Transmission electron microscope advantages

This kind of estimation of the relative concentration is the most widely used method for quantitative EELS analysis. It is advantageous because the dependence on the primary electron current, Iq, is cancelled out this is not easily determined in a transmission electron microscope under suitable analytical conditions. Eurthermore, in comparison with other methods, e. g. Auger electron spectroscopy and energy-disper-... 

Convergent-Beam eleetron Diffraction and Microdiffraction) become available on analytical transmission electron microscopes. Most of the electron diffraction techniques use a stationary incident beam, but some specific methods like the precession method take advantage of a moving incident beam. 

Another, more often applied, method is the irradiation of matrices with neutrons or charged particles (electrons, a-particles, heavy ions). Advantages of this method include short time of irradiation (min or h) direct visibility of structural damage (observed in situ during irradiation using a transmission electron microscope) and... 

Figure 14.1. Schematic diagram showing the principle of image formation and diffraction in the transmission electron microscope. The incident beam/o illuminates the specimen. Scattered and unscattered electrons are collected by the objective lens and foeused back to form first an electron diffraction pattern and then an image. For a 2D or 3D crystal, the electron-diffraetion pattern would show a lattice of spots, eaeh of whose intensity is a small fraetion of that of the incident beam. In praetiee, an in-focus image has no eontrast, so images are recorded with the objeetive lens slightly defocused to take advantage of the out-of-focus phase-contrast mechanism.

Resolution in the STEM is limited by the probe diameter, which is about 1 nm in equipment dedicated to this operating mode, at the cost of using a cold field emission gun requiring an ultravacuum. Because of the high-precision optics and the point-by-point image formation principle, the STEM combines the advantages of scanning electron microscope analysis with resolution performance levels similar to the transmission electron microscope. 

Recent development of transmission electron microscope enables us to make microscopic observation with sub-nanometer spatial resolution, because the electron beam can be focused within 1 nm diameter. The advantage of the use of EELS technique is that an investigation of electronic state at very small local space of sub-nanometer scale is feasible when it is combined with the high resolution electron microscopic(HREM) technique. A schematic illustration of the combination of HREM and ELNES is indicated in Fig. 17. 

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). 

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