Gap size-dependent plasmonic enhancement in electroluminescent tunnel junctions

Nanoscale plasmonic structures have been primarily characterized through scattering studies, but electroluminescence offers an exciting alternative from a technological standpoint by removing the need for optical excitation. In sub-nanometer biased junctions, electronic tunneling can serve as the excitation source for plasmon-coupled electroluminescence, but the gap size dependence to this plasmonic enhancement has not been characterized. Here, we simultaneously probe the electroluminescence and conductance of Au tunnel junctions. We find that plasmonic enhancement increases as the gap size is reduced for junctions biased between 1.4 and 1.8 V, consistent with the behavior of charge transfer plasmons. At biases above 1.9 V, we see decreasing plasmonic enhancement with the decreasing gap, showing quenching due to tunneling in remarkable agreement with the trends observed for high energy plasmons in scattering experiments. Critically, we find that plasmonic enhancement of electroluminescence is gap size-dependent and, furthermore, is in agreement with the nature of modes excited by scattering.