Conductance enhancement in nanographene-gold junctions by molecular pi-stacking.

First-principles calculations on gold-pentacene-gold and several gold-circumacene-gold nanojunctions indicate that their low-bias conductance is due to the onset of a HOMO-derived resonance, thus being quite sensitive to the detailed interaction between the molecule and the gold leads. It is also found that such interaction is dominated by the electrophilic binding of Au to the (circum)acene, in agreement with previous theoretical and experimental results on pentacene. Therefore, the alignment of the HOMO resonance with the Fermi level, and thus the conductance, increases as the ionization potential and the HOMO-LUMO gap of the molecular arrangement diminish. It is shown here that both quantities are inversely proportional to the molecular length and the number of molecules present on a pi stack. It is also found that the conductance depends dramatically on the amount of pi overlap between the molecules in the stack, as well as on the particular disposition of the metallic tips with respect to the molecule. The conclusions reached point toward pi-stacked arrangements of large circumacenes as potential candidates to build useful nanodevices for molecular electronics made out of nanographene-based materials.