6. Analytical Transmission Electron Microscopy

Abstract

INTRODUCTIONAnalytical transmission electron microscopy (TEM) is used to reveal sub-micrometer, internal ine structure (the microstructure or ultrastructure) and chemistry in minerals. The amount and scale of the information which can be extracted by TEM depends critically on four parameters; the resolving power of the microscope (usually smaller than 0.3 nm); the energy spread of the electron beam (of the order of an electron volt, eV); the thickness of the specimen (almost always signiicantly less than 1 mm), and the composition and stability of the specimen. An introductory text on all types of electron microscopy is provided by Goodhew et al. (2001), while more detailed information on transmission electron microscopy may be found in the comprehensive text of Williams and Carter (2009). INTRODUCTION TO ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY (TEM) Basic design of transmission electron microscopes (TEM)The two available modes of TEM-CTEM and STEM-differ principally in the way they address the specimen. Conventional TEM (CTEM) is a wide-beam technique, in which a close-to-parallel electron beam loods the whole area of interest and the image (or diffraction pattern), formed by an imaging (objective) lens after the thin specimen from perhaps 10 6 -10 7 pixels on a digital camera, is collected in parallel. Scanning TEM (STEM) deploys a ine focused beam, formed by a probe-forming lens before the thin specimen, to address each pixel (here, a dwell point) in series and form a sequential image as the probe is scanned across the specimen. Figures 1 and 2 summarize these different instrument designs; here it should be noted that many modern TEM instruments are capable of operating in both modes, rather than being instruments dedicated to one mode of operation.In both types of instrument analytical information from a small region is usually collected using a focused beam. The smallest region from which an analysis can be collected is deined by the diameter of this beam and hence the corresponding through-thickness volume in the Brydson, Brown, Benning, Livi specimen within which various elastic and inelastic scattering (energy-loss) processes take place.In both CTEM and STEM, electrons are produced from an electron emitter, focused and collimated into a beam and inally accelerated to a given beam energy. Key instrumental components, which affect the microscope resolution and analytical performance, are:• the electron emitter which can operate via either a thermionic or ield emission mechanism or a combination of the two; ield emission provides the brightest, most monochromatic and coherent source of electrons.• the accelerating voltage (E o , typically in the range 60-300 kV) and hence incident electron energy; the higher the accelerating voltage the higher the resolution and the larger the sample penetration (although, in certain cases depending on the elements present and their chemical bonding, sample damage via sputtering may be an issue above a certain threshold energy).In the CTEM (Fig. 1), two or more electro...