This is also known as Z-contrast imaging because there is a direct correlation between the local contrast and local mass-thickness, which depends on the atomic number Z. None of the elastically scattered electrons reach the detector, so it only images from inelastically scattered electrons. BF (bright-field) detector: small angles ( 50mrad.There are multiple detectors for STEM imaging: The transmitted signal is collected as a function of the beam location as it is rastered across the sample. While in TEM parallel electron beams are focused perpendicular to the sample plane, in STEM the beam is focused at a large angle and is converged into a focal point. STEM (Scanning transmission electron microscopy) Phase contrast: Several beams are admitted on the sample, and the interaction of the deflected beams with the transmitted beams give high-resolution images useful for determining crystal structure.Often this is used to examine crystal lattice defects. Diffraction contrast: Electrons are deflected according to Bragg’s law and depends on the crystal structure.This is often the case for biological materials. For example, contrast in amorphous materials arises from mass-density contrast. Mass-density contrast: Scattering increased with the atomic number and thickness of the sample.Imaging depends on contrast, which can arise from three processes: mass-thickness contrast, diffraction contrast, and phase contrast. Although atomic resolution is theoretically possible, it is difficult to achieve due to defects in the lenses. Either the transmitted electrons or the scattered electrons can be imaged, known as dark-field and light-field imaging, respectively (See Bright/Light field versus dark field and EDS). The electrons are elastically or inelastically scattered as they penetrate the sample. To obtain a TEM image, a thin sample of about 200 nm is subjected with a high energy electron beam., which is directed using electromagnetic lenses. 2 A high vacuum is needed to prevent collisions between the high-energy electrons and air molecules, which would absorb energy from the electrons. An electron beam is produced by heating a tungsten filament and is focused using magnetic fields. 1 The shorter wavelengths allow for the images to be better resolved, down to about 0.1 nm. While light microscopes use visible light (400-700 nm), electron microscopes use beams of electrons, which have wavelengths about 10,000 times shorter.
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