Microscopio Electronico De Transmision Tem

2. The TEM and its Optics

2.1 Introduction to the Transmission Electron Microscope
The transmission electron microscope (TEM) has become the premier tool for the microstructural characterizationof materials. In practice, the diffraction patterns measured by x-ray methods are more quantitative than electron

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2. The TEM and its Optics

diffraction patterns, but electrons have animportant advantage over x-rays – electrons can be focused easily. The optics of electron microscopes can be used to make images of the electron intensity emerging from the sample. For example, variationsin the intensity of electron diffraction across a thin specimen, called “diffraction contrast,” is useful for making images of defects such as dislocations, interfaces, and second phase particles.Beyond diffraction contrast microscopy, which measures the intensity of diffracted waves, in “high-resolution” transmission electron microscopy (HRTEM or HREM) the phase of the diffracted electron wave ispreserved and interferes constructively or destructively with the phase of the transmitted wave. This technique of “phase-contrast imaging” is used to form images of columns of atoms. TEM is such apowerful tool for the characterization of materials that some microstructural features are defined largely in terms of their TEM images. Besides diffraction and spatial imaging, the high-energy electrons inTEM cause electronic excitations of the atoms in the specimen. Two important spectroscopic techniques make use of these excitations. • In energy-dispersive x-ray spectrometry (EDS), an x-ray spectrumis acquired from small regions of the specimen illuminated with a focused electron beam, usually using a solid-state detector as described in Sect. 1.4.1. Characteristic x-rays from each element areused to determine the concentrations of the different elements in the specimen. • In electron energy-loss spectrometry (EELS), energy losses of the electrons are measured after the high-energy...