The principle of the STM is apparently simple : A sharp metallic tip is placed very close, around one nanometer away from the surface of the studied sample. At such a short distance the quantum tunneling phenomenon occurs : since the electron wave-functions have a rapid yet finite decay in vacuum, often referred to as “evanescent waves”, the electrons from the tip can “tunnel” through the tiny vacuum space into the surface. At such a tunnelling condition, if a small voltage difference is applied between the sample and the tip, a net electric current starts flowing - the tunnel current.
The intensity of the tunneling current depends on the overlap of evanescent electron waves of the tip and the surface. With increasing the sample-to-tip distance, the intensity of the tunneling current rapidly decays on a scale of one-two nanometers.
Importantly, the tunneling current flows very locally, only through the very apex of the tip, since, owing the rapid decay of tunnelling probability, other more distant parts of the tip give negligibly small contributions to it. This local aspect of the tunneling current flow in the STM tunneling geometry is the key for the exceptional spatial resolution of this method.
In the STM experiment the tip-to-sample distance is controlled in the real time by a feed-back loop which drives a piezo-electric transducer and approaches or retracts the tip (Z-direction) in order to keep the intensity of the tunnelling current constant. Due to the exponential current-distance dependence, the constant current intensity is usually associated with the constant tip-sample distance. To acquire “topographic” STM maps, another piezo-electric driver scans the tip in the two other (X,Y) directions along the sample surface, while the feed-back signal necessary to keep constant the tunnelling intensity is recorded as a function of lateral position ; it is then presented in a form of grey or artificial colour scale Z(X,Y) images.
In the spectroscopic mode one takes advantage of the bias dependence of the tunneling effect : if the position of the tip is kept fixed and the voltage across the junction is swept, the current intensity I(V) varies depending on local density of electronic states LDOS of the sample, at the positon of the tip.
More precisely, at low enough temperature (and if the LDOS of the tip is constant !) the tunneling conductance dI(V)/V is simply proportional to the sample LDOS. It is therefore possible to get spectroscopic maps, by performing LDOS spectroscopy in each sample location.
While the basic principles of STM are rather straightforward to understand, the fabrication of the apparatus and its use in the high-quality research are much more complex. Since the STM measures the current flowing between the tip apex and the very surface, the method is extremely sensitive to any contamination. That is why in most cases the STM unit is confined inside chambers in which ultrahigh vacuum conditions are achieved. Moreover, the tunneling current is very weak (from a few pico- to a few nano-amps) ; its detection requires a very low-noise electronics. Last but not least, the STM unit is very sensitive to mechanical noise and vibrations, and needs to be properly insulated. As a result, the STM requires a substantial budget and highly qualified man power to run.
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