Model-Based Control of the Scanning Tunneling Microscope: Enabling New Modes of Imaging, Spectroscopy, and Lithography

The principle of scanning tunneling microscopy is based on the quantum mechanical phenomenon of “tunneling,” whereby the wavelike properties of electrons allow them to “tunnel” beyond the surface of a solid into regions of space that are forbidden under the rules of classical physics [1]. This phenomenon is used in the scanning tunneling microscope (STM), where electrons tunnel from the apex of a sharp tip to a conducting surface held at a different potential (see “Summary”). The probability of electrons tunneling decreases exponentially as the distance between the two surfaces increases [2]. Hence, electrons tunnel mostly from the very last atom on the tip apex to the surface. The STM makes use of this extreme sensitivity to distance. A positioner brings the sharp tip of a tungsten probe to a distance of a few angstroms from the sample surface. A bias voltage is applied between the probe tip and the surface, causing electrons to tunnel across the gap.