Elliptical Nanoparticles Research Articles

Optical properties of a metal film perforated with coaxial elliptical hole arrays

The optical properties of a metal film perforated with coaxial elliptical hole arrays have been investigated experimentally and a simple analysis model that qualitatively explains the experimental results has been presented. In our structure, two localized excitations, i.e., the short- and long-axis localized surface-plasmon modes of the elliptical nanoparticles or nanoholes can be excited, which couples, respectively, with the surface-plasmon polariton modes and causes different optical response. As a consequence, the transmission features can be manipulated by the polarization state of the incident light.

Self-assembled necklace-like polyaniline nanochains from elliptical nanoparticles

High quality necklace-like polyaniline nanochains assembled by elliptical nanoparticles have been synthesized in chitosan aqueous solution by a facile dispersion polymerization method. The synthetic procedure is very simple and easy to scale up. The necklace-like nanochains coated by a layer of chitosan are typical doped polyaniline in its emeraldine salt form, which is easy to form stable polyaniline dispersion in water. Open-circuit potential measurements show that the rate of polymerization of aniline has a substantial decrease due to the steric effects of chitosan. The synthetic parameters, such as reaction times, and the concentrations of chitosan, aniline, and dopant acid, have profound influences on the sizes and morphologies of polyaniline nanostructures. A new self-assembly process of elliptical nanoparticles has been proposed for the formation of necklace-like polyaniline nanochains with fluctuant diameters, which is different from that of other one-dimensional polyaniline nanostructures including nanofibers and nanotubes.

Surface Plasmon Resonance and Enhanced Fluorescence Application of Single-step Synthesized Elliptical Nano Gold-embedded Antimony Glass Dichroic Nanocomposites

A novel series of elliptical gold (Au0) nanoparticles (18–40 nm) embedded antimony glass (K2O-B2O3-Sb2O3) dichroic nanocomposites have been synthesized by a single-step melt-quench in-situ thermochemical reduction technique. X-ray and selected area electron diffractions manifest growth of Au0 nanoparticles along the (111) and (200) crystallographic planes. The transmission electron microscopic image reveals elliptical Au0 nanoparticles having an aspect ratio varying in the range 1.2–2.1. The dichroic behavior of the nanocomposites arises due to elliptical shape of the Au0 nanoparticles. These nanocomposites show strong surface plasmon resonance (SPR) band of Au nanoparticles in the range 610–681 nm and it exhibit red-shifts with increasing Au concentration. They, when co-doped with Sm2O3 and excited at 949 nm, exhibit about sevenfold enhancement of the upconverted red emission transition of 4G5/2→6H9/2 at 636 nm due to local electric field enhancement effect of Au0 nanoparticles induced by its SPR. These nanocomposites are the promising materials for laser, display, and various nanophotonic applications.

Enhanced frequency upconversion of Sm3+ ions by elliptical Au nanoparticles in dichroic Sm3+: Au-antimony glass nanocomposites

Dichroic Sm(3+): Au-antimony glass nanocomposites are synthesized in a new reducing glass (dielectric) matrix (mol%) K(2)O-B(2)O(3)-Sb(2)O(3) (KBS) by a single-step melt-quench technique involving selective thermochemical reduction. X-ray diffraction (XRD) and selected area electron diffraction (SAED) results indicate that Au(0) nanoparticles are grown along the (200) plane direction. The transmission electron microscopic (TEM) image reveals the elliptical Au(0) nanoparticles having major axis range 12-17 nm. Dichroic behavior is due to elliptical shape of Au(0) nanoparticles of aspect ratio approximately 1.2. Au(0) NPs of concentration of 0.03 wt% (4.1 x 10(18)atoms/cm(3)) drastically enhances the intensity ( approximately 7-folds) of electric dipole (4)G(5/2)-->(6)H(9/2) red transition (636 nm) of Sm(3+) ions and then attenuates with further increase in Au(0) concentration. The magnetic dipole (4)G(5/2)-->(6)H(5/2) green (566 nm) and (4)G(5/2)-->(6)H(7/2) orange (602 nm) transitions remain almost unaffected by presence of nano Au(0). Local field enhancement (LFE) induced by Au(0) SPR and energy transfer (ET) from fluorescent Au(0)-->Sm(3+) ions are found to be responsible for the enhancement while reverse ET from Sm(3+)-->Au(0) and optical re-absorption due to Au(0) SPR for attenuation.

Scattering and Absorption Properties of Multiply Coated Magnetic Nanoparticles

The optical properties of a material are characterized by its electric and magnetic susceptibilities. Finding analytical expressions for those quantities for a nanoparticle of arbitrary shape is generally a formidable task. A great deal of insight in to the basic phenomena can be obtained by studying analytically solvable cases, for nanoparticles of ellipsoidal and spherical shapes. We present here our study on the scattering and absorptive properties of multiply coated magnetic spherical and elliptical nanoparticles as functions of frequency of an incident electromagnetic radiation. We will present our results based on the analytical expressions for the dielectric and magnetic susceptibilities derived for such multiply coated nanoparticles. To our knowledge, the latter results are new. We will also present our results based on Maxwell-Garnett theory for a material having uniform distribution of mono dispersed multiply coated nanoparticles.

Scaling analysis and application: Phase diagram of magnetic nanorings and elliptical nanoparticles

The magnetic properties of single-domain nanoparticles with different geometric shapes, crystalline anisotropies, and lattice structures are investigated. A recently proposed scaling approach is shown to be universal and in agreement with dimensional analysis coupled with the assumption of incomplete self-similarity. It is used to obtain phase diagrams of magnetic nanoparticles featuring three competing configurations: in-plane ferromagnetism, out-of-plane ferromagnetism, and vortex formation. The influence of the vortex core on the scaling behavior and phase diagram is analyzed. Three-dimensional phase diagrams are obtained for cylindrical nanorings depending on their height and outer and inner radii. The triple points in these phase diagrams are shown to be in a linear relationship with the inner radius of the ring. Elliptically shaped magnetic nanoparticles are also studied. A new parametrization for double vortex configurations is proposed, and regions in the phase diagram where the double vortex is a stable ground state are identified.

Near-field second-harmonic generation in single gold nanoparticles

Second-harmonic generation from single gold elliptical nanoparticles is experimentally investigated by a nonlinear scanning near-field optical microscope (SNOM). The near-field nonlinear response is found to be directly related to local surface plasmon resonances and to particle morphology. The combined analysis of linear and second-harmonic SNOM images provides discrimination among different light extinction particle behaviors, not achievable just with linear techniques. The polarization state of the emitted second harmonic is also investigated, providing experimental evidence of second-harmonic particle emission modes peculiar to near-field excitation.

Enhanced localized fluorescence in plasmonic nanoantennae

Pairs of gold elliptical nanoparticles form antennae, resonant in the visible. A dye, embedded in a dielectric host, coats the antennae; its emission excites plasmon resonances in the antennae and is enhanced. Far-field excitation of the dye-nanoantenna system shows a wavelength-dependent increase in fluorescence that reaches 100 times enhancement. Near-field excitation shows enhanced fluorescence from a single nanoantenna localized in a subwavelength area of ∼0.15μm2. The polarization of enhanced emission is along the main antenna axis. These observed experimental results are important for increasing light extraction from emitters localized around antennae and for potential development of a subwavelength sized laser.

Effects of size and interactions on the magnetic behaviour of elliptical (0 0 1)Fe nanoparticles

Arrays of elliptical particles with aspect ratio 1:3 and short axes 50, 100 and 150 nm were prepared by electron-beam lithography and ion-beam milling of epitaxial (0 0 1)Fe films of thicknesses 10 and 20 nm. The domain state of an individual particle imaged by magnetic force microscopy in zero field after demagnetization was observed to change from being bi-domain or multidomain (MD) to stable single domains (SD) as the lateral size and film thickness were decreased. The critical size for SD formation was found to be close to the actual lateral sizes of 100 nm×300 nm and 150 nm×450 nm for the thicknesses of 20 and 10 nm, respectively. Only in the 10 nm thick ellipses of lateral size 100 nm×300 nm, the magnetization reversal may take place through coherent rotation. For all other investigated samples, the experimental switching field is lower than what would be required for this process.

An FDTD method for the simulation of dispersive metallic structures

In this paper, we present a formulation of the finite-difference time-domain method for the simulation of metallic structures. The frequency dependent dielectric function of metals is approximated by a combined Drude–Lorentzian multi-pole expansion and fitting errors of only a few percent are obtained. An auxiliary differential equation technique is used to extend the standard FDTD algorithm with the dispersive material equations. The algorithm is validated by calculating reflection and transmission coefficients for thin metal layers, elliptical nano-particles and by simulating a surface plasmon resonance device. Excellent agreement between the FDTD simulations and exact theoretical results are obtained.

Transitions between the states with uniform and vortex distributions of magnetization in ferromagnetic nanoparticles under the action of an inhomogeneous magnetic field

Specific features of magnetization distribution in elliptical Co nanoparticles have been investigated by magnetic force microscopy. Reversible transitions (induced by the microscope magnetic probe) between the uniform and vortex magnetization states have been found. The possibility of controlling the vorticity direction in such particles is shown.

Near-field excitation of nanoantenna resonance

An array of paired elliptic nanoparticles designed to enhance local fields around the particle pair is fabricated with gold embedded in quartz. Light excites a coupled plasmon resonance in the particle pair and the system acts like a plasmonic nanoantenna providing an enhanced electromagnetic field. Near-field scanning optical microscopy and finite element modeling are used to study the local field effects of the nanoantenna system. Local illumination shows similar resonant properties as plane wave illumination: a strong, localized optical resonance for light polarized parallel to the main, center-to-center axis.

Synthesis and characterization of the biodegradable polycaprolactone‐graft‐chitosan amphiphilic copolymers

A novel synthetic approach to biodegradable amphiphilic copolymers based on poly (epsilon-caprolactone) (PCL) and chitosan was presented, and the prepared copolymers were used to prepare nanoparticles successfully. The PCL-graft-chitosan copolymers were synthesized by coupling the hydroxyl end-groups on preformed PCL chains and the amino groups present on 6-O-triphenylmethyl chitosan and by removing the protective 6-O-triphenylmethyl groups in acidic aqueous solution. The PCL content in the copolymers can be controlled in the range of 10-90 wt %. The graft copolymers were thoroughly characterized by 1H NMR, 13C NMR, FT-IR and DSC. The nanoparticles made from the graft copolymers were investigated by 1H NMR, DLS, AFM and SEM measurements. It was found that the copolymers could form spherical or elliptic nanoparticles in water. The amount of available primary amines on the surface of the prepared nanoparticles was evaluated by ninhydrin assay, and it can be controlled by the grafting degree of PCL.

Controlling the Nanostructure of RuO2 / Carbon Nanotube Composites by Gas Annealing

We fabricated nanotube (CNT) composites by coating the CNTs with an amorphous layer using metallorganic chemical vapor deposition followed by postannealing at a relatively low temperature of 350°C. It was found that the kind of gas used during the postannealing process should affect the nanostructure of the composites quite significantly. Starting with a thin coating layer, we could obtain relatively uniform-sized spherical and elliptical nanoparticles by using Ar and gas, respectively, during the postannealing. The growth behaviors of the nanoparticles could be explained in terms of surface self-diffusion. The surface diffusion process can provides a way to fabricate nanocomposites with a controllable geometrical shape and size, whose process can be used for diverse applications, such as high-performance supercapacitor electrodes, electrocatalysts, gas sensors, and new electrodevices. © 2004 The Electrochemical Society. All rights reserved.

Simulation of hysteresis in magnetic nanoparticles with Nosé thermostating.

The magnetic hysteresis of a two-dimensional lattice of rotors with four-way anisotropy interaction and a Heisenberg exchange interaction is studied. The Hamiltonian dynamics of the lattice is thermostated using the Nosé thermostat, resulting in a system that approaches thermal equilibrium and which under certain conditions can remain in metastable states. Using physically realistic values for the interactions in a nanoparticle of monolayer thickness, we locate the Curie temperature of our lattice by determining the peak of the heat capacity curve. We then compare the coercive field of our two-dimensional lattice below this Curie temperature to the coercive field of an elliptical cobalt nanoparticle measured in experiment. We find an order of magnitude agreement between our lattice model and the experimental results, even though the value of the anisotropy used is more appropriate for a monolayer film than for the nanoparticle.

Nanostructures of pyrolytic carbon from a polyacetylene thin film

Pyrolysis of a polyacetylene thin film has been performed in order to carbonize at temperatures of 500 to 1000 °C in vacuum. A trans-polyacetylene thin film was synthesized using a Ziegler–Natta catalyst. A black char below 20% in weight of the original PA film remained after pyrolysis. Structural properties and morphology of the black chars were investigated using Raman scattering spectrum, X-ray diffraction measurements, and scanning and transmission electron microscopy. Dehydrogenation and carbonization of the PA film were almost finished at a pyrolysis temperature of 800 °C. However, hollow spherical or elliptical nano-particles of tens of nanometers in size, which are composed of graphite structure, were included in the black chars obtained at all pyrolysis temperatures in this work. The formation mechanism of a graphite crystal in nanometer size from a PA crystal was discussed.

Shape-selective Raman scattering from surface phonon modes in aggregates of amorphous SiO2 nanoparticles

A spontaneous Raman scattering from the surface phonon modes (SPMs) of amorphous silica (SiO2) nanoparticles which is small compared with the excitation wavelength (514.5 nm) and aggregated in micron size clusters (globules) is utilized to determine the shape of particles. The excitation dynamics for these modes were studied for silica samples pretreated at different temperatures (30–1000 °C). The SPMs are located between Raman peaks corresponding to the bulk traverse-optical (TO) and longitudinal-optical (LO) asymmetrical and bending phonon modes. An anomalously large TO-LO splitting (∼185 cm−1) of bending modes was observed. The shape examination was based on the fact that three depolarization factors are equal in value for spherical nanoparticles, but they are different for an elliptical one. According to this, one can observe one or three SPMs, respectively. The position of corresponding Raman peaks allows one to determine the depolarization factors and subsequently the shape of nanoparticles. The single SPM is located between Raman peaks corresponding to the TO and LO asymmetrical phonon modes and can be attributed to small spherical particles. Six different SPMs, which are located between Raman peaks corresponding to the TO and LO bending phonon modes, were observed. These modes can be assigned to elliptical nanoparticles in two surrounding mediums of different effective dielectric constants. The intensity of these SPMs is decreased drastically by heat treatment at a temperature around 950 °C, suggesting that aggregates of nanoparticles form the bulk α-quartz structure. The bulk structure formation from small silica particles was supported by thermogravimetric measurements. The values of the fraction of the total sample volume occupied by particles were calculated theoretically. A good agreement with the theory was obtained by assuming the anomalously large TO-LO splitting of bending phonon modes in nanoparticles of the amorphous silica.

Spontaneous and Stimulated Raman Scattering from Surface Phonon Modes in Aggregated SiO2 Nanoparticles

A spontaneous and stimulated Raman scattering from the surface phonon modes (SPMs) in aggregated amorphous silica (SiO2) nanoparticles is reported for the first time. The SPMs are located between Raman peaks corresponding to the amorphous silica bulk transverse-optical (TO) and longitudinal-optical (LO) bending phonon modes. An anomalously large TO−LO splitting (∼185 cm-1) of bending modes for bulk amorphous silica was observed. Six different SPMs corresponding to elliptical nanoparticles in two mediums of different effective dielectric constants were detected by the spontaneous Raman scattering spectra. One can obtain a good agreement with the theory for SPMs by assuming the anomalously large TO−LO splitting of bending modes in nanoparticles of the amorphous SiO2. The stimulated Raman scattering from SPMs was observed under second harmonic excitation of the YAG:Nd pulsed laser. The threshold value was estimated as ∼60 MW cm-2.