What is "field ion microscope"

Field ion microscopy (FIM) is an analytical technique used in materials science. The field ion microscope is a type of microscope that can be used to image the arrangement of atoms at the surface of a sharp metal tip. It was the first technique by which individual atoms could be spatially resolved.

On October 11, 1955, Müller and Kanwar Bahadur (Pennsylvania State University) observed individual tungsten (W) atoms on the surface of a sharply pointed W tip by cooling it to 78 K and employing helium as the imaging gas. Müller & Bahadur were the first persons to observe individual atoms directly; to do so, they used an FIM, which Müller had invented in 1951.