Opportunities and limitations of transition voltage spectroscopy: a theoretical analysis

TL;DR

This paper provides a theoretical analysis of transition voltage spectroscopy (TVS) in molecular electronics, highlighting how screening effects and molecular length influence its effectiveness and limitations in probing molecular energy levels.

Contribution

It offers a detailed theoretical investigation into the dependence of transition voltage on molecular length and screening, clarifying the conditions under which TVS is predictive.

Findings

Transition voltage depends on molecular length and screening effects.

Strongly screening molecules show a constant ratio between transition voltage and frontier orbital.

TVS can estimate level position and asymmetry in single-level transport, but is limited with multiple levels.

Abstract

In molecular charge transport, transition voltage spectroscopy (TVS) holds the promise that molecular energy levels can be explored at bias voltages lower than required for resonant tunneling. We investigate the theoretical basis of this novel tool, using a generic model. In particular, we study the length dependence of the conducting frontier orbital and of the 'transition voltage' as a function of length. We show that this dependence is influenced by the amount of screening of the electrons in the molecule, which determines the voltage drop to be located at the contacts or across the entire molecule. We observe that the transition voltage depends significantly on the length, but that the ratio between the transition voltage and the conducting frontier orbital is approximately constant only in strongly screening (conjugated) molecules. Uncertainty about the screening within a molecule thus limits the predictive power of TVS. We furthermore argue that the relative length independence of the transition voltage for non-conjugated chains is due to strong localization of the frontier orbitals on the end groups ensuring binding of the rods to the metallic contacts. Finally, we investigate the characteristics of TVS in asymmetric molecular junctions. If a single level dominates the transport properties, TVS can provide a good estimate for both the level position and the degree of junction asymmetry. If more levels are involved the applicability of TVS becomes limited.