Scanning Tunneling Microscopy
The general principle behind Scanning Tunneling Microscopy (STM) is elegant and conceptually simple. A piezo-electric scanner is used to position (with sub-nm precision) an atomically sharp metal tip above a sample. Changing the position in the lateral (x,y) plane allows to scan continuously across the sample surface. Changing the vertical (z) position allows to maintain a desired tip-sample distance. In a typical STM experiment, a bias voltage is applied between the sample and the tip, which for sufficiently small tip-sample distances creates the tunneling current.
The tunneling current depends exponentially on the tip-sample separation. Therefore, if a feedback loop is used to adjust the vertical position of the tip to keep the current constant (constant current scanning mode), tip-sample separation can be kept constant with sub-nm precision. Alternatively the z-coordinate can be held constant and the tunneling current recorded. Since the current is proportional to the density of electronic states in the sample, the first method maps constant density of states contours and the second method maps the actual density of states.
In other words, the values of the three STM parameters: tip-sample distance, bias voltage, and tunneling current, are not independent, which allows to choose any one of them as the measured variable. Alternatively, the chosen parameter can be systematically adjusted, e.g., to increase the tip-sample distance, one can keep the tunneling current constant and increase the bias voltage.