Transmission Electron Microscope (AQA A Level Physics)

How does a TEM work?

• In the 1930s, experimental scientists realised that the much shorter wavelengths that electron waves offered could be used to construct microscopes with a higher resolving power
• The first to be constructed were transmission electron microscopes (TEMs)
  • Where light microscopes had used convex optical lenses, these microscopes focused beams of electrons using magnetic lenses
  • The electrons passed through a sample and formed an image on a fluorescent screen

A diagram showing the path of electrons through magnetic lenses

A cross-sectional diagram of a TEM. The dotted lines represent the paths of electrons - those travelling along the microscope's axis (the middle vertical line) are not deflected. Each magnetic lens has a different purpose.

• The electron gun emits electrons through thermionic emission
  • These are then accelerated to high speeds (and therefore short wavelengths) by a large potential difference
• The function of the condenser lens:
  • The condenser lens' magnetic field deflects the electrons into a wide beam travelling parallel to the axis of the microscope
  • This parallel beam is uniformly incident on the sample
• The function of the objective lens:
  • This lens forms an image of the sample
  • It deflects the outer electrons in the beam towards the central axis, much like a convex optical lens does for light
  • Electrons travelling along the microscope's axis are not deflected, again similarly to light in a convex lens
• The function of the projector lens:
  • This lens causes the beams from the objective lens to spread out, magnifying the image created by the objective lens
  • This magnified image is directed onto a fluorescent screen, emitting light where electrons are incident

Drawbacks of the TEM

• The level of detail available in an image depends on the resolving power
  • In an electron microscope, electrons need to be travelling as fast as possible to have the shortest wavelength and therefore highest resolving power
• In the TEM the electrons must pass through the sample
  • This reduces the speed of electrons, increasing wavelength and reducing resolving power so electron waves are unable to resolve as much detail as their short wavelength would allow
• Additionally, not all electrons emitted by thermionic emission have the same speed, and not all electrons are slowed by the sample to the same degree
• This means electrons in the beam have a range of speeds
  • Electrons travelling at different velocities through a magnetic field are deflected by different amounts
  • This means electrons passing through a single point in the sample are projected onto a range of locations on the fluorescent screen instead, forming a blurrier image