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Plasmons have been widely studied over the past several decades because of their ability to strongly absorb light and localize its electric field on the nanoscale, leading to applications in spectroscopy, biosensing, and solar energy storage. In a classical electrodynamics framework, a plasmon is defined as a collective, coherent oscillation of the conduction electrons of the material. In recent years, it has been shown experimentally that noble metal nanoclusters as small as a few nm can support plasmons. This work has led to numerous attempts to identify plasmons from a quantum mechanical perspective, including many overlapping and sometimes conflicting criteria for plasmons. Here, we shed light on the definitions of plasmons. We start with a brief overview of the well-established classical electrodynamics definition of a plasmon. We then turn to the experimental features used to determine whether a particular system is plasmonic, connecting the experimental results to the corresponding features of the classical electrodynamics description. The core of this article explains the many quantum mechanical criteria for plasmons. We explore the common features that these criteria share and explain how these features relate to the classical electrodynamics and experimental definitions. This comparison shows where more work is needed to expand and refine the quantum mechanical definitions of plasmons.
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Classical vs. quantum plasmon-induced molecular transformations at metallic nanojunctions
Chemical transformations near plasmonic metals have attracted increasing attention in the past few years. Specifically, reactions occurring within plasmonic nanojunctions that can be detected via surface and tip-enhanced Raman (SER and TER) scattering were the focus of numerous reports. In this context, even though the transition between localized and nonlocal (quantum) plasmons at nanojunctions is documented, its implications on plasmonic chemistry remain poorly understood. We explore the latter through AFM-TER-current measurements. We use two molecules: i) 4-mercaptobenzonitrile (MBN) that reports on the (non)local fields and ii) 4-nitrothiophenol (NTP) that features defined signatures of its neutral/anionic forms and dimer product, 4,4′-dimercaptoazobenzene (DMAB). The transition from classical to quantum plasmons is established through our optical measurements: It is marked by molecular charging and optical rectification. Simultaneously recorded force and current measurements support our assignments. In the case of NTP, we observe the parent and DMAB product beneath the probe in the classical regime. Further reducing the gap leads to the collapse of DMAB to form NTP anions. The process is reversible: Anions subsequently recombine into DMAB. Our results have significant implications for AFM-based TER measurements and their analysis, beyond the scope of this work. In effect, when precise control over the junction is not possible (e.g., in SER and ambient TER), both classical and quantum plasmons need to be considered in the analysis of plasmonic reactions
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- Award ID(s):
- 2004655
- PAR ID:
- 10550168
- Publisher / Repository:
- National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 121
- Issue:
- 14
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2319233121
