Electroluminescence and Current Rectification in Plasmonic Nanosphere-on-Mirror Tunnel Junctions

Robust inelastic electron tunneling electroluminescence (EL) and current rectification represent two key milestones in molecular electronics. We report strongly rectified EL and current (rectification ratio up to 102-103) in bottom-up fabricated tunnel junctions where a noble-metal nanosphere (20-200 nm) is bridged to a planar metal thin film by structurally symmetric molecules (1,4-benzenedithiol or 1,4-diethynylbenzene). Both electrodes are made of the same noble metal (M = Ag or Au), so the junctions are compositionally symmetric. The rectification ratio is strongly dependent on the electrode geometry and linker chemistry: The rectification behavior is strongly suppressed when the nanosphere is replaced by a nanocube of comparable size or when the nanosphere diameter increases from 20 to 200 nm. Pronounced rectification occurs with dithiol (M-S) and diethynyl (M-C≡C) linkers but not with diisocyanide (M-CN). These observations cannot be explained by simple electrostatic asymmetry of electrodes but instead arise from curvature-dependent metal-molecule coupling, specifically the enhanced electronic coupling at under-coordinated surface atoms of the nanosphere. Overall, the results show that a structurally symmetric molecule can rectify when placed between compositionally identical but geometrically inequivalent electrodes. These findings open a new route to scalable, bottom-up molecular optoelectronic diodes whose function is encoded in the geometry and chemistry of nominally symmetric junctions.