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We explore theoretically the electroluminescence of single molecules. Weadopt a local-electrode framework that is appropriate for scanning tunnelingmicroscopy (STM) experiments where electroluminescence originates fromindividual molecules of moderate size on complex substrates: Couplings betweenthe STM tip and molecule and between the molecule and multiple substrate sitesare treated on the same footing, as local electrodes contacting the molecule.Electron flow is modelled with the Lippmann-Schwinger Green function scatteringtechnique. The evolution of the electronic energy levels of the molecule underbias is modelled assuming the total charge of the molecule to be invariant,consistent with Coulomb blockade considerations, but that the electronicoccupations of the molecular HOMO and LUMO levels vary with changing bias. Thephoton emission rate is calculated using Fermi's golden rule. We apply thistheoretical approach to the STM/Zn-etioporphyrin/Al2O3/NiAl(110) system, andsimulate various configurations of coupling strength between the molecule andsubstrate. We compare our results to the experimental observations of Qiu,Nazin and Ho [Science 299, 542 (2003)] for this system and find that our modelprovides a comprehensive explanation of a multitude of previously unexplainedobservations. These include the different types of current-voltagecharacteristics (CVC's) that are observed experimentally, the observedassociation of electroluminescence with some CVC's and not others and keyproperties of the observed photon spectra. Theoretical predictions arepresented for further single-molecule electroluminescence experiments.

Understanding the electroluminescence emitted by single molecules in scanning tunneling microscopy experiments