ZANG group research

department of chemistry and biochemistry SIU

Scanning Tunneling Microscopy (STM)

As the first member of the scanning probe microscopy (SPM) family, scanning tunneling microscopy (STM) was invented in 1982 by Gerd Binnig and Heinrich Rohrer at IBM in Zurich (Phys. Rev. Lett., 1982, vol 49, p57). The invention won them the Nobel prize in physics just four years later. 

STM works by scanning a very sharp metal wire tip over a surface. The feedback of STM is based on electrical interaction between the tip and the surface. As the tip gets close to the surface (must be electrical conductive), a tunneling current can be generated between the tip and surface. This is so-called ‘tunneling effect’, which occurs only when the tip is close enough to the surface and an appropriate bias voltage is applied between the two. By monitoring the tunneling current, the feedback loop adjusts the tip height from the surface in such a way that the current always stays constant. Because the current is proportional to the local density of states, the scanning of the tip follows a contour of a constant density of surface states. The raster scanning of a whole area generates a 3-dimensional map of surface electronic states, similar tot the topographic image of AFM. The Z-direction (height of tip) represents the surface density of electronic states. Typically, when imaging a crystalline surface, the peak position (on X-Y) refers to a local atom.

Similarly to AFM, the spatial resolution of STM measurement depends on the precision of the piezo motion. The recent developed piezoelectric ceramics afford scanning step as small as a angstrom. This allows us to precisely control the tip position and height, and thus atomic resolution can be feasibly achieved, even at room temperature.

Tunneling is a typical quantum mechanical effect. Click here for more details about ‘tunneling’ effect.

(updated on june 24, 2004)