The peptide has four ligation sites for heme binding. The heme should provide a single electronic state near the fermi level for electron transport. The surrounding peptide material is generally considered to be insulating, so we expect the presence of hemes to enhance the tunneling current between the tip and substrate in a certain bias regime. Our constant-current topographic images should show a raising of the tip above the heme to compensate for the increased tunneling, as shown on the left. These give us information about the density of states in the peptide, and the effective tunnel barrier.
Some additional peptides are visible on top of the monolayer.
Tail' on lower left of peptide indicates that the molecule was displaced during imaging. Our substrates are gold which has been thermally evaporated onto mica, and then annealed in a gas flame. The peptides are then self-assembled onto these substrates from solution. We have found that although the peptide itself is expected to be insulating, it exhibits relatively strong contrast in STM images. Therefore, it is a fairly simple matter to image the peptide monolayers and study their structure, even under ambient conditions.