Dielectric Host Matrix Research Articles

Tunable optical properties of graphene wrapped ZnO@Ag spherical core-shell nanoparticles

In this paper, we studied theoretically and numerically the material’s response to incident electromagnetic wave of graphene wrapped zinc-oxide/silver (g − ZnO@Ag) core–shell spherical nanoparticles embedded in a dielectric host matrix. As the nanoparticles size is ∼30 nm, a size much smaller than the wavelength of light, the quasi-static approximation is utilized to obtain analytical expressions for the electric polarizability and the corresponding extinction cross-section. It is found that the spectra of the extinction cross-section of g − ZnO@Ag nanoparticles exhibit two sets of localized surface resonance peaks in the visible and near infra-red (NIR) spectral regions. The first set of peaks observed below ∼900 nm are due to the coupling of the energy gap of the ZnO core with the local surface plasmon resonances of Ag shell, and the second set of graphene-assisted narrow peaks located in the NIR region (above ∼900 nm) are attributed to the plasmons excited at the Ag/graphene interface. It is found that the intensity of the extinction cross-section as well as the positions of the resonance wavelengths are interesting that the graphene-assisted narrow peaks are strongly dependent on the number of layers (N g ) and the chemical potential (μ) of graphene. It means that the response of ZnO@Ag core–shell nanoparticles to electromagnetic fields are greatly enhanced when it is wrapped with graphene and can also be tuned in the therapeutic NIR spectral region by varying N g and μ. The results may be used for possible application in the medical fields, especially for cancer detection and drug delivery.

Silver Nanoparticles for Fluorescent Nanocomposites by High-Pressure Magnetron Sputtering

We report on the formation of silver nanoparticles by gas aggregation in a reaction chamber at room temperature. The size distribution of nanoparticles deposited on a silicon substrate for various lengths of an aggregation (high-pressure) chamber was investigated by atomic force microscopy. Nanoparticles were characterized by scanning and transmission electron microscopy and spectral ellipsometry. The physical shape of the nanoparticles and its distribution was correlated with their optical properties. Metal-dielectric nanocomposites were deposited employing simultaneous deposition of Ag NPs via high-pressure magnetron sputtering and the dielectric matrix was deposited via thermal evaporation. Pure and Eu-, Er-, and Yb-doped lithium fluoride was used as the dielectric host matrix. Optical transmittance of lithium fluoride containing silver nanoparticles was measured and their theoretical absorption cross-section calculated. The nanoparticles were also embedded in Eu3+-doped downshifting and Er3+- and Yb3+-doped up-conversion materials to study their influence on emission spectra. Spectra of identical layers with and without nanoparticles were compared. Their transmittance at various annealing temperatures is also presented.

Tailoring the refractive index of impedance-matched ferrite composites

Independent control of the magnetic and electric properties of two-part and three-part ferrite composites is demonstrated through variation of particle size and volume fraction of ferrite inclusions. This provides a route to creating broadband impedance-matched composites with tailored high refractive-index values. A two-part composite comprising NiZn ferrite in a PTFE dielectric host with approximately equal values of relative real permittivity and permeability up to 100 MHz is manufactured. The refractive index for NiZn–PTFE composites, measured at 20 MHz, is 6.1 for NiZn volume fraction of 50%vol. and 6.9 for NiZn volume fraction of 70%vol. Similarly, we have characterised a three-part composite with a refractive index of approximately 16 up to 60 MHz. The three-part composite comprises NiZn and MnZn ferrites in a PTFE dielectric host matrix with a percentage volume ratio of 65%: 15%: 20%, respectively.

Enhancement of the optical response of Fe3O4@Ag core-shell nanoparticles

In this work, the optical properties of Fe3O4@Ag core/shell spherical nanostructures embedded in a dielectric host matrix are investigated theoretically. The theoretical analysis is carried out based on the electrostatic approximation and Maxwell-Garnet effective medium theory to obtain the effective electric permittivity and magnetic permeability, as well as the corresponding refractive index and absorbance. Moreover, for a fixed size of NPs (of radius r2=30nm) numerical analysis is carried out to see the effect of varying the metal fraction (the volume fraction of the metallic shell) (β), the filling fraction (the volume fraction of inclusions in the composite) (f), and the permittivity (εh) of the host matrix on the optical properties of the nanostructures. The results show that graphs of real and imaginary parts of polarizability, refractive index and absorbance as a function of wavelength possess two sets of resonance peaks in the UV and visible regions. These sets of peaks arise due to the strong coupling/interactions of the surface plasmon oscillations of silver with the semiconductor/dielectric at the inner (Fe3O4/Ag) and outer (Ag/host) interfaces and/or to near-field inter-particle interaction. Moreover, the two set of resonance peaks are found to be enhanced with an increase of β,f, or εh; keeping two of these parameters constant at a time. The results obtained can be used in applications that are designed to integrate plasmonic effects of noble metals with magnetic semiconductors in a core/shell nanostructure.

Slow and Fast Lights in Metal/Dielectric Composite of Cylindrical Nanoinclusions in Passive and Active Linear Dielectric Host Matrices

This paper presents theoretical discussions and computational numerical results obtained from the study of extreme values of the speed of light in metal/dielectric composite, where the cylindrical nanoinclusions are uniformly distributed in the linear dielectric host matrix. The results testify that, within our approach, at the region of anomalous dispersion, light can travel with a group velocity greater than the speed of light in vacuum. In a composite with passive host matrix, the light pulse is absorbed within a very small distance. The problem of absorption can be reduced considerably by using an active host matrix.

Size dependent optical properties of ZnO@Ag core/shell nanostructures

In this paper, we studied the effect of size and thickness variation on the optical properties of a system that consists of spherical ZnO@Ag core/shell composite nanostructures embedded in a dielectric host matrix. The effective dielectric function, polarizability, refractive index, and absorbance of the composite nanostructures are determined using the Maxwell-Garnett effective medium theory within the framework of the electrostatic approximation. The numerical simulations using nanoinclusions of radii 20 nm show interesting behavior in the optical responses of the ensemble. In particular, it is shown that for different values of metal fraction and filling factor, the polarizability, refractive index, and optical absorbance of the ensemble exhibit two sets of resonance peaks in the UV (around 300 nm) and visible (between 400 and 640 nm) spectral regions. These peaks are attributed to the surface plasmon resonance of silver at the ZnO/Ag and Ag/host-matrix interface. Moreover, when the Ag shell thickness is increased, the observed resonance peaks are enhanced; accompanied with slight red shifts in the UV and blue shifts in the visible regions. The results obtained may be used in various applications such as sensors and nano-optoelectronics devices in optimizing material parameters to the ‘desired’ values.

Effects of Interfacial Layer on Refractive Index and Propagation of Waves in Small Spherical Metal/ Dielectric Composite

In this work we have studied the effect of interfacial layer on the refractive index, and propagation of waves in small spherical metal/dielectric composite separated by interfacial layers which is randomly embedded in a linear dielectric host matrix. The theoretical, and numerical descriptions are in terms of the interfacial factor (I) by incorporating Taylor expansion and Drude model. The result shows that both the interfacial layer property, and the percentage of the volume fraction of the metallic particles in the composite has an effect on the refractive index, and propagation of waves of the composite when the dielectric functions of the interfacial layer is more metal-like property than dielectric-like property.

Enhancement of local electric field in core-shell orientation of ellipsoidal metal/dielectric nanoparticles

In this paper it is shown that the enhancement factor of the local electric field in metal covered ellipsoidal nanoparticles embedded in a dielectric host matrix has two maxima at two different frequencies. The second maximum for the metal covered inclusions with large dielectric core (small metal fraction $p$) is comparatively large. This maximum strongly depends on the depolarization factor of the core $L_{z}^{(1)}$, keeping that of the shell $L_{z}^{(2)}$ constant and is less than $L_{z}^{(1)}$. If the frequency of the external radiation approaches the frequency of surface plasmons of a metal, the local field in the particle considerably increases. The importance of maximum value of enhancement factor $|A|^{2}$ of the ellipsoidal inclusion is emphasized in the case where the dielectric core exceeds metal fraction of the inclusion. The results of numerical computations for typical small silver particles are presented graphically.

Purcell effect at the percolation transition

We investigate the spontaneous emission rate of a two-level quantum emitter next to a composite medium made of randomly distributed metallic inclusions embedded in a dielectric host matrix. In the near-field, the Purcell factor can be enhanced by two-orders of magnitude relative to the case of an homogeneous metallic medium, and reaches its maximum precisely at the insulator-metal transition. By unveiling the role of the decay pathways on the emitter's lifetime, we demonstrate that, close to the percolation threshold, the radiation emission process is dictated by electromagnetic absorption in the heterogeneous medium. We show that our findings are robust against change in material properties, shape of inclusions, and apply for different effective medium theories as well as for a wide range of transition frequencies.

Local field enhancement at the core of cylindrical nanoinclusions embedded in a linear dielectric host matrix

In this paper we have discussed theoretical concepts and presented numerical results of local field enhancement at the core of different assemblages of metal/dielectric cylindrical nanoinclusions embedded in a linear dielectric host matrix. The obtained results show that for a composite with metal coated inclusions there exist two peak values of the enhancement factor at two different resonant frequencies. The existence of the second maxima becomes more important for a larger volume fraction of the metal part of the inclusion. For dielectric coated metal core inclusions and pure metal inclusions there is only one resonant frequency and one peak value of the enhancement factor. The enhancement of an electromagnetic wave is promising for the existence of nonlinear optical phenomena such as optical bistability which is important in optical communication and in optical computing such as optical switch and memory elements.

Nanoparticles in dielectric matrix for electronics and optics: from the fabrication to the devices

AbstractThe discovery of the nanosize‐related effects has led to tremendous efforts, numerous publications, international symposia and patents on the development and study of nanometer scale based structures. The achievements are specific and novel optical, electronic, and magnetic properties are reported that have never been obtained on a macroscopic scale. Among these nanostructured systems, nanoparticles embedded in a dielectric host matrix offer a wide range of applications in electronics and optics from non‐volatile memories to non‐linear optics, from light emitting diodes to enhanced photovoltaic sensors, and from optical amplifiers to plasmonic substrates. This requires a large range of skills in different fields such as chemistry, physics, and material science.This symposium aimed at gathering scientists and industrial partners from the above fields and involved in the fabrication and the study of such nanostructured systems and related applications. The goal was to share and discuss recent advances in fabrication, doping, optical and/or electrical properties, biosensing, photovoltaics, plasmonics, fundamental mechanisms of growth or excitation, devices that incorporate those properties, and applications of such materials systems. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effective non-retarded method as a tool for the design of tunable nanoparticle composite absorbers

We investigate the capabilities of an effective non-retarded formalism (ENR) for the exploration and design of nanoparticle composites with specific optical properties. We consider a composite material comprising periodically distributed metallic spheres in a dielectric host matrix. The effective macroscopic dielectric function of the composite medium is obtained by means of the ENR and is used to calculate the electromagnetic response of a slab made of an inhomogeneous material. This response is compared with that obtained by using the layer Korringa–Kohn–Rostoker wave calculation method (LKKR). We analyze the optical properties for different filling fractions, especially in the vicinity of the resonance frequencies of the macroscopic dielectric function. We notice that for dense systems within the long wavelength regime, the results of some analytical theories developed by other authors do not properly describe the multipolar excitations and interactions of orders higher than the dipole, in contrast with the results obtained by using an ENR. Therefore, those methods are not suitable for the design of compound films with novel properties. We show that by appropriately choosing the parameters of the composite, it is possible to achieve a tunable absorber film, and more generally, we show that ENR is a versatile tool for the design of nanoparticle composite materials with specific properties.

Effect of swift heavy ion irradiation on lead sulfide quantum dots embedded in polyvinyl alcohol

The present study compares structural and optical modifications of lead sulfide (PbS) quantum dots under swift heavy ion (SHI) irradiation. PbS quantum dots are prepared following an inexpensive chemical route using polyvinyl alcohol as the dielectric host matrix. SHI irradiation of the samples is carried out with 100 MeV Si7+ ion beam with fluences in the range from 1×1011 to 3×1013 ions cm−2 and their structural and optical properties, before and after irradiation, are compared by X-ray diffraction (XRD), photoluminescence (PL) and UV–Vis spectroscopy. The structural parameters of PbS quantum dots are studied by X-ray line profile analysis using the Williamson–Hall plot. The values of average crystallite sizes are found to vary from 8 to 18 nm. XRD studies confirmed the formation of the cubic nanocrystalline PbS. Improvement of crystalline quality for lower fluences is exhibited by an increase in the X-ray intensities of the films. The UV–Vis absorption spectra reveal blue shift relative to the bulk material. Size enhancement of PbS quantum dots after irradiation has been indicated in XRD line profiles of the samples which has also been supported by optical absorption spectra. PL studies of all the samples are carried out with excitation wavelength 350 nm. A broad PL emission with peak centered at 473 nm is observed in pristine as well as all the irradiated samples. PL study also shows that PL intensity increases with ion fluences.

Permittivity of Dielectric Composite Materials Comprising Graphene Nanoribbons. The Effect of Nanostructure

New lightweight, flexible dielectric composite materials were fabricated by the incorporation of several new carbon nanostructures into a dielectric host matrix. Both the permittivity and loss tangent values of the resulting composites were widely altered by varying the type and content of the conductive filler. The dielectric constant was tuned from moderate to very high values, while the corresponding loss tangent changed from ultralow to extremely high. The data exemplify that nanoscale changes in the structure of the conductive filler result in dramatic changes in the dielectric properties of composites. A microcapacitor model most explains the behavior of the dielectric composites.

Performance of electro-optical plasmonic ring resonators at telecom wavelengths

In this work we report on the characteristics of an electro-optical dielectric-loaded surface plasmon polariton waveguide ring resonator. By doping the dielectric host matrix with an electro-optical material and designing an appropriate set of planar electrodes, we measured a 16% relative change of transmission upon application of a controlled electric field. We have analyzed the temporal response of the device and conclude that electrostriction of the host matrix is playing a dominating role in the transmission response.

Low-Loss, High-Permittivity Composites Made from Graphene Nanoribbons

A new composite material was prepared by incorporation of graphene nanoribbons into a dielectric host matrix. The composite possesses remarkably low loss at reasonably high permittivity values. By varying the content of the conductive filler, one can tune the loss and permittivity to desirable values over a wide range. The obtained data exemplifies how nanoscopic changes in the structure of conductive filler can affect macroscopic properties of composite material.

Magnetic Behavior of Barium Hexaferrite Nanoparticles

An analysis of the magnetic behavior of Barium Hexaferrite nanoparticles is presented in this paper. The particles’ average size is around 200 nm. The SU-8 photoresist is used as dielectric host matrix in which the particles are embedded. The volume fraction of magnetic particles reached is around 10%. Microwave characterizations show the properties of the resulting nanocomposite. The external applied magnetic field plays the main role in orienting these magnetic particles in the composite. Coplanar CPW lines were used to investigate the magnetic material’s efficiency by its non-reciprocal effects. Magnetic nanocomposites seem promising material for future applications. Thus, higher magnetic particle volume fractions and better orientation are required.

Microwave Properties of Mechanosynthesized Fe-Paraffin Nanocomposites

The structure and microwave properties of composites consisting of paraffin as a dielectric host matrix and Fe particles as inclusions have been studied. The composites were prepared by the mechanochemical synthesis and contain 7.7% vol. concentration of the inclusions. The microwave properties were studied in 0.1 to 6 GHz frequency range. The microwave properties of the composites are shown to depend mostly on the size and shape of the inclusions, with the phase composition of the inclusions exerting only a slight effect. With decreasing the inclusions size, the low-frequency peak (300−400 MHz) attributed to the domain wall motion diminishes.

Effect of swift heavy ion irradiation on bare and coated ZnS quantum dots

The present study compares structural and optical modifications of bare and silica (SiO 2) coated ZnS quantum dots under swift heavy ion (SHI) irradiation. Bare and silica coated ZnS quantum dots were prepared following an inexpensive chemical route using polyvinyl alcohol (PVA) as the dielectric host matrix. X-ray diffraction (XRD) and transmission electron microscopy (TEM) study of the samples show the formation of almost spherical ZnS quantum dots. The UV–Vis absorption spectra reveal blue shift relative to bulk material in absorption energy while photoluminescence (PL) spectra suggests that surface state and near band edge emissions are dominating in case of bare and coated samples, respectively. Swift heavy ion irradiation of the samples was carried out with 160 MeV Ni 12+ ion beam with fluences 10 12 to 10 13 ions/cm 2. Size enhancement of bare quantum dots after irradiation has been indicated in XRD and TEM analysis of the samples which has also been supported by optical absorption spectra. However similar investigations on irradiated coated quantum dots revealed little change in quantum dot size and emission. The present study thus shows that the coated ZnS quantum dots are stable upon SHI irradiation compared to the bare one.

Counterintuitive thermo-optical response of metal-dielectric nanocomposite materials as a result of local electromagnetic field enhancement

Thermo-optical properties of composite materials consisting of noble metal nanoparticles spread in a dielectric host matrix are relevant for various fields in fundamental and applied nanosciences. We present a calculation of these properties in the framework of the Maxwell-Garnett effective medium model, exemplified by the case of gold nanoparticles in silica. The spectral variations of bulk gold thermo-optical coefficients, including implicitly the temperature dependence of intra- and interband transitions, are first extracted from experimental results of the literature. The composite material effective thermo-optical coefficients are then determined, accounting also for particle and matrix volume thermal expansion as well as temperature dependence of the matrix refractive index. These calculations lead to counterintuitive results in the vicinity of the surface plasmon resonance of the nanoparticles, originating from the local electromagnetic field enhancement. The findings are used to simulate the temperature dependence of the optical absorption spectrum of a Au:glass composite material. The result is compared with experimental data from the literature with good agreement.