Large surface enhanced Raman scattering enhancements from fracture surfaces of nanoporous gold

Article abstract of DOI:10.1063/1.2890164

We report the improved surface enhanced Raman scattering (SERS) of mechanically ruptured nanoporous gold. The SERS intensities of rhodamine 6G and crystal violet 10B molecules from the fracture surfaces of nanoporous gold are about one order of magnitude

Full text of DOI:10.1063/1.2890164

Large surface enhanced Raman scattering enhancements from fracture surfaces of
nanoporous gold
L. H. Qian, A. Inoue, and M. W. Chen
Citation: Applied Physics Letters 92, 093113 (2008); doi: 10.1063/1.2890164
View online: http://dx.doi.org/10.1063/1.2890164
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/92/9?ver=pdfcov
Published by the AIP Publishing
Articles you may be interested in
Recyclable surface-enhanced Raman scattering template based on nanoporous gold film/Si nanowire arrays
Appl. Phys. Lett. 105, 011905 (2014); 10.1063/1.4889850
ZnO/Si arrays decorated by Au nanoparticles for surface-enhanced Raman scattering study
J. Appl. Phys. 111, 033104 (2012); 10.1063/1.3682462
Geometric effect on surface enhanced Raman scattering of nanoporous gold: Improving Raman scattering by
tailoring ligament and nanopore ratios
Appl. Phys. Lett. 94, 213109 (2009); 10.1063/1.3143628
Surface enhanced Raman scattering of nanoporous gold: Smaller pore sizes stronger enhancements
Appl. Phys. Lett. 90, 153120 (2007); 10.1063/1.2722199
Surface-enhanced Raman scattering on nanoporous Au
Appl. Phys. Lett. 89, 053102 (2006); 10.1063/1.2260828
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
120.117.138.77 On: Mon, 15 Dec 2014 03:51:18

APPLIED PHYSICS LETTERS 92, 093113 ͑2008͒
Large surface enhanced Raman scattering enhancements from fracture
surfaces of nanoporous gold
L. H. Qian, A. Inoue, and M. W. Chena͒
Institute for Materials Research and WPI Advanced Institute for Materials Research, Tohoku University,
Sendai 980-8577, Japan
͑Received 29 December 2007; accepted 11 February 2008; published online 5 March 2008͒
We report the improved surface enhanced Raman scattering ͑SERS͒ of mechanically ruptured
nanoporous gold. The SERS intensities of rhodamine 6G and crystal violet 10B molecules from the
fracture surfaces of nanoporous gold are about one order of magnitude higher than those from the
as-prepared samples. Microstructural characterization reveals that the fracture surfaces contain
numerous sharp protrusions with 5–10 nm apexes, produced by localized plastic deformation of
gold ligaments during failure. The large SERS enhancements from the fracture surfaces are most
likely associated with the intensified electromagnetic fields around the nanosized protrusions and the
electromagnetic interaction between the protrusions.
© 2008 American Institute of Physics. ͓DOI: 10.1063/1.2890164͔
Surface enhanced Raman scattering ͑SERS͒ arising from
nanostructured noble metals is a promising technique for
molecular detections and diagnoses because of the signifi-
cantly improved Raman intensity of organic molecules and
the precise identification of molecular structures.1–3 It has
been found that the SERS enhancements remarkably rely on
the size, shape, and assembly of metallic nanostructures.4–10
Many efforts have been made to exploit nanostructured met-
als as ultrasensitive SERS substrates by microstructure
optimizations.11–16 Among various nanostructured metals, bi-
continuous nanoporous gold with tunable nanoporosity has
recently been developed as a promising SERS substrate by
optimizing their nanoporosity.17–19 Large SERS enhance-
ments have been achieved from ultrafine nanoporous gold
prepared by low-temperature dealloying and from coarsen
gold ligaments with nanosized surface “pimples” by high-
temperature annealing.17,19,20 However, the underlying
mechanisms on the SERS effects of nanoporous gold remain
unclear. In this letter, we report largely improved SERS en-
hancements from mechanically ruptured surfaces of nano-
porous gold, which paves a new way to improve the SERS
effects of nanoporous gold and is helpful to understand the
SERS mechanisms of nanoporous gold.
edge is still about micrometer-scale thick, which is much
larger than the nanosized gold ligaments. Thus, the failure of
each ligament is mainly by pulling and/or bending, resulting
in sharp tips of fractured ligaments. The ruptured surfaces
were placed faceup for SERS measurements. Two types of
molecules, rhodamine 6G ͑R6G͒ and crystal violet ͑CV͒
10B, were used to examine the SERS properties of the as
etched and fractured nanoporous gold. The detailed process
of the SERS experiments has been reported before.19 Scan-
ning electron microscope ͑SEM͒ and transmission electron
microscope ͑TEM͒ observations were conducted to charac-
terize the microstructure of the facture surfaces and the as-
prepared samples.
Representative SEM image, as shown in Fig. 1͑a͒, illus-
trates the microstructure of the as-prepared nanoporous gold
with open nanoporosity in three dimensions. Statistical
analysis by a rotational fast Fourier transform method21 sug-
Ag65Au35 ͑at. %͒ sheets with a thickness of ϳ100 ␮m
were used for fabricating nanoporous gold. Selective disso-
lution of silver atoms in 70% ͑mass ratio͒ HNO3 aqueous
solution at room temperature for 48 h leads to the formation
of bicontinuous nanoporosity across the entire sheets. A
nanoporous gold sheet was first placed on a glass slide and
then partly covered by an aluminum cube with smooth and
straight edges. The side length of the cube ͑20 mm͒ is much
larger than the width of nanoporous gold sheet ͑ϳ5 mm͒ and
the entire nanoporous sheet was sliced along an edge of the
aluminum cube by a sharp knife. To make smooth cutting
surfaces, the sharp edge of the knife is vertical to the surface
of nanoporous gold sheet during cutting. The overall rough-
ness of the cutting surfaces is smaller than submicron, which
can ensure the reproducible acquisition of Raman scattering
from the ruptured surfaces. Even the knife is very sharp, its
FIG. 1. ͑a͒ SEM micrograph of as-prepared nanoporous gold and ͑b͒ the
fracture surface of nanoporous gold. The inset shows the morphology of the
fractured ligaments with sharp tips. ͑c͒ Bright-field TEM image of fractured
gold ligaments and ͑d͒ HREM image of the nanosized apex of a fractured
ligament. Deformation twins and stacking faults are frequently observed in
the fractured ligaments.
a͒
Author to whom correspondence should be addressed. Electronic mail:
mwchen@imr.tohoku.ac.jp
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
0003-6951/2008/92͑9͒/093113/3/$23.00 92, 093113-1 © 2008 American Institute of Physics
120.117.138.77 On: Mon, 15 Dec 2014 03:51:18

093113-2
Qian, Inoue, and Chen
Appl. Phys. Lett. 92, 093113 ͑2008͒
FIG. 3. ͑Color online͒ ͑a͒ SEM micrograph of as-prepared nanoporous gold,
͑b͒ surface morphology of nanoporous gold with the deposited gold nano-
particles, and ͑c͒ the corresponding SERS spectra of 10−7 mol/l R6G mol-
ecules from the as-prepared nanoporous gold ͑a͒ and the deposited sample
͑b͒ detected by 514.5 nm laser.
To demonstrate that the improved SERS effects of R6G
from the fracture surfaces is an intrinsic phenomenon arising
from the microstructure of ruptured nanoporous gold, an-
other organic reagent, CV 10B, is also used as the test probe.
Figure 2͑b͒ illustrates the SERS spectra of 10−5 mol/l CV
molecules from the as-prepared and fractured nanoporous
gold. The analogous improvements can be observed, i.e., the
SERS enhancement from the mechanically ruptured surfaces
is about five times higher than that from the as-prepared
nanoporous gold with the exactly same acquisition param-
eters. The slightly lower improvement than that of the R6G
molecules ͓about ten times, as shown in Fig. 2͑a͔͒ is prob-
ably related to their diversities in the distinct absorption be-
haviors on gold surfaces and in the intrinsic enhancement
factors from gold substrates.
According to the SEM and TEM micrographs, the dis-
tinct difference between the as-prepared and the ruptured
nanoporous gold is the appearance of the sharp protrusions
on the fracture surfaces. Thus, the improved SERS enhance-
ments from the fracture surfaces are most likely associated
with the nanosized protrusions on the top surfaces. To con-
firm this supposition, gold nanoparticles were deposited onto
the as-prepared nanoporous gold surfaces by physical vapor
deposition with high purity gold as a sputtering target. Fig-
ures 3͑a͒ and 3͑b͒ show the morphological changes of the
nanoporous gold surfaces before and after the deposition. By
optimizing sputtering conditions, the top surface of the de-
posited nanoporous gold is decorated by gold nanoparticles
in the form of nanosized protrusions, similar to the fracture
surfaces. The size of the deposited gold nanoparticles,
5–10 nm, is fairly comparable to the diameters of the pro-
trusion apexes in the ruptured surfaces ͓Fig. 1͑b͔͒. Figure
3͑c͒ represents the Raman spectra of R6G molecules from
the nanoporous gold substrates with and without the depos-
ited gold nanoparticles. The analogous improvement in the
SERS enhancement is achieved from the nanoporous gold
with the decorated nanoparticles, which is also about one
order of magnitude higher than that from the as-prepared
porous gold. Therefore, we can conclude that the dramati-
cally improved SERS effects from the fractured nanoporous
gold surfaces originate from the nanosized protrusions pro-
duced by the broken gold ligaments.
FIG. 2. ͑Color online͒ Representative SERS spectra of organic molecules
absorbed onto the as-prepared and fractured nanoporous gold surfaces. ͑a͒
10−7 mol/l R6G molecules detected by 514.5 nm laser and ͑b͒ 10−5 mol/l
CV molecules detected by 632.8 nm laser.
gests that the characteristic length scale of the as-prepared
sample is ϳ60 nm in diameter for both nanopores and gold
ligaments. A facture surface of nanoporous gold is shown in
Fig. 1͑b͒, from which detectable change in the nanopore size
cannot be identified. However, a large number of protrusions
as the result of broken gold ligaments appear on the fracture
surfaces with a bright contrast ͓Fig. 1͑b͔͒. The spear-shaped
protrusions generally have sharp tips ͓the inset of Figs. 1͑b͒
and 1͑c͔͒, which are much smaller than the original ligament
diameter of ϳ60 nm. High-resolution electron microscopy
͑HREM͒ of the protrusions reveals that the apexes have a
diameter of 5–10 nm ͓for an example, Fig. 1͑d͔͒. Deforma-
tion defects, such as deformation twins and stacking faults,
can be frequently observed in the broken gold ligaments,
indicating the formation of the sharp tips are associated with
the localized plastic deformation of gold ligaments during
failure.22
The SERS spectra of R6G molecules on as-prepared
nanoporous gold are shown in Fig. 2͑a͒. Raman bands cor-
responding to the characteristic vibrational modes of R6G
molecules can be observed.19,23 However, the scattering in-
tensities are very weak and only major bands can be de-
tected. Dramatic improvement in the Raman scattering is
achieved from the fracture surfaces of nanoporous gold. With
the same experimental conditions, the intensities of the R6G
Raman bands are approximately ten times higher than those
from the as-prepared nanoporous gold. Moreover, the minor
SERS bands of R6G molecules that are too weak to be de-
tected from the as-prepared nanoporous gold can be well
identified from the fracture surfaces. Although the fracture
surfaces are not flat at the submicron scale, as shown by the
SEM image ͓Fig. 1͑b͔͒, the SERS intensities of R6G mol-
ecules from various regions do not change too much ͓Fig.
2͑a͔͒, indicating the microstructure of the fracture surfaces is
A number of experiments and simulations have demon-
strated that the localized electromagnetic fields play impor-
tant roles in the improved SERS effects24–28 and steep edges
of sharp protrusions can offer an additional enhancement in
comparison with the blunt edges.28 Accordingly, our obser-
vations of the largely improved SERS enhancements from
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
relatively homogeneous.
the fractured surfaces apparently result from a large number
120.117.138.77 On: Mon, 15 Dec 2014 03:51:18

093113-3
Qian, Inoue, and Chen
Appl. Phys. Lett. 92, 093113 ͑2008͒
large internal surface areas for molecular adsorptions and
electromagnetic enhancements from the localized surface
plasmon resonance. Additionally, both gold ligaments with a
positive curvature and nanopores with a negative curvature
are possible origins of the intensified surface plasmon reso-
nance. According to the present observations, the dramatic
improvements from the fractured surfaces strongly suggest
that the chemical contribution in the SERS effects of nano-
porous gold is minor and the dominant enhancements come
from the top surface layers with intensified surface plasmon
resonance and coupling. The gold ligaments with positive
curvatures appear to play a prevailing role in SERS effects of
nanoporous gold, rather than nanopores with negative curva-
tures. It is interesting to note that the enhancement factor of
the fractures surfaces is very close to that of ultrafine nano-
porous gold with the nanopore and ligament sizes of about
5–10 nm.19 Thus, the improved SERS enhancements with
reducing nanopore sizes, observed previously, most likely
result from the enhanced electromagnetic effect from small
gold ligaments.
FIG. 4. ͑Color online͒ ͑a͒ SEM micrograph of nanoporous gold annealed at
300 °C for 2 h, ͑b͒ the fracture surface of the coarsened nanoporous gold,
͑c͒ the sharp tip of a fractured ligament, and ͑d͒ the corresponding SERS
spectra of 10−7 mol/l R6G molecules from annealed nanoporous gold and
the fracture surfaces detected by 514.5 nm laser.
of the sharp protrusions produced by localized deformation
of gold ligaments during mechanical failure. The fractured
gold ligaments with sharp tips can provide intensified elec-
tromagnetic fields around the nanosized apexes.28 Although
the electromagnetic enhancements are localized around the
apex of each protrusion, a large number of ruptured liga-
ments with a high density on the fracture surfaces result in a
macroscopic effect for the overall improvements in SERS
enhancements.
1M. Moskovits, Rev. Mod. Phys. 57, 783 ͑1985͒.
2K. Kneipp, H. Kneipp, I. Itzkan, R. R. Dasari, and M. S. Feld, Chem. Rev.
͑Washington, D.C.͒ 99, 2957 ͑1999͒.
In addition to the localized electromagnetic effect arising
from individual protrusions, electromagnetic coupling
among the sharp tips with distance smaller than 10 nm may
also play an important role in the improved SERS enhance-
ments of ruptured nanoporous gold.15 To clarify the possible
coupling effect, we change the average interprotrusion dis-
tance by cutting a coarsened nanoporous gold sample that
was prepared by annealing the as-dealloyed sample at
300 °C for 2 h.19 Figure 4͑a͒ is the SEM micrograph of the
annealed nanoporous gold. The average size of nanopores
and ligaments is ϳ200 nm, which is about three to four
times larger than that of the as-dealloyed samples. The rup-
tured ligaments on the facture surface also have 5–10 nm
sharp tips but the density of the protrusions ͑5/␮m2͒ is about
three to four times lower than that ͑15ϳ20/␮m2͒ on the
fracture surface of the as-prepared nanoporous gold ͓Figs.
4͑b͒ and 4͑c͔͒. The interprotrusion distance is generally
larger than 100 nm and, in principle, the electromagnetic in-
teraction between the fractured ligaments should not be sig-
nificant. The SERS measurements reveal the SERS enhance-
ments of the fracture surfaces of the coarsened nanoporous
gold is about three times higher than that of the annealed
sample ͓Fig. 4͑d͔͒, further confirming that the fractured liga-
ments with a sharp tip can dramatically improve the SERS
effect of nanoporous gold. However, in comparison with the
SERS improvement of the fractured surfaces of the 60 nm
nanoporous gold, the enhancement factor of the fracture sur-
faces of the 200 nm sample is about 6–7 times smaller. Even
considering the density difference ͑about three to four times
difference͒ of the ruptured ligaments on the fracture surfaces
of the two samples, one can find that there is still 2–4 times
disparity in the SERS enhancement factors. Apparently, in
addition to the localized electromagnetic effect arising from
the sharp apex of individual fractured gold ligaments, the
electromagnetic coupling among the neighboring fractured
ligaments plays an important role in the large SERS en-
hancements of the fracture surfaces of nanoporous gold with
a small pore size.
3C. J. Murphy, T. K. San, A. M. Gole, C. J. Orendorff, J. X. Gao, L. Gou,
S. E. Hunyadi, and T. Li, J. Phys. Chem. B 109, 13857 ͑2005͒.
4U. Kreibig and L. Genzel, Surf. Sci. 156, 678 ͑1985͒.
5S. Link and M. A. El-Sayed, J. Phys. Chem. B 103, 8410 ͑1999͒.
6P. 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.
7A. Tao, F. Kim, C. Hess, J. Goldberger, R. R. He, Y. G. Sun, Y. N. Xia,
and P. D. Yang, Nano Lett. 3, 1229 ͑2003͒.
8E. Hao and G. C. Schatz, J. Chem. Phys. 120, 357 ͑2004͒.
9Y. J. Xiong, J. M. McLellan, J. Y. Chen, Y. D. Yin, Z. Y. Li, and Y. N. Xia,
J. Am. Chem. Soc. 127, 17118 ͑2005͒.
10C. H. Sun, N. C. Linn, and P. Jiang, Chem. Mater. 19, 4551 ͑2007͒.
11L. H. Lu, R. Capek, A. Kornowski, N. Gaponik, and A. Eychmuller,
Angew. Chem. 44, 5997 ͑2005͒.
12H. Wang, C. S. Levin, and N. J. Halas, J. Am. Chem. Soc. 127, 14992
͑2005͒.
13B. N. J. Persson, K. Zhao, and Z. Y. Zhang, Phys. Rev. Lett. 96, 207401
͑2006͒.
14X. Y. Zhang, J. Zhao, A. V. Whitney, J. W. Elam, and R. P. Van Duyne, J.
Am. Chem. Soc. 128, 10304 ͑2006͒.
15H. H. Wang, C. Y. Liu, S. B. Wu, N. W. Liu, C. Y. Peng, T. H. Chan,
C. F. Hsu, J. K. Wang, and Y. L. Wang, Adv. Mater. ͑Weinheim, Ger.͒ 18,
491 ͑2006͒.
16H. X. Xu, J. Aizpurua, M. Kall, and P. Apell, Phys. Rev. E 62, 4318
͑2000͒.
17S. O. Kucheyev, J. R. Hayes, J. Biener, T. Huser, C. E. Talley, and A. V.
Hamza, Appl. Phys. Lett. 89, 53102 ͑2006͒.
18M. C. Dixon, T. A. Daniel, M. Hieda, D. M. Smilgies, M. H. W. Chan, and
D. L. Allara, Langmuir 23, 2414 ͑2007͒.
19L. H. Qian, X. Q. Yan, T. Fujita, A. Inoue, and M. W. Chen, Appl. Phys.
Lett. 90, 153120 ͑2007͒.
20L. H. Qian and M. W. Chen, Appl. Phys. Lett. 91, 83105 ͑2007͒.
21T. Fujita and M. W. Chen, Jpn. J. Appl. Phys. 47, 1161 ͑2008͒.
22J. Biener, A. M. Hodge, and A. V. Hamza, Appl. Phys. Lett. 87, 121908
͑2005͒.
23P. Hildebrandt and M. Stockburger, J. Phys. Chem. 88, 5935 ͑1984͒.
24B. Pettinger, B. Ren, G. Picardi, R. Schuster, and G. Ertl, Phys. Rev. Lett.
92, 96101 ͑2004͒.
25C. E. Talley, J. B. Jackson, C. Oubre, N. K. Grady, C. W. Hollars,
S. M. Lane, T. R. Huser, P. Nordlander, and N. J. Halas, Nano Lett. 5,
1569 ͑2005͒.
26D. D. Evanoff and G. Chumanov, ChemPhysChem 6, 1221 ͑2005͒.
27F. Jackel, A. A. Kinkhabwala, and W. E. Moerner, Chem. Phys. Lett. 446,
339 ͑2007͒.
28W. H. Zhang, X. D. Cui, B. S. Yeo, T. Schmid, C. Hafner, and R. Zenobi,
In general, the SERS effects of nanoporous gold have
been attributed to possible chemical enhancements from
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
Nano Lett. 7, 1401 ͑2007͒.
120.117.138.77 On: Mon, 15 Dec 2014 03:51:18