Theoretical study of photon emission from molecular wires

TL;DR

This theoretical study models photon emission from single-molecule conductors using a tight-binding approach, highlighting conditions for electroluminescence and its potential to reveal electronic structure details.

Contribution

It introduces a simple way to include bias-dependent electronic structures and predicts photon emission sensitivity to contact asymmetries, aiding molecular electronics research.

Findings

Photon emission rate is more sensitive to contact asymmetries than current.

Electroluminescence can reveal HOMO-LUMO gap and Fermi level positions.

Feasibility of observing photon emission from Au/benzene-dithiolate wires is discussed.

Abstract

We explore theoretically the principles that govern photon emission from single-molecule conductors carrying electric currents between metallic contacts. The molecule and contacts are represented by a generic tight-binding model. The electric current is calculated using Landauer theory and the photon emission rate is obtained using Fermi's golden rule. The bias-dependence of the electronic structure of the molecular wire is included in the theory in a simple way. Conditions under which significant photon emission should occur are identified and photon spectra are calculated. We predict the photon emission rate to be more sensitive than the electric current to coupling asymmetries between the molecule and contacts. This has important implications for the design and interpretation of STM experiments searching for electroluminescence from individual molecules. We discuss how electroluminescence may be used to measure important characteristics of the electronic structure of molecular wires such as the HOMO-LUMO gap and location of the Fermi level of the contacts relative to the HOMO and LUMO. The feasibility of observing photon emission from Au/benzene-dithiolate molecular wires is also discussed.