213109-2
Lang et al.
Appl. Phys. Lett. 94, 213109 ͑2009͒
d(nm)
Raman bands and the characteristic lengths of GP-NPG, re-
vealing dramatical increase in SERS intensities by an ap-
proximately exponential relation with the decreasing d and
the increasing D. The detection limits for R6G and CV mol-
4
8
12 16 20
-7
R6G 10 M
(c)
(
a)
5
R6G
1
2
10
CV
7
10
5
8
1
0
−10
−8
ecules are ϳ10
and 10 M, respectively, much better
1
2
1
5
4
0
than AP-NPG. However, when the 632.8 nm laser is used, no
pore-size dependence of SERS spectra can be observed from
both R6G and CV molecules. Therefore, the improved SERS
effect of the GP-NPG films mainly arises from the EM field
enhancements caused by plasmon excitation through the
1
7
2
0
4
4
1
0
10
1
6
-5
(b)
CV 10 M
(d)
R6G
CV
1
0
1
2
8
4
0
1
0
1
2
1
2
5
Figure 2͑d͒ shows the relative Raman intensity
1
7
0
1
͑
ISERS,GP-NPG/ISERS,AP-NPG͒ as an exponential function of the
1200 1400 1600
0.2 0.4 0.6 0.8 1.0
d/D
-1
ratio of d/D, where I
SERS,GP-NPG
and ISERS,AP-NPG denote the
Raman shift ꢀcm ꢁ
SERS intensities of GP-NPG and AP-NPG films, respec-
tively. The SERS enhancements of GP-NPG are strongly de-
pendent on not only d but also the ratio of d/D. To elucidate
the size dependence, we investigated the UV-visible spectra
of GP-NPG films. Typical pore-size dependence of the wave-
length of the SP bands is shown in Fig. 3͑a͒. The wavelength
at a maximum extinction of the SP band ͑max͒ decreases
from 520 to 497 nm when d is reduced from ϳ20.5 to
ϳ7.1 nm ͓Fig. 3͑b͔͒. The blueshift of SP peaks of GP-NPG
FIG. 2. ͑Color online͒ SERS spectra of ͑a͒ R6G and ͑b͒ CV molecules
adsorbed on GP-NPG films with different d/D ratios. Laser excitation:
14.5 nm for both molecules. ͑c͒ Nanopore-size dependence of the inte-
grated SERS intensity of Raman bands of R6G and CV at 1650 and
175 cm , respectively. ͑d͒ The normalized SERS enhancements of GP-
NPG films ͑ISERS,GP-NPG/ISERS,AP-NPG͒ as a function of the d/D ratios.
5
−
1
1
The SERS effect of the GP-NPG films with different
d/D ratios was evaluated using rhodamine 6G ͑R6G͒ as
laser are presented in Fig. 2͑a͒, showing that as d decreases
and D increases, the SERS enhancements are remarkably
improved. Two resonance effects may be responsible for the
improved SERS effect, i.e., the resonance of the excitation of
light ͑514.5 nm͒ with the SPR of GP-NPG films ͕the plas-
half-height of SP bands, or ⌫ϰ1/D ͓Fig. 3͑c͔͒. Generally,
plasmon bandwidth is associated with dephasing of coherent
electron oscillation via ⌫=1/cT based on the two-level
and the dephasing time, respectively.
For the
NPG films, the random electron scattering is controlled by
ranging from 520 to 497 nm ͓see
max
the inner surfaces of NPG. Thus, the dephasing time of
Fig. 3͑a͔͖͒; and the resonances of R6G molecules ͑absorption
coherent electron oscillation increases with increasing D,
wavelength of ϳ524 nm͒ with both the SPR of GP-
abs
namely TϰD, which gives rise to increasing local-field
NPG films and the excitation of light. To clarify which factor
takes the paramount role in the strong SERS enhancements,
enhancement.
This effect is further illustrated by the
plot of max as a function of d ͓Fig. 3͑b͔͒, which reveals a
−
5
crystal violet ͑CV͒ ͑1ϫ10 M͒ was also used as testing
molecule. CV molecule with abs of ϳ590 nm is nonreso-
nant with respect to both the excitation laser wavelength and
good linear relationship for both AP-NPG and GP-NPG films
with the slops of ⌬max/⌬dϷ1.2 and 2.1, respectively. The
difference in the slopes implies that the surface plasmon po-
laritons of gold ligaments significantly influence the optical
properties of NPG in addition to the local SPR induced by
͑b͒ presents the SERS spectra of CV molecules adsorbed on
R6G molecules. Figure 2͑c͒ illustrates the quantitative rela-
tionship between the SERS intensities of R6G and CV
the curvatures of nanopores.
As shown in Fig. 2͑d͒, the relationship of
ISERS,GP-NPG/I
with the d/D ratio is very analo-
SERS,AP-NPG
gous to the general behavior of SERS enhancements in nano-
particles arrays, which is strongly dependent on the ratio of
1
5
2
9
6
3
180
(a)
Pore size
20nm
(
c)
1
1
60
40
1
gap/diameter owing to the enhanced near-field coupling be-
17
tween nanoparticles.
We qualitatively verify this by near-
1
5
120
1
2
1
8
00
0
1
7
0
0
4
60
key structural features of NPG ͓Fig. 4͑a͔͒ is introduced to
00 450 500 550 600 0.02
0.03
0.04
0.05
Wavelength (nm)
-1
qualitatively simulate the optical properties of NPG with the
1/D ꢀnm ꢁ
5
5
5
4
20 (b)
2.4
bulk dielectric function of Au. Under the plane wave with
wavelength of 514 nm propagating along the direction nor-
2
10
00
90
2
2
0
4
.2
2
=
͑i͒ for NPG films with d=10 nm and D=30 nm, d=D
10 nm, and d=D=20 nm, respectively, exhibiting that
E /E increases with the decrease in the d/D ratio. This is
4
6
8 10 12 14 16 18 20 22
d (nm)
2
2
0
qualitatively in accordance with experimental measurements
of the SERS enhancements. In order to evaluate the contri-
bution of the near-field coupling in the EM enhancements,
FIG. 3. ͑Color online͒ ͑a͒ UV-visible extinction spectra of GP-NPG films
with different pore sizes and d/D ratios. ͑b͒ Relationship of max of GP-
NPG ͑͒ and AP-NPG ͑᭞͒ films with d. ͑c͒ ⌫ of GP-NPG films as a
function of 1/D.
2
the electric field distributions of half pore ͑Eleft /E0 and
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