Apex-Enhanced Raman Spectroscopy Using Double-Hole Arrays in a

Figure 1 shows scanning electron microscope (SEM) images of single double-hole ... correspond to the (1, 1) and the (1, 0) Bragg resonances.15 These r...
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2007, 111, 2347-2350 Published on Web 01/20/2007

Apex-Enhanced Raman Spectroscopy Using Double-Hole Arrays in a Gold Film A. Lesuffleur,† L. K. S. Kumar,† A. G. Brolo,*,‡ K. L. Kavanagh,§ and R. Gordon*,† Department of Electrical and Computer Engineering, UniVersity of Victoria, P.O. Box 3055, Victoria, B.C., Canada, V8W 3P6, Department of Chemistry, UniVersity of Victoria, P.O. Box 3065, Victoria, B.C., Canada, V8W 3V6, and Department of Physics, Simon Fraser UniVersity, 8888 UniVersity DriVe, Burnaby, B.C., Canada, V5A 1S6 ReceiVed: NoVember 18, 2006; In Final Form: January 3, 2007

Periodic arrays of subwavelength double-hole apertures in a 100 nm gold film were used for surface-enhanced Raman scattering (SERS). The periodicity of the arrays and the double-hole overlap were varied to measure their effect on the Raman enhancement. The experiments were realized in two geometries: forward-scattering (transmission through the holes) and back-scattering. Significant enhancement in the SERS signal was observed in both geometries for the optimized substrates. Electromagnetic calculations verified that the local electromagnetic field intensity was indeed maximized for the arrays that produced the largest SERS effect. The double-hole structure was shown to produce controlled and predictable SERS due to the enhanced field localized at the apexes formed by the overlap between the holes.

In 1998, extraordinary transmission through an array of nanoholes in metal films was demonstrated.1 Since that work, the influence of the hole shape on the optical properties of subwavelength holes in metal has been studied.2,3 It has been shown that the aspect ratio of elliptical and rectangular holes influences the polarization, the transmission intensity, and the cutoff wavelength, both in arrays4 and in isolated holes.5,6 The orientation of arrays of elliptical and double holes showed separate basis and lattice contributions to the light transmission.7 Hole shapes can also be designed to produce regions that allow an extreme focusing of the electromagnetic (EM) field at the nanoscale. Enhanced local field intensity is required for surfaceenhanced Raman scattering (SERS),8 surface-enhanced secondharmonic generation (SHG),9 and super continuum generation.10 SERS is typically performed with random surface topologies that have the disadvantage of unpredictable local field enhancements.11,12 A primary motivation of exploring SERS in hole arrays is to allow for better predictability and control of the local field enhancement. Surface plasmon-based spectroscopy, including SERS, can be readily applied in biochemical and biomedical sciences for ultrasensitive chemical sensing; hence, the development of reproducible substrates is an area of general interest in this field. SERS has already been observed from arrays of circular nanoholes in a gold film with various periodicities.13 It was shown that the enhanced Raman scattering was due to the excitation of surface plasmons at the arrays by grating coupling. This was confirmed by the direct connection between the SERS enhancement and the periodicity of the arrays. SERS from periodic arrays of gold nanoparticles has also been demonstrated.14 * Corresponding authors. E-mail: [email protected]; [email protected]. † Department of Electrical and Computer Engineering, University of Victoria. ‡ Department of Chemistry, University of Victoria. § Simon Fraser University.

10.1021/jp067677e CCC: $37.00

We have explored the use of a double-hole structure to enhance the local electromagnetic field. The double-hole basis consists of two holes that overlap to produce two apexes (or cusps). These apexes are responsible for the subwavelength focusing of this structure. The double-hole structure permits enhancement of the local field intensity when compared to circular holes.15,16 The double-hole structure offers two degrees of freedom, shape and periodicity, for electric field optimization. Both of these degrees of freedom were explored in this work. First, the Raman signal was maximized by adjusting the hole-to-hole distance, thereby changing the structure of the apexes. Then, the effect of the periodicity on the SERS was explored. Both the forward-scattering (transmitted through the double holes) and the back-scattering geometries were studied to separate the effects of excitation via transmission through the holes and direct excitation by the source. Figure 1 shows scanning electron microscope (SEM) images of single double-hole bases (Figure 1a-c) and of arrays of double holes (Figure 1d and e). All structures were milled through a 100-nm-thick gold film with a 5 nm Cr adhesion layer on a glass substrate using a focused ion beam.15,16 Each array covered a 25 × 30 µm2 area. Figure 1a shows two 200 nm holes, and the separation between the hole centers, d, is 250 nm. This results in a double-hole structure where each hole is isolated. Just a short gold bridge is left between the holes when the distance between their centers is decreased to 210 nm (Figure 1b). Finally, as seen in Figure 1c, apexes are formed when the distance between the holes became smaller than 200 nm. Figure 1d and e shows the arrays of double holes with different periodicity. Figure 1d shows the array with 750 nm periodicity from a tilted angle to demonstrate that the taper in the array was not significant (around 10 nm) over the 100-nm-deep hole. Results for two sets of double-hole arrays will be presented. In the first set, the center-to-center hole distance, d, was varied © 2007 American Chemical Society

2348 J. Phys. Chem. C, Vol. 111, No. 6, 2007

Figure 1. SEM picture of double holes (d is the center-to-center hole distance): (a) double hole without apexes (d ) 250 nm, hole diameter ) 200 nm), (b) double hole with apexes and a taper (d ) 210 nm, hole diameter ) 200 nm), (c) double hole with sharp apexes (d ) 190 nm, hole diameter ) 200 nm), (d) array of double holes with 750 nm periodicity and tilted viewing to observe taper in hole (d ) 175 nm, hole diameter ) 180 nm), and (e) arrays of double holes with 550 nm periodicity (d ) 175 nm, hole diameter ) 180 nm).

for a fixed array periodicity of 750 nm and a hole-diameter of 200 nm. The second set had arrays with a fixed center-to-center hole distance, d ) 175 nm, but varying periodicities, with a hole diameter of 180 nm. This center-to-center hole distance produces sharp apexes, and the slightly smaller diameter allowed for a wider range of periodicities to be explored (from 750 down to 380 nm). The purpose of the first set was to study the effect of the apex structure on the SERS, whereas the second set offered further tuning of the SP resonance through the periodicity. Figure 2a shows the white light transmission for three centerto-center hole distances and a fixed periodicity of 750 nm. Figure 2b shows the transmission for various periodicities for doublehole structures with sharp apexes (175 nm center-to-center distance for a 180 nm hole diameter). In Figure 2a and b, the dotted vertical line corresponds to the laser wavelength (632.8 nm) used for the SERS measurements. The band around 500 nm in the transmission spectra can be attributed to the bulk plasmon,17 and it is a common feature observed in the light transmission through thin metallic films.18 The spectra in Figure 2a show resonance peaks around 600 and 800 nm, which correspond to the (1, 1) and the (1, 0) Bragg resonances.15 These resonances shifted to shorter wavelengths when the periodicity was decreased (Figure 2b). For periodicity equal to 490 nm or shorter, the two resonances are replaced by a broader peak, as described elsewhere.19 SERS spectra were taken from oxazine 720 adsorbed on all arrays of double holes. A 35 mW He-Ne laser was used as the excitation source with the wavelength of excitation at 632.8 nm. The laser was polarized along the apexes, which gives the highest field enhancement.16 In the forward-scattering geometry, the incident laser was transmitted from the glass side through the double-hole arrays to excite the molecules adsorbed on the gold-air interface. This geometry has been used in SERS from arrays of circular nanoholes.13 This forward-scattering geometry ensured that the excitation photons were part of the extraordinary transmission phenomenon. The laser was directed from the glass side of the substrate and focused onto the array of double holes using a 10× (NA ) 0.25) microscope objective, and the forward-scattered light was collected using a 50× (NA ) 0.90) objective mounted on a metallurgical microscope. The funda-

Letters mental laser light was blocked from the collected radiation using a Kaiser supernotch filter, and the remaining radiation was directed through a f/1.4 Kaiser spectrograph coupled with an thermoelectrically cooled CCD detector (Andor). In the backscattering experiments, the laser was focused directly onto the arrays using the 50× (NA ) 0.90) objective. The collection setup of the scattered light was the same as that for the forwardscattering geometry. Figure 3a shows the SERS intensity of the 591 cm-1 oxazine 720 band plotted versus the center-to-center hole distance for both geometries. In the past, corrections have been made to account for the filling factor.20 The data in Figure 3a was not corrected for the variations in the area of the aperture because the aperture shape plays a more important role in the optical properties of the nanostructure than the filling factor. The filling factor varies from 4.5% for the fully overlapped holes (d ) 0) to 9.0% for the two isolated holes (d ) 250 nm). A maximum in the SERS intensity was observed in Figure 3a when the distance between the holes was 190 nm in forward-scattering and 195 nm in back-scattering. These center-to-center hole distances corresponded to the sharpest apexes at the overlap between the holes. The maximum SERS signal for the double holes with apexes (d ) 190 nm) was 12 times larger than that observed for arrays of single holes (d ) 0 nm) in forwardscattering. The linear transmission spectra of the same two arrays are shown in Figure 2a. The dotted line is at 632.8 nm, which corresponds to the laser wavelength used in the SERS experiments. It can be seen in Figure 2a that the transmission at the laser wavelength increased by only a factor of 3 between these two arrays. The observed enhancement should then be attributed to local resonance in the near field due to the apexes. We have confirmed this by numerical simulations, which showed that by tuning the center-to-center hole distance the local electromagnetic field resonances changes both in wavelength and magnitude.16 In back-scattering, it was possible to compare the SERS from the arrays to the signal obtained from species adsorbed on the unpatterned gold surface. Oxazine 720 has a strong absorption band that peaks close to the laser excitation. This allows for the observation of resonance Raman scattering from small amounts of oxazine 720 adsorbed on the unpatterned gold substrate. The dashed lines in Figure 3a and b represent the measured Raman intensity from the unpatterned gold surface adjacent to an array of nanoholes. The enhancement factor of the maximized SERS from the double-hole arrays as compared with the unpatterned surface was measured to be 19 using this procedure. This 19× increase in Raman scattering relative to the resonance Raman was obtained by tuning only the hole-tohole distance for a fixed periodicity at 750 nm. The relationship with the SERS intensities and the periodicities of the arrays was also investigated and will be discussed next. Figure 3b shows the peak intensity of the SERS band at 591 cm-1 versus the periodicity of the arrays, both in forwardscattering and in back-scattering. The data in Figure 3b did not consider the filling factor, but it may readily be included as the inverse of the periodicity squared. The correction for the filling factor would not change the results qualitatively (i.e., the periodicity yielding the maximum SERS would remain the same). A maximum was observed for the 490 nm periodicity in both geometries. The ratio between the SERS signal obtained for the arrays with 490 nm (maximum SERS) and 750 nm (minimum SERS) periodicities was 11.9 in the forwardscattering geometry and 12.4 in the back-scattering geometry.

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Figure 2. Linear transmission measurements for double-hole arrays. (a) The periodicity was kept constant (750 nm), and the center-to-center hole distance was varied. (b) The center-to-center hole distance was kept constant (d ) 175 nm, hole diameter ) 180 nm), and the periodicity was varied. All spectra were baseline corrected.

Figure 3. SERS in forward- and back-scattering geometries as a function of (a) the center-to-center hole distance (for arrays with fixed periodicity at 750 nm) and (b) the periodicity (for arrays with fixed center-to-center hole distance at 175 nm). The intensity of the 591 cm-1 band of oxazine 720 was used. The dashed line shows the back-scattering results from the unpatterned surface.

By varying the periodicity of the arrays, it was possible to tune the Bragg resonance to further enhance the SERS signal. The SERS signal from the 490 nm periodicity was 35 times higher than that observed from the unpatterned gold surface, whereas the SERS signal from the 750 nm periodicity was only 2.8 times higher. This means that an extra 35× increase in the SERS enhancement over the resonance Raman effect was obtained by tuning the periodicity of the double-hole structure. Figure 3a and b shows differences between the forwardscattering and back-scattering geometries. The forward-scattering involves enhanced optical transmission through excitation of surface waves on both sides of the film and transmission through the hole. The back-scattering geometry is dominated by surface plasmon excitation on one side of the film and the excitation of localized resonances from the hole without transmission. These differences lead to distinct optimal centerto-center hole spacing and periodicities for the two different geometries. A more quantitative description of the interaction between the transmission, localized surface plasmon, and propagating surface plasmon is not straightforward and probably can only be properly described by comprehensive numerical simulation. It should be noted that the periodicity that gives resonant transmission at the excitation wavelength did not provide the maximum SERS signal. In Figure 3b, the maximum SERS enhancement was for the 490 nm periodicity, whereas the 545 nm periodicity array had the (1, 0) resonance peak closest to 632.8 nm (Figure 2b). In fact, the maximum SERS signal is expected not from the array with resonant transmission but from the one with maximum local field at the laser excitation. To further explore this effect, finite-difference time-domain (FDTD) calculations for two different array periodicities at 490 and 545

nm were performed (details of the calculation parameters are given elsewhere21). The transmission intensity and the local field enhancements for an excitation wavelength of 632.8 nm were analyzed from the FDTD results. The local electromagnetic field intensity at the surface was 2.5 times larger for the 490 nm periodicity when compared to the 545 nm periodicity, even though the 545 nm array had the linear transmission wavelength resonance closest to the excitation wavelength. Although there is a debate on the role of surface plasmons in the transmission properties of nanohole arrays,17,22-24 it has been proposed that the interference between localized and propagating surface plasmon modes plays a role in the overall transmission.15,19 The comprehensive electromagnetic calculations presented here show that the local field is enhanced for a different periodicity than the one that gives the peak transmission near the excitation wavelength, in agreement with the experimental results. In conclusion, we demonstrated that arrays of subwavelength double holes in a gold film can be used to systematically investigate SERS from adsorbed molecules. Two parameters were varied to maximize the SERS: the center-to-center hole distance and the periodicity. The center-to-center hole distance increased the SERS in the forward-scattering geometry with respect to the single hole by a factor of 12 for a fixed periodicity. The periodicity variation gave an extra 12.4 times enhancement with respect to a nonresonant periodicity. The back-scattering geometry allowed direct comparison between the SERS and the resonance Raman of oxazine 720 obtained from an unpatterned area of the substrate coated with the dye. The maximum SERS was 19 and 35 times larger than the resonance Raman obtained from the unpatterned surface for the optimized center-to-center and periodicity, respectively. The local field enhancement

2350 J. Phys. Chem. C, Vol. 111, No. 6, 2007 calculated by FDTD reached a maximum for the same array periodicity that gave the maximum SERS signal. The results shown here indicate that these structures with apexes could produce a new generation of SERS substrates that combines both reproducibility and high enhancing capabilities. References and Notes (1) Ebbesen, T. W.; Lezec, H. J.; Ghaemi, H. F.; Thio, T.; Wolff, P. A. Nature 1998, 391, 667-669. (2) Gordon, R.; Brolo, A. G.; McKinnon, A.; Rajora, A.; Leathem, B.; Kavanagh, K. L. Phys. ReV. Lett. 2004, 92, 037401. (3) Koerkamp, K. J. K.; Enoch, S.; Segerink, F. B.; Hulst, N. F. v.; Kuipers, L. Phys. ReV. Lett. 2004, 92, 183901. (4) van der Molen, K. L.; Klein Koerkamp, K. J.; Enoch, S.; Segerink, F. B.; van Hulst, N. F.; Kuipers, L. Phys. ReV. B 2005, 72, 045421. (5) Degiron, A.; Lezec, H. J.; Yamamoto, N.; Ebbesen, T. W. Opt. Commun. 2004, 239, 61-66. (6) Gordon, R.; Brolo, A. G. Opt. Express 2005, 13, 1933-1938. (7) Gordon, R.; Hughes, M.; Leathem, B.; Kavanagh, K. L.; Brolo, A. G. Nano Lett. 2005, 5, 1243-1246. (8) Moskovits, M. ReV. Mod. Phys. 1985, 57, 783-826. (9) Bozhevolnyi, S. I.; Beermann, J.; Coello, V. Phys. ReV. Lett. 2003, 90, 197403. (10) Muhlschlegel, P.; Eisler, H. J.; Martin, O. J. F.; Hecht, B.; Pohl, D. W. Science 2005, 308, 1607-1609.

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