Optical and spectral tunability of multilayer spherical and cylindrical nanoshells

This theoretical work presents a comparative study of the optical properties and spectral tunability of hybrid multilayer spherical and cylindrical nanoshells based on the quasi-static approximation of classical electrodynamics. The interband transitions have been considered using the Drude–Lorentz model for the complex dielectric function of metallic layers because the optical properties of metals arise from both the optical excitation of interband transitions and the free-electron response. A general formula for N-ayer concentric nanoshells is arranged, and numerical calculations are performed for the four-layer nanoshells as an example. We have analyzed in detail different configurations of nanoshells such as dielectric-metal-dielectric-metal with dielectric core, metal-dielectric-metal-dielectric with metal core and semiconductor-metal-dielectric-metal with semiconductor core because composition of nanoshells have dramatic influence on their optical properties. The absorbance spectrum behavior of the shell thicknesses, surrounding medium, shape and composition of each layer of the nanoshell is numerically investigated.     

Engineered inorganic core/shell nanoparticles

It has been for a long time recognized that nanoparticles are of great scientific interest as they are effectively a bridge between bulk materials and atomic structures. At first, size effects occurring in single elements have been studied. More recently, progress in chemical and physical synthesis routes permitted the preparation of more complex structures. Such structures take advantages of new adjustable parameters including stoichiometry, chemical ordering, shape and segregation opening new fields with tailored materials for biology, mechanics, optics magnetism, chemistry catalysis, solar cells and microelectronics. Among them, core/shell structures are a particular class of nanoparticles made with an inorganic core and one or several inorganic shell layer(s). In earlier work, the shell was merely used as a protective coating for the core. More recently, it has been shown that it is possible to tune the physical properties in a larger range than that of each material taken separately. The goal of the present review is to discuss the basic properties of the different types of core/shell nanoparticles including a large variety of heterostructures. We restrict ourselves on all inorganic (on inorganic/inorganic) core/shell structures. In the light of recent developments, the applications of inorganic core/shell particles are found in many fields including biology, chemistry, physics and engineering. In addition to a representative overview of the properties, general concepts based on solid state physics are considered for material selection and for identifying criteria linking the core/shell structure and its resulting properties. Chemical and physical routes for the synthesis and specific methods for the study of core/shell nanoparticle are briefly discussed.     

The effect of the character of electron reflection on the electromagnetic properties of an inhomogeneous spherical particle

The magnetic absorption cross section of a small spherical particle with a dielectric core and a metallic shell is calculated. The general case is considered when the ratio of the radius of the dielectric core to the total radius of the particle may take arbitrary values. The condition of specular-diffuse reflection of conduction electrons from the surfaces of the metal layer of the particle is chosen as the boundary conditions of the problem. The limit cases are considered, and the results are discussed.     

Metal Nanoparticle Coating of Oxide Nanospheres for Core‐Shell Structures

A low‐temperature route for coating oxide nanospheres with metal nanoparticles to achieve core‐shell structures is introduced. First results indicating a dense coverage of silica nanospheres of about 300 nm size with regularly arranged Ag and Au nanoparticles deposited by a modified incipient wetness impregnation procedure are presented. This synthesis works completely without external reducing agents or media, adhesive aids or functionalizing agents. Metal particles of only a few nanometers in size may serve as seeds for continuous metal coating of the oxide spheres by complementary processes. Structural characterization of the materials by transmission electron microscopy reveals a nearly spherical shape of the metal particles, the structure of which ranges from single crystalline to single twinned and multiply twinned configurations.     

Artificial isotropic magnetism and negative refraction of metamaterial consisting of dielectric spherical shells

We present a detailed study on the artificial magnetism of high-permittivity dielectric spherical shell array and the negative refraction behavior of the composite consisting of dielectric spherical shells and metal wire lattice. The spherical shell array in cubic lattice arrangement can exhibit an isotropic negative magnetism when excited into magnetic dipole resonance. When the dielectric spherical shell and metal wire lattice are combined in a body-centered cubic structure, full-wave simulations show that the bulk metamaterial with this structure can exhibit a negative refraction behavior. It is shown that artificial magnetic response can originate from differently structured dielectric elements, similarly to the conventional metamaterial constituents based on metal resonance structures.     

Growth of three-shell onionlike bimetallic nanoparticles

We show by molecular dynamics simulations on three systems (B/A=Pd/Ag, Cu/Ag, and Ni/Ag) that three-shell metallic nanoparticles made by a core of a metal A, an intermediate shell of metal B and an external shell of metal A (A-B-A nanoparticles) can be grown by deposition of B atoms onto an A core. The growth of the intermediate B shell is triggered by the fact that the most favorable positions for isolated B impurities inside A clusters are located just one layer below the cluster surface.     

Optical Resonances of the Systems of Nanoparticles with a Metallic Shell

Characteristic features of the formation of the plasma resonance absorption spectra of double-layer nanoparticles with a dielectric core and metal shell were investigated theoretically and experimentally. Two peaks of the surface plasma resonances were observed with the example of an AgI–Ag system. The model of the conductivity electron free path limitation suggested by Kreibig for describing the dimensional dependence of the optical constants of homogeneous spherical metal nanoparticles was extended to the case where metal is concentrated in the shell of the particle. It is established that allowance for the dimensional effect leads to a decrease in plasma resonance absorption and expansion, with the two-peak band structure being preserved. The influence of the metal shell granularity and the degree of the polydispersity of particles on the spectral position, halfwidth, and absolute value of absorption resonances was investigated.     

Synthesis of core–shell nanoparticles with a Pt nanoparticle core and a silica shell

A method to prepare a core–shell structure consisting of a Pt metal core coated with a silica shell (Pt(in)SiO₂) is described herein. A silica shell was grown on poly(vinylpyrrolidone) (PVP)-stabilized Pt nanoparticles 2–3 nm in size through hydrolysis and condensation reactions of tetraethyl orthosilicate (TEOS) in a water/ethanol mixture with ammonia as a catalyst. This process requires precise control of the reaction conditions to avoid the formation of silica particles containing multiple Pt cores and core-free silica. The length of PVP molecules, water content, concentration of ammonia and Pt nanoparticles in solution were found to significantly influence the core–shell structure. By optimizing these parameters, it was possible to prepare core–shell particles each containing a single Pt nanoparticle with a silica layer coating approximately 10 nm thick.     

Linear and nonlinear properties for a dilute suspension of coated ellipsoids

The effects of geometric structures of coated particles on the dielectric properties of composites are derived. For a dilute suspension of coated ellipsoids with dielectric core and metallic shell embedded in a linear dielectric host, we find that the optical absorption peak of maximum wavelength shifts due to the core-shell structure, and has a significant red shift from that of pure metallic particles, especially for thin metallic coating. Meanwhile, the shape of coated ellipsoid can tune the absorption peak in a wide frequency range by properly choosing the depolarization factor. When the composite is made of linear materials, we obtain the condition of partial resonance for coated ellipsoids, which is relative to the particle shape. Under such a condition, the property of the inner core can be extended to the outer shell. When the inner core is a weakly nonlinear material, the nonlinearity of the composite can be greatly enhanced at the linear partial resonance. To achieve the condition of partial resonance, a metallic shell may be suitable to reach the case if the frequency is far away from the plasmon frequency. It indicates that the nonlinearity can also be enhanced in terms of the geometric structure of materials as well as the properties of themselves.     

A novel two-phase thermal approach for synthesizing CdSe/CdS core/shell nanostructure

CdSe/CdS core/shell nanocrystals have been synthesized through a low cost and simple two-phase thermal route. The optical spectroscopy and structural characterization evidenced the core/shell structure of the CdSe/CdS nanoparticles. The X-ray diffraction patterns of CdSe and CdSe/CdS nanoparticles exhibited peak positions corresponding to those of their bulk cubic crystal structures. The X-ray photoelectron spectroscopy data confirmed the elemental composition of the CdSe/CdS nanoparticles. The absorption spectra of core/shell nanoparticles showed red shift with respect to the core CdSe nanoparticles. The photoluminescence study indicates that the intensity of the emission maximum is considerably increased in the core/shell structure as compared with the parent material, and the capping of CdS nanoparticles with CdSe material exhibit a near band-edge emission, indicating a successful passivation by removing surface defects. The high-resolution transmission microscope images of the bare and core/shell nanoparticles ascertained the monodispersed and well-defined spherical particles. The average particle sizes for CdSe and CdSe/CdS nanoparticles are 2.5 and 5 nm, respectively, thus confirming, the larger diameter of CdSe/CdS core/shell nanostructure than the core CdSe nanoparticles.     

Modelling of isotropic double negative media for microwave applications

A composite medium consisting of two sublattices of dielectric spherical particles of high permittivity and different radii embedded in a dielectric matrix of smaller permittivity are considered. It has been shown that such a composite medium reveals properties of an isotropic double negative media (DNG) in a limited frequency range, when resonance oscillations of H_III mode in one kind of particles and E_III mode in another kind of particles are excited simultaneously. The E_III resonance and the H_III resonance give rise to the magnetic dipole momentum and the electric dipole momentum correspondingly. Averaging the magnetic momentum and the electric momentum over the cells belonging to the appropriate spherical particles gives the negative permittivity and permeability. The model of diffraction of a plane electromagnetic wave on a dielectric sphere is presented and compared with the mixing rule based consideration. The results obtained are rather close. Distribution of the electromagnetic wave outside the sphere is calculated. Influence of the dispersion of the sphere size and the dielectric permittivity on the effective parameters of the DNG material is estimated.     

High quality and tuneable silica shell–magnetic core nanoparticles

Obtaining small (<50 nm), monodispersed, well-separated, single iron oxide core–silica (SiO₂) shell nanoparticles for biomedical applications is still a challenge. Preferably, they are synthesised by inverse microemulsion method. However, substantial amount of aggregated and multicore core–shell nanoparticles is the undesired outcome of the method. In this study, we report on the production of less than 50 nm overall size, monodispersed, free of necking, single core iron oxide–SiO₂ shell nanoparticles with tuneable shell thickness by a carefully optimized inverse microemulsion method. The high degree of control over the process is achieved by understanding the mechanism of core–shell nanoparticles formation. By varying the reaction time and precursor concentration, the thickness of silica layer on the core nanoparticles can be finely adjusted from 5 to 13 nm. Residual reactions during the workup were inhibited by a combination of pH control with shock freezing and ultracentrifuging. These high-quality tuneable core–shell nanocomposite particles exhibit superparamagnetic character and sufficiently high magnetization with great potential for biomedical applications (e.g. MRI, cell separation and magnetically driven drug delivery systems) either as-prepared or by additional surface modification for improved biocompatibility.     

Novel Two-Step Approach for the Synthesis of Cadmium Selenide/Zinc Sulfide Core/Shell Nanocomposites with Precursor Injection Technique

This work describes the synthesis of cadmium selenide/zinc sulfide core/shell nanoparticles from the cadmium oxide precursor through a convenient, two-step approach. This modified novel synthesis procedure for cadmium selenide/zinc sulfide nanoparticles in trioctylphosphine oxide and trioctyl phosphine provides better control over growth dynamics. The outer zinc sulfide shell provides efficient confinement of electron and hole wave functions inside the nanocrystals as well as high photochemical stability. The materials have been characterized using a range of optical and structural techniques. The high resolution transmission electron microscope micrographs of the cadmium selenide/zinc sulfide core/shell nanoparticles show well-defined spherical particles with an increase in diameter as compared to the parent cadmium selenide material. Thus, the present simulation and its extension can give insight to the understanding of the formation of core/shell or other heterostructures in different kinds of self-assembled aggregates.     

Modeling of light absorption and scattering by metal nanoparticles coated with organic dye J-aggregate

The results of the theoretical study of optical properties of composite nanoparticles consisting of a metal core (Ag, Au, Cu, Al, Ni, Cr) and a J-aggregate shell of organic dye are presented. Light extinction, absorption, and scattering coefficients in colloidal solutions were calculated within the model based on the Mie theory modified taking into account dimensional phenomena and complemented by calculations of complex dielectric functions of the metal core and J-aggregate shell. The model adequately explains the features observed in light absorption and scattering spectra by hybrid nanoparticles, associated with the plasmon resonance in the metal core and with electronic excitation of the J-aggregate. The strong dependence of the results on geometrical parameters of nanoparticles and dielectric constants of core and shell materials was demonstrated. Methods for controlling the effects of the plasmon-exciton interaction in the system and optical properties of composite materials developed based on nanoparticles under study are discussed.     

Nonlinear optical response from composites based on nanostructures consisting of an absorbing core and metallic shell near the plasmon resonance

The object of experimental study and numerical simulation is the nonlinear optical response from composites with nanoparticles consisting of a nonlinearly absorbing dielectric core and a metallic shell. It is shown that a small spread in nanoparticle sizes near the plasmon resonance may significantly change the dependence of the nonlinear optical response on the concentration of nanoparticles of each size.     

Optodynamic phenomena in aggregates of polydisperse plasmonic nanoparticles

We propose an optodynamical model of interaction of pulsed laser radiation with aggregates of spherical metallic nanoparticles embedded into host media. The model takes into account polydispersity of particles, pair interactions between the particles, dissipation of absorbed energy, heating and melting of the metallic core of particles and of their polymer adsorption layers, and heat exchange between electron and ion components of the particle material as well as heat exchange with the interparticle medium. Temperature dependence of the electron relaxation constant of the particle material and the effect of this dependence on interaction of nanoparticles with laser radiation are first taken into consideration. We study in detail light-induced processes in the simplest resonant domains of multiparticle aggregates consisting of two particles of an arbitrary size in aqueous medium. Optical interparticle forces are realized due to the light-induced dipole interaction. The dipole moment of each particle is calculated by the coupled dipole method (with correction for the effect of higher multipoles). We determined the role of various interrelated factors leading to photomodification of resonant domains and found an essential difference in the photomodification mechanisms between polydisperse and monodisperse nanostructures.     

Optical bistability in plasmonic nanoparticles: Effect of size,shape and embedding medium

We theoretically investigate the optical bistability, which one input signal allows two possible outputs, from single spherical/cylindrical nanoparticles and also nanoshells in the frame work of quasi-static formalism. It is shown that the bistability behavior greatly depends on several parameters such as the nanoparticle size, material and the surrounding dielectric environment. We demonstrated the width of the bistability region and also the bistable threshold depends on the geometrical parameters, and can be tuned by adjusting the size of nanoparticle, the shell thickness and the dielectric constant of the embedding medium. It is also shown that the optical bistable behavior depends strongly on the shape of plasmonic nanoparticles and nanoshells. However, these dependences of optical bistability of spherical/cylindrical nanoparticles and nanoshells on changing of their geometrical parameters can be used for realize optical switching and sensing purposes.     

Microwave-Assisted Synthesis of Core–Shell Nanoparticles—Insights into the Growth of Different Geometries

Microwave irradiation is utilized for the rapid synthesis of gold–silver core–shell bimetallic nanoparticles (NPs) in a two-step process. A strategy of establishing a bilayer organic barrier around the core using citrate and ascorbic acid as capping agents, providing a means to achieve a well-defined boundary layer between the core and the shell material, is reported. These boundary layers are essential for synthesizing different core–shell morphologies and the approach results in tunable bimetallic NPs with defined core–shell structures, both for spherical as well as for triangular seed cores. In addition, theoretical calculations of the plasmonic characteristics based on the boundary element method of different classes of NPs are conducted. These investigations enable conclusions to be drawn on the influence of the core morphology on the tunability of their localized surface plasmon resonances.     

Zn/ZnO纳米颗粒的表面声子Raman散射

Ｚｎ／ＺｎＯ纳米颗粒的表面声子Ｒａｍａｎ散射许建峰黄亚彬莫育俊（河南大学物理系开封４７５００１）ＲａｍａｎＳｃａｔｅｒｉｎｇｆｒｏｍＳｕｒｆａｃｅＰｈｏｎｏｎｓｉｎＺｎ／ＺｎＯＮａｎｏｐａｒｔｉｃｌｅｓＸｕＪｉａｎｆｅｎｇ，ＨｕａｎｇＹａｂｉｎ，Ｍｏ...     

Antimicrobial and antitumor activities of 1,2,4‐triazoles/polypyrrole chitosan core shell nanoparticles

Combination of natural biodegradable polymer with a synthetic polymer offers excellent properties for the support in drug delivery system. For this purpose, biodegradable conductive nanoparticle polypyrrole based on chitosan (PPC) has been prepared via oxidative polymerization of pyrrole in presence of chitosan using FeCl₃ as oxidant in acidic medium and used as a carrier for 1,2,4‐triazoles. The resultant nanoparticles were characterized by X‐ray diffraction, Fourier transform infrared analysis, transmission electron microscopy, scanning electron microscopy, and thermal gravimetric analysis. The results indicate that spherical nanoparticle of average diameter 52 ± 8 nm was successfully prepared. The spherical particles were composed of dark sphere surrounded by grey shell. A circumferential dark ring is observed in the shell after loading 1,2,4‐triazoles into PPC nanoparticles. The loaded triazoles were released almost linearly against time in a sustained fashion into different pH media. The mechanism of triazoles release was determined using different kinetics equations. The antibacterial activities against the gram‐negative and gram‐positive bacteria were examined. Furthermore, the antitumor activity of PPC nanoparticles loaded 1,2,4‐triazoles was also examined against Ehrlich ascites carcinoma cells and breast cancer cell line (MCF7). Polypyrrole chitosan loaded nanoparticles exhibited higher antitumor activity than 1,2,4‐triazoles.