Routing the Exciton Emissions of WS2 Monolayer with the High-Order Plasmon Modes of Ag Nanorods.

Locally routing the exciton emissions in two-dimensional (2D) transition-metal dichalcogenides along different directions at the nanophotonic interface is of great interest in exploiting the promising 2D excitonic systems for functional nano-optical components. However, such control has remained elusive. Herein we report on a facile plasmonic approach for electrically controlled spatial modulation of the exciton emissions in a WS2 monolayer. The emission routing is enabled by the resonance coupling between the WS2 excitons and the multipole plasmon modes in individual silver nanorods placed on a WS2 monolayer. Different from prior demonstrations, the routing effect can be modulated by the doping level of the WS2 monolayer, enabling electrical control. Our work takes advantage of the high-quality plasmon modes supported by simple rod-shaped metal nanocrystals for the angularly resolved manipulation of 2D exciton emissions. Active control is achieved, which offers great opportunities for the development of nanoscale light sources and nanophotonic devices.

Plasmon-Enhanced, Self-Traced Nanomotors on the Surface of Silicon.

Light-driven nanomotors have attracted much attention due to their potential applications. The movement of conventional nanomotors typically occurs in the solution phase, which limits their application fields. Utilizing visible light to drive nanomotors at the solid-liquid interface represents a grand challenge due to the large friction force between the nanomotor and the solid surface. Based on the attractive plasmon resonance of Au nanocrystals, for the first time, plasmon-enhanced Au nanocrystal-based nanomotors moving at the silicon-aqueous solution interface have been developed. Such nanomotors move with a clear trace engraved on the Si surface, representing an excellent and exceptional self-traced nanomotor system. In addition, the nanomotor trace on the Si surface also provides a unique and promising approach to the fabrication of nanoscale Si patterns, which is central to many applications, including microelectronics, sensing, information storage, and optoelectronics.

Colloidal Gold Nanorings and Their Plasmon Coupling with Gold Nanospheres.

Gold nanorings are attractive as plasmonic metal nanocrystals because they have a hollow inner cavity. Their enhanced electric field inside the ring cavity is accessible, which is highly desirable for assembling with other optical components and studying their plasmon-coupling behaviors. However, the lack of robust methods for synthesizing size-controllable and uniform Au nanorings severely impedes the study of their attractive plasmonic properties and plasmon-driven applications. Herein, an improved wet-chemistry method is reported for the synthesis of monodisperse colloidal Au nanorings. Using circular Au nanodisks with different thicknesses and diameters as templates, Au nanorings are synthesized with thicknesses varied from ≈30 to ≈50 nm and cavity sizes varied from ≈90 to ≈40 nm. The produced Au nanorings are assembled with colloidal Au nanospheres to yield Au nanoring-nanosphere heterodimers in sphere-in-ring and sphere-on-ring configurations on substrates. The sphere-in-ring heterodimers exhibit the interesting feature of plasmonic Fano resonance upon the excitation of the dark quadrupolar plasmon mode of the Au nanorings. The open cavity in a nanoring holds a great promise for studying plasmon-coupled systems, which will facilitate the construction of advanced metamaterials and high-performance Fano-based devices.

Mode-dependent energy exchange between near- and far-field through silicon-supported single silver nanorods.

Optical antenna effects endow plasmonic nanoparticles with the capability to enhance and control various types of light-matter interaction. Most reported plasmonic systems can be regarded as single-channel nanoantennas, which rely only on a bright dipole plasmon mode for energy exchange between near- and far-field. Herein we demonstrate a dual-channel plasmonic system that can separate the excitation and emission processes into two energy exchange pathways mediated by the different plasmon modes, offering a higher degree of freedom for the manipulation of light-matter interaction. Our system, consisting of high-aspect-ratio Ag nanorods and Si substrates, can support a series of bright and dark plasmon modes with distinct near- and far-field properties and generate relatively intensive local field enhancement in the gap region. As a proof-of-principle, we take plasmon-enhanced fluorescence of dye molecules as an example to reveal the energy exchange mechanism in the dual-channel plasmonic system. Such a system is potentially also useful for manipulating other types of light-matter interaction. Our work represents a step toward the utilization of a broader class of plasmon resonance for the development of optical antennas and various on-chip nanophotonic components.

Tipping Gold Nanobipyramids with Titania for the Use of Plasmonic Hotspots to Drive Amine Coupling.

Designing plasmonic photocatalysts with spatially controlled catalytic sites is an effective strategy to boost the sunlight-driven chemical transformation efficiency through plasmonic enhancement. Herein, we describe a facile method for the synthesis of TiO2-tipped Au nanobipyramids (NBPs) to give (Au NBP)/t-TiO2 nanodumbbells. The surfactant cetyltrimethylammonium bromide concentration is the key factor in the construction of this type of unique nanostructure. The photocatalytic aerobic oxidative coupling of amines using the plasmonic photocatalysts with the dumbbell-like and core@shell structures indicates that the TiO2-tipped ends for the photo-reduction and the exposed adjacent Au surface for the photo-oxidation on (Au NBP)/t-TiO2 can significantly improve the photocatalytic activity. The underlying mechanism of the photocatalytic oxidative coupling of benzylamine over (Au NBP)/t-TiO2 has been thoroughly investigated. Both experimental and simulation results for (Au NBP)/t-TiO2 and (Au nanorod)/t-TiO2 confirm the important effect of the plasmonic hotspots on the enhancement of the photocatalytic activity.

Facet- and Gas-Dependent Reshaping of Au Nanoplates by Plasma Treatment.

The reshaping of metal nanocrystals on substrates is usually realized by pulsed laser irradiation or ion-beam milling with complex procedures. In this work, we demonstrate a simple method for reshaping immobilized Au nanoplates through plasma treatment. Au nanoplates can be reshaped gradually with nearly periodic right pyramid arrays formed on the surface of the nanoplates. The gaseous environment in the plasma-treatment system plays a significant role in the reshaping process with only nitrogen-containing environments leading to reshaping. The reshaping phenomenon is facet-dependent, with right pyramids formed only on the exposed {111} facets of the Au nanoplates. The morphological change of the Au nanoplates induced by the plasma treatment leads to large plasmon peak redshifts. The reshaped Au nanoplates possess slightly higher refractive index sensitivities and largely increased surface-enhanced Raman scattering intensities compared to the flat, untreated nanoplates. Our results offer insights for studying the interaction mechanism between plasma and the different facets of noble metal nanocrystals and an approach for reshaping light-interacting noble metal nanocrystals.

Strengthening Fano resonance on gold nanoplates with gold nanospheres.

Plasmonic Fano resonance has attracted extensive attention due to its many applications, including plasmonic sensing, electromagnetically induced transparency, light trapping and stopping, due to its narrow linewidth and asymmetric spectral shape. However, many metal nanostructures are designed with complex geometries to generate Fano resonance and few of them can support a deep Fano dip. Herein we report on the strengthening of the Fano resonance on silicon-supported Au nanoplates through the formation of (Au nanosphere)-(Au nanoplate) heterodimers. The deposition of the Au nanosphere on the top can greatly strengthen the substrate-induced Fano resonance of the Au nanoplate with a deep dip. We also observe that the replacement of the Au nanosphere with a Au nanocube can suppress the excitation of the Fano resonance in the heterodimer. When the sharp corners and edges of the nanocubes gradually become rounded, the Fano resonance appears again with increasing asymmetry. Both the dip depth and wavelength of the Fano resonance can be independently tailored by varying the nanosphere diameter and the nanoplate thickness, respectively. We believe that our results provide an attractive and facile platform for modulating Fano dips and constructing Fano resonance-based devices.

Molecular Tunnel Junction-Controlled High-Order Charge Transfer Plasmon and Fano Resonances.

Quantum tunneling plays an important role in coupled plasmonic nanocavities with ultrasmall gap distances. It can lead to intriguing applications such as plasmon mode excitation, hot carrier generation, and construction of ultracompact electro-optic devices. Molecular junctions bridging plasmonic nanocavities can provide a tunneling channel at moderate gap distances and therefore allow for the facile fabrication of quantum plasmonic devices. Herein we report on the large-scale bottom-up fabrication of molecular junction-bridged plasmonic nanocavities formed from Au nanoplate-Au nanosphere heterodimers. When the molecular junction turns from insulating to conductive, a distinct spectral change is observed, together with the emergence of a high-order charge transfer plasmon mode. The evolution of the electron tunneling-induced plasmon mode also greatly affects the Fano resonance feature in the scattering spectrum of the individual heterodimers. The molecular conductance at optical frequencies is estimated. The molecular junction-assisted electron tunneling is further verified by the reduced surface-enhanced Raman intensities of the molecules in the plasmonic nanocavity. We believe that our results provide an interesting system that can boost the investigation on the use of molecular junctions to modulate quantum plasmon resonances and construct molecular plasmonic devices.

Deep Fano resonance with strong polarization dependence in gold nanoplate-nanosphere heterodimers.

Plasmonic Fano resonance arises from the destructive interference between a superradiant and a subradiant plasmon mode that overlap spectrally with each other. Because of its importance in revealing many physical phenomena and its applications in sensing, metamaterials, photoswitching and spectroscopy, a variety of metal nanostructures have been fabricated to generate Fano resonance. However, few metal nanostructures can support deep Fano resonance with strong polarization dependence. Herein, we report on the observation of deep Fano resonance with strong polarization dependence in Au nanoplate-nanosphere heterodimers. Experiments and simulations reveal that the presence of a nanosphere at one side edge or one vertex of the nanoplate causes distinct Fano resonance. With increasing nanosphere sizes, the shape of the scattering spectrum becomes more asymmetric, with the Fano dip getting deeper correspondingly. When the nanosphere diameter reaches 68 nm, the Fano dip almost reaches the spectral background. Moreover, the heterodimers with the nanosphere attached to one vertex of the nanoplate exhibit Fano resonance with strong polarization dependence. Such heterodimers are very attractive for constructing polarization-controlled plasmonic Fano switches.

Highly enhanced transverse plasmon resonance and tunable double Fano resonances in gold@titania nanorods.

Gold nanorods have attracted intensive interest owing to their localized surface plasmon resonance properties and enormous potential applications. The transverse plasmon of Au nanorods is usually weaker than the longitudinal one, hampering certain plasmonic applications. Herein we report on the intensification of the transverse plasmon resonance by coating TiO2 onto Au nanorods. The transverse plasmon mode of the resultant Au@TiO2 nanorods with a sufficiently thick shell can be comparable to or even stronger than the longitudinal one in intensity. Moreover, both the transverse and longitudinal plasmon resonances of the Au@TiO2 nanorods exhibit an asymmetric line shape on their scattering spectra. Electrodynamic simulations and analyses based on a coupled oscillator model suggest that the asymmetric line shape originates from the coupling between the Au core and TiO2 shell. Apart from the shell thickness, the plasmonic properties of the Au@TiO2 nanorods can also be tuned by the dimension of the Au nanorod core. In addition, the polarization-dependent light scattering from the individual Au@TiO2 nanorods has also been investigated. These results will be of high importance for understanding the interactions between noble metals and semiconductors in plasmonic hybrid nanosystems, and for designing novel plasmonic nanostructures with desired optical properties and functions.

Role of shape in substrate-induced plasmonic shift and mode uncovering on gold nanocrystals.

A number of plasmonic devices and applications, such as chemical and biological sensors, plasmon-enhanced solar cells, optical nanoantennas, metamaterials and metasurfaces, require the deposition of plasmonic metal nanocrystals on various substrates. Because the localized plasmon resonance modes, energies and strengths are strongly dependent on the dielectric function of the surrounding environment, the substrate is expected to largely affect the plasmonic properties of supported metal nanocrystals. Therefore, understanding the effects of the substrate on the plasmonic properties of metal nanocrystals and the roles of the involved factors will be crucial for designing various plasmonic devices that are made of metal nanocrystals deposited on different substrates. Herein we report on our study and results of the effects of substrates with distinct dielectric functions on the plasmonic properties of three types of Au nanocrystals. A combination of experiments and numerical simulations shows that the presence of a substrate causes plasmonic shifts as well as the appearance of new plasmon modes. The plasmonic shifts and the emergence of new plasmon modes are found to be dependent on the particle shape of Au nanocrystals and in turn on the fractional particle surface area that is in contact with the supporting substrate. For Au nanospheres and nanorods, plasmonic shifts, less than 100 nm, are observed on the scattering spectra by changing the supporting substrate from indium tin oxide to silicon. In comparison, a giant spectral shift of more than 300 nm is obtained for Au nanoplates. Moreover, silicon substrates induce the emergence of an out-of-plane quadrupolar mode of Au nanoplates, which interacts with an out-of-plane octupolar mode to give rise to a distinct Fano resonance. The Fano resonance is found to become stronger as the thickness of Au nanoplates is decreased. These results are of great importance for understanding the plasmonic properties of noble metal nanocrystals supported on various substrates, and for designing novel plasmonic nanostructures with desired optical properties and functions.