Using the scanning tunneling spectroscopy (STS), surfaces can be displayed in a high-resolution manner, since information on the density of states of the sample surface is contained in the tunnel current.
Schematic representation of the tunneling process.
The majority of the electrons tunnel at the Fermi edge of the tip through a potential peak at a positive sample voltage (see Fig.). If these electrons can tunnel at the appropriate voltage into an unoccupied surface state of the sample, the tunnel current increases dramatically. In the case of tunnel current spectroscopy, the dependence of the dI / dU signal on the applied sample voltage is investigated. The dI / dU signal is proportional to the density of states of the tip and sample and a material- or structure-dependent spectrum results.
For the measurement of tunnel current spectra, the tip is positioned at the point to be examined and the control loop is deactivated. This means that the peak-to-probe distance is constant during the measurement. The distance for the measurement of the spectra is determined by the choice of the stabilization parameters (gap voltage and tunnel current setpoint) before the start of the measurement. When measuring a spectrum, the voltage is varied within a predefined interval in discrete steps and the current is measured as a function of the voltage I (U). As shown in the figure below, a differential dI / dU (U) tunneling capability spectrum is obtained by subsequently differentiating the measured I (U) curve.
(Upper diagram) tunneling conductivity as a function of the applied voltage.
(Lower diagram) differential tunneling conductivity.
In order to obtain a spectroscopic image of the surface, the dI / dU signal is plotted pixel by pixel as brightness information in an image. If the sample voltage is selected so that, for example, atom B (see below) is located on a peak in the tunnel spectrum, regions of this atomic species appear to be lighter in the spectroscopic image than that of the atom A.
