Plasmon Transmutation: Inducing New Modes in Nanoclusters byAdding Dielectric NanoparticlesFangfang Wen,†,# Jian Ye,*,⊥,# Na Liu,‡,# Pol Van Dorpe,⊥ Peter Nordlander,‡,§,∥

and Naomi J. Halas*,†,‡,§,∥

†Department of Chemistry, ‡Department of Electrical and Computer Engineering, §Department of Physics and Astronomy, and∥Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States⊥IMEC vzw, Kapeldreef 75, Leuven, Belgium B-3001

ABSTRACT: Planar clusters of coupled plasmonic nano-particles support nanoscale electromagnetic “hot spots” andcoherent effects, such as Fano resonances, with unique nearand far field signatures, currently of prime interest for sensingapplications. Here we show that plasmonic cluster propertiescan be substantially modified by the addition of individual,discrete dielectric nanoparticles at specific locations on thecluster, introducing new plasmon modes, or transmutingexisting plasmon modes to new ones, in the resultingmetallodielectric nanocomplex. Depositing a single carbonnanoparticle in the junction between a pair of adjacent nanodisks induces a metal−dielectric−metal quadrupolar plasmon mode.In a ten-membered cluster, placement of several carbon nanoparticles in junctions between multiple adjacent nanoparticlesintroduces a collective magnetic plasmon mode into the Fano dip, giving rise to an additional subradiant mode in themetallodielectric nanocluster response. These examples illustrate that adding dielectric nanoparticles to metallic nanoclustersexpands the number and types of plasmon modes supported by these new mixed-media nanoscale assemblies.

KEYWORDS: Nanocluster, plasmon, electron beam-induced deposition (EBID), carbon nanoparticle (CNP), Fano resonance,magnetic mode

Just as molecules are formed by the arrangement of atomsinterconnected by chemical bonds, plasmonic nanoclusters

are formed by the assembly of noble metal nanoparticles andare coupled by local fields induced by illumination. In theseartificial plasmonic molecules, interparticle coupling can giverise to collective hybridized plasmon modes and also to higherorder electric and magnetic plasmon modes.1−6 Interferencebetween collective plasmon modes gives rise to coherentphenomena such as electromagnetically induced transparency(EIT)7 and Fano resonances.6,8−31 Because of their rich andcomplex optical properties, plasmonic nanoclusters have beenthe subject of intense recent research interest, primarily fortheir potential use in sensing applications, where their opticalresponse is unusually sensitive to small changes in localdielectric environment.32−36 The subradiant magnetic plasmonmodes observed on these structures8 have also been identifiedas a means to propagate surface plasmons with remarkably lowloss.32 In sensing, plasmonic nanoclusters with Fano resonancesare of particular interest with the Fano dip exhibitingremarkably large LSPR shifts due to homogeneous changes inthe dielectric environment in which it is immersed orembedded. Recently, we showed that a single, discrete dielectricnanoparticle, when placed on a plasmonic cluster, can alsoinduce significant changes in both its near and far field opticalresponse.36 This effect that may be very important whendesigning sensors for the detection of discrete analytes in the

10−100 nm size range, such as large biomolecular complexes,bacteria, or viruses.Here, we demonstrate that the addition of individual

dielectric nanoparticles at specific locations on a plasmoniccluster can introduce entirely new plasmon modes, ordramatically modify existing modes, in the resulting metallodi-electric nanostructure. We show two examples of this effect.First, the addition of a single discrete dielectric nanoparticle tothe interparticle gap of an individual plasmonic dimer,consisting of two adjacent Au nanodisks, introduces a newquadrupolar plasmon mode in addition to the collective dipolardimer mode associated with the dimer “hot spot”. In a larger,Fano resonant plasmonic cluster consisting of a central disksurrounded by a ring of nine adjacent disks, or “decamer”, thedeposition of four dielectric nanoparticles at specificinterparticle locations gives rise to a near-IR collective magneticplasmon mode. The far-field signature of this new modeappears as a second minimum in the Fano resonance of theresulting metallodielectric complex. These examples not onlyillustrate the ease with which plasmonic properties in thesestructures can be modified, they also define a new, metallodi-electric variant of planar plasmonic clusters, where both

Received: July 28, 2012Revised: August 22, 2012Published: August 27, 2012

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dielectric and metallic nanoscale components together give riseto new optical properties in the resulting mixed-mediananostructure.Plasmonic dimer and decamer nanoclusters were fabricated

by e-beam lithography using a scanning electron microscope(SEM, FEI QUANTA 400). The nanostructures were definedby e-beam patterning in the positive resist poly(methylmethacrylate) (PMMA) spin-coated onto a Si substrate coatedwith a 100 nm layer of SiO2. The gold structures werefabricated by e-beam evaporation of an initial 1 nm Ti adhesionlayer, then a 30 nm evaporation of gold onto the substrate,followed by a standard lift-off process. The thickness of theSiO2 layer was chosen to minimize substrate effects on theplasmonic properties of the nanoclusters. The deposition ofcarbon nanoparticles onto the structure was also performedwith the same SEM. The carbon nanoparticles were depositedindividually by electron beam-induced deposition (EBID)37−39

with the electron beam operated at 30 kV with a current of 40pA at the deposition location of choice on the nanostructure.An acceleration voltage of 30 KV with a beam spot size of 2.1nm at a 7 mm working distance was used. The process of EBIDonto an interparticle region of a single plasmonic nanocluster isshown schematically in Figure 1 (not to scale). The highly

focused electron beam dissociates the carbonaceous residues inthe SEM chamber into nonvolatile carbon, resulting in thedeposition of an amorphous carbon nanoparticle on thesubstrate surface. The size of the deposited carbon nanoparticleis controlled by the time period of e-beam exposure. Thelocation of the deposited carbon nanoparticle can be positionedwith high accuracy due to the spatial resolution of the SEM. Inthis study, 25−35 nm diameter carbon nanoparticles weredeposited using e-beam irradiation times of 15 and 25 s,respectively.We begin by examining the effect of the deposition of a

single carbon nanoparticle at different locations on anindividual Au dimer (Figure 2). The dimer was composed oftwo gold disks, 150 nm in diameter, separated by a gap of 15

nm. The disk height was 30 nm. The same dimer was usedduring the course of this experiment to ensure a high level ofconsistency in the dimer optical properties. At each stage of theexperiment, a carbon nanoparticle was deposited onto a specificregion within the dimer (Figure 2a). (All the carbonnanoparticles were ∼25 nm, indicated by the colored circlesin the SEM images in the figure.) Then, the correspondingdark-field scattering spectrum of the resulting nanostructurewas obtained (Figure 2b) and compared with numericalsimulations of the dark-field spectrum for the same geometries(Figure 2c). Between the placements of carbon nanoparticles 1and 2, and 2 and 3, an oxygen plasma cleaning (FischioneInstruments, Model 1020) was performed to remove the firstcarbon nanoparticle without significantly modifying the opticalproperties of the plasmonic cluster (data not shown). Thesystem was illuminated with unpolarized white light, and bothhorizontally (longitudinal polarization) and vertically (trans-verse) polarized scattering signals were collected.With the placement of a carbon nanoparticle on the side of a

dimer cluster (nanoparticle 1), only a pronounced redshift inthe collective dimer mode, the main feature in the scatteringspectrum, from 770 to 800 nm, was observed for longitudinalpolarization. When carbon nanoparticle 2 was placed asym-metrically within the dimer gap, however, a new spectral featurecould be observed at 600 nm, in addition to an additionalredshift of the bonding plasmon mode (to 845 nm).Repositioning a carbon nanoparticle more symmetrically inthe gap (nanoparticle 3) only slightly intensifies this newspectral feature. The addition of a carbon nanoparticle on theside of the “loaded” dimer structure (nanoparticle 4) induces anaddition redshift in the dimer mode (from 870 to 890 nm) anda smaller redshift of nominally 10 nm of the new spectralfeature induced by the deposition of carbon nanoparticle 3.This new resonance is not observed for spectra obtained fortransverse polarization (Figure 2b, right). This qualitativebehavior correlates well with theoretical simulations of thesecluster geometries (Figure 2c). (Finite-difference time domain(FDTD, Lumerical Solutions, Inc.) simulations were performedusing the empirical dielectric functions for Au,40 the supportingsubstrate,41 and the carbon nanoparticle.41) When comparingexperimental and theoretical spectra, slightly larger spectralshifts were observed experimentally than were theoreticallyobtained; this is due to the accompanying deposition of a thin(1−2 nm) layer of carbon over the nanoparticle during carbonnanoparticle growth. Since the deposition time is the same foreach nanoparticle, the extraneous carbon coating also producedis likely to be very similar in each case.The case of adding a dielectric nanoparticle to a dimer

junction clearly illustrates the role of the added nanoparticle inmodifying the properties of a plasmonic cluster. The increaseddielectric function in the interparticle gap region increases thecoupling between the two metallic nanoparticles where it waspositioned. For example, both spectral changes resulting fromthe added dielectric nanoparticle (2) or (3) in Figure 2, aspectral redshift of the dipolar dimer resonance and theappearance of a quadrupolar mode to the dimer spectrum,result from an increased coupling between the two metallicnanoparticles. The same spectral changes would be seen byreducing the distance between the metallic nanoparticles. Sothe addition of a dielectric nanoparticle to a metallic cluster canbe seen as a way of increasing the interparticle couplingbetween two specific elements of the cluster without reducingthe physical distance between metallic nanoparticles.

Figure 1. Schematic illustration (not to scale) of EBID of a carbonnanoparticle at a precise position on a single gold nanocluster. Carbonprecursor molecules are illustrated schematically, and the carbonnanoparticle is represented by the larger black nanosphere on thecluster. Local exposure to the electron beam induces thedecomposition of the gaseous carbon precursor molecules, resultingin the growth of a carbon nanoparticle in the location where theelectron beam is incident on the cluster.

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Since plasmonic clusters are well-known to have resonantlineshapes sensitive to their dielectric environment,10,42 a localdielectric perturbation will shift the scattering spectrum withrespect to the pristine nanostructure with the spectral shiftbeing more prominent if this perturbation is in a region ofstrongly enhanced electromagnetic field.43−45 The nature of thespectral shift provides valuable information regarding thedielectric function of the local perturbation. For example, inthe case of a plasmonic dimer with a conductive junction, ablueshifted spectrum is anticipated due to the generation of acharge transfer plasmon.46 In these experiments, we onlyobserve red-shifted perturbations in the spectrum, confirmingthat the carbon nanoparticles placed on these nanostructuresare localized dielectric perturbations.The different spectral shifts induced by a single carbon

nanoparticle positioned at the locations of nanoparticles 1, 2,and 3 on a plasmonic dimer can be understood from the local

electric field magnitudes at the various positions around thenanostructure. In the FDTD simulations for longitudinalpolarization (Figure 2d), we see that the field intensity is thestrongest in the interparticle gap (nanoparticle 3), decreasesslightly in the offset gap position (nanoparticle 2), and is theweakest at the side of the structure (nanoparticle 1). Figure2d(ii), corresponding to transverse polarization, shows why thecarbon nanoparticles placed in these three regions inducesnegligible spectral shifts; here the positions of nanoparticles 1,2, and 3 are all in regions of virtually no field enhancement.Both the local field (Figure 2e(iii)) and charge distributions

(Figure 2f(iii)) obtained from our FDTD analysis provideinsight into the properties of the new plasmon mode inducedby the presence of the dielectric nanoparticle in the gap of thedimer nanostructure. The charge density plot clearly shows thatthe new spectral feature is a hybridized bonding quadrupolardimer plasmon mode. For a standalone dimer with the present

Figure 2. Optical properties of a single Au dimer before and after carbon nanoparticle deposition. (a) SEM images of a single Au dimer before andafter the deposition of carbon nanoparticles (1−4) at different positions. The carbon nanoparticles 1−4 are each marked in the image with distinctdashed color circles. The scale bar is 100 nm. The diameter and the thickness of the individual nanodisks comprising the dimer structure are 150 and30 nm, respectively, with an interparticle gap of 15 nm. All experiments shown here were performed on the same dimer for consistency. Each carbonnanoparticle is ∼25 nm. (b) Experimentally obtained dark-field scattering spectra. The system is excited with unpolarized white light, and thepolarized scattering light is collected. (c) Theoretical scattering cross-section spectra, calculated using FDTD. (d) Simulated electric field intensity(log(|E|2)) distribution at resonances (i) and (ii). (e) Simulated electric field intensity (log(|E|2)) distribution at resonances (iii) and (iv). (f)Simulated surface charge distributions at resonances (iii) and (iv).

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relatively large interparticle spacing, this mode cannot bedirectly excited by light since the dipole moment of the mode isnegligible. However, when the carbon particle is present, thelocal field induced by the quadrupolar plasmon polarizes thedielectric particle and the system acquires a net dipole momentthat renders the mode excitable by light. In contrast, thedipolar, bright plasmon mode of the dimer is only slightlyinfluenced by the presence of the dielectric nanoparticle in thegap region (Figure 2e(iv) and Figure 2f(iv)). Comparing thelocal field distribution of these two modes, we also see that thepresence of the dielectric nanoparticle provides an additionalfield confinement effect in the hot spot for the case of the new,quadrupolar plasmon mode.

For plasmonic clusters that exhibit coherent phenomena, theaddition of dielectric nanoparticles to the nanostructure givesrise to more dramatic changes in the plasmon modes. Toexamine this, we deposit four carbon nanoparticles sequentiallyon a Fano resonant decamer nanocluster. This plasmonicnanocluster consists of ten disks with a single central disksurrounded by a ring of nine peripheral disks in close proximityto each other and to the center particle.6,9,10 Coupling betweenthe superradiant plasmon mode (bright mode) and thesubradiant plasmon mode (dark mode) of this structure givesrise to a single pronounced Fano resonance in the scatteringspectrum.6,8−10,14 The diameter of the central disk is 178 nmwhile the diameter of the surrounding disks is 85 nm, ensuringa constant gap size of ∼15 nm between the disks. The height of

Figure 3. Scattering spectral evolution of a single Au decamer decorated with four sequentially deposited 35 nm carbon nanoparticles. (a) SEMimage of a single decamer with four 35 nm carbon nanoparticles (1−4) already deposited at four different positions on the nanostructure. Thecarbon nanoparticles are marked inside the colored, dashed circles, and enumerated in order of deposition. The scale bar is 100 nm. The diameters ofthe peripheral disks and the central disk are 85 and 178 nm, respectively. The thickness of the structure is 30 nm. All gap sizes are ∼15 nm. (b)Experimental scattering spectra obtained before (black) and after the deposition of each carbon nanoparticle in sequence, showing both horizontaland vertically polarized scattered light from the cluster. (c) Corresponding simulated (FDTD) scattering cross-section spectra. (d) Simulated surfacecharge density plots for the Au decamer decorated with four ∼35 nm carbon nanoparticles at the resonances (i), (ii), and (iii), as indicated by arrowsin (c).

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the structure is kept at 30 nm. A sequence of carbonnanoparticles was deposited onto specific interparticle regionsof the cluster, one at a time. Following the deposition of eachcarbon nanoparticle, the scattering spectrum of the modifiedcomplex was obtained using dark-field microspectroscopy.Figure 3a shows an SEM image of the decamer decoratedwith the four sequentially deposited carbon nanoparticles. Thecarbon nanoparticles are each indicated by colored dashedcircles with the Arabic numerals indicating the depositionsequence. Each carbon nanoparticle deposited was ∼35 nm indiameter. All measurements in the sequence were performed onthe same decamer.Following the deposition of each nanoparticle, the dark-field

scattering spectrum of the complex was obtained (Figure 3b).The black curves indicate the scattering spectrum of the pristinegold decamer with its characteristic strong Fano dip locatedwithin a broad superradiant resonance profile. For thehorizontally polarized spectrum (Figure 3b, left), the spectralposition of the Fano resonance shifts toward longer wave-lengths with each sequential placement of an additional carbonnanoparticle. For vertical polarization, this effect is significantlyweaker with larger shifts in the experimental than the simulatedspectra again due to the deposition of a nanoscale carbon layeronto the structure during the nanoparticle deposition process.Primarily, however, we observe that the addition of dielectric

nanoparticles to the structure induces a new plasmon feature inthe spectrum. The first indication of the formation of this newstate is the appearance of a modulation at 670 nm, near theFano minimum in the scattering spectrum, following thedeposition of the first dielectric nanoparticle. With theintroduction of the second nanoparticle, this spectral featurebecomes more distinct and increases in prominence still furtherfor the deposition of nanoparticles 3 and 4. With the depositionof the fourth dielectric nanoparticle, the strength of this spectralmodulation becomes comparable to the original Fanoresonance itself. This feature is observed only in the

horizontally polarized scattered light. FDTD simulations ofthe experiment show very good agreement with the spectralfeature introduced by the dielectric nanoparticles. Thesimulated far field spectrum indicates that the Fano resonanceis split into two subradiant modes following the deposition ofdielectric nanoparticle 1, and that the new feature increases instrength as the other three nanoparticles are added to thecomplex.Charge density plots obtained at three specific wavelengths

(Figure 3c(i,ii,iii)) for the final metallodielectric complex revealmore of the nature of the new subradiant plasmon mode(Figure 3d). The first spectral feature (i) at ∼950 nmcorresponds to a charge distribution characteristic of asuperradiant plasmon mode, that is, where all constituentnanoparticles in the cluster are in phase. The spectral feature(ii) at ∼860 nm corresponds to the original Fano minimum ofthe nanocluster, now red shifted in wavelength due to theaddition of the dielectric nanoparticles. The charge distributionis characteristic of a strong Fano resonance, modified slightly bythe presence of the dielectric nanoparticles. For spectral feature(iii) at ∼740 nm, a distinct rotational symmetry of theperipheral nanoparticles and a corresponding suppression ofthe charge oscillation in the center particle can clearly be seen.This charge distribution is characterized by a displacementcurrent that circulates in the peripheral nanoparticles. It can beidentified as a magnetic plasmon mode.4,8,32 In the absence ofthe carbon particles, this mode possesses no net dipole momentand cannot be excited by incident light at near-normalincidence.In Figure 4, we examine the “transmutation” of the

subradiant plasmon mode into the magnetic plasmon modein greater detail. The placement of the first dielectricnanoparticle on the decamer breaks the symmetry of thecluster, perturbing the local charge distribution and thereforethe local electromagnetic field. The charge distribution on themetal nanodisks adjacent to nanoparticle (1) begins to lose its

Figure 4. Transmuting the Fano resonance of the Au decamer (0) into a magnetic plasmon resonance (4) with the addition of the first (1), second(2), third (3), and fourth (4) nanoparticles as in the experiment: (a) simulated charge distribution; (b) local E component of electromagnetic field;(c) local H component of electromagnetic field at the magnetic resonance indicated in Figure 3c(iii).

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dipolar character and acquire a quadrupolar character, similar towhat was seen in Figure 2f(iii). This reduces the local E fieldand increases the local H field. This symmetry-breaking alsolifts the mode degeneracy seen in the optical spectrum, where anew subradiant spectral feature at higher energies relative to theunperturbed Fano minimum can now be observed (Figure3b,c). The addition of nanoparticle (2) depolarizes one of itstwo adjacent metal nanodisks in an out-of-phase relationshipwith the depolarized particle induced by nanoparticle (1). Atthis point, the three nanodisks linked by the two dielectricnanoparticles now have a weaker collective dipole, screened bythe quadrupolar character of the two end nanodisks. Nano-particle (3) is placed between the nearest unlinked peripheraldisk and the larger, central disk, which still retains some of itsstrong dipolar character. The placement of this nanoparticlereorients the dipole on the peripheral disk, inducing a change inthe charge distribution for the chain of three nanodisks linkedtogether by nanoparticles (1) and (2). The dipole of this linkedchain of nanodisks is flipped, and we see the first indication ofthe excitation of the magnetic plasmon mode. The presence ofnanoparticle (4) increases the symmetry of the emergentmagnetic plasmon mode, resulting in a more uniform, reducedsymmetry field distribution and a more uniform chargedistribution for the peripheral ring of nanodisks.In summary, we have shown that the plasmonic properties of

metallic nanoclusters can be substantially modified by theaddition of dielectric nanoparticles at specific locations on thenanostructure. For a plasmonic dimer, deposition of a dielectricnanoparticle in the interparticle gap makes a quadrupolarplasmon mode appear in the spectrum. For a plasmonicdecamer, a dielectric nanoparticle placed in one of theinterparticle gaps results in a similar change in local chargedistribution as that seen in the simpler dimer structure.However, in this case we observe that the sequential depositionof dielectric nanoparticles in interparticle gaps gives rise to acollective magnetic plasmon mode in the nanocluster,essentially converting the Fano resonance into this newmode. Not only does this study reveal a simple but strikingway to create and/or convert plasmon modes to new modeswith different symmetries and properties, it also introduces anew family of metallodielectric nanocomplexes obtained bydepositing dielectric nanoparticles onto prepatterned plasmonicclusters of coupled nanodisks. This result is of both practicaland fundamental importance. For plasmonic clusters, since thestrength of the magnetic mode is comparable to that of theFano resonance, this deposition-transmutation technique couldhave potential application in high-density digital data storagedevices with optical readout. It is highly likely that this e-beamnanoparticle deposition method gives rise to other examples oftransmuted plasmon modes in this new family of coherentmetallodielectric nanoclusters. While the effects we report havebeen demonstrated with planar structures fabricated by e-beam-based methods, advances in the directed chemical assembly of2D and 3D nanoparticle complexes with copolymer-based orDNA-based approaches should enable the synthesis of newfamilies of metallodielectric nanocomplexes with propertiessimilar to those demonstrated in this work.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: (J.Y.) jian.ye@imec.be; (N.J.H.) halas@rice.edu.Author Contributions#These authors contributed equally to this work.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge Sven Hein for material visualizations andYumin Wang for helpful discussions. This work was financiallysupported by the Robert A. Welch Foundation under Grants C-1220 (N.J.H.) and C-1222 (P.N.), the National SecurityScience and Engineering Faculty Fellowship (NSSEFF)N00244-09-1-0067, the Defense Threat Reduction Agency(DTRA) HDTRA1-11-1-0040, and the Air Force Office ofScientific Research (AFOSR) FA9550-10-1-0469. J.Y. andP.V.D. acknowledge financial support from the FWO ofFlanders.

■ REFERENCES(1) Sonnefraud, Y.; Verellen, N.; Sobhani, H.; Vandenbosch, G. A. E.;Moshchalkov, V. V.; Van Dorpe, P.; Nordlander, P.; Maier, S. A. ACSNano 2010, 4 (3), 1664−1670.(2) Brown, L. V.; Sobhani, H.; Lassiter, J. B.; Nordlander, P.; Halas,N. J. ACS Nano 2010, 4 (2), 819−832.(3) Sheikholeslami, S.; Jun, Y.-w.; Jain, P. K.; Alivisatos, A. P. NanoLett. 2010, 10 (7), 2655−2660.(4) Sheikholeslami, S. N.; Garcia-Etxarri, A.; Dionne, J. A. Nano Lett.2011, 11 (9), 3927−3934.(5) Artar, A.; Yanik, A. A.; Altug, H. Nano Lett. 2011, 11 (9), 3694−3700.(6) Hentschel, M.; Dregely, D.; Vogelgesang, R.; Giessen, H.; Liu, N.ACS Nano 2011, 5 (3), 2042−2050.(7) Liu, N.; Weiss, T.; Mesch, M.; Langguth, L.; Eigenthaler, U.;Hirscher, M.; Soennichsen, C.; Giessen, H. Nano Lett. 2010, 10 (4),1103−1107.(8) Fan, J. A.; Wu, C.; Bao, K.; Bao, J.; Bardhan, R.; Halas, N. J.;Manoharan, V. N.; Nordlander, P.; Shvets, G.; Capasso, F. Science2010, 328 (5982), 1135−1138.(9) Hentschel, M.; Saliba, M.; Vogelgesang, R.; Giessen, H.;Alivisatos, A. P.; Liu, N. Nano Lett. 2010, 10 (7), 2721−2726.(10) Lassiter, J. B.; Sobhani, H.; Fan, J. A.; Kundu, J.; Capasso, F.;Nordlander, P.; Halas, N. J. Nano Lett. 2010, 10 (8), 3184−3189.(11) Luk’yanchuk, B.; Zheludev, N. I.; Maier, S. A.; Halas, N. J.;Nordlander, P.; Giessen, H.; Chong, C. T. Nat. Mater. 2010, 9 (9),707−715.(12) Francescato, Y.; Giannini, V.; Maier, S. A. ACS Nano 2012, 6(2), 1830−1838.(13) Gallinet, B.; Martin, O. J. F. ACS Nano 2011, 5 (11), 8999−9008.(14) Lassiter, J. B.; Sobhani, H.; Knight, M. W.; Mielczarek, W. S.;Nordlander, P.; Halas, N. J. Nano Lett. 2012, 12 (2), 1058−1062.(15) Chen, H. J.; Shao, L.; Man, Y. C.; Zhao, C. M.; Wang, J. F.;Yang, B. C. Small 2012, 8 (10), 1503−1509.(16) Liu, H. L.; Wu, X. J.; Xi, C.; Zhang, G.; Zheng, L. Appl. Phys.Lett. 2012, 100, 153114.(17) Lopez-Tejeira, F.; Paniagua-Dominguez, R.; Rodriguez-Oliveros,R.; Sanchez-Gil, J. A. New J. Phys. 2012, 14, 023035.(18) Cui, Y. H.; Zhou, J. H.; Tamma, V. A.; Park, W. ACS Nano2012, 6 (3), 2385−2393.(19) Ho, J. F.; Luk’yanchuk, B.; Zhang, J. B. Appl. Phys. A 2012, 107(1), 133−137.(20) Frimmer, M.; Coenen, T.; Koenderink, A. F. Phys. Rev. Lett.2012, 108, 077404.(21) Wu, D. J.; Jiang, S. M.; Liu, X. J. J. Chem. Phys. 2012, 136,034502.(22) Liu, H.; Li, G. X.; Li, K. F.; Chen, S. M.; Zhu, S. N.; Chan, C. T.;Cheah, K. W. Phys. Rev. B 2011, 84, 235437.(23) Rahmani, M.; Lukiyanchuk, B.; Nguyen, T. T. V.; Tahmasebi,T.; Lin, Y.; Liew, T. Y. F.; Hong, M. H. Opt. Mater. Express 2011, 1(8), 1409−1415.

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dx.doi.org/10.1021/nl302799h | Nano Lett. 2012, 12, 5020−50265025

(24) Wu, D. J.; Jiang, S. M.; Liu, X. J. J. Phys. Chem. C 2011, 115(48), 23797−23801.(25) Gallinet, B.; Martin, O. J. F. Opt. Express 2011, 19 (22), 22167−22175.(26) Dregely, D.; Hentschel, M.; Giessen, H. ACS Nano 2011, 5(10), 8202−8211.(27) Yang, Z. J.; Zhang, Z. S.; Hao, Z. H.; Wang, Q. Q. Appl. Phys.Lett. 2011, 99, 081107.(28) Chen, H. J.; Shao, L.; Ming, T.; Woo, K. C.; Man, Y. C.; Wang,J. F.; Lin, H. Q. ACS Nano 2011, 5 (8), 6754−6763.(29) Woo, K. C.; Shao, L.; Chen, H. J.; Liang, Y.; Wang, J. F.; Lin, H.Q. ACS Nano 2011, 5 (7), 5976−5986.(30) Yang, Z. J.; Zhang, Z. S.; Zhang, L. H.; Li, Q. Q.; Hao, Z. H.;Wang, Q. Q. Opt. Lett. 2011, 36 (9), 1542−1544.(31) Zhou, Z. K.; Peng, X. N.; Yang, Z. J.; Zhang, Z. S.; Li, M.; Su, X.R.; Zhang, Q.; Shan, X. Y.; Wang, Q. Q.; Zhang, Z. Y. Nano Lett. 2011,11 (1), 49−55.(32) Liu, N.; Mukherjee, S.; Bao, K.; Brown, L. V.; Dorfmueller, J.;Nordlander, P.; Halas, N. J. Nano Lett. 2012, 12 (1), 364−369.(33) Wu, C.; Khanikaev, A. B.; Adato, R.; Arju, N.; Yanik, A. A.;Altug, H.; Shvets, G. Nat. Mater. 2012, 11 (1), 69−75.(34) Pasquale, A. J.; Reinhard, B. M.; Dal Negro, L. ACS Nano 2011,5 (8), 6578−6585.(35) Ou, F. S.; Hu, M.; Naumov, I.; Kim, A.; Wu, W.; Bratkovsky, A.M.; Li, X.; Williams, R. S.; Li, Z. Nano Lett. 2011, 11 (6), 2538−2542.(36) Ye, J.; Wen, F.; Sobhani, H.; Lassiter, J. B.; Van Dorpe, P.;Nordlander, P.; Halas, N. J. Nano Lett. 2012, 12 (3), 1660−1667.(37) Ye, J.; Chen, C.; Lagae, L.; Maes, G.; Borghs, G.; Van Dorpe, P.Phys. Chem. Chem. Phys. 2010, 12 (37), 11222−11224.(38) Koops, H. W. P.; Kretz, J.; Rudolph, M.; Weber, M.; Dahm, G.;Lee, K. L. Jpn. J. Appl. Phys., Part 1 1994, 33 (12B), 7099−7107.(39) van Dorp, W. F.; van Someren, B.; Hagen, C. W.; Kruit, P. NanoLett. 2005, 5 (7), 1303−1307.(40) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6 (12), 4370−4379.(41) Palik, E. D. Handbook of the optical constants of solids; AcademicPress: New York, 1985.(42) Sherry, L. J.; Chang, S. H.; Schatz, G. C.; Van Duyne, R. P.;Wiley, B. J.; Xia, Y. N. Nano Lett. 2005, 5 (10), 2034−2038.(43) Willets, K. A.; Van Duyne, R. P. Localized surface plasmonresonance spectroscopy and sensing. In Annual Review of PhysicalChemistry; Annual Reviews: Palo Alto, 2007; Vol. 58, pp 267−297.(44) Hao, F.; Sonnefraud, Y.; Van Dorpe, P.; Maier, S. A.; Halas, N.J.; Nordlander, P. Nano Lett. 2008, 8 (11), 3983−3988.(45) Lassiter, J. B.; Aizpurua, J.; Hernandez, L. I.; Brandl, D. W.;Romero, I.; Lal, S.; Hafner, J. H.; Nordlander, P.; Halas, N. J. NanoLett. 2008, 8 (4), 1212−1218.(46) Perez-Gonzalez, O.; Zabala, N.; Borisov, A. G.; Halas, N. J.;Nordlander, P.; Aizpurua, J. Nano Lett. 2010, 10 (8), 3090−3095.

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