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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 5 — Feb. 28, 2011
  • pp: 4337–4345
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Optimizing electromagnetic enhancement of flexible nano-imprinted hexagonally patterned surface-enhanced Raman scattering substrates

D. Z. Lin, Y. P. Chen, P. J. Jhuang, J. Y. Chu, J. T. Yeh, and J.-K. Wang  »View Author Affiliations


Optics Express, Vol. 19, Issue 5, pp. 4337-4345 (2011)
http://dx.doi.org/10.1364/OE.19.004337


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Abstract

The production of inexpensive, large-scale, uniform substrates for surface-enhanced Raman scattering (SERS) is a key to popularize its usage in chemical and biological detection. We demonstrate a flexible nano-imprinted hexagonally patterned SERS-active substrate. Its electromagnetic enhancement factor was optimized by the thickness adjustment of its silver over-coated film. The experimental data show a good correspondence with the theoretical prediction. Such substrate was shown to exhibit high uniformity and reproducibility with a variation of less than 2%, offering a potential of greatly exploiting such substrate in infield biocide monitoring.

© 2011 OSA

1. Introduction

The basic principle of making SERS-active substrates lies on producing subwavelength structures that enable the resonant interaction with applied electromagnetic radiation (plasmon resonance) in both absorption and emission [17

17. A. Campion and P. Kambhampati, “Surface-enhanced Raman scattering,” Chem. Soc. Rev. 27(4), 241–250 (1998). [CrossRef]

], thus enhancing local electric field for Raman scattering. Based on this principle, we show here another large-scale, cheap fabrication method of SERS-active substrates based on imprinted two-dimensional hexagonally patterned structures with a silver nanometer-scaled over-coating to induce plasmon resonance. The intention is to use this structure to enhance the absorption of the excitation laser beam while performing Raman measurements. The silver film thickness was adjusted to optimize its SERS performance. The absorption spectra of the substrates with different silver film thicknesses presented a good correspondence with electrodynamic calculation based on finite difference time-domain method. The relationship between the SERS signal and the absorption spectrum is discussed. With such substrate, the trace detection of malachite green (MG) in water is demonstrated down to 10−8 M, corresponding to <5 μg/l. MG is highly effective against protozoal and fungal infections and had been extensively used as biocide in aquaculture industry, scientific evidences have however indicated that residual MG might cause carcinogenesis and teratogenesis [18

18. S. Srivastava, R. Sinha, and D. Roy, “Toxicological effects of malachite green,” Aquat. Toxicol. 66(3), 319–329 (2004). [CrossRef] [PubMed]

, 19

19. E. Sudova, J. Machova, Z. Svobodova, and T. Vesely, “Negative effects of malachite green and possibilities of its replacement in the treatment of fish eggs and fish: a review,” Vet. Med. 52, 527–539 (2007).

]. For that reason, in Ireland, the concentration of MG in fish farm water effluent should not exceed 100 μg/l. This study thus shows the potential usage of SERS with the developed inexpensive, large-scale substrates in on-site monitoring of biocide concentration in water systems. The method developed in this study is to be compared with other relatively low-speed, bulky techniques, such as liquid chromatography [20

20. J. L. Allen, J. R. Meinertz, and J. E. Gofus, “Determination of malachite green and its leuco form in water,” J. AOAC Int. 77, 646 (1992).

], amperometer [21

21. K. Sagar, M. Smyth, J. Wilson, and K. McLaughin, “High-performance liquid chromatographic determination of the triphenylmethane dye, malachite green, using amperometric detection at a carbon fibre microelectrode,” J. Chromatogr. A 659(2), 329–336 (1994). [CrossRef]

], capillary electrophoresis [22

22. C. H. Tsai, J. D. Lin, and C. H. Lin, “Optimization of the separation of malachite green in water by capillary electrophoresis Raman spectroscopy (CE-RS) based on the stacking and sweeping modes,” Talanta 72(2), 368–372 (2007). [CrossRef] [PubMed]

], and enzyme-linked immunosorbent assay [23

23. M.-C. Yang, J.-M. Fang, T.-F. Kuo, D.-M. Wang, Y.-L. Huang, L.-Y. Liu, P.-H. Chen, and T.-H. Chang, “Production of antibodies for selective detection of malachite green and the related triphenylmethane dyes in fish and fishpond water,” J. Agric. Food Chem. 55(22), 8851–8856 (2007). [CrossRef] [PubMed]

].

2. Experiments and calculation

The two-dimensional hexagonally patterned mold was fabricated by two-beam laser interference plus nickel electroless plating, which was reported previously [24

24. T.-L. Chang, K.-Y. Cheng, T.-H. Chou, C.-C. Su, H.-P. Yang, and S.-W. Luo, “Hybrid-polymer nanostructures forming an anti-reflection film using two-beam interference and ultraviolet nanoimprint lithography,” Microelectron. Eng. 86(4-6), 874–877 (2009). [CrossRef]

]. Briefly, spin-coated photo-resist film on silicon wafers was triply exposed with two 351 nm Ar-ion laser beams, crossing at about 80°. The substrates were rotated laterally by 60° between exposures, resulting in hexagonal periodic patterns. After a development process, the pattern was then formed on the surface of the silicon wafers with reactive ion etching. The subsequent nickel electroless plating process then created nickel-plated mold with the hexagonal pattern. Before nano-imprinting, a self-assembled monolayer of fluoroalkyl silane was deposited on the mold surface with atmospheric-plasma chemical surface treatment to decrease its surface energy, thus improving its anti-adhesive property. The patterned mold was then imprinted onto the poly-ethylenetelephthalate (PET) substrates by hot embossing process. This was followed by raising the temperature of the polymer just above its glass transition temperature. The PET substrates were then separated from the mold, creating the replicated hexagonally patterned structure on their surfaces. Silver films of different thicknesses were subsequently deposited on the substrates. Surface profilometric calibration on separate silver films that were co-deposited on flat silicon substrates gave the resultant silver film thicknesses of d = 13.1, 21.8, 30.6, 43.7, 65.5 and 87.3 nm. Figure 1
Fig. 1 (a) Cross-sectional schematic, (b) top-view scanning electron microscopic image (zoom-in view in the inset), and (c) atomic force microscopic topography image of a nano-imprinted SERS-active substrate with a 25 nm silver over-coating. The figure in the lower half of (c) shows the height profile along the red dashed line.
shows the schematic cross-section, top-view scanning electron microscopic image, and topographic atomic force microscopic (AFM) image of a typical nano-imprinted SERS-active substrate with a 25 nm silver over-coating. The period (p) and modulation depth (h) of the resultant structures determined with the AFM image are about 280 nm and 140 nm, respectively.

The SERS-active substrates were placed inside an integrating sphere of a UV-visible spectrophotometer for light absorption characterization. Their absorptance spectra in air were determined by the subtraction of the measured reflection and transmission spectra from unity. Dropping a water droplet on the surface and then placing a cover glass slip on top forms a uniform water layer above the substrate. The corresponding absorptance spectra of the substrates with a top water layer were also determined similarly. Distinct quantities of MG molecules were dissolves in water to prepare solutions of different concentrations. Rhodamine 6G (R6G) molecules in water was also prepared to compare its SERS performance with that of MG. 1 μL of the sample solution was introduced to the substrate for Raman measurements. The following application of a cover glass on the substrate prevented water evaporation during the measurements. All the Raman measurements were performed with a commercial micro-Raman system. A 515 nm solid-state laser (632.8 nm HeNe laser) served as the excitation source for MG (R6G) to avoid direct photoexcitation, making the Raman measurements being nearly non-resonant and thus minimizing the disturbance of laser-induced fluorescence and resonance Raman scattering. With a quarter-wave plate, the excitation polarization was set to be circular, thus eliminating the possible polarization dependence caused by the directional character of the hexagonally ordered pattern of the substrates. The 1 mW laser beam was focused with a 20× objective lens, resulting in a focal spot diameter of about 6.5 μm and therefore an illuminating intensity of 3 kW/cm2. The scattered radiation was collected in a backward direction with the same objective lens and was sent through a spectrograph to a liquid nitrogen-cooled charge-coupled device for spectral recording. The spectral resolution was calibrated to be less than 0.68 cm−1. The signal integration time was varied from 1 to 10 sec, depending on the sample concentration.

Finite-difference time-domain calculations were performed to simulate the electrodynamic behaviors of the substrates whereas the two-dimensional hexagonally patterned structure is approximated with periodical hemispheres, as depicted in Fig. 1(a). The refractive index of PET in the wavelength of interest was set to 1.58. The incident direction of the excitation light source is downward towards the silver film. The simulation was performed in a unit cell with anti-symmetry or symmetry boundary conditions on the substrate surface — depending on the polarization of the excitation light source — and with perfect matching layers on both the upper and lower boundaries of the calculation domain. The calculations were performed for the polarization direction of the incident optical wave being parallel with and perpendicular to the axis along the two adjacent hemispheres, in which the linear polarization of the excitation source was assumed to elucidate the underlying optical characteristics. The mesh sizes are 1 nm in the lateral direction but 1.5 nm in the vertical direction. The extracted transmission and reflection spectra for the two polarization configurations were averaged to mimic the absorptance spectra with a random polarization of the lamp source in the absorption measurement, as no chiral asymmetry is expected in the hexagonal ordered structure.

3. Results and discussion

Figure 2
Fig. 2 Experimental absorption spectra of SERS-active substrates deposited with different silver film thicknesses in (a) air and (b) water; calculated absorptance spectra of SERS-active substrates with different silver film thicknesses in (c) air and (d) water. d represents the thickness of silver film. Red, orange, green, blue, indigo and violet lines in (a) and (b) represent d = 13.1, 21.8, 30.6, 43.7, 65.5 and 87.3 nm, respectively. Red, orange, green, blue, and indigo dotted lines in (c) and (d) represent d = 15, 25, 35, 50, and 75 nm, respectively.
shows the absorption spectra of the SERS substrates with different silver film thicknesses (d) in air and with a top water layer. All the spectra exhibit a prominent peak in the wavelength range from 400 nm to 900 nm, reflecting plasmon resonant excitation. The absorption peak wavelength is decreased with the increment of d and reaches a plateau as d > 65 nm. The absorption peak wavelength obtained with a top water layer is red-shifted with respect to the one obtained in air, which is caused by the extra dielectric screening effect from the top water layer. The calculation also shows an absorption peak at a shorter wavelength (not shown here). The consistency between the experimental and calculation results is demonstrated in Fig. 3
Fig. 3 Extracted resonant wavelengths of symmetric (filled symbols) and asymmetric (open symbols) modes of silver-coated hexagonally patterned substrates vs. silver film thickness (d) from experimental and calculated results. Squares and circles represent the substrates in air and with a top water layer, respectively. Filled red symbols represent experimental data. Lines are guides to eye.
. The resonant wavelengths extracted from the absorption spectra of the substrates at different silver film thicknesses agree with the ones obtained from calculations. The two resonant peaks correspond to two localized plasmon modes: symmetric and asymmetric [25

25. F. Pigeon, I. F. Salakhutdinov, and A. V. Tishchenko, “Identity of long-range surface plasmons along asymmetric structures and their potential for refractometric sensors,” J. Appl. Phys. 90(2), 852–859 (2001). [CrossRef]

]. Their identification has been similarly pointed out in the investigation of light transmission through corrugated silver films at normal incidence by Dvoynenko, Samoylenko and Wang [26

26. M. M. Dvoynenko, I. I. Samoylenko, and J. K. Wang, “Suppressed light transmission through corrugated metal films at normal incidence,” J. Opt. Soc. Am. A 23(9), 2315–2319 (2006). [CrossRef]

]. The symmetric mode corresponds to the low-frequency resonant peak, while the asymmetric mode corresponds to the high-frequency one. They have different dispersion relations. Similar to a flat metallic slab, as the slab thickness is increased, the asymmetric mode undergoes red-shifting, while the symmetric mode experiences blue-shifting [27

27. J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B Condens. Matter 33(8), 5186–5201 (1986). [CrossRef] [PubMed]

, 28

28. P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon. 1(3), 484–588 (2009). [CrossRef]

]. This trend, as demonstrated in Fig. 3, can be interpreted with plasmon hybridization theory [29

29. E. Prodan, C. Radloff, N. J. Halas, and P. Nordlander, “A hybridization model for the plasmon response of complex nanostructures,” Science 302(5644), 419–422 (2003). [CrossRef] [PubMed]

] in which the coupling between these two modes decreases with the increment of d. As a final note, the experimentally observed absorption spectra were broader than that obtained from the calculation. This may be due to the following two facts. Firstly, the surface roughness of the deposited silver films, as shown in the inset of Fig. 1(b), provides extra electron scattering mechanism that facilitates the dephasing of plasmon oscillation and therefore increases the spectral width of plasmon resonance [30

30. H. Hövel, S. Fritz, A. Hilger, U. Kreibig, and M. Vollmer, “Width of cluster plasmon resonances: Bulk dielectric functions and chemical interface damping,” Phys. Rev. B Condens. Matter 48(24), 18178–18188 (1993). [CrossRef] [PubMed]

]. Secondly, the slight non-uniformity in the fabricated hexagonally patterned structure renders an additional inhomogeneous contribution to the overall spectral characteristics. Owing to the spherical symmetry of the hemispheric protrusion in the structure, the electrical field component expressed in spherical coordinate — in which the origin is at the spherical center of the hemisphere (Fig. 1) — is anticipated to reveal its spatial symmetric properties. Figure 4
Fig. 4 Cross-sectional view of radial electric field component of SERS-active substrate deposited with a 15 nm silver film in air at (a) 338 nm and (b) 643 nm. Their respective rescaled field distributions are shown at (c) and (d). The centre of the hemisphere is the origin of the spherical coordinate.
shows a cross-sectional view of calculated radial electric field components of the SERS-active substrate with a 15 nm silver film at the two resonant wavelengths of 338 and 643 nm, corresponding to asymmetric (Erasym) and symmetric (Ersym) modes, respectively. The chosen thickness of the silver over-coated film is thin enough to portray the two distinct plasmon modes that are sustained by such hemispheric structure. Note that Erasym changes phase across the PET-silver and silver-air interfaces along the radial direction, while Ersym changes phase across the silver film. Such distinct characteristics are illustrated in the rescaled distributions in Fig. 4. As a final note, these two plasmon modes are not propagating surface plasmon modes. The excitation of such delocalized electromagnetic modes requires the satisfaction of both the dispersion relation of surface plasmon at the silver-air (or silver-water) interface and the Bragg diffraction condition. The calculation yields a period of >349 nm for both excitation wavelengths (515nm and 633nm), which is larger than the period of our substrates (280 nm).

Figure 5
Fig. 5 Raman spectra of 10−5 M of R6G in water on 25-nm silver-coated nano-imprinted hexagonally patterned SERS-active substrate (red dots) and that of 10−3 M of R6G in water on 25-nm silver-coated flat glass slide (blue dots). The excitation wavelength is 632.8 nm.
shows the Raman spectrum of R6G solution obtained on the SERS-active substrate with a 25-nm silver over-coating film which is compared with that acquired on the flat glass substrate with the same silver coating. Note that the Raman signature of 10−3 M of R6G in water on the silver-coated flat glass substrate is barely recognizable. The Raman enhancement factor is estimated to be larger than 105. The performance of the SERS-active substrate with a 25-nm silver coating in the detection of MG in water is illustrated in Fig. 6
Fig. 6 (a) Mean Raman spectrum of 10−6 M of MG in water averaged over ten spectra measured at ten different locations of single SERS-active substrate. Its standard error, Δ, is shown below. (b) Raman intensity vs. concentration of MG in water. Solid line is a guide to eye.
. The standard error obtained by analyzing ten Raman spectra of 10−6 M of MG in water measured at ten different locations of a typical SERS-active substrate shows less than 2% of percentage error at the most intense Raman peak of 1620 cm−1. MG is a triarylmethane dye with dimethylamino substituents at the para positions of two of the three phenyl rings. In the range from 1100 to 1700 cm−1, nine prominent peaks emerge and match with the peaks observed with normal Raman scattering excited at 514.5 nm [31

31. H. B. Lueck, D. C. Daniel, and J. L. McHale, “Resonance Raman Study of Solvent Effects on a series of Triarylmethane Dyes,” J. Raman Spectrosc. 24(6), 363–370 (1993). [CrossRef]

]. In the concentration curve shown in Fig. 6(b), the recorded Raman intensity at 1620 cm−1 increases linearly with the MG concentration ranging from 10−8 to 10−6 M. The intensity then saturates from 10−6 to 10−5 M, suggesting that the average MG molecule does not enjoy the same amount of Raman enhancement as multilayers of MG molecules accumulate on the substrate surface. This concentration curve represents the detection of MG with this SERS substrate reaching less than 5 μg/L and a dynamical range of 100.

As shown in Fig. 7(b), the SERS-active substrate with a silver film thickness of 25 nm gives the optimal performance in the case of using the 632.8 nm laser as the excitation source in Raman measurements performed in water. In this case, the resonant wavelength of the low-frequency symmetric surface plasmon mode coincides with the excitation wavelength. The calculated electric field intensity, shown in Fig. 8
Fig. 8 (a) Top view of electric field intensity at 630 nm of SERS-active substrate deposited with a 25 nm silver film in water; (b) cross-sectional electric field intensity along the white dashed line in (a); (c) electric field intensity enhancement factor along the dashed line in (b).
, exhibits high values at the gaps between adjacent hemispherical protrusion structures. It is anticipated that the molecules residing at these so-called “hot spots” enjoy the maximal Raman enhancement factor. This scenario can be compared with the case of the SERS substrates based on two-dimensional silver nanoparticle arrays grown on nanochannels in anodic alumina [5

5. H. H. Wang, C. Y. Liu, S. B. Wu, N. W. Liu, C. Y. Peng, T. H. Chan, C. F. Hsu, J. K. Wang, and Y. L. Wang, “Highly Raman-enhancing substrates based on silver nanoparticle arrays with tunable sub-10 nm gaps,” Adv. Mater. 18(4), 491–495 (2006). [CrossRef]

, 36

36. B. Y. Lin, H. C. Hsu, C. H. Teng, H. C. Chang, J. K. Wang, and Y. L. Wang, “Unraveling near-field origin of electromagnetic waves scattered from silver nanorod arrays using pseudo-spectral time-domain calculation,” Opt. Express 17(16), 14211–14228 (2009). [CrossRef] [PubMed]

], except that, in such case, the maximal plasmon-mediated localized field is created by adjacent silver nanoparticles. Similarly, the electrical field intensity in this case, shown in Figs. 8(b) and (c), decays drastically away from the top surface of the silver over-coated film, exhibiting an evanescent character. The intensity enhancement factor at the hot spots of our SERS substrates reaches its maximal value of >1600 at the silver-water interface, as presented in Fig. 8(c), resulting in an expected Raman enhancement factor of more than 2×106. Such theoretical prediction can be compared with the experimental Raman data presented in Fig. 5, in which the experimental enhancement factor confers the average electromagnetic enhancement contribution over the whole silver surface.

4. Conclusions

Surface-enhanced Raman scattering has been successfully demonstrated on silver-coated, two-dimensional hexagonally patterned, hemispherically protruded structures fabricated with nano-imprint on flexible PET substrates. Their optical properties have been analyzed experimentally and investigated with electrodynamic calculation. The surface plasmon polariton modes have been identified with the calculations, and their resonant wavelengths agree well with the experimental results. Based on the acquired absorptance data, we show that the dependence of the SERS signal on the silver film thickness is in good agreement with the corresponding dependence of the calculated electromagnetic enhancement factor. We demonstrate that the SERS detection of malachite green with such substrates can reach 10−8 M, indicating that this technique can potentially become a quick and simple detection method of residual antiseptics in aquaculture industry.

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S. Srivastava, R. Sinha, and D. Roy, “Toxicological effects of malachite green,” Aquat. Toxicol. 66(3), 319–329 (2004). [CrossRef] [PubMed]

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E. Sudova, J. Machova, Z. Svobodova, and T. Vesely, “Negative effects of malachite green and possibilities of its replacement in the treatment of fish eggs and fish: a review,” Vet. Med. 52, 527–539 (2007).

20.

J. L. Allen, J. R. Meinertz, and J. E. Gofus, “Determination of malachite green and its leuco form in water,” J. AOAC Int. 77, 646 (1992).

21.

K. Sagar, M. Smyth, J. Wilson, and K. McLaughin, “High-performance liquid chromatographic determination of the triphenylmethane dye, malachite green, using amperometric detection at a carbon fibre microelectrode,” J. Chromatogr. A 659(2), 329–336 (1994). [CrossRef]

22.

C. H. Tsai, J. D. Lin, and C. H. Lin, “Optimization of the separation of malachite green in water by capillary electrophoresis Raman spectroscopy (CE-RS) based on the stacking and sweeping modes,” Talanta 72(2), 368–372 (2007). [CrossRef] [PubMed]

23.

M.-C. Yang, J.-M. Fang, T.-F. Kuo, D.-M. Wang, Y.-L. Huang, L.-Y. Liu, P.-H. Chen, and T.-H. Chang, “Production of antibodies for selective detection of malachite green and the related triphenylmethane dyes in fish and fishpond water,” J. Agric. Food Chem. 55(22), 8851–8856 (2007). [CrossRef] [PubMed]

24.

T.-L. Chang, K.-Y. Cheng, T.-H. Chou, C.-C. Su, H.-P. Yang, and S.-W. Luo, “Hybrid-polymer nanostructures forming an anti-reflection film using two-beam interference and ultraviolet nanoimprint lithography,” Microelectron. Eng. 86(4-6), 874–877 (2009). [CrossRef]

25.

F. Pigeon, I. F. Salakhutdinov, and A. V. Tishchenko, “Identity of long-range surface plasmons along asymmetric structures and their potential for refractometric sensors,” J. Appl. Phys. 90(2), 852–859 (2001). [CrossRef]

26.

M. M. Dvoynenko, I. I. Samoylenko, and J. K. Wang, “Suppressed light transmission through corrugated metal films at normal incidence,” J. Opt. Soc. Am. A 23(9), 2315–2319 (2006). [CrossRef]

27.

J. J. Burke, G. I. Stegeman, and T. Tamir, “Surface-polariton-like waves guided by thin, lossy metal films,” Phys. Rev. B Condens. Matter 33(8), 5186–5201 (1986). [CrossRef] [PubMed]

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P. Berini, “Long-range surface plasmon polaritons,” Adv. Opt. Photon. 1(3), 484–588 (2009). [CrossRef]

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E. Prodan, C. Radloff, N. J. Halas, and P. Nordlander, “A hybridization model for the plasmon response of complex nanostructures,” Science 302(5644), 419–422 (2003). [CrossRef] [PubMed]

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H. Hövel, S. Fritz, A. Hilger, U. Kreibig, and M. Vollmer, “Width of cluster plasmon resonances: Bulk dielectric functions and chemical interface damping,” Phys. Rev. B Condens. Matter 48(24), 18178–18188 (1993). [CrossRef] [PubMed]

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H. B. Lueck, D. C. Daniel, and J. L. McHale, “Resonance Raman Study of Solvent Effects on a series of Triarylmethane Dyes,” J. Raman Spectrosc. 24(6), 363–370 (1993). [CrossRef]

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OCIS Codes
(240.6680) Optics at surfaces : Surface plasmons
(220.4241) Optical design and fabrication : Nanostructure fabrication
(240.6695) Optics at surfaces : Surface-enhanced Raman scattering

ToC Category:
Optics at Surfaces

History
Original Manuscript: December 21, 2010
Revised Manuscript: February 7, 2011
Manuscript Accepted: February 9, 2011
Published: February 22, 2011

Citation
D. Z. Lin, Y. P. Chen, P. J. Jhuang, J. Y. Chu, J. T. Yeh, and J.-K. Wang, "Optimizing electromagnetic enhancement of flexible nano-imprinted hexagonally patterned surface-enhanced Raman scattering substrates," Opt. Express 19, 4337-4345 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-5-4337


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