Extended Quantum Tunnelling Distance in Planar Plasmonic Gaps

Light can drive charge transfer across metal nanogaps via quantum tunnelling processes, underpinning diverse molecular optoelectronic devices. However, such optical quantum tunnelling effects are usually significant at the subnanometer scale, which is not readily accessible. Here, we demonstrated that the threshold tunnelling gap distance, below which the quantum tunnelling effects become significant, strongly depends on the gap morphologies and can be significantly extended to >1.3 nm in planar plasmonic gaps. Furthermore, a phenomenological model was developed to illustrate how this threshold gap distance correlates with gap morphology. These findings provide new insights into quantum tunnelling effects in plasmonic nanogaps and point out how to relax the size requirement for accessing quantum tunnelling effects in plasmonic nanosystems, significant for quantum plasmonics and relevant applications.