The scanning tunnelling microscope (STM) uses a process called quantum tunnelling - AQA - A-Level Physics - Question 3 - 2020 - Paper 7

Quantum tunnelling is a phenomenon where an electron can pass through a potential barrier, despite not having enough energy to surmount it classically. This occurs within the STM when the electron from the sample tunnels through a gap to the conducting tip of the probe. This tunnelling effect is significant at very small distances, allowing STM to detect and image surfaces at the atomic level. A diagram can illustrate this process:

          electron
             |
          ____|____
         |         |
      probe     sample
       tip       atom
      (A)       (B)

In this diagram, the tunnelling occurs between the tip and the atom on the surface as the electron transitions between regions of different energy levels.