How does quantum tunneling allow a scanning tunneling microscope to work?

A scanning tunneling microscope (STM) operates by utilizing the quantum mechanical phenomenon of tunneling, where electrons move through a potential barrier that would be impassable according to classical physics.12

In an STM, a sharp conductive tip is positioned just a few angstroms from a sample surface. When a small voltage is applied between the tip and the sample, electrons "tunnel" across the vacuum gap between them. This process relies on the wavelike properties of electrons; because the probability of finding these tunneling electrons decreases exponentially as the distance from the surface increases, the instrument is extremely sensitive to the gap spacing.12

As the probe scans the surface, it registers variations in the resulting tunneling current. Because this current is directly related to the electron density at the surface, the data can be processed to map the surface's topography, allowing for the imaging of individual atoms.1