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

) 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.