Multipole plasmon resonances in self-assembled metal hollow-nanospheres

Cylindrical Al Nano-Dimer Induced Polarization in Deep UV Region

The polarization properties of asymmetric plasmonic nanostructures originating from optical anisotropy show great application prospects in many fields, such as display, sensing, filtering, and detection. Here, we report the realization of polarization control in the deep ultraviolet (UV) region using Al nano-dimer structures. The simulation results indicated that the polarization effect was generated by the modulation of inter-coupling between the quadrupole plasmon resonances of the asymmetric dimer. By further optimizing the size and gap of the dimer, the extinction in the 200-nm deep UV region obtained a polarization ratio of 18%. This research is helpful for understanding the resonance hybridization of high-order surface plasmons in UV region and is of great significance to the emerging polarized micro-nano photonics fields, such as spin optoelectronics and deep UV optoelectronic devices.

Dual-Mode Plasmonic Coupling-Enhanced Color Conversion of Inorganic CsPbBr3 Perovskite Quantum Dot Films

Plasmonic coupling has been demonstrated to be an effective manipulation strategy for emission enhancement in low-dimensional semiconductor materials. Here, dual-mode plasmonic resonances based on a metal dimer structure were proposed to simultaneously enhance the absorption under short-wavelength excitation and excitons' emission at longer wavelengths for CsPbBr₃ perovskite quantum dots (QDs). Large-area metal nanodimer arrays with well-controlled local surface plasmon resonance were facilely fabricated by a simple method combined with metal angular deposition and nanosphere lithography. With the addition of an optimized polymethyl methacrylate spacer, the effective plasmonic coupling and interfacial passivation of QDs were successfully achieved in the hybrid system. As a result, the QD films exhibited a significant and approximately 3.95-fold overall fluorescence enhancement when using blue light excitation, showing the novel advantages of dual-mode plasmonic coupling of semiconductor quantum structures for color conversion applications.

Nanostructured, ultrathin silver-based transparent electrode with broadband near-infrared plasmonic resonance

A nanostructured transparent electrode with high average visible transmittance of 76%, low sheet resistance of 7.0 Ω/sq and steep transmittance drop in the near-infrared (NIR) range is investigated by simulations and experiments. The electrode is composed of a nanostructured substrate, on which a trilayer, consisting of an ultrathin 14 nm thick silver film embedded between thin films of TiO₂ and Al-doped ZnO, is deposited. Directional silver deposition results in the formation of a disk-hole array without additional lift-off or etching steps. While the trilayer approach enables increased visible transmittance, the transmittance in the NIR regime is decreased by a broadband plasmonic dipole excitation in the disk-hole array. Moreover, a rich mode spectrum of weaker multipole surface plasmon excitations is observed in the nanodisk- and nanohole array. The presented electrode holds great potential for applications in optoelectronic devices, solar control coatings and solar cells.

Synergetic SERS Enhancement in a Metal-Like/Metal Double-Shell Structure for Sensitive and Stable Application

Because of either thermal/chemical instability or high optical loss in noble metal nanostructures, searching for alternative plasmonic materials is becoming more and more urgent, considering the practical biosensing applications under various extreme conditions. In this work, titanium nitride (TiN), a low-loss metal-like material with both excellent thermal and excellent chemical stabilities, was proposed to composite with Ag hollow nanosphere (HNS) nanostructures as an effective surface-enhanced Raman scattering (SERS) substrate to realize both highly sensitive and highly stable molecular detection. Because of the multiple-mode local surface plasmon resonance around the spherical composite nanospheres and the "gap effect" derived from the ultrasmall nanogaps within the precisely controlled plasmonic arrays, an intensively enhanced local field was successfully induced on this SERS substrate. Combined with the unique charge transferring process between Ag and TiN, a synergistically enhanced SERS sensitivity involving both physical and chemical mechanisms was achieved. Especially, with the isolation of TiN, a time-durable Raman detection on these TiN-Ag HNS arrays was accomplished, showing great potential for practical applications.

Effective approach to strengthen plasmon resonance localized on top surfaces of Ag nanoparticles and application in surface-enhanced Raman spectroscopy

The spatial distribution of localized surface plasmon resonance (LSPR) plays a key role in many plasmonic applications. Based on the thermal stability of alumina templates, this work reports a novel approach to manipulate the distribution of LSPR and exhibits its significance for an important plasmonic application, the surface-enhanced Raman spectroscopy (SERS). A suitable thermal annealing sharpens the edges in top surfaces (far from the substrates) of Ag nanoparticles, which significantly strengthens the distal mode (DM) with the LSPR excited on the top surfaces. Because the top surface is the major place to adsorb probe molecules, this manipulation greatly improves the detection sensitivity of SERS. Our research provides a new way to improve the sensitivity of SERS, which also indicates that great care has to be taken on special LSPR mode which is largely responsible for a certain plasmonic application (e.g., the DM for SERS although it is not the major mode).

Theory of tailorable optical response of two-dimensional arrays of plasmonic nanoparticles at dielectric interfaces

Two-dimensional arrays of plasmonic nanoparticles at interfaces are promising candidates for novel optical metamaterials. Such systems materialise from ‘top–down’ patterning or ‘bottom–up’ self-assembly of nanoparticles at liquid/liquid or liquid/solid interfaces. Here, we present a comprehensive analysis of an extended effective quasi-static four-layer-stack model for the description of plasmon-resonance-enhanced optical responses of such systems. We investigate in detail the effects of the size of nanoparticles, average interparticle separation, dielectric constants of the media constituting the interface, and the nanoparticle position relative to the interface. Interesting interplays of these different factors are explored first for normally incident light. For off-normal incidence, the strong effects of the polarisation of light are found at large incident angles, which allows to dynamically tune the reflectance spectra. All the predictions of the theory are tested against full-wave simulations, proving this simplistic model to be adequate within the quasi-static limit. The model takes seconds to calculate the system’s optical response and makes it easy to unravel the effect of each system parameter. This helps rapid rationalization of experimental data and understanding of the optical signals from these novel ‘metamaterials’, optimised for light reflection or harvesting.

Significant metal enhanced fluorescence of Ag2S quantum dots in the second near-infrared window

The amplification of light in NIR-II from Ag2S QDs via metal enhanced fluorescence (MEF) is reported for the first time. Significant fluorescence enhancement of over 100 times for Ag2S QDs deposited on Au-nanostructured arrays, paves the way for novel sensing and imaging applications based on Ag2S QDs, with improved detection sensitivity and contrast enhancement.

Broadband and Low-Loss Plasmonic Light Trapping in Dye-Sensitized Solar Cells Using Micrometer-Scale Rodlike and Spherical Core-Shell Plasmonic Particles

Dielectric scattering particles have widely been used as embedded scattering elements in dye-sensitized solar cells (DSCs) to improve the optical absorption of the device. Here we systematically study rodlike and spherical core-shell silica@Ag particles as more effective alternatives to the dielectric scattering particles. The wavelength-scale silica@Ag particles with sufficiently thin Ag shell support hybrid plasmonic-photonic resonance modes that have low parasitic absorption losses and a broadband optical response. Both of these features lead to their successful deployment in light trapping in high-efficiency DSCs. Optimized rodlike silica@Ag@silica particles improve the power conversion efficiency of a DSC from 6.33 to 8.91%. The dimension, surface morphology, and concentration of these particles are optimized to achieve maximal efficiency enhancement. The rodlike silica particles are prepared in a simple one-pot synthesis process and then are coated with Ag in a liquid-phase deposition process by reducing an Ag salt. The aspect ratio of silica rods is tuned by adjusting the temperature and duration of the growth process, whereas the morphology of Ag shell is tailored by controlling the reduction rate of Ag salt, where slower reduction in a polyol process gives a smoother Ag shell. Using optical calculations, the superior performance of the plasmonic core-shell particles is related to the large number of hybrid photonic-plasmonic resonance modes that they support.

Bonding and Anti-bonding Modes of Plasmon Coupling Effects in TiO2-Ag Core-shell Dimers

Bonding and anti-bonding modes of plasmon coupling effects are numerically investigated in TiO2-Ag core-shell nano dimers. First, splitting phenomena of the coupled anti-bonding modes are observed under the longitudinal polarization when the distance between the monomers decreases to a certain level. Second, one of the split resonance modes is identified to be formed by the dipole anti-bonding mode of the monomers from charge density distribution patterns. Those split modes have similar redshift behaviors as the coupled dipole bonding modes in the same situations. Furthermore, the intensities of those anti-bonding modes weaken with decreasing distance between the monomers, because of the interaction of the induced dipole moment in the monomers and the charge distribution variation on the facing surfaces of the gap by the coulomb attraction. Other split bands are the higher-order mode (octupole-like or triakontadipole-like), which do not have obvious peak-shift behavior, and the intensities have very little attenuation with decreasing distance. Finally, the coupling of the bonding and anti-bonding modes under the longitudinal polarization is symmetric (bonding).

Quantifying the Sensitivity of Multipolar (Dipolar, Quadrupolar, and Octapolar) Surface Plasmon Resonances in Silver Nanoparticles: The Effect of Size, Composition, and Surface Coating

The effect of composition, size, and surface coating on the sensitivity of localized multipolar surface plasmon resonances has been spectroscopically investigated in high-quality silver colloidal solutions with precisely controlled sizes from 10 to 220 nm and well-defined surface chemistry. Surface plasmon resonance modes have been intensively characterized, identifying the size-dependence of dipolar, quadrupolar, and octapolar modes. Modifications of the NP's surface chemistry revealed the higher sensitivity of large sizes, long molecules, thiol groups, and low-order resonance modes. We also extend this study to gold nanoparticles, aiming to compare the sensitivity of both materials, quantifying the higher sensitivity of silver.

Hollow Au/Ag nanostars displaying broad plasmonic resonance and high surface-enhanced Raman sensitivity

Bimetallic Au/Ag hollow nanostar (HNS) nanoparticles with different morphologies were prepared in this work. These nanoplatforms were obtained by changing the experimental conditions (concentration of silver and chemical reductors, hydroxylamine and citrate) and by using Ag nanostars as template nanoparticles (NPs) through galvanic replacement. The goal of this research was to create bimetallic Au/Ag star-shaped nanoparticles with advanced properties displaying a broader plasmonic resonance, a cleaner exposed surface, and a high concentration of electromagnetic hot spots on the surface provided by the special morphology of nanostars. The size, shape, and composition of Ag as well as their optical properties were studied by extinction spectroscopy, hyperspectral dark field microscopy, transmission and scanning electron microscopy (TEM and SEM), and energy dispersive X-ray spectroscopy (EDX). Finally, the surface-enhanced Raman scattering (SERS) activity of these HNS was investigated by using thioflavin T, a biomarker of the β-amyloid fibril formation, responsible for Alzheimer's disease. Lucigenin, a molecule displaying different SERS activities on Au and Ag, was also used to explore the presence of these metals on the NP surface. Thus, a relationship between the morphology, plasmon resonance and SERS activity of these new NPs was made.

Leveraging Nanocavity Harmonics for Control of Optical Processes in 2D Semiconductors

Optical cavities with multiple tunable resonances have the potential to provide unique electromagnetic environments at two or more distinct wavelengths--critical for control of optical processes such as nonlinear generation, entangled photon generation, or photoluminescence (PL) enhancement. Here, we show a plasmonic nanocavity based on a nanopatch antenna design that has two tunable resonant modes in the visible spectrum separated by 350 nm and with line widths of ∼60 nm. The importance of utilizing two resonances simultaneously is demonstrated by integrating monolayer MoS2, a two-dimensional semiconductor, into the colloidally synthesized nanocavities. We observe a 2000-fold enhancement in the PL intensity of MoS2--which has intrinsically low absorption and small quantum yield--at room temperature, enabled by the combination of tailored absorption enhancement at the first harmonic and PL quantum-yield enhancement at the fundamental resonance.

Highly Controllable Surface Plasmon Resonance Property by Heights of Ordered Nanoparticle Arrays Fabricated via a Nonlithographic Route

Perfectly ordered nanoparticle arrays are fabricated on large-area substrates (>cm(2)) via a cost-effective nonlithographic route. Different surface plasmon resonance (SPR) modes focus consequently on their own positions due to the identical shape and uniform size and distance of these plasmonic metallic nanoparticles (Ag and Au). On the basis of this and FDTD (finite-difference time-domain) simulation, this work reveals the variation of all SPR parameters (position, intensity, width, and mode) with nanoparticle heights, which demonstrates that the effect of heights are different in various stages. On increasing the heights, the major dipole SPR mode precisely blue-shifts from the near-infrared to the visible region with intensity strengthening, a peak narrowing effect, and multipole modes excitation in the UV-vis range. The intensity of multipole modes can be manipulated to be equal to or even greater than the major dipole SPR mode. After coating conformal TiO2 shells on these nanoparticle arrays by atomic layer deposition, the strengthening of the SPR modes with increasing the heights results in the multiplying of the photocurrent (from ∼2.5 to a maximum 90 μA cm(-2)) in this plasmonic-metal-semiconductor-incorporated system. This simple but effective adjustment for all SPR parameters provides guidance for the future design of plasmonic metallic nanostructures, which is significant for SPR applications.