3.2 The principles of scanning probe microscopes
Scanning probe microscopy is a term that is applied to a set of imaging methods based on a common element: a fine stylus. In many ways, what scanning probe microscopes do is similar to what a gramophone does. A gramophone stylus scans a spiral groove (by travelling along it) on which information has been encoded in the form of undulations in the groove wall. Side-to-side and up-and-down movements of the stylus (which is mounted on one end of a rod supported and pivoted at its centre) as it follows these undulations are transferred, see-saw fashion, to a magnet mounted on the other end of the rod. This magnet lies in the space between coils mounted vertically and horizontally. As the magnet moves, it induces currents in the coils that are then amplified in the audio system, ultimately reproducing the sound encoded in the groove of the record.
In the case of scanning probe microscopy, the scanning is done by moving the stylus over the surface to be imaged in a raster pattern. To move the probe tip and the sample laterally relative to one another, either the probe or the sample is mounted on the top of a piezoelectric tube (see Figure 6).
The phenomenon of piezoelectricity, in which certain materials change their dimensions in response to an electric field and vice versa, is described in more detail in Section 4 Piezoelectricity: motion from crystals.
This tube bends slightly in the left–right or forwards–backwards direction when voltages are applied across appropriate electrodes on it. It is possible to control the deflection of this type of actuator to a very high degree of precision and, because it is made from a single piece of material, it suffers much less than any other electromechanical system from inaccuracy of motion due to things such as friction, backlash, etc., that plague gears and bearings.
By measuring either a current through, or a deflection of, the stylus as it scans across the sample, a signal related to some aspect (not necessarily just the height) of the surface is then recovered. By giving the stylus a precisely shaped tip with a tiny radius of curvature, it is possible in some cases to obtain images that show individual atoms on the surface of the sample being imaged.