Revealing molecular orbital gating by transition-voltage spectroscopy

Recently, Song et al [Nature 462, 1039 (2009)] employed transition-voltage spectroscopy to demonstrate that the energy $\varepsilon_H$ of the highest occupied molecular orbital (HOMO) of single-molecule transistors can be controlled by a gate potential $V_G$. To demonstrate the linear dependence $\varepsilon_H-V_G$, the experimental data have been interpreted by modeling the molecule as an energy barrier spanning the spatial source-drain region of molecular junctions. Since, as shown in this work, that crude model cannot quantitatively describe the measured $I$-$V$-characteristics, it is important to get further support for the linear dependence of $\varepsilon_H$ on $V_G$. The results presented here, which have been obtained within a model of a point-like molecule, confirm this linear dependence. Because the two models rely upon complementary descriptions, the present results indicate that the interpretation of the experimental results as evidence for a gate controlled HOMO is sufficiently general.