High brightness electron guns

The technique involves a scanning transmission electron microscope (STEM) and a high brightness electron source, a field emission gun (FEG to ensure sufficient current in a l-2nm diameter probe to excite a useful X-ray intensity. 

Additional advantages enjoyed by the dedicated STEM units are their clean, high vacuum systems in use and their very bright electron source in the field emission gun. The high vacuum (I0 " torr in the gun chamber and 10 to l(E torr in the sample chamber) allows prolonged observation of the sample without contamination, and the bright source allows viewing of data collection at TV scan rates. 

In summary, to produce a useful probe of subnanometer size, a high-brightness gun is essential together with optimised use of the probe-forming aperture to limit the aberration effects. Among the choice of the various electron sources, the field emission gun stands out. The penalty to be paid for a field emission source is the necessity to use ultra-high vacuum techniques. Such instruments were usually restricted to the dedicated STEM , but nowadays field emission sources are also popular in analytical TEMs. The improvement in the latter has blurred the distinction between dedicated STEM and TEM-STEM . 

The electron microscopes can be divided into two types (166) scanning electron microscopes (SEM), which use a 10-nm electron beam at the specimen surface, and transmission electron microscopes (TEM). With current TEMs, resolution of about 0.2 nm can be achieved, provided very thin (<20 nm) samples are available. With conventional inorganic oxide-supported metal catalysts, particles of approximately 1 nm can be detected. Scanning transmission electron microscopes (STEM) use a high brightness dark-field emission gun to produce a probe about 0.3 nm in diameter and combine the techniques of SEM and TEM. Further experimental and theoretical aspects of electron microscopy applied to catalysis have been reviewed recently (113, 167-169). 

Bazarov IV, Ouzounov DG, Dunham BM, Belomestnykh SA, Li Y, Liu X, et al. Efficient temporal shaping of electron distributions for high-brightness photoemission electron guns. Phys Rev Spec Top Accel Beams 2008 11 040702. 

The electron gun should provide a source brightness as high as possible and the energy spread in the emitted beam should be as low as possible. The two principal aberrations of concern in a SEM are spherical and chromatic aberration, and they should be as low as possible. 

Table 2 summarizes some important characteristics of three electron sources currently used in SEMs. Clearly the tungsten field emitter is most attractive, due to its high brightness and low energy spread. There are, however, a number of drawbacks associated with this type of source. In the first place, a gun vacuum of 10 torr is required, which can result in problems when examining a specimen which outgasses. The emission current is unstable. The maximum... 

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