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• Scanning Electron Microscope (SEM)


  A type of electron microscope that scans surfaces of microorganisms using a beam of electrons moving at low energy to focus and scan specimens

  
• The first Scanning Electron Microscope was initially made by Mafred von Ardenne


  1937

  
• Aim of Mafred von Ardenne


  • To surpass the transmission electron Microscope
  • To use high-resolution power to scan a small raster using a beam of electrons that were focused on the raster
  • To reduce the problems of chromatic aberrations images produced by the Transmission electron Microscopes
• Cambridge Scientific Instrument Company developed a fully constructed Scanning electron Microscope and named it a Stereoscan


  1965
• Scanning Electron Microscope (SEM)

  • Uses a beam of electrons moving at low energy to focus and scan specimens
  • Scans surfaces of microorganisms

  
• Wavelength


  Electron microscopes have short wavelengths in comparison to the light microscope which enables better resolution power
• Transmission Electron Microscope


  Uses transmitted electrons
• Scanning Electron Microscope


  Uses emitted electrons

  
• Principle of the Scanning Electron Microscope

  1. Applying kinetic energy to produce signals on the interaction of the electrons

  2. Using secondary electrons, backscattered electrons and diffracted backscattered electrons to view crystallized elements and photons

  3. Using secondary and backscattered electrons to produce an image

  
• How the Scanning Electron Microscope (SEM) works

  1. Electrons are emitted after thermal energy is applied to the electron source and allowed to move in a fast motion to the anode

  2. The beam of electrons activates the emission of primary scattered (Primary) electrons at high energy levels and secondary electrons at low-energy levels from the specimen surface

  3. The beam of electrons interacts with the specimen to produce signals that give information about the surface topography and composition of the specimen

  4. Specimens need fixation, dehydration, and drying to maintain structural features and prevent collapsing

  5. Samples are mounted and coated with a thin layer of heavy metal elements to allow spatial scattering of electric charges and better image production

  6. Scanning is attained by tapering a beam of electrons back and forth over a thin section of the microscope

  7. Secondary electrons strike a scintillator which emits flashes of light that get converted into an electric current, sending a signal to the cathode ray tube to produce an image

  
• Major components of the Scanning Electron Microscope (SEM)

  • Electron Source
  • Lenses
  • Scanning Coil
  • Detector
  • Display device (data output devices)
  • Power supply
  • Vacuum system

  
• Like the transmission electron Microscope, the Scanning electron microscope should be free from vibrations and any electromagnetic elements

  
• Applications of the Scanning Electron Microscope (SEM)

  • Spot chemical analysis in energy-Dispersive X-ray Spectroscopy
  • Analysis of cosmetic components which are very tiny in size
  • Study of the filament structures of microorganisms
  • Study of the topography of elements used in industries

  
• Advantages of the Scanning Electron Microscope (SEM)

  • Easy to operate and has user-friendly interfaces
  • Used in a variety of industrial applications to analyze surfaces of solid objects
  • Some modern SEMs are able to generate digital data that can be portable
  • Easy to acquire data from the SEM, within a short period of time of about 5 minutes

  
• Limitations of the Scanning Electron Microscope (SEM)

  • Very expensive to purchase
  • Bulky to carry
  • Must be used in rooms that are free of vibrations and free of electromagnetic elements
  • Must be maintained with a consistent voltage
  • Should be maintained with access to cooling systems

  
• Scanning-Transmission Electron Microscope (STEM)


  Uses a convergent beam of electrons to focus on a probe on the specimen, and the probe is then scanned on its surface collecting signals which are then collected as point-to-point to form an image