Light Emission in Single-Molecule Molecular Junctions

The basic mechanism for light emission in Scanning Tunneling Microscopes (STM) is described by inelastic tunneling electrons which couple to localized surface plasmons. There is also growing interest in charge transfer plasmons which appear when electrically coupling metallic nanoparticles. Here, we use single molecules to form conductive bridges between an STM tip and substrate and study the complex relationship between tunneling electrons, charge transfer plasmons, and luminescence in molecular junctions. We employ the STM break junction technique to form thousands of Au point contacts and molecular junctions at ambient conditions and room temperature and measure the light emitted from these junctions using a Si photomultiplier. Since the current, junction bias, and emission signals are collected simultaneously, fluctuations in light emission can be correlated with changes in current and junction bias. Because the break junction procedure continuously modifies the local geometry, limits on the emission efficiency are experimentally determined. Finally, we explore the effect of different molecules on light emission.