Scanning tunneling microscope

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. The invention garnered the two a Nobel prize for physics in 1986. The STM allows scientists to see and position individual atoms with higher resolution than its related cousin, the atomic force microscope (AFM). Both the STM and the AFM fall under the class of scanning probe microscopy instruments.

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.

Related Topics:
Quantum mechanics - Tunnel

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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.

Related Topics:
Feedback loop - Piezoelectric element

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