Why must the probe tip of a scanning tunneling microscope be extremely close to the sample surface?

The probe tip of a scanning tunneling microscope (STM) must be positioned extremely close to the sample surface—typically within a few angstroms—because the instrument relies on the quantum mechanical phenomenon of tunneling.1

In this process, electrons pass through a potential barrier that would be impassable under the rules of classical physics. The probability of these electrons tunneling across the gap between the probe tip and the sample surface decreases exponentially as the distance between them increases. Because the STM is designed to measure this tunneling current to create topographical images of atomic structures, the extreme sensitivity to distance requires the tip to remain in very close proximity to the surface to ensure a measurable and consistent flow of electrons.21