Scanning Tunneling Microscopy

Scanning tunneling microscopy (STM) is a scanning probe technique for the imaging of surfaces and interfaces of solids or liquids in real space.

Measuring principle

STMThe measuring principle of the STM is based on the quantum mechanical tunneling effect. A thin metal tip and an electrically conductive sample will be approximated to about 3 to 10 Å, so that the wave functions of the tip may overlap with those of the sample. Ideally, the tip consists of a single atom. After applying a voltage U (in the order of a couple of V) an exponentially dependent on the distance d current I (in the order of nA) of tunnel electrons is measured. In a simple estimate this current I can be described as follows:

                     I(d) ~ exp(-A*√Phi*d)

Here, A ≈ 1 / (Å * √eV) is a constant and Phi represents the barrier height of the tunnel junction, which corresponds to the average work function (only in this approximation) of tip and sample. The tunnel current I is mainly caused by electrons near the Fermi level EF for which the effective barrier height is lowest. The ideally mono-atomic tip is moved line by line over the sample using piezoceramic actuators. The interaction variable can be kept constant by a control loop. The manipulated variable of the control loop as a function of x and y position of the probe represents a contour image of constant interacting forces. For homogeneous samples this represents in a good approximation the actual topography. In order to prevent a rapid contamination of the surface most of the measurements are carried out under vacuum.

The control loop

In order to keep the distance between the tunneling tip and the sample surface constant, the tunneling current I is kept constant by an electronic control loop. The control loop measures the instantaneous tunneling current I (actual value) and compares it with the preset value (target value). If these differ the controller attempts to eliminate the difference via its output (control value). The required changes are transmitted by piezoelectric elements which can move the cantilever in the three spatial directions. The controller settings depend on the entire system and therefore cannot be set arbitrarily high. Moreover, the control loop in itself represents an oscillatory system with a cutoff frequency above which natural oscillations occur. These natural oscillations can damage cantilever and sample. The scanning speed may greatly affect the quality of the AFM image. Too high a speed usually degrades the image. One must also consider that the control rate is closely coupled to the scanning speed.

The tunneling tip

The tunneling tips used have according to electron micrographs radii of curvature of some 10 nm at the tip. Since atomic resolution on the STM can be achieved this can be explained only by the presence of so-called mini tips. This means that almost the whole tunnel current is flowing solely over the closest to the sample mini tip. The arrangement of these mini tips on the cantilever is by no means stable, but can change during the measurements. This can have both positive and negative effects on the resolution and the overall quality of the images. The tunneling tips are usually made of tungsten (etching a piece of wire) or a platinum-iridium alloy (tearing off a piece of wire).

For more information see here.


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