Analytical electron microscopy usefulness

This chapter discusses the range of analytical methods which use the properties of X-rays to identify composition. The methods fall into two distinct groups those which study X-rays produced by the atoms to chemically identify the elements present, and X-ray diffraction (XRD), which uses X-rays of known wavelengths to determine the spacing in crystalline structures and therefore identify chemical compounds. The first group includes a variety of methods to identify the elements present, all of which examine the X-rays produced when vacancies in the inner electron shells are filled. These methods vary in how the primary vacancies in the inner electron shell are created. X-ray fluorescence (XRF) uses an X-ray beam to create inner shell vacancies analytical electron microscopy uses electrons, and particle (or proton) induced X-ray emission (PIXE) uses a proton beam. More detailed information on the techniques described here can be found in Ewing (1985, 1997) and Fifield and Kealey (2000). 

The STEM instrument itself can produce highly focused high-intensity beams down to 2 A if a field-emission source is used. Such an instrument provides a higher spatial resolution compositional analysis than any other widely used technique, but to capitalize on this requires very thin samples, as stated above. EELS and EDS are the two composition techniques usually found on a STEM, but CL, and even AES are sometimes incorporated. In addition simultaneous crystallographic information can be provided by diffraction, as in the TEM, but with 100 times better spatial resolution. The combination of diffraction techniques and analysis techniques in a TEM or STEM is termed Analytical Electron Microscopy, AEM. A well-equipped analytical TEM or STEM costs well over 1,000,000. 

D. B. Williams. Practical Analytical Electron Microscopy in Materials Science. Verlag Chemie International, Weinheim, 1984. A good monograph discussing the use and applications of AEM, especially at intermediate voltages. The discussion on EDS is an excellent primer for using X-ray analysis on a TEM. 

Introduction to Analytical Electron Microscopy (J. J. Hren, J. I. Goldstein, and D. C. Joy, eds.) Plenum, New York, 1979. Somewhat dated text that is still useful for certain chapters including those of Cowley, Steeds, and Isaacson, et al. 

The combined use of energy-dispersive X-ray spectroscopy and TEM/STEM is a routine method of analytical electron microscopy enabling both qualitative and quantitative chemical analysis of interfaces and interlayers with high lateral resolution. Reso-... 

Analytical electron microscopy (AEM) permits elemental and structural data to be obtained from volumes of catalyst material vastly smaller in size than the pellet or fluidized particle typically used in industrial processes. Figure 1 shows three levels of analysis for catalyst materials. Composite catalyst vehicles in the 0.1 to lOim size range can be chemically analyzed in bulk by techniques such as electron microprobe, XRD, AA, NMR,... 

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

Lichtenberger O, Neumann D. Analytical electron microscopy as a powerful tool in plant cell biology examples using electron energy loss spectroscopy and x-ray microanalysis. Eur J Cell Biol 1997 73 378-386. 

The origins of analytical electron microscopy go back only about 15 years when the first x-ray spectra were obtained from submicron diameter areas of thin specimens in an electron microscope [1]. Characterization of catalyst materials using AEM is even more recent[2,3] but is currently a very active research area in several industrial and academic laboratories. The primary advantage of this technique for catalyst research is that it is the only technique that can yield chemical and structural information from individual submicron catalyst particles. 

Analytical electron microscopy by x-ray emission spectroscopy can be extremely useful as a qualitative analysis tool, e.g. to determine which elements are present in lOnm diameter areas of the specimen. However, the greatest impact of AEM comes from quantitative chemical profiles across minute regions or features in the specimen, information that usually cannot be obtained by other means. 

An interesting development of analytical electron microscopy, and a potentially very useful one for mineralogical research, has been made by Spence and Tafto (1982,1983). The technique, known as ALCHEMI (atom location by channeling enhanced microanalysis), is the electron analogue of an x-ray technique originally used by Batterman (1969). The theoretical basis of the technique was discussed in Chapter 4, but it is appropriate to summarize that discussion before considering the ALCHEMI technique in detail. 

The size and distribution of pores and the size, distribution, and identity of minerals in coal specimens from an eastern Kentucky splint coal and the Illinois No. 6 coal seam were determined by means of transmission electron microscopy (TEM) and analytical electron microscopy (AEM). The observed porosity varies with the macerals such that the finest pores (<2-5 nm) are located in vitrinite, with a broad range of coarser porosity (40-500 nm) associated with the macerals exinite and inertinite. Elemental analyses, for elements of atomic number 11 or greater, in conjunction with selected area diffraction (SAD) experiments served to identify the source of the titanium observed in the granular material as the mineral rutile. Only sulfur could be de-tected in the other coal macerals. Dark-field microscopy is introduced as a means for determining the domain size of the coal macerals. This method should prove useful in the determination of the molecular structure of coal. 

Electronic microscopic measurements were curried out with a Jeol Temscan 100 CX electron microscope equipped with a Kevex 5100 C cnergy-dispersive spectrometer. Conventional transmission and analytical electron microscopy were used. 

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