Theoretical calculations of electron transport in molecular junctions: Inflection behavior in Fowler-Nordheim plot and its origin

We investigated the origin of an inflection behavior appearing in Fowler-Nordheim (F-N) plot of current-voltage characteristics for molecular junctions using two different levels of calculation methods: nonequilibrium Green's-function technique combined with the density-functional theory and tight-binding approximation. Although the inflection has so far been interpreted from the naive model that the charge transport mechanism transits from a direct to the F-N tunneling, our results indicated that the inflection does not necessarily mean the transition between the two regimes. We found from the close examination of the relation between the behavior of the F-N curve and the transmission function that the inflection takes place when the molecular level responsible for electric currents approaches to the edge of the bias window. While our interpretation for the inflection drastically differ from the conventional model, the F-N plots obtained from our calculations showed closely similar behavior as those from the recent experiments.