Scanning Tunnelling Microscope (STM)

The Scanning Tunnelling Microscope is one of the most important tools invented that allowed for nanotechnology to evolve into what it is today. It was invented in 1981 by two physicists, Heinrich Rohrer and Gerd Binnig, in IBM’s Zurich Lab in Switzerland. In 1986, they were awarded a Nobel Prize in physics for their accomplishment.
The STM belongs to a class of scanning probe microscopy instruments, and allows scientists to see conductive surfaces at 0.2 nm. Furthermore, it also allows for the manipulation of the material observed through the removal and replacing of ions.
How It Functions
A non-optical microscope, the STM instead employs the principles of quantum mechanics. As the tip of the STM is moved over the surface of the observed material, a voltage is applied between a probe in the tip and the surface. Depending on the voltage, the electrons will tunnel (jump) from the tip to the surface or the surface of the tip, creating a weak electric current, whose magnitude is dependent on the distance between the tip and the surface. However, since this can only occur if the scanned substance is electrically conductive, insulators cannot be scanned by the STM and another method must be applied. By adjusting the distance between the surface and the tip with a piezoelectric element (a crystal that generates a voltage as a response to an applied mechanical stress) so that the current is constant, the surface observed can be reconstructed.

Diagram: Scanning Tunnelling Microscope

Applications in Nanotechnology
To avoid contamination and other complications, the STM is frequently operated in vacuums with pressures lower than 10-9 torr (we normally live with an atmospheric pressure of approximately 760 torr) and temperatures around 4K (almost absolute 0) Because it is both an observing and modification tool, the STM can be used in many functions including dragging atoms on the surface with its tip and examining the charge transport mechanisms of nanoparticles. 

References

"Molecules that suck." PhysOrg.com 21 Nov 2005. 25 Nov 2005 http://www.physorg.com/news8356.html