Prebus and James Hillier at the University of Toronto in 1938 using concepts developed earlier by Max Knoll and Ernst Ruska. In the past, light microscopes have been used mostly for imaging due to their relative ease of use. However, the maximum resolution that one can image is determined by the wavelength of the photons that are being used to probe the sample. In the early days of microscopy nothing smaller than the wavelength being used could be resolved, whereas nowadays the law of RESOLFT sets the limit for optical microscopes employing such concepts (see microscope). Visible light has wavelengths of 400 700 nanometers; larger than many objects of interest. Ultraviolet could be used, but soon runs into problems of absorption. Even shorter wavelengths, such as X-rays, exhibit a lack of interaction: both in focusing (nothing interacts strongly enough to act as a lens) and actually interacting with the sample. Bacterial cells of Staphylococcus aureus captured by a transmission electron microscope, magnified 50000x. Like all matter, electrons have both wave and particle properties (as theorized by Louis-Victor de Broglie), and their wave-like properties mean that a beam of electrons can in some circumstances be made to behave like a beam of radiation. The wavelength is dependent on their energy, and so can be tuned by adjustment of accelerating fields, and can be much smaller than that of light, yet they can still interact with the sample due to their electrical charge. Electrons are generated by a process known as thermionic discharge in the same manner as the cathode in a cathode ray tube, or by field emission; they are then accelerated by an electric field and focused by electrical and magnetic fields onto the sample. The electrons can be focused onto the sample providing a resolution far better than is possible with light microscopes, and with improved depth of vision. Details of a sample can be enhanced in light microscopy by the use of stains; similarly with electron microscopy, compounds of heavy metals such as lead or uranium can be used to selectively deposit heavy atoms in the sample and enhance structural detail, the dense electron clouds of the heavy atoms interacting strongly with the electron beam. The electrons can be detected using a photographic film, or fluorescent screen among other technologies. An additional class of these instruments is the electron cryomicroscope, which includes a specimen stage capable of maintaining the specimen at liquid nitrogen or liquid helium temperatures. This allows imaging specimens prepared in vitreous ice, the preferred preparation technique for imaging individual molecules or macromolecular assemblies. Another type of TEM is the scanning transmission electron microscope (STEM), where the beam can be rastered across the sample to form the image.
2 of 5 10/24/2006 12:55 PM In analytical TEMs the elemental composition of the specimen can be determined by analysing its X-ray spectrum or the energy-loss spectrum of the transmitted electrons. Modern research TEMs may include aberration correctors, to reduce the amount of distortion in the image, allowing information on features on the scale of 0.1 nm to be obtained (resolutions down to 0.08 nm have been demonstrated, so far). Monochromators may also be used which reduce the energy spread of the incident electron beam to less than 0.15 ev. Contents 1 Applications of the TEM 2 Imaging in the TEM 3 Limitations 4 See also 5 External links Applications of the TEM The TEM is used heavily in both material science/metallurgy and the biological sciences. In both cases the specimens must be very thin and able to withstand the high vacuum present inside the instrument. For biological specimens, the maximum specimen thickness is roughly 1 micrometre. To withstand the instrument vacuum, biological specimens are typically held at liquid nitrogen temperatures after embedding in vitreous ice, or fixated using a negative staining material such as uranyl acetate or by plastic embedding. Typical biological applications include tomographic reconstructions of small cells or thin sections of larger cells and 3-D reconstructions of individual molecules via Single Particle Reconstruction. In material science/metallurgy the specimens tend to be naturally resistant to vacuum, but must be prepared as a thin foil, or etched so some portion of the specimen is thin enough for the beam to penetrate. Preparation techniques to obtain an electron transparent region include ion beam milling and wedge polishing. The focused ion beam (FIB) is a relatively new technique to prepare thin samples for TEM examination from larger specimens. Because the FIB can be used to micro-machine samples very precisely, it is possible to mill very thin membranes from a specific area of a sample, such as a semiconductor or metal. Furthermore, a less time consuming and far more common technique is the deposition of a dilute sample containing the specimen on copper or nickel grids just to name a few. These grids consist of thin films of the respective elements which rest atop sturdier grid bars and act as relatively transparent supports for specimens. Samples are normally desposited as a suspension in a volatile solvent such as ethanol.
3 of 5 10/24/2006 12:55 PM The imaging techniques explained below are particularly important in materials science. Faults in crystals affect both the mechanical and the electronic properties of materials, so understanding how they behave gives a powerful insight. By carefully selecting the orientation of the sample, it is possible not just to determine the position of defects but also to determine the type of defect present. If the sample is orientated so that one particular plane is only slightly tilted away from the strongest diffracting angle (known as the Bragg Angle), any distortion of the crystal plane that locally tilts the plane to the Bragg angle will produce particularly strong contrast variations. However, defects that produce only displacement of atoms that do not tilt the crystal to the Bragg angle (i.e. displacements parallel to the crystal plane) will not produce strong contrast. SEM image of a thin TEM sample milled by FIB. The thin membrane is suitable for TEM examination; however, at ~300 nm thick, it would not be suitable for high-resolution TEM without further milling. SEM image of a thin TEM sample milled by FIB. The thin membrane is suitable for TEM examination; however, at ~300 nm thick, it would not be Furthermore, the HRTEM technique (see below) allows the direct observation of suitable for high-resolution TEM crystal structure and therefore has an advantage over other methods e.g. there is no without further milling. displacement between the location of a defect and the contrast variation caused in the image. However, it is not always possible to interpret the lattice images directly in terms of sample structure or composition. This is because the image is sensitive to a number of factors (specimen thickness and orientation, objective lens defocus, spherical and chromatic aberration), and although quantitative interpretation of the contrast shown in lattice images is possible, it is inherently complicated and may require extensive simulation of the images. Computer modeling of these images has added a new layer of understanding to the study of crystalline materials. Imaging in the TEM The contrast in a TEM image is not like the contrast in a light microscope image. A crystalline material interacts with the electron beam mostly by diffraction rather than absorption, although the intensity of the transmitted beam is still affected by the volume and density of the material through which it passes. The intensity of the diffraction depends on the orientation of the planes of atoms in a crystal relative to the electron beam at certain angles the electron beam is diffracted strongly, sending electrons away from the axis of the incoming beam, while at other angles the beam is largely transmitted. Modern TEMs are often equipped with specimen holders that allow the user to tilt the specimen to a range of angles in order to obtain specific diffraction conditions, and apertures placed below the specimen allow the user to select electrons diffracted in a particular direction. A high contrast image can therefore be formed by blocking electrons deflected away from the optical axis of the microscope by placing the aperture to allow only unscattered electrons through. This produces a variation in the electron intensity that reveals information on the crystal structure, and can be viewed on a fluorescent screen, or recorded on photographic film or captured electronically. This technique (known as Bright Field or Light Field) is particularly sensitive to extended crystal lattice defects in an otherwise ordered crystal. As the local distortion of the crystal around the defect changes the angle of the crystal plane, the intensity of the scattering will vary around the defect. As the image is formed by the distortion of the crystal planes around the defect, the contrast in these images does not normally coincide exactly with the defect, but is slightly to one side. It is also possible to produce an image from electrons deflected by a particular crystal plane. By either moving the aperture to the position of the deflected electrons, or tilting the electron beam so that the deflected electrons pass through
4 of 5 10/24/2006 12:55 PM the centred aperture, an image can be formed of only deflected electrons, known as a Dark Field image. In the most powerful diffraction contrast TEM instruments, crystal structure can also be investigated by High Resolution Transmission Electron Microscopy (HRTEM), also known as phase contrast imaging as the images are formed due to differences in phase of electron waves scattered through a thin specimen. Resolution of the HRTEM is limited by spherical and chromatic aberration, but a new generation of aberration correctors has been able to overcome spherical aberration. Software correction of spherical aberration has allowed the production of images with sufficient resolution to show carbon atoms in diamond separated by only 0.89 Ångström units and atoms in silicon at 0.78 Ångströms (one Ångström is 0.000 000 000 1 of a meter) at magnifications of 50 million times. Improved resolution has also allowed the imaging of lighter atoms that scatter electrons less efficiently lithium atoms have been imaged in lithium battery materials. The ability to determine the positions of atoms within materials has made the HRTEM an indispensable tool for nanotechnology research and development in many fields, including heterogeneous catalysis and the development of semiconductor devices for electronics and photonics. Limitations There are a number of drawbacks to the TEM technique. Many materials require extensive sample preparation to produce a sample thin enough to be electron transparent, which makes TEM analysis a relatively time consuming process with a low throughput of samples. The structure of the sample may also be changed during the preparation process. Also the field of view is relatively small, raising the possibility that the region analysed may not be characteristic of the whole sample. There is potential that the sample may be damaged by the electron beam, particularly in the case of biological materials. See also Scanning electron microscope Electron microscope Transmission Electron Aberration-corrected Microscope Energy filtered transmission electron microscopy Wikibooks has more about this subject: Nanowiki External links The National Center for Electron Microscopy, Berkeley California USA (http://ncem.lbl.gov) The National Center for Macromolecular Imaging, Houston Texas USA (http://ncmi.bcm.tmc.edu) http://nsm1.fullerton.edu/~skarl/em/microscopy/temmicroscopy.html http://www.unl.edu/cmracfem/em.htm http://www.matter.org.uk/tem/ http://www.microscopy.ethz.ch Nanotechnology, Institute of Physics Publishing (http://www.nanotechnology.iop.org) Retrieved from "http://en.wikipedia.org/wiki/transmission_electron_microscopy" Categories: Articles to be merged since October 2006 Microscopes This page was last modified 03:06, 9 October 2006.
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