Experimental Setup

The most crucial part of the STM is its metal tip, which is usually made from tungsten or platinum/iridium alloy wires. The sample is mounted opposite this tip on a sample holder that can be driven in x-,y- and z-direction by means of piezo-motors. A feedback loop monitors the tunneling current continuously, allowing controlled movement of the surface relative to the tip with an accuracy of a few Å(see fig. 4).

Figure 4: General setup of an STM

Most STMs can be operated in scanning modes (see fig. 5):

Figure 5: The different scanning modes applied in STM

• Constant Current Mode
  The tip is moved across the surface while a constant tunneling current is maintained by the feedback loop. The voltage U$_{z}$ applied to the piezo-driven z-actuator to keep constant current depends on the electronic properties of the sample surface at the position of the tip. As the electronic structure of the surface is correlated with its atomic structure, an image of the sample surface s obtained by plotting maps of U$_{z}$ as a function of the x- and y-coordinates.

• Constant Distance Mode
  In this mode, U$_{z}$ is kept at a constant value, i.e., the tip is moved at a constant z-position. The distance between tip and sample surface thus varies as the tip is moved over the sample. The tunneling current is a function of the distance between tip and sample so that it can be used to show the structures. Note that this method can only be applied for samples flat at an atomic scale, as otherwise there is a high risk of crashing the tip into corrugations at the sample surface. A major advantage of this mode is its higher scan rate, which can help to reduce the effect of thermal drift on the measurement.