Monitoring of Energy Conservation and Losses in Molecular Junctions through Characterization of Light Emission

Emission of visible light from large area molecular junctions provides a direct measure of the energy of carriers when they encounter a conducting contact and stimulate photon emission. For carbon/molecule/carbon molecular junctions containing aromatic molecular layers with thicknesses less than 5 nm, transport is elastic, and the maximum emitted photon energy (i.e., “cut-off” energy, hvco) is equal to eVapp, where Vapp is the bias across the molecular junction. hvco increases monotonically with Vapp, is symmetric with polarity, but is weakly dependent on the nature of the contact material. Light emission from molecular junctions containing oligomeric films of anthraquinone, nitroazobenzene, naphthalene diimide, and bis-thienyl benzene with thicknesses of 4.5–59 nm is observed as a function of bias. For layers thicker than 5–7 nm, hvco < eVapp, indicating loss of energy and therefore inelastic transport. The energy loss depends strongly on molecular structure and is linear with molecular layer thickness. When the molecular layer thickness exceeds 5–7 nm, the results provide strong evidence for a transition from elastic to inelastic transport and for stepwise, activationless transport up to 65 nm molecular layer thicknesses. Such information proves valuable for determining transport mechanisms and ultimately designing molecular junctions with desirable electronic properties.