How does it work?

If a bias voltage is applied to two electrodes a few tenths of a nm apart, a current flows between them even though they are not in contact. This is due to the quantum mechanical process of electron tunneling. The resulting current depends exponentially on the electrode separation s so that even minute, subatomic variations in s produce measurable current changes. In 1981 Gerd Binnig and Heinrich Rohrer at IBM in Zurich realised that by shaping one of the electrodes as a sharp tip and scanning it across the surface of the other, quantum tunneling can be used to build a microscope with ultra high spatial resolution. Moreover, since the current depends also on the electronic properties of the electrodes, they recognised that this microscope has the ability to probe the electronic density of states of surfaces at the atomic scale. A few years later, Don Eigler at IBM in Almaden, showed that, due to the extremely localised interaction between tip and sample, it is even possible to use this instrument to manipulate individual atoms, to position them at arbitrary locations and therefore to build artificial structures atom-by-atom.

Variations of 1 Å in the tip-sample separation induce changes in the tunneling probability of one order of magnitude. This places stringent requirements on the precision by which the tip position must be controlled, as well as on the suppression of vibrational noise and thermal drift. Moreover, typical tunneling currents are in the 0.01-10 nA range, requiring high gain and low noise electronic components.

STM can be used to analyse clean and adsorbate covered metal surfaces, semiconductors, superconductors, thin insulating layers, small and large organic molecules, individual atoms, liquid-solid interfaces, magnetic layers and surfaces, quasicrystals, polymers, biomolecules, nanoclusters and carbon nanotubes. Sharp metal tips with a low aspect ratio are essential to optimise the resolution of the STM images.

STMs can operate in various environments such as air, inert atmosphere (N₂, Ar), vacuum, high pressure, liquid or in an electrochemical cell.

STM can also be performed at different temperatures (in vacuum or controlled atmosphere chambers): variable temperature STMs (VT-STM) typically cover the 5-700 K range, while low temperature STMs (LT-STM) can operate at 77 K (liquid N₂), 5 K (liquid He) and even at milli-Kelvin (³He-⁴He dilution refrigerators).