3 Building atomic force microscope probes
The atomic force microscope is a key visualisation tool for the ‘invisible’ world of micro and nano technology. Within it, right at the heart, is a probe tip that is itself a triumph of nanotechnology.
This section is going to begin with a fair amount of detail about how scanning probe microscopes of various types work, starting with a description of the scanning tunnelling microscope (STM). After that I want to concentrate on its close relative, the atomic force microscope. Then we can get down to micromechanics to see how the atomic force probe is expected to perform, finally addressing the question of how to make the probe tip. It is worth knowing some of the differences between the ways in which different probes interrogate surfaces, because although superficially the pictures look similar (in that they tend to be bumpy relief maps that look like mountain landscapes), what they show is not always the shape of the surface. In the case of the STM, for instance, the image is a combination of the topography of the surface and something called the surface distribution of the local density of states – often abbreviated to LDOS – which is essentially where the electrons of the surface atoms are most likely to be.
Because these energy states (electron orbitals) are centred on the atoms themselves, the images produced look very like those of atoms on a surface. This can be deceptive, as atoms can disappear from view if they are bonded to others in such a way that they don't produce a strong density-of-states signal. A good example of this is the graphite surface shown in Figure 5, where half the carbon atoms are absent from the image. This is because not all carbon atoms are equal in this case, with some, on what are known as the A sites, being bonded more strongly than the others to the sheet of carbon atoms on the layer below.