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Excitation wavelength dependent surface enhanced Raman scattering of4-aminothiophenol on gold nanorings

Jian Ye,*ab James Andell Hutchison,b Hiroshi Uji-i,b Johan Hofkens,b Liesbet Lagae,a Guido Maes,b

Gustaaf Borghsa and Pol Van Dorpea

Received 20th November 2011, Accepted 20th December 2011

DOI: 10.1039/c2nr11805j

Detailed understanding of the underlying mechanisms of surface enhanced Raman scattering (SERS)

remains challenging for different experimental conditions. We report on an excitation wavelength

dependent SERS of 4-aminothiophenol molecules on gold nanorings. SERS and normal Raman

spectra, combined with well-characterized surface morphology, optical spectroscopy and

electromagnetic (EM) field simulations of gold nanoring substrates indicate that the EM enhancement

occurs at all three excitation wavelengths (532, 633 and 785 nm) employed but at short wavelengths

(532 and 633 nm) charge transfer (CT) results in additional strong enhancements of particular Raman

transitions. These results pave the way to further understanding the origin of the SERS mechanism.

Introduction

Raman scattering is typically very weak due to the small scat-

tering cross-section of molecules. However, the surface enhanced

Raman scattering (SERS) effect can result in the enhancement of

Raman scattering by molecules adsorbed on rough metal

surfaces. The enhancement factor (EF) can reach a level that

enables single molecule detection.1 The mechanisms behind

SERS still remain a matter of controversy, but two mechanisms

are extensively mentioned in the literature. In the electromag-

netic (EM) mechanism, the excitation of localized surface plas-

mons results in strongly enhanced local electric fields around

the metal nanostructure, leading to a more intense Raman

scattering of molecules near the metal surface.2–4 In the EM

mechanism, the EF of each molecule is approximately given by

EF ¼ |EFex|2|EFscat|

2, where EFex is the local electric field (EF) at

the excitation wavelength and EFscat is the corresponding EF at

the Stokes-shifted Raman scattering wavelength. More often,

this expression is simplified by assuming that EFex and EFscat are

the same, and hence EF ¼ |EFex|4. The second popular mecha-

nism, called charge transfer (CT), involves the excitation of CT

between analyte molecules and metal structures to give rise to

a resonance Raman enhancement process.5,6

A SERS substrate can be any plasmon-resonance-supporting

nanostructure, for example, nanoshells,7 nanocubes,8 nano-

triangles,9 nanostars,10 nanobowls,11 nanosphere dimers12 and

aggregates.13 Recently, plasmonic nanoring structures have also

been proposed and demonstrated for SERS application,14–16

because of their highly tunable optical properties over a broad

aimec vzw, Kapeldreef 75, Leuven, Belgium. E-mail: Jian.Ye@imec.be;Fax: +32 1628 1097; Tel: +32 1628 8795bChemistry Department, Katholieke Universiteit Leuven, Leuven, Belgium

1606 | Nanoscale, 2012, 4, 1606–1611

spectral range and their efficient concentration of EM fields.14 A

particular advantage of the nanorings is that they can be fabri-

cated by the nanosphere lithography technique,14,16 which is

a simple, productive, scalable and cheap method.

Because the SERS effect mainly arises from the plasmon

resonances of the nanostructures, it is typically wavelength

dependent. A particular SERS substrate will therefore exhibit

good enhancement in a limited excitation wavelength range. For

example, a detailed excitation wavelength-scanned SERS spec-

troscopic study of benzenethiol adsorbed on silver (Ag) nano-

particle arrays, fabricated by nanosphere lithography, has been

carried out by McFarland et al.17 The SERS spectra were

correlated with the corresponding localized surface plasmon

resonance (LSPR) spectra of the nanoparticle arrays. The

maximum SERS enhancement factor (EFmax) was shown to

occur for excitation wavelengths which are slightly blue-shifted

with respect to the LSPR wavelength of nanoparticle arrays. This

is in agreement with the predictions of the EM enhancement

mechanism of SERS. Additional work by Etchegoin’s group18

and Van Duyne’s group19,20 has made it extensively accepted that

the EFmax occurs at excitation wavelengths quite close to the

spectral locations of the LSPR extinction maximum for indi-

vidual non-interacting nanostructures. However, the extinction

spectrum is not a good indicator of the maximum SERS

enhancement in the case of complex nanostructures, e.g. dimers,

quadrimers and non-ordered aggregates.18 If other effects, such

as CT or chemical transformation, are involved, the SERS

mechanism becomes even more complicated. The full under-

standing of the SERS mechanism remains a challenge.

4-Aminothiophenol (4-ATP), a Raman active probe, has been

widely used in SERS because of its strong chemical affinity to Au

and Ag and the large SERS signal.21–28 Another reason of 4-ATP

becoming increasingly important is that its b2-type Raman

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bands, for example, at 1140, 1384 and 1432 cm�1 have been

considered as crucial evidence of the CT effect in SERS.21–25

However, recent reports have reinterpreted that these Raman

bands are from 4,40-dimercaptoazobenzene which is formed

by photo-induced chemical transformation or plasmon-assisted

(or ‘‘hot electrons’’) catalytic reaction of 4-ATP.26,27 Addition-

ally, one has come up with a new observation for the appearance

of these bands that a threshold energy is required.28 In this paper,

we examine the SERS properties of 4-ATP and 4-methoxy-

thiophenol (4-MOTP) with laser excitation at 532, 633 and

785 nm on hollow Au nanorings. Au nanorings are fabricated by

the nanosphere lithography technique. Their uniform, high-

density patterning and accessibility of the surface morphology to

the surrounding media are favorable for SERS measurements.

With a similar molecular structure, 4-ATP and 4-MOTP show

distinct SERS spectra at different excitation wavelengths in

terms of the relative intensity of the Raman bands. In particular,

the intensity of b2-type Raman bands of 4-ATP is relatively

enhanced at 532 and 633 nm. A full understanding of Au

nanorings’ morphology, optical properties and local electric field

properties may help us assess the different origins of the SERS

of 4-ATP.

Experimental

The Au nanorings were fabricated according to a slightly

modified nanosphere lithography method (Fig. 1).14,29 Prior to

use, the quartz substrate was cleaned for 5 min by subsequently

submerging it in acetone (45� 2 �C) and 2-propanol (60� 2 �C),respectively, then rinsed well with deionized H2O, and dried in

a nitrogen stream. Next, the substrate surface was functionalized

by pipetting a 1 mL aqueous solution containing 0.2% poly-

diallyldimethyl-ammonium (PDDA, Mw 100 000–200 000,

Sigma Aldrich) onto the substrate, followed by careful rinsing

with deionized H2O in order to remove all excess of PDDA and

drying in a nitrogen stream. Deposition of an aqueous suspen-

sion containing 100 nm size polystyrene (PS) beads (0.2% solid,

Interfacial Dynamics Corp.) could result in the substrate surface

covered by a submonolayer of uniformly distributed PS beads as

sacrificial masks. A 30 nm thick Au film was deposited subse-

quently on top of the PS beads by sputtering, followed by a Xe

ion milling process for 1 min.30 Finally, a 30 min oxygen plasma

treatment was used to remove the remaining PS beads.

Fig. 1 Schematic illustration of the fabrication procedure for hollow Au

nanorings.

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The extinction spectrum was measured using a Shimadzu UV-

1601PC spectrophotometer. The SEM image was taken using

a Philips XL30 FEG instrument. The AFM image was acquired

in a tapping mode on a Dimension 3100/Nanoscope IV, VEECO

under ambient conditions. All Raman spectra were recorded on

a Horiba Jobin Yvon LabRam HR 800 system in a backscat-

tering configuration using a laser illumination of 532 (0.98 mW),

633 (0.97 mW), and 785 nm (0.8 mW). A 100� objective (NA ¼0.9) mounted on an Olympus BXFM-ILHSmicroscope was used

to focus the laser. The Raman band of a silicon wafer at 520 cm�1

was used to calibrate the spectrometer. The spectral resolution of

the system was 1.5 cm�1. In order to chemisorb a monolayer of

molecules on Au nanorings, the nanoring substrates were cleaned

by UV ozone and were soaked in a fresh ethanol solution of

4-ATP (0.3 mM, Acros Organics) or 4-MOTP (1.7 mM, Acros

Organics) overnight, rinsed thoroughly with ethanol, and dried

in a nitrogen stream before the Raman measurement. All Raman

spectra were plotted and normalized to [0, 1] for comparison

except for the EF calculation.

Simulation

Numerical simulations were performed by employing a finite

difference time domain (FDTD) method (Lumerical Solutions,

Inc.). The Au nanoring is illuminated by normal incident light

with its polarization along the X-direction (Fig. 1). The whole

simulation region is assumed in air. We have used the Lumer-

ical’s multi-coefficient model (MCM) to fit the empirical Au

dielectric function from CRC.31,32

Results and discussion

Fig. 1 outlines the procedure to fabricate Au nanorings. During

the deposition of PS particles suspension, the electrostatic

Fig. 2 (A) SEM image and (B) experimental extinction spectrum of Au

nanorings. The inset in (B) is the AFM topographic 3D image of Au

nanorings. Vertical lines in (B) indicate the excitation laser wavelengths at

(red) 532, (green) 633 and (blue) 785 nm.

Nanoscale, 2012, 4, 1606–1611 | 1607

Fig. 3 (A) The FDTD calculated local electric field enhancement (log |EF|2) of a Au nanoring at the XY plane and (B and D) normalized SERS and (C

and E) normal Raman (NR) spectra of (B and C) 4-ATP and (D and E) 4-MOTPmolecules at different excitation wavelengths of (top) 532, (middle) 633

and (bottom) 785 nm. Insets in (B)–(E) are the corresponding structures of molecules bound on the Au surface or bulk molecules.

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repulsive force between PS particles and the attractive force

between PDDAmolecules and PS particles allow the PS particles

to form a submonolayer on the substrate as a sacrificial mask.

This is a crucial step to make large-area non-aggregated nanor-

ing structures. Fig. 2A is a low magnification top-view SEM

image, which shows the obtained Au nanorings over a relatively

large area. The nanorings are distributed randomly but are iso-

lated with a similar inter-particle distance of 200–300 nm and

a density of�11 nanorings per mm2. The AFMmeasurement was

used to investigate the surface morphology of Au nanorings in

further detail. From the AFM topographic 3D image in the inset

of Fig. 2B, we can clearly see ideally separated Au nanorings with

an open and sharp edge on the substrate, which is consistent with

the observations from the SEM image. From the AFM

measurement, the height of the Au nanorings was estimated to be

�65 nm and the total diameter �135 nm.

A thorough investigation of the optical properties of Au

nanorings is crucial to understand their SERS properties. The

experimental extinction spectrum of Au nanorings on quartz in

Fig. 2B shows a strong plasmon band at 800 nm, which can be

attributed to a dipolar resonance mode.14,29 In our SERS

experiments, we use laser excitation at 532, 633 and 785 nm (see

solid lines indicated in Fig. 2B). It is clear from Fig. 2B that only

785 nm is an on-resonance excitation. As there is no complex

particle geometry leading to (dark) multipolar plasmon excita-

tions, and as there is no strong inter-particle coupling between

the Au nanorings in our sample, the electric field enhancement

should have a similar spectral dependence as the extinction

spectrum, indicating that the electric field enhancement will be

maximal for the 785 nm excitation. This has been confirmed by

the simulated local electric fields of a Au nanoring at the XY

1608 | Nanoscale, 2012, 4, 1606–1611

plane at 532, 633 and 785 nm in Fig. 3A. A strong electric field

enhancement is present on the top of the nanoring for 785 nm

excitation and reaches an enhancement factor in the order of 103

to 104. Furthermore, the electric field profile corroborates the

dipolar character of the plasmon resonance. The field intensity

enhancement decreases to 102 to 103 at 633 nm, and further to 101

to 102 at 532 nm.

To examine the SERS behavior of 4-ATP, we fulfilled SERS

and bulk normal Raman (NR) measurements of 4-ATP on Au

nanorings at three excitation wavelengths of 532, 633 and 785 nm

(in Fig. 3B and C). Fig. 3B clearly highlights the remarkable

differences between the SERS spectra of 4-ATP in terms of the

relative intensity of the Raman bands at different excitation

wavelengths. Two strong Raman bands at 1078 and 1589 cm�1

exist under 785 nm excitation laser, which correspond to the

vibration modes of nCS(a1) and nCC(a1).21 At a shorter excita-

tion laser wavelength of 633 or 532 nm, multiple Raman bands

between 1078 and 1589 cm�1 are strongly enhanced. In partic-

ular, three Raman bands at 1140, 1384 and 1432 cm�1 display an

apparent enhancement relative to the 1078 cm�1 band. Previous

work has extensively assigned these three Raman bands at 1140,

1384 and 1432 cm�1 to the b2 modes of 4-ATP.21–25,28,33–35 One

should mention that 4-ATP shows satisfactorily similar NR

spectra at all excitation wavelengths, which is rather consistent

with the spectrum at 785 nm excitation (Fig. 3C). This specific

Raman band enhancement effect of 4-ATP has also been

observed for other plasmonic nanostructures.23

Using the calculationmethod,35EF¼ (ISERS/Nads)/(Ibulk/Nbulk)

(where ISERS and Ibulk are the intensity of a vibrationalmode in the

SERS spectrum and bulk sample, and Nads and Nbulk are the

number of molecules adsorbed on the gold nanorings and bulk

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Table 1 FDTD calculated and experimental SERS enhancement factor (EF) of gold nanorings and SERS I1078/I1432 for 4-ATP and 4-MOTPmoleculeson gold nanorings at different excitation wavelengths

Excitation wavelength/nm

Experimental EF FDTD EF SERS I1078/I1432c

EF1a (4-ATP) EF2

b (4-ATP) EF1a (4-MOTP) EF1

a EF2b 4-ATP

532 3.0 � 102 5.0 � 105 8.3 � 102 3.8 � 104 5.2 � 104 0.29633 1.0 � 105 1.0 � 107 1.1 � 103 1.1 � 105 1.3 � 105 0.76785 2.6 � 105 1.5 � 105 4.4 � 104 5.5 � 106 3.3 � 106 8.00

a EF1 was experimentally or theoretically calculated by the Raman band at 1078 cm�1. b EF2 was experimentally or theoretically calculated by theRaman band at 1432 cm�1. c Raman intensity ratio between the Raman band at 1078 and 1432 cm�1.

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molecules exposed to the laser focal area, respectively), we

calculated two experimental EFs of 4-ATP molecules (see Table

1), termed EF1 and EF2, by using the Raman bands of 1078 and

1432 cm�1, respectively. This is because the Raman band at

1078 cm�1 has the highest intensity in the range of 950–1650 cm�1

in the NR spectrum and the SERS spectrum at 785 nm and the

Raman band at 1432 cm�1 has the highest intensity in the SERS

spectra at 633 and 532 nm. The laser spot size was calculated to be

0.72mmfor 532nm, 0.858mmfor 633nmand1.064mmfor 785nm.

The experimental EF1 of 4-ATP was calculated to be 3.0� 102 at

532 nm, 1.0 � 105 at 633 nm and 2.6 � 105 at 785 nm excitation,

respectively. Alternatively, the experimental EF2 of 4-ATP

was calculated to be 5.0� 105 at 532 nm, 1.0� 107 at 633 nm and

1.5� 105 at 785 nm excitation, respectively. We additionally used

4-MOTP as a Raman probe since it possesses a similar molecular

structure to 4-ATP (insets in Fig. 3D and E). For comparison, the

SERS and bulkNR spectra of 4-MOTPon theAunanorings were

measured as well at 532, 633 and 785 nm (Fig. 3D and E), and the

corresponding experimental EF1 was calculated to be 8.3 � 102,

1.1 � 103 and 4.4 � 104, respectively. To clearly analyze the

Fig. 4 Combination of (black) the extinction spectrum of Au nanorings a

wavelengths of (red) 532, (green) 633 and (blue) 785 nm. Vertical solid lines ind

and vertical dashed lines indicate the wavelength positions of the scattering

excitation wavelengths, respectively. All corresponding FDTD calculated elect

scattering wavelengths are shown on the top of the panel.

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different origins of the SERS EFs, the FDTD simulation method

was applied to calculate the electric field (EF), which contributes

to the EM mechanism. The calculation was followed by

EF ¼ |EFex|2|EFscat|

2 in order to improve the accuracy since the

Raman scattering wavelength and the excitation wavelength can

be separated by over 100 nm for the near-infrared (NIR) SERS.

We have combined the extinction spectrum of Au nanorings and

the SERS spectra of 4-ATPmolecules as a function of wavelength

in Fig. 4. From Fig. 4, the wavelength positions of the scattering

Raman bands at 1078 and 1432 cm�1 were identified to be 564 (ii)

and 576 nm (iii) for 532 nm excitation (i), 679 (v) and 696 nm (vi)

for 633 nmexcitation (iv), and 858 (viii) and 884nm(ix) for 785 nm

excitation (vii) (see vertical solid and dashed lines in Fig. 4). The

corresponding electric field enhancement at the XZ plane at both

the excitation (|EFex|2) and scattering wavelengths (|EFscat|

2) is

calculated and shown on the top of Fig. 4 (i–ix). Therefore,

the FDTD EF1 (based on the Raman band at 1078 cm�1) and

EF2 (1432 cm�1) were calculated to be 3.8 � 104 and 5.2 � 104

at 532 nm, 1.1 � 105 and 1.3 � 105 at 633 nm, and 5.5 � 106 and

3.3 � 106 at 785 nm, respectively (see Table 1).

nd normalized SERS spectra of 4-ATP molecules excited by different

icate the excitation laser wavelengths at (i) 532, (iv) 633 and (vii) 785 nm,

Raman bands of (ii, v, viii) 1078 and (iii, vi, ix) 1432 cm�1 for the three

ric field enhancement profiles at the XZ plane of the ring at excitation and

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It is apparent that the SERS EF (experimental EF1) of

4-MOTP molecules on Au nanorings at 532, 633 and 785 nm

excitation qualitatively follows the theoretical prediction by the

FDTD EF1, i.e. a smaller SERS experimental EF1 at a shorter

excitation wavelength with one order of magnitude in between. It

is quite common that the experimental EF1 is 101 to 102 smaller

than the numerically calculated FDTD EF1, because the

maximum FDTD EF1 typically occurs at specific positions (e.g.,

hot spots) on the surface of the structures and only those analytes

immobilized there can profit from this large amplification.36 By

contrast, the experimental EF1 is averaged over all exposed

surfaces on the structure. The reasonable agreement between the

theoretical and experimental wavelength dependence of the EF

indicates that the SERS of 4-MOTP molecules on Au nanorings

is mainly dominated by the EM effect. However, the situation for

4-ATP is more complicated. At 633 nm, 4-ATP molecules exhibit

SERS experimental EF1 and EF2 of 1.0 � 105 and 1.0 � 107,

respectively. For EF1, there is a mere factor of the difference

between 633 and 785 nm (compared to 40 for 4-MOTP), while

for EF2, the experimental value at 633 nm greatly exceeds the one

at 785 nm (1.0 � 107 compared to 1.5 � 105), despite the FDTD

EF1 being significantly lower with 633 nm excitation. Therefore,

it is necessary to consider other effects, such as the CT effect, to

understand the SERS EF of 4-ATP molecules on Au nanorings.

We use the ratio between the Raman band at 1078 and

1432 cm�1 (I1078/I1432) in order to perform a further comparison

for all SERS spectra of 4-ATP at different wavelengths. Table 1

shows that when the excitation wavelength shifts from 785 nm to

633 nm, I1078/I1432 correspondingly decreases from 8.00 to 0.76

and the experimental EF2 increases by around two orders of

magnitude from 1.5 � 105 to 1.0 � 107. In contrast, the FDTD

EF2 shows a decrease from 3.3� 106 to 1.3� 105. It suggests that

an extra non-EM enhancement effect occurs at 633 nm for

4-ATP. This enhancement becomes even more pronounced

(I1078/I1432 ¼ 0.29) when the excitation wavelength shifts to

532 nm. In comparison, Fig. 3D and E show that all SERS

and NR spectra for 4-MOTP are similar regarding the position

and relative intensity of the Raman bands at different

wavelengths.

It can be seen from Fig. 4 that the electric field enhancement

(i–ix) has a similar wavelength dependency as the extinction

spectrum of Au nanorings. The overall profile of the SERS

spectrum at each wavelength range is very similar to the shape

of the extinction spectrum in the same range. All these would

suggest that the EM effect may play a role in the overall shape

conversion of the SERS spectrum of the 4-ATP molecules at

different excitation wavelengths.37 The comparison of the

FDTD EF1 and EF2 (see Table 1) shows that the EM effect is

stronger for the 1432 cm�1 band than the one for the 1078 cm�1

band at 532 nm and becomes weaker at 785 nm, which further

demonstrates the contribution of the EM effect to the shape

conversion of the SERS spectrum. It can also be noticed in

Fig. 4 that for the molecular vibrations under study, the

difference in wavelength between the excitation and scattering

wavelengths amounts up to 100 nm for 785 nm excitation. The

corresponding electric field enhancement at the excitation (vii in

Fig. 4) and the scattering wavelength (ix in Fig. 4) exhibit

a much larger difference as well. This indicates that the

approximation often used for the EM effect, EF ¼ |EFex|4, may

1610 | Nanoscale, 2012, 4, 1606–1611

be not suitable for the excitation at the NIR region, for

example, for Au nanorings.

There has been a number of articles about exploring the SERS

mechanisms of 4-ATP molecules on metallic nanostructures

including Ag/Au nanoparticles,22,23 Ag/Au electrodes,34 core–

shell nanoparticles,33 sandwich structures24,25,35 and nanorods.38

Most of the authors of those articles consider that both EM and

CT mechanisms may contribute to the SERS effect of 4-ATP

molecules to some extent, but their relative contribution is quite

sensitive to the experimental conditions, the composition, the

morphology of the metallic nanostructures and so on. Recent

reinterpretations of the b2-type modes have given more possible

explanation of the SERS mechanism of 4-ATP.26–28,37,39

To obtain more insights into the wavelength dependent SERS of

4-ATP in this work, we performed additional laser power and

wavelength variable Raman measurements. In the former

experiment, we have found that the relative amplitude of the b2-

type mode of 4-ATP (compared to the band at 1078 cm�1) does

not depend on the laser power in the range of 30 mW to 4 mW for

all three excitation wavelengths of 532, 633 and 785 nm. This

indicates that the threshold energy for the b2-type mode of

4-ATP may be required only under certain conditions. In the

second experiment, we switched the laser wavelength in

a sequence of 785, 633, 532, and 785 nm (with a roughly constant

power of 1 mW) at the same spot on the substrate. We have

found that the b2-type modes of 4-ATP only appear when excited

by 633 and 532 nm laser and are both absent when excited by 785

nm laser for the first and second time. This spectrum reversible

behavior conflicts with the explanation of photo-induced or

plasmon-assisted chemical transformation where the chemical

reaction is not reversible only by changing the laser wavelength

particularly under a dried-sample condition. It seems that in this

work the CT is a more reasonable explanation for the effect that

the b2-type modes of 4-ATP are enhanced with 633 and 532 nm

excitation.

In the general CT mechanism, a direct resonant Raman scat-

tering can be excited when the laser energy matches the energy

gap between the LUMO andHOMO of molecules. Alternatively,

an indirect coupling can be realized by CT through the metal,

appearing as a ‘resonance Raman-like’ process. In this work, the

absorption spectra of 4-ATP and 4-MOTP molecules with

absorption bands shorter than 400 nm ensure that no resonance

Raman contribution is involved for all SERS enhancements

when visible and NIR excitations are employed. The excitation

wavelength dependent SERS results of 4-ATP confirm the CT

mechanism where we could only observe a strong CT effect when

we applied higher energy laser excitation. The same CT obser-

vations for 4-ATP molecules have also been reported by Baia

et al.23 and Kim et al.;24 4-ATP molecules feature electron-

donating (amino) and electron-accepting (thiol) groups con-

nected by a conjugated p system. When they are adsorbed on the

Au surface through Au–S bonds, two resonance structures exist:

benzenoid and quinonoid forms.25 The quinonoid form of the

adsorbed 4-ATP plays an important role in the CT process in this

complex system, because the dipolar direction of the 4-ATP

molecules is parallel to the CT direction from the molecule to the

metal, which may be helpful for the overall CT process. This also

explains why the CT effect disappears completely if slightly

different molecules (e.g., 4-MOTP) are employed.

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Conclusions

We have demonstrated the fabrication of Au nanorings by

a nanosphere lithography technique as an efficient SERS

substrate. Two types of similar structural molecules, 4-ATP and

4-MOTP, as Raman probes have been examined on Au nanor-

ings by the excitation lasers of 532, 633 and 785 nm, respectively.

An excitation wavelength dependent SERS effect was clearly

observed only for 4-ATP. Well-characterized structural

morphology, optical spectra and electric field distribution of Au

nanorings, combined with SERS and NR spectra, indicate that

the EM effect mainly contributes to the SERS mechanism for 4-

MOTP at all wavelengths and for 4-ATP at 785 nm but the CT

effect is dramatically increased at shorter excitation wavelengths

of 633 and 532 nm for 4-ATP.

Acknowledgements

J.Y. and P.V.D. acknowledge financial support from the FWO of

Flanders.

Notes and references

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