The Conventional Transmission Electron Microscope

The analytical electron microscope (AEM) is fitted with a spectrometer for X-ray microanalysis and also for electron energy-loss analysis (q.v. pp. 185). 

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The transmission electron microscope is now well established as a useful tool for the characterization of supported heterogeneous catalysts(l). Axial bright-field imaging in the conventional transmission electron microscope (CTEM) is routinely used to provide the catalyst chemist with details concerning particle size distributions, 3), particle disposition over the support material(2-6) as well as particle morphology(7). Internal crystal structure(8-10), and elemental compositions(ll) may be inferred by direct structure imaging. 

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

A spectrometer for the measurement of characteristic secondary X-rays was perhaps the first peripheral analytical device to be attached to a conventional Transmission Electron Microscope (TEM)... 

A conventional transmission electron microscope is required as well as an electron energy analyser to measure the electron energy loss spectra. It is often preferable to keep the energy of the detected electrons constant and to sweep the energy of incident electrons instead to improve signal to noise and energy resolution. 

Conventional transmission electron microscopes (CTEM or TEM) are electron optical instnunents analogous to light microscopes, where the specimen is illuminated by an electron beam. This requires operation in a vacuum because air scatters electrons. High resolution... 

XAS requires synchrotron radiation and a relatively large amount of material but no vacuum condition. On the other hand, EELS can be performed directly using an electron spectrometer fitted to a scanning transmission electron microscope (STEM). Here, the main advantage is the high spatial resolution attainable. (The incident electron beam can be as. small as I nm in diameter.) EELS can also be coupled with conventional transmission electron microscope (TEM) facilities and particularly high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS). 

As noted earlier, most electron diffraction studies are perfonned in a mode of operation of a transmission electron microscope. The electrons are emitted themiionically from a hot cathode and accelerated by the electric field of a conventional electron gun. Because of the very strong interactions between electrons and matter, significant diffracted intensities can also be observed from the molecules of a gas. Again, the source of electrons is a conventional electron gun. 

This type of electron microscope is completely different in principle and application from the conventional transmission-type electron microscope. In the scanning instrument, the surface of a solid sample is bombarded with a fine probe of electrons, generally less than 100 A in diameter. The sample emits secondary electrons that are generated by the action of the primary beam. These secondary electrons are collected and amplified by the instrument. Since the beam strikes only one point on the sample at a lime, the beam must be scanned over the sample surface in a raster pattern to generate a picture of the surface sample. The picture is displayed on a cathode ray tube from which it can be photographed. 

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