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Optical Properties of Plasmon Resonances with Ag/S^/Ag Multi-Layer Composite Nanoparticles

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Optical Properties of Plasmon Resonances with Ag/SiO2/Ag Multi-Layer

Composite Nanoparticles *

MA Ye-Wan(^#^)**, ZHANG Li-Hua(^L^), WU Zhao-Wang(^kE), ZHANG School of Physics and Electric Engineering, Anqing Teachers College, Anqing 246011

(Received 8 March 2010)

Optical properties of plasmon resonance with Ag/SiO2/Ag multi-layer nanoparticles are studied by numerical simulation based on Green's function theory. The results show that compared with single-layer Ag nanoparticles, the multi-layer nanoparticles exhibit several distinctive optical properties, e.g. with increasing the numbers of the multi-layer nanoparticles, the scattering efficiency red shifts, and the intensity of scattering enhances accordingly. It is interesting to find out that slicing an Ag-layer into multi-layers leads to stronger scattering intensity and more "hot spots" or regions of stronger Geld enhancement. This property of plasmon resonance of surface Raman scattering has greatly broadened the application scope of Raman spectroscopy. The study of metal surface plasmon resonance characteristics is critical to the further understanding of surface enhanced Raman scattering as well as its applications.

PACS: 42. 25. -p, 42. 25. Hz, 42. 62. -b DOI: 10.1088/0256-307X/27/6/064204

Due to the quantum size and surface effects, noble metal nanoparticles have different optical, electromagnetic and chemical properties from bulk materials.!1-4] All along, the unique optical properties have been important study subjects of optics, electronics, biomedical science, and materials science. The intense optical absorption and scattering effects induced by surface plasmon resonance, which occur when a light wave is incident onto a metal surface, have attracted particularly strong study in recent years.[5-7] Plasmon-resonance-induced optical absorption at a metal surface is related to the movement of free electrons. When the plasmon is under certain electromagnetic disturbance, according to metallic electrical theory, the charge density may not be zero in some regions, and a restoring force will be generated to induce oscillating charge distribution. When the frequencies of the electromagnetic disturbance and the plasma oscillation match each other, resonance will happen. The oscillation frequency is determined by four factors: the density of electrons, the electron mass, and the size and shape of the charge distribution. In the macro scale, this resonance manifests as optical absorption by metal nanoparticles. The metal surface plasmon resonance is the main factor in determining the optical properties of metal nanoparticles. Many unique optical properties can be achieved when adjusting the structure, morphology, size and composition of the metal nanoparticles.[8-15] Consequently, manufacturing and application of metallic nanoparti-cles have become very active topics in materials science. By adjusting the structure and size of nanopar-ticles, we can derive new optical properties, and pro-

duce new nano-materials to serve the needs of society.

In this Letter, we present our numerical study on a type of Ag/SiO2/Ag multi-layer nanoparticles. The Ag multi-layer nanoparticles consist of piled Ag/SiO2/Ag nanoparticles, and provide a reference for manufacturing novel nano-materials. The results show that the plasmon resonances of such nanoparticles are augmented and exhibited significantly stronger light scattering at plasmon resonance wavelengths. We give each component of the scattering intensity for clarity, in addition, the incident angles and polarizations of the wave are also studied.

In order to thoroughly study the optical characteristics of Ag particles, we concentrate on the main features of the theoretical scattering formalism with Green's tensort16'17] on which the numerical simulation is based and associated with the numerical methods. The silver dielectric constants are taken from Ref. [18] for this study, and the dielectric constants of SiO2 is set to 2.25 assuming a bulk refractive index of 1.50. The Ag bulk is illuminated by incident light under total internal reflection on a glass substrate with an incident angle 60° in order to excite an evanescent wave.[19] The substrate which has significant effects on the plasmon resonance is a homogeneous medium with a refractive index of 1.5, e.g. glass. Firstly the influences of the numbers of Ag multi-layer nanoparti-cles on the scattering efficiency are studied, as shown in Fig. 1. With increasing numbers of Ag-SiO2-multi-layer nanoparticles from 1-layer to 8-layers, the resonant peak wavelengths of individual nanoparticles were seen to be significantly red-shifted from 390 nm to 430 nm. The shift of these peaks can be explained

* Supported by the Scientific Research Fund of Anhui Provincial Education Department under Grant Nos 2005KJ232 and KJ2008B83ZC.

** Email: ma_yewan@sohu.com

©2010 Chinese Physical Society and IOP Publishing Ltd

as the variation of the near-field plasmon coupling between Ag-layers with the change of the dielectric constant, thus the plasmon resonance shifts to a longer wavelength as the dielectric constant of the surrounding medium increases. However, the scattering intensity is significantly enhanced firstly and then attenuated. From Fig. 1 we can also find out that the scattering coefficients have a local minimum at about 320 nm, which is because both the real and imaginary parts of the Ag dielectric parameter almost reach zero at that wavelength. Its spectral feature is inherent to the Ag material properties, independent of the particle's geometries and sizes. To obtain a good study on the influence of the dielectric on plasmon resonance, we also change the dielectric SiO2 thickness from 5nm to 30 nm while keeping the Ag thickness constant, the resonant peak wavelengths of individual nanoparticles are observed to shift only about several nanometers (which are not given here). In contrast, the resonant peak wavelengths of individual nanopar-ticles are observed to blue shift for increasing Ag thickness but keeping the dielectric constant. This is equal to change the Ag-nanoparticle height.[20] Compared with nanoparticle growth of different sizes and geometries, we could easily reach the required plasmon resonance frequency by the numbers of multi-layers.

scattering intensity is almost symmetrical for both x and £ components in the z direction.

Fig. 2. Simulated scattering intensity of Ag-metal layers with TIR 60o in the x—z plane. Each Ag thickness is 32nm. (a) Total scattering intensity distribution (|E|), (b) contour of scattering intensity (\E|), (c) \E|, \EX |, \Ey |, (d) \EZ\.

1 Ag layer

2 Ag layers

3 Ag layers 5 Ag layers 8 Ag layers

400 À (nm)

Fig. 1. Simulated scattering spectra for different numbers of Ag-metal layers with TIR. Each dielectric or Ag thickness is 20 nm.

Secondly, the scattering intensity distribution of the Ag-layer with an x — z plane by TM polarization under total internal incident angle 60° is calculated, as shown in Fig. 2. Compared with the dielectric, the intensity is significantly enhanced, especially in Ag-corner regions. The top intensity is much more superior to the bottom. In order to reach a better understanding about the contributions of each component (Ex, Ey and Ez) to the scattering field, each component of scattering intensity is also given in Fig. 2(b). It is seen clearly that the main contributions to scattering are the x and z components, while the y component is very small. We can also find out that the

x (nm)

Fig. 3. Scattering intensity of local electrical

field distribution with nanoparticles of four Ag-layers (Ag/SiO2)3/Ag at resonant frequency in the x — z plane.

It is interesting to find out the comparison of the scattering spectra and local field distribution between Ag nanoparticles of single and multi-layers. The results show that by "slicing" an Ag-layer evenly into several layers, the scattering intensity can be significantly enhanced. The local electrical field intensity is significantly enhanced for the multi-layer nanopar-ticles as a result of adding more sharp corners or singularities which play an important role in scattering intensity, as shown in Fig. 3. Furthermore, the electrical fields for Ag-layer nanoparticles are mainly intensified at its corners especially on the top, and as a result, the electrical field within the SiO2 nanopar-ticles is also enhanced much more strongly than the

single Ag-layer nanoparticle. This could be explained such that different layers are oscillating in different multi-polar modes. The local field intensity is asymmetrical in the z direction, as the Ey component plays an important role greater than the one-layer. This unique electrical field distribution may be employed for nonlinear optical applications, for example, a nonlinear material is used to replace SiO2, this multi-layer nanoparticle should exhibit an enhanced nonlinear effect. The scattering intensity enhances obviously with more Ag-layers. In order to obtain a stronger enhancement, we should give more layers.

0 fi»i»i»i»

300 350

400 450

A (nm)

Fig. 4. Spectra calculated for different total internal reflection angles with TM polarization.

Thirdly, the influences of the polarization and its incident angles on plasmon resonance are also studied. The individual main peak of localized plasmon resonance is shown clearly for both TE and TM waves. However, the TE wave has a second and smaller peak. This is because the field component which is vertical to the metal protuberant film plays an important role in the TM wave. Compared with the TM wave, due to the exponential damping of electrical field intensity and plasmon corner or singularity enhancement, the x- and z-components are parallel to the metal protuberant film by the TE wave. Thus there are two peaks by the TE wave. In addition, dependence on the incident angle is also studied for TM waves. When varying the incident angle of light, the localized plasmon resonance peaks, both spectral locations and shapes, do not seem to change at all, but the electrical intensity changes with the variation of the angles because

the exponential damping of electrical field intensity attenuates with light angles. The larger the angles, the smaller the intensity.

In summary, we have presented the optical properties of plasmon resonance of a type of multi-layer nanoparticles. It is observed that by changing the number of multi-layer nanoparticles, the peaks of plsmon resonance red shift, which could be tunable to the relevant wavelength for plasmon resonance. In addition, by slicing an Ag-layer into multi-layers, the scattering intensity is significantly enhanced due to the addition of more corners and singularities. The manufacturing of novel metal nanoparticles and synthesis of new structures have opened new fields for study in material science, e.g., environmental monitoring, medical diagnosis and treatment, Raman scattering and optics etc. However, development in this field is still in the beginning stage, with many problems waiting to be solved. For example, manufacturing techniques are far from mature (adjustment of response time, nano-size and concentration etc.). More broad applications in related fields have occurred to meet the demands of social and scientific development.

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