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As featured in:

See M.-M. Jiang et al., J. Mater. Chem. C, 2014, 2, 56.

Showcasing research from State Key Laboratory of

Luminescence and Applications, Changchun Institute of Optics,

Fine Mechanics and Physics, Chinese Academy of Sciences.

Title: Hybrid quadrupolar resonances stimulated at short wavelengths using coupled plasmonic silver nanoparticle aggregation

Hybrid quadrupolar resonance in the short wavelength region has been achieved using the coupled plasmonic modes of silver nanoparticle aggregation, which may be used to improve the performance of wide bandgap semiconductor devices.

Journal ofMaterials Chemistry C

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View Article OnlineView Journal | View Issue

aState key Laboratory of Luminescence an

Optics, Fine Mechanics and Physics, Chines

3888, Changchun, 130033, People’s RepublibUniversity of the Chinese Academy of Scien

China

Cite this: J.Mater. Chem.C, 2014, 2, 56

Received 28th September 2013Accepted 25th October 2013

DOI: 10.1039/c3tc31910e

www.rsc.org/MaterialsC

56 | J. Mater. Chem. C, 2014, 2, 56–63

Hybrid quadrupolar resonances stimulated at shortwavelengths using coupled plasmonic silvernanoparticle aggregation

Ming-Ming Jiang,a Hong-Yu Chen,ab Bing-Hui Li,a Ke-Wei Liu,a Chong-Xin Shana

and De-Zhen Shen*a

The coupling interaction among metal nanoparticles can cause asymmetric distribution of the surface

charges, which can lead to the cleavage of surface plasmon resonance. Higher order resonance modes

in plasmonic nanostructures exhibit sharp resonance and dramatic line shape shifts. These unique

resonant behaviours existing in nanostructures can be used to improve the performance of wide band

gap semiconductor devices, such as increasing the output power of lasers. According to the theoretical

calculation and simulation, silver nanoparticles have been prepared experimentally, and the

corresponding extinction spectra were also tested to verify the simulation results. It is found that the

hybrid quadrupolar resonance can indeed occur in the short wavelength region.

1 Introduction

Higher order resonance modes result from interference of broadand narrow excitation modes, and they are typically more sensi-tive to the coupling interaction of metal nanostructures as well aschanges of the refractive index of the environment,1 such as Fanoresonance, which is mainly due to the coupling of dark quad-rupolar and higher order modes with bright dipolar modes ofnanoparticles. In the past decade, it has been reported that Fanoresonances can also be generated in plasmonic nano-structures.2–4 Thus, considerable interest has emerged withmanypromising applications in physical, chemical, and biologicalscience.5–8 Among the topics related to enhancement of thesensitivity, Fano resonance and higher order resonance modeshave become the focus in the implementation of enhancementsin recent years.9–14 Coupled plasmonic nanostructures providegood tunability to the generation and resonance intensity ofhigher-order surface plasmon resonance. Among the practicalexamples reported, disk ring (DR) plasmonic nanostructures arethe most typical coupling plasmonic nanostructures, where thering provides higher order multipolar resonance modes (sub-radiant modes or narrow dark mode) which are coupled to thedisk dipolarmode (superradiantmode or broad brightmode).13,15

Breaking the symmetry by displacing the disk with respect to thering provides a crucial mechanism for enhancing the coupling ofthe plasmon modes. It is reported that quadrupolar resonance

d Applications, Changchun Institute of

e Academy of Sciences, Dongnanhu Road

c of China. E-mail: [email protected]

ces, Beijing, 100049, People’s Republic of

modes are excited at the normal incidence while the excitation ofthe higher order modes (e.g., octupolar and hexadecapolarmodes) needs an oblique incidence. By variation of the incidentangle, the shape of Fano resonance can be altered from asym-metric to symmetric. Despite the design of plasmonic structuresexhibiting Fano resonances at specic wavelengths is a chal-lenging task because of their complex nature. A central issue inthis design is the spectral engineering of the resonances viacontrolled hybridization of the available modes.

Many of the original studies on plasmonic Fano resonanceswere carried out on metallic arrays, dolmen-type slab arrange-ments and the non-concentric ring/disk cavity.1 To implementthe Fano resonance, one of the most important things is toproduce higher order resonance modes. According to thepublished results of Fano resonances, experimental conditionsare harsh. In metal nanoscale aggregation, the coupling effectamong metal nanoparticles can lead to the energy level splittingof surface plasmon resonances, and a shi of the resonancepeaks. However, this is difficult in systems where higher ordermodes are excited in the short wavelength spectral range ofinterest.2,16 For example, the random distribution of metalnanostructures, such as metal nanospheres, nanorods, trianglenanoparticles and so on, whether higher order resonancemodes can be achieved or not is still a topic worth studying.

In recent years, much attention has been given to wideband gap semiconductors, such as ZnO, and GaN, for theirwide applications in blue and ultraviolet light emitters anddetectors.17–19 However, there is still a lot of room forimprovement of the efficiency of luminescence and lasers.20–27

How to make resonance peaks appear in the ultraviolet regimeproves to be the dilemma for improvement of the laser lumi-nous efficiency.28–30

This journal is © The Royal Society of Chemistry 2014

http://dx.doi.org/10.1039/C3TC31910E

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http://pubs.rsc.org/en/journals/journal/TC?issueid=TC002001

Paper Journal of Materials Chemistry C

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. View Article Online

In this article, the extinction spectra of the dimer and theheptamer and their corresponding electromagnetic elddistribution were studied by using the Finite Difference TimeDomain (FDTD) method. Asymmetric distribution of thesurface charges, as well as the cleavage of surface plasmonresonance will be caused by the interparticle coupling interac-tion in the silver nanoparticle aggregates under certain condi-tions. Due to the energy level splitting, formation ofquadrupolar resonance mode and other higher multipolarresonance modes may occur in the short wavelength region.The calculation of the extinction spectrum of the aggregationand the electromagnetic eld distribution provide an intuitiveapproach to study the random distribution of the silver nano-particles. Experimentally, as long as the size and spacingdistance of silver nanoparticles are within the range oftheoretical calculation results, as well as the concentration oftrimers and tetramers extracted from random silver nano-particles, hybrid quadrupolar resonances can be produced byusing coupled plasmonic silver nanoparticle aggregation in theshort wavelength region.

Fig. 1 Extinction (scattering + absorption) spectra of silver nano-particles with a gap between the central and surrounding Ag nano-particles ranging from 5–30 nm, and the corresponding (a)–(f) arefrom the dimer to the heptamer respectively. The radius of the centralnanoparticle is denoted as r1, the radius of the surrounding nano-particles is denoted as r2, and the gap distance between the centralnanoparticle and the surrounding nanoparticle is denoted as g.

2 Results and discussion2.1 Theoretical section

The size, shape, and surrounding medium of metal nano-particles have a very important inuence on the localizedsurface plasmon resonances. Surface plasmon (SP) couplingamong nanoparticles can induce redistribution of collectiveoscillations of the conduction electrons.15,16 The coupling canlead to plasmon-induced photoluminescence. A nanoparticle isone of the most promising nanostructures to form Fanoresonances and higher-order dipole resonance. The spectraloverlap and destructive interference of these two modes, dipoleand quadrupole modes, leads to the formation of the higher-order dipole resonance. Reported results also show that bymodifying the interparticle separation, relative particle size, orby breaking the symmetry of plasmonic nanoparticles, themodulation depth as well as the spectral position of the surfaceplasmon resonances can be highly tuned.31–36

Recently, much progress on the Fano resonance has beenmade as demonstrated by a variety of experimental and theo-retical studies in different physical settings.1,3,4,13,15 Feng Haoet al. (2009) demonstrated the tunability of subradiant dipolarand Fano-type plasmon resonances in metallic ring/diskcavities.15 This nanostructure consisted of a ring and a disk, andsustained both subradiant and superradiant dipolar mode setup via hybridization of individual disk and ring plasmons. Thesubradiant mode exhibits a substantially narrower line widthand higher eld enhancement than the parent plasmons.Plasmonic heptamers made of silver and all-dielectric oligo-mers exhibit well-pronounced Fano resonances with strongsuppression of the scattering cross-section.2,16,31 The analysisreveals that this type of Fano resonance originates from theoptically induced coupling interaction between dipole andquadrupole modes of individual nanoparticles. Based on theresults, this asymmetric type coupling interaction between


nanoparticles will lead to plasmon redistribution and shiingof the resonance peak position.

The strength of the interaction between the central and thesurrounding plasmons is controlled by the distance betweenthe nanospheres.2 Simultaneously, for the sake of promptingthe multipole resonance occurred in the short wavelengthregion, we should focus on the coupling interaction between thecentral nanoparticle and the surrounding nanoparticles, andthe coupling interaction among the surrounding nanoparticlesof aggregation.

In order to understand the interparticle coupling interactionof a random distribution of metal nanoparticles, a simple andideal nite element model consisting of a central sphere andsurrounding spheres was built to calculate the extinctionspectra using the FDTD method based on the quasi-particleapproximation as shown in Fig. 1. The metallic particles in thisstudy are assumed to be silver spheres, and will be modeled byusing experimental dielectric data (JC).37 The refractive index ofthe environment is dened as 1.33. The aggregate changes fromthe dimer to the heptamer, the radius of the central Ag nano-particle r1 ¼ 40 nm, the radius of the surrounding Ag nano-particles r2 ¼ 30 nm, and the radius of silver nanoparticlesshown in Fig. 1(b) is denoted as r1 ¼ 40 nm. The gap betweenthe central and the surrounding nanoparticles of differentcross-sections of Ag nanoparticle aggregates g ranged from5 nm to 30 nm, and the type of aggregation from dimer to

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Fig. 2 Extinction (scattering + absorption) spectra of silver nano-particle aggregation: (a) shows the normalized extinction of thetetramer with the radius of the central nanoparticle r1 ¼ 40 nm, theradius of the surrounding nanoparticles r2 ¼ 30 nm, the distancebetween the central nanoparticle and the surrounding nanoparticlesg ¼ 10 nm; (b) shows the normalized extinction of the heptamer withthe radius of the central nanoparticle r1 ¼ 40 nm, the radius of thesurrounding nanoparticles r2 ¼ 30 nm, and the distance between thecentral nanoparticle and the surrounding nanoparticles g ¼ 10 nm.

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heptamer. Fig. 1(a) and (b) demonstrate that when the gap is ina certain range, such as 5 nm, the interaction between thenanoparticles is much strong such that interparticle couplingwill lead to shiing of the spectral position of the plasmonresonance compared to the case of an isolated particle. Theextra format is the quadrupole resonance at around 350 nm.Fig. 1(c) shows that the interparticle coupling between thecentral nanoparticle and the surrounding nanoparticles isstronger than the interaction between the surrounding nano-particles of the tetramer. The quadrupole resonance of thetetramer is also relatively strong compared with the dipolemode. So the resonance peak located in the short-wave regionwill dominate in the extinction spectrum. Fig. 1(d)–(f) reveal theextinction spectra of the pentamer, hexamer and heptamerrespectively. From these extinction spectra, we can get that theemergence of the special line shape rooted in the couplinginteraction between the plasmons originated from theinterparticle coupling between the central nanoparticle and thesurrounding nanoparticles, and the plasmons originated fromthe interparticle coupling between the surrounding nano-particles. The plasmon hybridization can be used to gure outthe multifeatured plasmon response of more complex metallicnanostructures, such as dimers or other nanoparticleaggregates. An aggregate is composed of a central silver nano-particle and several surrounding nanoparticles, and theassociated interaction between the central nanoparticle and thesurrounding nanoparticles, leading to higher order resonancemodes.

When the gap g is in certain ranges, such as 5 nm, and 10 nm,we can observe the special linear spectral lines. The extinctionspectra reveal a special narrow feature in the short wavelengthregion as shown in Fig. 1(d)–(f). These types of spectral lines arein agreement with the results published in previous studies,10,38

which are called Fano resonance. The quadrupole resonancesappear naturally in such systems when the interparticle couplinginteraction occurred between the central nanoparticle and thesurrounding nanoparticles.3,16 When the two oscillator modesare of similar frequency the interference results in a symmetricantiresonance. If the frequencies are different, the typicalasymmetric multiple resonances appear.2 The quadrupole modeof the metal nanoparticle aggregation can overlap with thesuperradiant bright mode due to the small energy gap, whichmay help to form Fano resonances.1

The interparticle interaction results in the splitting of theplasmon resonances into two new resonances: the lower energysymmetric and the higher energy antisymmetric plasmons. Theirinteraction results in other hybridized plasmon resonances asdemonstrated in Fig. 1; we have shown that the plasmonresponse of metal-based nanostructures can be viewed as thecollection of plasmons arising from simpler geometries to forman interacting system. The plasmonics of the metallic nano-structures are determined by the electromagnetic interactionbetween these free plasmons, which leads to mixing (hybridiza-tion), splitting, and shiing of the plasmon energies.2,16,31

For further exploration of factors affecting the shi of thepeak position, Fig. 2 shows the extinction spectra of thetetramer and heptamer placed on a sapphire substrate

58 | J. Mater. Chem. C, 2014, 2, 56–63

surrounded by dielectric embedding media of differentpermittivities. Fig. 2(a) demonstrates the quadrupole resonancerooted in the interparticle coupling interaction between thecentral silver nanoparticle and the surrounding nanoparticles.Fig. 2(b) demonstrates the Fano resonance. Both panelsdemonstrate that the effect of dielectric screening is a strongred-shi of the multiple resonances. Particularly for thetetramer, the LSPR shi for the multiple resonances in Fig. 2(a)is from 250 nm to 350 nm. As the shi of the multipolar formatpeak position is in the control range of the short wavelengthregion, the dielectric function of the environment mediumshould not be too big, for example 3d # 2 as shown in Fig. 2.

In the following, we used the FDTD method to simulate thedistribution of the electromagnetic eld for the aggregationthrough the plasmon hybridization picture. For small particles,the diameter of the particle and the interparticle distance d, g,under the condition d, g � li. These interparticle interactionsamong ensembles are essentially of dipolar nature, and theparticle ensemble can in the rst approximation be treated as an




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ensemble of interacting dipoles. However, when the size anddistribution of nanoparticles in the aggregate appears as asym-metric distribution, along with the size increase of the nano-structure, the electric eld of the light can no longer be assumedto be uniform inside the particles, and higher order (quadru-pole, octupole, etc.) plasmon modes can directly couple with theelectric eld of the light simply due to the phase retardationeffect. Excitation of multipole plasmon resonances is alsocaused by the asymmetric distribution of the surface plasmonsexcited by the electromagnetic interactions between the local-izedmodes.When the gap distance among nanoparticles is largeenough, every metal nanoparticle exists as an individual, so theinteraction between nanoparticles is very weak. When the gap issmall in certain ranges the aggregation almost exists as a whole.So in both cases it is impossible to cause the energy splitting andthe resonance peak position shiing. Therefore, the corre-sponding resonance peak was located in the short wavelengthregion as shown in Fig. 1. We have simulated the aggregationfrom dimers to heptamers at the incident wavelength li ¼ 350nm as shown in Fig. 3. The radius of the central nanoparticle is40 nm, and the radius of the surrounding nanoparticles is 30nm. Close inspection of ring structures of aggregates showssuperposed bright features identied as the electromagneticinteraction formed by the quadrupole plasmons and dipoleplasmons that is thought to be the hybridization couplinginteraction between the dipole modes and the quadrupolemodes. Fig. 3(a)–(c) demonstrate that the interparticle couplingamong the surrounding nanoparticles is very weak, the maininteraction existed between the central nanoparticle and its

Fig. 3 The spatial distribution of the electric field intensity in aggre-gates with the gap g¼ 10 nm between the central nanoparticle and thesurrounding nanoparticles; the incident wavelength is li ¼ 350 nm; thelight is vertically incident on the cross-section of the aggregate.


surrounding nanoparticles. Fig. 3(d)–(f) demonstrate that theinterparticle coupling interaction exists between the centralnanoparticle and its surrounding nanoparticles, and the inter-particle interaction exists between the surrounding nano-particles. We may safely draw the conclusion that higher ordermodes, such as quadrupolar resonance, octupolar resonance,hexadecapolar resonance and so on, are derived from thecoupling interaction between the central nanoparticle and itssurrounding nanoparticles.

Take the heptamer for example, the nanoparticles of theheptamer are silver nanospheres with the diameter d ¼ 50 nm.When the gap distance between nanoparticles is in a certainrange, such as 10–30 nm, electromagnetic eld distribution wassimulated, as shown in Fig. 4. At the incident wavelength li ¼500 nm, with gap g ¼ 20 nm and g ¼ 30 nm, the distribution ofthe electromagnetic eld line is completely symmetrical; thecorresponding resonant peak line is also completely symmet-rical as shown in Fig. 1. At the incident wavelength li ¼ 350 nm,with gap g ¼ 20 nm and g ¼ 30 nm, the asymmetric distributionof the electromagnetic eld line appears accordingly. Thissituation will directly lead to the splitting of energy levels.Compared with Fig. 4(a) and (b) and Fig. 4(c) and (d), there is asuperimposed region resulting from the interaction shown inFig. 1(f). The plasmon hybridization picture can be used todescribe the sensitive structural tunability of the plasmonresonance as the interaction between plasmons is supported bya nanoscale central nanoparticle and the surrounding nano-particles. This simple and intuitive picture can also be used tounderstand the plasmon resonance behavior of compositemetallic nanostructures of greater geometrical complexity. Theplasmon hybridization picture is important because it providesthe nanoscientist with a powerful and general design principlethat can be applied both qualitatively and quantitatively, andguides the design of metallic nanostructures and predict theirresonant properties.

Fig. 4 The spatial distribution of the electric field intensity in theheptamer with the diameter of the nanoparticle being 50 nm: (a) and(b) show that at the incident wavelength li ¼ 500 nm, with differentgaps among the nanoparticles 20 nm, 30 nm, compared with theincident wavelength li ¼ 350 nm (c) and (d).

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From the results above, the hybrid quadrupolar resonancestimulated in the short wavelength region should meet somerequirements: the interparticle coupling interaction among thesurrounding nanoparticles should be weaker than the inter-particle coupling interaction between the central nanoparticleand the surrounding nanoparticles. As the distance between thecentral nanoparticle and the surrounding nanoparticles issmaller than the distance between the surrounding nano-particles, the number of the surrounding nanoparticles shouldnot be too many, the best is two or three. The effect of asurrounding dielectric medium on the extinction spectrum ofthe silver nanosphere aggregation also demonstrated that therefractive index of the environment medium should not be toohigh, otherwise the peak positions of the spectrum will be redshied. Compared with silver, we also studied gold nano-particles and found that gold nanoparticle aggregates canexhibit the multipole resonance phenomenon, but not in theshort wavelength region. So gold is not suitable for study in theshort wavelength band.36,37

Fig. 5 SEM images of nanoparticle aggregates with different sput-tering times 3 min, 7 min, 8 min, 9 min, 11 min, and 12 min respectively.

2.2 Experimental section

In this article, Ag (99.99%) was deposited on the c-face sapphireby a radio-frequency magnetron sputtering technique at roomtemperature with a pressure of 5 � 10 Pa, and subsequentlyannealed in a N2 atmosphere at 350 �C to form Ag nanoparticles.The morphology of the samples was characterized using a scan-ning electronmicroscope (SEM), as shown in Fig. 5 with differentsputtering times from 1 min to 12 min. The sputtering time willdirectly determine the spacing distance among the silver nano-particles and the size of the nanoparticles. Although, silvernanoparticles are randomly distributed during the preparationprocess, the central silver nanoparticle and the surroundingsilver nanoparticle ring-like aggregate can be extracted from therandom distribution. With the sputtering time of 3 minutes,silver nanoparticles existed are mainly dimers. With the exten-sion of the sputtering time, the aggregate of silver nanoparticlesextracted from dimer to trimer to tetramer can appear even as aheptamer. Detailed SEM images are shown in Fig. 5. Fig. 5(a)demonstrates that the dimer and trimer of silver nanoparticlescan easily be extracted as an aggregate, the diameter ranged from50 nm to 90 nm, and the corresponding gap distance rangedfrom 10 nm to 40 nm. Naturally, the hybrid quadrupolar reso-nance can be generated in the short wavelength region accordingto our calculated results. Just because the sputtering time isrelatively short, it leads to the relatively large spacing distanceamong silver nanoparticles. When we extend the time of sput-tering of silver nanoparticles, the concentration of trimers andtetramers collected from random distribution of silver nano-particles will continue to improve, the corresponding hybridquadrupolar resonance will also continue to increase. The bestexperimental results as shown in Fig. 5 involve a sputtering timeof 9 minutes. Hence, the diameter change of the silver nano-particles is not too much, ranged from 50 nm to 100 nm, and themain changes are in the spacing distance among silver nano-particles and the concentration of trimers and tetramerscollected from random distribution of silver nanoparticles, as

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shown in Fig. 5. So the spacing distance among the silvernanoparticles and the concentration of trimers and tetramers arethe most directly inuencing factors of the hybrid quadrupolarresonance intensity. Thus two types of dipolar resonances areformed in this structure: hybridized bonding and antibondinglinear combinations of the central localized and the surroundinglocalized modes.2

The extinction spectra of random silver nanoparticles weremeasured using a Shimadzu UV-3101PC scanning spectropho-tometer. Fig. 6 demonstrates the extinction spectra of Agnanoparticles sputtered on the c-plane sapphire with differentsputtering times from 1 to 12 min. There are two resonances ataround 350 and 500 nm, which are the quadrupole and dipolemodes, respectively. From the extinction spectrum graphics,one can know that when the sputtering time is short, such as 1–3 min, the asymmetric prole curve is not very obvious. So thereis only one of the main resonance peak positions in theextinction spectra, which is conrmed by the theoreticalcalculation and simulation results. Silver nanoparticles mainlyexist in the form of single nanoparticles and dimer aggregates,but with a large gap distance. As the sputtering time isincreased, the multipolar resonance prole curve becomesmore and more obvious. So in the short wavelength region,another resonance peak appears, which is caused by the energylevel splitting.3,4 And the corresponding surface plasmon reso-nance is identied as the hybrid quadrupolar resonance. Fromthe extinction spectral curve, the concentration of trimers andtetramers collected from random distribution of silver nano-particles as well as the gap distance among silver nanoparticles



Fig. 6 Extinction spectra of silver nanoparticles sputtered on the c-plane sapphire with different sputtering times from 1 min to 12 min.

Fig. 7 Extinction spectra of gold nanoparticles sputtered on the c-plane sapphire with different sputtering times of 1 min, 3 min, 6 min,and 9 min respectively.


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are the crucial factors for the generation and intensity of thehybrid quadrupolar resonance. Therefore, under certainexperimental conditions, the hybrid quadrupolar resonance canbe produced in the short wavelength region using the randomdistribution of silver nanoparticles.

Compared with silver, gold nanoparticles are more suitablefor study in the 500 nm to 600 nm wavelength region.39 Goldnanoparticles also were prepared under the same experimentalconditions and environment with different sputtering times of 1min, 3 min, 6 min, and 9 min respectively.37,40,41 The corre-sponding extinction spectra are shown in Fig. 7. The obviousquadrupolar resonance peak cannot be observed. There is onlyone resonance at around 550 nm when sputtering for 1 min,this is dipole resonance mode. When sputtering for 3 min,enhancement of dipole resonance can be derived from strong


interparticle interactions among gold nanoparticles. Theincrease of sputtering time will lead to the decrease of thedistance between the nanoparticles. Compared with silveraggregation, the controllability of gold nanoparticles can onlyenhance the dipole resonance without the cleavage of surfaceplasmon resonance. With the further extension of the sputter-ing time of 9 min, the decrease in the spacing distance betweennanoparticles will make gold nanoparticles lose individualexistence. Therefore, when sputtering for 9 min, obvious dipoleresonance cannot be observed. Therefore, hybrid quadrupolarresonance cannot be produced in the short wavelength regionusing coupled plasmonic gold nanoparticle aggregation.

The hybrid quadrupolar resonances appear naturally in suchsystems when the surrounding localized plasmon modes arecoupled to and spectrally overlapped with the central localizedplasmon modes. The effective interaction between such twomodes is dispersive and can result in a strong interference inthe oscillator amplitudes which in turn inuences the radiationemitted from the system. When the two oscillator modes havesimilar frequency, the interference results in a symmetricantiresonance. If the frequencies are different, the typicalhybrid quadrupolar resonance appears.38

The peak wavelength, the peak width, and the effect ofsecondary resonances yield a unique spectral ngerprint for a

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plasmonic nanoparticle with a specic size and shape. Addi-tionally, UV-Visible spectroscopy provides a mechanism tomonitor the changes in nanoparticles over time. When silvernanoparticles aggregate, the metal nanoparticles becomeelectronically coupled and this coupled system has a differentSPR from the individual nanoparticles.27,42 In the case of amulti-nanoparticle aggregate, the plasmon resonance will beblue-shied to a shorter wavelength than the resonance of anindividual nanoparticle, and aggregation is observable as anintensity increase in the blue/violet region of the spectrum. Thiseffect can be observed in Fig. 1–4, which displays the opticalresponse of a silver nanoparticle solution destabilized by theaddition of saline. Carefully monitoring the UV-Visible spec-trum of the silver nanoparticles with the sputtering time is asensitive technique used for determining if any nanoparticleaggregation has occurred.43

3 Conclusions

In summary, we have shown that the interparticle couplinginteraction among the random distribution of silver nano-particles can lead to mixing (hybridization), splitting, andshiing of the plasmon energies, as well as the hybrid quad-rupolar resonances formed by the interaction between theplasmons of the exocyclic metal nanoparticles and the centralmetal nanoparticle. This interaction also results in the splittingof the plasmon resonances into two new resonances: the lowerenergy symmetric plasmon and the higher energy antisym-metric plasmon. Particularly the higher energy antisymmetricplasmon embodies the hybrid quadrupolar resonances and canbe produced in the short-wavelength region. The physicalproperties can be applied in wide band gap semiconductorbased lasers and detectors. Our work affords a theoretical andexperimental method, and provides evidence that silver nano-particle aggregation produces hybrid quadrupolar resonancesin the short-wavelength region.

Acknowledgements

This work was supported by the National Basic ResearchProgram of China (973 Program) (no. 2011CB302006,2011CB302004), the National Natural Science Foundation ofChina (no. 10974197, 11174273, 11104265, 21101146), and the100 Talents Program of the Chinese Academy of Sciences.

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