Observation of inhomogeneous plasmonic field distribution in a nanocavity

Precise measurements in the ångström level for plasmonic fields in nanocavities along the longitudinal direction lead to the discovery of the plasmon comb that is attributed to the self-focusing effect of individual molecules in self-assembly monolayers.

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Plasmonics that confined in a nanocavity has become an emerging research area in optics, which has shown an extraordinary ability to allow visualization of the inner structure of a single molecule with sub-nanometer resolution (Nature 498, 82-86 (2013); Nature 531, 623-627 (2016)) and chemical identification a single molecule in real space (Nat. Nanotech. 10, 865-869 (2015); Nature 2019, 568, 78-82). In all these appealing applications, the distribution of plasmon plays a decisive role. Conceptually, it is known that the plasmons in the nanocavity should be inhomogeneous. However, the actual field distribution in the nanocavity in the longitudinal direction, probably the last puzzle piece, has never been experimentally obtained, causing heated debates on the mechanism for achieving super-high resolution Raman images in literature.

Figure 1. a. Schematic diagram of a plasmonic coupling nanocavity between a Au(111) surface and a shell-isolated nanoparticle (SHIN), where each molecule in the hotspot experiences a highly localized plasmonic field. b. Calculated molecular conformations for a series of V+· viologens adsorbed on Au(111), where the position of the 4,4-bipyridinium probe moiety (red dotted rectangle) is shifted by altering its location along the alkyl chain via C2H4 units (longitudinal resolution, ~2 Å). c. Normalized electric field amplitude for V SAMs in the cavity, simulated via first-principles calculations showing the contributions from a Au SHIN antenna and the Au(111) substrate. cps, counts per second; a.u., arbitrary units.

In this work, we present a rational design to experimentally overcome this great difficulty, which enables us to measure the field distribution in a nanocavity with ~2 ångström spatial resolution in the longitudinal direction. It is done by placing a self-assembly viologen molecular monolayer with a specific probe moiety in a nanocavity (Fig. 1a). The position of the probe moiety is continuously shifted along the viologen molecule, differing by two two methylene units (Fig. 1b). The field distribution in the nanocavity can be directly determined through the proper deconvolution of the experimentally measured Raman signals. We surprisingly find a large field inhomogeneity (~5.2 fold between largest and smallest field, Fig. 1c) inside the self-assembly monolayers, which cannot be explained by the widely adopted continuous media approximation for molecular monolayer. Consequently, a plasmon comb forms owing to the self-focusing of the individual molecule (Fig. 1a), leading to a plasmonic force that can result in a trapping potential around 10 kBT to stabilize a small molecule.

Our findings not only provide the solution to a long-standing problem in the field and enrich our fundamental understanding of plasmonics, but also have strong implications for a variety of applications, such as photo-selective bond dissociation, sub-nanometer chemical recognition, and optical force mediated assembly of nano-objects.

This work was recently published in Nature Nanotechnology:

Observation of inhomogeneous plasmonic field distribution in a nanocavity

Chao-Yu Li, Sai Duan, Bao-Ying Wen, Song-Bo Li, Murugavel Kathiresan, Li-Qiang Xie, Shu Chen, Jason R. Anema, Bing-Wei Mao, Yi Luo, Zhong-Qun Tian, and Jian-Feng Li

Nature Nanotechnology (2020), https://www.nature.com/articles/s41565-020-0753-y

Jian-Feng Li

Prof., Xiamen University