1. Article
Long-Standing Stability of Silver
Nanorod Array Substrates
Functionalized Using a Series of
Thiols for a SERS-Based Sensing
Application
Ranjit De, Yong-Seok Shin, Chang-Lyoul Lee, and
Myoung-Kyu Oh
Abstract
Silver nanorod (AgNR) array substrates were fabricated using an oblique angle thermal evaporation technique; their long-
term stability, surface uniformity and reproducibility, which are primary requirements for their widespread realistic appli-
cation and commercialization, were assessed using surface-enhanced Raman scattering (SERS) spectroscopy. The nanorod
surfaces were functionalized using a series of organic thiols, which range from hydrophilic to hydrophobic, to mimic various
conditions that often arise during detection of hydrophilic/phobic analytes in a realistic application field. A group of these
functionalized substrates was stored in ambient laboratory atmosphere; another in light minimized, moisture-free vacuum;
while another was stowed carefully and neatly in water to mimic realistic conditions. The effects of these storing con-
ditions were studied. A surfactant was added to the water to maintain consistent surface wetting in the third group. SERS
spectra of nanorod substrates prior to functionalization were also recorded to investigate the effect of adventitious
carbonaceous contaminants. A meticulous systematic study on the reproducibility of SERS signals was carried out:
spot-to-spot, substrate-to-substrate, batch-to-batch, day-to-day. The relative standard deviation (RSD) shown by the
SERS signals acquired from various spots of a single substrate was less than 3%, which is very similar to the only account
reported so far, in which RSD is reported as 2%. The wetting behavior of these thiol functionalized AgNR substrates are
investigated using static contact angle measurements. The functionalized substrates have exhibited excellent long-standing
stability over a period of six months when stored appropriately; hence, they are highly suitable for mass production
towards realistic application.
Keywords
Surface-enhance Raman scattering, SERS, nanorod arrays, stability, reproducibility, uniformity, relative standard deviation
Date received: 14 July 2015; accepted: 6 November 2015
Introduction
Since its inception in the 1970s, surface-enhanced Raman
scattering (SERS) spectroscopy has emerged as a widely
used powerful molecular spectroscopic technique for
ultra-sensitive detection of molecules in various chemical
environments, even when the analyte is present at trace or
single molecular level.1–2
SERS was first reported by
Fleischmann et al. in 1974, though the interpretation was
different to the present understanding.3
This was followed
by Jeanmaire and Van Duyne4
and Albrecht and Creighton,5
thereafter passing through a number of stages to result in
the present form, where enhancement factors of 106
to
1014
of Raman intensities are reported.2
Presently, it is
known that this enhancement is predominantly contributed
by long-range electromagnetic (EM) fields existing in nano-
gaps, called hotspots, between plasmonic metal nanostruc-
tures.6–9
The other mechanism that also contributes to the
total enhancement, though to a comparatively smaller
extent, is the short-range chemical effect (CM), which
Advanced Photonics Research Institute (APRI), Gwangju Institute of
Science and Technology (GIST), South Korea
Corresponding author:
Myoung-Kyu Oh, Advanced Photonics Research Institute (APRI),
Gwangju Institute of Science and Technology (GIST),
Gwangju 500 712, South Korea.
Email: omkyu@gist.ac.kr
Applied Spectroscopy
0(0) 1–13
! The Author(s) 2016
Reprints and permissions:
sagepub.co.uk/journalsPermissions.nav
DOI: 10.1177/0003702816652327
asp.sagepub.com

2. originates due to a charge transfer between a metallic sur-
face and the molecules attached to it or through the for-
mation of adsorbate electronic resonance over a localized
area.10–11
Of the noble metals, Au and Ag have been most
widely employed in SERS substrate fabrication: using these
as an upper limit of the SERS enhancement factor (EF) has
already been realized.2
However, no systematic studies have
been carried out to assess their surface uniformity, repro-
ducibility, and long-term stability, especially for nanorod
array substrates fabricated by the oblique thermal evapor-
ation technique.
Recently, feasibility of trace level as well as single
molecular level detection of a wide range of analytes has
become the driving force behind various attempts to fabri-
cate smart SERS substrates. There have been various stra-
tegies employed to fabricate SERS substrates with
considerable surface roughness to exploit the electromag-
netic behavior of metal nanostructures. Among the early
efforts, strategies like chemical etching, electrochemical
oxidation-reduction cycles, etc. were used but they have
been found to often suffer from poor uniformity, morph-
ology reproducibility, stability and also low surface enhance-
ment. Thereafter, most extensively studied SERS substrates
were designed using colloidal metal nanoparticles which
exhibited the desired advantages, such as biomolecule com-
patibility and high surface enhancement, but in many cases
the surface morphology was found to be vulnerable to the
laser field12
and interparticle spacing was challenging to
control; the colloidal stability was also often affected by
temperature, pH, and presence of adsorbates and reprodu-
cibility was often questioned. SERS signal intensity is highly
sensitive to the size of colloidal particles and varies from
sample to sample as reproducibility of monodisperse col-
loidal sol preparation is tedious and demands a great deal of
effort.13
Thus, high signal reproducibility could not be
achieved whereas realistic application requires it. Earlier,
it has been shown by theoretical as well as practical inves-
tigations that nanorods and nanowires have higher aspect
ratio and can cause greater surface enhancement.14
Thus,
researchers have fabricated such nanostructures using vari-
ous methods like seeding growth, electrochemical etching
and electroplating, wet chemical reaction, templating, etc.
Using Maxwell’s equations on adaptive meshes, Garcia-Vidal
and Pendry15
showed, through their theoretical investiga-
tion, that arrays of touching silver nanoparticles can result
in huge electromagnetic coupling while Zou and Schatz16
practically showed that the silver nanoparticle array struc-
tures can produce huge electromagnetic enhancement. A
combined understanding of these strategies to fabricate
nanostructures with high aspect ratios, along with arrayed
morphology, supports the present trend of fabricating
nanorod array structures to which high density has been
incorporated. Investigators have been using various meth-
ods like electron beam lithography (EBL), templates, chem-
ical or physical deposition, etc. to fabricate such
morphology. To the best of our knowledge, based on a
literature survey, there has only been one report where
Hankus et al.17
fabricated highly uniform and reproducible
nanostructure arrays by acid etching of an optical fiber
bundle and subsequently silver was deposited onto it at
less than 2% relative standard deviation (RSD) in SERS
intensity when a spot-to-spot scanning was carried out
on a single substrate. It is notable that, despite the ability
to produce metal nanostructures with a high aspect ratio
and sufficient surface roughness, most of the above-men-
tioned methods require complicated preparation protocols
and lack stability. EBL is also an expensive technique and
requires a special arrangement.
Keeping the above-mentioned constraints in mind, to
fabricate smart substrates with large and uniform SERS
active surface area, high reproducibility and long-standing
stability, we employed oblique angle physical vapor depos-
ition, which provides the opportunity to control the aspect
ratio with arrayed morphology and the use of various
source materials, such as Ag, Au, Cu, etc., as long as
these can be evaporated and a simple standard thermal
evaporator can be used. A small change in the evaporation
chamber can provide the opportunity to fabricate various
nanostructures. Thus, we have chosen this method to fab-
ricate silver nanorod arrays as SERS substrate by integrating
oblique angle deposition (OAD) technique into a standard
thermal evaporation system. This is a simple, single-step
and chemical-free green process. In our earlier publication,
we presented the details of the fabrication procedure and
the effect of substrate temperature on the morphology of
OAD nanorod arrays.18
In the present work, we have con-
fined our interest to checking the long-standing stability, uni-
formity, and reproducibility of these OAD AgNR arrays for
SERS spectroscopy-based sensing application. Earlier it has
been reported that Ag nanorod arrays fabricated via oblique
angle vapor deposition can provide large SERS active surface
area but almost no works have been performed to assess
their uniformity, reproducibility and stability. There are vari-
ous SERS substrates that exhibit high surface enhancement
and can be used as promising candidates for ultra-sensitive
trace level molecular detection. However, commercializa-
tion of these substrates has often been challenged due to
poor stability, reproducibility of morphology, uniformity and
spectral intensity. In addition, Ag surfaces are susceptible to
contamination regardless of fabrication procedure, which
can interfere in trace level molecular recognition and
reduce stability, as well as reproducibility.
Here, we have presented SERS responses of OAD AgNR
arrays and demonstrated their large area morphological uni-
formity, spectral reproducibility and long-term stability by
studying those spot-to-spot, substrate-to-substrate, batch-
to-batch and day-to-day spectral responses, respectively.
Nanorod surfaces were functionalized using various organic
thiols: namely, carboxythiophenol (CTP), methoxythiophe-
nol (MTP), aminothiophenol (ATP), nitrothiophenol (NTP),
2 Applied Spectroscopy 0(0)

3. thiophenol (TP), and propanethiol (PT), that ranged from
hydrophilic to hydrophobic in nature to mimic the realistic
conditions faced during the sensing of a variety of hydrophilic
or hydrophobic analytes. Static contact angle measurement
was performed to investigate the wetting behavior of these
thiol functionalized AgNR substrates. It is known that these
thiol molecules get anchored on the metal surfaces through
a strong metal–sulfur bond formation which thereby con-
structs a self-assembled monolayer (SAM) and is stable
enough to withstand drastic conditions such as the variation
of pH over a long range. SAMs can also protect metal sur-
faces from various types of adventitious materials that are
often present in the immediate ambient atmosphere. A
group of all these functionalized substrates was stored in
an ambient laboratory atmosphere, another in a light-mini-
mized, moisture-free vacuum chamber, another in water to
mimic some realistic applications; a surfactant was used to
maintain uniform surface wettability.
Experimental
Fabrication of Silver Nanorod Arrays
Oblique angle deposited (OAD) silver-nanorod (AgNR)
arrays were fabricated on a silicon wafer using a bell-jar
type thermal evaporator (GVtech, Inc.). Prior to deposition,
the pressure in the vacuum chamber was brought down
below 10À6
Torr and maintained the same throughout the
whole fabrication process. Substrate cooling system
equipped with copper tube, through which water circu-
lated, was employed and the temperature was maintained
at 20
C to reduce the heating due to the blackbody radi-
ation from the tungsten boat used as silver source holder.
The vapor deposition angle (flux angle), which is the angle
between the normal substrate surface and the vapor flux
direction, was maintained at 86 Æ 1
. Appropriately sized
(1 cm  1 cm) pieces of silicon (100) wafer were cleaned
in a solution of H2O:H2O2:NH4OH ¼ 5:1:1 at 70
C for
15 min followed by water rinsing. Highly pure Milli-Q
water was used wherever required. A schematic presenta-
tion of AgNR fabrication by OAD process is in Scheme 1.
A layer of silver film with thickness $50 nm, monitored by
quartz crystal microbalance (QCM) placed at normal inci-
dence to vapor flux, was first deposited on the cleaned Si-
wafer keeping the flux angle at 0
on which the OAD AgNR
arrays were fabricated. The metal source used was silver
grains of high purity (99.999%) procured from Alfa Aesar.
Evaporation rate was maintained at 5 A˚ /s and the overall
thickness of silver deposited was constantly monitored by
QCM. Substrates were allowed to cool in vacuum before
removing from the chamber after the desired AgNR film
thickness of about 3 mm was achieved. These substrates
were stored in a clean vacuum chamber to protect from
adventitious materials prior to functionalization whenever
required.
Characterization of Silver Nanorod Arrays
The morphology of AgNRs fabricated via the above-men-
tioned OAD method was characterized by field emission
scanning electron microscopy (Hitachi, S-4700). To assess
crystallographic purity, X-ray diffraction (XRD) spectrum
was obtained using a computer controlled Rigaku diffract-
ometer with a Cu radiation ( ¼ 0.15406 nm) running at
40 kV and 40 mA. Surface plasmon property was character-
ized by recording spectra in reflection mode using a
ultraviolet–(UV-Vis) spectrometer (Jasco, V-570).
Functionalization of Silver Nanorod Arrays
AgNR array substrates were functionalized in groups by six
thiols: namely, 4-aminothiophenol (97%), 4-nitrothiophenol
(80%), 4-carboxythiophenol (99%), thiophenol (99%), 4-
methoxythiophenol (97%), and 1-propanethiol (99%)
which were purchased from Aldrich. The substrates were
immersed in 10 mM ethanoic solution of thiols for about
12 h. Absolute ethanol, HPLC grade was purchased from
Fisher. Functionalized OAD AgNR array substrates were
rinsed with copious amount of ethanol to remove any phy-
sisorbed thiol molecules from the substrate to result in a
monolayer formation. These functionalized substrates were
then dried under a gentle stream of nitrogen gas. All func-
tionalization processes were carried out at room tempera-
ture. The morphology of nanorods remained intact after
functionalization. A group of the functionalized substrates
were stored in the ambient laboratory atmosphere;
another was stored at room temperature in moisture
Scheme 1. Schematic presentation of oblique angle deposition
technique used for AgNR array fabrication.
Oh et al. 3

4. free vacuum chamber wrapped with silver foil to minimize
effect of UV light while another group was stored in 5 mM
aqueous solution of sodium dodecylsulfate (!98.5%, Sigma).
Substrates fabricated and functionalized in the same batch
were considered in all these cases to compare the effect of
storing condition on substrate stability, uniformity, and spec-
tral reproducibility. Prior to the measurement of SERS spec-
tra, each time the substrates stored in aqueous SDS solution
were gently rinsed with water and dried.
SERS Spectra Measurement
Surface-enhanced Raman spectra of thiol-coated AgNR
array substrates were acquired using a home-built optical
fiber-coupled micro-Raman spectrometer employing a
computed tomography spectrometer (Spectro, Inc.) with
a cooled Si-CCD array detector (Andor, Inc., model
name: iVac). The focal length of the spectrometer is
20 cm and the resolution of the spectrometer is 0.3 nm
(10.6 cmÀ1
) at 532 nm when an optical fiber bundle (inlet:
circular with 700 mm diameter, outlet: slit with 200 mm
width ) is used to couple the Raman signal to the spectrom-
eter. The wavelength of the excitation source used is
532 nm (continuous wave diode laser) and the linewidth
of the laser is smaller than 0.3 nm at full width half max-
imum. The spot size of the laser beam focused on Ag sur-
face was $30 mm in diameter. Rayleigh scattering was
reduced using a Notch filter (Semrock, Inc.) having the
bandwidth as 17 nm and optical density 6. The objective
used has the numerical aperture value of $0.5. Both the
incidence and collection angles of pump laser and Raman
signal were normal to the substrate. A motorized xy-trans-
lation stage was used to scan the substrates. Spectra were
collected over the range 3000 to 200 cmÀ1
. For each spec-
trum, a minimum of three spectra were recorded and the
average was considered for analysis. The laser power was
10 mW and the acquisition time was one second for each
spectrum acquisition. It is worth mentioning that no pre-
processing has been used for correction of spectra; hence,
all the spectra used in this study are referred to as raw. The
AgNR substrate size used in this study was 1 cm  1 cm and
each substrate was scanned over an approximate area of
60 000 mm2
(30 Â 2000 mm2
) during spectral line scan with
spots considered randomly.
Contact Angle Measurements
The wetting behaviors of thiol functionalized nanorod sub-
strates were investigated through contact angle measure-
ments by gently dispensing 2 mL of molecular grade water
using a micropipette on the substrate surface, which was
mounted on a stage. Drop size was maintained the same in
all the substrates as the contact angle is also dependent on
the drop size.19
The pictures were taken using a common
camera.
Results and Discussion
Morphology of AgNR Arrays
SEM images (top and side view) of Ag thin film and AgNR
arrays fabricated on this thin film via thermal evaporation,
using the OAD technique, are shown in Figure 1a and b
respectively. According to these images, the average length
of these nanorods is 800 Æ 50 nm with an average diameter
of 70 Æ 10 nm. The density of these nanorods was found to
be 20 Æ 1 rods/mm2
with an average distance of
120 Æ 20 nm between two nanorods. The nanorods were
aligned at 70 Æ 5
with respect to the substrate surface
normal. The individual rods were mostly cylindrical with
random nanoscale irregularities on their surfaces; which
often contributes to surface enhancement, as was also
observed by an earlier group.20
A detailed study of morph-
ology and SERS response of the AgNR array substrates
fabricated at various substrate temperatures are presented
elsewhere.18
The crystallinity of a fabricated AgNR array was studied
by investigating the obtained XRD spectrum presented in
Figure 2. The diffraction peaks observed at 38.10
, 44.22
,
64.50
, and 77.40
corresponding to (111), (200), (220),
and (311) planes suggests the AgNR to have a face-centered
cubic (fcc) lattice structure. The Ag is polycrystalline in
Figure 1. SEM images of (a) silver thin film and (b) silver nanorod arrays. The top (scale bar 2 mm) and cross-sectional (inset, scale bar
1 mm) view.
4 Applied Spectroscopy 0(0)

5. nature with a stronger (111) phase suggesting dominant
out-of-plane growth along this crystal orientation.21
Thus,
these Ag nanorods fabricated via thermal evaporation tech-
nique are free from crystallographic contaminations as no
counterfeiting diffraction is observed.22
According to the UV-Vis reflectance spectrum presented
in Figure 3, the fabricated AgNR arrays have shown plasmon
resonance over a wide range of wavelengths which thereby
justifies the choice of pump laser at 532 nm (indicated by
vertical dashed line) during SERS line scan. This wide range
of wavelengths in the long tail of the plasmon band should be
the consequence of notable presence of nanorods with
larger average diameter and long lengths along the major
axis (high aspect ratio), as also seen in the SEM image
(Figure 1b), because the rate of radiative decay of surface
plasmon mode increases for larger sized particles.23
It had also been previously pointed out by various
researchers that SERS activity of nanorod array substrates
strongly depends on the optical response of their struc-
tures. Zhao et al.,24
for example, had shown that the optical
response of nanorods deposited on glass substrates
depends on the length of the nanorods; in another work,
Liu et al.25
have thoroughly investigated the dependence of
optical response on the nanorod length and deposition
angle. Thus, the reflectance spectrum is informative when
characterizing the optical properties of the nanorod array
substrates fabricated by thermal evaporation. Since the
nanorod arrays were deposited on a silver film which was
on a Si wafer, we could only characterize the optical reflect-
ance of the nanorod substrates. The shape of the reflect-
ance spectrum, shown in Figure 3, is observed to be
consistant with that shown by Liu et al., which is charac-
teristic of nanorods with such length, diameter and density,
as observed in our samples through SEM images (Figure
1b). Their group also investigated the reflectance spectra
of silver nanorods with different lengths, fabricated using a
fixed angle of deposition as well as a fixed length at different
deposition angles, and finding that reflectance depends on
both the length and deposition angle. The larger the depos-
ition angle used, the lower the reflectance was observed
and the SERS enhancement factor increases with the
decrease of reflectance at a particular excitation wave-
length. The optimized deposition angle maintained during
our fabrication process was the largest one that we could
use to produce nanorods with optimal morphology, which
is thus evident through the reflectance spectrum obtained
from the fabricated nanorod substrate.
Effect of Contamination
The effect of contamination of a nanorod on SERS spectra
are presented in Figure 4. These spectra show how con-
tamination by adventitious materials can affect the sub-
strate. Figure 4a presents the SERS spectrum of bare
(without functionalization) AgNR array substrate acquired
immediately after removal from the fabrication chamber;
Figure 4b represents a SERS spectrum which was acquired
from an AgNR array substrate functionalized by propa-
nethiol immediately after fabrication. The broad peak
observed at around 1618 cmÀ1
in Figure 4a may be attrib-
uted to the organic materials that might have outgassed
during fabrication in the chamber and this disappeared in
Figure 4b after functionalization, which thereby suggests
that those organic materials were substituted by propa-
nethiol molecules during the process of functionalization.
Another bare AgNR array substrate was kept in the ambi-
ent atmosphere for about a month prior to functionaliza-
tion: the SERS spectrum of that bare AgNR is presented in
Figure 4c, while Figure 4d represents the spectrum
acquired after functionalizing the same substrate by propa-
nethiol. A comparison of spectra in Figure 4c and d shows
that in this case the process of functionalization could not
substitute some of the contaminant materials completely as
Figure 3. UV-Vis reflectance spectrum of Ag nanorod arrays
with thin Ag film underneath. The excitation wavelength used for
SERS measurements is indicated by the dashed line.
Figure 2. XRD spectrum of Ag nanorod array with stronger
(111) phase.
Oh et al. 5

6. the peaks of those materials interfered with the spectrum
of this propanethiol functionalized substrate (indicated by
dashed lines). The substrate stored in an ambient labora-
tory atmosphere had various peaks in its SERS spectrum at
480, 928, 1188, 1383, 1601 and 2852 cmÀ1
. It is reported
that the main peaks (1383 and 1601 cmÀ1
) are primarily due
to the graphitic carbonaceous contaminant materials.26
Graphite mainly has two characteristic peaks at
1340 cmÀ1
(D band) and 1580 cmÀ1
(G band) and it is
also known that G band shifts towards higher frequency
as the dimension of graphitic materials decreases.27
The
broad peak at 2852 cmÀ1
is due to n(C–H) modes of ali-
phatic alkanes.28
The other bands in the spectrum might be
attributed to the organic impurities which most likely out-
gassed in the vacuum chamber during fabrication, as men-
tioned earlier. Bare metal surfaces have a tendency to
adsorb adventitious materials as such adsorption can
reduce the free energy of the interface between the
metal surface and ambient environment.29
Thus, prior to
functionalization it is important to store substrates in a
controlled environment, whenever required, to protect
those from any undesired morphological changes that
may arise due to atmospheric humidity, temperature, con-
taminants or UV radiation. It is known that, once functio-
nalized, the metal surfaces can remain protected from such
adventitious materials as long as the surface is covered by
self-assembled monolayers (SAMs) formed during the func-
tionalization process.
Earlier, the presence of carbonaceous contaminants on
SERS-active substrates has also been observed by research-
ers and found to be present irrespective of fabrication
method.30
This often poses challenges by affecting the
reproducibility of measurements and limiting the sensitivity
of substrates in high sensitive analytical applications. Various
protocols, such as electrochemical cleaning,31
solid CO2
snow jet,32
Ar plasma33–35
or O3 treatment,36
to name
some, have been used to clean the surfaces but it is to be
mentioned that there have been none which can success-
fully remove the contaminants eliminating background con-
tamination on SERS substrates completely. Thus, while
these processes could be adopted it is to be noted that
during the cleaning process the SERS signal intensity can get
reduced33
and there is a risk of distortion in SERS-sub-
strates’ nanostructured morphology, if proper attention is
not paid, which is often effort demanding. A more detailed
study on this was beyond the present scope and might be
considered in an upcoming work.
SERS Spectra of AgNR Array Substrates
Functionalized Using Various Thiols
The molecular structures of thiols used in this study are
presented in Scheme 2. Representative spectra of AgNR
array surfaces functionalized by aminothiophenol (ATP),
nitrothiophenol (NTP), carboxythiophenol (CTP), thiophe-
nol (TP), methoxythiophenol (MTP), and propanethiol (PT)
are shown in Figure 5. The absence of any spurious peak in
the spectra of functionalized substrates suggests that the
thiols are organically pure and if there was any adventitious
material adsorbed on the metal surface, it can be assumed
that such materials were substituted by thiols owing to
their high affinity towards silver, as discussed above. It has
been shown earlier that surface enhancement is greater in
nanorod substrates with underlying Ag thin film in compari-
son to the nanorod substrate without such film;37
hence,
the former is employed in all the experiments here to
Figure 4. (a) SERS spectrum of bare AgNR array acquired immediately after fabrication, (b) SERS spectrum of AgNR array functio-
nalized by propanethiol immediately after fabrication, (c) SERS spectrum of bare AgNR array acquired after keeping it at ambient
atmosphere and (d) SERS spectrum of AgNR array functionalized by propanethiol after keeping the bare AgNR at ambient atmosphere.
6 Applied Spectroscopy 0(0)

7. achieve maximized enhancements. The Raman enhance-
ment factor (EF) of fabricated AgNR substrates and func-
tionalized by different thiol probe molecules are calculated
using the following equation.
EF ¼
ISERS=Ibulk
NSAM=Nbulk
ð1Þ
where ISERS and Ibulk are the intensities of Raman band
measured in the SERS-active medium and bulk state,
respectively. NSAM and Nbulk are the numbers of probe
molecules in the self-assembled monolayer (SAM) and
bulk state contributing to Raman signal, respectively.18
The ISERS=Ibulk values for each substrate were obtained
from their corresponding SERS spectrum by investigating
a particular peak intensity enhancement in comparison to
same peak intensity of the bulk probe molecules. The pos-
itions of the peaks involved in this calculation are men-
tioned in the parenthesis of each probe molecule. The
area occupied by each TP molecule is considered as
Figure 5. Representative SERS spectra of AgNR arrays functionalized by: (a) nitrothiophenol, (b) aminothiophenol, (c) carboxythio-
phenol, (d) thiophenol, (e) methoxythiophenol, and (f) propanethiol. Some strong peaks are specified in each spectrum.
Scheme 2. Molecular structures of thiols used in this investigation.
Oh et al. 7

8. 0.2 nm2
, which is the well known value, and is considered
the same for all the remaining molecules for simpli-
city.18,38–39
Though the morphology of the AgNR film sur-
face is highly rough, still we have considered the surface as
flat, which makes the calculation simple and expresses the
sensitivity or detection capability, explicitly. Thus, the EFs of
the substrates functionalized by NTP (at 1336 cmÀ1
), ATP
(at 1470 cmÀ1
), CTP (at 1586 cmÀ1
), TP (at 1573 cmÀ1
),
MTP (at 1591 cmÀ1
) and PT (at 780 cmÀ1
) are estimated
as 1.7 Â 108
, 1.4 Â 108
, 1.1 Â 108
, 9.0 Â 107
, 6 Â 107
, and
2.1 Â 107
respectively. It is to mention that SERS enhance-
ment is caused by a joint contribution of CM and EM effect
where EM can contribute from 104
to 1012
of the total, the
prominent contributor, and CM can contribute from 100
to
102
of the total.39
Uniformity and Reproducibility of SERS Substrates
and Signal Intensity
Spot-to-Spot. The uniformity of SERS signal intensities
and nanorod morphology were investigated. For this pur-
pose, the surface of a thiol-functionalized substrate was
thoroughly scanned out of which randomly selected spots
were considered to record SERS spectra. It is important to
mention that each spectrum is an average of a minimum of
three spectra and the spots were randomly selected in a
relatively large area; same process was repeated for all the
six types of thiol functionalized substrates. This spot-to-
spot signal intensity variation has shown the relative stand-
ard deviation (RSD) less than 3% which is very close to the
value observed by Hankus et al.17
and better than the
values shown by nanorod arrays fabricated using electron-
beam/sputtering evaporation method, where this RSD
value was found to vary in the range of 8 to 11% obtained
using trans-1,2-bis(4-pyridyl)-ethene as Raman probe mol-
ecules on the nanorods of comparable average length
868 nm.37
This wide range is attributed to the variations
in the length, which was calculated to be about 11%. In
our work, the variation of nanorod length, calculated via
similar way by investigating of AgNR SEM images (side view)
in different substrates,37
was found to vary less ($7%) in
comparison to the above-mentioned value, and hence pro-
duced nanorod arrays with better homogeneity in terms of
both the length and distribution along the silver thin film
surface on the silicon wafer. Thus, this high morphological
homogeneity of the nanorods is the reason for such good
spot-to-spot reproducibility. The results were verified using
various thiol molecules and are summarized in Figure 6 and
Table 1. A representative figure (Figure S1) can also be
found in the supporting information. This shows that the
disparity in SERS signal intensity at different spots across
the fabricated substrate is within the acceptable range and
the variation of nanorod morphology along a substrate is
very low, which is also visible in the side view of the SEM
image (Figure 1b, inset).
Substrate-to-Substrate. The nanorod fabrication
chamber, where the silver source was evaporated, can
house several (1 cm  1 cm) substrates, enabling mass pro-
duction in a single batch. Hence, it is important to confirm
whether all substrates can produce similar SERS signal
intensity and bear comparable AgNR morphology. To evalu-
ate this, six such (1 cm  1 cm) substrates fabricated in the
first batch, were randomly selected and functionalized
maintaining same condition parameters. An average of
three SERS signals was accepted in each substrate and the
evaluation was carried out considering the bands at
1079 cmÀ1
and 1573 cmÀ1
for TP functionalized substrates
which showed RSD values of 4.88% and 4.94%, respectively
(Figure 7 and Table 1). A similar investigation was carried
out using the remaining five thiols. As we moved from sub-
strate to substrate functionalized by the thiols used in this
investigation, the repeatability of SERS signal intensity was
found to show RSD values of less than 5% (Table 1). This
demonstrates that the repeatability of SERS signal intensity,
and thereby the AgNR morphology, was satisfactory even
when randomly picked up substrates, prepared in a batch,
were considered. This also supports the great possibility of
mass production of AgNR array substrates using the ther-
mal evaporation technique.
Batch-to-Batch. In this approach, four substrates
were collected from four different fabrication batches and
every effort was made to keep condition parameters similar
during the AgNR array fabrication in each batch. The pur-
pose of this was to understand the reproducibility of the
SERS signal provided by substrates which were fabricated in
different batches, which is challenging. Here too, the same
SERS signal bands mentioned above were considered for
analysis. It was found that batch-to-batch reproducibility of
signal intensity for TP functionalized substrates has shown
the RSD value of 5.81% and 5.89% for the bands at
1079 cmÀ1
and 1573 cmÀ1
, respectively (Figure 7).
Substrates fabricated in four different batches and functio-
nalized by ATP, NTP, CTP, MTP and PT were also
Figure 6. Spot-to-spot SERS signal intensity variation in thiol
functionalized AgNR substrates.
8 Applied Spectroscopy 0(0)

9. investigated and the highest RSD value was found to be
about 7% as listed in Table 1. It is notable that, in all the
above cases, spectra obtained from different substrates, or
various spots of a single substrate, varied only in intensity,
while the basic spectral shape remained unaltered. This also
shows that, to meet the practical need-based production,
fabrication of nanorod array substrates can be considered
from various batches as long as similar condition param-
eters during fabrication process are maintained.
Day-to-Day. One of the greatest challenges in the field
of widespread application of SERS substrates is lack of long-
term stability. There are various substrates in dispersion as
well as on solid dry support fabricated via various methods
and many of them can show extremely high surface
enhancement as well as trace-level sensitivity. But their
popularity in realistic application is always challenged due
to poor long-term stability. To evaluate long-standing sta-
bility and the effect of storing conditions, the functionalized
substrates were stored in three different environments: (i)
ambient laboratory atmosphere, (ii) moisture free, light
minimized vacuum chamber, and (iii) aqueous surfactant
solution. The substrates considered for these investigations
were fabricated in a single batch to ensure similar nanorod
morphology and their functionalization was also done in a
single batch to maintain same condition parameters. After
their SERS responses were recorded, one group of sub-
strates functionalized by different thiol molecules was
stored in ambient conditions, another was stored in a
vacuum chamber, with a third in aqueous surfactant solu-
tion of sodium dodecylsulfate (5 mM). This surfactant was a
judicious choice as it does not show any peak in the inves-
tigated spectral region and hence is not expected to affect
the intensity and sensitivity of the functionalized substrates.
The aqueous surfactant solution assists in preserving uni-
form wettability and mimics many realistic applications.40
SERS spectra were recorded using the substrates from all
Table1.TheRSDvaluesofSERSspectraacquiredfromvariousAgNRarraysubstratesfunctionalizedbydifferentthiols.
ThiolsNTPATPCTPTPMTPPT
AgNRSubstrates
%RSDvaluesat
1336cmÀ1
1576cmÀ1
1188cmÀ1
1470cmÀ1
1144cmÀ1
1586cmÀ1
1079cmÀ1
1573cmÀ1
1081cmÀ1
1591cmÀ1
780cmÀ1
1088cmÀ1
Spot-to-spot2.512.442.532.462.552.462.492.442.832.872.322.37
Substrate-to-substrate4.154.204.314.364.644.594.884.943.933.884.334.23
Batch-to-batch6.776.785.895.976.906.965.815.896.096.156.116.06
Day-to-day11.6911.5411.2411.3610.8910.779.9410.019.129.058.698.73
Figure 7. Relative intensity of thiophenol functionalized SERS
substrate fabricated in four different batches (RSD ¼ 5.89%). Six
bars in Batch 1 represent relative intensities of six substrates
randomly selected from first batch (RSD ¼ 4.94%).
Oh et al. 9

10. three groups at an interval of a few days and continued for
about three months for the first group stored at ambient
laboratory atmosphere, while it was continued for six
months to generate a lifetime plot. The substrates kept in
ambient atmosphere showed their stability for about three
months. One of the reasons for this comparatively low
stability could be due to the aerial oxidation of the sub-
strates, which often can occur in the presence of UV light.
Just to mention, the relative intensities in all these cases
were produced by comparing the signal intensities of differ-
ent days with respect to that of the first day. The life-time
plots for the second and third groups produced in the same
way are presented in Figure 8. This figure illustrates the
long-term stability of all six thiol-functionalized substrates
stored in the remaining two conditions: one in a moisture
free, external light minimized vacuum chamber (Figure S2);
the other in aqueous surfactant solution (Figure 8). The
scattering of the points originated around the straight line
could be due to variation in positioning the substrate during
the record of SERS spectra: it is not possible to maintain
the same spot when spectra is recorded on day-to-day
basis. To minimize error, thorough scanning of the substrate
was performed during the record of spectra and an average
was considered. The slopes for each of these substrates
provide a measure of their long-term stability. It is also
observed that the reproducibility of the SERS intensity pro-
duced by the SERS substrates stored in surfactant solution
provided similar longevity to the substrates stored in a
chamber, which suggests that such AgNR array substrates,
fabricated by oblique angle thermal evaporation, can be
used successfully for realistic application purposes. It also
shows that storing of such substrates in aqueous solution of
SDS could also be a decent option, as long as the water
quality is good with low dissolved oxygen and it can protect
Figure 8. Lifetime plot showing long-term stability of AgNR array substrates functionalized by (a) NTP, (b) ATP, (c) CTP, (d) TP,
(e) MTP, and (f) PT. Functionalized substrates were stored in surfactant solution.
Table 2. Adjusted R2
, slope and % RSD for the lifetime plot of thiol functionalized AgNR array substrates.
Thiols
(Peak position)
Surfactant solution Vacuum Ambient atmosphere
Adjusted
R2
Slope
(Rel. Int.) % RSD
Adjusted
R2
Slope
(Rel. Int.) % RSD
Adjusted
R2
Slope
(Rel. Int.) % RSD
NTP (1576 cmÀ1
) 0.685 –0.001 10.46 0.922 –0.002 10.56 0.881 –0.012 9.89
ATP (1470 cmÀ1
) 0.864 –0.002 10.24 0.862 –0.001 10.57 0.861 –0.013 10.01
CTP (1144 cmÀ1
) 0.874 –0.001 10.57 0.887 –0.001 10.97 0.884 –0.011 10.61
TP (1573 cmÀ1
) 0.876 –0.002 10.33 0.898 –0.002 11.06 0.816 –0.012 10.49
MTP (1591 cmÀ1
) 0.750 –0.001 10.98 0.832 –0.002 10.79 0.849 –0.014 10.71
PT (1088 cmÀ1
) 0.932 –0.004 11.31 0.971 –0.003 11.74 0.892 –0.017 11.51
10 Applied Spectroscopy 0(0)

11. the substrate from aerial oxidation, which is often acceler-
ated by light. The Adjusted R2
, slope and % RSD values of
the lifetime plots for all the substrates are summarized in
Table 2. As SERS signal intensity is concerned, these slope
values for the second and third groups showed that the
substrates are stable for more than six months, though
the slope shown by the propanethiol functionalized sub-
strate was found to be slightly lower; which shows that
the spectral reproducibility is slightly lower in the case of
propanethiol functionalized substrates in comparison to the
other thiol molecules, though overall stability was found to
be long enough.
Hydrophilic/Hydrophobic Probe Molecules
and Wetting Behavior
In addition to all the above-mentioned factors, the choice
of different probe molecules with variable hydrophilic as
well as hydrophobic characters represents the various real-
istic conditions often faced during detection of numerous
analytes; hence it is important to know the stability and
reproducibility of SERS substrates functionalized by such
molecules. As these analytes could be hydrophilic as well
as hydrophobic, during the choice of analyte specific probe
molecules for functionalization of our SERS substrates, a
series of thiol molecules (Scheme 2), with variable hydro-
philic and hydrophobic characters, were chosen and their
long-term stability and reproducibility investigated. The
static contact angle measurements have shown the
change in wetting behavior of the AgNR substrates functio-
nalized by different thiols (Figure 9). The AgNR substrates
functionalized by PT, and TP have shown very high contact
angles—close to 180
, showing their hydrophobic nature.
On the other hand, AgNR substrates functionalized by CTP,
MTP, ATP, NTP showed relatively lower contact angles,
especially with a CTP functionalized substrate, where it
was lower than 30
– showing their hydrophilic surfaces
and the spreading of the water drop to be comparatively
faster. By the definition of contact angle, it is known that
with an increase in hydrophobicity the contact angle
increases but decreases with the increase in
hydrophilicity.41
It is also well known that the wetting
behaviors of these types of pristine nanostructured metallic
surfaces are highly dependent on the aspect ratio of the
nano-array.33,42
According to the aforementioned analysis,
where the aspect ratios of all AgNR array substrates used
here are almost same, it is clear that the variation in hydro-
phobicity and hydrophilicity is the result of surface functio-
nalization caused by different thiols; this type of strategy
can be successfully used for the sensing of a number of
analytes with variable hydrophobic and hydrophilic behav-
ior. Thus, we could correlate the wetting behavior of dif-
ferent thiols on AgNR substrates using contact angle
measurements.
On the whole, this study has shown that in all these
cases the substrates were similarly stable over the span
of six months and produced reproducible spectra
(Table 2) which present these substrates to be highly suit-
able for various analyte detections.
It is also to be noted that the change in distance between
source and substrate inside the evaporation chamber can
bring variation in signal intensity, and this primarily could
be attributed to the variation in nanorod length and morph-
ology.43–44
Effective distance between the source and sub-
strate in our fabrication chamber was maintained at $35 cm.
The effect of source to substrate distance is beyond the
scope of this work and will be pursued in a forthcoming
communication.
These results suggest that the nanorod array substrates
fabricated by oblique-angle thermal deposition have shown
large surface uniformity, high reproducibility, and long-term
stability in terms of nanorod morphology, as well as SERS
signal intensity. These findings also comply with the results
found in earlier investigations.37,45
Thus, these substrates
are highly reliable and analytically sensitive to be used as
SERS substrates with good performances. A further study is
underway to apply these nanorod substrates in trace level
hazardous gas sensing as a part of our future work. This
investigation also proposes that these nanorod array sub-
strates, fabricated by oblique-angle thermal evaporation,
have excellent potential to be used for various SERS
based practical application purposes in large scale.
Figure 9. Contact angle pictures of 2 mL molecular grade water dispensed on the surface of AgNR substrates functionalized using
(a) CTP, (b) MTP, (c) ATP, (d) NTP, (e) TP, and (f) PT showing the wetting behavior.
Oh et al. 11

12. Conclusion
Oblique angle deposited silver nanorod array substrates
with large SERS active surface areas were fabricated using
a cost-effective and environmentally friendly single-step
thermal evaporation technique, which has been shown to
be an excellent SERS substrate. These nanorods were
found to be free from crystallographic and chemical con-
tamination, with an average length of 800 Æ 50 nm and
density 20 Æ 1 rods/mm2
. The effect of contamination on
the nanorod surface was discussed. Various probe mol-
ecules, namely aminothiophenol, nitrothiophenol, carbox-
ythiophenol, thiophenol, methoxythiophenol and
propanethiol, with variable hydrophilic–hydrophobic prop-
erties, have been used to investigate long-term stability,
surface uniformity, and reproducibility of the functionalized
nanorod array films, which also mimics many realistic appli-
cations. In most cases, the functionalization process was
also shown to be successful in substituting the contamin-
ants, if present, on the silver surface. The spot-to-spot
signal intensity variation was found to vary less than RSD
value of 3% for the functionalized substrates showing high
uniformity of nanorod morphology with large SERS active
surface area. The effect of storing conditions on long-term
stability of functionalized substrates was studied. It has been
found that the substrates properly stored in dry, as well as
wet, conditions has shown high SERS signal reproducibility
during the investigated period of six months, as long as the
substrate morphology was not disturbed and the substrates
are protected from extensive contamination and deterior-
ation. This study demonstrated that silver nanorod arrays
fabricated by the oblique-angle thermal evaporation tech-
nique possess a uniformly distributed large SERS active sur-
face area, a highly reproducible morphology and thereby
signal intensity, and are stable over a period of six
months. Thus, we propose that these reliable and reprodu-
cible substrates are suitable for mass production and util-
ization in SERS-based sensor technology for a wide range of
analyte detection often present in ultra-trace level rather
than meagre identification.
Conflict of Interest
None declared.
Funding
The authors gratefully acknowledge support from the grant (grant
number 2009-0077245) through the National Research
Foundation of Korea, funded by the Ministry of Science, ICT and
Future Planning, Korea and the grant of Gwangju Institute of
Science and Technology under the ‘‘Ultrashort Quantum Beam
Facility Program’’ in 2016.
Supplemental Material
All supplemental material mentioned in the text, consisting of two
figures, is available in the online version of the journal, at http://
www.s-a-s.org
References
1. R. Kodiyath, S.T. Malak, Z.A. Combs, T. Koenig, A.M. Mahmoud, A. El-
Sayed, V.V. Tsukruk. ‘‘Assemblies of Silver Nanocubes for Highly
Sensitive SERS Chemical Vapor Detection’’. J. Mater. Chem. A. 2013.
1(8): 2777–2788.
2. S. Nei, S.R. Emory. ‘‘Probing Single Molecules and Single Nanoparticles
by Surface-Enhanced Raman Scattering’’. Science. 1997. 275(5303):
1102–1106.
3. M. Fleischmann, P.J. Hendra, A.J. McQuillan. ‘‘Raman Spectra of
Pyridine Adsorbed at a Silver Electrode’’. Chem. Phys. Lett. 1974.
26(2): 163–166.
4. D.L. Jeanmaire, R.P. Van Duyne. ‘‘Surface Raman
Spectroelectrochemistry. Part I. Heterocyclic, Aromatic, and
Aliphatic Amines Adsorbed on the Anodized Silver Electrode’’.
J. Electroanal. Chem. 1977. 84(1): 1–20.
5. M.G. Albrecht, J.A. Creighton. ‘‘Anomalously Intense Raman Spectra
of Pyridine at a Silver Electrode’’. J. Am. Chem. Soc. 1977. 99(15):
5215–5217.
6. L. Tong, H. Xu, M. Kall. ‘‘Nanogaps for SERS Applications’’. MRS
Bulletin. 2014. 39(02): 163–168.
7. J.L. Abell, J.D. Driskell, Y. Zhao. ‘‘Controllable and Reversible Hot Spot
Formation on Silver Nanorod Arrays’’. Chem. Commun. 2014. 50(1):
106–108.
8. H. Im, K.C. Bantz, N.C. Lindquist, C.L. Haynes, S.-H. Oh. ‘‘Vertically
Oriented Sub-10-nm Plasmonic Nanogap Arrays’’. Nano Lett. 2010.
10(6): 2231–2236.
9. A. Shiohara, Y. Wang, L.M. Liz-Marzan. ‘‘Recent Approaches towards
Creation of Hot Spots for SERS Detection’’. J. Photochem. Photobiol
C. 2014. 21: 2–25.
10. L. Guerrini, D. Graham. ‘‘Molecularly-mediated Assemblies of
Plasmonic Nanoparticles for Surface-Enhanced Raman
Spectroscopy’’. Chem. Soc. Rev. 2012. 41(21): 7085–7107.
11. P. Negri, R.A. Dluhy. ‘‘Ag Nanorod Based Surface-enhanced Raman
Spectroscopy Applied to Bioanalytical Sensing’’. J. Biophotonics.
2013. 6(1): 20–35.
12. R.P. Cooney, M.R. Mahoney, M.W. Howard. ‘‘Auger and Raman
Spectroscopic Studies of Visible Laser Microzones in the
Electrochemical SERS System: Silver/2,20
-Bipyridine’’. Langmuir. 1985.
1(3): 273–277.
13. N. Shirtcliffe, U. Nickel, S. Schneider. ‘‘Reproducible Preparation of
silver Sols with Small Particle Size Using Borohydride Reduction: for
Use as Nuclei for Preparation of Larger Particles’’. J. Colloid Interface
Sci. 1999. 211(1): 122–129.
14. C.J. Orendorff, L. Gearheart, N.R. Jana, C.J. Murphy. ‘‘Aspect Ratio
Dependence on surface Enhanced Raman Scattering Using silver and
Gold Nanorod Substrates’’. Phys. Chem. Chem. Phys. 2006. 8(1): 165–170.
15. F.J. Garcia-Vidal, J.B. Pendry. ‘‘Collective Theory for Surface Enhanced
Raman Scattering’’. Phys. Rev. Lett. 1996. 77(6): 1163–1166.
16. S. Zou, G.C. Schatz. ‘‘Silver Nanoparticle Array Structures that
Produce Giant Enhancements in Electromagnetic Fields’’. Chem.
Phys. Letts. 2005. 403(1–3): 62–67.
17. M.E. Hankus, H. Li, G.J. Gibson, B.M. Cullum. ‘‘Surface-Enhanced
Raman Scattering-Based Nanoprobe for High Resolution, Non-
Scanning Chemical Imaging’’. Anal. Chem. 2006. 78(21): 7535–7546.
18. M.-K. Oh, Y.-S. Shin, C.-L. Lee, R. De, H. Kang, N.E. Yu, B.H. Kim,
J.H. Kim, J.-K. Yang. ‘‘Morphological and SERS Properties of Silver
nanorod Array Films Fabricated by Oblique Thermal Evaporation at
Various Substrate Temperatures’’. Nanoscale Res. Lett. 2015. 10(1): 259.
19. W.J. Herzberg, J.E. Marian. ‘‘Relationship between Contact Angle and
Drop Size’’. J. Colloid. Interface Sci. 1970. 33(1): 161–163.
20. J.A. Sanchez-Gil, J.V. Garcia-Ramos. ‘‘Local and Average
Electromagnetic Enhancement in Surface-enhanced Raman Scattering
from Self-affine Fractal Metal Substrates with Nanoscale
Irregularities’’. Chem. Phys. Lett. 2003. 367(3–4): 361–366.
12 Applied Spectroscopy 0(0)

13. 21. C. Khare, C. Patizig, J.W. Gerlach, B. Rauschenbach. ‘‘Influence of
Substrate Temperature on Glancing angle Deposited Ag Nanorods’’.
J. Vac. Sci. Technol. A. 2010. 28(4): 1002–1009.
22. R. Varshney, S. Bhadauria, M.S. Gaur. ‘‘Biogenic Synthesis of Silver
Nanocubes and Nanorods Using Sundried Stevia rebaudiana Leaves’’.
Adv. Mat. Lett. 2010. 1(3): 232–237.
23. J.B. Khurgin, G. Sun. ‘‘Plasmonic Enhancement of the Third Order
Nonlinear Optical Phenomena: Figures of Merit’’. Opt. Express.
2013. 21(22): 27460–27480.
24. Y.-P. Zhao, S.B. Chaney, Z.Y. Zhang. ‘‘Absorbance Spectra of Aligned
Ag Nanorod Arrays Prepared by Oblique Angle Deposition’’. J. Appl.
Phys. 2006. 100(6): 063527.
25. Y.-J. Liu, H.Y. Chu, Y.-P. Zhao. ‘‘Silver Nanorod Array Substrates
Fabricated by Oblique Angle Deposition: Morphological, Optical, and
SERS Characterization’’. J. Phys. Chem. C. 2010. 114(18): 8176–8183.
26. C.E. Taylor, S.D. Garvey, J.E. Pemberton. ‘‘Carbon Contamination at
silver Surfaces: Surface Preparation Procedures Evaluated by Raman
Spectroscopy and X-ray Photoelectron Spectroscopy’’. Anal. Chem.
1996. 68(14): 2401–2408.
27. W. Lu, C.L. Feldman, Y. Song, S. Dhar, W.E. Collins, W.C. Mitchel,
J.R. Williams. ‘‘Graphitic Features on SiC Surface Following
Oxidation and Etching Using Surface Enhanced Raman
Spectroscopy’’. Appl. Phys. Lett. 2004. 85(16): 3495–3497.
28. D. Lin-Vien, N. Colthup, W. Fately, J. Grasselli. The Handbook of
Infrared and Raman Characteristic Frequencies of Organic
Molecules. New York: Academic Press, 1991.
29. A.W. Adamson, A.P. Gast. ‘‘Physical Chemistry of Surfaces’’.
New York: Wiley-Interscience, 1997. 6th ed.
30. C.E. Taylor, S.D. Garvey, J.E. Pemberton. ‘‘Carbon Contamination at
Silver Surfaces: Surface Preparation Procedures Evaluated by Raman
Spectroscopy and X-ray Photoelectron Spectroscopy’’. Anal. Chem.
1996. 68(14): 2401–2408.
31. R.P. Cooney, M.R. Mahoney, M.W. Howard. ‘‘Intense Raman Spectra of
Surface Carbon and Hydrocarbons on silver Electrodes’’. Chem. Phys.
Lett. 1980. 76(3): 448–452.
32. J.T. Hugall, J.J. Baumberg, S. Mahajan. ‘‘Disentangling the Peak and
Background Signals in Surface-Enhanced Raman Scattering’’. J. Phys.
Chem. C. 2012. 116(10): 6184–6190.
33. P. Negri, N.E. Marotta, L.A. Bottomley, R.A. Dluhy. ‘‘Removal of
Surface Contamination and Self-Assembled Monolayers (SAMs) from
Silver (Ag) Nanorod Substrates by Plasma Cleaning with Argon’’. Appl.
Spectrosc. 2011. 65(1): 66–74.
34. N. Nuntawong, P. Eiamchai, B. Wong-ek, M. Horprathum,
K. Limwichean, V. Patthanasettakul, P. Chindaudom. ‘‘Shelf Time
Effect on SERS Effectiveness of Silver Nanorod Prepared by OAD
Technique’’. Vacuum. 2013. 88(1): 23–27.
35. K.L. Norrod, K.L. Rowlen. ‘‘Removal of Carbonaceous Contamination
from SERS–Active Silver by Self-Assembly of Decanethiol’’. Anal.
Chem. 1998. 70(19): 4218–4221.
36. K.L. Norrod, K.L. Rowlen. ‘‘Ozone-Induced Oxidation of Self-
Assembled Decanethiol: Contributing Mechanism for
‘Photooxidation’’’? J. Am. Chem. Soc. 1998. 120(11): 2656–2657.
37. J.D. Driskell, S. Shanmukh, Y. Liu, S.B. Chaney, X.-J. Tang, Y.P. Zhao,
R.A. Dluhy. ‘‘The Use of Aligned Silver Nanorod Arrays Prepared by
Oblique Angle Deposition as Surface Enhanced Raman Scattering
Substrates’’. J. Phys. Chem. C. 2008. 112(4): 895–901.
38. H.G. Kang, H.W. Lee, Y.J. Kang, M. Hara, J.G. Noh. ‘‘Two-dimensional
Ordering of Benzenethiol Self-assembled Monolayers Guided by
Displacement of Cyclohexanethiols on Au(111)’’. Chem. Commun.
2008. 41: 5197–5199.
39. A. Martin, A. Pescaglini, C. Schopf, V. Scardaci, R. Coull, L. Byrne,
D. Iacopino. ‘‘Surface-Enhanced Raman Scattering of 4-
Aminobenzenethiol on Au Nanorod Ordered Arrays’’. J. Phys.
Chem. C. 2014. 118(24): 13260–13267.
40. T.O. Deschaines, K.T. Carron. ‘‘Stability and Surface Uniformity of
Selected Thiol-coated SERS Surfaces’’. Appl. Spectrosc. 1997. 51(9):
1355–1359.
41. T. Smith. ‘‘The Hydrophilic Nature of a Clean Gold Surface’’. J. Colloid.
Interface Sci. 1980. 75(1): 51–55.
42. J. Bico, C. Marzolin, D. Quere. ‘‘Pearl Drop’’. Europhys. Lett. 1999.
47(2): 220–226.
43. J. Wang, J. Shao, K. Yi, Z. Fan. ‘‘Layer uniformity of Glancing Angle
Deposition’’. Vacuum. 2005. 78(1): 107–111.
44. W.N.W. Mustapha, S.A.R.S.A. Hamid, S.D. Hutagalung, N.V. Hieu,
K. Mohamed, C.K. You. ‘‘Synthesis and Gas Sensing Properties of
SnO2 Nanostructures by Thermal Evaporation’’. Adv. Mater. Res.
2013. 620: 350–355.
45. M.A. De Jesus, K.S. Giesfeldt, J.M. Oran, N.A. Abu-Hatab, N.V. Lavrik,
M.J. Sepaniak. ‘‘Nanofabrication of Densely Packed Metal–Polymer
Arrays for Surface-Enhanced Raman Spectrometry’’. Appl.
Spectrosc. 2005. 59(12): 1501–1508.
Oh et al. 13