Scanning Tunneling Microscopy

7. Scanning Tunneling Microscopy

The scanning tunneling microscope (STM) has the capability to directly image atoms on a surface. It was developed in the early 1980’s by two scientists at IBM. Since its invention, the STM has become a widely-used tool to study surfaces at the atomic scale. Among other applications, it has been used to investigate deposition and etching of materials, to map surface electronic structure, and to investigate electron transport in thin films. The STM can even be used in some cases to manipulate individual atoms and molecules and build designer atomic structures.

The STM is a non-optical microscope which employs principles of quantum mechanics. A very fine probe is moved over the surface of the material under study, and a voltage is applied between probe and the surface. Depending on the voltage and its characteristics electrons will "tunnel" (this is a quantum-mechanical effect) or jump from the probe to the surface (or vice-versa depending on the polarity), resulting in a weak electric current. The size of this current is exponentially dependent on the distance between probe and the surface. Obviously, for a current to occur the substrate being scanned must be conductive. Insulators can not be scanned through the STM. By scanning the probe over the surface and measuring the current, one can thus reconstruct the surface structure of the material under study. Adjustments of the distance between probe and surface are done using a servo loop and converse piezoelectricity.

The topography of the surface is displayed on a computer screen, typically as a gray-scale image, see Fig. 1(a). The gray-scale image is similar to a black-and-white television picture. Usually, the bright spots represent high z values (protrusions), and the dark spots represent low z values (depressions). The z values corresponding to the gray levels are indicated by a scale bar. For a more quantitative representation of the topography, a contour plot along a given...