Theoretical study of photon emission from molecular wires

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 the 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 scanning tunneling microscopy 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 (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap and the 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.