Metal Nanospheres Research Articles

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.

Confronting theoretical results of localized and additional surface plasmon resonances in silver nanoparticles with electron energy-loss spectroscopy measurements

Raza et al. recently observed the extraordinarily large energy blueshifts of localized surface plasmon resonances and additional surface plasmon resonances in silver nanoparticles encapsulated in silicon nitride, which are not fully understood yet. By using the quantum model consisting of two subsystems respectively for describing the center of mass and intrinsic motions of conduction electrons of a metallic nanosphere and a coupling occurred between the center of mass and conduction electrons outside the metallic nanosphere, we firstly deduced the general energy and line broadening size-dependence of localized surface plasmon resonances, which removes the divergent defect of usual 1/R size-dependence. Secondly, we proposed that the additional surface plasmon resonance in a metallic nanosphere originates from the transition of the first excited state to the ground state of the center of mass subsystem with energy levels corrected by degenerate state pairs of the system composed of the center of mass and intrinsic motions of conduction electrons. Then, we implemented this generation mechanism of additional surface plasmon resonances for silver nanoparticles encapsulated in silicon nitride and the calculated results are well consistent with experimental results. Furthermore, we obtained a new energy expression of localized surface plasmon resonances, with which we successfully explained the extraordinarily large energy blueshifts of localized surface plasmon resonances in few-nanometer silver nanoparticles encapsulated in silicon nitride. Finally, we calculated the localized and additional surface plasmon resonance energies of silver nanoparticles resting on carbon films and the calculated results perfectly explain the experimental measurements of Scholl et al..

Tunable Size Dependence of Quantum Plasmon of Charged Gold Nanoparticles.

The quantum behavior of surface plasmons has received extensive attention, benefiting from the development of exquisite nanotechnology and the diverse applications. Blueshift, redshift, and nonshift of localized surface plasmon resonances (LSPRs) have all been reported as the particle size decreases and enters the quantum size regime, but the underlying physical mechanism to induce these controversial size dependences is not clear. Herein, we propose an improved semiclassical model for modifying the dielectric function of metal nanospheres by combining the intrinsic quantized electron transitions and surface electron injection or extraction to investigate the plasmon shift and LSPR size dependence of the charged Au nanoparticles. We experimentally observe that the nonmonotonic blueshift of LSPRs with size for Au nanoparticles is turned into an approximately monotonic blueshift by increasing the electron donor concentration in the reduction solution, and it can also be transformed to an approximately monotonic redshift after surface passivation by ligand molecules. Moreover, we demonstrate controlled blueshift and redshift for the electron and hole plasmons in Cu_{2-x}S@Au core-shell nanoparticles by injecting electrons. The experimental observations and the theoretical calculations clarify the controversial size dependences of LSPR reported in the literature, reveal the critical role of surface electron injection or extraction in the transformation between the different size dependences of LSPRs, and are helpful for understanding the nature of surface plasmons in the quantum size regime.

Quantum electrodynamics description of localized surface plasmons at a metal nanosphere

A canonical quantization scheme for localized surface plasmons (LSPs) in a metal nanosphere is presented based on a microscopic model composed of electromagnetic fields, oscillators that describe plasmons, and a reservoir that describes excitations other than plasmons. The eigenmodes of this fully quantum electrodynamic theory show a spectrum that includes radiative depolarization and broadening, including redshifting from the quasistatic LSP modes with increasing particle size. These spectral profiles correctly match those obtained with exact classical electrodynamics (Mie theory). The present scheme provides the electric fields per plasmon in both near- and far-field regions whereby its utility in the fields of quantum plasmonics and nano-optics is demonstrated.

Vibrations of Dimers of Mechanically Coupled Nanostructures: Analytical and Numerical Modeling

The coupling effects affecting the vibrations of two close nanostructures (e.g., two metal nanoplates or nanospheres separated by a thin dielectric layer) may considerably alter their vibrational eigenfrequencies, as demonstrated by several recent experimental studies. In this work, we present theoretical investigations of these coupling processes based on a continuum mechanics approach, considering various systems composed by two identical nanostructures mechanically coupled by a spacer made of a different material and computing their eigenfrequencies as a function of the spacer thickness. We first discuss the vibrations of stacked slabs, a one-dimensional problem which can be treated analytically. The more complex configurations of dimers of rods or spheres coupled by a finite cylindrical spacer are then treated numerically. In all cases, the frequency shifts occurring for thin spacers can be simply interpreted as a modification of the boundary conditions of the problem as compared to the single nanostructure case, while those predicted near specific spacer thicknesses are ascribed to an avoided crossing effect, happening when the individual building blocks of the dimers (nanostructures and spacer) present common eigenfrequencies.

Comparative study of thermoplasmonic effects of gold and silver metal nanoparticle

We investigated the plasmonic heating effect of noble metal nanoparticles in a water environment using the first-principles approach. In this approach, we have solved the heat transfer equation in the steady state to obtain the heat generation and temperature profile corresponding to two different types of metals. Metals exhibited a surface plasmon resonance property in which maximum absorption of light for smaller size nanoparticles is observed, which can be used to heat up the surrounding environment. Inspired by the same, we have simulated the absorption cross section of different sizes of a metal nanosphere and observed the threshold value of the radius below which absorption is dominant. The maximum absorption of light by the nanosphere produces a hotspot, which can be visualized in terms of the electric field distribution plot. This electric field distribution profile of silver and gold metal nanoparticles is computed under the resonance wavelength using the boundary element method The results thus obtained in terms of the optical cross section are compared with those of the numerical model to establish their veracity. These theoretical works aim to further develop the fundamental understanding of the heating mechanism of plasmonic geometries, which can be used in several applications.

Refined effective-medium model for the optical properties of nanoparticles coated with anisotropic molecules

This work aims to provide a simple yet complete effective dielectric function for an anisotropic layer of polarizable molecules adsorbed on a metallic surface. This effective medium model considers the important and nontrivial case of nonvacuum embedding media and accounts for orientation effects, coverage dependence through dipole-dipole interactions, and image-dipole effects. To check the model's validity, we focus in particular on the experimentally relevant case of dyes adsorbed on metallic nanospheres. We can then use anisotropic Mie theory, together with the effective dielectric function describing the molecular coating, to calculate their optical properties. We show that this effective medium description is in very good agreement with more elaborate and computationally intensive microscopic calculations based on coupled-dipole models. The effective medium model therefore provides a simple means to investigate orientation effects and coverage dependence, including in more complex systems such as dyes adsorbed on nonspherical or ensembles of nanoparticles. This model can readily be used to further our theoretical understanding of dye-nanoparticle systems, for example in the context of dye-plasmon resonance coupling or surface-enhanced Raman and fluorescence spectroscopy.

Elastically tuned defect mode within a cholesteric elastomer doped with metallic nano-spheres

A composite material, formed by a cholesteric elastomer slab, doped with randomly but uniformly distributed metallic nano-spheres, is studied. The cholesteric elastomer has a twist jump defect consisting of a finite discontinuity in the helical angle of the structure. The transmittance and reflectance spectra of the elastomer sample are studied when circularly polarized electromagnetic waves are impinging, while it is stretched by the action of an externally imposed agent. It has been observed that the co-polarized and cross-polarized transmittance and reflectance depend on the wavelength of the impinging wave and the amount of metallic spheres parameterized by the filling factor, strain, and magnitude of the twist defect. Additionally, we have found that the position of the line defect within the band reflection is red-shifted either by increasing the filling factor or stretching the sample.

Vanadium dioxide nanostructures with remarkable surface-enhanced Raman scattering activity.

Structure and size control are always considered to be effective routes to enhance the sensitivity of materials. Herein, rough VO2 (D) nanostars and nanospheres with highly dense and rough surfaces were synthesized. Accompanied by the properties inherited from the rough VO2 nanospheres, i.e., high adsorption and strong plasma resonance, these VO2 (D) nanospheres exhibit highly sensitive activity as a surface enhanced Raman spectrum substrate. The detection limit of Rhodamine 6G on this semiconductor SERS substrate is as low as 10-9 M.

Design of high efficiency perovskite/silicon tandem solar cells based on the plasmonic enhancement of metal nanosphere

Design of high efficiency perovskite/silicon tandem solar cells based on the plasmonic enhancement of metal nanosphere

On the Calculation of Optical Characteristics and Dimensional Shifts of Surface Plasmons of Spherical Bimetallic Nanoparticles

Formulas of efficient relaxation time are obtained for the cases when the free length of electrons is less or comparable with the characteristic dimensions of metallic regions. The frequency dispersion of optical characteristics of spherical bimetallic particles is calculated near plasma resonance in the absence of quantum-size effects. Retaining the style of common description of metallic particles based on the Mie–Drude theories, the frequency dependence of electric dipole polarizability of a two-layered metallic nanosphere was analyzed. The appearance of two polarizability maxima is the result of a difference in core and shell metals. The calculations are conducted for the following particles immersed into teflon: Au@Ag, Ag@Au, Au@Pt, Pt@Au, and Pt@Pd. It is demonstrated that the optical characteristics of bimetallic particles can be controlled by means of changing their composition and volumetric metal content. Cross-sections of absorption and scattering, as well as optical radiation efficiency of the particles in a wide spectrum range are calculated. Possible temperature of bimetallic particles at the absorption of an electromagnetic wave (for the photothermal therapy of malignant tumors) is estimated.

Non-Hermitian plasmonic antennas and waveguides: Superradiant and dark states

One-dimensional structures of non-Hermitian plasmonic metallic nanospheres are studied in this paper. For a single sphere, solving Maxwell's equations results in quasi-stationary eigenmodes with complex quantized frequencies. Coupled mode theory is employed in order to study more complex structures. The similarity between the coupled mode equations and the effective non-Hermitian Hamiltonians governing open quantum systems allows us to translate a series of collective phenomenon emerging in condensed matter and nuclear physics to the system of plasmonic spheres. A nontrivial physics emerges as a result of strong non-radiative near field coupling between adjacent spheres. For a system of two identical spheres, this occurs when the width of the plasmonic resonance of the uncoupled spheres is twice the imaginary component of the coupling constant. The two spheres then become coupled through a single continuum channel and the effect of coherent interaction between the spheres becomes noticeable. The eigenmodes of the system fall into two distinct categories: superradiant states with enhanced radiation and dark states with no radiation. The transmission through one-dimensional chains with an arbitrary number of spheres is also considered within the effective Hamiltonian framework which allows us to calculate observables such as the scattering and transmission amplitudes. This nano-scale waveguide can undergo an additional superradiance phase transition through its coupling to the external world. It is shown that perfect transmission takes place when the superradiance condition is satisfied.

Thermoplasmonic-biosensing demonstration based on the photothermal response of metallic nanoparticles

In this paper, we describe how it is possible to harness the photo-thermal effects occurring in metallic nanoparticles to develop molecular sensors. In particular, we numerically demonstrate how a change of the surrounding medium affects the localized surface plasmon resonance with a consequent shift of the corresponding resonance wavelength, enabling the detection of analytes on metal nanospheres. At the same time, the photothermal response of the nanostructures is modified because of the shift of the plasmonic resonance due to the variation of the refractive index of the host medium. By monitoring the photothermal response, it is possible to quantify the concentration of the analytes binding at the metal nanoparticle surface.

К расчету оптических характеристик и размерных сдвигов поверхностных плазмонов сферических биметаллических наночастиц

Formulas are obtained for the effective relaxation time when the electron mean free path is less or comparable with the characteristic dimensions of the metallic regions. The frequency dispersion of the optical characteristics of spherical bimetallic particles near plasma resonances is calculated in the absence of quantum-size effects. Maintaining the style of the generally accepted description of monometallic particles based on the theories of Drude and Mie, the frequency dependence of the electric dipole polarizability of a two-layer metal nanosphere is analyzed. The appearance of two maxima of polarizability is a consequence of the difference between the metals of the core and the shell. The calculations were performed for Au @ Ag, Ag @ Au, Au @ Pt, Pt @ Au, and Pt @ Pd particles immersed in Teflon. The possibility of controlling the optical characteristics of bimetallic particles by changing their composition and volumetric content of metals has been demonstrated. The calculations of the absorption and scattering cross sections, as well as the optical radiation efficiency of particles in a wide spectral range, have been performed. The possible temperature of bimetallic particles upon absorption of an electromagnetic wave (for the purposes of photothermal therapy of malignant tumors) has been estimated. Key words: bimetallic nanoparticle, surface plasmon, polarizability, absorption cross section, scattering cross section, relaxation time.

Trapping nanospheres within graphene-based heterogeneous plasmonic nano-trench

We theoretically investigate a graphene-based heterogeneous plasmonic nano-trench for trapping noble metal and non-metal nanospheres. We propose heterogeneous plasmonic modeling geometry consisting of silver, gold and graphene for functioning plasmonic trapping operating at the near-infrared spectrum. For our designed model, the vertical potential well is 32 times larger than the Brownian motion energy KBT, indicating that the nanosphere can be stably trapped using the graphene covered gold and silver nano-trench structure. By varying the incident laser wavelength and angle of illumination, the gold nanosphere can be trapped stably 40 nm above the bottom of the proposed nano-trench structure. The graphene layer enhances the localized electric field of the nano-trench in comparison to a pure silver and gold nano-trench structure without a graphene layer on top.

The Synthesis of Metal Nanowires in Liquid Nitrogen

It is shown that nanosized wires with a length of up to ~1 cm or longer from metal nanospheres observed in liquid helium can be efficiently self-assembled in liquid nitrogen, which is a much cheaper and more technologically sophisticated method than the other available methods. The negative effect of the higher temperature can be compensated by applying a uniform electric field with the intensity of ~104 V/cm.

Improvement of Optical Extinction by Various Wideband Dielectrics with Ag and Aunanoparticle on Metal-dielectric-semiconductor Structure in Solar Cell

Background: In this article, photonic extinction performance of Metal Dielectric Semiconductor (MDS) nanostructure has been improved by noble metallic nanosphere such as gold, silver and copper into various wideband dielectrics like silicon di-oxide, aluminum oxide and silicon nitride. Methods: Presently, Plasmonics gives very much interest and closely involves in the main domains of nanophotonics that can control of optical fields at the nanoscale level as well as it can concentrate and enhance the electromagnetic field on the nanometer scale especially in metal dielectric semiconductor (MDS) nanostructure. Results: In plasmonics, noble metals used as nanoparticle where density of electron gas which oscillates at surface Plasmon frequency so we investigate impact of various wideband dielectrics with nanoparticle size for enhancement of extinction in terms of absorption and scattering by using surface Plasmon resonance. Conclusion: At Plasmonic resonance the efficiency will be maximum. If the size of the metallic nanosphere increases, initially the efficiency increases up to a certain wavelength then it becomes sharply decreased.

Plasmonic Nanostructures for Energy Application

The present work furnishes the optical properties of metal nanostructures and their application in energy. Metal supports plasmonic resonances which can be tuned in desired range of solar spectrum by changing its size, shape and surrounding environment. Two different systems, isolated and coupled gold metal nanosphere are taken into account to study the optical properties. These optical properties of chosen nanostructure are analysed in terms of surface plasmon resonances using Boundary Element Method (BEM). The tuning of surface plasmon resonances under different parameter domain and their applications in energy is the main thrust of this work. The simulated extinction spectrum and corresponding spectral width of metallic nanogeometries could be used in several applications like solar still, solar collector and solar cell.

Electrostatic wave propagation in an array of metallic wires

Propagation of electrostatic waves in an array of parallel infinitely long metallic wires is investigated. General expressions for the dispersion relation and polarization are presented. Existence of electrostatic oscillations is due to the anisotropic property of the system. Similar results can be found for the electrostatic plasma waves propagating at an angle to a static magnetic field in an array of metallic nanospheres.

Self-Assembled Arrays of Gold Nanorod-Decorated Dielectric Microspheres with a Magnetic Dipole Response in the Visible Range for Perfect Lensing and Cloaking Applications

Photonic nanostructures made of a dielectric sphere covered with many metallic nanospheres fabricated by self-assembly constitute a basic building block for optical metamaterials with a magnetic re...