Electrically induced light emission via nanoscale vacuum discharge in nanogap electrodes without luminescent molecules

Understanding the mechanisms of electrically induced light emission in nanoscale junctions is crucial in molecular-scale optoelectronics. In this study, we investigated the light emission from nanogap electrodes with an insulating and nonluminescent material (polystyrene). Electrically induced light emission was observed over a wide wavelength range of 400–600 nm. The threshold voltages for the light emission ranged from 3 to 15 V, depending on the device, indicating that the electric field induced at the nanogap, rather than the applied potential, is the driving force for the light emission. The intensity of the light emission varies exponentially with the current with an exponent higher than 1.6, and the light emission is driven by a thermally activated current. These characteristics are inconsistent with the mechanisms reported for solid-state nanogap devices. We conclude that light emission in the nanogap electrodes is attributed to nanoscale vacuum discharge with metal plasma. The light emission from this mechanism can conflict with the other mechanisms ever reported since it occurs at such low voltages. The results also suggest intrinsic instability in molecular-scale electronic devices based on nanogap electrodes because the vacuum discharge is accompanied by the plasticization of the electrode.