Bias-Controlled Selective Excitation of Vibrational Modes in Molecular Junctions: A Route Towards Mode-Selective Chemistry

TL;DR

This paper demonstrates that by applying an external bias voltage to asymmetric molecular junctions, specific vibrational modes can be selectively excited, enabling mode-specific chemical reactions.

Contribution

It introduces a theoretical framework showing how bias voltage can control vibrational mode excitation in single-molecule junctions, advancing mode-selective chemistry techniques.

Findings

Selective vibrational excitation is achievable in asymmetric junctions.

Both Green's function and master equation approaches agree on key transport features.

Non-resonant processes like co-tunneling influence vibrational energy distribution.

Abstract

We show that individual vibrational modes in single-molecule junctions with asymmetric molecule-lead coupling can be selectively excited by applying an external bias voltage. Thereby, a non-statistical distribution of vibrational energy can be generated, that is, a mode with a high frequency can be stronger excited than a mode with a lower frequency. This is of particular interest in the context of mode-selective chemistry, where one aims to break specific (not necessarily the weakest) chemical bond in a molecule. Such mode-selective vibrational excitation is demonstrated for two generic model systems representing asymmetric molecular junctions and/or scanning tunneling microscopy experiments. To this end, we employ two complementary theoretical approaches, a nonequilibrium Green's function approach and a master equation approach. The comparison of both methods reveals good agreement in describing resonant electron transport through a single-molecule contact, but also highlights the role of non-resonant transport processes, in particular co-tunneling and off-resonant electron-hole pair creation processes.