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

Area of Science:

• Molecular electronics
• Nanotechnology
• Materials Science

Background:

• Inelastic electron tunneling electroluminescence (EL) and current rectification are key to molecular electronics.
• Achieving robust rectification in molecular junctions is a significant challenge.

Purpose of the Study:

• To investigate current rectification and EL in molecular tunnel junctions with geometrically asymmetric electrodes.
• To explore the influence of electrode geometry and linker chemistry on rectification behavior.

Main Methods:

• Fabrication of tunnel junctions using noble-metal nanospheres (20-200 nm) bridged to planar metal films by symmetric molecules (1,4-benzenedithiol or 1,4-diethynylbenzene).
• Compositionally symmetric junctions were created using identical noble metals (Ag or Au) for both electrodes.
• Systematic variation of electrode geometry (nanosphere vs. nanocube, nanosphere diameter) and linker chemistry (dithiol, diethynyl, diisocyanide).

Main Results:

• Achieved strong current rectification (ratio 10^2-10^3) and EL.
• Rectification strongly depended on electrode geometry, decreasing with nanocube replacement or increased nanosphere diameter.
• Pronounced rectification observed with dithiol and diethynyl linkers, but not diisocyanide.
• Observed rectification attributed to curvature-dependent metal-molecule coupling and enhanced electronic coupling at under-coordinated nanosphere surface atoms, not electrostatic asymmetry.

Conclusions:

• Structurally symmetric molecules can rectify current between compositionally identical but geometrically inequivalent electrodes.
• This work presents a new pathway for scalable, bottom-up molecular optoelectronic diodes.
• Device function can be controlled by the geometry and chemistry of nominally symmetric junctions.