Surface Plasmon Polaritons on Silver Gratings for Optimal SERS

Article pubs.acs.org/JPCC

Surface Plasmon Polaritons on Silver Gratings for Optimal SERS Response Yevgeniya Kalachyova,† David Mares,‡ Oleksiy Lyutakov,*,† Martin Kostejn,§ Ladislav Lapcak,∥ and Vaclav Švorčík† †

Department of Solid State Engineering, University of Chemistry and Technology, Prague, Czech Republic Department of Microelectronics, Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic § Institute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Prague, Czech Republic ∥ Central Laboratories, University of Chemistry and Technology, Prague, Czech Republic ‡

ABSTRACT: One of the most important obstacles for the application of surface enhanced Raman spectroscopy (SERS) is the poor reproducibility of SERS active substrates: SERS intensity can be varied from one substrate to another and moreover along the one substrate surface. Reproducible SERS substrate can be prepared through introduction of highly ordered metal array, where light focusing is achieved through excitation of surface plasmon-polaritons (SPPs). In this work, excimer laser patterning of poly(methyl methacrylate) followed by silver evaporation is proposed as an effective way for the creation of reproducible and effective surface plasmon-polaritons (SPP)-based SERS substrate. Detailed theoretical and experimental studies were performed to optimize structure parameter for effective SPP excitation. It was found that the narrow range of grating periodicity and metal thickness exist, where SPPs can be most efficiently excited. Despite the fact that SERS response was almost always achieved, the enhancement factor was found to vary more with the effectivity of SPP excitation. When the real structure parameters were set to optimal for SPP excitation, a SERS enhancement factor was achieved up to four times.

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INTRODUCTION Surface enhanced Raman spectroscopy (SERS) is a subject of extensive research owing to its potential in various practical applications, such as biological sensing and trace analysis,1,2 as well as in situ surveys of chemical and structural information.3 High enhancement of the near-field intensity is the key factor for ultrasensitive SERS realization.4−6 For more effective photon-plasmon energy transfer and light focus, the SERS excitation wavelength must be close to the maximum of the plasmon-related absorption band.7 A common approach of SERS active substrate preparation consists in the deposition of noble metal nanoparticles (NPs) from colloidal suspensions onto suitable substrate.7−11 However, uncontrolled NPs aggregation leads to poor reproducibility of the SERS signal and significantly limits their wide application. Presently, it is more significant and beneficial to control the electromagnetic amplifications on the SERS active substrate in a more consistent and reproducible manner. One way is to engineer substrates with highly regular hot spots distribution.12 This can be achieved by ordered arrangement of NPs in different structures13 or by deposition of thin metal layer onto previously patterned template.14−16 © 2015 American Chemical Society

Template patterning can be done by modification with laser beam,17 optical, or electron beam lithographies, making use of hydrodynamic instabilities18,19 or nanosphere template lithography.20,21 Precise tailoring of template design allows engineering of plasmon modes and tuning of optimal SERS excitation wavelength.22 As a rule, better periodicity of the prime template results in a higher SERS signal intensity and more appropriate signal uniformity.8 Especially interesting are highly ordered 2D gold or silver arrays with subwavelength dimension.23 This periodical metal structures enable one to excite Bloch wave surface plasmon polaritons (SPPs), which “focus” light energy into the close to surface volume. In this case, electric field enhancement occurs not only at the local, surface plasmon related places, but also, because of surface polariton excitation and propagation, it is spread over the surface with a spatially modulated intensity.24 So, large-area electromagnetic enhancement occurs, which strongly contrasts with the traditional “hot spots”. This largeReceived: February 23, 2015 Revised: April 10, 2015 Published: April 10, 2015 9506

DOI: 10.1021/acs.jpcc.5b01793 J. Phys. Chem. C 2015, 119, 9506−9512

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The Journal of Physical Chemistry C

Fourier space. The convergence study was performed by increasing of the number of harmonics and comparison with simulated absorption spectra. We found that the positions and absolute values of absorption maxima obtained with 50 and 100 applied harmonics differ by less than 1%, so 50 harmonics is enough.

area enhancement on strongly periodical metal structures relieves the rigorous requirement of the close-to-hot-spot location of sample molecules.25 In this way, the main disadvantage of SERSits poor reproducibilitycan be overcome. Moreover, by standardization of SERS active substrate and probe deposition methods, SERS can be extended to the level of a quantitative analytical method. In this work, we present potentially reliable SERS substrates composed of silver grating with sinusoidal shape. The periodic disturbances act as diffraction gratings for exciting surface plasmon polaritons (SPPs). Dependences between morphology of periodical template, thickness of the deposited metal film, position of plasmon peak, and SERS enhancement factor were studied from theoretical and experimental points of view.

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RESULTS AND DISCUSSION Periodical metal grating illuminated by light can couple photons into the surface plasmon-polariton mode in the case of matching of the photon and surface plasmon wave vectors. For the fixed grating periodicity, the incident light of certain wavelength can excite SPPs, which travel along the metal− dielectric interface.32−34 Schematically, this process is depicted in Figure 1, where illumination by x-polarized plane wave from

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EXPERIMENTAL SECTION Materials and Samples Preparation. The Fast Reddoped poly(methyl methacrylate) (PMMA) films were prepared by separately dissolving of PMMA, (molecular weight Mw ≈ 1500 K) and the Fast Red ITR (FR) in 1,2dichloroethane. Then, 7.0 wt % PMMA and 2.8 wt % FR solutions were mixed and spin-coated onto glass substrates (supplied by Glassbel Ltd., CZ). The prepared samples were dried under ambient conditions for 24 h. Dried films were patterned by excimer laser under following parameters: angle from surface normal, from 0 to 50°; laser fluency, 12 mJ.cm−2; and number of laser pulses, from 50 to 350. As a result, the periodic surface structures were created on PMMA surface.26−28 Experimental conditions were optimalized to achieve a 50 nm value of grating amplitude (this is maximal value, which can be prepared in our case without disruptance of periodicity). Silver was then deposited onto a patterned surface by vacuum sputtering (DC Ar plasma, gas purity 99.995%, gas pressure of 4 Pa, discharge power of 7.5 W, sputtering time ranging from 5 to 180 s.). The deposition of silver was accomplished from Ag target (purity 99.99%, provided by Safina, CZ). Rhodamine 6G (R6G) was supplied from SigmaAldrich and dissolved in methanol (10−7 M solution). R6G solution was added by dropping solution onto prepared surface and spinning at 1000 rpm for 10 s. Measurement Techniques. Surface properties were analyzed using atomic force microscopy (AFM) using a VEECO CP II device (“tapping” mode, probe RTESPA-CP; spring constant 50 N·m−1). Both surface morphology and phases (materials differences) were extracted from AFM scans. UV−Vis absorption spectra were measured using PerkinElmer’s Lambda 25 UV/vis/NIR Spectrometer in the spectral range from 300−1100 nm at scanning rate of 240 nm·min−1 and a data collection interval of 1 nm. Raman scattering was measured on Nicolet Almega XR spectrometer (Laser power 15 mW) and Advantage NIR (DeltaNu, U.S.A., Laser power 60 mW) Raman spectrometers with 470, 532, and 785 nm excitation wavelength. Spectra were measured 10 times, each of them with 30 s accumulation time. Simulation Techniques. The behavior of a metal nanograting and consequent surface plasmon-polaritons (SPPs) phenomenon was simulated using the RSoft photonic simulation suite. To an efficient, rigorous, and fully vectorial solution of Maxwell’s equations solution in periodic dielectric structures Rigorous Coupled Wave Analysis (RCWA)29,30 algorithm enhanced with Modal Transmission Line (MTL)31 was employed. Applied software chooses a default number of harmonics to expand the refractive index and the field in

Figure 1. Schematic representation of surface plasmon-polariton excitation, propagation, and interaction with analyte molecules. Insert gives the calculated electric field intensity for two-dimensional array of periodical silver structures illuminated by normally incident xpolarized plane wave from air.

air leads to excitation of SPPs. It must be noted that SPPs are shorter in wavelength than the incident light (photons), so light focusing takes place at the small, close to the surface area (see Figure 1, insert). This “focused” mode of electro-magnetic wave propagation can interact with organic molecules deposited onto periodical metal surface and excite their SERS response (Figure 1). Unlike the localized surface plasmon related SERS, excitation of SPPs can “spread” the energy more evenly over the surface, without tying it to a specific hot spot. As a result, more homogeneous and reproducible SERS signals can be obtained. Optimal excitation wavelength depends strongly on the grating periodicity, metal properties, and surface structure arrangements. In our case, we applied the excimer laser surface treatment for the preparation of surface grating, which was further covered by silver. For the changing of the grating periodicity, we perform the polymer modification under a different incidence angle of laser beam. Results for the laser treatment under different angles with respect to the sample surface normal are presented in Figure 2. This method allows preparation of grating with periodicity from 250 to 400 nm. It should be also noted, that periodicity range can be potentially tuned by using another polymer. In the next step, silver was evaporated onto the patterned metal surface. Silver is one of the best materials for SERS because of its high enhancement factor and the wide available 9507

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transmission minimum. However, because plasmon-absorption is wide, this shift can be neglected. Simulated and measured positions of maxima of plasmon absorption peaks as the function of silver film thickness for different periods of grating are plotted in Figure 4. There are two rather different areas regarding to the correlation of measured and calculated data. For “thicker” silver films, there is an apparent discrepancy between both data. This can be explained by silver structure on patterned PMMAfor 5 and 7 nm thick silver films do not form continuous layer and the position of plasmon absorption is determined by the local plasmon resonance at the silver clusters. For the silver films with thicknesses above 10 nm, very good agreement between calculated and real data are observed. From Figure 3, it is also clearly visible that both grating period and silver thickness affect the position of the SPP related absorption maximum. With increasing silver film thickness, the absorption maximum apparently decreases. However, increasing the grating periodicity leads to the shift of absorption maximum to longer wavelengths, such that an optimization of the periodical structure with the aim to excite SPPs as strong as possible must include both geometrical parameters of the prepared template and the thickness of the added metal film. It must also be noted that one should expect efficient excitation of SPP waves with the thickest silver film and not the thinner one. However, efficient SPP excitation depends on the interplay of light absorption, reflection, and transmission. When the silver film is thin, the photon absorption is less probable because of the small interaction distance. Oppositely, when the silver film is too thick, the reflection becomes dominant. So an “optimal” range of silver thicknesses must exist for which the probability of photon reflection and transmission are tolerable, and the probability of photon absorption (and SPPs excitation) is high enough. More detailed optimization of two-dimensional array of periodical silver structures illuminated by perpendicularly incident light for three different wavelengths (470, 532, and 785 nm), typical for Raman spectrophotometer, are presented in Figure 5. In particular, Figure 5 gives the two-dimensional plot, where the efficiency of SPP excitation is evaluated for the range of periods and silver thicknesses. For such analysis, the available thicknesses of continuous silver film (from 10 to 100 nm) and grating periods were taken into account, and after performing the initial simulation, an adjustment of initial parameters into more effective ones was performed. It is evident from Figure 5 that for effective excitation of SPPs by 470 nm wavelength, the grating structure with a periodicity up to 260 nm and with about a 15 nm thick silver layer should be prepared. In the case of the 532 nm excitation wavelength, optimal periodicity is slightly shifted to higher values (up to 275 nm). The silver thickness should be in the 15−20 nm narrow range. For SPP excitations with 785 nm wavelength, separated optimal places in Figure 5 are visible. The best structure for SPP excitation has a grating periodicity of 340 nm and a silver thickness of about 15 nm. SPP excitations are closely related with the effective pumping of light energy into SPP modes and, as can be expected, with stronger SERS response. SERS spectra of a typical dye for surface activity evaluationR6G are shown in Figure 6. Typical peaks of R6G are well visible at 1653 and 1367 cm−1. It is also apparent that the intensities of peaks are rather affected by the SERS active substrate. For the calculation of the SERS effectivity typical parameter, the SERS enhancement factor

Figure 2. AFM images of periodical structures prepared on PMMA thin films patterned by laser beam coming under different angles of incidence relative to the surface normal: (A) 45°; (B) 40°; (C) 35°; (D) 30°; and (E) 20°.

wavelength range. Prepared gratings were covered by silver films of different thicknesses (from 5 to 100 nm). Simultaneously, theoretical simulations of the SPP excitation on prepared structures were performed, and their results were compared with actual measured transmission spectra. The examples of calculated and measured light transmission curves are presented in Figure 3A. Good agreement between

Figure 3. (A) Measured and calculated transmission spectra of silver grating with the following parameters: P = 248 nm, d = 25 nm and (B) expansion of transmission spectrum into reflection and absorption part.

experimental and calculated results is clearly visible. In the case of silver, which is a highly reflective material, it is quite difficult to estimate plasmon-related absorption from transmission spectra, because the light is rather reflected than coupled to the SPP mode. For this reason, Figure 3B also shows the expansion of the transmission spectrum into reflection and absorption spectra. Comparison between Figure 3, parts A and B, indicates that the actual position of the plasmon absorption is slightly red-shifted relative to the 9508

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Figure 4. Calculated and measured plasmon peak position as a function of silver thickness for different periods (P) of grating (constant amplitude, 50 nm).

Figure 5. Theoretical fit of optimal combination of grating period and silver thickness for different SERS excitation wavelength (constant amplitude, 50 nm).

Figure 6. Typical SERS spectra for gratings with different periods (constant amplitude, 50 nm) covered by a 25 nm thick silver layer and excited by a 532 nm laser wavelength.

(EF) was introduced and calculated through the evaluation of signal intensities from SERS-active and passive substrates (flat silicon substrate was used as a reference).35 SERS measurements were performed on the samples with different periodicity and silver thickness for three excitation wavelengths (470, 532, and 785 nm). Results are presented in Table 1. Comparison between the 2D maps (see Figure 5) and the real value of SERS EF clearly indicates very good agreement between calculated optimum of SPPs excitation and effectivity of SERS response. Some disagreement occurs for thinner silver films, where continuous film is not formed. From Table 1, it is evident that the SERS effect almost always appears. However, the SERS enhancement factor mostly depends on structure parameters. In the case of effective SPP excitation, the strongest SERS responses were observed. By varying the periodicity and metal thickness, the enhancement factor can either be increased or compressed by up to 4 orders of magnitude, so that we can conclude that for effective pumping of light energy into SPPs and optimal SERS response, an interplay of grating periodicity, metal thickness, and excitation wavelength must be taken into account.

For comparison, SERS EF measured on the flat silver substrate (silver was evaporated onto nonpatterned polymer) with the appropriate thicknesses were also added in Table 1. As can be expected, the EF of flat silver is higher when a shorter Raman excitation wavelength is used. The EF factor also increases with increasing silver thickness, achieves a maximum value on the 25 nm thick silver layer, and then decreases. In the case where the surface plasmon-polariton is not efficiently excited, the value of EF is on the same order of magnitude as that on the flat substrate. So, the SERS phenomenon in these cases can be attributed to local enhancement of the electric field achieved on the randomly rugate silver layer. The grating amplitude is another parameter that can affect the SERS response. To take this parameter into account, calculation of the maximum absorption value dependent on the silver thickness for different amplitudes with constant periodicity (260 nm) was performed. In this case, values of grating amplitudes were restricted to the 30−50 nm range, where the best (more ordered) periodic structure is achievable. Results are presented in Figure 7. In all cases, the position (silver thickness) of the absorbance maximum is well 9509

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Table 1. SERS Enhancement Factor at Three Excitation Wavelengths As a Function of Silver Thickness and Grating Periodicity silver thickness (nm) excitation wavelength (nm)

periodicity (nm)

5

7

10

15

25

50

100

λex = 785 nm

248 260 270 343 380 flat Ag 248 260 270 343 380 flat Ag 248 260 270 343 380 flat Ag

9 5 4 7 12 4 4 12 41 14 12 5 1902 1319 2262 1654 1411 101

143 328 300 560 240 5 8 23 105 18 16 11 2743 1361 1223 1432 1322 140

960 1150 1170 3490 600 4 19 510 316 150 19 12 3836 1902 381 189 44 170

74 230 325 16 400 2154 9 15 780 14 780 10 518 440 17 14 8851 5627 1026 79 23 190

59 80 95 1105 723 15 1157 11 800 9161 2180 11 27 3509 2977 1811 55 11 180

5 17 18 28 18 9 71 70 107 315 14 25 1402 1235 549 68 14 102

5 12 9 16 12 7 61 34 33 17 12 22 17 15 16 22 27 41

λex = 532 nm

λex = 470 nm

Figure 7. Dependence of calculated absorbance maximum on silver thickness for different grating amplitudes.

Table 2. SERS Enhancement Factor at Three Excitation Wavelengths as a Function of Grating Amplitude amplitude (nm)

EF (λex = 470 nm)

EF (λex = 532 nm)

EF (λex = 785 nm)

30 35 40 45 50

3863 4151 4937 5136 5627

12 232 12 912 13 420 13 972 14 780

855 950 922 971 1150

pronounced and shifts slightly upward with increasing grating amplitude. Simultaneously, the absolute value of the absorption maximum increases. With increasing wavelengths of excitation light, the absorbance peak becomes narrower but the dependence on the silver thickness remains unchanged. This result corresponds well with the data from Table 1. Effect of grating amplitude was also experimentally examined. Series of gratings with the amplitudes from 30 to 50 nm were

prepared, coated by a 15 nm thick silver layer, and examined for SERS response by excitation at three wavelengths. The results are presented in Table 2. In general, experimental results confirm the theoretical simulation, i.e., increasing the grating amplitude leads to more effective excitation of SPP and enhances the SERS signal. Reproducibility of SERS response was checked on the three samples with different periods of grating. The SERS response 9510

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University in Prague under Grant No. SGS14/145/OHK3/2T/ 13

was measured at seven points on each sample and randomly distributed on the SERS active area, and the results were compared. In the cases of 260, 270, and 343 nm grating periods, differences in the data do not exceed 10% of absolute value of the SERS response. Slightly worse results were obtained on the samples with 380 and 248 nm grating periods. In these cases, the SERS response deviates by up to 30% from an average value from different samples. However, good agreement (similar to previous case) along the one sample surface was found. Most of the observed EF in conventional SERS lies in the 104−108 range,17 but the theoretical calculation predicts enhancements of up to 1014 for nanoparticles with precise shape and interparticle gap.36 Such high values are commonly measured on the random arrays of nanoparticles, where their random distribution leads to the emergence of local “optimal” places. Certainly, this approach leads to very high mismatch in the obtained SERS results. An alternative way is the application of advanced techniques, such as electron beam lithography, for the preparation of SERS active substrate with well-defined metal nanostructure shape, interparticle distance, and assembly configuration.37,38 In this case, however, the preparation procedure requires sophisticated equipment, and fabrication of large samples for a common purpose is questionable. In our case, we achieved EF = 105, which is far from the maximum reported in the literature. However, this value was obtained over the whole sample surface, rather than at discrete points. In addition, we believe that further optimization in terms of samples geometry and materials used will further increase the EF.

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CONCLUSIONS Theoretical and experimental investigation of SPP excitation on the two-dimensional periodical silver array was performed with the aim to make SERS response as high as possible. Theoretical study indicates that there are narrow areas of structure parameters (grating periodicity and silver thickness) where SPPs can be more effectively excited. The areas are specific for each wavelength used for SPP excitation. Positions of these areas shift to higher periodicity with increasing excitation wavelength. Optimal SPP excitation leads to the most effective focusing of light energy into close-to-surface areas, and subsequently high SERS response. In general, the SERS response was almost always achieved on our periodical structures. Enhancement factor, however, was found to vary significantly with structure parameters and effectivity of SPPs excitation. It was demonstrated experimentally, that the enhancement factor increases by up to 4 orders of magnitude when the structure parameters achieve optimal values found by our analysis.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by a Grant Agency of the Czech Republic under Project Nos. 14-18131S and 15-19209S and Student Grant Competition of the Czech Technical 9511

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