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A scanning transmission electron microscope (STEM) is a type of transmission electron microscope. With it, the electrons pass through the specimen, but, as in scanning electron microscopy, the electron optics focus the beam into a narrow spot which is scanned over the sample in a raster.

The rastering of the beam across the sample makes these microscopes suitable for analysis techniques such as mapping by energy-dispersive X-ray (EDX) spectroscopy, electron energy loss spectroscopy (EELS) and annular dark-field imaging (ADF). These signals can be obtained simultaneously, allowing direct correlation of image and quantitative data.

By using a STEM and a high-angle detector, it is possible to form atomic resolution images where the contrast is directly related to the atomic number. This is in contrast to the conventional high-resolution transmission electron microscopy technique, which uses phase-contrast, and therefore produces results which need interpretation by simulation.

Contents

History

The first STEM was built in 1938 by Baron Manfred von Ardenne,[1][2] working in Berlin for Siemens. However, the results were inferior to that of TEM at the time, and von Ardenne only spent two years working on the problem. The microscope was destroyed in an air raid in 1944, and von Ardenne did not return to the field after WWII.[3]

The technique did not become developed until the 1970s, with Albert Crewe at the University of Chicago developing the field emission gun[4] and adding a high quality objective lens to create the modern STEM, and demonstrated the ability to image atoms using ADF.

Crewe and coworkers at the University of Chicago developed the cold field emission electron source and built a STEM able to visualize single heavy atoms on thin carbon substrates.[5]

Atomic resolution chemical analysis using the STEM was first reported in 1993.[6][7]

Biological Application

The first application of this method to the imaging of biological molecules was demonstrated in 1971.[8] The motivation for STEM imaging of biological samples is particularly to make use of dark-field microscopy, where the STEM is more efficient than a conventional TEM, allowing high contrast imaging of biological samples without requiring staining. The method has been widely used to solve a number of structural problems in molecular biology.[9][10][11]

Low voltage electron microscope (LVEM)

The low voltage electron microscope (LVEM) is a combination of SEM, TEM and STEM in one instrument, which operated at relatively low electron accelerating voltage of 5 kV. Low voltage increases image contrast which is especially important for biological specimens. This increase in contrast significantly reduces, or even eliminates the need to stain. Sectioned samples generally need to be thinner than they would be for conventional STEM (20-70nm). Resolutions of a few nm are possible in TEM, SEM and STEM modes. [12][13]

See also

References

  1. ^ von Ardenne, M (1938). "Das Elektronen-Rastermikroskop. Theoretische Grundlagen". Z Phys 109: 553–572.  
  2. ^ von Ardenne, M (1938). "Das Elektronen-Rastermikroskop. Praktische Ausführung". Z tech Phys 19: 407–416.  
  3. ^ D. McMullan, SEM 1928 - 1965
  4. ^ Crewe, Albert V; Isaacson, M. & Johnson, D. (1969). "A Simple Scanning Electron Microscope". Rev. Sci. Inst. 40: 241–246. doi:10.1063/1.1683910.  
  5. ^ Crewe, Albert V; Wall, J. & Langmore, J. (1970). "Visibility of a single atom". Science 168: 1338–1340. doi:10.1126/science.168.3937.1338. PMID 17731040.  
  6. ^ Browning, N. D.; Chisholm M. F. & Pennycook S. J. (1993). "Atomic-resolution chemical analysis using a scanning transmission electron microscope". Nature 366: 143–146. doi:10.1038/366143a0.  
  7. ^ Browning, N. D.; Chisholm M. F. & Pennycook S. J. (2006). "Corrigendum: Atomic-resolution chemical analysis using a scanning transmission electron microscope". Nature 444: 235. doi:10.1038/nature05262.  
  8. ^ Wall, J.S., 1971 "A high resolution scanning electron microscope for the study of single biological molecules" PhD thesis, University of Chicago
  9. ^ Wall JS, Hainfeld JF (1986). "Mass mapping with the scanning transmission electron microscope". Annu Rev Biophys Biophys Chem 15: 355–76. PMID 3521658.  
  10. ^ Hainfeld JF, Wall JS (1988). "High resolution electron microscopy for structure and mapping". Basic Life Sci 46: 131–47. PMID 3066333.  
  11. ^ Wall JS, Simon MN (2001). "Scanning transmission electron microscopy of DNA-protein complexes". Methods Mol Biol 148: 589–601. PMID 11357616.  
  12. ^ Nebesářová1, Jana; Vancová, Marie (2007). "How to Observe Small Biological Objects in Low Voltage Electron Microscope". Microscopy and Microanalysis 13 (3): 248–249. doi:10.1017/S143192760708124X.  
  13. ^ Drummy, Lawrence, F.; Yang, Junyan; Martin, David C. (2004). "Low-voltage electron microscopy of polymer and organic molecular thin films". Ultramicroscopy 99: 247–256. doi:10.1016/j.ultramic.2004.01.011.  

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