Simulated study of plasmonic coupling in noble bimetallic alloy nanosphere arraysAmit Bansal and S. S. Verma

Citation: AIP Advances 4, 057104 (2014); doi: 10.1063/1.4875759 View online: http://dx.doi.org/10.1063/1.4875759 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/4/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Plasmonic properties of Au-Ag nanoparticles: Distinctiveness of metal arrangements by optical study J. Appl. Phys. 115, 053517 (2014); 10.1063/1.4864428 Ion irradiation synthesis of Ag–Au bimetallic nanospheroids in SiO2 glass substrate with tunable surface plasmonresonance frequency J. Appl. Phys. 114, 054308 (2013); 10.1063/1.4817725 Optimization of the bimetallic gold and silver alloy nanoshell for biomedical applications in vivo Appl. Phys. Lett. 97, 061904 (2010); 10.1063/1.3479053 Do nanomaterials form truly homogeneous alloys? J. Appl. Phys. 107, 104325 (2010); 10.1063/1.3428455 Surface plasmon resonance-based gas sensor with chalcogenide glass and bimetallic alloy nanoparticle layer J. Appl. Phys. 106, 103101 (2009); 10.1063/1.3255972

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AIP ADVANCES 4, 057104 (2014)

Simulated study of plasmonic coupling in noble bimetallicalloy nanosphere arrays

Amit Bansala and S. S. VermaDepartment of Physics, Sant Longowal Institute of Engineering & Technology,Longowal-148106, District-Sangrur, Punjab (India)

(Received 7 January 2014; accepted 29 April 2014; published online 7 May 2014)

The plasmonic coupling between the interacting noble metal nanoparticles plays animportant role to influence the optical properties of arrays. In this work, we haveextended the Mie theory results of our recent communication to include the effect ofparticle interactions between the alloy nanoparticles by varying interparticle distanceand number of particles. The localized surface plasmon resonance (LSPR) peak posi-tion, full width at half maxima (FWHM) and scattering efficiency of one dimensional(1D) bimetallic alloy nanosphere (BANS) arrays of earlier optimized compositionsi.e. Ag0.75Au0.25, Au0.25Cu0.75 and Ag0.50Cu0.50 have been studied presently by us-ing discrete dipole approximation (DDA) simulations. Studies have been made tooptimize size of the nanosphere, number of spheres in the arrays, material and theinterparticle distance. It has been found that both the scattering efficiency and FWHM(bandwidth) can be controlled in the large region of the electromagnetic (EM) spec-trum by varying the number of interacting particles and interparticle distance. Incomparison to other alloy arrays, Ag0.50Cu0.50 BANS arrays (each of particle radius50 nm) shows larger tunability of LSPR with wide bandwidth (essential conditionfor plasmonic solar cells). C© 2014 Author(s). All article content, except where other-wise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4875759]

I. INTRODUCTION

In noble metal nanostructures, the light trapping at nanoscale using surface plasmons has becomeone of the most expanding fields to improve the efficiency of thin-film solar cells. Solar cells arethe materials that lead to the production of electric energy directly from the conversion of lightenergy. The third generation solar cells are based on thin-film technology in which thin-film of theabsorbing materials is used. No doubt the thin-film of the materials used corresponds to the lowcost but with large reduction in the absorption of light; thus reducing the efficiency of thin-filmsolar cells. Therefore, to increase the efficiency of thin-film solar cells with low cost, the scatteringfrom plasmonic metal nanostructures over the broad spectral region makes a considerable attraction.Plasmonic nanostructures play a crucial role to increase the efficiency of thin-film solar cells neartheir localized surface plasmon resonance (LSPR) by placing them on the surface of absorbinglayer.1–4 Plasmonic noble metal nanoparticles such as silver (Ag), gold (Au) and copper (Cu) exhibita special kind of property called surface plasmon resonances and this property originates whenEM waves are incident on the surface of noble metal nanoparticles. The energy associated with theelectric component of the incident EM waves is transferred to the conduction electrons in the noblemetal nanoparticles and with this energy, the conduction electrons starts oscillating with respect to thepositive background core. This causes charge separation at the surface of noble metal nanoparticle,which leads to the production of restoring force due to the columbic interaction between the electron

aEmail: amit.bansal133@yahoo.com

2158-3226/2014/4(5)/057104/14 C© Author(s) 20144, 057104-1

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057104-2 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

cloud and positive core. The LSPR in metal nanoparticles takes place when electron cloud oscillateswith the same frequency as that of incident EM waves, where the optical cross-sections (scattering,absorption and extinction) are resonantly enhanced as compared to their geometrical cross-sections.This is due to the large EM field enhancement around the surface of metal nanoparticle. The LSPR innoble metal nanoparticles can be controlled and tuned throughout entire region of the EM spectrumby varying various factors such as shape, size, surrounding medium, metal composition, interactionbetween the particles and polarization direction etc.5–7 With this flexibility in tunability of theiroptical properties, noble metal nanoparticles can be used in variety of emerging applications suchas thin-film solar cells,2, 4 plasmon sensors,8 surface enhanced Raman spectroscopy (SERS)9 andbiomedical applications10 etc.

The optical properties of individual noble metal nanoparticles (Ag, Au and Cu) have beenstudied by many researchers11–13 but their bimetallic nanoparticles such as alloys14, 15 and core-shellnanostructures16 have attracted much attention, because of their large tunability in LSPR as comparedto the individual noble metals. The optical properties of alloy nanoparticles can be tuned by varyingthe metal and their composition in the alloy nanoparticles. The formation of alloys results in thebroadening of resonance peak with an LSPR that lies in between the LSPRs of the individual metals.Moreover, the advancement in the bimetallic nanostructures synthesis of Au-Cu,14 Ag-Au17 and Ag-Cu18 with controlled optical properties makes them suitable for practical applications. In our recentstudy,19 the LSPR based scattering efficiency for noble metals Au, Ag and Cu with all their possibleBANSs such as Au-Cu, Ag-Au and Ag-Cu have been studied by using Mie theory and it was foundthat Ag0.50-Cu0.50 BANSs are the best material of choice in comparison to the other two alloys, toenhance the efficiency of plasmonic thin-film solar cells. In order to enhance the efficiency, the metalalloy nanoparticles are embedded directly on the top of a thin-film semiconductor absorber layer.The noble metal nanoparticles are deposited on the thin-film substrate using several experimentaltechniques like thermal evaporation method,20 Porous alumina template technique,21 Nanoimprintlithography method22 and Electro-deposition method.23 Similar experimental techniques may also beused to deposit noble metal alloy nanoparticles. However, for practical applications of nanoparticlesin the form of arrays, in addition to the size, surrounding medium and metal composition in thealloy nanoparticles, particle interaction is also an important parameter. The array of nanoparticlesgives different resonant peak from that of individual nanoparticles due to the plasmonic couplingamong the nanoparticles. In arrays, the plasmonic coupling plays an important role to tune the LSPRpeak position and bandwidth as a function of number of nanospheres and interparticle distance.24

Plasmonic coupling takes place when the particles are close to each other and therefore, the plasmonresonance of each particle is affected by plasmon resonance of its neighboring particle. The strengthof plasmonic coupling depends on the interparticle distance and number of interacting particles.25, 26

Under parallel polarization i.e. when polarization is along the interparticle axis, as the gap betweenparticles is reduced, red-shift in the plasmon resonance takes place along with broadening of peak.Under perpendicular polarization, a small blue-shift with decrease in the gap between the particlesis observed.5, 27 The electronic and structural properties of bimetallic arrays have been found inliterature28, 29 but their optical properties also need to be optimized for their possible practicalapplications. In our best knowledge by reviewing the literature, no work has been carried out tostudy the scattering properties of BANS arrays for plasmonic solar cells. Therefore, in the presentstudy, the scattering properties of BANS arrays have been studied from the view point of solar cellsapplications. In solar cells, the essential condition to enhance the efficiency of the cells is to increasethe scattering from the nanoparticles with wide bandwidth (FWHM) in the photoactive layer ofthin film solar cells.30 However, analytical methods are unable to study the plasmonic interactionsin arrays therefore several numerical methods like discrete dipole approximation (DDA), finitedifference time domain method (FDTD), finite element method (FEM) etc. have been proposed butDDA is considered to be the most powerful method to study the optical properties of arrays.26, 27

Therefore in this paper, DDA method31 is used to optimize the scattering efficiency and LSPR peakposition over a broad spectral region of wavelengths so that the absorption efficiency of thin-filmsolar cells can be enhanced. We have investigated the effect of particle interactions on the scatteringefficiency and FWHM for the simplest form of array i.e., one dimensional (1D) BANS arrays becausethe coupling of two or more nanoparticles is also a crucial parameter for plasmonic solar cells and

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057104-3 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

for designing future plasmonic devices. However, similar studies can be extended for two or threedimensional BANS arrays in future course of work.

The present work uses the DDA method to simulate the excitation of LSPR in BANS arrays inwhich the radius (R) of the nanoparticles are varied from 10 nm to 50 nm, number of interactingparticles are varied from 2 to 8 with varying interparticle distance in BANS dimer. In presentcalculations, the polarization direction is considered to be along the interparticle axis i.e. alongy-axis because of maximum electric-field (EF) enhancement in this direction.26 Here, the size ofindividual nanoparticle, surrounding medium refractive index and size of the arrays are optimized forhigher scattering and larger FWHM, so that the entire region of the visible and infrared is coveredwhich is the essential condition to increase the efficiency of plasmonics solar cells. In previousstudy,19 the effect of size, surrounding medium, and alloy composition on the scattering efficiencyand FWHM has been already reported for single BANSs but for plasmonic solar cells applications,the interaction between the particles also plays an important role, therefore, the study of interactionsbetween these BANSs is the main aim of present study. In this paper, we first compare the scatteringspectra of individual BANSs calculated by using Mie theory with DDA simulations and further thescattering spectra for BANS dimer has been studied by varying the size of individual particle andtheir interparticle distance. Finally, we studied the scattering spectra of BANS arrays by varying thenumber of interacting spheres.

II. SIMULATION METHOD

Numerical simulations are being performed by using DDA method implemented in source codeDDSCAT 7.2,32 which is freely available and download from [http://code.google.com/p/ddscat/].The DDA is a discrete dipole approximation method for solving Maxwell’s equations, which isable to calculate the scattering and absorption of EM plane waves by isolated targets, or periodictargets in one or two dimensions.33 In this approximation, the given target is divided into the cubicarray of N polarizable dipoles with lattice spacing d and each cube having a definite polarizability.The interactions between the dipoles in each cube and with the incident EF giving rise to a systemof linear equations, which are solved by using fast Fourier transform (FFT) method to obtain thepolarizability of each cube and hence the optical properties of nanostructures can be calculated. Inorder to obtain the accurate results, the target should consist of large number of polarizable dipolesN with minimum dipole spacing d. In present simulations, the numbers of dipoles used were nearly∼ 8 × 104 and size of the target is represented by an equal volume sphere of effective radius aeff

= (3V/4π )1/3. In simulations, the results are calculated by considering the parallel polarization ofincident light along the interparticle axis i.e. along y-axis and the direction of propagation along x-axis. The refractive index of 1.33 (as used in earlier communication)19 has been taken as surroundingmedium. The complete detail of mathematical description of the DDA can be found in.31

III. RESULTS AND DISCUSSION

The behavior of surface plasmons of noble metal nanostructures is described on the basis offrequency dependent complex dielectric function and is given by sum of the dielectric functioncontribution from intraband transitions in the bulk material (as Drude model)34 and the interbandtransition of bound electrons i.e. from d to sp-band transitions which becomes important in the visibleregion of the EM spectrum. The experimentally obtained complex dielectric functions for Ag, Auand Cu are given by Palik35 and for alloys it is calculated by using average weighted method.14, 15 Thereal and imaginary parts of the metal dielectric functions and their alloys of selected compositionsare shown in Figure 1.

The selected alloy compositions are Ag0.75Au0.25, Au0.25Cu0.75 and Ag0.50Cu0.50, because inour previous study,19 we already show that these compositions are best for plasmonic solar cellsapplications in comparison to other compositions. The real part of the metal dielectric functiondetermines the LSPR peak position in response to the surrounding medium refractive index andthe imaginary part leads to the broadening of resonance peak. Both the real and imaginary partsof the dielectric functions shows a decreasing trend in the order of Ag0.50Cu0.50 > Ag0.75Au0.25 >

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057104-4 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

FIG. 1. (a) Real and (b) Imaginary parts of the dielectric functions for pure and alloy nanoparticles.

Ag with increase in the Ag composition. Therefore, it is expected to increase in the scatteringrelative to that of absorption in the extinction spectra and narrowing the bandwidth. However, theoptical properties of Au, Cu and their alloys are more damped. For solar cells applications, we needmore tunability in scattering efficiency over the larger bandwidth but both these parameters have tobe compromised with each other because Ag0.75Au0.25 has high scattering efficiency with narrowbandwidth and Ag0.50Cu0.50 has comparatively less scattering efficiency with reasonable bandwidthbut the cost effectiveness of Ag0.50Cu0.50 in comparison to Ag0.75Au0.25 BANSs makes them usefulfor plasmonic solar cells. In Ag-Au alloy nanoparticles, 25% composition of Au drastically increasesthe stability of Ag.36 However, the lower air stability of both Ag and Cu nanoparticles affects theirLSPR properties especially of Cu nanoparticles but synthesis of Ag, Cu and their alloy nanoparticlesby using a suitable stabilizer or capping agent can make them oxidation free.18, 37

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057104-5 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

During the synthesis of Ag-Cu alloy nanoparticles, the main problem encountered is the ox-idation of Cu nanoparticles which leads to the formation of copper oxide Cu2O layer on Ag-Cualloy nanoparticles. Cysteine was used as stabilizer agent to prevent the oxidation of Ag-Cu alloynanoparticles.18 However, Pal et al.38 reported that the plasmonic properties of Cu nanoparticles areenhanced on formation of small shell layer thickness of copper oxide on Cu nanoparticles. Therefore,the plasmonic properties of Ag-Cu alloy nanoparticles would also be expected to enhance due to theformation of Cu2O layer (Ag0.50Cu0.50@Cu2O). Moreover, due to the low standard potential of Cuthan that of Ag, the reduction of Cu nanoparticle is slower in comparison to the Ag nanoparticleswhich lead to the formation of Ag-Cu alloy core Cu shell nanoparticles, as Tsuji et al.39 has reportedthe synthesis of Ag-Cu alloy core Cu shell nanoparticles by using polyol method and discussed itsoptical properties with better stability. On the other hand, to avoid the oxidation of Ag-Cu nanoparti-cles, the addition of protection layer might be done. Therefore, the control over LSPR peak positionand intensity of Ag-Cu alloy nanoparticles are also studied by coating them with Au as protectionlayer (Ag0.50Cu0.50@Au). The effect of copper oxide (Cu2O) and Au layers on the optical propertiesof Ag-Cu alloy nanoparticles have been shown in Figure 2. Here, the Ag-Cu alloy nanoparticles coreradius was fixed at 40 nm but with varying shell thickness (2 to 10 nm) of both the Cu2O and Aulayers so that the total radius remains up to 50 nm. In case of Cu2O shell (Figure 2(a)), the scatteringefficiency is comparable to the bare Ag-Cu alloy nanoparticles with shell thickness of about 6 nm andfurther increase in thickness leads to enhancement in scattering efficiency with maximum at shellthickness of 10 nm. The scattering efficiency is comparable to Ag-Cu alloy nanoparticles in case ofAu as shell even at Au shell thickness of 10 nm but with better stability (Figure 2(b)). Therefore, theeffect of oxidation and protection layer on the optical properties of Ag-Cu alloy nanoparticles hasbeen included and it has been found that the oxide layer with thickness in the range of 6 to 10 nmenhances the optical properties whereas comparable in case of Au.

A. Single nanosphere: Comparison of DDA with Mie theory

The theoretical calculations using DDA method have been performed to obtain the scatter-ing spectra of single BANSs of selected compositions. To check the accuracy of DDA method,Figure 3 shows the DDA simulated scattering spectra comparison of single Ag0.75Au0.25, Au0.25Cu0.75

and Ag0.50Cu0.50 BANSs with the results obtained by Mie theory.19 The same input parameters asused in Mie theory are taken as input for DDA simulations i.e. the radius of BANS is assumed to be50 nm in the surrounding medium of refractive index 1.33. It can be concluded that for all the threeBANSs, DDA calculations are in good agreement with the results obtained by Mie theory. A smalldip near 320 nm in both the scattering spectra of Ag0.75Au0.25 and Ag0.50Cu0.50 alloy nanospheres isdue to the characteristics of Ag material.

B. Bimetallic alloy nanosphere (BANS) arrays

An individual alloy particle shows a well-defined LSPR between the LSPRs of individualconstituent particles depending upon the composition of metal.15, 17 In case of arrays, the local EFgets modified by the scattering from individual alloy particles which results in the shifting (redor blue) and broadening (narrowing) of LSPR depending upon the size, interparticle distance andnumber of interacting particles etc.40 For small particles with small separations, the EF in the gap isstrongly enhanced by suppressing the scattering for polarization along the interparticle axis but forlarger particles with larger particle separations, the EF in the gap is reduced with increased scatteringwhich results in the broadening of LSPR.41 To study the interactions between the particles, the chosentarget is illustrated in Figure 4 with increase in the size of array, in the form number of identicalcoupled spheres (Figure 4(a)) and with different interparticle distance (edge to edge separations) innanosphere dimer (Figure 4(b)). The incident electromagnetic wave propagates in the x-directionand polarization is assumed to be along the interparticle axis i.e. along y-axis in all the cases.

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057104-6 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

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FIG. 2. Scattering spectra of single Ag0.50Cu0.50 BANSs having radius 40 nm coated with (a) Cu2O as oxidation effect and(b) Au as protection layer, with varying thickness (t) from 2 to 10 nm in surrounding medium refractive index 1.33.

1. Nanosphere dimer: effect of nanosphere size

Figure 5 shows the scattering spectra for the Ag0.50Cu0.50 BANS dimer of various sizes (radius)as an example to optimize the size of individual interacting particle for higher scattering over thelarge spectral region. It has been found that for small size particles, scattering is very less and asthe particle size increases the scattering starts dominate but for plasmonics solar cell applications,scattering is required in comparison to the absorption. Therefore, the larger size is responsible formaximum scattering efficiency and it is optimized to be 50 nm because for too large particles, theradiation damping and dynamic depolarization effects become dominating. Therefore, due to thisthe EF no longer remains uniform around the surface of nanoparticle and hence, the conductionelectrons start to oscillate out of phase with each other which results in reduced scattering.42 Hence,in further study the radius of the individual interacting particles is taken to be 50 nm.

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057104-7 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

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FIG. 3. Scattering spectra of single BANSs having radius of 50 nm calculated by Mie theory (solid lines) and DDAsimulations (dashed lines) with 1.33 as surrounding medium refractive index.

FIG. 4. Target of identical BANSs (R = 50 nm) in an array (a) with no gap between them and varying the number of spheresfrom 2 to 8 (b) Nanosphere dimer with varying interparticle distance g.

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057104-8 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

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FIG. 5. Scattering spectra of Ag0.50Cu0.50 BANS dimer with varying radius (R) of the individual particle.

2. Nanosphere dimer: effect of interparticle distance

In this section, we have studied the optical properties of all the three selected BANS dimers withvarying edge to edge gap distances. Figure 6 shows the scattering spectra in which the interparticledistance is varied from 0 (touching) to 250 nm. In all the three BANS dimers, when the gap betweenthe interacting particles is reduced from 250 to 150 nm, a blue shift in wavelength is found withdecrease in scattering efficiency. However, for Au-Cu dimer the blue shift is very small as comparedto the shift in other two (Figure 7(b)). Further, decreasing the distance between the interactingparticles leads to the red-shift in wavelength. A strong red-shift as well as broadening in dipolarresonance is found when particles are nearly touching i.e., with no gap between them (g = 0 nm). Thisis due to the strong coupling between the individual dipole resonances of the interacting particles.40

The coupling strength increases as the interparticle distance decreases. A strong red-shift has beenfound for the Ag0.50Cu50 BANS dimer with increasing bandwidth over the visible region of the EMspectrum.

A continuous decrease in the scattering efficiency with increase in FWHM (Figure 7(a)) hasbeen found for all the three BANS dimers as the coupling between the interacting particles increaseswith reduced gap between nanoparticles. The increase in FWHM leads to cover the large spectralregion of the EM spectrum which enhances the efficiency of plasmonic solar cells but at the cost ofreduced scattering. Further, the scattering efficiency can be increased by controlling the surroundingmedium environment. In nanosphere dimers, the more tunability in LSPR has been seen in case ofAg0.50-Cu0.50 BANS dimer in comparison to the other two and with larger FWHM in comparison tothe single Ag0.50-Cu0.50 alloy nanoparticle of same size in same surrounding medium. In conclusion,the FWHM can be controlled from 120 to 330 nm, scattering efficiency from 3 to 7.5 a.u. and LSPRwavelength from 510 to 600 nm by varying the interparticle distance and changing metal in thenanosphere dimer.

3. One dimensional array: effect of number of spheres

The effect of number of interacting particles on the scattering efficiency and FWHM for 1DBANS arrays has been studied. Figure 8 represents the scattering spectra of BANS arrays withincrease in number of interacting particles in a surrounding medium of refractive index 1.33. Forthe maximum coupling in arrays, the particles are considered to be touching with each other i.e. nogap between them (g = 0 nm) and the number of interacting particles are varied to 2, 4, 6 and 8.

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057104-9 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.800.0

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FIG. 6. Scattering spectra of (a) Au0.25Cu0.75 (b) Ag0.50Cu0.50 and (c) Ag0.75Au0.25 BANS dimers with varying interparticledistance (g).

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057104-10 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

Scatterin

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FIG. 7. (a) Scattering efficiency and FWHM, and (b) LSPR wavelength of BANS dimers as a function of the interparticledistance. The solid lines represent the scattering efficiency whereas the dashed lines represent the FWHM. The alloy dimersare Ag0.75Au0.25 (triangle), Au0.25Cu0.75 (star) and Ag0.50Cu0.50 (square).

The LSPR of all the three arrays is blue-shifted with increasing number of interacting particles incomparison to LSPR of an isolated nanoparticle with a significant increase in the FWHM. This isbecause of increased radiation damping due to larger volume of array with increased number ofinteracting particles. It is also found that as the numbers of interacting particles are increased up to4, the scattering efficiency reduces. Further increase in number of interacting particles leads to theincrease in scattering efficiency with nearly same (for 8 number of spheres) as for single BANSs(Figure 9(a)). The increase in the FWHM with nearly same scattering efficiency for any numberof interacting spheres from 2 to 8 is useful for plasmonic solar cell applications because larger theFWHM, more spectral region will be covered which implies to increase the efficiency of thin-filmplasmonic solar cells.

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057104-11 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

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6 spheresSca

tter

ing

eff

icie

ncy

(a.

u.)

8 spheres

Wavelength (um)

1 sphere

0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.800.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

2 spheres1 sphere

4 spheresSca

tter

ing

eff

icie

ncy

(a.u

.)

6 spheres

Wavelength (um)

8 spheres

(a)

(b)

(c)

FIG. 8. Scattering spectra of (a) Au0.25Cu0.75 (b) Ag0.50Cu0.50 and (c) Ag0.75Au0.25 BANS arrays with varying number ofinteracting particles.

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057104-12 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

Number of particles

2.5

3.0

3.5

4.0

4.5

5.0

5.5

2.5

3.0

3.5

4.0

4.5

5.0

5.5

2 4 6 80.15

0.20

0.25

0.30

0.35

0.40

FW

HM

(um

)

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Scatterin

g efficien

cy (a.u.)

2 4 6 80.46

0.48

0.50

0.52

0.54

0.56

0.58

0.60

0.62

LS

PR

wav

elen

gth

(u

m)

Number of particles

(a)

(b)

FIG. 9. (a) Scattering efficiency and FWHM and (b) LSPR wavelength of BANS arrays as a function of number of interactingparticles. The solid lines represent the scattering efficiency whereas the dashed lines represent the FWHM. The arrays areAg0.75Au0.25 (triangle), Au0.25Cu0.75 (star) and Ag0.50Cu0.50 (square).

The increase in FWHM has been found in all the three BANS arrays with increase in thenumber of interacting particles (Figure 9(a)). This is due to the coupling strength between theplasmon resonances of interacting particles with the increase in the number of particles. Higher thecoupling strength, larger is the plasmon damping and hence, larger FWHM. This increase in FWHMcovers the large spectral region of the EM spectrum which is an essential condition to increase theefficiency of plasmonic solar cells.4 However, Ag-Cu array shows the larger tunability of LSPR inthe visible region of the EM spectrum (Figure 9(b)) with comparable FWHM to other arrays but atthe cost of reduced scattering. Moreover, the scattering efficiency can be controlled by controllingthe surrounding medium environment.43 The FWHM of BANS arrays can be controlled from 175

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057104-13 Amit Bansal and S. S. Verma AIP Advances 4, 057104 (2014)

to 365 nm, scattering efficiency from 2.5 to 5.2 a.u. and LSPR wavelength from 460 to 600 nm byvarying the number of interacting particles and changing metal in the BANS array.

The LSPR tunability and scattering efficiency of Au-Cu BANSs is less for both i) varyingthe distance between the particles in nanosphere dimer and ii) number of interacting particles inarray in comparison to the other two BANSs of selected compositions. This is due to the effect ofinterband transitions in both Au and Cu which results in the damping of plasmon resonance andhence, reduces scattering efficiency and broadens the plasmon resonance. However, the effect ofinterband transitions in Cu is large as compared to Au; therefore more damping of plasmon resonanceis observed in case of Ag-Cu in comparison to Ag-Au. From Figures 6 to 9, it is concluded thatthe Ag-Cu BANSs have less scattering efficiency in comparison to Ag-Au but large tunability ofLSPR in the larger spectral region of the EM spectrum which makes them useful to increase theefficiency and cost effectiveness of plasmonic solar cells. More theoretical study will be requiredfor better understanding of light scattering from these arrays and further trapping of this scatteredlight into thin-film solar cells. Moreover, our results based on theoretical simulations will help theexperimentalists to synthesize the presently finalized noble metal alloy nanoparticles on thin-filmsubstrate.

IV. CONCLUSION

The plasmon coupling in BANS arrays of selected compositions have been studied with varyinginterparticle distance and number of interacting particles by using DDA simulations. It has beenfound that LSPR and scattering efficiency can be controlled over the large spectral region of the EMspectrum by varying these two parameters. Although the LSPR shift is smaller due to the plasmoniccoupling in comparison to the shift in individual alloy particles of same size in same surroundingmedium but their FWHM is larger. This larger FWHM results in the use of arrays for plasmonicsolar cells applications. In nanosphere dimers, when particles are nearly touching with each other astrong red-shift as well as broadening in dipolar resonance is found for Ag0.50-Cu0.50. In nanospherearrays with varying number of interacting particles, the more tunability in LSPR has been foundfor Ag0.50-Cu0.50 with larger FWHM. Therefore, it has been found that Ag0.50-Cu0.50 BANS arraysmight be used as a cost effective material for efficient plasmonic solar cells.

ACKNOWLEDGMENTS

The author, Amit Bansal, would like to thank SLIET Longowal for the financial support in theform of institute fellowship towards his Ph.D.

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