Probing Nanogap-Dependent Plasmonic Coupling in Gold Nanoparticle Superlattices by Scanning Tunneling Microscopy Induced Light Emission

Plasmonic nanogaps can support confined and enhanced electromagnetic fields. In this work, we use scanning tunneling microscopy-induced light emission (STM-LE) to study the localized surface plasmon resonance of gold nanoparticle superlattices, which consist of high-density nanogaps of tunable sizes (from 0.1 to 2.3 nm). By analyzing the far-field light emission, we discover that two distinct plasmon modes, i.e., the transverse dipolar plasmon mode (TDP) and bonding dipolar plasmon (BDP) mode, can be selectively excited depending on the location of the STM tip. As the interparticle gap distance decreases, the BDP mode excited at the plasmonic nanogaps shows a monotonous red-shift and broadening, indicating continuously enhanced interparticle plasmonic coupling. Moreover, we observe a stronger radiative strength of the BDP mode compared to the TDP mode excited at the nanoparticles, demonstrating that the plasmonic nanogaps act as electromagnetic hot spots. The intensity ratio of the BDP mode to the TDP mode is enhanced with decreased gap size down to the unprecedent 0.1 nm, revealing the extreme field confinement that can be achieved in plasmonic superlattices. Our results advance the understanding of near-field enhancement in subnanometer plasmonic gaps and shall benefit the design of plasmonic structures for applications in many fields, including surface enhance Raman spectroscopy, photocatalysis, and optoelectronics.