Adjusting the inter-particle spacing of a nanoparticle array at the sub-nanometre scale by thermal annealing

Quantitative Insights into a Plasmonic Ruler Equation from the Perspective of Enhanced Near Field

The plasmonic shift of resonance wavelength induced by near-field coupling enables one to measure nanoscale distances optically. Empirically, the well-known ruler equation correlating plasmon shift with interparticle spacing was proposed. Though it has been widely used in analyzing simulation and experimental outcomes, little is known about the underlying physical mechanism of the characteristic exponential form of the plasmon ruler equation and the universal decay constant therein. In this work, we attempt to decrypt these from the perspective of plasmon near-field enhancement. Based on an analytical quasi-normal mode formula for plasmon shifts, we proved that the exponential decaying electric field is the critical reason that results in the exponential form of the plasmon ruler equation and quantitatively, we found that the universal decay constant in the plasmon ruler equation actually reflects the range of the enhanced near field. This work hopefully helps to deepen the understanding of the mechanism of light-matter interaction in corresponding plasmonic processes.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Au Nanoparticles-Doped Polymer All-Optical Switches Based on Photothermal Effects

This article demonstrated the Au nanoparticles-doped polymer all-optical switches based on photothermal effects. The Au nanoparticles have a strong photothermal effect, which would generate the inhomogeneous thermal field distributions in the waveguide under the laser irradiation. Meanwhile, the polymer materials have the characteristics of good compatibility with photothermal materials, low cost, high thermo-optical coefficient and flexibility. Therefore, the Au nanoparticles-doped polymer material can be applied in optically controlled optical switches with low power consumption, small device dimension and high integration. Moreover, the end-pumping method has a higher optical excitation efficiency, which can further reduce the power consumption of the device. Two kinds of all-optical switching devices have been designed including a base mode switch and a first-order mode switch. For the base mode switch, the power consumption and the rise/fall time were 2.05 mW and 17.3/106.9 μs, respectively at the wavelength of 650 nm. For the first-order mode switch, the power consumption and the rise/fall time were 0.5 mW and 10.2/74.9 μs, respectively at the wavelength of 532 nm. This all-optical switching device has the potential applications in all-optical networks, flexibility device and wearable technology fields.

A Novel Strategy for Fabricating a Strong Nanoparticle Monolayer and Its Enhanced Mechanism

This work presents the preparation of cross-linking Au nanoparticle (NP) monolayer membranes by the thiol exchange reaction and their enhanced mechanical properties. Dithiol molecules were used as a cross-linking mediator to connect the adjacent nanoparticles by replacing the original alkanethiol ligand in the monolayer. After cross-linking, the membrane integrity was maintained and no significant fracture was observed, which is crucial for the membrane serving as a nanodevice. TEM (Transmission Electron Microscopy), UV–Vis absorption spectrum, and GISAXS (grazing incidence small angle X-ray scattering) were performed to characterize the nanostructure before and after cross-linking. All results proved that the interparticle distance in the monolayer was controllably changed by using dithiols of different lengths as the cross-linking agent. Moreover, the modulus of the cross-linking monolayer was measured by atomic force microscopy (AFM) and the result showed that the membrane with a longer dithiol molecule had a larger modulus, which might derive from the unbroken and intact structure of the cross-linking monolayer due to the selected appropriately lengthed dithiol. This study provides a new way of producing a nanoparticle monolayer membrane with enhanced mechanical properties.

In Situ Probing of Laser Annealing of Plasmonic Substrates with Surface-Enhanced Raman Spectroscopy

In this work, we in situ monitor the laser annealing of template-fabricated silver substrates using surface-enhanced Raman scattering (SERS) and 4-mercaptobenzoic acid (4-MBA) as a molecular probe. The annealing process, which exhibits a strong dependence on the laser power, yields a large (>50×) increase in the SERS of the immobilized 4-MBA. This increased SERS response is correlated with the changing substrate morphology using optical and scanning electron microscope images. We attribute the large enhancement to the formation of nanogaps facilitated by binding of the 4-MBA through both thiol and COO^- groups in a sandwich structure, resulting in both electromagnetic and chemical enhancement. This annealing effect, associated with the continuous increase of SERS intensity, was not limited to the AgNP arrays but included Ag films deposited on a variety of nanoporous templates. This study provides a simple strategy for in situ optimization of plasmonic SERS substrates.

Quantum mechanical effects in plasmonic structures with subnanometre gaps

Metallic structures with nanogap features have proven highly effective as building blocks for plasmonic systems, as they can provide a wide tuning range of operating frequencies and large near-field enhancements. Recent work has shown that quantum mechanical effects such as electron tunnelling and nonlocal screening become important as the gap distances approach the subnanometre length-scale. Such quantum effects challenge the classical picture of nanogap plasmons and have stimulated a number of theoretical and experimental studies. This review outlines the findings of many groups into quantum mechanical effects in nanogap plasmons, and discusses outstanding challenges and future directions.

Gold Nanoparticle Monolayers with Tunable Optical and Electrical Properties

Centimeter-scale gold nanoparticle (Au NP) monolayer films have been fabricated using a water/organic solvent self-assembly strategy. A recently developed approach, drain to deposit, is demonstrated to be most effective in transferring the Au NP films from the water/organic solvent interface to various solid substrates while maintaining their integrity. The interparticle spacing was tuned from 1.4 to 3.1 nm using alkylamine ligands of different lengths. The ordering of the films increased with increasing ligand length. The surface plasmon resonance and the in-plane electrical conductivity of the Au NP films both exhibit an exponential dependence on the interparticle spacing. These findings show great potential in scaling up the manufacturing of high-performance optical and electronic devices based on two-dimensional metallic nanoparticle superlattices.

A General Method for Solvent Exchange of Plasmonic Nanoparticles and Self-Assembly into SERS-Active Monolayers

We present a general route for the transfer of Au and Ag nanoparticles of different shapes and sizes, from water into various organic solvents. The experimental conditions for each type of nanoparticles were optimized by using a combination of thiolated poly(ethylene glycol) and a hydrophobic capping agent, such as dodecanethiol. The functionalized nanoparticles were readily transferred into organic dispersions with long-term stability (months). Such organic dispersions efficiently spread out on water, leading to self-assembly at the air/liquid interface into extended nanoparticle arrays which could in turn be transferred onto solid substrates. The dense close packing in the obtained nanoparticle monolayers results in extensive plasmon coupling, rendering them efficient substrates for surface-enhanced Raman scattering spectroscopy.