Abstract
Scanning transmission electron microscopy (STEM) offers a powerful extension to the conventional transmission electron microscopy (TEM) by combining high resolution microanalysis with a variety of imaging and diffraction modes. Bright-field (BF) and dark-field STEM imaging (with or without energy filtering) can be used to obtain structural images or to locate small particles and other inhomogeneous sample areas for microanalysis. The microdiffraction technique can provide crystallographic information on nanometer-scale small particles. Secondary electron (SE) images with a resolution approaching the probe size (0.5 nm) can be obtained in STEM instruments, providing topographic information on the samples studied. High-angle annular dark field (HAADF) imaging is also available in dedicated STEM instruments. The intensity of high-angle scattered electrons depends strongly on the atomic number (Z). This unique property makes the HAADF imaging a powerful microscopic technique in characterizing supported catalyst systems which often comprize high atomic number particles supported on low atomic number substrates.
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