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.

中文翻译：

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通过扫描隧道显微镜诱导光发射探测金纳米粒子超晶格中纳米间隙依赖性等离子体耦合


等离子体纳米间隙可以支持受限和增强的电磁场。在这项工作中，我们使用扫描隧道显微镜诱导发光 （STM-LE） 来研究金纳米粒子超晶格的局部表面等离子体共振，该超晶格由可调尺寸（从 0.1 到 2.3 nm）的高密度纳米间隙组成。通过分析远场光发射，我们发现两种不同的等离激元模式，即横向偶极等离激元模式 （TDP） 和键合偶极等离激元 （BDP） 模式，可以根据 STM 针尖的位置选择性激发。随着粒子间间隙距离的减小，在等离子体纳米间隙处激发的 BDP 模式表现出单调的红移和展宽，表明粒子间等离子体耦合不断增强。此外，我们观察到 BDP 模式的辐射强度比在纳米粒子上激发的 TDP 模式更强，这表明等离子体纳米间隙充当电磁热点。BDP 模式与 TDP 模式的强度比随着间隙尺寸减小到前所未有的 0.1 nm 而增强，揭示了在等离子体超晶格中可以实现的极端场限制。我们的结果促进了对亚纳米等离子体间隙中近场增强的理解，并将有利于等离子体结构的设计，用于许多领域的应用，包括表面增强拉曼光谱、光催化和光电子学。