Image of substitutional Cr impurities (small bumps) in the Fe(001) surface.
The scanning tunneling microscope (not to be confused with scanning electron microscopes), or STM, was invented in 1981 by Gerd Binnig and Heinrich Rohrer of IBM's Zurich Lab in Zurich, Switzerland. Although initially greeted with considerable scepticism by materials scientists in the early 1980s, the invention garnered the two a Nobel Prize in Physics in 1986. The STM allows scientists to visualize regions of high electron density and hence infer the position of individual atoms, where previously arduous study of diffraction patterns from prior methods lead to much debate as to the real, spatial lattice structure of the item in question. The STM has higher resolution than its slightly later invented but nevertheless related cousin, the atomic force microscope (AFM). Both the STM and the AFM fall under the class of scanning probe microscopy instruments.
It is used to obtain images of conductive surfaces at an atomic scale 2 × 10−10 m or 0.2 nanometre. It can also be used to alter the observed material by manipulating individual atoms, triggering chemical reactions, and creating ions by removing individual electrons from atoms and then reverting them to atoms by replacing the electrons.
The acronym STM is used for both scanning tunneling microscope and scanning tunneling microscopy.
Overview
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. Obviously, for a current to occur the substrate being scanned must be conductive. Insulators cannot be scanned through the STM.
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.
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