The scanning tunneling microscope (STM) is widely used in both industrial and fundamental research to obtain atomic-scale images of metal surfaces. It provides a three-dimensional profile of the surface which is very useful for characterizing surface roughness, observing surface defects, and determining the size and conformation of molecules and aggregates on the surface. Examples of advanced research using the STM are provided by current studies in the Electron Physics Group at NIST and at the IBM Laboratories. Several other recently developed scanning microscopies also use the scanning technology developed for the STM.
The electron cloud associated with metal atoms at a surface extends a very small distance above the surface. When a very sharp tip--in practice, a needle which has been treated so that a single atom projects from its end--is brought sufficiently close to such a surface, there is a strong interaction between the electron cloud on the surface and that of the tip atom, and an electric tunneling current flows when a small voltage is applied. At a separation of a few atomic diameters, the tunneling current rapidly increases as the distance between the tip and the surface decreases. This rapid change of tunneling current with distance results in atomic resolution if the tip is scanned over the surface to produce an image.
Russell D. Young, of the National Bureau of Standards, was the first person to combine the detection of this tunneling current with a scanning device in order to obtain information about the nature of metal surfaces. The instrument which he developed between 1965 and 1971, the Topografiner, altered the separation between the tip and the surface (z) so that, at constant voltage, the tunneling current (or, at constant current, the tunneling voltage) remained constant as the tip was scanned over the surface. The x, y, and z coordinates of the tip were recorded. (For details of the design and operation of the Topografiner, see the references given in the Bibliography.) The same principle was later used in the scanning tunneling microscope. The remaining barrier to the development of that instrument was the need for more adequate vibration isolation, in order to permit stable positioning of the tip above the surface. This difficult problem in mechanical design was surmounted through the work of Gerd Binnig and Heinrich Rohrer, IBM Research Laboratory, Zurich, Switzerland, who in 1986 shared in the Nobel Prize in Physics for their discovery of atomic resolution in scanning tunneling microscopy. In their announcement of the award, the Royal Swedish Academy of Sciences recognized the pioneering studies of Russell Young.