First-principles study of quasiparticle energies of a bipolar molecule in a scanning tunneling microscope measurement

Quasiparticle energies of a bipolar molecule, bithiophene naphthalene diimide (BND), on a substrate under conditions in typical scanning tunneling microscopy (STM) measurements are computed within the GW approximation. The calculated HOMO–LUMO gap is compared with the value determined from the dI/dV spectra of STM experiments. Electron screening from the substrate and STM tip are included within an image charge framework. It is shown that the influence of a metallic substrate is strongly modified by an atomically thin dielectric film and that the presence of the STM tip gives a non-negligible contribution to the quasiparticle energies. Quantitative agreement with experiment is achieved after taking into account the aforementioned effects.