Gold Split-Ring Resonators (SRRs) as Substrates for Surface

Article pubs.acs.org/JPCC

Gold Split-Ring Resonators (SRRs) as Substrates for SurfaceEnhanced Raman Scattering Weisheng 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, Saudi Arabia ABSTRACT: We used gold split ring resonators (SRRs) as substrates for surface-enhanced Raman scattering (SERS). The arrays of SRRs were fabricated by electron-beam lithography in combination with plasma etching. In the detection of rhodamine 6G (R6G) molecules, SERS enhancement factors of the order of 105 was achieved. This SERS enhancement increased as the size of the split gap decrease as a consequence of the matching between the resonance wavelength of the SRRs and the excitation wavelength of SERS. As the size of the split gap decreased, the localized surface plasmon resonance shifted to near the excitation wavelength and, thus, resulted in the increase in the electric field on the nanostructures. We used finite integration method (FIT) to simulate numerically the electromagnetic properties of the SRRs. The results of the simulation agreed well with our experimental observations. We anticipate this work will provide an approach to manipulate the SERS enhancement by modulating the size of split gap with SRRs without affecting the area and structural arrangement.

1. INTRODUCTION The interaction between metallic nanostructures and light can lead to interesting optical properties due to excitation of either the surface plasmon polariton (SPP) or localized surface plasmons (LSP). The LSPs result in strong local electromagnetic (EM) fields that enhance various optical processes.1 A typical example is surface-enhanced Raman scattering (SERS), where the localized EM enhancement generated by LSP resonance (LSPR) of metallic nanostructures, can enhance the inherently weak Raman process by orders of magnitude, allowing even single-molecule detection.2,3 Importantly, the LSPR greatly depends on the geometrical shape, size, and surrounding medium of nanostructures.4,5 These properties of LSPR afford opportunities to manipulate the SERS enhancement by designing suitable nanostructures. There is significant interest in designing and optimizing SERS-oriented substrates. So far, the majority of the SERS substrates use metallic nanoparticles prepared chemically.6 Those chemically synthesized metallic nanoparticles especially colloidal nanoparticles have been reported to have high SERS enhancement efficiency. Enhancement factors (EFs) for Raman signals as high as 1010−1011 were achieved by using colloidal metallic nanoparticles.7,8 However, chemical synthesis lack sufficient control over the size, shape, separation, and distribution of metallic nanoparticles. The limitations of chemical synthesis can be overcome by nanofabrication technique. By utilizing advanced nanofabrication techniques like electron beam lithography and focused ion beam lithography, it is possible to well define geometries and arrangement of nanostructures. A wide range of such nanostructures, such as nanodisc arrays, nanohole arrays, bowtie nanoantennas, and triangular prisms, can be fabricated © 2013 American Chemical Society

and used for SERS enhancement and other photonic applications.9−12 Among various photonic structures, split-ring resonators (SRRs) are widely known basic structures of metamaterials that provide a negative magnetic response to light.13 The main advantage of using SRRs as resonant nanostructures as compared to nanoparticles or nanodots is the precise control of the resonance by nanofabrication of precise geometries, spatial arrangements and sizes.14 The tunability of resonance has made SRRs good candidates for sensing applications.15 For example, Sun et al.16 and Lahiri et al.17 demonstrated the potential applications of SRRs as refractive index sensors. We also previously demonstrated tunable properties of SERS enhancement with U-shaped SRR structures by changing the bottom width of the U-shape SRRs.18 On the other hand, the EM field is particularly strong in the gap regions of resonant SRRs. This field enhancement can be used to detect lowconcentration molecules that are adsorbed on the surfaces of SRRs. Cubukcu et al. described the detection of self-assembled monolayers of 1-octadecanthiol with zeptomole sensitivity by using circular SRRs.19 Clark et al. used silver SRRs as molecular sensors to detect a self-assembled monolayer of 2-mercaptopyridine.20 However, there are limited reported works available on 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 high SERS enhancement, we demonstrate the tuning of SERS enhancement through the size of split gap. To do so, we Received: May 2, 2013 Revised: September 25, 2013 Published: September 26, 2013 21908

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

The Journal of Physical Chemistry C

Article

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 at 1649 cm−1 (b).

resist. The exposed part remained on the substrate and unexposed areas were removed after development. (4) The resist pattern then worked as a mask for the plasma etching. The plasma etching process was performed with an Oxford PlasmaLab100 etcher. (5) After the etching, the residual MaN2400 resist was cleaned with acetone immersion followed by oxygen plasma cleaning. Each pattern was fabricated over an area of ∼50 × 50 μm2. The detailed fabrication process is described in the literature.21 Rhodamine 6G (R6G, from Sigma-Aldrich) was used as the probe molecule for SERS detection. R6G has good photostability 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 R6G by immersing the sample into the R6G solution for 3 h. The substrates were rinsed with DI water and dried with a nitrogen stream. Raman spectra were collected in a backscattering mode using LabRAM HR800 (Horiba Jobin Yvon) micro-Raman spectrometer. The excitation source was a 532 nm diode pumped solid-state (DPSS) laser. A 50× objective was used to focus light on the sample surface and collect the scattered light from the surface. The focused beam spot was around 1.5 μm in

fabricated arrays of SRRs with different split-gap sizes using electron-beam (e-beam) lithography and subsequent plasma etching. We then conducted SERS studies on SRR substrates using R6G as the probe molecules. We observed the influence of the size of the split gap on the LSPR and hence the SERS enhancement. We performed finite integration technique (FIT) based numerical calculations to understand the SERS enhancement mechanism. The numerical results were in good agreement with our experimental observations.

2. EXPERIMENTAL METHODS We fabricated the SRR arrays using e-beam lithography in combination with a plasma etching process. The fabrication procedures are briefly described as follows: (1) A 8 nm titanium (Ti) layer followed by a 50 nm gold (Au) layer was deposited on the silicon substrate using sputtering. The Ti layer worked as the promotion layer to improve adhesion between the Au film and the silicon surface. (2) Negative tone e-beam resist Ma-N2400 (Microresist Technology Gmbh) was spincoated onto the Au surface. (3) SRR patterns were exposed to the resist by e-beam writer (CRESTEC 9500C). The exposed resists were developed with MaD525 developer (Microresist Technology Gmbh). The Ma-N2400 resist was a negative 21909



Article

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

resonance enhancement at 532 nm excitation.23,24 For comparison, a normal Raman spectrum of solid R6G was measured and is presented in Figure 3. The normal Raman spectrum was obtained at 532 nm excitation and the fluoresce background was subtracted. It is notable that the peak intensity at the 1649 cm−1 band is the highest in the SERS spectra. We used the intensity at the 1649 cm−1 band as a benchmark for comparing the SERS performance among various samples. Shown in Figure 2b are the integrated SERS intensities at the 1649 cm−1 band on the different patterns. The integration was under the peak area of the 1649 cm−1 band. The error bars in the graph originate from intensity variation of the three measured points on the same sample. The bar graph shows that pattern D had the highest SERS intensity, while pattern A had the lowest SERS intensity. The difference in SERS intensity was likely related to the split-gap size, as pattern D had the smallest split gap, whereas pattern A had the largest split gap. In addition, we compared the SERS intensity of SRRs with split gaps (A−D) to that of the closed-square rings (pattern E). We found that all the SRRs except for pattern A had higher SERS intensity than that of the closed-square rings. This result is reasonable because the gapped nanostructures are able to concentrate their electric fields and can produce large enhancement, as has been shown by Au particle dimers and Au bowties.25,26 However, the localized EM fields in the gaps have only slight influenced the SERS enhancement of the SRR arrays compared to the reported particle dimers, as the gap sizes were relatively large to have strong field coupling between the two tips. This was why the SERS intensity increased slightly with the decrease in the gap size. The total SERS enhancement was the contribution of the electric field at the gap and on the gold wire. The decrease in the gap size on one hand strengthened the field in the gap, and the field intensity on the Au wire on the other hand as the LSPR resonance approached the excitation wavelength. The change in the total field enhancement with the gap size is discussed in more detail in Numerical Simulations. It was worth mentioning that there was resonant Raman scattering where the excitation corresponds to an electronic transition in the molecule, as the

diameter, the typical laser power was 0.6 mW at the sample surface, and the acquisition time was 1 s. To achieve better uniformity, Raman spectra were collected from three different regions on each patterned area. The final spectra were the average spectra obtained from the three regions.

3. RESULTS AND DISCUSSION 3.1. SERS Properties of SRRs. The intent of this study was to investigate experimentally the effect of the split gap of SRRs on SERS intensity. Figure 1 shows scanning electron microscopy (SEM) images of the SRRs (a−d) with different gap sizes. An array of square-rings is shown in Figure 1e. Figure 1f presents a schematic of one unit SRR. The periodicity of the arrays (p = 750) are the same for all the patterned arrays. The length of the SRR arms was L = 540 nm, the width of the SRR was a = 540 nm, the width of Au wire was w = 85 nm, and the thickness of the Au was 50 nm. Five different patterns (a−e) were designed. The split gap size, g, of the SRRs varied from 320 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 as those of the SRRs. They can be considered as SRRs with a g = 0 gap size. All five patterns were fabricated on the same chip with different dies. The SEM images indicated the good quality and uniformity of the SRRs in the arrays. Both horizontally polarized (x-polarized) and vertically polarized (y-polarized) sources were used to excite the Raman signals. We first discuss the SERS properties under xpolarized excitation. Figure 2 shows SERS of R6G on SRRs (a− d) and the square rings (e). These spectra were normalized to the acquisition time and the background was subtracted. Raman measurements were also performed on the blank area outside the patterned arrays but no obvious Raman peaks were measured. This confirms that the strong Raman signals were the result of the enhancement of the nanostructures. Four dominant peaks at 1281, 1360, 1508, and 1649 cm−1 in the SERS spectra were characteristic Raman signals from symmetric models of in-plane C−C stretching vibration because of the 21910



Article

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 the excitation wavelength.

significantly influence field enhancement in the split gap. The closed square-rings were geometrically symmetrical in the xand y-directions and, therefore, the SERS intensities under the two polarization directions were at the same intensity level. 3.2. Numerical Simulations. Numerical simulations of the spectra’s responses and the electrical field distribution of the five structures were performed by employing FIT method with CST Microwave Studio.32 Periodic boundary conditions were applied to the lateral sides of the elementary lattice (Figure 1f). The dispersion properties of the Au were treated as the Drude model by ε(ω) = ε∞ + ω2p/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 the simulation method are presented in our previous work.34 The simulated reflection spectra of the A−E patterns are shown in Figure 5. Figure 5a presents the reflection spectra under x-polarized excitation. There is a strong reflection dip at around 532 nm in the reflection spectra, which indicates the LSP mode. With the decrease in the gap size from pattern A−D, the resonance shifts to lower wavelengths and approaches the excitation wavelength of 532 nm. This blue-shift of resonance is associated with the LSP interaction between the two tips. The LSP interaction is strengthened as the gap size decreases and thus the resonance dip shifts to lower wavelengths.35 The closed square-rings (pattern E) have no such LSP coupling and hence the resonance redshifts compared to the SRRs (B−D). The tunable property of LSP resonance affords us the opportunity to adjust the SERS enhancement. When the excitation wavelength overlaps with the resonance wavelength, there is a maximum EM field enhancement and thus a maximum SERS enhance-

excitation wavelength of 532 nm was near the R6G absorption maximum of 530 nm.27 The strong fluorescence of R6G was quenched due to nonradioactive interactions with the metal surface.28 The contribution of the resonance Raman process is beyond the scope of this study. We also studied the SERS properties of the SRRs under ypolarized excitation. In this case, the incident electric field was parallel to the two arms of the SRRs. Figure 4a shows the SERS spectra on the SRRs and Figure 4b is a bar graph of the SERS intensity (integration) at the 1649 cm−1 band. While strong SERS intensity of R6G was achieved on the SRRs under ypolarized excitation, it was only slightly lower than that measured under x-polarized excitation. This observation suggests that the polarization direction did not change the SERS enhancement of the SRRs significantly. This result was surprising, because we expected much weaker electric field coupling under y-polarized excitation than under x-polarized excitation, as reported in the literature.29,30 When the SRRs were excited under the y-polarized source, the charges were less acuminated that under x-polarized excitation. Therefore, the localized field on the tips of the SRRs under y-polarized excitation was weaker than that under x-polarized excitation. This was why the SERS intensity of the SRRs obtained under ypolarized excitation was weaker than that under x-polarized excitation. With small gaps, field coupling effects is normally strong between the tips. Such field coupling will greatly enhance the field in the gap region greatly. However, in our work, the split-gap size was relatively large and so the field coupling between the two tips was weak compared to the reported dimer structures and similar SRR structures.25,26,31 Therefore, the change in the excitation polarization did not 21911



Article

intensity was measured on pattern D, flowed by pattern C, B, E, and A. Thus, we can see a correlation between the SERS enhancement and the LSP resonance position: the closer the resonance dip to the excitation wavelength, the stronger the SERS enhancement. This correlation was in agreement with that obtained under x-polarized excitation. Figure 6 presents the map of simulated electric field enhancement |E/E0| on the surface of the SRRs and squarerings. E is local electric field at the surface and E0 is the incident electric field. The field was calculated at the excitation wavelength of 532 nm. The arrows in the figure indicate the polarization directions of the incidence. The electric field distributions of the structures under x-polarized excitation are shown in the first row (1a−1e) and the electric field distributions of the structures under y-polarized excitation are shown in the second row (2a−2e). In the field distribution (1a−1e) in Figure 6, intense hotspots can be observed on the two arms of the SRRs and on the opposing sides of the squarering that are perpendicular to the direction of polarization. These intense hotspots correspond to the near-field enhancement of SERS signals. With the decrease of gap size from patterns A−D (1a−1d)), the maximum intensity increased slightly. This increase may be related to the discrepancy in the resonance of the structure to the excitation light. With the approach of the resonance position to the excitation wavelength, the LSP was more excited and thus the electric field was more intense. This observation is in agreement with the experimental observation of SERS intensity on the SRRs. By comparing the electric field distributions on the SRRs and squaring-ring, we can see that the electric field distribution modes on the two vertical arms are very similar to on the two vertical sides of the square-ring. Additional field coupling between the gaps contributes additional field enhancement over the square-rings. Therefore, higher SERS enhancement was observed on the SRRs than on the square-rings. With the decrease in the size of the split gap, the electric field coupling between the two tips of the split became obvious. This variation trend of in the field coupling in the gaps is in agreement with the measured SERS enhancement variation trend on the SRRs from pattern A−D. It is worth mentioning that the electric field distribution in the SRRs and square-rings expanded on the two arms rather than being highly localized like that at sharp tips or corners. This field distribution model is very similar to the nearfield electric field distribution models on square-rings proposed

ment.2 The resonance wavelength of pattern D was closest to the excitation wavelength, whereas the resonance wavelength of pattern A was furthest from the excitation wavelength. Therefore, the highest SERS intensity was observed on pattern D due to the best match between the SRR resonance and the excitation wavelength. As the resonance dips of all the patterns were near the excitation wavelength, the SERS enhancement does not vary significantly. A similar dependence of the SERS intensity on LSP resonance was also observed on the arrays of gold nanowells by Li et al.36 Figure 5b shows the simulated reflection spectra of the SRRs under y-polarized excitation. The resonance frequencies of the patterns are similar to those of the x-polarized excitation. This suggests that the periodicity dominates the resonance position of arrays of the SRRs and square-rings. Because the resonance wavelengths of the SRRs under y-polarized excitation were similar to those under x-polarized excitation, the SERS intensity of the SRRs under y-polarized excitation was very close to that under x-polarized excitation. However, the resonance dips shown in Figure 5b are not as sharper as those in the Figure 5a. The sharpness of the resonance dip can be expressed by a quality factor (Q-factor), Q = (λmax/Δλ), where λmax is the resonance wavelength and Δλ is the half-intensity dip width. The Q-factors for patterns A−E are listed in Table 1. Table 1. Q-Factors of the Resonance Dips in Figure 5 pattern Q-factors

x-polarization y-polarization

A

B

C

D

E

49.3 38.7

41.7 28.2

41.3 26.8

41.1 25.5

38.8 38.8

Obviously, the Q-factors of the spectra shown in Figure 5b are generally smaller than those shown in Figure 5a. The quality factor is related to the excitations of LSPRs and a higher Qfactor is associated with higher excitation efficiency.37 The interaction between the gap structure and the incident electrometric field was stronger under x-polarized excitation than that under y-polarized excitation and resulted in sharper resonance dips and therefore higher SERS enhancement. It can be clearly observed in Figure 5b that the resonance dip shift to lower wavelength with the decrease of split gap size from patterns A−D. The resonance position of the pattern D is closest to the excitation wavelength 532 nm, followed by patterns C, B, E, and A. Correspondingly, the highest SERS

Figure 6. Numerically simulated electric field distribution on the surface of SRRs under x-polarized excitation(1a−1e); under y-polarized excitation (2a−2e). 21912



Article

by Valev et al.38 The expanded localized field distribution is interesting because it may increase the opportunity for interaction of local charges with molecules. Figure 6 (2a−2e) shows the electric field distribution on the SRR nanostructures under y-polarized excitation. There is an intense electric field distributed on the bottom of the SRRs (2a−2d) and the two sides in the polarization direction of the square-ring (2e). Obviously, the total area of the intense electric field under y-polarized excitation was less than that under xpolarized excitation. The electric-field contribution comes from the intense field on the bottom and at the tips of the splits. Therefore, it is reasonable that the SERS intensity under ypolarized excitation is slightly less than that under x-polarized excitation. It is worth mentioning that the electric field under both x- and y-polarized excitations decay into the surrounding medium. These parts of the decaying field may also contribute to the SERS enhancement. 3.3. SERS Enhancement Factors (EFs). To evaluate the SERS enhancement ability of the substrates of SRRs and square-rings, SERS enhancement factors (EFs) were calculated on the five patterns. A general equation for the Raman enhancement factor is EF = (ISERSNbulk)/(IbulkNSERS),39 where ISERS is the SERS intensity, NSERS is the number of molecules illuminated by the laser source on the SERS substrates, Ibulk is the normal Raman intensity of the solid samples, and Nbulk is the number of molecules in the laser excitation volume in the solid samples. The reference Raman signal of R6G Ibulk was obtained by measuring solid R6G and the spectrum is presented in Figure 3. The Nbulk was estimated by considering the laser spot as 1.5 μm in diameter and the laser penetration depth in solid R6G as 2 μm. Taking the density of solid R6G (1.25 g/cm3) into account, Nbulk was calculated to be about 5.59 × 109. The SERS intensity of the band at 1649 cm−1 was used for the calculations. The estimated SERS EF was on the order of 105 for all the patterns. We compared the EFs achieved for the SRR arrays and square-rings are to the EFs reported in the literatures. The EFs in this work are of the same order of the EFs obtained on nanostructures like nanopillars and nanorings fabricated with nanofabrication techniques (104−106),40,41 but they are lower than the EFs of the nanopaticles synthesized chemically (108−1011).42,43 Figure 7 shows a plot of measured EFs of the SRRs and the square-rings for x- and y-polarized excitation. The highest EF is 5 × 105 for the SRR with split gap

of 50 nm under x-polarized excitation and with the smallest split gap size. The SERS EF under x-polarized excitation is higher than that under y-polarized excitation. The discrepancy in the EF of the square-rings between the x- and y-polarized excitation was attributed to the imperfect repeatability of the Raman measurement system. EM enhancement was calculated based on the electric field distributions shown in Figure 7. It is known that SERS enhancement is related to near-field electric field enhancement by the following formula:44,45 E E0

EF ∝

4

(1)

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

E̅ =

1 S

∫ E(x , y)dxdy

(2)

The value of E̅ is less than the maximum surface field but is more in line with actual measurements. Therefore, the SERS enhancement factor can be rewritten as E̅ EF ∝ E0

4

(3)

The EM enhancement factor |E̅/E0|4 is plotted in Figure 7. For the purpose of comparison, the simulated EFs were multiplied by 50 such that they are at the similar height as the measured EFs in the plot. The variation trend of the simulated EFs is in line with the measured EFs. The simulated EM EFs vary more significantly with gap size from pattern A to E than do the experimental results due to the perfect condition in the simulation. Indeed, the simulated EM EFs were ∼50 times smaller than the measured EFs. There are three possible reasons leading to the discrepancy in the EF between the calculated EM EFs and the experimental EFs. First, the measured SERS EFs include the effects of both chemical and EM enhancements and the measured EFs were higher than the EM EFs. Second, only the EM fields on the top surface of the nanostructures were calculated. The EM fields that evanescently decay into the surrounding medium make contributions to SERS enhancement. Such contributions were not considered in the calculations. Third, the surface roughness of the nanostructures can lead to higher local EM enhancement and thus SERS enhancement. This part of contribution were not included in the calculation.46

4. CONCLUSIONS In conclusion, we have investigated SERS responses on arrays of SRRs with various split-gap sizes. The SERS enhancement exhibits a tunable dependence on the gap size where a smaller gap results in an increase in SERS enhancement. With a decrease in the size of the split-gap, the resonance shift close to the wavelength of the excitation light and thus the SERS enhancement increases. The SRRs (patterns B, C, and D) exhibited higher SERS enhancement than did the square-rings due to localized electric fields on the tips of the splits. This work demonstrated a promising approach to manipulating the SERS enhancement by modulating the split gap size of SRRs

Figure 7. Measured (m) SERS EFs and simulated (s) electric field EFs of the patterns under x-polarized (X-polar) and y-polarized (Y-polar) excitations, respectively. The electric field EFs were multiplied by 50 to enable comparison with the experimental data. 21913



Article

(19) Cubukcu, E.; Zhang, S.; Park, Y. S.; Bartal, G.; Zhang, X. Split Ring Resonator Sensors for Infrared Detection of Single Molecular Monolayers. Appl. Phys. Lett. 2009, 95, 043113. (20) Clark, A. W.; Glidle, A.; Cumming, D. R. S.; Cooper, J. M. Nanophotonic Split-Ring Resonators as Dichroics for Molecular Spectroscopy. Appl. Phys. Lett. 2008, 93, 023121. (21) Yue, W.; Wang, Z.; Yang, Y.; Chen, L.; Syed, A.; Wong, K.; Wang, X. Electron-Beam Lithography Of Gold Nanostructures for Surface-Enhanced Raman Scattering. J. Micromech. Microeng. 2012, 22, 125077. (22) Stamplecoskie, K. G.; Scaiano, J. C.; Tiwari, V. S.; Anis, H. Optimal Size of Silver Nanoparticles for Surface-Enhanced Raman Spectroscopy. J. Phys. Chem. C 2011, 115, 1403−1409. (23) Watanabe, H.; Hayazawa, N.; Inouye, Y.; Kawata, S. DFT Vibrational Calculations of Rhodamine 6G Adsorbed on Silver: Analysis of Tip-Enhanced Raman Spectroscopy. J. Phys. Chem. B 2005, 109, 5012−5020. (24) Upender, G.; Satyavathi, R.; Raju, B.; Alee, K. S.; Rao, D. N.; Bansal, C. Silver Nanocluster Films as Novel SERS Substrates for Ultrasensitive Detection of Molecules. Chem. Phys. Lett. 2011, 511, 309−314. (25) Zhu, W.; Banaee, M. G.; Wang, D.; Chu, Y.; Crozier, B. Lithographically Fabricated Optical Antennas with Gaps Well Below 10 nm. Small 2011, 7, 1761−1766. (26) Kim, S.; Jin, J.; Kim, Y.-J.; Park, I.-Y.; Kim, Y.; Kim, S.-W. HighHarmonic Generation by Resonant Plasmon Field Enhancement. Nature 2008, 453, 757−760. (27) Zhao, J.; Jensen, L.; Sung, J.; Zou, S.; Schatz, G. C.; Van Duyne, R. P. Interaction of Plasmon and Molecular Resonances for Rhodamine 6G Adsorbed on Silver Nanoparticles. J. Am. Chem. Soc. 2007, 129, 7647−7656. (28) Meyer, S. A.; Le Ru, E. C.; Etchegoin, P. G. Quantifying Resonant Raman Cross Sections with SERS. J. Phys. Chem. A 2010, 114, 5515−5519. (29) Yao, Y.; Kats, M. A.; Genevet, P.; Yu, N.; Song, Y.; Kong, J.; Capasso, F. Broad Electrical Tuning of Graphene-Loaded Plasmonic Antennas. Nano Lett. 2013, 13, 1257−1264. (30) Atay, T.; Song, J.-H.; Nurmikko, A. V. Strongly Interacting Plasmon Nanoparticle Pairs: From Dipole−Dipole Interaction to Conductively Coupled Regime. Nano Lett. 2004, 4, 1627−1631. (31) Aydin, K.; Pryce, I. M.; Atwater, H. A. Symmetry Breaking and Strong Coupling in Planar Optical Metamaterials. Opt. Express 2010, 18, 13407−13417. (32) CST website: http://www.cst.com/. (33) Lei, D. Y.; Li, J.; Fernández-Domínguez, A. I.; Ong, H. C.; Maier, S. A. Geometry Dependence of Surface Plasmon Polariton Lifetimes in Nanohole Arrays. ACS Nano 2010, 4, 432−438. (34) Yue, W.; Yang, Y.; Wang, Z.; Han, J.; Syed, A.; Chen, L.; Wong, K.; Wang, X. Improved Surface-Enhanced Raman Scattering on Arrays of Gold Quaisi-3D Nanoholes. J. Phys. D: Appl. Phys. 2012, 45, 425401. (35) Wu, H.-Y.; Choi, C. J.; Cunningham, B. T. Plasmonic NanogapEnhanced Raman Scattering Using a Resonant Nanodome Array. Small 2012, 8, 2878−2885. (36) Li, K.; Clime, L.; Tay, L.; Cui, B.; Geissler, M.; Veres, T. Multiple Surface Plasmon Resonances and Near-Infrared Field Enhancement of Gold Nanowells. Anal. Chem. 2008, 80, 4945−4950. (37) West, P. R.; Ishii, S.; Naik, G. V.; Emani, N. K.; Shalaev, V. M.; Boltasseva, A. Searching for Better Plasmonic Materials. Laser Photonics Rev. 2010, 4, 795−808. (38) Valev, V. K.; De Clercq, B.; Biris, C. G.; Zheng, X.; Vandendriessche, S.; Hojeij, M.; Denkova, D.; Jeyaram, Y.; Panoiu, N. C.; Ekinci, Y.; et al. Distributing the Optical Near-Field for Efficient Field-Enhancement in Nanostructures. Adv. Mater. 2012, 24, OP208− OP215. (39) Reilly, T. H., III.; Chang, S.-H.; Corbman, J. D.; Schatz, G. C.; Rowlen, K. L. Quantitative Evaluation of Plasmon Enhanced Raman Scattering from Nanoaperture Arrays. J. Phys. Chem. C 2007, 111, 1689−1694.

without affecting the area and structural arrangement of the patterns.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +966 12 8082549. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Kauranen, M.; Zayats, A. V. Nonlinear Plasmonics. Nat. Photonics 2012, 6, 737−748. (2) Sharma, B.; Frontiera, R. R.; Henry, A.-I.; Ringe, E.; Van Duyne, R. P. SERS: Materials, Applications and the Future. Mater. Today 2012, 15, 16−25. (3) Anker, J. N.; Hall, W. P.; Lyandres, O.; Shah, N. C.; Zhao, J.; Van Duyne, R. P. Biosensing with Plasmonic Nanosensors. Nat. Mater. 2008, 7, 442−453. (4) Henzie, J.; Lee, J.; Lee, M. H.; Hasan, W.; Odom, T. Nanofabrication of Plasmonic Structures. Annu. Rev. Phys. Chem. 2009, 60, 147−165. (5) Girard, C.; Dujardin, E.; Baffou, G.; Quidant, R. Shaping And Manipulation of Light Fields With Bottom-Up Plasmonic Structures. New J. Phys. 2008, 10, 105016. (6) Fana, M.; Andradec, G. F. S.; Brolo, A. G. A Review on the Fabrication of Substrates for Surface Enhanced Raman Spectroscopy and Their Applications in Analytical Chemistry. Anal. Chim. Acta 2011, 693, 7−25. (7) Le Ru, E. C.; Blackie, E.; Meyer, M.; Etchegoin, P. G. Surface Enhanced Raman Scattering Enhancement Factors: A Comprehensive Study. J. Phys. Chem. C 2007, 111, 13794−13803. (8) Le Ru, E. C.; Etchegoin, P. G. Single-Molecule Surface-Enhanced Raman Spectroscopy. Annu. Rev. Phys. Chem. 2012, 63, 65−87. (9) Yu, Q.; Guan, P.; Qin, D.; Golden, G.; Wallace, P. M. Inverted Size-Dependence of Surface-Enhanced Raman Scattering on Gold Nanohole and Nanodisk Arrays. Nano Lett. 2008, 8, 1923−1928. (10) Anema, J. R.; Brolo, A. G.; Marthandam, R.; Gordon, R. Enhanced Raman Scattering From Nanoholes in a Copper Film. J. Phys. Chem. C 2008, 112, 17051−17055. (11) Hatab, N. A.; Hsueh, C.-H.; Gaddis, A. L.; Retterer, S. T.; Li, J.H.; Eres, G.; Zhang, Z.; Gu, B. Free-Standing Optical Gold Bowtie Nanoantenna with Variable Gap Size for Enhanced Raman Spectroscopy. Nano Lett. 2010, 10, 4952−4955. (12) Stosch, R.; Yaghobian, F.; Weimann, T.; Brown, R. J. C.; Milton, M. J. T.; Guttler, B. Lithographical Gap-Size Engineered Nanoarrays for Surface-Enhanced Raman Probing Of Biomarkers. Nanotechnology 2011, 22, 105303. (13) Enkrich, C.; Wegener, M.; Linden, S.; Burger, S.; Zschiedrich, L.; Schmidt, F.; Zhou, J. F.; Koschny, T.; Soukoulis, C. M. Magnetic Metamaterials at Telecommunication and Visible Frequencies. Phys. Rev. Lett. 2005, 95, 203901. (14) Sarau, G.; Lahiri, B.; Banzer, P.; Gupta, P.; Bhattacharya, A.; Vollmer, F.; Christiansen, S. Enhanced Raman Scattering of Graphene Using Arrays Split Ring Resonators. Adv. Opt. Mater. 2013, 1, 151− 157. (15) Jeppesen, C.; Mortensen, N. A.; Kristensen, A. The Effect of Ti and ITO Adhesion Layers on Gold Split-Ring Resonators. Appl. Phys. Lett. 2010, 97, 263103. (16) Sun, Y.; Xia, X.; Feng, H.; Yang, H.; Gu, C.; Wang, L. Modulated Terahertz Responses of Split Ring Resonators by Nanometer Thick Liquid Layers. Appl. Phys. Lett. 2008, 92, 221101. (17) Lahiri, B.; Khokhbar, A. Z.; De La Rue, R. M.; McMeekin, S. G.; Johnson, N. P. Asymmetrical Split Ring Resonators for Optical Sensing of Organic Materials. Opt. Express 2009, 17, 1107−1115. (18) Yue, W.; Yang, Y.; Wang, Z.; Chen, L.; Wang, X. SurfaceEnhanced Raman Scattering on Gold Nanorod Pairs with Interconnection Bars of Different Widths. Sens. Actuators, B 2012, 171−172, 734−738. 21914



Article

(40) Caldwell, J. D.; Glembocki, O.; Bezares, F. J.; Bassim, N. D.; Rendell, R. W.; Feygelson, M.; Ukaegbu, M.; Kasica, R.; Shirey, L.; Hosten, C. Plasmonic Nanopillar Arrays for Large-Area, HighEnhancement Surface-Enhanced Raman Scattering Sensors. ACS Nano 2011, 5, 4046−4055. (41) Banaee, M. G.; Crozier, K. B. Gold Nanorings as Substrates for Surface-Enhanced Raman Scattering. Opt. Lett. 2010, 35, 760−762. (42) Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (43) Li, Q.; Jiang, Y.; Han, R.; Zhong, X.; Liu, S.; Li, Z.-Y.; Sha, Y.; Xu, D. High Surface-Enhanced Raman Scattering Performance of Individual Gold Nanoflowers and Their Application in Live Cell Imaging. Small 2013, 9, 927−932. (44) Li, K.; Clime, L.; Cui, B.; Veres, T. Surface Enhanced Raman Scattering on Long-Range Ordered Noble-Metal Nanocrescent Arrays. Nanotechnology 2008, 19, 145305. (45) Ru, E. C. Le; Meyer, M.; Blackie, E.; Etchegoin, P. G. Advanced Aspects of Electromagnetic SERS Enhancement Factors at A Hot Spot. J. Raman Spectrosc. 2008, 39, 1127−1134. (46) Rodríguez-Fernández, J.; Funston, A. M.; Pérez-Juste, J.; Alvarez-Puebla, R. A.; Liz-Marzán, L. M.; Mulvaney, P. The Effect of Surface Roughness on the Plasmonic Response of Individual SubMicron Gold Spheres. Phys. Chem. Chem. Phys. 2009, 11, 5909−5914.

21915


Gold Split-Ring Resonators (SRRs) as Substrates for Surface

Recommend Documents