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Home Search Collections Journals About Contact us My IOPscience Optical Properties of Plasmon Resonances with Ag/SiO2/Ag Multi-Layer Composite Nanoparticles This content has been downloaded from IOPscience Please scroll down to see the full text 2010 Chinese Phys Lett 27 064204 (http://iopscience.iop.org/0256-307X/27/6/064204) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 137.99.31.134 This content was downloaded on 19/05/2015 at 03:29 Please note that terms and conditions apply CHIN PHYS LETT Vol 27, No (2010) 064204 Optical Properties of Plasmon Resonances with Ag/SiO2 /Ag Multi-Layer Composite Nanoparticles * MA Ye-Wan(马业万)** , ZHANG Li-Hua(章礼华), WU Zhao-Wang(吴兆旺), ZHANG Jie(张杰) School of Physics and Electric Engineering, Anqing Teachers College, Anqing 246011 (Received 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 field 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 nanoparticles have become very active topics in materials science By adjusting the structure and size of nanoparticles, 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 tensor[16,17] 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 nanoparticles on the scattering efficiency are studied, as shown in Fig With increasing numbers of Ag-SiO2 -multilayer 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 c 2010 Chinese Physical Society and IOP Publishing Ltd ○ 064204-1 CHIN PHYS LETT Vol 27, No (2010) 064204 scattering intensity is almost symmetrical for both 𝑥 and 𝑧 components in the 𝑧 direction (a) z (nm) (b) 50 50 25 25 Ag Ag 0 50 100 x (nm) (c) 0 50 50 25 25 Ag 0 Ag 0.15 50 0.10 25 50 0.05 25 Ag Ag 0 50 100 x (nm) 50 100 x (nm) Fig Simulated scattering intensity of Ag-metal layers with TIR 60∘ in the 𝑥–𝑧 plane Each Ag thickness is 32 nm (a) Total scattering intensity distribution (|𝐸|), (b) contour of scattering intensity (|𝐸|), (c) |𝐸|, |𝐸𝑥 |, |𝐸𝑦 |, (d) |𝐸𝑧 | Ag Ag Ag Ag Ag 30 layer layers layers layers layers 200 25 20 z (nm) Qsca (arb units) 8 Ag 15 SiO2 Ag 100 10 SiO2 Ag 0 350 400 450 SiO2 Ag 300 100 x (nm) 50 z (nm) 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 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 nm 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 nanoparticles 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 50 100 x (nm) 500 λ (nm) Fig 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 Compared with the dielectric, the intensity is significantly enhanced, especially in Agcorner 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 Fig 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 nanoparticles as a result of adding more sharp corners or singularities which play an important role in scattering intensity, as shown in Fig 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 nanoparticles is also enhanced much more strongly than the 064204-2 CHIN PHYS LETT Vol 27, No (2010) 064204 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 Qsca (arb units) 10 50* 60* 70* 300 350 400 450 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 500 λ (nm) Fig 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, not seem to change at all, but the electrical intensity changes with the variation of the angles because References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] 064204-3 Nie S et al 1997 Science 275 1102 Barnes L W et al 2003 Nature 424 824 Shuford K L et al 2005 J Chem Phys 123 114713 Murray W A et al 2007 Adv Mater 19 3771 Maier S A 2007 Plasmonics: Fundamentals and Application (Berlin: Springer) Kreibig U and Vollmer M 1995 Optical Properties of Metal Clusters (Berlin: Springer) Novotny L 2006 Principle of Nano-Optics (Cambridge: Cambridge University) Brioude A et al 2005 J Phys Chem B 109 23371 Kelly K L et al 2003 J Phys Chem B 107 668 Kottmann J P et al 2001 Opt Express 655 Su K H et al 2006 Appl Phys Lett 88 063118 Wu D J et al 2008 J Chem Phys 129 074711 Hoflich K et al 2009 J Chem Phys 131 164704 Wang J Q et al 2009 Chin Phys Lett 26 084208 Ma Y W et al 2010 Chin Phys Lett 27 024207 Girard C et al 1995 Phys Rev B 52 2889 Martin O J F et al 1995 Phys Rev Lett 74 526 Johnson P B et al 1972 Phys Rev B 12 4370 Ma Y W et al 2008 Chin Phys Lett 25 2473 Ma Y W et al 2009 J Appl Phys 105 103101 ... et al 20 09 Chin Phys Lett 26 08 420 8 Ma Y W et al 20 10 Chin Phys Lett 27 024 207 Girard C et al 1995 Phys Rev B 52 2889 Martin O J F et al 1995 Phys Rev Lett 74 526 Johnson P B et al 19 72 Phys... 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... Provincial Education Department under Grant Nos 20 05KJ2 32 and KJ2008B83ZC ** Email: ma yewan@sohu.com c 20 10 Chinese Physical Society and IOP Publishing Ltd ○ 06 420 4-1 CHIN PHYS LETT Vol 27 ,
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