The image in a TEM on the other hand is generated by the electrons that have

The image in a tem on the other hand is generated by

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specimen. The image in a TEM, on the other hand, is generated by the electrons that have transmitted through a thin specimen. Let us see how these two microscopes work and what kind of information they can provide: 2.3.2 Scanning electron microscope Figure 8 shows a simplified schematic diagram of a SEM. The electrons produced by the electron gun are guided and focused by the magnetic lenses on the specimen.
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Figure 7 . The focused beam of electrons is then scanned across the surface in a raster fashion (Figure 9). This scanning is achieved by moving the electron beam across the specimen surface by using deflection/scanning coils. The number of secondary electrons produced by the specimen at each scanned point are plotted to give a two-dimensional image. Figure 8 In principle, any of the signals generated at the specimen surface can be detected. Most electron microscopes have the detectors for the secondary electrons and the backscattered electrons. Figure 10 shows the interaction volume within the specimen showing the regions of secondary electrons (energy < 50 eV) and backscattered electrons.
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Figure 9 A secondary electron detector is biased with positive potential to attract the low energy secondary electrons. Detector for backscattered electrons is not biased; the high energy backscattered electrons strike the unbiased detector. As backscattered electrons come from a significant depth within the sample (Figure 3), they do not provide much information about the specimen topology. However, backscattered electrons can provide useful information about the composition of the sample; materials with higher atomic number produce brighter images. 2.3.3 Transmission electron microscope The first electron microscope was developed by Knoll and Ruska in 1930s. It was a transmission electron microscope; the electrons were focused on a thin specimen and the electrons transmitted through the specimen were detected. Figure 4 shows a simplified optical diagram comparing a light microscope with a transmission electron microscope.
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Figure 10 Transmission electron microscopes usually have thermionic emission guns and electrons are accelerated anywhere between 40 – 200 kV potential. However, TEM with >1000 kV acceleration potentials have been developed for obtaining higher resolutions. Owing to their brightness and very fine electron beams, field emission guns are becoming more popular as the electron guns. A scanning transmission electron microscope or STEM is a transmission electron microscope that works in the scanning mode like a SEM. An electron beam is focused to a small spot and scanned across the specimen exactly as done in SEM. A STEM allows detecting the transmitted as well as secondary and backscattered electrons. This mode of electron microscope provides spatially resolved information about the specimen. All other types of electron microscopes are the modifications of SEM or TEM.
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  • Two '17
  • Electron, Scanning Tunneling Microscope, Scanning electron microscope, Scanning probe microscopy, Atomic force microscopy

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