Mapping the SERS Efficiency and Hot-Spots Localization on Gold Film over NanospheresSubstrates

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 confirmedclass 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 SERSenhancements on gold films over nanospheres (AuFoN) substrates. By constructing SERS imaging mapswith a resolution down to the diffraction limit, the local SERS efficiency is found to vary on two differentscales. First, the local SERS efficiency is periodically modulated (intensity ratios of 2-3) by the periodicAuFoN surface topography (as demonstrated by correlation with atomic force microscopy imaging of thesame sample area); second, randomly distributed SERS hot-spots are observed, at which the SERS intensityis 1 to 2 orders of magnitude larger than at adjacent regions. Furthermore, these hot-spots exhibit fluctuatingbehavior, characteristic of single-molecule SERS sensitivity. These results are particularly useful for furtheringcurrent understanding of SERS on AuFoN substrates. More generally, the SERS maps provide a direct visualdemonstration 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 thecharacterization of ordered noble-metal plasmonic structures.

Introduction

Surface Enhanced Raman Scattering (SERS) is an opticalspectroscopic technique which amplifies the sensitivity ofclassical Raman spectroscopy, by exploiting the enhancedelectromagnetic (EM) fields associated to surface plasmons (SP)excitation on noble metal nanostructures. SERS is of greatinterest in chemistry, biochemistry, and biophysics,1 especiallydue to its capability to detect and identify very low analyteconcentrations. In particular, short-range electromagnetic inter-actions at so-called “hot spots” have led to enormous Ramanenhancement factors, allowing detection and identification ofsingle molecules under ambient conditions.2-4

SERS investigations are performed both with colloidalnanoparticles in the liquid phase and with solid nanostructuredmetallic films supported by a substrate. Being robust, stable,and reproducible in most cases, solid SERS substrates arepreferred over solution-phase nanoparticles and their aggregatesfor many specific applications. Most of the efficiently used solidSERS-active substrates, like etched noble metal electrodes,island films, or colloidal particles mono- and multilayers, exhibitrandom surface texture, which obscure the identification ofprecise morphological details which determine the high EM fieldenhancement.5,6 On the contrary, regular arrays of nanoparticlesand periodically patterned thin films are more promisingcandidates for such investigations.7 Ordered SERS substratesconsisting of regular arrays of noble metal nanoparticles orperiodically patterned thin films benefit in their turn from highreproducibility and a controlled morphology. These allow foridentification of correlations between morphology, optical/plasmonic properties, and SERS activity of nanostructures, andthus for a rational development of SERS applications.

A particular class of proposed SERS substrates are metal filmsover nanospheres (MFoN), which are obtained by evaporatingnoble-metal films over a self-organized ordered array of colloidalnanospheres.8,9 Extensive work in Van Duyne’s group demon-strated that these type of SERS substrates are very efficient,robust and highly stable, for example, to extremely negativepotential excursions.8 Previous work in our group demonstratedthat, although their efficiency depends on the laser excitationenergy, MFoN substrates are SERS-active and exploitable overa broad spectral range, from visible to NIR.9 Moreover, we haveshown that the plasmonic response of MFoN can be tuned bythe proper choice of polystyrene sphere diameter, and that theirapplicability can be extended also to molecular fluorescenceenhancement.10 The high applicative potential of MFoN SERSsubstrates has also been demonstrated; AgFoN substrates wereapplied to SERS-based detection of lactate,11 glucose12 oranthrax spores.13

In this paper, we investigate the spatial distribution of theSERS efficiency on AuFoN substrates. By combining scanningconfocal SERS with atomic force microscopy (AFM), weprovide correlations between local SERS activity and localtopography of the periodically structured Au film. Surprisingly,the SERS signal intensity distribution maps show also theexistence of multiple hot-spots, randomly located over thesurface, but always in between neighboring metallic half-shells.The time evolution of SERS spectra at these hot-spots is alsomonitored. By electrodynamics simulations, we attempt toattribute the higher enhancement at the hot spots to the particularlocal geometry of the metallic crevice formed by adjacent Auhalf-shells.

Experimental Methods

SERS Substrate Preparation. Ordered two-dimensionalarrays of polystyrene spheres were prepared onto a glass support

* To whom correspondence should be addressed. E-mail: (C.F.)cfarcau@phys.ubbcluj.ro; (S.A.) sastil@phys.ubbcluj.ro.

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

10.1021/jp100861w 2010 American Chemical SocietyPublished on Web 06/17/2010

by a convective self-assembly technique, employing a home-built apparatus, as described previously.14 Glass slide substrateswere treated in piranha solution (3:1 mixture of 95% H2SO4/30% H2O2) for 24 h followed by rinsing with copious amountsof ultrapure water (18MΩ), until a neutral pH is obtained. Weused polystyrene microspheres (450 nm diameter and ∼2.4%size dispersion) from Polysciences, which were concentratedby centrifugation to 10% prior to deposition. The AuFoN SERSsubstrates 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. Areference flat Au film is simultaneously obtained on the samesubstrate due to metal deposition on substrate region free ofpolystyrene spheres. After metallization, the sample was incu-bated with 1 mM p-aminothiophenol in methanol solution for3 h, followed by rinsing the sample in pure methanol, to ensuredesorption of nonspecifically bounded molecules. The well-known high chemical affinity of thiol groups (-SH) to the metalensures the formation of a monolayer of pATP molecules onthe gold surface.15

Optical and Morphological Characterization. Opticalreflectance measurements were performed at quasi-normalincidence (∼8°) on a Jasco V-530 UV-vis spectrophotometer,using an interchangeable reflectivity modulus. AFM measure-ments were performed in intermittent contact mode. The AFMhead is mounted on the same Witec microscope used for Ramanmeasurements on the objectives turret; therefore, by rotatingthis latter, one can easily switch from spectroscopic to AFMmode, without physically moving or transferring the sample.Scanning electron microscopy (SEM) (JEOL JSM-5510) wasalso employed to observe the morphology of the samples.

SERS Measurements. SERS measurements were performedunder the 632.8 nm excitation line of a He-Ne laser, on aWiTec alpha300 R Raman microscope. For SERS imaging, theRaman signal was collected through a confocal pinhole of 25µm diameter by using 1.4 NA oil-immersion objective of 100×magnification. The setup provides a spatial resolution down tothe optical diffraction limit (∼300 nm). A total of 4096 recordedSERS spectra were acquired within the scan range of 4 × 4µm divided into an acquisition grid of 64 spectra × 64 lines.For a laser power of 0.6 mW measured at the microscopeentrance, the integration time was 0.4 s/spectrum. The software-controlled piezoscanner allows us to address the desiredparticular locations immediately after the SERS imaging scanof a selected area is finished. Time series of SERS spectra werealso recorded at specific locations on the sample with anacquisition time of 0.4 s/spectrum.

Results

Structure and Optical Properties of SERS Substrate. Thepolystyrene template for SERS substrate fabrication was orga-nized onto a glass surface by a dragging technique in whichthe drying front of a droplet of aqueous microsphere solutionis swept across a microscope glass slide. Typically, thecrystallization process leads to the formation of adjacent two-dimensional (2D) single-domain colloidal crystals ranging froma few to a hundred of micrometers in width. The SERSsubstrates were produced by thermal evaporation of gold filmon top of as prepared layer of polystyrene microspheres.Roughness analysis yields values of Rq ) 3.7 nm for the flatAu film and of 38 nm for the AuFON. Note that on the AuFONthe value is so large due to the periodic corrugation imposedby the spheres. The structure of fabricated SERS substrate isquite complex due to the superimposition of two metallic

gratings consisting of interconnected16 gold half-shells (HSs)(right inset in Figure 1) on the upper side of the polymerictemplate and truncated tetrahedral particles on the glass slide(left inset in Figure 1); the latter is directly deposited throughthe openings between the polystyrene spheres. In contrast withthe well-known reflectivity of a flat film, the AuFoN structuredfilm exhibits a deep and broad reflectivity dip (see Figure 2),which is a strong indicator of plasmon activity, favorable forthe enhancement of the Raman signal. It was indeed shown byprevious studies that by tuning the reflectivity minimumaccording to the laser excitation, the efficiency of MFoN canbe optimized.17 As seen in Figure 2 the reflectivity minimum iswell centered between the excitation laser line and the spectralrange of the SERS spectrum.

Averaged SERS Response. As a first step, we checked theSERS activity of as prepared substrate under the 632.8 nmexcitation laser line by using a low numerical aperture objective(NA ) 0.4) and a pinhole of 100 µm diameter for collectingthe signal. Several spectra were recorded on different single-crystalline domains of the AuFoN array. Characteristic Ramanbands of pATP, such as the 1081 cm-1 > C-S stretching andthe 1589 cm-1 -CdC- stretching vibrations18 are present inthe spectra. Spectral positions of the bands are stable, whilesome variations of their intensities can be observed, but are

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

Figure 2. Reflectivity of AuFoN (100 nm Au film deposited over anarray of 450 nm polystyrene microspheres); the position of maximumplasmon absorption falls in between the wavelength of the laser (dottedline) and that of Stokes scattered photons (hatched area).

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however comprised within a (15% interval. No SERS signalcould be collected from the reference flat Au film, confirmingthe important role of surface topography imparted by the spherestemplate, and the negligible role of film surface roughness forthe SERS enhancement. Note also that pATP is a nonresonantmolecule. According to the procedure described in ref 9, a bulkenhancement factor (EF) of about 106 was estimated from theseinvestigations, a level of SERS activity commonly observed inmany experimental systems.19

Mapping the Local SERS Response. Further we proceedto a microscopic investigation, aimed at correlating the localSERS activity and local topography. Figure 3a shows the AFMtopography (intermittent contact mode) of investigated crystal-line SERS substrate. The region marked by the rectangle inFigure 3a was actually scanned to generate a SERS image. Theselected sample area intentionally includes a crystallizationdefect 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 SERSimages. The most prominent Raman bands located at 1081 and1589 cm-1 (see Figure 4a), known to be enhanced at theexcitation laser line mainly by the electromagnetic mechanism,were used to probe the strength of the local electromagneticfield. Figure 3b shows the overall SERS image reconstructedby plotting the distribution of 1081 cm-1 Raman band intensityover the scanned area. The reconstructed SERS enhancementmap exhibits a regular pattern of lateral periodicity whichcorrelates perfectly with the AFM topography in Figure 3a. Overthe whole scanned area, the intensity of SERS bands is aboutthree times larger at the interstitial region between half-metalizedspheres than at the pole of the HSs (see the cross sections inFigure 3c). Note however that these variations lie probablywithin a much larger interval, the value obtained here beinglowered by the averaging over a large number of molecules inthe area covered by the focal spot. It is worth mentioning thatthe recorded SERS image is reproducible and many similarpatterns were registered from different areas on the substrate.

The SERS enhancement map in Figure 3b suggests that highlyenhanced EM fields are localized around the rims and atjunctions between HSs. The found distribution of the SERSenhancements is in good agreement with the distribution of EMfields found by Garcia-Vidal and Pendry20 through electrody-namics calculations on metallic crevices with a similar cross-sectional profile. Recent theoretical studies have shown thatisolated half-shell structures can support several plasmonresonances that are responsible for the high-field enhancementat their circular thinned edge.21 As the metallic HSs are here in

close contact, the local electromagnetic field in between the HSsis expected to increase at the sharp crevices formed by adjacentHSs. The localization of enhanced EM fields in the gaps betweenarrayed metallic nanoshells has been demonstrated also byelectromagnetic simulations.22 In our case, the metallic HSs areplaced in a periodic arrangement and between each HS and itssix neighborings a metallic bridge is formed. It is known thatwhen metallic nanoparticles are brought into physical contactan abrupt redistribution of plasmon modes can occur.23 More-over, the existence of two metallic gratings, interconnected HSsand truncated tetrahedral particles, spatially separated by the

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

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

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layer of polystyrene spheres, makes it difficult to accuratelyassign the individual contributions from each other.

In addition, the SERS distribution map in Figure 3b exhibitsa number of very high intensity bright spots, which appearexclusively in between adjacent metallic HSs and never on topof them. The half-maximum widths of about 250-300 nmcorrespond with the diffraction limited resolution. We assumethat 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 ordersof magnitude higher in the hot-spot than in the spectrumcollected outside of this area (see Figure 4a), from here resultinga measured EF of about 108, if compared with the low numericalaperture objective measurement. Previous theoretical studieshave shown24 that the actual electromagnetic EF experiencedby individual molecules placed at the center of the hot-spot couldbe 200-300 times larger than the measured EF. It is thereforereasonable that the local electromagnetic EF in hot-spots canreach values of 1010 and such giant electromagnetic EF allowsthe detection of single molecules. Several theoretical andexperimental studies have described highly localized electro-magnetic fields (hot-spots) at the junction of metal nanoparticles,where a local EF as high as 1010-1011 can operate at a level ofSERS activity able to detect single molecules.25 Moreover, itwas recently demonstrated26 for a similar SERS substrate (silver-coated microspheres), exhibiting an average SERS EF of 8.5× 105, that about 24% of the total measured signal is given byless than 0.007% of the total molecules, which are subjected tolocal EFs larger than 109. However the work cited above didnot identify the spatial location of the highest enhancementfactors.

Time-Evolution of SERS at Hot-Spots. In the next step,we proceed to monitor and analyze the time evolution of brightsites in Figure 3b. Time series of SERS spectra were recordedwith an acquisition time of 0.4s/spectrum and revealed fluctuat-ing behaviors of some peak intensities and peak/peak intensityratios (Figure 4b). Since the metallic structure was stable andhighly reproducible, the electromagnetic contribution to SERSshould remain constant if the illumination is controlled. Instudying time-lapse pATP spectra, we found that the modes at1081 and 1590 cm-1 are not only the most prominent bands inthe spectrum but they are also the most stable over time. On

the contrary, the bands at 1140, 1389, 1432 cm-1, which aremuch less intense, exhibited dramatic intensity swings. Thedistinct behaviors of the two classes of bands (almost steady-state and “blinking”) could be attributed to the particularsymmetry of the vibrational modes relative to the polarizationof the giantly enhanced EM fields. Indeed the bands at 1081and 1590 are due to totally symmetric a1 vibrations,27 theirRaman scattering being not very polarized, while the othermentioned bands are of b2 symmetry and their scattering is morepolarized. Therefore the former band intensities are practicallyinsensitive to molecular orientation fluctuations, while the latterchange more dramatically. Similar fluctuating behavior has beenreported in literature and assigned to SERS signature of singlemolecules, which are rendered visible only in the presence ofstrongly enhanced local fields. Although the recorded spectrumaverages over a large number of molecules, it is dominated byspectral fluctuations of a small number of molecules adsorbedat 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 geometriesof highly active sites. We compute electromagnetic fieldsdistribution on dimers of Au HSs on top of polystyrene spheres.Because of the heavy computational requirements of full three-dimensional (3D) FDTD simulations, we performed two-dimensional FDTD modeling of the electric field enhancement.30

Thus we obtain the distribution of electric fields in a planesectioning the two Au HSs through their poles and centers. AFMmicroscopy does not offer the possibility to measure the angleformed in the crevice by the two adjacent gold caps, or toidentify nanometric gaps. Thus, since most of the caps areinterconnected,16 we inquire about the hot-spot geometry bystudying theoretically the effect of the sharpness of the crevicebetween the two HSs. For simulation, we used a Drude dielectricfunction for bulk gold31 and a refractive index of 1.55 for theunderlying polystyrene spheres. Commercially available soft-ware, Lumerical FDTD Solutions, was used. Figure 5 showsthe distribution of near field intensity (|E|2 relatively to E0) atwavelength 632.8 nm for two selected cases: a relatively large(40 nm) and a small (5 nm) overlap (measured on the directioncontaining their centers) between the two Au HSs. Note thatthese different overlappings were introduced to modify the angle

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

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of the crevice formed by the two Au HSs. First observation isthat enhanced EM fields are located at the junction betweenthe two Au HSs, which is in good agreement with the measureddistribution of SERS intensities. The electric field intensityapproaches values of 103 and 104 in the case of large and smalloverlaps, respectively. If converted into Raman enhancementfactors, they yield values of 106 and 108. These results alreadyprovide qualitative agreement with the experimental measuredenhancement factors. Therefore we suggest that a very sharpangle of intersection (i.e., very sharp crevice) between neighbor-ing Au HSs could provide favorable geometrical circumstancesfor electromagnetic fields localization and very large SERSenhancements. Detailed investigations of all the parametersinfluencing the simulation results are beyond the scope of thepresent work, and will be conducted in future studies.

Discussion

Corroborating the experimental and theoretical results pre-sented above, we point out that most of the SERS signal onAuFoN substrates originates from the excitation of plasmonsconfined around the rims and in junctions between intercon-nected Au HSs. However, the presence of hot-spots suggeststhe existence of rare geometrical configurations with a muchhigher electromagnetic enhancement. Generally, only twoclasses of systems have been predicted and demonstrated sofar in the literature to produce superenhancing locations: pairsof nanoparticles32 or large fractal aggregates33 in which the hotspots arise from the symmetry breaking that occurs when thefractal cluster that possesses scaling symmetry is excited withan electromagnetic field that does not.34 Here we provide forthe first time an experimental evidence of the localization ofhot-spots between periodic arrays of metallic features with lateralsize comparable with the laser wavelength. However, theexistence of hot-spots on such periodic metallic microstructuresshould not be so surprising. In fact some particular geometricalconfigurations of inter-HSs junctions could be fabricated andselected from a large heterogeneous population of interstices,that is, either when the edges of HSs are in very close vicinityor they form a very sharp angle of intersection. Such a particularnanomorphology is beneficial for a strong enhancement andlocalization of plasmon fields, required for the observed intenseSERS signals. One could argue that surface roughness aroundthe cap edge can be responsible for the presence of hot-spots.This would be sustained by the decreasing thickness of theevaporated metallic film toward the half-shell rim. We outrulethis posibility, because this type of roughness, if existing, wouldbe present on all the spheres all across the sample’s surface. Ifislandlike particles would be formed during the Au evaporation,they would be present on each Au cap in the sample, aroundits circumference. Therefore, we believe that enhancement dueto roughness around the cap edge is not compatible with thelow density of observed hot-spots.

In the case of weakly interacting nanoparticles arranged inregular arrays, that is, interparticle distance larger than particledimensions, it was experimentally shown that all particlescontribute equally to the Raman signal, and the enhanced EMfields are located at the particles sites.35 On the other hand, forregular arrays of closely spaced nanoparticles it was demon-strated that interparticle EM coupling effects dominate theRaman enhancement.36,37 Theoretical models suggested thatsome periodic arrays of nanoparticles could exhibit even higherSERS efficiency because retardation or damping effects are lesscritical when compared to randomly nanostructured metalsurfaces.38,39 The results in this paper complete the above

findings by a direct experimental demonstration of the EM fieldslocalization, and its dependence on topological surface features.We wish also to emphasize the important role played by orderedSERS substrates (possessing a reproducible and controlledtopography) in the identification of correlations between optical/plasmonic properties, topography, and SERS activity.

Conclusion

To conclude, we combined scanning confocal Raman spec-troscopy, atomic force microscopy, and electromagnetic fieldcalculations on AuFoN to show that the surface-enhancedRaman signal is strongly correlated with the local topography.Mostly enhanced EM field is confined at the edges and sharpcrevices between adjacent metallic HSs. Some crevices exhibitmuch larger field enhancements and approach single-moleculesensitivity in SERS. Besides, the SERS maps offer an unam-biguous, visual confirmation that in SERS only a small fractionof the total exposed metallic surface provides the major part ofthe enhancement, and measured SERS signals. Such detailedknowledge of the SERS enhancements distribution, with respectto topography, should prove useful for more correct futureevaluations of SERS enhancement factors and a more rationaldesign of efficiency-optimized SERS substrates. In particular,these results can be useful for a deeper understanding of SERSon AuFoN substrates. From a practical point of view, as forexample in a SERS-based sensing chip, the results can impactthe proper selection of the working microscope objective in orderto achieve a good trade-off between sampled area (small NA) larger sampled area ) more hot-spots) and collectionefficiency (high NA ) smaller sampled area ) less hot-spots).More generally, by mapping the distribution of SERS enhance-ments we provided an image of the plasmon fields’ localizationon two-dimensional plasmonic nanostructure. The ability to mapthe local EM field distribution by simple far-field techniques isuseful for designing plasmonic-photonic microstructures withdesired spectroscopic functionality.

Acknowledgment. This work was supported by CNCSIS-UEFISCSU, 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.; Abraha-

msson, 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.

SERS Enhancements on Gold Films over Nanospheres J. Phys. Chem. C, Vol. 114, No. 27, 2010 11721

(17) Farcau, C.; Astilean, S. J. Optoelectron. AdV. Mater. 2007, 9, 772.(18) Hu, X.; Wang, T.; Wang, L.; Dong, S. J. Phys. Chem. C 2007,

111, 6962.(19) Ko, H.; Singamaneni, S.; Tsukruk, V. V. Small 2008, 4, 1576.(20) Garcia-Vidal, F. J.; Pendry, J. B. Phys. ReV. Lett. 1996, 77, 1163.(21) Cortie, M.; Ford, M. Nanotechnology 2007, 18, 235704.(22) Le, F.; Brandl, D. W.; Urzhumov, Y. A.; Wang, H.; Kundu, J.;

Halas, N. J.; Aizpurua, J.; Nordlander, P. ACS Nano 2008, 2, 707.(23) Rechberger, W.; Hohenau, A.; Leitner, A.; Krenn, J. R.; Lamprecht,

B.; Aussenegg, F. R. Opt. Commun. 2003, 220, 137.(24) Le Ru, E. C.; Etchegoin, P. G.; Meyer, M. J. Chem. Phys. 2006,

125, 204701.(25) Xu, H.; Aizpurua, J.; Kall, M.; Apell, P. Phys. ReV. E 2000, 62,

4318.(26) Fang, Y; Seong, N.-H.; Dlott, D. D. Science 2008, 321, 388.(27) Yoon, J. H.; Park, J. S.; Yoon, S. Langmuir 2009, 25, 12475.(28) Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K.

J. Am. Chem. Soc. 2007, 129, 1658.(29) Ward, D. R.; Grady, N. K.; Levin, C. S.; Halas, N. J.; Wu, Y.;

Nordlander, P.; Natelson, D. Nano Lett. 2007, 7, 1396.(30) Saj, W. M. Opt. Express 2005, 13, 4818.

(31) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1972, 6, 4370.(32) Camden, J. P.; Dieringer, J. A.; Wang, Y.; Masiello, D. J.; Marks,

L. D.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2008, 130, 12616.(33) Jiang, J.; Bosnick, K.; Maillard, M.; Brus, L. J. Phys. Chem. B

2003, 107, 9964.(34) Gresillon, S.; Aigouy, L.; Boccara, A. C.; Rivoal, J. C.; Quelin,

X.; Desmarest, C.; Gadenne, P.; Shubin, V. A.; Sarychev, A. K.; Shalaev,V. M. Phys. ReV. Lett. 1999, 82, 4520.

(35) Laurent, G.; Felidj, N.; Grand, J.; Aubard, J.; Levi, G.; Hohenau,A.; Aussenegg, F. R.; Krenn, J. R. Phys. ReV. B 2006, 73, 245417.

(36) Gunnarsson, L.; Bjerneld, E. J.; Xu, H.; Petronis, S.; Kasemo, B.;Kall, M. Appl. Phys. Lett. 2001, 78, 802.

(37) Lee, S. J.; Guan, Z.; Xu, H.; Moskovits, M. J. Phys. Chem. C 2007,111, 17985.

(38) Genov, D. A.; Sarychev, A. K.; Shalaev, V. M.; Wei, A. NanoLett. 2004, 4, 153.

(39) Wang, Z. B.; Luk’yanchuk, B. S.; Guo, W.; Edwardson, S. P.;Whitehead, D. J.; Li, L.; Liu, Z.; Watkins, K. G. J. Chem. Phys. 2008,128, 094705.

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