-
Simulation of Raman Enhancement
in SERS-Active Substrates with Au Layer
Considering Different Geometry of
Nanoparticles
Hui-Wen Cheng1 and Yiming
Li1,2,3,*
Institute of Communications
Engineering, National Chiao Tung University, 1001
Ta-
Hsueh Road, Hsinchu 300, Taiwan 2
Department of Electrical Engineering,
National Chiao Tung University, 1001 Ta-Hsueh
Road, Hsinchu 300, Taiwan
3
National Nano Device Laboratories,
Hsinchu 300, Taiwan
*
Tel:
+886 3 5712121 ext 52974; Fax: +886 3 5726639;
E-mail:
ymli@
Abstract-In
this work, we study surface enhanced Raman
identification of Rhdamine
6G (R6G) are
examined. This paper spectroscopy (SERS) active
substrates for the
detection of is
organized as follows. In Sec. II, we introduce the
fabrication Rhodamine
6G. To examine
the electromagnetic enhancement, process and
computational
technique for the SERS-
active with different shape of nanoparticle, we
apply the finite-
substrates. In Sec.
III, the local field enhancements of
difference timedomain (FDTD) algorithm
to analyze the nanoparticle with different
shapes are calculated by
three-
structures by solving a set of
coupled Maxwell’s
equations in
dimensional (3D) finite-difference time-domain
(FDTD) differential form.
The field
enhancements are investigated in the numerical
simulation. Finally, we draw
the
conclusions and visible regime with the wavelength
of 633 nm. In the suggest the
future
work.
experimental measurement, the
surface enhanced Raman scattering signals from the
surface of substrates with 12-hour
II. FABRICATION AND COMPUTATIONAL
TECHNIQUE hydrothermal treatment
and
without treatment are performed
and
compared. Through the three-dimensional (3D) FDTD
For the flow of fabrication,
as shown
in Fig. 1, first, calculation, the enhancements
with different shape of buffered
oxide
etchant (BOE) and standard RCA cleaning are
nanoparticle
are tested and obtained which are nanoparticle,
carried out to prepare clean
silicon
substrates (Boron-doped
gold nanocage and gold/silver alloy for
spherical, cubic and pyramidical shapes. The
results show that the enhancement of
(i)spherical and cubic shapes can be
much improved by nanocage
and
gold/siliver alloy structures.
Keywords- Surface-Enhanced Raman
spectroscopy (SERS), electromagnetic
enhancement, nanoparticle, gold
nanocage, gold/silver alloy, finite-difference
time-
domain, hydrothermally treated
substrate.
(ii)
I.
INTRODUCTION
Surface-enhanced Raman
Scattering (SERS) is one of the characterization
techniques,
which is sensitive to the
enhanced electromagnetic fields [1-6]. SERS-active
substrates
have recently attracted a
great deal of attention for rapid identification
of chemical and
bacterial samples
[5-7]. The fabricated nanostructures for both
bottom-up and top-down
approaches have
been reported. And, the degree of Raman
enhancement is strongly
dependent on
the morphology of formulated nanostructures [8].
Recently, a top-down
approach for the
fabrication of SERS-active substrate was proposed
[9-12]. However,
the expensive
substrate, equipments and complicated process are
needed. Therefore, a
low cost,
environment friendly and simple fabrication for
SERS-active substrates will
be of great
interest for basic and clinical researchers as
well as for biotechnologies. In
this
study, we experimentally and computationally study
the local field enhancements
of nanoparticles on
hydrothermally roughened SERS-active substrates,
where the
effects of shape and size of
Au particles and application of the fabricated
samples in
(iii)
(iv)
Figure 1.
Schematic representation for the fabrication of
SERS-active substrate. First,
silicon
wafers were cleaned by BOE and standard RCA
cleaning procedures. Then, Ti
films
were deposited on the pre-cleaned silicon wafers
using reactive DC magnetron
sputtering
system. The asdeposited samples were cleaved and
treated under
hydrothermal conditions
for various durations. Subsequently, Au was
thermal
evaporated onto the
hydrothermally roughened substrates for sensing. .
Figure. 2 (a)
The AFM image of titanium thin films treated under
hydrothermal
condition for 12 hours
treatment duration. (b) The plot of simulated
substrate which is
part of real
substrate, where the matrix of nanoparticles is 3
x 5 due to periodical
property of the
simulated structure.
p<100>). Then,
100-nm-thick titanium films are deposited on the
pre-cleaned silicon
wafers using
reactive DC magnetron sputtering system. The as-
deposited sample is
cleaved into 0.5 cm
x 1 cm squares and rinsed with ethanol, and de-
ionized water.
Subsequently, the sample
is put into a 23 mL Teflon-lined stainless steel
autoclave
filled with 20 mL distilled
water, which is sealed, and heated at 200oC for 2,
4, 6, 8, 10,
and 12 hours,
respectively. Then the treated sample is cooled to
room temperature
naturally, washed with
distilled water for several times, and dried with
a stream of
cylinder air. For example,
the image of Fig. 2(a) shows the AFM images
represent
titanium thin films treated
under hydrothermal conditions for 12 hours
treatment
duration.
The
image of Fig. 2(b) shows the plane view of the
gold-coated nanoparticular structure,
where the matrix of nanoparticles is 3
x 5 due to periodical property of the simulated
structure. Numerical simulation using a
3D FDTD method is conducted to investigate
the local field enhancement of
substrate [13-
15]. The Maxwell’s curl
equations in linear,
isotropic,
nondispersive, lossy materials are
?B
K
KK?=?
?
×E, (1)
?EKKt?t=?J1KK
ε+με
?
×
B, (2)
?
?
BK
=0,
(3)
F
igure 3. The simulation
procedure of solving the Maxwell’s
equations.
?
K
?
EK=ρ
ε
, (4)
where EK and BK
are the
vectors of electric and magnetic fields,
respectively,
?
and μ are permeability and
permittivity and JK and ρ are the
current density vector and c
harge
density. For a
globally defined
curvilinear space, Maxwell’s equations are easily
implemented in their
differential form,
where Faraday’s law is Eq. (1) and Ampere’s law is
Eq. (2).
The FDTD method
solves Maxwell’s equations by first discretizing
all
equations via
central
differences in time and space. Then, based upon a
3D Yee’s mesh and
components of the
electric and magnetic fields at points, the
discretized spacing in the x,
y, and z
directions adopted in our simulation are |x| =
0.01 um, |y| = 0.01 um and |z| =
0.01
um, where the time step Δt is 0.0004 and the time
duration T is 3 in units of
femtoseconds. The discretized equations
are iteratively solved in a leapfrog manner,
alternating between computing the E and
H fields at subsequent Δt/2
inte
rvals, as shown in Fig. 3. Notably,
we employ the
perfectly matched layer
as the simulation domain boundaries in which both
electric and
magnetic conductivities
are introduced in such a way that wave impedance
remains
constant, absorbing the energy
without
inducing reflections. III.
RESULTS AND DISCUSSION
In order to have
less light absorption, the larger scattering of
substrate is better to
achieve larger
field enhancement. For chemical sensing, the
hydrothermally roughened
substrates are
treated with aqueous solutions of 10-4 M R6G. The
The chemical
structure of R6G is shown
in Fig. 4(a). Fig. 4(b) shows that the
characteristic Raman
vibrational modes
of R6G immobilized on the substrate with or
without hydrothermal
treatment. The
substrate with hydrothermal treatment shows
-
-
-
-
-
-
-
-
-
上一篇:一年级小学生简短日记大全
下一篇:小学一年级日记30字30篇