Optical Properties of Plasmon Resonances with Ag/SiO2/Ag Multi-Layer Composite Nanoparticles View the table of contents for this issue, or go to the journal homepage for more 2010 Chines
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Optical Properties of Plasmon Resonances with Ag/SiO2/Ag Multi-Layer Composite Nanoparticles
View the table of contents for this issue, or go to the journal homepage for more
2010 Chinese Phys Lett 27 064204
(http://iopscience.iop.org/0256-307X/27/6/064204)
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Trang 2CHIN PHYS LETT Vol 27, No 6 (2010) 064204
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 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 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
proper-ties 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
absorp-tion at a metal surface is related to the movement
of free electrons When the plasmon is under
cer-tain electromagnetic disturbance, according to
metal-lic electrical theory, the charge density may not be zero
in some regions, and a restoring force will be
gener-ated 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
distribu-tion In the macro scale, this resonance manifests as
optical absorption by metal nanoparticles The metal
surface plasmon resonance is the main factor in
deter-mining the optical properties of metal nanoparticles
Many unique optical properties can be achieved when
adjusting the structure, morphology, size and
compo-sition of the metal nanoparticles.[ 8 − 15 ] Consequently,
manufacturing and application of metallic
nanoparti-cles have become very active topics in materials
sci-ence 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 nanoparti-cles The Ag multi-layer nanoparticles consist of piled Ag/SiO2/Ag nanoparticles, and provide a ref-erence for manufacturing novel nano-materials The results show that the plasmon resonances of such nanoparticles are augmented and exhibited signifi-cantly stronger light scattering at plasmon resonance wavelengths We give each component of the scat-tering intensity for clarity, in addition, the incident angles and polarizations of the wave are also studied
In order to thoroughly study the optical charac-teristics of Ag particles, we concentrate on the main features of the theoretical scattering formalism with Green’s tensor[ 16 , 17 ] on which the numerical simula-tion is based and associated with the numerical meth-ods The silver dielectric constants are taken from Ref [18] for this study, and the dielectric constants of SiO2is set to 2.25 assuming a bulk refractive index of 1.50 The Ag bulk is illuminated by incident light un-der 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 multi-layer to 8 multi-layers, the res-onant 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
Trang 3CHIN PHYS LETT Vol 27, No 6 (2010) 064204
as the variation of the near-field plasmon coupling
be-tween Ag-layers with the change of the dielectric
con-stant, thus the plasmon resonance shifts to a longer
wavelength as the dielectric constant of the
surround-ing medium increases However, the scattering
in-tensity is significantly enhanced firstly and then
at-tenuated 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
parti-cle’s geometries and sizes To obtain a good study
on the influence of the dielectric on plasmon
reso-nance, we also change the dielectric SiO2 thickness
from 5 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
nanopar-ticles are observed to blue shift for increasing Ag
thick-ness but keeping the dielectric constant This is equal
to change the Ag-nanoparticle height.[ 20 ] Compared
with nanoparticle growth of different sizes and
geome-tries, we could easily reach the required plasmon
res-onance frequency by the numbers of multi-layers
0
2
4
6
8
λ (nm)
1 Ag layer
2 Ag layers
3 Ag layers
5 Ag layers
8 Ag layers
Fig 1 Simulated scattering spectra for different
num-bers 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
supe-rior to the bottom In order to reach a better
under-standing about the contributions of each component
(𝐸𝑥, 𝐸𝑦 and 𝐸𝑧) to the scattering field, each
compo-nent of scattering intensity is also given in Fig.2(b)
It is seen clearly that the main contributions to
scat-tering are the 𝑥 and 𝑧 components, while the 𝑦
com-ponent is very small We can also find out that the
scattering intensity is almost symmetrical for both 𝑥 and 𝑧 components in the 𝑧 direction
0 25 50
0 4 6 8
Ag
x (nm)
x (nm)
x (nm)
x (nm)
0 25 50
Ag
(a)
(c)
(b)
0 25 50
0 4 6 8
0 25 50
3 5 7
0 25 50
0.05 0.10 0.15
0 25 50
0 3 6
Fig 2 Simulated scattering intensity of Ag-metal lay-ers 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|, |𝐸 𝑥 |, |𝐸𝑦|, (d) |𝐸 𝑧 |.
0 100 200
0 5 10 15 20 25 30
Ag SiO 2
Ag Ag Ag
SiO 2
SiO 2
Fig 3 Scattering intensity of local electrical field distribution with nanoparticles of four Ag-layers (Ag/SiO 2 ) 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 re-sults show that by “slicing” an Ag-layer evenly into several layers, the scattering intensity can be signif-icantly enhanced The local electrical field intensity
is significantly enhanced for the multi-layer nanopar-ticles as a result of adding more sharp corners or sin-gularities which play an important role in scattering intensity, as shown in Fig.3 Furthermore, the elec-trical fields for Ag-layer nanoparticles are mainly in-tensified 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 064204-2
Trang 4CHIN PHYS LETT Vol 27, No 6 (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
asym-metrical in the 𝑧 direction, as the 𝐸𝑦component plays
an important role greater than the one-layer This
unique electrical field distribution may be employed
for nonlinear optical applications, for example, a
non-linear material is used to replace SiO2, this multi-layer
nanoparticle should exhibit an enhanced nonlinear
ef-fect The scattering intensity enhances obviously with
more Ag-layers In order to obtain a stronger
enhance-ment, we should give more layers
0
2
4
6
8
10
λ (nm)
50 *
60 *
70 *
Fig 4 Spectra calculated for different total internal
re-flection 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
reso-nance 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
protu-berant film by the TE wave Thus there are two peaks
by the TE wave In addition, dependence on the
inci-dent angle is also studied for TM waves When
vary-ing 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
inten-sity 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 prop-erties 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 syn-thesis of new structures have opened new fields for study in material science, e.g., environmental moni-toring, medical diagnosis and treatment, Raman scat-tering and optics etc However, development in this field is still in the beginning stage, with many prob-lems waiting to be solved For example, manufactur-ing techniques are far from mature (adjustment of re-sponse 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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