Revealing molecular orbital gating by transition-voltage spectroscopy

TL;DR

This paper confirms that the energy of the HOMO in single-molecule transistors varies linearly with gate potential, supporting the interpretation of transition-voltage spectroscopy data as evidence of molecular orbital gating.

Contribution

It provides a theoretical validation of the linear dependence of HOMO energy on gate voltage using a point-like molecule model, supporting previous experimental interpretations.

Findings

HOMO energy varies linearly with gate voltage

Model confirms experimental observations

Supports the interpretation of molecular orbital gating

Abstract

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.