Scanning tunnelling microscope head. Physicist Ali Yazdani looking at the head of a variable temperature scanning tunnelling microscope (STM). The STM allows scientists to visualize regions of high electron density and hence infer the position of individual atoms and molecules on the surface of a lattice. It operates by bringing a fine point electrode, up to one atom wide at the tip, to within a few ten-millionths of a millimetre of the sample's surface. As the electrons of the atoms in the sample and tip interact, a small tunnelling current is created. The electrode is scanned across the sample, vertical movements maintaining tip-to-surface distance. These movements are then processed by computer to give a map of the surface.