Electrochemical NanoScience Group
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STM scheme STM tunneling

Scanning tunneling microscopy (STM)

The technique of scanning tunneling microscopy (STM) was a first one which made possible visualization of nanoscale structures such as atoms and molecules on the surface. Invented already 25 years ago in IBM Zürich Research Laboratory, it quickly became a popular tool to characterize surfaces in ultra-high vacuum (UHV) and solid-liquid interfaces. In 1986 original inventors of STM, Gerd Binnig and Heinrich Rohrer were awarded a Nobel Prize in Physics for their invention. The STM technique is based on the effect of electron tunneling, which is well known from quantum mechanics. When a freely moving electron incidents an energy barrier, it may penetrate through it with a certain probability, which depends on barrier height and distance between electrodes.

In STM a sharp metal tip, or probe, is brought into contact with a conductive surface. The potential difference (bias V) is applied between tip and sample. The current flowing between tip and sample is proportional to tunneling probability and bias.

The probe is moved over nanoscale distance by motors based on ceramics exhibiting piezo-effect: expansion or shrinking under applied voltage. The tip is scanned across the surface with the help of the piezo-elements moving it in lateral directions x and y while the tip-sample separation z is controlled by another piezo element. Two operational modes of STM are possible. In constant current mode the setpoint tunnel current IT is kept constant during the lateral scanning via a feedback circuit by adjusting and recording vertical displacement of the tip. In constant height mode the vertical position of tip is kept constant during scanning while tunneling current is recorded. z or IT as a function of tip position may be presented as a 3D or 2D image. STM allows resolution in sub-nm scale. The resolution ability of STM is due to the strong dependence of the tunnel current on the distance between tip and sample. With a typical barrier height of few eV, an increase of z by a size of single atom of metal (0.2-0.5 nm) leads tunnel current up to three orders of magnitude. Due to the distance sensitivity only the most protruding part of the sharp tip acts as a probe. Few additional prerequisites are required to use STM under electrochemical control. The potential of tip and sample must be controlled independently with respect to reference electrode, which requires special device (bipotentiostat). To avoid current of electrode reaction, STM tip must be isolated except of its very apex. STM experiments in our group are carried out with a single crystal substrates, which are specially treated before experiments.
Revised: 14.12.2007     ©: 2005-2007