World Journal Of Engineering
SURFACE PLASMON RESONANCE OF Ag
NANOCRYSTALLINES IN OPAA TEMPLATE
Han Shan-shan, Yang Xiu-Chun, Liu Yan, Hou Jun-wei,
Li Xiao-ning, Lu Wei
Department of Materials Science and Engineering, Tongji University, Shanghai 200092, China
spectrophotometer was used to measure optical absorption
spectra. Quanta 200 FEG scanning electron microscope
(FESEM) with an energy-dispersive x-ray spectroscope (EDS)
was used to characterize the morphology and elemental
composition. H-800 transmission electron microscope (TEM)
was used to analyze the morphology and microstructure of
these samples. TEM samples were prepared by immersing a
small piece of Ag/OPAA composite in 2 mol/L NaOH solution
for about 5 h (60 ℃) in order to dissolve the OPAA template.
Ag NCs were afterwards separated out of the solution by
centrifugal effects. Finally, the deposit was ultrasonically
dispersed in 3–5 mL ethanol, and a drop of the suspended
solution was placed on a Cu grid with carbon membrane for
TEM observation.
Introduction
When metal nanocrystallines (NCs) are irradiated by light, the
oscillating electric field causes the free conduction electrons
to oscillate coherently, which is denoted as surface plasmon
resonance (SPR) [1]. Since the establishment of Mie’s theory in
1908, a mass of studies [2-5] have been reported to understand
the nature and the influence factors of SPR due to its
important applications in areas such as surface enhanced
Raman scattering (SERS) [6-8], nonlinear optics [9-11], and
photonic crystals [12].
Recently, Zong et al. [13] reported the SPR properties of Ag
nanowire arrays filled in OPAA template by alternating
current (AC) electrodeposition. However, direct current (DC)
electrodeposition was seldom used to fabricate metal/OPAA
composites for optical research because it was difficult for the
electrons to tunnel through the barrier layer [14]. Therefore a
new procedure was developed to electrochemically fill OPAA
where porous alumina remained on the aluminum substrate
and the barrier layer was largely thinned by using a
step-by-step voltage decrement process [15] or a twice constant
current anodization process [16]. The thinning leaded to a
considerable decrease in the potential barrier for the electrons
to tunnel through the barrier layer, when the metal was
deposited at the pore tips. Using DC electrodeposition, we
have prepared 30-μm-long Cu and Ag nanowire arrays [17, 18].
However, due to the great length of Cu and Ag nanowires, the
composites are optically opaque, which makes the optical
property studies of the composite difficult.
Herein, we successfully synthesized transparent Ag
nanocrystallines/OPAA composites film by this modified DC
electrodeposition method and investigated the influences of
size, shape and volume fraction of Ag nanocrystallines on the
maxima and intensities of SPR peaks.
Results and discussion
Figure 1 presents the photos of OPAA templates before and
after electrodepositing silver. The background is a piece of
white paper printed with the logo of Tongji University. As
shown in Figure 1(a), the opaque border is Al matrix, which
can be used as the framework to support the brittle OPAA
template. The inner region is the OPAA template. The logo
under the OPAA template can be seen clearly, indicating that
the OPAA template is virtually transparent. Figure 1(b) shows
that the OPAA template becomes orange-red after depositing
Ag NCs, and the logo under the composite can be seen clearly,
indicating that the filled composite is still transparent.
Extending the electrochemical deposition time to 80 s, the
composite becomes greenish-brown and is still transparent.
Fig.1 Camera photos of OPAA template (a) and sample S1
(b) with the logo under these samples.
Figure 2 gives FESEM photographs and EDS spectra of
sample S1.
Figure 2(a) indicates that pore channels can be divided into
ordered pore layer as shown in region 1 and branched pore
layer as shown in region 2. In the ordered pore layer, the pore
channels are straight and parallel to each other. The straight
pores branch out at the formation front because the pore
diameter is proportional to the anodizing potential and the
pore density is inversely proportional to the square of the
anodizing potential [17, 19]. The thickness of the branched pore
layer is about 500nm. Figure 2(b) is the top-view of the OPAA
template, which shows that the honeycomb-like template is
highly ordered with circular holes and hexagonal structure cell.
The ordered pore diameters range from 88 nm to 98 nm. No
silver can be found in region 1 as shown in Figure 2(c). Figure
2(d) indicates the existence of Ag atoms in region 2. Al is
from the OPAA template, and Au is from the Au film
deposited on the observed surface.
Experimental
A highly ordered and large-area OPAA template was
fabricated by a two-step anodization process plus a
step-by-step voltage decrement method as described
previously [17, 18].
Electrodeposition was performed on LK98II electrochemical
system (Lanlike, China). In the electrodeposition cell, the
OPAA template with Al substrate, Pt plate and saturated
calomel electrode (SCE) were used as the working electrode,
the counter electrode and the reference electrode, respectively.
Constant voltage (-6.5 V) DC electrochemical deposition was
employed in a mixing electrolyte of 0.01 mol/L AgNO3 and
0.1 mol/L H3BO3, here H3BO3 was used as buffer reagent.
Samples S1, S2 and S3 were electrochemically deposited for
10 s, 40 s and 80 s, respectively. After deposition, the
as-prepared samples were rinsed with deionized water, and
then the Al substrate was removed by CuCl2 solution.
Optical photographs of the OPAA template and the Ag/OPAA
film were taken by Sony camera. Hitachi 3310 UV-Vis
401
the longitudinal resonance [20]. For sample S1, the peak of
longitudinal resonance is very weak, because most Ag NCs in
sample S1 are small spheres due to the shorter deposition time.
For sample S3, with prolonging deposition time, the volume
fraction and aspect ratio of Ag nanorods increases as
demonstrated by TEM photographs, accordingly, the
longitudinal resonace enhances and has red-shift. According to
Gans theory[20], the maximum of longitudinal mode shifts to
longer wavelength with increasing aspect ratio of the
nanorods.
Fig.2 FESEM photographs and EDS spectra of S1: the
cross-section image (a), the top-view (b), EDS spectra for
region 1 (c) and region 2 (d)
Figure 3(a) indicates that the diameters of the Ag NCs in
samples S2 are 45-55 nm with lengths of 60-90nm. With
increasing deposition time, the length of the Ag NCs can be up
to 200 nm without changing the diameter as shown in Figure
3(b). Since the diameter of the ordered pore channels is 88-98
nm, it can be deduced that Ag NCs should only be deposited
in the branched pore channels, which coincides with EDS
results in Figure 2. The selected area electron diffraction
(SAED) of sample S3 inserted on the top of Figure 3(b)
indicates that Ag NC is monocrystalline with face center cubic
structure.
Conclusion
Optically transparent Ag NCs/OPAA composites are
successfully
fabricated
by
constant
voltage
DC
electrodeposition. Ag NCs/OPAA composite shows a
significant SPR absorption, which can be divided into
transverse quadrupole resonance, transverse dipole resonance
and longitudinal resonance. Most Ag NCs in sample S1 are
sphere, and the volume fraction of Ag nanorods increases with
increasing deposition time, accordingly, the relative intensity
ratio of longitudinal to transverse dipole resonance becomes
larger. All of the resonance modes have red-shifts and their
intensities enhance with increasing deposition time.
References
Fig.3 TEM images of Ag NCs in samples S2 (a) and S3 (b).
Figure 4(a) gives optical absorption spectra of OPAA template,
samples S1, S2 and S3. Figure 4(b) gives Lorentzian fits for
the experimental spectra of samples S1 and S3.
Fig.4 Optical absorption spectra of OPAA template and
samples S1, S2, S3 (a) and Lorentzian fits of samples S1 and
S3 (b).
For the empty OPAA template, its optical absorption is very
weak at the wavelength longer than 350 nm, indicating that it
can be an excellent matrix for fabrication of optical devices.
For samples S1, S2 and S3, noticeable and broad absorption
peaks have been observed, which can be denoted as the SPR
absorption of Ag NCs [1-2].
As well known, SPR absorption is affected by size, shape and
volume fraction of the metal NCs [1-5]. When the diameter of
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In our experiment, the diameters of Ag NCs are larger than
45nm and the aspect ratios range from 1 to 5. Therefore, the
broad SPR spectrum of each sample could be divided into
three peaks by using Lorentzian fits as shown in Figure 4(b):
transverse quadrupole resonance at around 360 nm, transverse
dipole resonance at around 420 nm and longitudinal resonance
at around 520 nm, respectively [4, 13].
With increasing electrodeposition time, Ag volume fraction in
the OPAA template increases, which induces an enhanced SPR
peak. This is in good agreement with Mie’s theory, which
predicts a proportional relationship between metal volume
fraction and intensity of SPR absorption.
For the transverse dipole resonance, the fitted peak in sample
S3 has a little red shift compared to that in sample S1. This is
consistent with Link’s reports that the maximum of the
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NCs [2, 3].
For the transverse quadrupole resonance peaks, maxima shift
to longer wavelength with increasing deposition time. It could
also be explained as the size dependence effort [2, 3].
The aspect ratio of metal NCs plays an important role to affect
402
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