Surface-Enhanced Raman Scatteringfrom Individual Au Nanoparticles andNanoparticle Dimer SubstratesChad E. Talley, †,§ Joseph B. Jackson, |,# Chris Oubre, |,# Nathaniel K. Grady, ⊥,#

Christopher W. Hollars, †,§ Stephen M. Lane, ‡,§ Thomas R. Huser, †,‡,§

Peter Nordlander, |,# and Naomi J. Halas* ,⊥,#

Chemistry and Materials Science Directorate and Physics and AdVanced TechnologiesDirectorate, Lawrence LiVermore National Laboratory, 7000 East AVenue LiVermoreCalifornia 94550, NSF Center for Biophotonics Science and Technology, UniVersity ofCalifornia, DaVis, 4800 Second AVenue, Suite 2600, Sacramento, California 95817,and Department of Physics and Astronomy, Department of Electrical and ComputerEngineering, and Laboratory for Nanophotonics, Rice UniVersity, 6100 Main Street,Houston, Texas 77005

Received May 18, 2005; Revised Manuscript Received June 17, 2005

ABSTRACT

Surface-enhanced Raman scattering (SERS) intensities for individual Au nanospheres, nanoshells, and nanosphere and nanoshell dimerscoated with nonresonant molecules are measured, where the precise nanoscale geometry of each monomer and dimer is identified throughin situ atomic force microscopy. The observed intensities correlate with the integrated quartic local electromagnetic field calculated for eachspecific nanostructure geometry. In this study, we find that suitably fabricated nanoshells can provide SERS enhancements comparable tonanosphere dimers.

It is now well known that the plasmon resonances ofilluminated metallic substrates provide the intense opticalfrequency fields responsible for the electromagnetic contri-bution to surface-enhanced Raman scattering (SERS).1-5

When metallic nanoparticles are used as SERS substrates,dramatic variations in the degree of enhancement, often bymany orders of magnitude, have been observed. Thisvariation has been attributed to the random formation oflocalized plasmons or “hot spots” at the junctions betweennanoparticles, giving rise to large enhancements that enableSERS detection at or near single molecule sensitivity.1,4,6-9

The potential for structural control of the large local fieldsof junction plasmons has generated much interest in thefabrication, assembly, and properties of structures withnanoscale gaps.10,11 A pair of closely spaced metallicnanoparticles supports “dimer” plasmons that can be viewedas the bonding and antibonding hybridization of the indi-

vidual nanoparticle plasmons, in analogy with eigenstatesof homonuclear diatomic molecules.6

Developments in the fabrication and synthesis of Au andAg nanostructures of controlled size, composition, and shapehave led to a class of well-defined nanostructures whoseplasmon resonant frequencies can be manipulated throughcontrol of the geometry of the individual nanoparticles.12-14

One such nanoparticle is a dielectric core surrounded by athin metallic shell, known as a nanoshell. Metallic nanoshellspossess optically excitable plasmon resonances that can betuned throughout the visible and into the infrared regions ofthe electromagnetic spectrum by changing their aspect ratio,r1/r2, wherer1 and r2 are the inner and outer radius of themetallic shell layer.14-17 In this case, the internal geometryof the individual nanoparticle controls its plasmon resonance,thus controlling its local electromagnetic field. The SERSresponse of individual nanoshells covered with nonresonantmolecules has been shown to be a function of their aspectratio.18,19

Correlating the optical response of specific individualnanoparticles with their nanoscale structure permits a moredetailed understanding of the optical properties of nanopar-ticles and enables a closer connection with theory.3,20-32 Thusfar, single nanoparticle spectroscopy experiments have

* Corresponding author. E-mail: halas@rice.edu.† Chemistry and Materials Science Directorate, Lawrence Livermore

National Laboratory.‡ Physics and Advanced Technologies Directorate, Lawrence Livermore

National Laboratory.§ University of California, Davis.| Department of Physics and Astronomy, Rice University.⊥ Department of Electrical and Computer Engineering, Rice University.# Laboratory for Nanophotonics, Rice University.

NANOLETTERS

2005Vol. 5, No. 81569-1574

10.1021/nl050928v CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 06/28/2005

focused almost exclusively on direct spectral measurementsof elastically scattered light. Here we report measurementsof the inelastically scattered light from a monolayer of Ramanactive molecules bound to the surfaces of individual nano-particles and nanoparticle dimers. A comparison of theintegrated Stokes intensities of the nonresonant moleculesof p-mercaptobenzoic acid (4-MBA) adsorbed ontoindi-Vidual solid Au nanospheres, nanoshells, and individual pairsof adjacent Au nanospheres and nanoshells is performed. Theadsorbate molecules are nonresonant with respect to boththe excitation laser wavelength and the plasmon resonantenergies of each nanostructure. The SERS intensities arecorrelated in situ with the AFM topographic images of eachspecific individual nanoparticle or nanoparticle dimer. Al-though the SERS response arising from Au nanospheres canbe detected only for nanosphere dimers, SERS signals ofsimilar magnitudes can be observed from individual Aunanoshells as well as from dimers of directly adjacentnanoshells. The observed Stokes intensities of each nano-particle and nanoparticle dimer are compared with calcula-tions of the local electromagnetic field at the surface of eachgeometry performed using Mie scattering theory and thefinite difference time domain (FDTD) method. Calculationsof the integrated local field∫|E|4 ds at the incident laserwavelength are found to follow the qualitative trends of theSERS signals measured from each individual single nano-particle and nanoparticle dimer. The effect of nanoscalesurface roughness on the SERS response of individualnanoshell substrates is also examined.

Solid Au nanospheres of nominally 30-nm average radiuswere purchased from Polysciences. The Au nanoshells werefabricated as described elsewhere.14 The (49, 70) nanoshellswere fabricated with a silica core and Au shell, withdimensions that were characterized by agreement betweenthe optical extinction spectrum and Mie scattering theory16

and verified using transmission electron microscopy (TEM).The normalized UV/Visible/NIR extinction spectra of thenanospheres and nanoshells in solution, the calculated spectraof individual nanospheres and nanoshells in aqueous solutionand in ambient air, and the calculated dimer spectra ofnanospheres and nanoshells in air are shown in Figure 1.The dielectric function of the metal used in these calculationswas a modified Drude model with parameters adjusted toapproximate the empirical Au dielectric function33 withoutthe inclusion of chemical interface damping, finite pathlength corrections, or interband effects. The SERS excitationlaser wavelength of 633 nm used in the experiment ismarked. When deposited on a glass substrate and measuredin ambient air, the plasmon resonant wavelengths of all ofthe nanoparticles in solution are shifted toward shorterwavelengths because of the reduced index of refractionrelative to the solution phase medium.34-36 It is seen clearlyin Figure 1 that although the plasmon resonance of theisolated nanoshell is tuned to the excitation laser wavelength,the plasmon resonances of the isolated nanosphere and thenanosphere and nanoshell dimers are nonresonant with theexcitation laser. In this Figure, the dimer spectra correspond-ing to polarization of the input light along the interparticle

axis is shown, consistent with excitation of the dimerplasmon. This affects the near field intensities on each

Figure 1. Bulk UV/Visible/NIR extinction spectra (black line) of(a) Au nanoshell and (b) Au nanosphere aqueous solutions used inthis experiment prior to deposition. Magenta lines in a and b areextinction spectra calculated in aqueous solution and dotted bluelines are extinction spectra calculated using Mie scattering theoryin ambient air. Insets: TEM image of representative nanoparticles.(c) Extinction spectra of (green) nanoshell and (blue) nanospheredimers oriented parallel to the polarization of the incident light andimmersed in air calculated using FDTD. The vertical lines denotethe 633-nm excitation laser wavelength.

1570 Nano Lett., Vol. 5, No. 8, 2005

nanostructure surface and the SERS response of adsorbatemolecules bound to their surfaces.37

The Raman confocal images were acquired using acustom-built AFM/Confocal microscope with linearly polar-ized 633-nm wavelength excitation laser illumination de-scribed in detail elsewhere.29 In Figure 2a, a confocalmicrograph of the inelastically scattered light from the4-MBA coated solid Au nanospheres is shown. An atomicforce micrograph of the same field of view is shown in Figure2b. The highest intensity optical response observed corre-sponds to the dimers of adjacent nanospheres in the AFMimage; the Raman response for isolated Au nanospheres wasnot observable. The weak features observed in this imageappeared to have no topographic counterparts, and it is likelythat they arise from impurities either in or on the glasssubstrate. The insets in Figure 2b show high-resolution AFMimages of dimers of solid Au nanospheres. Figure 2c is theSERS spectrum of the leftmost nanosphere dimer highlightedin Figure 2b.

From the atomic force micrograph of the Au nanoshellsshown in Figure 3a, it is apparent that both individualnanoshells and dimers of adjacent nanoshells give rise toreadily observable SERS responses in a range of similarintensities, shown in Figure 3b and c, respectively. Again,some weak additional optical signals with no detectabletopographic counterparts were observable, most likely be-cause of impurities on or within the glass substrate. The insetsin Figure 3a show high-resolution topographic AFM images.In Figure 3b, a confocal micrograph of the inelasticallyscattered light observed from individual 4-MBA function-alized Au nanoshells is shown. The SERS spectrum of thepair of adjacent nanoshells (I) is shown in Figure 3c, andthat of the individual nanoshell (II) is shown in Figure 3d.The signal strengths for isolated nanoshells and for dimersof adjacent nanoshells appear to be qualitatively similar, thatis, within 1 order of magnitude of each other. The topo-graphic images of each nanoshell reveal its internal structure,indicating nanoscale roughness of the multicrystalline shell.

A theoretical examination of the local electromagneticproperties of these geometries allows us to assess the relativecontributions of each individual nanostructure and nano-structure pair to the experimentally observed SERS intensi-ties. The finite difference time domain (FDTD) method hasrecently been shown to be highly useful in the study of theelectromagnetic properties of metallic nanostructures ofalmost arbitrary complexity.37,38 FDTD calculations wereperformed to evaluate the local electromagnetic fields at theexcitation wavelength of 633 nm for all nanostructuregeometries. A realistic dielectric function for Au was usedin these calculations.37,38 Near field distributions for Aunanospheres and nanoshells (Mie theory) and nanosphere andnanoshell dimers (FDTD) for polarizations along and or-thogonal to the interparticle axis are shown in Figure 4.Amplitudes of the near field were normalized relative to theamplitude of the input plane wave exciting the nanostructure.These calculations permit a qualitative comparison of theelectromagnetic contribution to the SERS response in eachof the experimentally examined nanoparticle geometries. This

Figure 2. (a) Confocal micrograph of inelastically scattered lightfrom 4-MBA coated Au nanospheres immobilized on silane coatedglass substrates. (b) Atomic force micrograph of same field as in(a). Insets show high-resolution images of adjacent nanosphere pairs.(c) SERS spectrum of leftmost nanoparticle pair. Scale bar in insetsdenotes 100 nm.

Nano Lett., Vol. 5, No. 8, 2005 1571

method also allows us to calculate the local electromagneticfield on highly complex geometries, such as a nanoshell withnanoscale roughness, to determine the contribution of thelocal fields to the SERS response. The local fields for acontinuous nanoshell with simulated surface roughness isshown in Figure 4c.

Field values for all four geometries in Figure 4 are givenin Table 1. For the case of individual nanospheres andnanoshells (Figure 4a and b) the local fields were calculatedusing Mie scattering theory. Max|E|, max |E|4, and thesurface integrated∫|E| ds and ∫|E|4 ds at the excitationwavelength over each nanoparticle and nanoparticle pair aretabulated along with the experimentally obtained Stokesintensities of each geometry.

From this tabulation we can directly compare the SERSsignal intensities obtained from the various nanostructure

substrate geometries studied with the calculated field valuesfor each nanoparticle and nanoparticle dimer. Because theexperimental intensities are due to the total SERS signals ofthe molecules coating the nanoparticle surfaces, we wouldanticipate that∫|E|4 dswould provide qualitative agreementwith the experimentally observed trends in SERS intensities.The total surface areas of the nanoparticles and nanoparticledimers analyzed are taken into account in the evaluation ofthese integrals. This analysis is indeed consistent with theobserved SERS intensities in this series of experiments. Forexample, the difference in calculated integrated quartic fields∫|E|4 ds between isolated nanospheres and nanoshells agreewith the experimental observation that SERS signals wereobtainable from isolated nanoshells but not from isolated Aunanospheres. For geometries and excitation laser wavelengthsconsistent with our experiment,∫|E4| ds for an isolated

Figure 3. (a) Atomic force micrograph of the Au nanoshells. Insets show high-resolution images of two nanoshells. Scale bar in insets Iand II denotes 100 nm. (b) Confocal micrograph of inelastically scattered light from 4-MBA coated nanoshells immobilized on silanecoated glass substrates. (c) SERS spectrum of adjacent nanoshell pair (I), and (d) SERS spectrum of individual nanoshell (II).

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nanoshell is greater by a factor of 119 relative to the solidnanosphere; for a roughened nanoshell, this increases to afactor of 548 relative to the solid nanosphere. In other words,the signal-to-noise for SERS from an isolated nanoshellwould need to be greater than 119 (smooth nanoshell) or548 (rough nanoshell) for the SERS signal from a solidnanosphere to be detectable in this experiment. From thesignal-to-noise in the spectra shown in Figure 3, this is clearlynot the case.

These calculated values also allow us to assess the trendsin experimental signals observed for nanoparticle dimers. Incomparing the local electromagnetic fields of the individualnanoparticles with those of the nanoparticle dimers, we seeimmediately that the maximum field for a nanostructure pairis much larger than the maximum field at the surface of each

isolated nanoparticle. This high field hot spot is excited onlywhen the polarization of the incident light has somecomponent parallel to the interparticle axis. In the case whenthe polarization is orthogonal to the interparticle axis, themaximum field at the surface of the adjacent nanoparticlepair can actually be slightly lower than that of an isolatednanoparticle. Here we see a large difference in∫|E4| dsvaluesdepending on polarization of the incident light with respectto the interparticle axis. A large variability of SERSintensities for nanosphere and nanoshell dimers was observedexperimentally, consistent with the random orientation of thenanostructure dimers with respect to the polarization ofincident light. The precise internanoparticle distance, un-known but apparently less than 20 nm from the AFM imagesof the nanostructure dimers studied, introduces an additional

Figure 4. Electromagnetic near field enhancement at the excitation laser’s wavelength (633 nm) for (a) an isolated Au nanosphere, (b) anisolated Au nanoshell, (c) a roughened Au nanoshell, (d) an adjacent nanosphere pair with an axis perpendicular to the incident polarization,(e) an adjacent nanoshell pair with interparticle axis perpendicular to the incident polarization, (f) an adjacent nanosphere pair with interparticleaxis parallel to the incident polarization, and (g) a nanoshell dimer with interparticle axis parallel to the incident polarization. The colorscale represents the electromagnetic field enhancement (|E|). In f and g the hot spot of the junction, or dimer, plasmon can clearly be seenbetween the two nanoparticles. a and b were calculated using Mie scattering theory. c-g were calculated using the FDTD method.

Table 1. Comparison of Local Electromagnetic Field Magnitudes within 1 nm of the Surface for Nanostructures of the SameDimensions as In the Experiment and Experimental Stokes Intensities for All Representative Nanostructures Studieda

FDTD simulation SERS (exptl)

particle type max |E| max |E|4∫|E| ds(µm2)

∫|E|4 ds(µm2)

signal(cts/30 s)

isolated nanosphere(30-nm radius)

2.7 53 0.005 0.042

nanosphere dimer (⊥) 4 256 0.009 0.15 537-3065nanosphere dimer (|) 43 3.4 × 106 0.019 21.0isolated nanoshell(r1, r2) ) 49, 70

6.1 1385 0.057 5.0 118-516

roughened nanoshell ∼14 3.8 × 104 0.049 23nanoshell dimer (⊥) 5 625 0.077 5.8 295-361nanoshell dimer (|) 85 5.2 × 107 0.086 4000

a All fields are dimensionless, normalized to the amplitude of the input plane wave incident on the nanostructure. Field values for the nanostructure pairswere calculated assuming a 3-nm interparticle distance. Fewer than 10 nanostructures of each geometry were studied.

Nano Lett., Vol. 5, No. 8, 2005 1573

limitation on our ability to make more precisely quantifiedcomparisons of local electromagnetic fields for nanostructuredimers and their measured SERS intensities. Recent theoreti-cal studies have shown that for nanoshells of similar sizeand resonant energies to this experiment, the internanoparticledistance must be less than 20 nm for the enhanced field ofthe dimer plasmon to contribute to the overall SERS responsefor the case of optimal excitation polarization.37

The low SERS intensities observed for nanoshell dimersrelative to the theoretically predicted range of relativeenhancements merits a specific discussion. Because of thestrong attractive forces between two nanoshells, it may bequite possible that in some nanoshell dimers examined theadjacent nanoparticles were touching and the interparticlegap was closed. Because the adsorbate molecules wereintroduced onto the nanoparticle substrates after thesestructures were in place, any nanoshell dimers with closedjunctions would not have molecules located between theirconstituent nanoparticles and indeed would not give rise tothe larger SERS intensities predicted for the nanoshell dimerstructure. Although it is true that individual nanoshellroughness may lead to increased SERS intensities, theenhancements predicted for rough individual nanoshellsappear to be smaller relative to the enhancements achievablein the hot spot of even a smooth nanoshell dimer. Currentexperimental studies are underway to examine these geo-metrical aspects in greater detail, based in part on our recentobservation that nanoscale roughness can be introduced ontothe surface of nanoshells in a highly controlled manner.39

In conclusion, we have performed a correlated confocalRaman optical and atomic force microscopy study ofindividual nanostructure SERS substrates consisting ofisolated nanoparticles and nanoparticle dimers. We are ableto obtain good qualitative agreement between the measuredSERS intensities and the surface integrated quartic fieldmagnitudes∫|E4| ds for the nanoparticles and nanoparticledimers studied. We would anticipate that SERS studies withresonant molecules would show significant deviations inrelative intensities because of contributions from the resonantmolecular response, apparently not present in this series ofexperiments. These results show that nanoshells can provideconsistent and strong SERS enhancements as individualnanoparticles without the requirement of nanoparticle ag-gregation.

Acknowledgment. We gratefully acknowledge the Na-tional Science Foundation (EEC-0304097), the Army Re-search Office (DAAD19-99-1-0315), the Air Force Officeof Scientific Research (F49620-03-C-0068), NASA (68371),and the Robert A. Welch Foundation (C-1220 and C-1222)for their generous support of these research efforts. Fundingfor this work at Lawrence Livermore National Laboratorywas provided by the Laboratory Directed Research andDevelopment Program. Work at LLNL was performed underthe auspices of the U. S. Department of Energy by theUniversity of California, Lawrence Livermore NationalLaboratory, under contract no. W-7405-Eng-48. We grate-fully acknowledge Mikael Ka¨ll for his excellent feedbackupon critical reading of this manuscript.

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