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

Quantum tunnelling refers to the phenomenon where electrons can pass through a potential barrier that they would not have enough energy to overcome classically. In the case of an STM, when the probe is brought very close to the sample's surface, the electrons can tunnel through the gap between the probe and the surface due to their wave-like nature. This process allows the STM to detect the tunnelling current, which is generated by the movement of electrons from the sample to the probe. The diagram of this process can illustrate the electron's wave function overlapping with the sample's surface, demonstrating the tunnelling effect.