Plasmonics (2012) 7137ndash141DOI 101007s11468-011-9286-4

Optical Plasmon Properties of Co-Ag NanocompositesWithin the Mean-Field Approximation

Hernando Garcia middot Ritesh Sachan middotRamki Kalyanaraman

Received 10 June 2011 Accepted 8 September 2011 Published online 27 September 2011copy Springer Science+Business Media LLC 2011

Abstract The optical properties of multi-metalnanocomposites made from Cobalt (Co) and Silver(Ag) are analyzed theoretically within the mean fieldapproximation and experimentally verified usingabsorption spectroscopy The experimental system wasmodeled as a thin layer composed of hemisphericalnanoparticles formed by grains of Co and Ag in contactwith air and the SiO2 substrate The main aspects ofthe absorption curve such as the shape and spectrallocation of the localized surface plasmon resonanceare well captured by a simple self-consistent mixingmodel using a modified Maxwell-Garnett approachand the Milton lower bound

Keywords Plasmon middot Nanoparticles middot Mean-field middotComposites middot Ag

H GarciaDepartment of Physics Southern Illinois UniversityEdwardsville IL 62026 USAe-mail hgarciasiueedu

R Sachan middot R Kalyanaraman (B)Department of Materials Science and EngineeringUniversity of Tennessee Knoxville TN 37996 USAe-mail ramkiutkedu

R Sachan middot R KalyanaramanSustainable Energy and Education Research CenterUniversity of Tennessee Knoxville TN 37996 USA

R KalyanaramanDepartment of Chemical and Biomolecular EngineeringUniversity of Tennessee Knoxville TN 37996 USA

Introduction

Hybrid materials for example ones which have simul-taneous ferromagnetism and localized plasmon reso-nance can be useful for many applications like opticalsignal isolation and switching [1] ultrasensitive biosens-ing [2] biocatalysis [3] and actively-controlled plas-monics [4] When such hybrid ferromagnetic-plasmonic(FP) materials are made from nanoparticles the spec-tral location of the plasmonic frequency as well as theferromagnetic hysteresis behavior will be determinedby the shape size spacing and composition [5ndash8] Pre-dicting the F-P behavior especially when the nanopar-ticles are made by combining several materials andplaced in varying dielectric environments is thereforeof significant interest Towards this end simple modelsthat provide useful physical insight such as mean fieldor effective medium models could be of great utilitytowards the design of promising materials providedthey accurately capture the physical properties

In this paper we present a mean field approxima-tion approach to predict the experimentally measuredoptical behavior including the spectral change of thelocalized surface plasmon resonance (LSPR) for com-posite nanoparticles consisting of Co and Ag made onan SiO2 surface While the problem of a metal inclusionin an otherwise homogeneous and transparent mediawas solved by Mie at the beginning of last century [9]the problem of more than one type of inclusion is stilla complex problem We have recently proposed a self-consistent approach to express the effective dielectricfunction of mixtures by utilizing a binary mixing process[10] Using this we obtained good agreement withlinear as well as non-linear optical behavior of bulksystems such as dielectric medium containing metal

138 Plasmonics (2012) 7137ndash141

particles [11 12] In this paper we apply a similarapproach to explain the transmission spectrum of ananostructured layer made from an array of compositenanoparticles

Experiment and model

The samples were prepared by a laser-induced dewet-ting self-organization process [13 14] Briefly thin filmsof Ag followed by Co were sequentially deposited byelectron beam evaporation under ultra high vacuum(sim1 times 10minus8 Torr) on commercially available opticalquality quartz (SiO2) wafers Bilayer films with individ-ual metal layers of identical thickness and of 2 3 and 4nm were prepared In addition 5 nm thick single layerfilms of Co and Ag were also deposited Following de-position the samples were irradiated in vacuum with auniform Nd-YAG laser beam (Quanta-Ray Lab-150-50Nd-YAG laser Spectra-physics Newport Corporation)operating at 266 nm wavelength with a pulse width of 9ns and repetition rate of 50 Hz to create the final stablearray of nanoparticles for all the single and bilayersThe Gaussian shaped beam size was approximately 15mm2 and the spatially averaged laser energy densitywas selected between 80 to 100 mJcm2 such that allthe film layers could be melted for all the thicknesscombinations to create the final stable nanoparticlesarrays The patterns were then characterized using aHitachi S-4300 scanning electron microscope (SEM)operating at 15KV A typical SEM image is shown inFig 1a exhibiting the Ag-Co nanoparticle arrays corre-sponding to 4 nm Co 4 nm Ag bilayer system on SiO2The inset image shows the fast fourier transform (FFT)indicating the short range order in the nanoparticle ar-ray SEM images of nanoparticle arrays obtained fromeach bilayer system were then analyzed to estimate

the average particle diameter D of the nanoparticlearrays which were 38 52 and 98 nm for the 22 33and 44 nm bilayers respectively Also average particlesize obtained for pure Co and Ag nanoparticle arrayswere 75 and 69 nm respectively Transmission electronmicroscopy imaging of nanoparticles prepared by thistechnique have shown previously that the nanoparticlesare granular with grains made from the individual met-als Ag and Co [15] This finding is consistent with thefact that these metals are immiscible with each otherFurthermore the nanoparticles were nearly hemispher-ical in shape consistent with the equilibrium contactangles for the two metals on SiO2 which is 82 forAg and 102 for Co [6] In Fig 1b a schematic modelof CoAg nanocomposite particles has been presentedwhich shows polycrystalline nanoparticles with an av-erage radius h (=D2) at a inter-particle spacing of

In Fig 2andashc the experimentally measured (solidlines) broadband optical transmission data in the rangeof 250 to 800 nm is shown for the nanoparticles pre-pared from the different bilayers The position of the lo-calized surface plasmon resonance (LSPR) correspondsto the minima in the transmission spectrum In orderto model this transmission data an optical model wasdeveloped Since the average size of the particles wasmuch smaller than the probing light wavelength λ eachparticle can be treated as a homogeneous system withan effective dielectric function that can be given by amean field or effective medium model [16ndash18] Here wemodel the behavior in the following manner the systemis considered to be a layer of thickness h (which isequal to the radius of the nanoparticles) made by incor-porating the hemispherical nanoparticles of effectivedielectric function εeff in a host medium of effective di-electric function εhost as shown in Fig 1b The effectivedielectric function of the particle is calculated using theMilton lower bound expressed as [17]

εeff = εAg(εAg minus 2εCo)[εCo + 2(1 minus fCo)εAg + 2εCo] minus 2(1 minus fCo)ζ(2εCo minus 2εAg)

2

(εAg minus 2εCo)[2εAg + (1 minus fCo)εCo + 2εAg] minus 2(1 minus fCo)ζ(2εCo minus εAg)2(1)

where εAg and εCo are the dielectric function of Ag andCo fCo is the Co volume fraction (here = 05) andfinally ζ is given by

ζ = fCo + (1 minus fCo)(9G minus 1)

2(2)

Here G is called the Miller parameter and it dependson the shape of the grains making up the nanoparticleand can have values ranging from [19 13] To obtainthe Miller parameter for the case of spherical grainswe ran a simulation comparing the above equation with

the Maxwell-Garnet form for spherical inclusions of Aggrains of 20 nm radius embedded in a SiO2 matrix andobtained G = 009876556 which is close to the lowervalue of its range of validity Next the host dielectricfunction was estimated as

εhost = εSiO2σ + (1 minus σ)εo (3)

where εSiO2 is the dielectric constant of SiO2 taken tobe 226 εo is the dielectric constant of air (=1) and σ

is the ratio of the contact area of the nanoparticle with

Plasmonics (2012) 7137ndash141 139

(a) (b) (c)

Fig 1 a SEM image of the array of composite nanoparticlesmade from a 4 nm Co4 nm Ag bilayer Inset image shows theFFT indicating spatial short range order b The schematic modelof the Co-Ag nanoparticle array layer formed by hemisphericalparticles of radius h on top of SiO2 substrate There is a 50

contact area with the substrate c LSPR position as a function ofparticle diameter for 50Ag-50Co nanocomposite particlesExperimental data is shown by solid symbols while the theoryprediction is shown by the line

the substrate to the nanoparticle with the surroundingambient which for our case of hemispherical shapedparticles is σ = 033 Finally to model the transmissiondata the system was considered to be a multilayerof airnanoparticle layerSiO2 which can be analyzedby the matrix transfer approach in which each layerwas represented by a characteristic 2 times 2 matrix Forexample the matrix for the nanoparticle layer willbe [18]

M(h) =[

m11 m12

m21 m22

](4)

where m11 =cos(β) m12 = minusin sin(β) m21 = minusin sin(β)

and m22 = cos(β) and β = (2πλ)hn for normal inci-dence and n = n + iκ is the complex index of refractionfor the nanoparticle layer Since the SiO2 substrate

layer is non-absorbing for purposes of the transmissioncalculations the layer thickness could be chosen to bearbitrarily small so that the higher harmonics resultingfrom large values of β could be minimized in the co-sine terms In our calculations we assumed a thicknessof 5 nm for the SiO2 layer and a dielectric constantof εSiO2 = 226 Finally the transmission coefficient interms of the elements of the characteristic matrix of thesystem can be written as

t = 2no

m11no + m12nSiO2 no + m21 + m22nSiO2

(5)

where no is the refractive index of air (=1) and thetransmittance can be expressed as

T = ttlowast = |t|2 (6)

(a) (b) (c)

Fig 2 Optical transmission () as a function of wavelength for50Ag-50Co nanocomposite particles a Particles made from2 nm Co2 nm Ag bilayer b Particles made from the 3 nm Co3nm Ag bilayer c Particles made from the 4 nm Co4 nm Ag

bilayer The experimental measurements in are shown by solidlines while the theory predictions are shown by the dashed linesThe minima in the curves corresponds to the LSPR position Theinset shows the measured particle size distribution

140 Plasmonics (2012) 7137ndash141

Table 1 Data used in the fitting program In the final optimization step the free parameters were h fmix and the radius (r) of thegrains of Ag and Co within the nanoparticles

f exptmix f fit

mix hexpt (nm) hfit (nm) rCo (nm) rAg (nm) expt (nm) fit (nm)

2Co-2Ag 014 plusmn 006 013 19 plusmn 5 18 7 8 74 plusmn 5 783Co-3Ag 009 plusmn 004 0085 25 plusmn 7 25 8 11 128 plusmn 10 1304Co-4Ag 0080 plusmn 003 0077 50 plusmn 12 50 12 14 260 plusmn 18 260

The quantities with superscript lsquoexptrsquo correspond to the experimental values while those with superscript lsquofitrsquo correspond to valuesobtained from optimization

Results and conclusion

The theoretical analysis of the experimental opticaltransmission data was performed as follows First theMiller parameter G was obtained by the best fit of Eq 6to the experimental data for each of the three samplesstudied (Table 1) The only other free parameters werethe radius of the grains within the particles while theexperimentally measured values of h and fmix wereused (Table 1) Here fmix is the volume fraction ofnanoparticles within the layer of thickness h calculatedas fmix = 2π

3 ( h

)2 In addition a size-dependent cor-rection to the dielectric function for Co and Ag [19]was made using the analysis provided in [20] Fromthis analysis the average value of G was obtained tobe 027091 and this value was then used in the nextstep where the final theory fits shown in Fig 2 wereobtained In this final optimization step the free para-meters were h and fmix which were allowed to varybetween the bounds of the experimentally measureduncertainties (Table 1) as well as the radius r of theindividual grains of Co and Ag Table 1 summarizes theexperimental data used as well as the best fit valuesobtained for h fmix and r The values obtainedfrom the fit for h and fmix are consistent withthe experimentally measured values In addition the rvalues are reasonable in context of the overall size ofthe nanoparticles

As shown in Fig 2andashc very good agreement betweenthe experimental transmission curve (solid lines) andthe model prediction (dashed line) was obtained Keyfeatures such as the LSPR location as well as the gen-eral shape of the optical behaviour were well capturedby the model However the quadrupole effect visibleas the shoulder around 375 nm in the experimentalpeak could not be predicted by this approach becausethe model does not incorporate higher order multipoleeffects Nevertheless the model is very good at explain-ing the dipole contribution corresponding to the LSPRAs shown in Fig 1c the experimentally measuredLSPR position as a function of particle diameter (solidsymbols) has similar trend to the theory prediction Inaddition the actual quantitative difference in theory

prediction to the experimental LSPR position is onlyabout 15 nm which is very good given the simplicityof the model The model was also used to analyzethe experimental transmission spectrums of pure Coand Ag nanoparticle arrays as shown in Fig 3 Goodagreement was obtained for the shape of the Co spec-trum (from particles with average size of 75 plusmn 9 nm)as well as for the location of the spectral features inthe Ag spectrum (from particles with average size of69 plusmn 12 nm) This confirmed that the modeling ap-proach can be used for composite as well as elementallypure nanoparticles

In conclusion we have developed a simple approachto understand the optical response of composite metalnanoparticles lying in contact with substrate and sur-rounding dielectric medium We showed that within theMilton approach the value for the Miller parameterG is 027091 for hemispherical particles and this iswithin the valid range of [19 13] This number canbe compared to the case of 0137516 for the case ofcubical cells and of 0098765 for the case of sphericalinclusions There is very good agreement between themodel and experimentally measured optical transmis-sion data for nanoparticles made from Co-Ag In ad-dition the model also predicts very well the trend and

Fig 3 Experimental and modeled optical transmission () spec-tra as a function of wavelength for pure Co particles (withaverage size 75 plusmn 9 nm) and pure Ag particles (with average size69 plusmn 12 nm)

Plasmonics (2012) 7137ndash141 141

position of the LSPR as a function of particle size forCo-Ag composite nanoparticles Such models can bevery useful towards a rapid and efficient design of com-posite or hybrid nanoparticles with potentially usefuloptical responses

Acknowledgements HG acknowledges support by the NSFthrough grant CMMI-0757547 while RK acknowledges supportby NSF grants CMMI-0855949 DMR-0805258 and the NSFsupported TN-SCORE program

References

1 Eldada L (2001) Advances in telecom and datacom opticalcomponents Opt Eng 401165ndash1178

2 Sepuacutelveda B Calle A Lechuga LM Armelles G (2006)Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor Opt Lett 311085

3 Zayats M Pogorelova S Kharitonov A Lioubashevski OKatz E Willner I (2003) An nanoparticle-enhanced surfaceplasmon resonance sensing of biocatalytic transformationsChem A - Eur J 96108ndash6114

4 Gonzalez-Diaz JB Garcia-Martin A Garcia-Martin JMCebollada A Armelles G Sepulveda B Alaverdyan Y KallM (2008) Plasmonic AuCoAu nanosandwiches with en-hanced magneto-optical activity Small 4202ndash205

5 Maier S (2007) Plasmoncs Fundamentals and applicationsSpringer New York

6 Krishna H Miller C Longstreth-Spoor L Nussinov ZGangopadhyay AK Kalyanaraman R (2008) Unusual size-dependent magnetization in near hemispherical Co nano-magnets on SiO2 from fast pulsed laser processing J ApplPhys 103073902

7 Krishna H Gangopadhyay AK Strader J Kalyanaraman R(2011) Nanosecond laser-induced synthesis of nanoparticles

with tailorable magnetic anisotropy J Magn Magn Mater323356ndash362

8 Moskovits M Srnova-Sloufova I Vlckova B (2002) Bimetal-lic Ag-Au nanoparticles Extracting meaningful optical con-stants from the surface-plasmon extinction spectrum J ChemPhys 116(23)10435

9 Mie G (1908) Articles on the optical characteristics of turbidtubes especially colloidal metal solutions Ann Phys 25377

10 Garcia H Trice J Kalyanaraman R Sureshkumar R (2007)Self-consistent determination of plasmonic resonances internary nanocomposites Phys Rev B 75045439

11 Magruder R III Osborne H Jr Zuhr R (1994) Nonlinear-optical properties of nanometer dimension ag-cu particles insilica formed by sequential ion-implantation J Non-Cryst Sol176299

12 Garcia H Kalyanaraman R Sureshkumar R (2009) Nonlin-ear optical properties of multi-metal nanocomposites in aglass matrix J Phys B - At Mol Opt Phys 42175401

13 Krishna H Shirato N Yadavali S Sachan R Strader J Kalya-naraman R (2011) Self-organization of nanoscale multi layerliquid metal films experiment and theory ACS Nano 5470ndash476

14 Favazza C Trice J Gangopadhyay AK Garcia HSureshkumar R Kalyanaraman R (2006) Nanoparticleordering by dewetting of Co on SiO2 J Electron Mater351618ndash1620

15 Krishna H Favazza C Gangopadhyay AK Kalyanaraman R(2008) Functional nanostructures through nanosecond laserdewetting of thin metal films JOM 6037ndash42

16 Garnett J (1904) Color in metal glasses and in metal filmsPhilos Trans R Soc Lond Ser A 203385

17 Sihvola A (2002) How strict are theoretical bounds for dielec-tric properties of mixtures IEEE Trans Geosci Remote Sens40880

18 Haija A Freeman W Roarty T (2006) Effective characteris-tic matrix of ultathin multilayer structures Opt Appl 2639

19 wwwsopra sacom SOPRA database20 Kreibig U Vollmer M (1995) Optical properties of metal

clusters Springer series in material science Germany

138 Plasmonics (2012) 7137ndash141

particles [11 12] In this paper we apply a similarapproach to explain the transmission spectrum of ananostructured layer made from an array of compositenanoparticles

Experiment and model

The samples were prepared by a laser-induced dewet-ting self-organization process [13 14] Briefly thin filmsof Ag followed by Co were sequentially deposited byelectron beam evaporation under ultra high vacuum(sim1 times 10minus8 Torr) on commercially available opticalquality quartz (SiO2) wafers Bilayer films with individ-ual metal layers of identical thickness and of 2 3 and 4nm were prepared In addition 5 nm thick single layerfilms of Co and Ag were also deposited Following de-position the samples were irradiated in vacuum with auniform Nd-YAG laser beam (Quanta-Ray Lab-150-50Nd-YAG laser Spectra-physics Newport Corporation)operating at 266 nm wavelength with a pulse width of 9ns and repetition rate of 50 Hz to create the final stablearray of nanoparticles for all the single and bilayersThe Gaussian shaped beam size was approximately 15mm2 and the spatially averaged laser energy densitywas selected between 80 to 100 mJcm2 such that allthe film layers could be melted for all the thicknesscombinations to create the final stable nanoparticlesarrays The patterns were then characterized using aHitachi S-4300 scanning electron microscope (SEM)operating at 15KV A typical SEM image is shown inFig 1a exhibiting the Ag-Co nanoparticle arrays corre-sponding to 4 nm Co 4 nm Ag bilayer system on SiO2The inset image shows the fast fourier transform (FFT)indicating the short range order in the nanoparticle ar-ray SEM images of nanoparticle arrays obtained fromeach bilayer system were then analyzed to estimate

the average particle diameter D of the nanoparticlearrays which were 38 52 and 98 nm for the 22 33and 44 nm bilayers respectively Also average particlesize obtained for pure Co and Ag nanoparticle arrayswere 75 and 69 nm respectively Transmission electronmicroscopy imaging of nanoparticles prepared by thistechnique have shown previously that the nanoparticlesare granular with grains made from the individual met-als Ag and Co [15] This finding is consistent with thefact that these metals are immiscible with each otherFurthermore the nanoparticles were nearly hemispher-ical in shape consistent with the equilibrium contactangles for the two metals on SiO2 which is 82 forAg and 102 for Co [6] In Fig 1b a schematic modelof CoAg nanocomposite particles has been presentedwhich shows polycrystalline nanoparticles with an av-erage radius h (=D2) at a inter-particle spacing of

In Fig 2andashc the experimentally measured (solidlines) broadband optical transmission data in the rangeof 250 to 800 nm is shown for the nanoparticles pre-pared from the different bilayers The position of the lo-calized surface plasmon resonance (LSPR) correspondsto the minima in the transmission spectrum In orderto model this transmission data an optical model wasdeveloped Since the average size of the particles wasmuch smaller than the probing light wavelength λ eachparticle can be treated as a homogeneous system withan effective dielectric function that can be given by amean field or effective medium model [16ndash18] Here wemodel the behavior in the following manner the systemis considered to be a layer of thickness h (which isequal to the radius of the nanoparticles) made by incor-porating the hemispherical nanoparticles of effectivedielectric function εeff in a host medium of effective di-electric function εhost as shown in Fig 1b The effectivedielectric function of the particle is calculated using theMilton lower bound expressed as [17]

εeff = εAg(εAg minus 2εCo)[εCo + 2(1 minus fCo)εAg + 2εCo] minus 2(1 minus fCo)ζ(2εCo minus 2εAg)

2

(εAg minus 2εCo)[2εAg + (1 minus fCo)εCo + 2εAg] minus 2(1 minus fCo)ζ(2εCo minus εAg)2(1)

where εAg and εCo are the dielectric function of Ag andCo fCo is the Co volume fraction (here = 05) andfinally ζ is given by

ζ = fCo + (1 minus fCo)(9G minus 1)

2(2)

Here G is called the Miller parameter and it dependson the shape of the grains making up the nanoparticleand can have values ranging from [19 13] To obtainthe Miller parameter for the case of spherical grainswe ran a simulation comparing the above equation with

the Maxwell-Garnet form for spherical inclusions of Aggrains of 20 nm radius embedded in a SiO2 matrix andobtained G = 009876556 which is close to the lowervalue of its range of validity Next the host dielectricfunction was estimated as

εhost = εSiO2σ + (1 minus σ)εo (3)

where εSiO2 is the dielectric constant of SiO2 taken tobe 226 εo is the dielectric constant of air (=1) and σ

is the ratio of the contact area of the nanoparticle with

Plasmonics (2012) 7137ndash141 139

(a) (b) (c)

Fig 1 a SEM image of the array of composite nanoparticlesmade from a 4 nm Co4 nm Ag bilayer Inset image shows theFFT indicating spatial short range order b The schematic modelof the Co-Ag nanoparticle array layer formed by hemisphericalparticles of radius h on top of SiO2 substrate There is a 50

contact area with the substrate c LSPR position as a function ofparticle diameter for 50Ag-50Co nanocomposite particlesExperimental data is shown by solid symbols while the theoryprediction is shown by the line

the substrate to the nanoparticle with the surroundingambient which for our case of hemispherical shapedparticles is σ = 033 Finally to model the transmissiondata the system was considered to be a multilayerof airnanoparticle layerSiO2 which can be analyzedby the matrix transfer approach in which each layerwas represented by a characteristic 2 times 2 matrix Forexample the matrix for the nanoparticle layer willbe [18]

M(h) =[

m11 m12

m21 m22

](4)

where m11 =cos(β) m12 = minusin sin(β) m21 = minusin sin(β)

and m22 = cos(β) and β = (2πλ)hn for normal inci-dence and n = n + iκ is the complex index of refractionfor the nanoparticle layer Since the SiO2 substrate

layer is non-absorbing for purposes of the transmissioncalculations the layer thickness could be chosen to bearbitrarily small so that the higher harmonics resultingfrom large values of β could be minimized in the co-sine terms In our calculations we assumed a thicknessof 5 nm for the SiO2 layer and a dielectric constantof εSiO2 = 226 Finally the transmission coefficient interms of the elements of the characteristic matrix of thesystem can be written as

t = 2no

m11no + m12nSiO2 no + m21 + m22nSiO2

(5)

where no is the refractive index of air (=1) and thetransmittance can be expressed as

T = ttlowast = |t|2 (6)

(a) (b) (c)

Fig 2 Optical transmission () as a function of wavelength for50Ag-50Co nanocomposite particles a Particles made from2 nm Co2 nm Ag bilayer b Particles made from the 3 nm Co3nm Ag bilayer c Particles made from the 4 nm Co4 nm Ag

bilayer The experimental measurements in are shown by solidlines while the theory predictions are shown by the dashed linesThe minima in the curves corresponds to the LSPR position Theinset shows the measured particle size distribution

140 Plasmonics (2012) 7137ndash141

Table 1 Data used in the fitting program In the final optimization step the free parameters were h fmix and the radius (r) of thegrains of Ag and Co within the nanoparticles

f exptmix f fit

mix hexpt (nm) hfit (nm) rCo (nm) rAg (nm) expt (nm) fit (nm)

2Co-2Ag 014 plusmn 006 013 19 plusmn 5 18 7 8 74 plusmn 5 783Co-3Ag 009 plusmn 004 0085 25 plusmn 7 25 8 11 128 plusmn 10 1304Co-4Ag 0080 plusmn 003 0077 50 plusmn 12 50 12 14 260 plusmn 18 260

The quantities with superscript lsquoexptrsquo correspond to the experimental values while those with superscript lsquofitrsquo correspond to valuesobtained from optimization

Results and conclusion

The theoretical analysis of the experimental opticaltransmission data was performed as follows First theMiller parameter G was obtained by the best fit of Eq 6to the experimental data for each of the three samplesstudied (Table 1) The only other free parameters werethe radius of the grains within the particles while theexperimentally measured values of h and fmix wereused (Table 1) Here fmix is the volume fraction ofnanoparticles within the layer of thickness h calculatedas fmix = 2π

3 ( h

)2 In addition a size-dependent cor-rection to the dielectric function for Co and Ag [19]was made using the analysis provided in [20] Fromthis analysis the average value of G was obtained tobe 027091 and this value was then used in the nextstep where the final theory fits shown in Fig 2 wereobtained In this final optimization step the free para-meters were h and fmix which were allowed to varybetween the bounds of the experimentally measureduncertainties (Table 1) as well as the radius r of theindividual grains of Co and Ag Table 1 summarizes theexperimental data used as well as the best fit valuesobtained for h fmix and r The values obtainedfrom the fit for h and fmix are consistent withthe experimentally measured values In addition the rvalues are reasonable in context of the overall size ofthe nanoparticles

As shown in Fig 2andashc very good agreement betweenthe experimental transmission curve (solid lines) andthe model prediction (dashed line) was obtained Keyfeatures such as the LSPR location as well as the gen-eral shape of the optical behaviour were well capturedby the model However the quadrupole effect visibleas the shoulder around 375 nm in the experimentalpeak could not be predicted by this approach becausethe model does not incorporate higher order multipoleeffects Nevertheless the model is very good at explain-ing the dipole contribution corresponding to the LSPRAs shown in Fig 1c the experimentally measuredLSPR position as a function of particle diameter (solidsymbols) has similar trend to the theory prediction Inaddition the actual quantitative difference in theory

prediction to the experimental LSPR position is onlyabout 15 nm which is very good given the simplicityof the model The model was also used to analyzethe experimental transmission spectrums of pure Coand Ag nanoparticle arrays as shown in Fig 3 Goodagreement was obtained for the shape of the Co spec-trum (from particles with average size of 75 plusmn 9 nm)as well as for the location of the spectral features inthe Ag spectrum (from particles with average size of69 plusmn 12 nm) This confirmed that the modeling ap-proach can be used for composite as well as elementallypure nanoparticles

In conclusion we have developed a simple approachto understand the optical response of composite metalnanoparticles lying in contact with substrate and sur-rounding dielectric medium We showed that within theMilton approach the value for the Miller parameterG is 027091 for hemispherical particles and this iswithin the valid range of [19 13] This number canbe compared to the case of 0137516 for the case ofcubical cells and of 0098765 for the case of sphericalinclusions There is very good agreement between themodel and experimentally measured optical transmis-sion data for nanoparticles made from Co-Ag In ad-dition the model also predicts very well the trend and

Fig 3 Experimental and modeled optical transmission () spec-tra as a function of wavelength for pure Co particles (withaverage size 75 plusmn 9 nm) and pure Ag particles (with average size69 plusmn 12 nm)

Plasmonics (2012) 7137ndash141 141

position of the LSPR as a function of particle size forCo-Ag composite nanoparticles Such models can bevery useful towards a rapid and efficient design of com-posite or hybrid nanoparticles with potentially usefuloptical responses

Acknowledgements HG acknowledges support by the NSFthrough grant CMMI-0757547 while RK acknowledges supportby NSF grants CMMI-0855949 DMR-0805258 and the NSFsupported TN-SCORE program

References

1 Eldada L (2001) Advances in telecom and datacom opticalcomponents Opt Eng 401165ndash1178

2 Sepuacutelveda B Calle A Lechuga LM Armelles G (2006)Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor Opt Lett 311085

3 Zayats M Pogorelova S Kharitonov A Lioubashevski OKatz E Willner I (2003) An nanoparticle-enhanced surfaceplasmon resonance sensing of biocatalytic transformationsChem A - Eur J 96108ndash6114

4 Gonzalez-Diaz JB Garcia-Martin A Garcia-Martin JMCebollada A Armelles G Sepulveda B Alaverdyan Y KallM (2008) Plasmonic AuCoAu nanosandwiches with en-hanced magneto-optical activity Small 4202ndash205

5 Maier S (2007) Plasmoncs Fundamentals and applicationsSpringer New York

6 Krishna H Miller C Longstreth-Spoor L Nussinov ZGangopadhyay AK Kalyanaraman R (2008) Unusual size-dependent magnetization in near hemispherical Co nano-magnets on SiO2 from fast pulsed laser processing J ApplPhys 103073902

7 Krishna H Gangopadhyay AK Strader J Kalyanaraman R(2011) Nanosecond laser-induced synthesis of nanoparticles

with tailorable magnetic anisotropy J Magn Magn Mater323356ndash362

8 Moskovits M Srnova-Sloufova I Vlckova B (2002) Bimetal-lic Ag-Au nanoparticles Extracting meaningful optical con-stants from the surface-plasmon extinction spectrum J ChemPhys 116(23)10435

9 Mie G (1908) Articles on the optical characteristics of turbidtubes especially colloidal metal solutions Ann Phys 25377

10 Garcia H Trice J Kalyanaraman R Sureshkumar R (2007)Self-consistent determination of plasmonic resonances internary nanocomposites Phys Rev B 75045439

11 Magruder R III Osborne H Jr Zuhr R (1994) Nonlinear-optical properties of nanometer dimension ag-cu particles insilica formed by sequential ion-implantation J Non-Cryst Sol176299

12 Garcia H Kalyanaraman R Sureshkumar R (2009) Nonlin-ear optical properties of multi-metal nanocomposites in aglass matrix J Phys B - At Mol Opt Phys 42175401

13 Krishna H Shirato N Yadavali S Sachan R Strader J Kalya-naraman R (2011) Self-organization of nanoscale multi layerliquid metal films experiment and theory ACS Nano 5470ndash476

14 Favazza C Trice J Gangopadhyay AK Garcia HSureshkumar R Kalyanaraman R (2006) Nanoparticleordering by dewetting of Co on SiO2 J Electron Mater351618ndash1620

15 Krishna H Favazza C Gangopadhyay AK Kalyanaraman R(2008) Functional nanostructures through nanosecond laserdewetting of thin metal films JOM 6037ndash42

16 Garnett J (1904) Color in metal glasses and in metal filmsPhilos Trans R Soc Lond Ser A 203385

17 Sihvola A (2002) How strict are theoretical bounds for dielec-tric properties of mixtures IEEE Trans Geosci Remote Sens40880

18 Haija A Freeman W Roarty T (2006) Effective characteris-tic matrix of ultathin multilayer structures Opt Appl 2639

19 wwwsopra sacom SOPRA database20 Kreibig U Vollmer M (1995) Optical properties of metal

clusters Springer series in material science Germany

Plasmonics (2012) 7137ndash141 139

(a) (b) (c)

Fig 1 a SEM image of the array of composite nanoparticlesmade from a 4 nm Co4 nm Ag bilayer Inset image shows theFFT indicating spatial short range order b The schematic modelof the Co-Ag nanoparticle array layer formed by hemisphericalparticles of radius h on top of SiO2 substrate There is a 50

contact area with the substrate c LSPR position as a function ofparticle diameter for 50Ag-50Co nanocomposite particlesExperimental data is shown by solid symbols while the theoryprediction is shown by the line

the substrate to the nanoparticle with the surroundingambient which for our case of hemispherical shapedparticles is σ = 033 Finally to model the transmissiondata the system was considered to be a multilayerof airnanoparticle layerSiO2 which can be analyzedby the matrix transfer approach in which each layerwas represented by a characteristic 2 times 2 matrix Forexample the matrix for the nanoparticle layer willbe [18]

M(h) =[

m11 m12

m21 m22

](4)

where m11 =cos(β) m12 = minusin sin(β) m21 = minusin sin(β)

and m22 = cos(β) and β = (2πλ)hn for normal inci-dence and n = n + iκ is the complex index of refractionfor the nanoparticle layer Since the SiO2 substrate

layer is non-absorbing for purposes of the transmissioncalculations the layer thickness could be chosen to bearbitrarily small so that the higher harmonics resultingfrom large values of β could be minimized in the co-sine terms In our calculations we assumed a thicknessof 5 nm for the SiO2 layer and a dielectric constantof εSiO2 = 226 Finally the transmission coefficient interms of the elements of the characteristic matrix of thesystem can be written as

t = 2no

m11no + m12nSiO2 no + m21 + m22nSiO2

(5)

where no is the refractive index of air (=1) and thetransmittance can be expressed as

T = ttlowast = |t|2 (6)

(a) (b) (c)

Fig 2 Optical transmission () as a function of wavelength for50Ag-50Co nanocomposite particles a Particles made from2 nm Co2 nm Ag bilayer b Particles made from the 3 nm Co3nm Ag bilayer c Particles made from the 4 nm Co4 nm Ag

bilayer The experimental measurements in are shown by solidlines while the theory predictions are shown by the dashed linesThe minima in the curves corresponds to the LSPR position Theinset shows the measured particle size distribution

140 Plasmonics (2012) 7137ndash141

Table 1 Data used in the fitting program In the final optimization step the free parameters were h fmix and the radius (r) of thegrains of Ag and Co within the nanoparticles

f exptmix f fit

mix hexpt (nm) hfit (nm) rCo (nm) rAg (nm) expt (nm) fit (nm)

2Co-2Ag 014 plusmn 006 013 19 plusmn 5 18 7 8 74 plusmn 5 783Co-3Ag 009 plusmn 004 0085 25 plusmn 7 25 8 11 128 plusmn 10 1304Co-4Ag 0080 plusmn 003 0077 50 plusmn 12 50 12 14 260 plusmn 18 260

The quantities with superscript lsquoexptrsquo correspond to the experimental values while those with superscript lsquofitrsquo correspond to valuesobtained from optimization

Results and conclusion

The theoretical analysis of the experimental opticaltransmission data was performed as follows First theMiller parameter G was obtained by the best fit of Eq 6to the experimental data for each of the three samplesstudied (Table 1) The only other free parameters werethe radius of the grains within the particles while theexperimentally measured values of h and fmix wereused (Table 1) Here fmix is the volume fraction ofnanoparticles within the layer of thickness h calculatedas fmix = 2π

3 ( h

)2 In addition a size-dependent cor-rection to the dielectric function for Co and Ag [19]was made using the analysis provided in [20] Fromthis analysis the average value of G was obtained tobe 027091 and this value was then used in the nextstep where the final theory fits shown in Fig 2 wereobtained In this final optimization step the free para-meters were h and fmix which were allowed to varybetween the bounds of the experimentally measureduncertainties (Table 1) as well as the radius r of theindividual grains of Co and Ag Table 1 summarizes theexperimental data used as well as the best fit valuesobtained for h fmix and r The values obtainedfrom the fit for h and fmix are consistent withthe experimentally measured values In addition the rvalues are reasonable in context of the overall size ofthe nanoparticles

As shown in Fig 2andashc very good agreement betweenthe experimental transmission curve (solid lines) andthe model prediction (dashed line) was obtained Keyfeatures such as the LSPR location as well as the gen-eral shape of the optical behaviour were well capturedby the model However the quadrupole effect visibleas the shoulder around 375 nm in the experimentalpeak could not be predicted by this approach becausethe model does not incorporate higher order multipoleeffects Nevertheless the model is very good at explain-ing the dipole contribution corresponding to the LSPRAs shown in Fig 1c the experimentally measuredLSPR position as a function of particle diameter (solidsymbols) has similar trend to the theory prediction Inaddition the actual quantitative difference in theory

prediction to the experimental LSPR position is onlyabout 15 nm which is very good given the simplicityof the model The model was also used to analyzethe experimental transmission spectrums of pure Coand Ag nanoparticle arrays as shown in Fig 3 Goodagreement was obtained for the shape of the Co spec-trum (from particles with average size of 75 plusmn 9 nm)as well as for the location of the spectral features inthe Ag spectrum (from particles with average size of69 plusmn 12 nm) This confirmed that the modeling ap-proach can be used for composite as well as elementallypure nanoparticles

In conclusion we have developed a simple approachto understand the optical response of composite metalnanoparticles lying in contact with substrate and sur-rounding dielectric medium We showed that within theMilton approach the value for the Miller parameterG is 027091 for hemispherical particles and this iswithin the valid range of [19 13] This number canbe compared to the case of 0137516 for the case ofcubical cells and of 0098765 for the case of sphericalinclusions There is very good agreement between themodel and experimentally measured optical transmis-sion data for nanoparticles made from Co-Ag In ad-dition the model also predicts very well the trend and

Fig 3 Experimental and modeled optical transmission () spec-tra as a function of wavelength for pure Co particles (withaverage size 75 plusmn 9 nm) and pure Ag particles (with average size69 plusmn 12 nm)

Plasmonics (2012) 7137ndash141 141

position of the LSPR as a function of particle size forCo-Ag composite nanoparticles Such models can bevery useful towards a rapid and efficient design of com-posite or hybrid nanoparticles with potentially usefuloptical responses

Acknowledgements HG acknowledges support by the NSFthrough grant CMMI-0757547 while RK acknowledges supportby NSF grants CMMI-0855949 DMR-0805258 and the NSFsupported TN-SCORE program

References

1 Eldada L (2001) Advances in telecom and datacom opticalcomponents Opt Eng 401165ndash1178

2 Sepuacutelveda B Calle A Lechuga LM Armelles G (2006)Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor Opt Lett 311085

3 Zayats M Pogorelova S Kharitonov A Lioubashevski OKatz E Willner I (2003) An nanoparticle-enhanced surfaceplasmon resonance sensing of biocatalytic transformationsChem A - Eur J 96108ndash6114

4 Gonzalez-Diaz JB Garcia-Martin A Garcia-Martin JMCebollada A Armelles G Sepulveda B Alaverdyan Y KallM (2008) Plasmonic AuCoAu nanosandwiches with en-hanced magneto-optical activity Small 4202ndash205

5 Maier S (2007) Plasmoncs Fundamentals and applicationsSpringer New York

6 Krishna H Miller C Longstreth-Spoor L Nussinov ZGangopadhyay AK Kalyanaraman R (2008) Unusual size-dependent magnetization in near hemispherical Co nano-magnets on SiO2 from fast pulsed laser processing J ApplPhys 103073902

7 Krishna H Gangopadhyay AK Strader J Kalyanaraman R(2011) Nanosecond laser-induced synthesis of nanoparticles

with tailorable magnetic anisotropy J Magn Magn Mater323356ndash362

8 Moskovits M Srnova-Sloufova I Vlckova B (2002) Bimetal-lic Ag-Au nanoparticles Extracting meaningful optical con-stants from the surface-plasmon extinction spectrum J ChemPhys 116(23)10435

9 Mie G (1908) Articles on the optical characteristics of turbidtubes especially colloidal metal solutions Ann Phys 25377

10 Garcia H Trice J Kalyanaraman R Sureshkumar R (2007)Self-consistent determination of plasmonic resonances internary nanocomposites Phys Rev B 75045439

11 Magruder R III Osborne H Jr Zuhr R (1994) Nonlinear-optical properties of nanometer dimension ag-cu particles insilica formed by sequential ion-implantation J Non-Cryst Sol176299

12 Garcia H Kalyanaraman R Sureshkumar R (2009) Nonlin-ear optical properties of multi-metal nanocomposites in aglass matrix J Phys B - At Mol Opt Phys 42175401

13 Krishna H Shirato N Yadavali S Sachan R Strader J Kalya-naraman R (2011) Self-organization of nanoscale multi layerliquid metal films experiment and theory ACS Nano 5470ndash476

14 Favazza C Trice J Gangopadhyay AK Garcia HSureshkumar R Kalyanaraman R (2006) Nanoparticleordering by dewetting of Co on SiO2 J Electron Mater351618ndash1620

15 Krishna H Favazza C Gangopadhyay AK Kalyanaraman R(2008) Functional nanostructures through nanosecond laserdewetting of thin metal films JOM 6037ndash42

16 Garnett J (1904) Color in metal glasses and in metal filmsPhilos Trans R Soc Lond Ser A 203385

17 Sihvola A (2002) How strict are theoretical bounds for dielec-tric properties of mixtures IEEE Trans Geosci Remote Sens40880

18 Haija A Freeman W Roarty T (2006) Effective characteris-tic matrix of ultathin multilayer structures Opt Appl 2639

19 wwwsopra sacom SOPRA database20 Kreibig U Vollmer M (1995) Optical properties of metal

clusters Springer series in material science Germany

140 Plasmonics (2012) 7137ndash141

Table 1 Data used in the fitting program In the final optimization step the free parameters were h fmix and the radius (r) of thegrains of Ag and Co within the nanoparticles

f exptmix f fit

mix hexpt (nm) hfit (nm) rCo (nm) rAg (nm) expt (nm) fit (nm)

2Co-2Ag 014 plusmn 006 013 19 plusmn 5 18 7 8 74 plusmn 5 783Co-3Ag 009 plusmn 004 0085 25 plusmn 7 25 8 11 128 plusmn 10 1304Co-4Ag 0080 plusmn 003 0077 50 plusmn 12 50 12 14 260 plusmn 18 260

The quantities with superscript lsquoexptrsquo correspond to the experimental values while those with superscript lsquofitrsquo correspond to valuesobtained from optimization

Results and conclusion

The theoretical analysis of the experimental opticaltransmission data was performed as follows First theMiller parameter G was obtained by the best fit of Eq 6to the experimental data for each of the three samplesstudied (Table 1) The only other free parameters werethe radius of the grains within the particles while theexperimentally measured values of h and fmix wereused (Table 1) Here fmix is the volume fraction ofnanoparticles within the layer of thickness h calculatedas fmix = 2π

3 ( h

)2 In addition a size-dependent cor-rection to the dielectric function for Co and Ag [19]was made using the analysis provided in [20] Fromthis analysis the average value of G was obtained tobe 027091 and this value was then used in the nextstep where the final theory fits shown in Fig 2 wereobtained In this final optimization step the free para-meters were h and fmix which were allowed to varybetween the bounds of the experimentally measureduncertainties (Table 1) as well as the radius r of theindividual grains of Co and Ag Table 1 summarizes theexperimental data used as well as the best fit valuesobtained for h fmix and r The values obtainedfrom the fit for h and fmix are consistent withthe experimentally measured values In addition the rvalues are reasonable in context of the overall size ofthe nanoparticles

As shown in Fig 2andashc very good agreement betweenthe experimental transmission curve (solid lines) andthe model prediction (dashed line) was obtained Keyfeatures such as the LSPR location as well as the gen-eral shape of the optical behaviour were well capturedby the model However the quadrupole effect visibleas the shoulder around 375 nm in the experimentalpeak could not be predicted by this approach becausethe model does not incorporate higher order multipoleeffects Nevertheless the model is very good at explain-ing the dipole contribution corresponding to the LSPRAs shown in Fig 1c the experimentally measuredLSPR position as a function of particle diameter (solidsymbols) has similar trend to the theory prediction Inaddition the actual quantitative difference in theory

prediction to the experimental LSPR position is onlyabout 15 nm which is very good given the simplicityof the model The model was also used to analyzethe experimental transmission spectrums of pure Coand Ag nanoparticle arrays as shown in Fig 3 Goodagreement was obtained for the shape of the Co spec-trum (from particles with average size of 75 plusmn 9 nm)as well as for the location of the spectral features inthe Ag spectrum (from particles with average size of69 plusmn 12 nm) This confirmed that the modeling ap-proach can be used for composite as well as elementallypure nanoparticles

In conclusion we have developed a simple approachto understand the optical response of composite metalnanoparticles lying in contact with substrate and sur-rounding dielectric medium We showed that within theMilton approach the value for the Miller parameterG is 027091 for hemispherical particles and this iswithin the valid range of [19 13] This number canbe compared to the case of 0137516 for the case ofcubical cells and of 0098765 for the case of sphericalinclusions There is very good agreement between themodel and experimentally measured optical transmis-sion data for nanoparticles made from Co-Ag In ad-dition the model also predicts very well the trend and

Fig 3 Experimental and modeled optical transmission () spec-tra as a function of wavelength for pure Co particles (withaverage size 75 plusmn 9 nm) and pure Ag particles (with average size69 plusmn 12 nm)

Plasmonics (2012) 7137ndash141 141

position of the LSPR as a function of particle size forCo-Ag composite nanoparticles Such models can bevery useful towards a rapid and efficient design of com-posite or hybrid nanoparticles with potentially usefuloptical responses

Acknowledgements HG acknowledges support by the NSFthrough grant CMMI-0757547 while RK acknowledges supportby NSF grants CMMI-0855949 DMR-0805258 and the NSFsupported TN-SCORE program

References

1 Eldada L (2001) Advances in telecom and datacom opticalcomponents Opt Eng 401165ndash1178

2 Sepuacutelveda B Calle A Lechuga LM Armelles G (2006)Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor Opt Lett 311085

3 Zayats M Pogorelova S Kharitonov A Lioubashevski OKatz E Willner I (2003) An nanoparticle-enhanced surfaceplasmon resonance sensing of biocatalytic transformationsChem A - Eur J 96108ndash6114

4 Gonzalez-Diaz JB Garcia-Martin A Garcia-Martin JMCebollada A Armelles G Sepulveda B Alaverdyan Y KallM (2008) Plasmonic AuCoAu nanosandwiches with en-hanced magneto-optical activity Small 4202ndash205

5 Maier S (2007) Plasmoncs Fundamentals and applicationsSpringer New York

6 Krishna H Miller C Longstreth-Spoor L Nussinov ZGangopadhyay AK Kalyanaraman R (2008) Unusual size-dependent magnetization in near hemispherical Co nano-magnets on SiO2 from fast pulsed laser processing J ApplPhys 103073902

7 Krishna H Gangopadhyay AK Strader J Kalyanaraman R(2011) Nanosecond laser-induced synthesis of nanoparticles

with tailorable magnetic anisotropy J Magn Magn Mater323356ndash362

8 Moskovits M Srnova-Sloufova I Vlckova B (2002) Bimetal-lic Ag-Au nanoparticles Extracting meaningful optical con-stants from the surface-plasmon extinction spectrum J ChemPhys 116(23)10435

9 Mie G (1908) Articles on the optical characteristics of turbidtubes especially colloidal metal solutions Ann Phys 25377

10 Garcia H Trice J Kalyanaraman R Sureshkumar R (2007)Self-consistent determination of plasmonic resonances internary nanocomposites Phys Rev B 75045439

11 Magruder R III Osborne H Jr Zuhr R (1994) Nonlinear-optical properties of nanometer dimension ag-cu particles insilica formed by sequential ion-implantation J Non-Cryst Sol176299

12 Garcia H Kalyanaraman R Sureshkumar R (2009) Nonlin-ear optical properties of multi-metal nanocomposites in aglass matrix J Phys B - At Mol Opt Phys 42175401

13 Krishna H Shirato N Yadavali S Sachan R Strader J Kalya-naraman R (2011) Self-organization of nanoscale multi layerliquid metal films experiment and theory ACS Nano 5470ndash476

14 Favazza C Trice J Gangopadhyay AK Garcia HSureshkumar R Kalyanaraman R (2006) Nanoparticleordering by dewetting of Co on SiO2 J Electron Mater351618ndash1620

15 Krishna H Favazza C Gangopadhyay AK Kalyanaraman R(2008) Functional nanostructures through nanosecond laserdewetting of thin metal films JOM 6037ndash42

16 Garnett J (1904) Color in metal glasses and in metal filmsPhilos Trans R Soc Lond Ser A 203385

17 Sihvola A (2002) How strict are theoretical bounds for dielec-tric properties of mixtures IEEE Trans Geosci Remote Sens40880

18 Haija A Freeman W Roarty T (2006) Effective characteris-tic matrix of ultathin multilayer structures Opt Appl 2639

19 wwwsopra sacom SOPRA database20 Kreibig U Vollmer M (1995) Optical properties of metal

clusters Springer series in material science Germany

Plasmonics (2012) 7137ndash141 141

position of the LSPR as a function of particle size forCo-Ag composite nanoparticles Such models can bevery useful towards a rapid and efficient design of com-posite or hybrid nanoparticles with potentially usefuloptical responses

Acknowledgements HG acknowledges support by the NSFthrough grant CMMI-0757547 while RK acknowledges supportby NSF grants CMMI-0855949 DMR-0805258 and the NSFsupported TN-SCORE program

References

1 Eldada L (2001) Advances in telecom and datacom opticalcomponents Opt Eng 401165ndash1178

2 Sepuacutelveda B Calle A Lechuga LM Armelles G (2006)Highly sensitive detection of biomolecules with the magneto-optic surface-plasmon-resonance sensor Opt Lett 311085

3 Zayats M Pogorelova S Kharitonov A Lioubashevski OKatz E Willner I (2003) An nanoparticle-enhanced surfaceplasmon resonance sensing of biocatalytic transformationsChem A - Eur J 96108ndash6114

4 Gonzalez-Diaz JB Garcia-Martin A Garcia-Martin JMCebollada A Armelles G Sepulveda B Alaverdyan Y KallM (2008) Plasmonic AuCoAu nanosandwiches with en-hanced magneto-optical activity Small 4202ndash205

5 Maier S (2007) Plasmoncs Fundamentals and applicationsSpringer New York

6 Krishna H Miller C Longstreth-Spoor L Nussinov ZGangopadhyay AK Kalyanaraman R (2008) Unusual size-dependent magnetization in near hemispherical Co nano-magnets on SiO2 from fast pulsed laser processing J ApplPhys 103073902

7 Krishna H Gangopadhyay AK Strader J Kalyanaraman R(2011) Nanosecond laser-induced synthesis of nanoparticles

with tailorable magnetic anisotropy J Magn Magn Mater323356ndash362

8 Moskovits M Srnova-Sloufova I Vlckova B (2002) Bimetal-lic Ag-Au nanoparticles Extracting meaningful optical con-stants from the surface-plasmon extinction spectrum J ChemPhys 116(23)10435

9 Mie G (1908) Articles on the optical characteristics of turbidtubes especially colloidal metal solutions Ann Phys 25377

10 Garcia H Trice J Kalyanaraman R Sureshkumar R (2007)Self-consistent determination of plasmonic resonances internary nanocomposites Phys Rev B 75045439

11 Magruder R III Osborne H Jr Zuhr R (1994) Nonlinear-optical properties of nanometer dimension ag-cu particles insilica formed by sequential ion-implantation J Non-Cryst Sol176299

12 Garcia H Kalyanaraman R Sureshkumar R (2009) Nonlin-ear optical properties of multi-metal nanocomposites in aglass matrix J Phys B - At Mol Opt Phys 42175401

13 Krishna H Shirato N Yadavali S Sachan R Strader J Kalya-naraman R (2011) Self-organization of nanoscale multi layerliquid metal films experiment and theory ACS Nano 5470ndash476

14 Favazza C Trice J Gangopadhyay AK Garcia HSureshkumar R Kalyanaraman R (2006) Nanoparticleordering by dewetting of Co on SiO2 J Electron Mater351618ndash1620

15 Krishna H Favazza C Gangopadhyay AK Kalyanaraman R(2008) Functional nanostructures through nanosecond laserdewetting of thin metal films JOM 6037ndash42

16 Garnett J (1904) Color in metal glasses and in metal filmsPhilos Trans R Soc Lond Ser A 203385

17 Sihvola A (2002) How strict are theoretical bounds for dielec-tric properties of mixtures IEEE Trans Geosci Remote Sens40880

18 Haija A Freeman W Roarty T (2006) Effective characteris-tic matrix of ultathin multilayer structures Opt Appl 2639

19 wwwsopra sacom SOPRA database20 Kreibig U Vollmer M (1995) Optical properties of metal

clusters Springer series in material science Germany