Peeling Back Graphite with a Needle
For the final lab of the term, the task was the scanning tunneling microscope. STM is a tool for surfaces, and graphite makes an ideal subject. A flake is prepped by peeling off layers with Scotch tape and mounting the freshly exposed face on the microscope bed.
The instrument itself sounds almost impossibly delicate. A needle is sharpened until its tip is only a few atoms wide, then given a small electric charge and brought to within a few angstroms of the sample. Electrons cross this gap by quantum tunneling, producing a faint current. As the needle sweeps across the surface via piezo crystals, the height or current is recorded at each point. Filtering that data through an FFT yields a clear picture of what the tip sees.
The result shows the graphite's regular hexagonal pattern in sharp relief. But appearances deceive: this is not a map of individual atoms. The carbon atoms in a graphite layer arrange in open hexagons, yet the image shows no holes at the centers. The scale confirms the mismatch—peak-to-peak spacing here measures about 0.26 nanometers, while the actual interatomic distance is 0.142 nanometers.
What the STM resolves is the electron cloud's probability distribution, a structure shaped by the underlying hexagonal lattice into a perfect triangular tessellation. The atoms themselves remain invisible beneath that cloud; the microscope reads the charge, not the nucleus.
