STM: SCANNING TUNNELING MICROSCOPE

STM (Scanning Tunneling Microscope) is a kind of scanning microscope able to detect the surfaces of the analyzed samples to the atomic level; the principle of operating is based on the tunnel effect. In practice, an ultra-thin probe, usually an electric wire, with a tip composed of a very few atoms of tungsten, platinum-iridium or gold, is approached to the atoms of the sample surface, at a distance approximately equal to 1 nanometer; in this way the overlap is obtained between the electron cloud of the atom, at the end of the probe, and the electron cloud of the atom of the sample surface that has to be analyzed. If a small potential difference is applied to the STM, an electric current will flow between the two electronic clouds; this current is due to quantum tunneling effect and depends on the distance between the probe and the sample. A software keeps constant the current while the probe is raised and lowered, following the contours of the electronic clouds, in order to obtain a topography of the sample surface. This kind of microscope provides maps of conductive samples in high resolutions up to 0,2 nm - 0,6 nm.