Abstract
Abstract Plasmonic core–molecule–shell (CMS) nanojunctions provide a versatile platform for studying electron transport through conductive molecules under light excitation. In general, the impact of electron transport on the near-field response of CMS nanojunctions is more prominent than on the far-field property. In this work, we use two-photon luminescence (TPL) spectroscopy to probe the effect of electron transport on the plasmonic properties of gold CMS nanojunctions. Theoretical calculations show that the TPL response of such nanojunctions is closely related to the near-field enhancement inside the metal regions, and can be strongly affected by the electron transport through the embedded molecules. TPL excitation spectroscopy results for three CMS nanojunctions (0.7, 0.9 and 1.5 nm junction widths) reveal no perceivable contribution from their low-energy plasmon modes. This observation can be well explained by a quantum-corrected model, assuming significant conductance for the molecular layers and thus efficient charge transport through the junctions. Furthermore, we explore the charge transport mechanism by investigating the junction width dependent TPL intensity under a given excitation wavelength. Our study contributes to the field of molecular electronic plasmonics through opening up a new avenue for studying quantum charge transport in molecular junctions by non-linear optical spectroscopy.
Highlights
As a promising solution for further miniaturization of electronic devices towards the sub-nanometer scale, molecular electronics has experienced a rapid growth over the past decade [1, 2]
Here the gold CMS nanojunction is assumed to be free-standing in air and an insulating junction is considered by setting the refractive index of the medium in the gap between the shell and the core to 1.60
We have investigated the two-photon luminescence (TPL) response of gold CMS nanojunctions embedded with different molecules and found that the charge transport in the junctions has strong impact on the non-linear optical response of such quantum plasmonic systems
Summary
As a promising solution for further miniaturization of electronic devices towards the sub-nanometer scale, molecular electronics has experienced a rapid growth over the past decade [1, 2]. Quantum plasmonics studies ultra-strong and enhanced light–matter interactions at atomic scale, for example room-temperature strong coupling between plasmons and excitons in two-dimensional materials coupled nanocavities [16] These quantum effects may dominate in metallic nanostructures with feature sizes on the same order of the length scale where molecular electronics operates, i.e. ranging from a few nanometers down to sub-nanometer range. Among these structures, plasmonic nanocavities fabricated by the molecular selfassembly technique have similar configurations as that of metal–molecular–metal junctions applied in molecular electronics [17]. While scanning over the sample, TPL emission signals were detected simultaneously by a HyD detector
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