Transmission electron microscope dedicated

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. 

The dedicated scanning transmission electron microscope (STEM) is an integral tool for characterizing catalysts because of its unique ability to image and analyze nano-sized volumes. This information is valuable in optimizing catalyst formulations and determining causes for reduced catalyst performance. For many commercial catalysts direct correlations between structural features of metal crystallites and catalytic performance are not attainable. When these instances occur, determination of elemental distribution may be the only information available. In this paper we will discuss some of the techniques employed and limitations associated with characterizing commercial catalysts. 

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. 

Fig. 3.1. A schematic diagram of electron-optics in a dedicated Scanning Transmission Electron Microscope (VG HB501 STEM).

Scanning transmission electron microscopy gives essentially the same type of results and has the same type of difficulties as the conventional TEM. There are two types of instruments, the dedicated STEMs, which generally have a UHV column, and the TEM based instruments mostly known as AEMs (analytical electron microscopes). A detailed comparison of STEM and TEM was given in Section 2.4.1.3. There are some advantages in using the STEM on polymer samples in particular it seems that thicker samples can be used. However, the added complexity and cost, combined with lower resolution in the AEM STEM mode, make it unlikely that either kind of instrument would be purchased for polymer studies. 

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