Electronic structure and STM image of C-60-C-70 adsorbed on the Cu(111) surface

A recent scanning tunneling microscopy (STM) experiment has revealed that C-60-C-70 mixture on the Cu(111)-(1 x 1) substrate shows specific bias-voltage-dependent images which reflect intramolecular structure. In order to understand this observation, we perform a band-structure calculation. In this study, we adopt a simple model assuming a two-dimensional (2D) molecular crystal and a proper adsorption geometry. By comparing the calculated charge distributions of the lowest unoccupied molecular orbital (LUMO)-induced bands and the highest occupied molecular orbital (HOMO)-induced bands with the observed STM images, we find that the shapes of the intramolecular structures which change with the bias voltage strongly reflect the positions of pentagonal and hexagonal rings.