Scanning Tunneling Microscopy (STM)

The scanning tunneling microscope (STM) is the ancestor of all scanning probe microscopes. Invented in 1981 at IBM Zurich, it was the first instrument to generate real-space images of surfaces with atomic resolution. The inventors, Gerd Binnig and Heinrich Rohrer, were awarded the Nobel Prize in Physics five years later for this invention.

STM is based on the fact that the tunneling current between a conductive tip and sample is exponentially dependent on their separation. This can be represented by the equation: I ~ Ve ^-cdwhere:

• I = tunneling current
• V = bias voltage between tip and sample
• c = constant
• d = tip/sample separation distance

As the tip scans the sample surface, it encounters sample features of different heights, resulting in an exponential change in the tunneling current. A feedback loop is used to maintain a constant tunneling current during scanning by vertically moving the scanner at each (x,y) data point until a “setpoint” current is reached. The vertical position of the scanner at each (x,y) data point is stored by the computer to form the topographic image of the sample surface.

Figure 1: Feedback loop maintains constant tunneling current.

This technique is typically limited to conductive and semiconducting surfaces.

Applications:

• Atomic resolution imaging (STM is the only technique that detects atomic-scale defects)
• Electrochemical STM
• Scanning Tunneling Spectroscopy
• Low-current imaging of poorly conductive samples

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