gold split-ring resonators (srrs) as substrates for surface-enhanced raman scattering

Gold Split-Ring Resonators (SRRs) as Substrates for Surface-Enhanced Raman ScatteringWeisheng Yue,* Yang Yang, Zhihong Wang, Longqing Chen, and Xianbin Wang

Advanced Nanofabrication Core Lab, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, SaudiArabia

ABSTRACT: We used gold split ring resonators (SRRs) assubstrates for surface-enhanced Raman scattering (SERS). Thearrays of SRRs were fabricated by electron-beam lithography incombination with plasma etching. In the detection ofrhodamine 6G (R6G) molecules, SERS enhancement factorsof the order of 105 was achieved. This SERS enhancementincreased as the size of the split gap decrease as a consequenceof the matching between the resonance wavelength of theSRRs and the excitation wavelength of SERS. As the size of thesplit gap decreased, the localized surface plasmon resonanceshifted to near the excitation wavelength and, thus, resulted in the increase in the electric field on the nanostructures. We usedfinite integration method (FIT) to simulate numerically the electromagnetic properties of the SRRs. The results of the simulationagreed well with our experimental observations. We anticipate this work will provide an approach to manipulate the SERSenhancement by modulating the size of split gap with SRRs without affecting the area and structural arrangement.

1. INTRODUCTIONThe interaction between metallic nanostructures and light canlead to interesting optical properties due to excitation of eitherthe surface plasmon polariton (SPP) or localized surfaceplasmons (LSP). The LSPs result in strong local electro-magnetic (EM) fields that enhance various optical processes.1 Atypical example is surface-enhanced Raman scattering (SERS),where the localized EM enhancement generated by LSPresonance (LSPR) of metallic nanostructures, can enhance theinherently weak Raman process by orders of magnitude,allowing even single-molecule detection.2,3 Importantly, theLSPR greatly depends on the geometrical shape, size, andsurrounding medium of nanostructures.4,5 These properties ofLSPR afford opportunities to manipulate the SERS enhance-ment by designing suitable nanostructures.There is significant interest in designing and optimizing

SERS-oriented substrates. So far, the majority of the SERSsubstrates use metallic nanoparticles prepared chemically.6

Those chemically synthesized metallic nanoparticles especiallycolloidal nanoparticles have been reported to have high SERSenhancement efficiency. Enhancement factors (EFs) for Ramansignals as high as 1010−1011 were achieved by using colloidalmetallic nanoparticles.7,8 However, chemical synthesis lacksufficient control over the size, shape, separation, anddistribution of metallic nanoparticles. The limitations ofchemical synthesis can be overcome by nanofabricationtechnique. By utilizing advanced nanofabrication techniqueslike electron beam lithography and focused ion beamlithography, it is possible to well define geometries andarrangement of nanostructures. A wide range of suchnanostructures, such as nanodisc arrays, nanohole arrays,bowtie nanoantennas, and triangular prisms, can be fabricated

and used for SERS enhancement and other photonicapplications.9−12

Among various photonic structures, split-ring resonators(SRRs) are widely known basic structures of metamaterials thatprovide a negative magnetic response to light.13 The mainadvantage of using SRRs as resonant nanostructures ascompared to nanoparticles or nanodots is the precise controlof the resonance by nanofabrication of precise geometries,spatial arrangements and sizes.14 The tunability of resonancehas made SRRs good candidates for sensing applications.15 Forexample, Sun et al.16 and Lahiri et al.17 demonstrated thepotential applications of SRRs as refractive index sensors. Wealso previously demonstrated tunable properties of SERSenhancement with U-shaped SRR structures by changing thebottom width of the U-shape SRRs.18 On the other hand, theEM field is particularly strong in the gap regions of resonantSRRs. This field enhancement can be used to detect low-concentration molecules that are adsorbed on the surfaces ofSRRs. Cubukcu et al. described the detection of self-assembledmonolayers of 1-octadecanthiol with zeptomole sensitivity byusing circular SRRs.19 Clark et al. used silver SRRs as molecularsensors to detect a self-assembled monolayer of 2-mercapto-pyridine.20 However, there are limited reported works availableon the use of SRRs as SERS substrates.In this work, we report on the use of SRR nanostructures as

substrate for SERS enhancement. Instead of pursuing highSERS enhancement, we demonstrate the tuning of SERSenhancement through the size of split gap. To do so, we

Received: May 2, 2013Revised: September 25, 2013Published: September 26, 2013

Article

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© 2013 American Chemical Society 21908 dx.doi.org/10.1021/jp404332n | J. Phys. Chem. C 2013, 117, 21908−21915

pubs.acs.org/JPCC

fabricated arrays of SRRs with different split-gap sizes usingelectron-beam (e-beam) lithography and subsequent plasmaetching. We then conducted SERS studies on SRR substratesusing R6G as the probe molecules. We observed the influenceof the size of the split gap on the LSPR and hence the SERSenhancement. We performed finite integration technique (FIT)based numerical calculations to understand the SERS enhance-ment mechanism. The numerical results were in goodagreement with our experimental observations.

2. EXPERIMENTAL METHODS

We fabricated the SRR arrays using e-beam lithography incombination with a plasma etching process. The fabricationprocedures are briefly described as follows: (1) A 8 nmtitanium (Ti) layer followed by a 50 nm gold (Au) layer wasdeposited on the silicon substrate using sputtering. The Ti layerworked as the promotion layer to improve adhesion betweenthe Au film and the silicon surface. (2) Negative tone e-beamresist Ma-N2400 (Microresist Technology Gmbh) was spin-coated onto the Au surface. (3) SRR patterns were exposed tothe resist by e-beam writer (CRESTEC 9500C). The exposedresists were developed with MaD525 developer (MicroresistTechnology Gmbh). The Ma-N2400 resist was a negative

resist. The exposed part remained on the substrate andunexposed areas were removed after development. (4) Theresist pattern then worked as a mask for the plasma etching.The plasma etching process was performed with an OxfordPlasmaLab100 etcher. (5) After the etching, the residual Ma-N2400 resist was cleaned with acetone immersion followed byoxygen plasma cleaning. Each pattern was fabricated over anarea of ∼50 × 50 μm2. The detailed fabrication process isdescribed in the literature.21

Rhodamine 6G (R6G, from Sigma-Aldrich) was used as theprobe molecule for SERS detection. R6G has good photo-stability and has been extensively used in the studies of SERS.22

An R6G solution with a concentration as low as 1 × 10−5 mol/L was prepared by dissolving R6G powder into deionized (DI)water. The substrates with SRRs were functionalized with R6Gby immersing the sample into the R6G solution for 3 h. Thesubstrates were rinsed with DI water and dried with a nitrogenstream. Raman spectra were collected in a backscattering modeusing LabRAM HR800 (Horiba Jobin Yvon) micro-Ramanspectrometer. The excitation source was a 532 nm diodepumped solid-state (DPSS) laser. A 50× objective was used tofocus light on the sample surface and collect the scattered lightfrom the surface. The focused beam spot was around 1.5 μm in

Figure 1. SEM images of an array of SRRs with different gap sizes (a−e) and the schematic of a unit cell (f).

Figure 2. SERS spectra on SRR substrates under x-polarized excitation (a) and a bar graph of integrated SERS intensity (peak area) of the band at1649 cm−1 (b).

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp404332n | J. Phys. Chem. C 2013, 117, 21908−2191521909

diameter, the typical laser power was 0.6 mW at the samplesurface, and the acquisition time was 1 s. To achieve betteruniformity, Raman spectra were collected from three differentregions on each patterned area. The final spectra were theaverage spectra obtained from the three regions.

3. RESULTS AND DISCUSSION

3.1. SERS Properties of SRRs. The intent of this study wasto investigate experimentally the effect of the split gap of SRRson SERS intensity. Figure 1 shows scanning electronmicroscopy (SEM) images of the SRRs (a−d) with differentgap sizes. An array of square-rings is shown in Figure 1e. Figure1f presents a schematic of one unit SRR. The periodicity of thearrays (p = 750) are the same for all the patterned arrays. Thelength of the SRR arms was L = 540 nm, the width of the SRRwas a = 540 nm, the width of Au wire was w = 85 nm, and thethickness of the Au was 50 nm. Five different patterns (a−e)were designed. The split gap size, g, of the SRRs varied from320 to 0 nm. Figure 1a−e shows the following patterns: a (g =370 nm), b (g = 230 nm), c (g = 120 nm), and d (g = 50 nm).The square rings have the same periodicity and line width asthose of the SRRs. They can be considered as SRRs with a g = 0gap size. All five patterns were fabricated on the same chip withdifferent dies. The SEM images indicated the good quality anduniformity of the SRRs in the arrays.Both horizontally polarized (x-polarized) and vertically

polarized (y-polarized) sources were used to excite theRaman signals. We first discuss the SERS properties under x-polarized excitation. Figure 2 shows SERS of R6G on SRRs (a−d) and the square rings (e). These spectra were normalized tothe acquisition time and the background was subtracted. Ramanmeasurements were also performed on the blank area outsidethe patterned arrays but no obvious Raman peaks weremeasured. This confirms that the strong Raman signals werethe result of the enhancement of the nanostructures. Fourdominant peaks at 1281, 1360, 1508, and 1649 cm−1 in theSERS spectra were characteristic Raman signals from symmetricmodels of in-plane C−C stretching vibration because of the

resonance enhancement at 532 nm excitation.23,24 Forcomparison, a normal Raman spectrum of solid R6G wasmeasured and is presented in Figure 3. The normal Ramanspectrum was obtained at 532 nm excitation and the fluorescebackground was subtracted. It is notable that the peak intensityat the 1649 cm−1 band is the highest in the SERS spectra. Weused the intensity at the 1649 cm−1 band as a benchmark forcomparing the SERS performance among various samples.Shown in Figure 2b are the integrated SERS intensities at the1649 cm−1 band on the different patterns. The integration wasunder the peak area of the 1649 cm−1 band. The error bars inthe graph originate from intensity variation of the threemeasured points on the same sample. The bar graph shows thatpattern D had the highest SERS intensity, while pattern A hadthe lowest SERS intensity. The difference in SERS intensity waslikely related to the split-gap size, as pattern D had the smallestsplit gap, whereas pattern A had the largest split gap. Inaddition, we compared the SERS intensity of SRRs with splitgaps (A−D) to that of the closed-square rings (pattern E). Wefound that all the SRRs except for pattern A had higher SERSintensity than that of the closed-square rings. This result isreasonable because the gapped nanostructures are able toconcentrate their electric fields and can produce largeenhancement, as has been shown by Au particle dimers andAu bowties.25,26 However, the localized EM fields in the gapshave only slight influenced the SERS enhancement of the SRRarrays compared to the reported particle dimers, as the gapsizes were relatively large to have strong field coupling betweenthe two tips. This was why the SERS intensity increased slightlywith the decrease in the gap size. The total SERS enhancementwas the contribution of the electric field at the gap and on thegold wire. The decrease in the gap size on one handstrengthened the field in the gap, and the field intensity onthe Au wire on the other hand as the LSPR resonanceapproached the excitation wavelength. The change in the totalfield enhancement with the gap size is discussed in more detailin Numerical Simulations. It was worth mentioning that therewas resonant Raman scattering where the excitation corre-sponds to an electronic transition in the molecule, as the

Figure 3. Normal Raman spectrum of solid R6G (the inset shows the molecular structure of R6G).



excitation wavelength of 532 nm was near the R6G absorptionmaximum of 530 nm.27 The strong fluorescence of R6G wasquenched due to nonradioactive interactions with the metalsurface.28 The contribution of the resonance Raman process isbeyond the scope of this study.We also studied the SERS properties of the SRRs under y-

polarized excitation. In this case, the incident electric field wasparallel to the two arms of the SRRs. Figure 4a shows the SERSspectra on the SRRs and Figure 4b is a bar graph of the SERSintensity (integration) at the 1649 cm−1 band. While strongSERS intensity of R6G was achieved on the SRRs under y-polarized excitation, it was only slightly lower than thatmeasured under x-polarized excitation. This observationsuggests that the polarization direction did not change theSERS enhancement of the SRRs significantly. This result wassurprising, because we expected much weaker electric fieldcoupling under y-polarized excitation than under x-polarizedexcitation, as reported in the literature.29,30 When the SRRswere excited under the y-polarized source, the charges were lessacuminated that under x-polarized excitation. Therefore, thelocalized field on the tips of the SRRs under y-polarizedexcitation was weaker than that under x-polarized excitation.This was why the SERS intensity of the SRRs obtained under y-polarized excitation was weaker than that under x-polarizedexcitation. With small gaps, field coupling effects is normallystrong between the tips. Such field coupling will greatlyenhance the field in the gap region greatly. However, in ourwork, the split-gap size was relatively large and so the fieldcoupling between the two tips was weak compared to thereported dimer structures and similar SRR structures.25,26,31

Therefore, the change in the excitation polarization did not

significantly influence field enhancement in the split gap. Theclosed square-rings were geometrically symmetrical in the x-and y-directions and, therefore, the SERS intensities under thetwo polarization directions were at the same intensity level.

3.2. Numerical Simulations. Numerical simulations of thespectra’s responses and the electrical field distribution of thefive structures were performed by employing FIT method withCST Microwave Studio.32 Periodic boundary conditions wereapplied to the lateral sides of the elementary lattice (Figure 1f).The dispersion properties of the Au were treated as the Drudemodel by ε(ω) = ε∞ + ωp

2/ω(ω − iγ). The plasma frequency,ωp = 1.375 × 1016 Hz, and damping frequency, γ = 1.174 × 1014

Hz, were adapted from the literature.33 Details on thesimulation method are presented in our previous work.34 Thesimulated reflection spectra of the A−E patterns are shown inFigure 5.Figure 5a presents the reflection spectra under x-polarized

excitation. There is a strong reflection dip at around 532 nm inthe reflection spectra, which indicates the LSP mode. With thedecrease in the gap size from pattern A−D, the resonance shiftsto lower wavelengths and approaches the excitation wavelengthof 532 nm. This blue-shift of resonance is associated with theLSP interaction between the two tips. The LSP interaction isstrengthened as the gap size decreases and thus the resonancedip shifts to lower wavelengths.35 The closed square-rings(pattern E) have no such LSP coupling and hence theresonance redshifts compared to the SRRs (B−D). The tunableproperty of LSP resonance affords us the opportunity to adjustthe SERS enhancement. When the excitation wavelengthoverlaps with the resonance wavelength, there is a maximumEM field enhancement and thus a maximum SERS enhance-

Figure 4. SERS spectra of SRRs under y-polarized excitation (a) and integrated SERS intensity (peak area) of the band at 1649 cm−1 (b).

Figure 5. Simulated reflectance spectra of the SRRs under x-polarized excitation (a) and y-polarized excitation (b).The gray dashed-line indicates theexcitation wavelength.



ment.2 The resonance wavelength of pattern D was closest tothe excitation wavelength, whereas the resonance wavelength ofpattern A was furthest from the excitation wavelength.Therefore, the highest SERS intensity was observed on patternD due to the best match between the SRR resonance and theexcitation wavelength. As the resonance dips of all the patternswere near the excitation wavelength, the SERS enhancementdoes not vary significantly. A similar dependence of the SERSintensity on LSP resonance was also observed on the arrays ofgold nanowells by Li et al.36

Figure 5b shows the simulated reflection spectra of the SRRsunder y-polarized excitation. The resonance frequencies of thepatterns are similar to those of the x-polarized excitation. Thissuggests that the periodicity dominates the resonance positionof arrays of the SRRs and square-rings. Because the resonancewavelengths of the SRRs under y-polarized excitation weresimilar to those under x-polarized excitation, the SERS intensityof the SRRs under y-polarized excitation was very close to thatunder x-polarized excitation. However, the resonance dipsshown in Figure 5b are not as sharper as those in the Figure 5a.The sharpness of the resonance dip can be expressed by aquality factor (Q-factor), Q = (λmax/Δλ), where λmax is theresonance wavelength and Δλ is the half-intensity dip width.The Q-factors for patterns A−E are listed in Table 1.

Obviously, the Q-factors of the spectra shown in Figure 5bare generally smaller than those shown in Figure 5a. The qualityfactor is related to the excitations of LSPRs and a higher Q-factor is associated with higher excitation efficiency.37 Theinteraction between the gap structure and the incidentelectrometric field was stronger under x-polarized excitationthan that under y-polarized excitation and resulted in sharperresonance dips and therefore higher SERS enhancement. It canbe clearly observed in Figure 5b that the resonance dip shift tolower wavelength with the decrease of split gap size frompatterns A−D. The resonance position of the pattern D isclosest to the excitation wavelength 532 nm, followed bypatterns C, B, E, and A. Correspondingly, the highest SERS

intensity was measured on pattern D, flowed by pattern C, B, E,and A. Thus, we can see a correlation between the SERSenhancement and the LSP resonance position: the closer theresonance dip to the excitation wavelength, the stronger theSERS enhancement. This correlation was in agreement withthat obtained under x-polarized excitation.Figure 6 presents the map of simulated electric field

enhancement |E/E0| on the surface of the SRRs and square-rings. E is local electric field at the surface and E0 is the incidentelectric field. The field was calculated at the excitationwavelength of 532 nm. The arrows in the figure indicate thepolarization directions of the incidence. The electric fielddistributions of the structures under x-polarized excitation areshown in the first row (1a−1e) and the electric fielddistributions of the structures under y-polarized excitation areshown in the second row (2a−2e). In the field distribution(1a−1e) in Figure 6, intense hotspots can be observed on thetwo arms of the SRRs and on the opposing sides of the square-ring that are perpendicular to the direction of polarization.These intense hotspots correspond to the near-field enhance-ment of SERS signals. With the decrease of gap size frompatterns A−D (1a−1d)), the maximum intensity increasedslightly. This increase may be related to the discrepancy in theresonance of the structure to the excitation light. With theapproach of the resonance position to the excitation wave-length, the LSP was more excited and thus the electric field wasmore intense. This observation is in agreement with theexperimental observation of SERS intensity on the SRRs. Bycomparing the electric field distributions on the SRRs andsquaring-ring, we can see that the electric field distributionmodes on the two vertical arms are very similar to on the twovertical sides of the square-ring. Additional field couplingbetween the gaps contributes additional field enhancement overthe square-rings. Therefore, higher SERS enhancement wasobserved on the SRRs than on the square-rings. With thedecrease in the size of the split gap, the electric field couplingbetween the two tips of the split became obvious. This variationtrend of in the field coupling in the gaps is in agreement withthe measured SERS enhancement variation trend on the SRRsfrom pattern A−D. It is worth mentioning that the electric fielddistribution in the SRRs and square-rings expanded on the twoarms rather than being highly localized like that at sharp tips orcorners. This field distribution model is very similar to the near-field electric field distribution models on square-rings proposed

Table 1. Q-Factors of the Resonance Dips in Figure 5

pattern A B C D E

Q-factors x-polarization 49.3 41.7 41.3 41.1 38.8y-polarization 38.7 28.2 26.8 25.5 38.8

Figure 6. Numerically simulated electric field distribution on the surface of SRRs under x-polarized excitation(1a−1e); under y-polarized excitation(2a−2e).



by Valev et al.38 The expanded localized field distribution isinteresting because it may increase the opportunity forinteraction of local charges with molecules.Figure 6 (2a−2e) shows the electric field distribution on the

SRR nanostructures under y-polarized excitation. There is anintense electric field distributed on the bottom of the SRRs(2a−2d) and the two sides in the polarization direction of thesquare-ring (2e). Obviously, the total area of the intense electricfield under y-polarized excitation was less than that under x-polarized excitation. The electric-field contribution comes fromthe intense field on the bottom and at the tips of the splits.Therefore, it is reasonable that the SERS intensity under y-polarized excitation is slightly less than that under x-polarizedexcitation. It is worth mentioning that the electric field underboth x- and y-polarized excitations decay into the surroundingmedium. These parts of the decaying field may also contributeto the SERS enhancement.3.3. SERS Enhancement Factors (EFs). To evaluate the

SERS enhancement ability of the substrates of SRRs andsquare-rings, SERS enhancement factors (EFs) were calculatedon the five patterns. A general equation for the Ramanenhancement factor is EF = (ISERSNbulk)/(IbulkNSERS),

39 whereISERS is the SERS intensity, NSERS is the number of moleculesilluminated by the laser source on the SERS substrates, Ibulk isthe normal Raman intensity of the solid samples, and Nbulk isthe number of molecules in the laser excitation volume in thesolid samples. The reference Raman signal of R6G Ibulk wasobtained by measuring solid R6G and the spectrum ispresented in Figure 3. The Nbulk was estimated by consideringthe laser spot as 1.5 μm in diameter and the laser penetrationdepth in solid R6G as 2 μm. Taking the density of solid R6G(1.25 g/cm3) into account, Nbulk was calculated to be about5.59 × 109.The SERS intensity of the band at 1649 cm−1 was used for

the calculations. The estimated SERS EF was on the order of105 for all the patterns. We compared the EFs achieved for theSRR arrays and square-rings are to the EFs reported in theliteratures. The EFs in this work are of the same order of theEFs obtained on nanostructures like nanopillars and nanoringsfabricated with nanofabrication techniques (104−106),40,41 butthey are lower than the EFs of the nanopaticles synthesizedchemically (108−1011).42,43 Figure 7 shows a plot of measuredEFs of the SRRs and the square-rings for x- and y-polarizedexcitation. The highest EF is 5 × 105 for the SRR with split gap

of 50 nm under x-polarized excitation and with the smallestsplit gap size. The SERS EF under x-polarized excitation ishigher than that under y-polarized excitation. The discrepancyin the EF of the square-rings between the x- and y-polarizedexcitation was attributed to the imperfect repeatability of theRaman measurement system.EM enhancement was calculated based on the electric field

distributions shown in Figure 7. It is known that SERSenhancement is related to near-field electric field enhancementby the following formula:44,45

∝ EE

EF0

4

(1)

where E is the local electric field at the SRR surface and E0 isincident electric field. Because the molecules are adsorbed onthe whole surface (S) of SRRs, the value of the electric field isthe average of the electric field over the total surface of SRRs.

∫ =ES

E x y x y1

( , )d d(2)

The value of E is less than the maximum surface field but ismore in line with actual measurements. Therefore, the SERSenhancement factor can be rewritten as

∝ EE

EF0

4

(3)

The EM enhancement factor |E/E0|4 is plotted in Figure 7. For

the purpose of comparison, the simulated EFs were multipliedby 50 such that they are at the similar height as the measuredEFs in the plot. The variation trend of the simulated EFs is inline with the measured EFs. The simulated EM EFs vary moresignificantly with gap size from pattern A to E than do theexperimental results due to the perfect condition in thesimulation. Indeed, the simulated EM EFs were ∼50 timessmaller than the measured EFs. There are three possiblereasons leading to the discrepancy in the EF between thecalculated EM EFs and the experimental EFs. First, themeasured SERS EFs include the effects of both chemical andEM enhancements and the measured EFs were higher than theEM EFs. Second, only the EM fields on the top surface of thenanostructures were calculated. The EM fields that evan-escently decay into the surrounding medium make contribu-tions to SERS enhancement. Such contributions were notconsidered in the calculations. Third, the surface roughness ofthe nanostructures can lead to higher local EM enhancementand thus SERS enhancement. This part of contribution werenot included in the calculation.46

4. CONCLUSIONSIn conclusion, we have investigated SERS responses on arraysof SRRs with various split-gap sizes. The SERS enhancementexhibits a tunable dependence on the gap size where a smallergap results in an increase in SERS enhancement. With adecrease in the size of the split-gap, the resonance shift close tothe wavelength of the excitation light and thus the SERSenhancement increases. The SRRs (patterns B, C, and D)exhibited higher SERS enhancement than did the square-ringsdue to localized electric fields on the tips of the splits. Thiswork demonstrated a promising approach to manipulating theSERS enhancement by modulating the split gap size of SRRs

Figure 7.Measured (m) SERS EFs and simulated (s) electric field EFsof the patterns under x-polarized (X-polar) and y-polarized (Y-polar)excitations, respectively. The electric field EFs were multiplied by 50 toenable comparison with the experimental data.



without affecting the area and structural arrangement of thepatterns.

■ AUTHOR INFORMATIONCorresponding Author*Tel.: +966 12 8082549. E-mail: [email protected] authors declare no competing financial interest.

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gold split-ring resonators (srrs) as substrates for surface-enhanced raman scattering

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