How far is a scanning tunneling microscope tip held above a sample surface?

In a scanning tunneling microscope (STM), the sharp probe tip is positioned a few angstroms above the sample surface. This extremely small spacing—often comparable to the distance between neighboring atoms in a lattice—is critical because the probability of tunneling electrons decreases exponentially as the distance from the surface increases. When the gap is reduced to this scale, typically around five angstroms, electrons can tunnel between the sample and the probe tip, allowing the instrument to map the surface topography with atomic resolution.1