Article pubs.acs.org/JPCC
Surface-Enhanced Resonance Raman Scattering on Gold Concentric Rings: Polarization Dependence and Intensity Fluctuations Gustavo F. S. Andrade,†,§ Qiao Min,‡ Reuven Gordon,*,‡ and Alexandre G. Brolo*,† †
Department of Chemistry, University of Victoria, P.O. Box 3065, Victoria, B.C., Canada V8W 3V6 Department of Electrical and Computer Engineering, University of Victoria, P.O. Box 3055, Victoria, B.C., Canada V8W 3P6
‡
ABSTRACT: The spatial and temporal dependence of the surface-enhanced resonance Raman scattering (SERRS) from Nile Blue A (NBA) adsorbed on concentric rings on a gold film was probed. The SERRS intensity mapping of the concentric rings, obtained using linearly polarized excitation, was well-fitted by a sin2 θ function. This spatial variation of SERRS intensity from the nanostructure was in good qualitative agreement with finite-difference time-domain numerical calculations. Histograms of the time-dependent SERRS intensities showed a normal-distribution for concentrations of NBA in the micromolar range and a tailed distribution for diluted solutions (nanomolar range). Time-dependent fluctuations of SERRS intensities were observed when the concentrations of NBA reached 200 nM. The maximum relative SERRS intensity and its absolute variation, obtained from the experiments with diluted solutions, was much lower than previously observed from random substrates that support SERRS, such as colloids immobilized on glass. This suggests that organized nanostructures, such as the concentric nanorings, present a more uniform distribution of regions of localized enhanced electric field.
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INTRODUCTION Nanometric metallic rings are a paradigmatic system in plasmonics.1 The focusing properties of single rings milled on Ag and Au have been explored recently because of the possibility of enhancing the longitudinal electric field at the center of the structure.2−7 Several groups have showed that the engineering of multiple ring structures led to constructive interference of the plasmon waves at the center of the motifs.8−12 The localizing properties of the concentric rings can result in a Bessel-shaped beam for excitation with radially polarized excitation or in two peaks around the center of the rings for a linearly polarized excitation.5,6 Nanostructures milled in Au thin films by focused ion beam (FIB) have been used as surface-enhanced Raman scattering (SERS) substrates13 and as surface plasmon resonance (SPR)based biosensors.14,15 The SERS performance of arrays of nanoholes in gold films is strongly modulated by the size and shape of the nanostructures.16,17 The focusing properties of the concentric ring arrays were recently combined to the increased local field enhancement caused by a double-hole structure, resulting in a high-performance SERS substrate.17 In this work, we present the evaluation of the performance of concentric rings on Au films as substrates for surface-enhanced resonance Raman scattering (SERRS) using a linearly polarized laser excitation. Mapping of the spatial distribution of SERRS intensities for this structure shows a strong dependence of the Raman efficiency on the relative orientation of the grooves for a given laser polarization. The SERRS intensity dependence was probed for two perpendicular laser polarization orientations, and it was fitted by sin2 θ functions. The experimental results © 2011 American Chemical Society
qualitatively agreed with finite-difference time-domain (FDTD) numerical calculations. The SERRS performance of the concentric ring structure with low concentrations of Nile Blue A (NBA) showed temporal fluctuations. These timedependent SERS intensities fluctuations, for a 200 nM NBA solution, presented a tailed distribution of intensities, which is characteristic of a regime where just a few molecules are probed at the regions of large local field intensities of the nanostructure.18,19
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EXPERIMENTAL SECTION The concentric rings were milled in a thermally deposited 300 nm Au thin film by FIB. The details of the experimental procedure can be found in a previous work.14 Figure 1 shows the optical and the SEM image of one of the concentric rings. The width and the depth of the grooves are 435 and 100 nm, respectively, and the periodicity is 870 nm, with an 870 nm diameter central circle. The SERRS spectra were obtained with the sample mounted on the computer-driven motorized stage of a Renishaw inVia microspectrometer and using the 632.8 nm line of a HeNe laser as exciting radiation. The results are indicated as SERRS because the NBA dye presents an absorption band at 625 nm (resonance Raman effect). For the spatial mapping, the SERRS spectra were obtained in a predefined area of the sample using 1 μm steps. All experiments were performed with the sample Received: September 9, 2011 Revised: December 30, 2011 Published: December 31, 2011 2672
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Figure 2 presents a polar plot of the SERRS intensity average calculated for a series of angles θ relative to the x axis and the
Figure 1. (A) Optical microscopy of a concentric rings structure milled on a 300 nm thick gold film. The ring periodicity is 870 nm, and the strip width is 435 nm. The scale bar is for 20 μm. (B) Scanning electron microscopy image of the rings. The scale bar is 20 μm. 2D-mapping of the surface-enhanced Raman scattering intensity of the 594 cm−1 band for 1 μM Nile Blue on the Au concentric rings structure for the laser light polarized (C) in the y direction and (D) in the x direction, as indicated in panel A. The spectra were obtained at each 1 μm in both x and y directions.
immersed in aqueous solutions of the NBA dye. Figure 1A presents the optical micrograph of the concentric rings, viewed using a 63× (NA = 0.75) water-immersion objective together with a definition of an axis system for the discussion of the SERRS results.
Figure 2. . (A) Representative SERRS spectrum of 10 μM Nile Blue dye on a high-intensity point of the substrate (peak positions (in cm−1) are indicated in the spectrum). (B) Polar plot of the fitting of a sin2 θ function (red line) to the average intensity for different angles relative to the x axis (circles).
RESULTS AND DISCUSSION Figure 1C present the SERRS mapping (1 μm steps in both x and y directions) of the structure shown in Figure 1A immersed in a 1 μM NBA solution for the polarization of the laser light along the y axis. Figure 1D is the result when the laser polarization was set to be parallel to the x axis. Both SERRS mappings, in Figure 1C,D, show a “butterfly”-like distribution of SERRS intensities around the center of the concentric rings microstructure but with a 90° shift relative to each other. The relative orientation of the nanostructures toward the incident polarized laser light changes with the curvature of the rings, leading to the spatial variation of SERRS intensities shown in Figures 1C,D. Taking one section of the concentric rings illuminated by the laser light, the structure within that section can be approximated as a few straight gold strips. (Assuming a 3 μm2 laser spot, it will be, on average, three Au strips illuminated in each step of the mapping.) Using this approximation, it is possible to visualize that the concentric rings act as a series of small straight lines that slowly change orientation relative to the laser polarization with the curvature of the ring. Because the SERRS intensity from nanowires is expected to be at a maximum when their smaller axis is parallel to the laser polarization, a butterfly-like pattern is expected. This pattern is expected to shift 90° when the laser polarization is turned 90°, as observed in Figure 1C,D.
fitting of a sin2θ function to the experimental data. The sin2 θ function fits well the experimental results, and this dependence agrees with previous results for scratched Au films,20−23 for Agoriented nanowires,20 and for Ag-aggregated nanoparticles.24 It should be noted that there is some deviation from the sin2 θ fit and that the SERRS intensity does not vanish for the 0° orientation relative to the x axis, as expected. These discrepancies can be attributed to defects on the Au rings surface that cause additional field localization, leading to polarization-independent SERRS activity; the most important defects are the thin gold film roughness (both intrinsic or caused by redepositions during the FIB milling process), although other possible sources of defects may also play a minor role, such as the implantation of Ga+ ions during the FIB milling.25 The polarization-independent field enhancement caused by random surface roughness has been proposed as one possible additional mechanism of enhancement for nanostructures milled on metallic films.26 The spatial control of the SERRS intensity obtained using the concentric rings may be used to eliminate the solution phase spectrum from SERRS measurement, as it was proposed for anisotropic scratches on Au films.23,27 This can be achieved in the concentric rings substrate by measuring the SERRS spectrum in two regions: the substrate would produce the
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plasmons at the excitation frequency are driven by a polarized source leading to an efficient coupling when the proper polarization is used. The second contribution arises from a radiative enhancement modulated by the presence of the excited metallic surface close to the molecular dipole, which enhances the scattered field.29 However, the Raman scattering is isotropic; therefore, only a fraction of the scattered field possesses the proper polarization to couple efficiently to the surface plasmons of the oriented nanostructures. An important agreement between the experimental data (Figure 1) and the simulations (Figure 3) is that strong SERRS signal is predicted from the central area of the pattern. This result could be expected based on the well-known grating effect of the concentric rings that will direct the incident field toward the center of the feature, enhancing additionally the SERRS performance;17 in the present experimental setup, the laser beam has an area of 3 mm2, and only the first few rings around the center would contribute to this grating effect. The experimental results indicated a slight increase in the SERRS intensity of NBA at the central portion of the rings relative to other parts of the feature (as can be observed in Figure 1C,D). Therefore, we decided to focus on that central area next and explore the efficiency of the local field intensity for this fabricated structure. An interesting aspect of SERRS is related to the timedependent fluctuations in intensities observed from diluted solutions. These fluctuations have been linked to the behavior of a small number of molecules (even single molecules) visiting regions of strongly enhanced local field intensity. The regions of high curvature present a much larger SERRS performance (more field localization) than different regions of the substrate, which correlates with the maximum enhancement factor (EF).29 It should be pointed out that the most efficient hot spots found in the SERS literature are those from gaps between metallic nanostructures.30−32 Early observations of the SERRS intensity fluctuations were from Ag aggregates, which are considered to be a very efficient SERRS substrate. Recently, similar time dependence has been found from other types of substrates,19,33 confirming that these fluctuations are not unique for a particular type of nanostructured platform. This means that even substrates that do not present a very large average SERRS EF might also have a few regions of highly enhancing electric field that support the intensity fluctuations. Most of the time-dependent fluctuations of SERRS and SERS intensities reported so far were from random substrates (metallic aggregates, roughened electrodes, and so on).34 Figure 4 presents the histograms for the timedependent SERRS intensity fluctuations of the 594 cm−1 band of NBA for the concentric ring structure. A 1500 time-sequence of SERRS spectra was taken from the same spot at the center of the concentric ring structure to generate each histogram. The experiments were performed under the conditions of “high” concentration of NBA, 1 μM, and also using 200 nM NBA solutions. The statistical procedure for the construction of the histograms was reported in a previous work.19 For the 1 μM solution, the intensity distribution maximizes around the average, with fwhm = 0.4, which can be considered to be a narrow distribution around the average intensity. The intensity histogram for this solution is well-represented by a log-normal function, as can be seen by the fitting of this function to the experimental data (black line in Figure 4A). The log-normal curve has been proposed for the fitting of the SERRS intensity fluctuations in the high-coverage limit.19,35 The laser spot
maximum SERRS intensity in one of the regions and the minimum SERRS intensity in the other. Both measurements should be realized with the substrate immersed in a solution containing the potentially interfering substances. The subtraction of the last spectrum from the first should be enough to remove the interference from the solution species.27 One advantage of the concentric rings substrate is that both polarization measurements may be acquired without necessity of polarization modulation or reorientation of the substrate, although a background correction scheme can be devised by taking spectra at the two polarization directions from the region of highest anisotropy. In this case, the spectrum obtained with the polarization that produces the largest SERRS should be subtracted from the one obtained with the polarization that suppresses the SERRS. Figure 3 presents the profile of the fourth power of the electric field in the presence of the nanostructure normalized by
Figure 3. Profile of the FDTD-calculated fourth power of the electrical field (normalized by the field in the absence of the nanostructure) at the surface of Au concentric rings with 870 nm separation excited at 632.8 nm polarized in the x direction. The simulation region in this Figure is 30 × 30 μm2.
the field in the absence of the nanostructure ((E/E0)4). The results for Figure 3 were calculated by the FDTD numerical method for different excitation positions on 30 × 30 um2 central area of concentric rings. An x-polarized Gaussian source was chosen at 632.8 nm wavelength, and a power monitor was placed on the gold surface to record all field components. The simulation was set with the same parameters as the experiment (Figure 2). It can be noticed that the FDTD simulation result shows strong electric field in the area where the ring curves are perpendicular to the laser polarization and weak electric field when the ring curves are parallel to the excitation polarization, which agrees well with the “butterfly” pattern in the Raman mapping result (Figure 1C,D). Note that there are still some quantitative discrepancies in amplitude scales between the simulation and the experiment results, which are caused by the imperfection of the fabrication, which leads to the surface roughness and inaccuracy. The SERS intensity is generally approximated as being proportional to the fourth power of the electric field localized at the metallic surface (E4);28 hence, the polarization dependence should follow a fourth power dependence of the sine of the angle between the light polarization and the nanostructure. This was not the case shown in Figure 2, and it can be attributed to a more significant contribution to the field enhancement at the excitation frequency rather than at the scattered frequency, as discussed in ref 17. Basically, the surface 2674
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efficiency compared with random structures.13 The strongest electric field intensities in these structures are localized on the edges of the rings because these are the regions with higher curvature. Because the dimensions of the region of field localization are expected to be smaller than the laser spot and most of the SERRS intensity comes from the molecules adsorbed on those regions,35 the actual number of molecule being probed is much smaller than the estimated number obtained considering only the diameter of the laser spot. The maximum SERRS intensity obtained for the concentric ring structure immersed in 200 nM solution of NBA is 3.8× the average. In experiments with random substrates, such as aggregated metallic nanoparticles, SERRS signal much larger than average is observed for a few events for two reasons: (1) there is a large spatial distribution of hot-spot efficiency in a random substrate and (2) the most efficient hot spots are rare.19,35 For the concentric rings reported here, the maximum SERRS intensity is not as large compared with the average value because the regions of high electric field localization are more equivalent in a fabricated substrate (compared with random substrates, such as colloids). Random gold nanostructures possess the advantage of highperformance for SERS and SERRS, but they still have to face some serious issues for application: (1) the poor repetitiveness and the (2) difficulties for control, which are the very merits of well-designed, organized nanostructures. However, these advantages of fabricated substrate come to a cost in terms of average enhancement for the substrate reported here. We demonstrated here that even with a lower average enhancement a fabricated substrate is still efficient enough to sustain similar intensity fluctuations, as observed in single-molecule experiments. Moreover, it is shown that the SERRS intensity from fabricated substrates can be affected by both the substrate periodic patterns and the excitation polarization. It should also be noted that surface roughness plays different roles in random substrates and designed (fabricated) substrates. In the former case, surface roughness contributes to the enhancement because it is the surface feature responsible for the localized plasmon resonance, and it is key to the efficiency of the hot spots.37,38 For well-patterned periodic substrates, surface roughness and imperfection provide additional channels for the scattering of the propagating surface plasmon, resulting in decreased SERS and SERRS signals. For instance, this could explain some of the amplitude discrepancies between the experimental results and the simulations presented in this work. The average SERRS signal of the concentric ring substrate presented here is not as strong as some random substrates,36,39,40 but it can probably be improved by using better fabrication methods that induce less surface roughness and by adding more sophisticated structures that allow stronger field localization.17,41
Figure 4. Histogram of the band at 594 cm−1 of Nile Blue intensities for 1500 SERRS spectra for the dye concentrations: (A) 1 μM and (B) 200 nM. The accumulation time for each spectrum was 1 s for (A) and 3 s for (B). The black line in panel A is a log-normal fitting to the intensity histogram.
probes the distribution of a large amount of adsorbed molecules, and the SERRS signal observed is an average for all molecules within the illuminated area.18 Similar behavior has been recently verified experimentally for R6G adsorbed on electrochemically roughened Ag surfaces.19 The SERRS measurement from the concentric rings substrate immersed in 200 nM NBA solution shows a wider range of intensity fluctuations, as demonstrated in the histogram presented in Figure 4B. The strong variation in the observed values of the SERRS intensities was in agreement to the results from diluted solutions of dyes adsorbed in aggregated colloids. The SERRS intensity distribution from diluted solutions of NBA (Figure 4B) presents a great number of events with intensity close to zero and no detectable maximum around the average. The distribution is tailed, with a maximum intensity ca. 3.8× the average. The type of fluctuation shown in Figure 4B, characterized by a large number of spectra with a very low intensity and some spectra with intensity much greater than the average, is well-known in the SERS literature, and this type of behavior have been linked to single-molecule experiments. Under our experimental conditions, the number of molecules within the laser spot during the intensity fluctuations is estimated, based on the assumption of a Langmuir behavior, as previously reported,36 to be of the order of at least tens of thousands. (The number of molecules within the laser spot was estimated considering a 3 μm2 spot and a saturation coverage of 10−11 mol/cm2. The adsorption constant was considered to be 105, and the concentration of the dye was 200 nM.) It should be pointed out that the concentric rings are a substrate with ca. 100× lower average SERRS
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CONCLUSIONS
We reported a concentric ring SERRS substrate showing a “butterfly” shape Raman mapping pattern dependent on the polarization of the exciting radiation, which can be fit to sin2 θ distribution. This substrate also shows a time-dependent SERRS and the tailed distribution of SERRS intensities in low concentration, which are results previously linked to single molecule SERS events. 2675
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AUTHOR INFORMATION
Corresponding Author
*Tel: (250) 721-7167. Fax: (250) 721-7147. E-mail: agbrolo@ uvic.ca;
[email protected]. Present Address
́ Departamento de Quimica, Universidade Federal de Juiz de Fora, Brazil. §
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ACKNOWLEDGMENTS We thank Mr. Jonathan Rudge for help in obtaining the SEM images. This work was supported by operating grants from NSERC and by the NSERC Strategic Network for Bioplasmonic Systems (BiopSys), Canada. The equipment grant was provided by the Canada Foundation for Innovation (CFI), the British Columbia Knowledge and Development Fund (BCKDF), and the University of Victoria through the New Opportunities Program. G.F.S.A. thanks Canadian Bureau for International Education - Department of Foreign Affairs and International Trade (CBIE-DFAIT) of Canada for a postdoctoral fellowship.
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