Scanning Tunneling Microscope
6
Scanning Probe Microscope Training Notebook
Rev. F
Figure 2.0a
Feedback Loop Maintains Constant Tunneling Current
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
-cd
•
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.
This technique is typically limited to conductive and semiconducting surfaces
