Surface-enhanced Raman scattering on ordered gold nanodot arrays prepared from anodic porous alumina mask

Article abstract of DOI:10.1246/cl.2008.466

We show surface-enhanced Raman scattering (SERS) on an ordered array of Au nanodots prepared from a highly ordered anodic porous alumina mask. The intensity of SERS signals is strongly dependent on the height of the Au dots, indicating the existence of optimum geometrical structures for SERS. The fabrication process of the nanodot array described in the present study enables easy preparation of SERS substrates. Copyright

Full text of DOI:10.1246/cl.2008.466

466
Chemistry Letters Vol.37, No.4 (2008)
Surface-enhanced Raman Scattering on Ordered Gold Nanodot Arrays Prepared
from Anodic Porous Alumina Mask
Toshiaki Kondo,1;2 Futoshi Matsumoto,1 Kazuyuki Nishio,1 and Hideki MasudaÃ1
1Department of Applied Chemistry, Tokyo Metropolitan University, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397
2Japan Society for the Promotion of Science, 1-1 Minamiosawa, Hachioji, Tokyo 192-0397
(Received January 7, 2008; CL-080014; E-mail: masuda-hideki@c.metro-u.ac.jp)
We show surface-enhanced Raman scattering (SERS) on an
sion of the dots in addition to the ease of preparation. Here, we
describe the dependence of the SERS efficiency of pyridine on
the geometrical structures of Au nanodot arrays on a Si substrate.
The procedure for the preparation of Au dot arrays is almost
the same as reported previously.8 Briefly, the anodic porous
alumina used for the mask was prepared by the anodization of
Al (99.99% purity) in acidic solution after the electrochemical
polishing of Al. Porous alumina masks with through holes
were obtained by the selective dissolution of Al in saturated I2
methanol solution followed by the dissolution of the bottom
part of the porous alumina films in phosphoric acid solution.
The hole sizes of the alumina mask were adjusted by postetching
treatment in phosphoric acid solution.
Au was vacuum deposited onto a Si substrate using a vacu-
um evaporator through the alumina mask, which was set up on
the substrate. The thickness of the alumina mask was typically
240 nm. After the vacuum deposition, the Au nanodot array on
the substrate was obtained by removing the alumina mask
mechanically.
Raman scattering spectra of pyridine were obtained using a
Raman scattering spectrometer equipped with a He-Ne laser
(633 nm) as a light source. The size of laser spot was 13 mm.
The laser power was 1.6 mW. The duration of irradiation and
accumulation of signals were 3 s and 10 times, respectively.
The Si substrate with the Au dot array was dipped in pyridine
solution (HPLC grade, >99:9%) and dried in air before the
measurement.
Figure 1 shows the SEM images of the typical Au nanodot
array formed on the Si substrate. From the surface view in
Figure 1a, it can be confirmed that uniformly sized dots are in
an ordered arrangement with regular intervals. The size and
interval of the Au dots were 70 and 100 nm, respectively.
These values corresponded to the geometrical structure of the
ordered array of Au nanodots prepared from a highly ordered
anodic porous alumina mask. The intensity of SERS signals is
strongly dependent on the height of the Au dots, indicating
the existence of optimum geometrical structures for SERS.
The fabrication process of the nanodot array described in the
present study enables easy preparation of SERS substrates.
The fabrication of functional devices based on the localized
surface plasmon (LSP) in small metal particles has attracted
increasing attention due to its application in various fields, such
as chemical or biological sensing.1–6 Precise control of the sizes
and shapes of the small particles is important because the prop-
erties of LSP are substantially dependent on the geometrical
structure of the metal particles.7,8 In our previous work, we re-
ported the fabrication of ordered nanodot arrays of metals on
the substrate using anodic porous alumina masks and showed
LSP properties of the obtained metal nanodot arrays.9–12 In this
process, metal nanodot arrays are prepared by vacuum deposi-
tion of metals through an anodic porous alumina mask with a
highly ordered hole array structure. In the present work, we
show, for the first time, SERS on the ordered array of Au nano-
dots prepared by vacuum deposition through a highly ordered
anodic porous alumina mask. SERS on the metal fine structures
is a typical phenomenon originating from the enhancement of
the electric field of incident light. There have been numerous re-
ports on the preparation of an ordered nanostructure of metals for
optimizing the efficiency of SERS.13,14 However, processes for
easy precise control of the structure of metal particles have not
been established. The advantage of the use of vacuum deposition
using an anodic porous alumina mask for the preparation of the
metal nanostructure for SERS is the controllability of the dimen-
Figure 1. SEM image of Au dot array prepared using porous
alumina mask; surface view (a) and oblique view (b).
Figure 2. Cross-sectional SEM views of Au dot array; dot height
of 20 (a), 60 (b), and 100 nm (c).
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.4 (2008)
467
Figure 3. SERS spectra from Au dots array with different dot
heights.
Figure 4. Dependence of SERS intensity on Au dot height.
yield the highest efficiency of SERS. For such optimization of
SERS efficiency, the process described in the present study
will be useful because of its ease and controllability in the prep-
aration of ordered Au nanodot arrays.
porous alumina mask used for evaporation. The oblique view
in Figure 1b shows the shape of the Au dots. The height of
Au dots was 40 nm. The conical shape of the Au dots results
from the shadowing effect of the mask during the vacuum
deposition of Au.
The SERS intensity from ordered Au nanodot arrays pre-
pared using an anodic porous alumina mask was examined.
The intensity was strongly dependent on the height of the Au
dots, indicating the existence of optimum geometrical structures
for SERS. The process described in the present study allows
preparation of nanodot arrays of other metals on various kinds
of substrates. The present process allows the easier preparation
of SERS substrates compared to the process employing the elec-
tron beam lithographic technique. In addition, the geometrical
structures of dots can be tuned to optimize the enhancement
of SERS intensity. The obtained SERS substrates will be used
for the Raman spectra measurement with high sensitivity.
In the present study, the effect of the height of Au dots on
SERS was examined. Figure 2 shows SEM images indicating
the variation of the height of the Au dots. The height of Au nano-
dots was controlled by changing the nominal thickness of the
evaporated Au using a thickness monitor during the vacuum
evaporation. The cross-sectional SEM images in Figure 2
revealed that the height of the Au dots was controlled from 20
to 100 nm. These heights corresponded to the aspect ratio (the
height divided by the bottom size of dots) from 0.3 to 1.4.
Figure 3 shows the SERS spectra obtained from Au dot
arrays with different dot heights. The deviation of peak intensity
in the measurement was 20% for the same substrate. Compared
to the spectrum from the smooth Au surface, the intensity of
Raman scattering was confirmed to be enhanced on the Au
dot arrays; that is, two characteristic peaks from the adsorbed
pyridine molecules were observed at 1014 and 1040 cmÀ1. The
assignment of a slight peak observed at around 940 cmÀ1 could
not be achieved at the present stage. Interestingly, the intensity
of SERS is strongly dependent on the height of Au dots.
Figure 4 summarizes the dependence of SERS intensity on
the height of Au dots. In Figure 4, the Raman intensities at
1014 cmÀ1 was summarized against the dot heights. The intensi-
ties were normalized by setting the value at the dot height of
60 nm to unity. The Raman intensity increased with the height
of the Au dots and showed the highest value at the dot height
of 60 nm. Although the detailed mechanism of the dependence
of the SERS intensity on the height of Au dots is not clear at
the present stage, one possible explanation is that the efficiency
of absorption by LSP depends on the shape of dots and yields the
change of SERS intensity. Such dependence of the efficiency of
absorption by LSP on the shape of Au dots could be also
supported by the three-dimensional FDTD simulation. The result
in Figure 3 indicates that the optimal geometrical structures
This work was partially supported by Grant-in-Aid for
Scientific Research No. 19049013 on Priority Area ‘‘Strong
Photons–Molecules Coupling Fields (470)’’ from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
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Published on the web (Advance View) March 25, 2008; doi:10.1246/cl.2008.466