nanoREVTM STM – is a Scanning Tunneling Microscope for imaging surfaces at very high magnifications – down to the scale of individual atoms. The STM does this by mechanically scanning a pointed tip over the surface (sample to be scanned). Piezoelectric elements can provide the necessary infinitesimal translations of the tip.
The exponential dependence of tunneling current and the separation between the tip and the sample give the STMs its remarkable sensitivity.
The process of acquiring information about the surface profile happens during the scanning process. Two distinct STM scanning modes are described below:
CONSTANT HEIGHT MODE
The tip travels in a horizontal plane above the sample and the tunneling current varies depending on topography and the local electronic properties of the sample. The tunneling current measured at each location on the sample surface is represented as a pixel in the STM image.
CONSTANT CURRENT MODE
STM uses a feedback control mechanism to keep the tunneling current constant, around a reference, by adjusting the height of the tip at each measurement location. Thus, the feedback error voltage which is used to keep the tunnel current constant, represents the STM image.
You can see in the movie below, live images taken of an STM while scanning in Constant Current Mode, captured by a Scanning Electron Microscope.