How did the first scanning tunneling microscope image reveal atoms on gold?

The first scanning tunneling microscope (STM) image, produced by Gerd Binnig and Heinrich Rohrer in 1981, revealed atoms on a gold surface by utilizing the quantum mechanical phenomenon of tunneling.1

To capture this image, the researchers positioned the sharp tip of a tungsten needle a few angstroms from the gold surface and applied a small voltage between them. This caused electrons to tunnel across the gap. As the probe scanned the surface, it registered variations in the tunneling current, which were then processed to create a topographical map.1

Although the electronic charge in metals is typically distributed uniformly, the interaction between the probe tip and the sample perturbs the electron density. This causes the tunneling current to increase slightly when the tip is positioned directly above a surface atom. When displayed on a monitor, this process revealed rows of precisely spaced atoms and broad terraces separated by steps only one atom in height, providing a direct image of the surface's atomic structure.1