Scanning tunneling microscope

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Image of substitutional Cr impurities (small bumps) in the Fe(001) 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 Zurich, Switzerland. Although initially greeted with some scepticism 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.

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.

[edit] Overview

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. The scanning tunneling microscope (STM) is widely used in both industrial and fundamental research to obtain atomic-scale images of metal surfaces. It provides a three-dimensional profile of the surface which is very useful for characterizing surface roughness, observing surface defects, and determining the size and conformation of molecules and aggregates on the surface. Examples of advanced research using the STM are provided by current studies in the Electron Physics Group at NIST and at the IBM Laboratories. Several other recently developed scanning microscopies also use the scanning technology developed for the STM.

The electron cloud associated with metal atoms at a surface extends a very small distance above the surface. When a very sharp tip--in practice, a needle which has been treated chemically or mechanically so that a single atom projects from its end--is brought sufficiently close to such a surface, there is a strong interaction between the electron cloud on the surface and that of the tip atom, and an electric tunneling current flows when a small voltage is applied. At a separation of a few atomic diameters, the tunneling current rapidly increases as the distance between the tip and the surface decreases. This rapid change of tunneling current with distance results in atomic resolution if the tip is scanned over the surface to produce an image.

Russell D. Young, of the National Bureau of Standards (now NIST), was the first person to combine the detection of this tunneling current with a scanning device in order to obtain information about the nature of metal surfaces. The instrument which he developed between 1965 and 1971, the Topografiner, altered the separation between the tip and the surface (z) so that, at constant voltage, the tunneling current (or, at constant current, the tunneling voltage) remained constant as the tip was scanned over the surface. The x, y, and z coordinates of the tip were recorded. (For details of the design and operation of the Topografiner, see the references given in the Bibliography.) The same principle was later used in the scanning tunneling microscope. The remaining barrier to the development of that instrument was the need for more adequate vibration isolation, in order to permit stable positioning of the tip above the surface. This difficult problem in mechanical design was surmounted through the work of Gerd Binnig and Heinrich Rohrer, IBM Research Laboratory, Zurich, Switzerland, who in 1986 shared in the Nobel Prize in Physics for their discovery of atomic resolution in scanning tunneling microscopy. In their announcement of the award, the Royal Swedish Academy of Sciences recognized the pioneering studies of Russell Young.