STMs(Scanning Tunneling Microscopes) measure the amount of electrons that "tunnel" through a 10Å gap between the tip and the sample surface. The direction of this flow depends upon the sign of the applied bias voltage. (See Figure 1-2) The resulting tunneling current varies with tip-to-sample spacing, and it is the signal used to create an STM image. For tunneling to take place, both the sample and the tip must be conductors or semiconductors. Unlike AFMs, which are discussed in the next section, STMs cannot image insulating materials.

Figure1 : Schematic of tip and sample interaction (Figure Courtesy:Park Scientific Instruments)

STMs can be designed to scan a sample in either of two modes: constant-height or constant-current mode, as shown in Figure 2.

Figure 2. Comparision of constant-height and constant-current mode for STM (Figure Courtesy:Park Scientific Instruments)

In constant-height mode, the tip travels in a horizontal plane above the sample and the tunneling current varies depending on topography and the local surface electronic properties of the sample. The tunneling current measured at each location on the sample surface constitute the data set that can be reconstructed to form a topographic image of the sample surface. In constant-current mode, STMs use feedback to keep the tunneling current constant by adjusting the height of the scanner at each measurement point. For example, when the system detects an increase in tunneling current, it adjusts the voltage applied to the piezoelectric scanner to increase the distance between the tip and the sample. In constant-current mode, the motion of the scanner constitutes the data set. If the system keeps the tunneling current constant to within a few percent, the tip-to-sample distance will be constant to within a few hundredths of an angstrom. Each mode has advantages and disadvantages. Constant-height mode is faster because the system doesn't have to move the scanner up and down, but it provides useful information only for relatively smooth surfaces. Constant-current mode can measure irregular surfaces with high precision, but the measurement takes more time. As a first approximation, an image of the tunneling current maps the topography of the sample. More accurately, the tunneling current corresponds to the electronic density of states at the surface. STMs actually sense the number of filled or unfilled electron states near the Fermi surface, within an energy range determined by the bias voltage. Rather than measuring physical topography, it measures a surface of constant tunneling probability.