Diode-like I-V characteristics of a nonplanar polyaromatic compound: a spectroscopic study of isolated and stacked dibenzo[g,p]chrysene.

In this scanning-tunneling-microscopy/spectroscopy study (STM/STS), samples of isolated and close-packed dibenzo[g,p]chrysene (DBC), a nonplanar polyaromatic compound, are used as model systems to demonstrate the effect of intermolecular interactions on the electronic structures. For dropcast films, DBC molecules adopt an edge-on orientation in a close-packed structure on graphite. Isolated DBC molecules are prepared on graphite from a DBC-coated STM tip by a ca. 7 V/10 μs pulse. STS spectra for both isolated- and close-packed DBC molecules exhibit diode-like I-V curves in which the latter shows a turn-on voltage (0.47 V) smaller than that of the former (0.91 V). The diode-like behaviors are attributed to the more-facile tunneling of electrons through the HOMO of DBC than through the LUMO. The reduced turn-on voltage for the films is ascribed to the diminished HOMO-LUMO gap based on the results of DFT (density functional theory) simulations for the energy-level couplings of π-stacked DBC molecules.