Imaging with the Tunneling and Force Microscopes

Abstract

THE SCANNING TUNNELING MICROSCOPEThe STM is an instrument that is used for measuring surface structure. The new instrument was introduced when Rohrer, together with Binnig, discovered that a single atom at the end of a sharp tungsten needle could probe the electronic structure of smooth conducting surfaces. Their atomic probe was placed in close proximity to the surface and mechanically scanned over the image. The resolution is sufficient to define the spatial positions of the atomic sites with extraordinary precision. This instrument is ideal for characterizing small structures.The instrument operates over a wide range of temperatures - from liquid helium at 4K to room temperature and beyond. It works in a variety of atmospheres; vacuum, gases and liquids. Piezoelectric scanners are used to move the probe over the surface with the precision necessary to determine the atomic positions. The most favored scanner consists of a single piezotube with electrodes configured to provide motion in three dimensions.The scanning tip is the STM component, more than any other, that determines the resolving power. The tip is usually formed with electrochemical etching similar to that used in making tips for the field emission microscopes. However, more elaborate systems can be used. Fink has shown that it is feasible to fabricate, in a controlled way, a tip that consists of a single atom. He uses single crystal (111) tungsten and evaporates ions from the tip with a combination of heat and high field. This procedure reduces the apex of the tungsten rod to a single atom.