Surface enhanced Raman scattering of nanoporous gold: Smaller pore sizes stronger enhancements

Article abstract of DOI:10.1063/1.2722199

The authors report the surface enhanced Raman scattering (SERS) of nanoporous gold with nanopore sizes ranging from 5 to 700 nm. Their comprehensive investigations prove that the strongest SERS enhancement of nanoporous gold takes place from the samples w

Full text of DOI:10.1063/1.2722199

Surface enhanced Raman scattering of nanoporous gold: Smaller pore
sizes stronger enhancements
L. H. Qian, X. Q. Yan, T. Fujita, A. Inoue, and M. W. Chen
Citation: Appl. Phys. Lett. 90, 153120 (2007); doi: 10.1063/1.2722199
View online: http://dx.doi.org/10.1063/1.2722199
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APPLIED PHYSICS LETTERS 90, 153120 ͑2007͒
Surface enhanced Raman scattering of nanoporous gold: Smaller pore
sizes stronger enhancements
a͒
L. H. Qian, X. Q. Yan, T. Fujita, A. Inoue, and M. W. Chen
International Frontier Center for Advanced Materials, Institute for Materials Research, Tohoku University,
Sendai 980-8577, Japan
͑
Received 30 January 2007; accepted 12 March 2007; published online 12 April 2007͒
The authors report the surface enhanced Raman scattering ͑SERS͒ of nanoporous gold with
nanopore sizes ranging from 5 to 700 nm. Their comprehensive investigations prove that the
strongest SERS enhancement of nanoporous gold takes place from the samples with an ultrafine
nanopore size of ϳ5–10 nm. Both the enhancement factor and detection limit of the ultrafine
nanoporous substrate are one to two orders of magnitude better than those of coarsened nonporous
gold with smooth surfaces. Moreover, careful microstructure characterization reveals
that the anomalous SERS enhancement of the annealed nanoporous gold arises from rough
surfaces with characteristic surface irregularities. © 2007 American Institute of Physics.
͓
DOI: 10.1063/1.2722199͔
Surface enhanced Raman scattering ͑SERS͒ originates
stronger SERS enhancements and the anomalous enhance-
ment of the annealed nanoporous gold, observed by
Kucheyev et al., is most likely associated with rough gold
surfaces caused by contaminations rather than the nanopore
size effect.
from the improved inelastic scattering of the molecules ad-
1
,2
18
sorbed on nanostructured metals and alloys. The SERS en-
hancements generally depend on the nanoscale characteris-
3
,4
tics of the metallic substrates, such as surface morphology,
5
,6
and size and aggregated state of nanoparticles. Although
extremely high SERS enhancements with single molecule
detection have been achieved at “hot spots” where intense
electromagnetic fields are believed to generate at interpar-
ticle crevices, reliable SERS substrates with uniformly ul-
trahigh SERS enhancements remain to be developed for the
practical applications of the SERS effect. A number of at-
tempts have been made to fabricate high-performance SERS
substrates, for examples, engraving periodical nanosphere ar-
Two classes of Ag Au ͑at. %, 50/50 by weight͒ films
3
5
65
with the thicknesses of ϳ100 nm and ϳ60 ␮m, respectively,
were used to synthesize nanoporous gold by selective chemi-
cal and electrochemical dealloying in 70% HNO aqueous
3
7
–9
solution at the temperatures of −20, 0, and 25 °C, respec-
tively. By controlling the dealloying time and temperatures,
we have proven that the nanopore sizes can be tailored from
1
9
ϳ5 to ϳ33 nm for 100 nm thick films. The 60 ␮m thick
nanoporous gold with a pore size of ϳ55 nm, produced by
room-temperature dealloying for 48 h, was further annealed
at 200, 300, 400, 500, and 600 °C for 2 h in air, which
results in the coarsening of nanoporous gold with pore sizes
ranging from 90 to 700 nm. The nanoporous gold specimens
with various nanopore sizes were subjected to SERS experi-
ments with rhodamine 6G ͑R6G͒ and crystal violet ͑CV͒
10 B as the test molecules. The commercially pure reagents
were dissolved into pure water ͑18.2 M⍀ cm͒ and methanol,
respectively, and the Raman scattering of R6G/nanoporous
gold and CV/nanoporous gold was detected by a Ranishaw
1
0
rays by lithography, self-organizations of nanoparticles in
11
12–14
the solutions, and nanoporous substrates.
Amongst
them, nanoporous gold with excellent thermal stability and
chemical inactivity has recently been exploited as an attrac-
tive substrate for SERS applications because of its large sur-
face area and bicontinuous porous structure in three
1
5
dimensions. Nevertheless, conventional nanoporous gold
with a pore size of about tens of nanometers made by room-
temperature dealloying does not show a promising SERS
enhancement although nanostructured gold, such as gold
nanoparticles, are known to be SERS active for a number of
2
0
Raman microscope with 514.5 and 632.8 nm lasers.
1
6
organic and biological molecules. Thus, optimizing the mi-
crostructure is crucial to the improvement of the SERS en-
hancements and thereby the SERS application of nanoporous
gold, which has been made an effort by a few of research
Representative micrographs of the nanoporous gold
specimens with different pore sizes are illustrated in Figs.
1͑a͒–1͑f͒. Visible microstructure coarsening during high tem-
perature annealing starts from 200 °C. Thereafter, the porous
sizes monotonously increase with annealing temperatures
from ϳ90 nm at 200 °C to ϳ700 nm at 600 °C ͓Fig. 1͑f͔͒.
In the annealed samples, the nanoporous structure is still
retained, as shown in Fig. 1. It is worthy to note that the
surface roughness of gold ligaments strongly depends on the
annealing temperatures and surface contaminations. Rough
ligament surfaces with more or less characteristic surface
irregularities ͑pimples͒ always appear as the samples are an-
nealed at a temperature between 400 and 600 °C, accompa-
nying with the coarsening of nanopores and gold ligaments.
This surface roughening appears to be related to a pinning
effect in the surface diffusion of gold atoms at the medium
1
7–19
groups.
More recently, Kucheyev et al. reported that the
optimized SERS enhancement can be achieved in the an-
nealed nanoporous gold with a coarsened pore size of
1
8
ϳ250 nm. However, we found that the strongest SERS en-
hancement takes place from the nanoporous gold with an
ultrafine nanopore size of ϳ5–10 nm. In this letter we report
comprehensive investigations on the SERS effect of nano-
porous gold with pore sizes varying from 5 to 700 nm. We
prove that the nanoporous gold with a smaller pore size has
a͒Author to whom correspondence should be addressed; electronic mail:
mwchen@imr.tohoku.ac.jp
0003-6951/2007/90͑15͒/153120/3/$23.00
90, 153120-1
© 2007 American Institute of Physics
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153120-2
Qian et al.
Appl. Phys. Lett. 90, 153120 ͑2007͒
FIG. 2. ͑Color online͒ SERS spectra of nanoporous gold with different pore
−
7
−5
sizes for ͑a͒ 10 mol/l R6G aqueous solution and ͑b͒ 10 mol/l CV
methanol solution. ͑c͒ Pore size dependence of the integrated intensities of
−
1
−7
1
1
650 cm Raman band at the concentrations of 1ϫ10 mol/l R6G and
175 cm Raman band at the concentrations of 1ϫ10 mol/l CV. Laser
−
1
−5
excitation: 514.5 nm for R6G and 632.8 nm for CV.
FIG. 1. ͑Color online͒ Representative SEM micrographs of nanoporous gold
with various nanopore sizes. ͑a͒ Nanoporous gold film after 5 min dealloy-
ing at room temperature; ͑b͒ dealloyed at room temperature for 48 h; ͑c͒
thoroughly rinsed nanoporous gold annealed at 200 °C for 2 h; ͑d͒ 400 °C
for 2 h; ͑e͒ 500 °C for 2 h, and ͑f͒ 600 °C for 2 h. ͑g͒ Nanoporous gold
with short-time rinsing and annealed at 600 °C for 2 h. ͑h͒ The relationship
between nanopore sizes ͑D͒ of thoroughly rinsed nanoporous gold and the
annealing temperatures.
ferent regions of each sample with the same acquisition pa-
rameters. However, for the coarsened samples annealed at
the temperatures above 300 °C, the intensities of spectra col-
lected from different regions of a sample are inconsistent
with 5%–10% variation due to the coarsened nanopores and
gold ligaments. Thus, in this study the spectra ͓Fig. 2͑a͔͒ are
the average ones acquired from about 10–15 different spots
of each sample to show the representative enhancements of
nanoporous gold with various pore sizes. Our results prove
that the SERS enhancements of nanoporous gold with
smooth ligament surfaces are remarkably improved with the
reduction of nanopore sizes. The strongest SERS enhance-
ment of nanoporous gold is achieved from a sample with
ultrafine pore size of ϳ5 nm. The integrated intensity of
R6G 1650 cm− Raman band from 5 nm porous gold is more
than 24 times stronger than that from 200 nm porous gold
annealed at 300 °C ͓Fig. 2͑b͔͒. Thus, the detection limit of
the 5 nm sample approaches to 5ϫ10− mol/l, which is
about one to two orders of magnitude better than that of the
annealed samples with smooth gold surfaces. In this study,
we do observe the anomalous enhancements from annealed
nanoporous gold with a large pore size ͓Figs. 2͑a͒ and 2͑c͔͒,
temperatures, caused by the residual acid and silver nitrate
2
1
on the nanoporous surfaces. We have found that thoroughly
rinsing with distilled water for 20 times can dramatically
reduce the number of surface irregularities. In contrast, short-
time rinsing for only three times lead to the formation of
a serious heterogeneous morphology with a large number
of “pimples” and relatively small pore sizes after annealing
at a temperature between 400 and 600 °C ͓for example,
Fig. 1͑g͔͒.
1
Figure 2͑a͒ illustrates the SERS spectra of R6G mol-
ecules on nanoporous gold with various pore sizes. For the
as-dealloyed and low temperature annealed nanoporous gold
10
͑
Ͻ300 °C͒, the nanopores and gold ligaments are much
smaller than the laser beam spot ͑ϳ2 ␮m in diameter͒ and
the reproducible spectra can be uniformly attained from dif-
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153120-3
Qian et al.
Appl. Phys. Lett. 90, 153120 ͑2007͒
hancement from the rough ligament surfaces is analogous to
that of the ultrafine nanoporous gold with smooth ligament
surfaces ͑Fig. 3͒. Thus, our observations of the SERS en-
hancements of nanoporous gold confirm that the high SERS
enhancements result from smaller microstructure features,
either smaller pore sizes or the rough gold ligament surfaces,
which are believed to promote electromagnetic filed en-
hancements and provide more active sites for molecule
2
2,23
adsorption.
Because the formation and coarsening of nanoporous
gold are controlled by surface diffusion, the smaller pore
sizes formed by a short dealloying time at low temperatures
generally contain more or less residual silver. It is known
FIG. 3. ͑Color online͒ SERS spectra of nanoporous gold with different
ligament surface roughness for 10− mol/l R6G aqueous solution. The spec-
8
24
that the SERS effect of silver exceeds that of gold. Thus,
tra collected from
comparison.
5 and 700 nm nanoporous gold are plotted for
the stronger Raman scattering from the ultrafine nanoporous
gold may also include the extra contribution from the re-
sidual silver. However, the fact that the annealed nanoporous
gold ͑without detectable residual silver͒ containing a high
density of surface irregularities exhibits the analogous en-
hancement as the ultrafine nanoporous gold indicates that the
ultrafine structures, both small pore sizes and fine surface
pimple-type irregularities, play the major role in the strong
SERS enhancement of the nanoporous gold.
similar to the observations of Kucheyev et al. However, care-
ful microstructure characterization reveals that the anoma-
lous enhancements from the samples with large pore sizes
are associated with the rough surfaces of gold ligaments.
Figure 3 illustrates the representative SERS spectra of
−
8
10
mol/l R6G solution from the annealed nanoporous gold
with different surface roughnesses, as shown in Figs. 1͑e͒
and 1͑g͒. The Raman intensity of the porous sample with a
smooth gold surface is almost 15 times weaker than that of
Research was sponsored by the Grant-in-Aid for Explor-
atory Research, the Ministry of Education, Culture, Sports,
Science and Technology ͑MEXT͒, Japan, and by Mitsubishi
Science Foundation.
500 nm nonporous gold with a large number of surface ir-
regularities. Apparently, this huge difference in the SERS
enhancements cannot be attributed to the minor discrepancy
in pore sizes, and most likely results from the difference in
the gold surface roughness. Scanning electron microscopy
1
M. Moskovits, Rev. Mod. Phys. 57, 783 ͑1985͒.
2
A. Campion and P. Kambhampati, Chem. Soc. Rev. 27, 241 ͑1998͒.
3
S. Link and M. A. El-sayed, Int. Rev. Phys. Chem. 19, 409 ͑2000͒.
͑SEM͒ characterization reveals that the diameter of the
4
Y. C. Liu, C. C. Yu, and S. F. Sheu, J. Mater. Chem. 16, 3546 ͑2006͒.
pimples on the rough surfaces is of ϳ5–20 nm. Interest-
ingly, the SERS enhancement from the regions with a high
density of surface pimple is comparable with that from the
ultrafine nanoporous gold that possesses an equivalent size in
the nanopore diameter as that of the surface irregularities.
To further investigate the optimized SERS enhancements
of nanoporous gold, in addition to R6G CV, 10 B in metha-
nol was also used as the test molecules. Figure 2͑b͒ repre-
5
P. Hildebrandt and M. Stockburger, J. Phys. Chem. 88, 5935 ͑1984͒.
6
U. Kreibig and L. Genzel, Surf. Sci. 156, 678 ͑1985͒.
7
S. M. Nie and S. R. Emery, Science 275, 1102 ͑1997͒.
8
K. Kneipp, Y. Wang, H. Kneipp, L. T. Perelman, I. Itzkan, R. R. Dasari,
and M. S. Feld, Phys. Rev. Lett. 78, 1667 ͑1997͒.
H. Xu, E. J. Bjerneld, M. Kall, and L. Borjesson, Phys. Rev. Lett. 83,
9
104
357 ͑1999͒.
X. Y. Zhang, A. V. Whitney, J. Zhao, and R. P. Van Duyne, J. Nanosci.
Nanotechnol. 6, 1 ͑2006͒.
−
5
11
sents SERS spectra of 10 mol/l CV on nanoporous gold
with various pore sizes. Characteristic CV SERS bands can
be identified and their intensities gradually increase with the
reduction of nanopore sizes. Quantitative intensity measure-
P. M. Tessier, O. D. Velev, A. T. Kalambur, J. F. Rabolt, A. M. Lenhoff,
and E. W. Kaler, J. Am. Chem. Soc. 122, 9554 ͑2000͒.
L. H. Lu, R. Capek, A. Kornowski, N. Gaponik, and A. Eychmuller,
Angew. Chem. 117, 6151 ͑2005͒.
1
2
13
Y. Ding, A. Mathur, M. W. Chen, and J. Erlebacher, Angew. Chem., Int.
Ed. 44, 4002 ͑2005͒.
−
1
ments of Raman bands at 1175 cm ͓Fig. 2͑c͔͒ show the
same size effect as that of R6G, i.e., smaller pore sizes have
stronger SERS enhancements. The anomalous enhancement
from coarsened nanopores with an ϳ350 nm pore size but a
rough gold surface was also evidenced in the CV/nanoporous
gold system, as shown in Figs. 2͑b͒ and 2͑c͒. This anomalous
enhancement was found to quickly degrade with the smooth-
ening of gold ligament surfaces.
1
1
4
Y. Ding, Y. J. Kim, and J. Erlebacher, Adv. Mater. ͑Weinheim, Ger.͒ 16,
1
897 ͑2004͒.
5
T. Fujita, L. H. Qian, K. Inoke, J. Erlebacher, and M. W. Chen, Phys. Rev.
Lett. ͑in press͒.
16
K. Kneipp, A. S. Haka, H. Kneipp, K. Badizadegan, N. Yoshizawa, C.
Boone, K. E. Shafer-peltier, J. T. Motz, R. R. Dasari, and M. S. Feld,
Appl. Spectrosc. 56, 150 ͑2002͒.
1
1
7
8
M. C. Dixon, Ph.D. thesis, Pennsylvania State University, 2005.
S. O. Kucheyev, J. R. Hayes, J. Biener, T. Huser, C. E. Talley, and A. V.
Hamza, Appl. Phys. Lett. 89, 53102 ͑2006͒.
SERS enhancements of nanoporous gold for both R6G
and CV molecules prove that ultrafine nanopores possess the
strongest SERS enhancement. Although high enhancements
can be obtained in the annealed samples with a large pore
size of ϳ350 nm, similar to the observations of Kucheyev
et al., this anomalous enhancement originates from the ul-
trafine pimple-type irregularities on the rough gold surfaces.
SEM characterization reveals that the diameter of the surface
irregularities is of ϳ5–20 nm. Interestingly, the SERS en-
19
L. H. Qian, X. Q. Yan, T. Fujita, and M. W. Chen ͑unpublished͒.
X. Q. Yan, W. J. Li, T. Goto, and M. W. Chen, Appl. Phys. Lett. 88,
131905 ͑2006͒.
2
0
2
2
2
1
2
3
J. Erlebacher, ͑private communication͒.
J. Gersten and A. Nitzan, J. Chem. Phys. 73, 3023 ͑1980͒.
T. Jensen, L. Kelly, A. Lazarides, and G. C. Schatz, J. Cluster Sci. 10, 295
4͑
1999͒.
2
K. Kneipp, M. Moskovits, and H. Kneipp, Surface-Enhanced Raman Scat-
tering: Physics and Applications ͑Springer, Berlin, 2006͒, 103, p. 261
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