For the last lab of the term, we learned how to use the scanning tunneling electron microscope, or STM.

<img src=“/data/posts/93/graphite.png” style=“width: 100%”, alt=“Graphite STM image” />

This is an image of the surface of a flake of graphite–the same stuff pencil lead is made out of. We take a small block of graphite and peel off layers using Scotch tape, then mount them on the bed of the STM. An extraordinarily sharp needle, with a tip only a few atoms thick, is given a small electric charge, and positioned a few angstroms above the sample. Electrons then tunnel across the gap between the needle and the surface at a small rate, yielding a current. We sweep the needle across the surface using piezo crystals, and measure the height/current data at each point. That data yields an image, which we’ve filtered with an FFT to show the structure more clearly.

The regular hexagonal arrangement of the graphite sheets is clearly visible, but it turns out that this image is not of individual atoms. The atoms in a graphite layer form open hexagons, but this image doesn’t show the holes! In fact, the scale is off entirely: the peak-to-peak distance in this image is about .26 nanometers, but the distance between atoms is actually 0.142 nanometers. What this image shows is the distribution of electron cloud probabilities, which, thanks to the underlying hexagonal structure, does show perfect triangular tessellation.