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J. Phys. Chem. C 2010, 114, 11717–11722

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Mapping the SERS Efficiency and Hot-Spots Localization on Gold Film over Nanospheres Substrates Cosmin Farcau* and Simion Astilean* Faculty of Physics, Babes¸-Bolyai UniVersity, M. Kogalniceanu 1, 400084, Cluj-Napoca, Romania ReceiVed: January 28, 2010; ReVised Manuscript ReceiVed: May 30, 2010

Noble metal films deposited over two-dimensional arrays of polystyrene nanospheres constitute a confirmed class of efficient and cost-effective substrates for surface enhanced Raman scattering (SERS). In this paper, we perform scanning confocal SERS microscopy to investigate the spatial (lateral) variations of the SERS enhancements on gold films over nanospheres (AuFoN) substrates. By constructing SERS imaging maps with a resolution down to the diffraction limit, the local SERS efficiency is found to vary on two different scales. First, the local SERS efficiency is periodically modulated (intensity ratios of 2-3) by the periodic AuFoN surface topography (as demonstrated by correlation with atomic force microscopy imaging of the same sample area); second, randomly distributed SERS hot-spots are observed, at which the SERS intensity is 1 to 2 orders of magnitude larger than at adjacent regions. Furthermore, these hot-spots exhibit fluctuating behavior, characteristic of single-molecule SERS sensitivity. These results are particularly useful for furthering current understanding of SERS on AuFoN substrates. More generally, the SERS maps provide a direct visual demonstration that in SERS only a fraction of the metallic surface yields the major part of the SERS scattering. The evidence of clear correlations between SERS enhancement and topography can be relevant for the characterization of ordered noble-metal plasmonic structures. Introduction Surface Enhanced Raman Scattering (SERS) is an optical spectroscopic technique which amplifies the sensitivity of classical Raman spectroscopy, by exploiting the enhanced electromagnetic (EM) fields associated to surface plasmons (SP) excitation on noble metal nanostructures. SERS is of great interest in chemistry, biochemistry, and biophysics,1 especially due to its capability to detect and identify very low analyte concentrations. In particular, short-range electromagnetic interactions at so-called “hot spots” have led to enormous Raman enhancement factors, allowing detection and identification of single molecules under ambient conditions.2-4 SERS investigations are performed both with colloidal nanoparticles in the liquid phase and with solid nanostructured metallic films supported by a substrate. Being robust, stable, and reproducible in most cases, solid SERS substrates are preferred over solution-phase nanoparticles and their aggregates for many specific applications. Most of the efficiently used solid SERS-active substrates, like etched noble metal electrodes, island films, or colloidal particles mono- and multilayers, exhibit random surface texture, which obscure the identification of precise morphological details which determine the high EM field enhancement.5,6 On the contrary, regular arrays of nanoparticles and periodically patterned thin films are more promising candidates for such investigations.7 Ordered SERS substrates consisting of regular arrays of noble metal nanoparticles or periodically patterned thin films benefit in their turn from high reproducibility and a controlled morphology. These allow for identification of correlations between morphology, optical/ plasmonic properties, and SERS activity of nanostructures, and thus for a rational development of SERS applications. * To whom correspondence should be addressed. E-mail: (C.F.) [email protected]; (S.A.) [email protected].

A particular class of proposed SERS substrates are metal films over nanospheres (MFoN), which are obtained by evaporating noble-metal films over a self-organized ordered array of colloidal nanospheres.8,9 Extensive work in Van Duyne’s group demonstrated that these type of SERS substrates are very efficient, robust and highly stable, for example, to extremely negative potential excursions.8 Previous work in our group demonstrated that, although their efficiency depends on the laser excitation energy, MFoN substrates are SERS-active and exploitable over a broad spectral range, from visible to NIR.9 Moreover, we have shown that the plasmonic response of MFoN can be tuned by the proper choice of polystyrene sphere diameter, and that their applicability can be extended also to molecular fluorescence enhancement.10 The high applicative potential of MFoN SERS substrates has also been demonstrated; AgFoN substrates were applied to SERS-based detection of lactate,11 glucose12 or anthrax spores.13 In this paper, we investigate the spatial distribution of the SERS efficiency on AuFoN substrates. By combining scanning confocal SERS with atomic force microscopy (AFM), we provide correlations between local SERS activity and local topography of the periodically structured Au film. Surprisingly, the SERS signal intensity distribution maps show also the existence of multiple hot-spots, randomly located over the surface, but always in between neighboring metallic half-shells. The time evolution of SERS spectra at these hot-spots is also monitored. By electrodynamics simulations, we attempt to attribute the higher enhancement at the hot spots to the particular local geometry of the metallic crevice formed by adjacent Au half-shells. Experimental Methods SERS Substrate Preparation. Ordered two-dimensional arrays of polystyrene spheres were prepared onto a glass support

10.1021/jp100861w  2010 American Chemical Society Published on Web 06/17/2010

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by a convective self-assembly technique, employing a homebuilt apparatus, as described previously.14 Glass slide substrates were treated in piranha solution (3:1 mixture of 95% H2SO4/ 30% H2O2) for 24 h followed by rinsing with copious amounts of ultrapure water (18MΩ), until a neutral pH is obtained. We used polystyrene microspheres (450 nm diameter and ∼2.4% size dispersion) from Polysciences, which were concentrated by centrifugation to 10% prior to deposition. The AuFoN SERS substrates were produced by thermal evaporation of gold film (100 nm thick, as monitored by quartz crystal microbalance) on top of as-prepared layer of polystyrene nanospheres. A reference flat Au film is simultaneously obtained on the same substrate due to metal deposition on substrate region free of polystyrene spheres. After metallization, the sample was incubated with 1 mM p-aminothiophenol in methanol solution for 3 h, followed by rinsing the sample in pure methanol, to ensure desorption of nonspecifically bounded molecules. The wellknown high chemical affinity of thiol groups (-SH) to the metal ensures the formation of a monolayer of pATP molecules on the gold surface.15 Optical and Morphological Characterization. Optical reflectance measurements were performed at quasi-normal incidence (∼8°) on a Jasco V-530 UV-vis spectrophotometer, using an interchangeable reflectivity modulus. AFM measurements were performed in intermittent contact mode. The AFM head is mounted on the same Witec microscope used for Raman measurements on the objectives turret; therefore, by rotating this latter, one can easily switch from spectroscopic to AFM mode, without physically moving or transferring the sample. Scanning electron microscopy (SEM) (JEOL JSM-5510) was also employed to observe the morphology of the samples. SERS Measurements. SERS measurements were performed under the 632.8 nm excitation line of a He-Ne laser, on a WiTec alpha300 R Raman microscope. For SERS imaging, the Raman signal was collected through a confocal pinhole of 25 µm diameter by using 1.4 NA oil-immersion objective of 100× magnification. The setup provides a spatial resolution down to the optical diffraction limit (∼300 nm). A total of 4096 recorded SERS spectra were acquired within the scan range of 4 × 4 µm divided into an acquisition grid of 64 spectra × 64 lines. For a laser power of 0.6 mW measured at the microscope entrance, the integration time was 0.4 s/spectrum. The softwarecontrolled piezoscanner allows us to address the desired particular locations immediately after the SERS imaging scan of a selected area is finished. Time series of SERS spectra were also recorded at specific locations on the sample with an acquisition time of 0.4 s/spectrum. Results Structure and Optical Properties of SERS Substrate. The polystyrene template for SERS substrate fabrication was organized onto a glass surface by a dragging technique in which the drying front of a droplet of aqueous microsphere solution is swept across a microscope glass slide. Typically, the crystallization process leads to the formation of adjacent twodimensional (2D) single-domain colloidal crystals ranging from a few to a hundred of micrometers in width. The SERS substrates were produced by thermal evaporation of gold film on top of as prepared layer of polystyrene microspheres. Roughness analysis yields values of Rq ) 3.7 nm for the flat Au film and of 38 nm for the AuFON. Note that on the AuFON the value is so large due to the periodic corrugation imposed by the spheres. The structure of fabricated SERS substrate is quite complex due to the superimposition of two metallic

Farcau and Astilean

Figure 1. SEM image of the AuFoN SERS substrate. (Left inset) SEM image of an array of truncated tetrahedral Au nanoparticles formed on the glass support. (Right inset) SEM image of an array of interconnected Au HSs, as seen from the concave side; inset images were obtained on a sacrificial sample.

Figure 2. Reflectivity of AuFoN (100 nm Au film deposited over an array of 450 nm polystyrene microspheres); the position of maximum plasmon absorption falls in between the wavelength of the laser (dotted line) and that of Stokes scattered photons (hatched area).

gratings consisting of interconnected16 gold half-shells (HSs) (right inset in Figure 1) on the upper side of the polymeric template and truncated tetrahedral particles on the glass slide (left inset in Figure 1); the latter is directly deposited through the openings between the polystyrene spheres. In contrast with the well-known reflectivity of a flat film, the AuFoN structured film exhibits a deep and broad reflectivity dip (see Figure 2), which is a strong indicator of plasmon activity, favorable for the enhancement of the Raman signal. It was indeed shown by previous studies that by tuning the reflectivity minimum according to the laser excitation, the efficiency of MFoN can be optimized.17 As seen in Figure 2 the reflectivity minimum is well centered between the excitation laser line and the spectral range of the SERS spectrum. Averaged SERS Response. As a first step, we checked the SERS activity of as prepared substrate under the 632.8 nm excitation laser line by using a low numerical aperture objective (NA ) 0.4) and a pinhole of 100 µm diameter for collecting the signal. Several spectra were recorded on different singlecrystalline domains of the AuFoN array. Characteristic Raman bands of pATP, such as the 1081 cm-1 > C-S stretching and the 1589 cm-1 -CdC- stretching vibrations18 are present in the spectra. Spectral positions of the bands are stable, while some variations of their intensities can be observed, but are

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Figure 3. (a) AFM micrograph of AuFoN SERS substrate. (b) False-color map obtained by plotting the intensities of the pATP SERS spectra collected inside the area marked by the rectangle in A. (c) Cross-section profiles through the AFM (lower graph, right scale) and SERS (upper graph, left scale) maps, along the lines marked in A and B.

however comprised within a (15% interval. No SERS signal could be collected from the reference flat Au film, confirming the important role of surface topography imparted by the spheres template, and the negligible role of film surface roughness for the SERS enhancement. Note also that pATP is a nonresonant molecule. According to the procedure described in ref 9, a bulk enhancement factor (EF) of about 106 was estimated from these investigations, a level of SERS activity commonly observed in many experimental systems.19 Mapping the Local SERS Response. Further we proceed to a microscopic investigation, aimed at correlating the local SERS activity and local topography. Figure 3a shows the AFM topography (intermittent contact mode) of investigated crystalline SERS substrate. The region marked by the rectangle in Figure 3a was actually scanned to generate a SERS image. The selected sample area intentionally includes a crystallization defect to serve as a control mark for positioning the scanner. The Raman signal was collected in a confocal scanning mode, through a 100× magnification objective, to generate SERS images. The most prominent Raman bands located at 1081 and 1589 cm-1 (see Figure 4a), known to be enhanced at the excitation laser line mainly by the electromagnetic mechanism, were used to probe the strength of the local electromagnetic field. Figure 3b shows the overall SERS image reconstructed by plotting the distribution of 1081 cm-1 Raman band intensity over the scanned area. The reconstructed SERS enhancement map exhibits a regular pattern of lateral periodicity which correlates perfectly with the AFM topography in Figure 3a. Over the whole scanned area, the intensity of SERS bands is about three times larger at the interstitial region between half-metalized spheres than at the pole of the HSs (see the cross sections in Figure 3c). Note however that these variations lie probably within a much larger interval, the value obtained here being lowered by the averaging over a large number of molecules in the area covered by the focal spot. It is worth mentioning that the recorded SERS image is reproducible and many similar patterns were registered from different areas on the substrate. The SERS enhancement map in Figure 3b suggests that highly enhanced EM fields are localized around the rims and at junctions between HSs. The found distribution of the SERS enhancements is in good agreement with the distribution of EM fields found by Garcia-Vidal and Pendry20 through electrodynamics calculations on metallic crevices with a similar crosssectional profile. Recent theoretical studies have shown that isolated half-shell structures can support several plasmon resonances that are responsible for the high-field enhancement at their circular thinned edge.21 As the metallic HSs are here in

Figure 4. (a) pATP SERS spectra at three specific points: on top of Au HS (black), in between adjacent HSs (blue), and at hot-spot (red). (b) Waterfall plot of time-series (integration time 0.4 s/spectrum) of SERS spectra at a selected hot-spot.

close contact, the local electromagnetic field in between the HSs is expected to increase at the sharp crevices formed by adjacent HSs. The localization of enhanced EM fields in the gaps between arrayed metallic nanoshells has been demonstrated also by electromagnetic simulations.22 In our case, the metallic HSs are placed in a periodic arrangement and between each HS and its six neighborings a metallic bridge is formed. It is known that when metallic nanoparticles are brought into physical contact an abrupt redistribution of plasmon modes can occur.23 Moreover, the existence of two metallic gratings, interconnected HSs and truncated tetrahedral particles, spatially separated by the

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Figure 5. (Top) Schematic representation of the simulated structure with a gold HS over a dielectric sphere. (Bottom) Maps of |E|2 distribution obtained from 2D FDTD simulations; Au HSs overlapping 40 nm (left) and 5 nm (right) in the x direction.

layer of polystyrene spheres, makes it difficult to accurately assign the individual contributions from each other. In addition, the SERS distribution map in Figure 3b exhibits a number of very high intensity bright spots, which appear exclusively in between adjacent metallic HSs and never on top of them. The half-maximum widths of about 250-300 nm correspond with the diffraction limited resolution. We assume that these distinct bright spots are effective SERS “hot-spots”, as frequently invoked in randomly organized nanostructures. The intensity of Raman band at 1081 cm-1 is almost 2 orders of magnitude higher in the hot-spot than in the spectrum collected outside of this area (see Figure 4a), from here resulting a measured EF of about 108, if compared with the low numerical aperture objective measurement. Previous theoretical studies have shown24 that the actual electromagnetic EF experienced by individual molecules placed at the center of the hot-spot could be 200-300 times larger than the measured EF. It is therefore reasonable that the local electromagnetic EF in hot-spots can reach values of 1010 and such giant electromagnetic EF allows the detection of single molecules. Several theoretical and experimental studies have described highly localized electromagnetic fields (hot-spots) at the junction of metal nanoparticles, where a local EF as high as 1010-1011 can operate at a level of SERS activity able to detect single molecules.25 Moreover, it was recently demonstrated26 for a similar SERS substrate (silvercoated microspheres), exhibiting an average SERS EF of 8.5 × 105, that about 24% of the total measured signal is given by less than 0.007% of the total molecules, which are subjected to local EFs larger than 109. However the work cited above did not identify the spatial location of the highest enhancement factors. Time-Evolution of SERS at Hot-Spots. In the next step, we proceed to monitor and analyze the time evolution of bright sites in Figure 3b. Time series of SERS spectra were recorded with an acquisition time of 0.4s/spectrum and revealed fluctuating behaviors of some peak intensities and peak/peak intensity ratios (Figure 4b). Since the metallic structure was stable and highly reproducible, the electromagnetic contribution to SERS should remain constant if the illumination is controlled. In studying time-lapse pATP spectra, we found that the modes at 1081 and 1590 cm-1 are not only the most prominent bands in the spectrum but they are also the most stable over time. On

the contrary, the bands at 1140, 1389, 1432 cm-1, which are much less intense, exhibited dramatic intensity swings. The distinct behaviors of the two classes of bands (almost steadystate and “blinking”) could be attributed to the particular symmetry of the vibrational modes relative to the polarization of the giantly enhanced EM fields. Indeed the bands at 1081 and 1590 are due to totally symmetric a1 vibrations,27 their Raman scattering being not very polarized, while the other mentioned bands are of b2 symmetry and their scattering is more polarized. Therefore the former band intensities are practically insensitive to molecular orientation fluctuations, while the latter change more dramatically. Similar fluctuating behavior has been reported in literature and assigned to SERS signature of single molecules, which are rendered visible only in the presence of strongly enhanced local fields. Although the recorded spectrum averages over a large number of molecules, it is dominated by spectral fluctuations of a small number of molecules adsorbed at the sites with highest enhancements.28,29 Electromagnetic Field Distribution by FDTD Calculations. In the following, we provide by finite difference time domain (FDTD) calculations further insight into the possible geometries of highly active sites. We compute electromagnetic fields distribution on dimers of Au HSs on top of polystyrene spheres. Because of the heavy computational requirements of full threedimensional (3D) FDTD simulations, we performed twodimensional FDTD modeling of the electric field enhancement.30 Thus we obtain the distribution of electric fields in a plane sectioning the two Au HSs through their poles and centers. AFM microscopy does not offer the possibility to measure the angle formed in the crevice by the two adjacent gold caps, or to identify nanometric gaps. Thus, since most of the caps are interconnected,16 we inquire about the hot-spot geometry by studying theoretically the effect of the sharpness of the crevice between the two HSs. For simulation, we used a Drude dielectric function for bulk gold31 and a refractive index of 1.55 for the underlying polystyrene spheres. Commercially available software, Lumerical FDTD Solutions, was used. Figure 5 shows the distribution of near field intensity (|E|2 relatively to E0) at wavelength 632.8 nm for two selected cases: a relatively large (40 nm) and a small (5 nm) overlap (measured on the direction containing their centers) between the two Au HSs. Note that these different overlappings were introduced to modify the angle

SERS Enhancements on Gold Films over Nanospheres of the crevice formed by the two Au HSs. First observation is that enhanced EM fields are located at the junction between the two Au HSs, which is in good agreement with the measured distribution of SERS intensities. The electric field intensity approaches values of 103 and 104 in the case of large and small overlaps, respectively. If converted into Raman enhancement factors, they yield values of 106 and 108. These results already provide qualitative agreement with the experimental measured enhancement factors. Therefore we suggest that a very sharp angle of intersection (i.e., very sharp crevice) between neighboring Au HSs could provide favorable geometrical circumstances for electromagnetic fields localization and very large SERS enhancements. Detailed investigations of all the parameters influencing the simulation results are beyond the scope of the present work, and will be conducted in future studies. Discussion Corroborating the experimental and theoretical results presented above, we point out that most of the SERS signal on AuFoN substrates originates from the excitation of plasmons confined around the rims and in junctions between interconnected Au HSs. However, the presence of hot-spots suggests the existence of rare geometrical configurations with a much higher electromagnetic enhancement. Generally, only two classes of systems have been predicted and demonstrated so far in the literature to produce superenhancing locations: pairs of nanoparticles32 or large fractal aggregates33 in which the hot spots arise from the symmetry breaking that occurs when the fractal cluster that possesses scaling symmetry is excited with an electromagnetic field that does not.34 Here we provide for the first time an experimental evidence of the localization of hot-spots between periodic arrays of metallic features with lateral size comparable with the laser wavelength. However, the existence of hot-spots on such periodic metallic microstructures should not be so surprising. In fact some particular geometrical configurations of inter-HSs junctions could be fabricated and selected from a large heterogeneous population of interstices, that is, either when the edges of HSs are in very close vicinity or they form a very sharp angle of intersection. Such a particular nanomorphology is beneficial for a strong enhancement and localization of plasmon fields, required for the observed intense SERS signals. One could argue that surface roughness around the cap edge can be responsible for the presence of hot-spots. This would be sustained by the decreasing thickness of the evaporated metallic film toward the half-shell rim. We outrule this posibility, because this type of roughness, if existing, would be present on all the spheres all across the sample’s surface. If islandlike particles would be formed during the Au evaporation, they would be present on each Au cap in the sample, around its circumference. Therefore, we believe that enhancement due to roughness around the cap edge is not compatible with the low density of observed hot-spots. In the case of weakly interacting nanoparticles arranged in regular arrays, that is, interparticle distance larger than particle dimensions, it was experimentally shown that all particles contribute equally to the Raman signal, and the enhanced EM fields are located at the particles sites.35 On the other hand, for regular arrays of closely spaced nanoparticles it was demonstrated that interparticle EM coupling effects dominate the Raman enhancement.36,37 Theoretical models suggested that some periodic arrays of nanoparticles could exhibit even higher SERS efficiency because retardation or damping effects are less critical when compared to randomly nanostructured metal surfaces.38,39 The results in this paper complete the above

J. Phys. Chem. C, Vol. 114, No. 27, 2010 11721 findings by a direct experimental demonstration of the EM fields localization, and its dependence on topological surface features. We wish also to emphasize the important role played by ordered SERS substrates (possessing a reproducible and controlled topography) in the identification of correlations between optical/ plasmonic properties, topography, and SERS activity. Conclusion To conclude, we combined scanning confocal Raman spectroscopy, atomic force microscopy, and electromagnetic field calculations on AuFoN to show that the surface-enhanced Raman signal is strongly correlated with the local topography. Mostly enhanced EM field is confined at the edges and sharp crevices between adjacent metallic HSs. Some crevices exhibit much larger field enhancements and approach single-molecule sensitivity in SERS. Besides, the SERS maps offer an unambiguous, visual confirmation that in SERS only a small fraction of the total exposed metallic surface provides the major part of the enhancement, and measured SERS signals. Such detailed knowledge of the SERS enhancements distribution, with respect to topography, should prove useful for more correct future evaluations of SERS enhancement factors and a more rational design of efficiency-optimized SERS substrates. In particular, these results can be useful for a deeper understanding of SERS on AuFoN substrates. From a practical point of view, as for example in a SERS-based sensing chip, the results can impact the proper selection of the working microscope objective in order to achieve a good trade-off between sampled area (small NA ) larger sampled area ) more hot-spots) and collection efficiency (high NA ) smaller sampled area ) less hot-spots). More generally, by mapping the distribution of SERS enhancements we provided an image of the plasmon fields’ localization on two-dimensional plasmonic nanostructure. The ability to map the local EM field distribution by simple far-field techniques is useful for designing plasmonic-photonic microstructures with desired spectroscopic functionality. Acknowledgment. This work was supported by CNCSISUEFISCSU, project number PNII-ID_PCCE_129/2008. References and Notes (1) Porter, M. D.; Lipert, R. J.; Siperko, L. M.; Wang, G.; Narayanan, R. Chem Soc. ReV. 2008, 37, 1001. (2) Nie, S. M.; Emory, S. R. Science 1997, 275, 1102. (3) Michaels, A. M.; Nirmal, M.; Brus, L. E. J. Am. Chem. Soc. 1999, 121, 9932. (4) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667. (5) Wang, Z.; Pan, S.; Krauss, T. D.; Du, H.; Rothberg, L. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 8638. (6) Kalkan, A. K.; Fonash, S. J. Appl. Phys. Lett. 2006, 89, 233103. (7) Grand, J.; Lamy de la Chapelle, M.; Bijeon, J. L.; Adam, P. M.; Vial, A.; Royer, P. Phys. ReV. B 2005, 72, 033407. (8) Dick, L. A.; McFarland, A. D.; Haynes, C. L.; Van Duyne, R. P. J. Phys. Chem. B 2002, 106, 853. (9) Baia, L.; Baia, M.; Popp, J.; Astilean, S. J. Phys. Chem. B 2006, 110, 23982. (10) Farcau, C.; Astilean, S. Appl. Phys. Lett. 2009, 95, 193110. (11) Shah, N. C.; Lyandres, O.; Walsh, J. T.; Glucksberg, M. R.; Van Duyne, R. P. Anal. Chem. 2007, 79, 6927. (12) Ranjit Yonzon, C.; Haynes, C. L.; Zhang, X.; Walsh, J. T.; Van Duyne, R. P. Anal. Chem. 2004, 76, 78. (13) Zhang, X.; Zhao, J.; Whitney, A. V.; Elam, J. W.; Van Duyne, R. P. J. Am. Chem. Soc. 2006, 128, 10304. (14) Kuttesch, A.; Farcau, C.; Neda, Z.; Astilean, S. Proc. SPIE 2007, 6785, 67850O. (15) Loren, A.; Engelbrektsson, J.; Eliasson, C.; Josefson, M.; Abrahamsson, J.; Abrahamsson, K. Nano Lett. 2004, 4, 309. (16) Zhan, P.; Wang, Z.; Dong, H.; Sun, J.; Wu, J.; Wang, H.-T.; Zhu, S.; Ming, N.; Zi, J. AdV. Mater. 2006, 18, 1612.

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