12.2.11 Scanning Tunnelling Microscope

How Does an STM Work?

• By the 1980s, many advances had been made in research technology and in physics on the scale of the atom
• The scanning tunnelling microscope (STM) was created
  • This microscope is able to resolve objects just 0.001 nm apart
  • This means it can produce images showing individual rows of atoms
• A probe with a very fine tip (only a few atoms across) is held a few nanometres above the surface of an object
  • The probe is moved by pieces of equipment called piezoelectric transducers
  • These are able to move the tip in any direction by tiny increments of 0.001 nm
• The probe is held at a constant potential difference with the surface of the sample
  • Because the tip is so close to the surface of the sample, some electrons are able to jump, or tunnel, to the tip
  • This produces a tunnelling current which is recorded
• To be able to scan this surface to produce an image of it:
  • When the tip reaches a raised atom, the distance from the tip to the surface decreases, and more electrons tunnel so the tunnelling current increases
  • Likewise, when the tip reaches a dip in the surface, the tunnelling current decreases because the gap is larger and fewer electrons tunnel
  • These changes in tunnelling current are used to produce a map of the surface of a sample

Diagram showing the probe of the STM and the sample surface

Due to the potential difference across the gap and its short distance, some electrons tunnel across the gap to the tip of the probe, generating a current. The size of this current depends on the width of the gap it must cross.

Constant Height or Constant Current?

• STMs operate in two modes: constant height or constant current
• In constant height mode:
  • The tip does not move vertically up or down while scanning areas
  • This means that changes in the surface increase or decrease the gap size
  • The tunnelling current therefore varies
  • This is used to produce an image of the surface
• In constant current mode:
  • When a change in tunnelling current is detected, the tip moves up or down to keep the current constant
  • This means the gap size is always the same
  • The vertical motion of the probe is used to map an image of the surface

Quantum Tunnelling

• Quantum tunnelling is a result of the wave-like behaviour of particles such as electrons
• The gap between the surface and the tip acts as a barrier for electrons
• The amplitude of the matter-wave of the electrons is decreased by this barrier, but on the other side of the gap, this amplitude is non-zero
  • This effect only occurs if the barrier is weak enough (i.e. the distance is small enough)
  • This is like thin surfaces not being opaque to visible light because some of the wave can pass through
• This results in a current passing from sample to probe
  • This current is very sensitive to changes in gap distance, which allows the STM to have a great resolving power