Scanning tunneling microscope

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Image of reconstruction on a clean Au(100) surface.

The scanning tunneling microscope (STM) is a non-optical microscope that scans an electrical probe over a surface to be imaged to detect a weak electric current flowing between the tip and the surface. The STM (not to be confused with the scanning electron microscope) was invented in 1981 by Gerd Binnig and Heinrich Rohrer of IBM's Zurich Lab in Switzerland. Although initially greeted with some skepticism by materials scientists, the invention garnered the two a Nobel Prize in Physics (1986). The STM allows scientists to visualize regions of high electron density and hence infer the position of individual atoms and molecules on the surface of a lattice. Previous methods required arduous study of diffraction patterns and required interpretation to obtain spatial lattice structures. The STM is capable of higher resolution than its somewhat newer cousin, the atomic force microscope (AFM). Both the STM and the AFM fall under the class of scanning probe microscopes.

The STM can obtain images of conductive surfaces at an atomic scale 2 × 10⁻¹⁰ m or 0.2 nanometer, and also can be used to manipulate individual atoms, trigger chemical reactions, or reversibly produce ions by removing or adding individual electrons from atoms or molecules.

A closeup of a simple scanning tunneling microscope head at the University of St Andrews scanning MoS2 using a Platinum-Iridium stylus.

A closeup of a simple scanning tunneling microscope head at the University of St Andrews scanning MoS₂ using a Platinum-Iridium stylus.

The acronym STM can mean either scanning tunneling microscope or scanning tunneling microscopy. This microscope has an extremely sharp stylus that scans the surface. The stylus is so sharp that its tip consists only of one atom. Strictly, as the tunnelling current is such a short ranged phenomenon (which is what gives STM its impressive resolution), tunnelling normally only occurs through the furthest extremity of the stylus - which might itself appear to be rather blunt on a larger scale.

Schematic view of an STM

The STM is a non-optical microscope which employs principles of quantum mechanics. An atomically sharp probe (the tip) is moved over the surface of the material under study, and a voltage is applied between probe and the surface. Depending on the voltage electrons will "tunnel" (this is a quantum-mechanical effect) or jump from the tip 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. For a current to occur the substance being scanned must be conductive (or semiconductive). Insulators cannot be scanned through the STM, as the electron has no available energy state to tunnel into or out of due to the band gap structure in insulators.

A servo loop (feedback loop) keeps the tunneling current constant by adjusting the distance between the tip and the surface (constant current mode). This adjustment is done by placing a voltage on the electrodes of a piezoelectric element. By scanning the tip over the surface and measuring the height (which is directly related to the voltage applied to the piezo element), one can thus reconstruct the surface structure of the material under study. High-quality STMs can reach sufficient resolution to show single atoms. The STM will get within a few nanometers of what it is observing.