Plasmon resonance absorption in sulfide-coatedgold nanorods

K. Chatterjee, S. Basu, and D. Chakravortya)

Indian Association for the Cultivation of Science, Kolkata 700 032, India

(Received 9 February 2005; accepted 7 June 2005)

Gold nanorods 100 nm in diameter were grown within polycarbonate membranes astemplates by the electrodeposition technique. A low-temperature sulfidation processwas used to make gold sulfide nanoshells around the nanorods with a thickness of∼7 nm. Optical absorption measurements were carried out on sulfide-coated goldnanorods obtained by dissolving the polycarbonate membrane. Several peaks wereobserved. These were analyzed on the basis of longitudinal and transverse modes ofgold nanorods, the core–shell structure of gold–gold sulfide, and the presence ofnanoparticles of gold. Theoretical analysis was carried out using a modified Miescattering formalism. Satisfactory agreement between experimental results andtheoretical fits were obtained.

I. INTRODUCTION

Surface plasmon resonance (SPR) in noble metalnanostructures has generated wide interest in recent yearsbecause of the unusual properties they exhibit and theirpotential applications in chemical/biological sensors,1–3

data storage,4 and optoelectronic devices.5–7 A numberof researchers8–10 have studied the optical properties ofnanostructured gold particles. The absorption character-istics have been analyzed by Mie scattering theory11 orthe effective medium formalism.12 The optical propertiesof nonspherical nanostructures10,13 have also drawn quitea bit of researchers’ attention. A rather interesting aspectin this context is the spectroscopic investigation of metalnanowire with a coating of a semiconducting phase. In-terest in these core–shell nanostructures originated fromthe expectation of a large nonlinear response due to thesurface plasmon resonance.14,15 We had previously stud-ied the shifts in plasmon resonance peak in metal-core-metal-oxide-shell spherical nanostructures grown withina silica gel matrix.16

We have been able to synthesize sulfide coated goldnanorods by a simple chemical method. Optical absorp-tion characteristics of this material have been studied.The experimental data have been analyzed successfullyusing a modified Mie scattering formalism. The detailsare reported in this paper.

II. EXPERIMENTAL

Gold nanorods were synthesized by electrodeposition.Figure 1 shows schematically the setup used for such

growth. The electrolyte used was tetrachloroauric acid(HAuCl4·xH2O, supplied by Aldrich, Milwaukee, WI). Apolycarbonate membrane (supplied by Whatman, Nucle-pore, Clifton, NJ) of pore diameter 100 nm and mem-brane thickness 1 �m was taken as the template, theoneside of which was painted by silver paste (supplied byAcheson Colloiden B.V. Holland, Scheemda, The Neth-erlands) to be used as cathode. A gold plate was used asthe anode. For the growth of the gold nanorods 1.5 V wasapplied for 5 min. Aligned gold nanorods ∼100 nm weregrown within polycarbonate template. The sample wasused for the electrical measurement of gold nanorods.Silver paste electrodes were applied on the two oppositefaces of a polycarbonate membrane in which gold nano-rods were grown. The painted electrodes formed ohmiccontacts with the gold nanorods as was confirmed by thelinear voltage-current characteristics exhibited by thesystem Sec. III. For the optical measurement of the lat-ter, the polycarbonate film with gold rods was dissolvedin chloroform. Prior to this step, the silver paste on themembrane was washed off thoroughly with acetone. Inthe process, the gold wires break up into smaller lengthsforming nanorods. In our subsequent discussion, we referto the optical properties of gold-gold sulfide core–shellstructured nanorods. For the sulfidation treatment thepolycarbonate film impregnated with gold nanorods wasdissolved in 10 ml chloroform. The solution was mixedwith 2 mg thiourea [(NH2)2 CS] and a few drops ofammoniacal solution. It was then heated at 343 K. In thisreaction process, the surfaces of the gold nanorods wereoxidized and became coated with Au2S.

Optical absorption spectra of these Au2S coated goldnanorods dispersed in chloroform were recorded at dif-ferent temperatures in a UV-2101 PC ultraviolet–visible

a)Address all correspondence to this author.e-mail: mlsdc@mahendra.iacs.res.in

DOI: 10.1557/JMR.2006.0032

J. Mater. Res., Vol. 21, No. 1, Jan 2006 © 2006 Materials Research Society34

(UV-vis) scanning spectrophotometer manufactured byShimadzu, Kyoto, Japan. Thus the nanorods were takenout of the polycarbonate structure and randomly distrib-uted in a liquid medium for optical measurement. Beforetaking the spectrum in this case and also for the earliercase of pure gold nanorods dispersed in chloroform, thebase line was corrected and absorption was measuredkeeping a pure polycarbonate film dissolved in chloro-form at the reference beam position. From the opticalabsorption data, the extinction coefficient was calculated.In the subsequent discussion, we refer only to the extinc-tion coefficient.

The microstructure of different samples before andafter sulfidation of gold nanorods was studied by a JEOL2010 transmission electron microscope (Tokyo, Japan).

III. RESULTS AND DISCUSSION

Figure 2(a) shows the microstructure of gold nanorodsbefore the sulfidation treatment and Fig. 2(b) shows thatof Au2S coated gold nanorod. Figure 2(c) is the electrondiffraction pattern obtained from Fig. 2(b). The observa-tion of diffraction rings implies that the nanorod is com-posed of large number of nanocrystals of gold. It hasbeen shown earlier in the case of electrodeposited silverwithin a glass template that the metallic phase grownconsists of metallic nanoparticles.17–19 It is thus con-cluded that in the present case (gold rod) nanoparticles ofgold also form the nanorod. The interplanar spacing dhkl

values were calculated from the diameters measuredfrom the different rings. The positions of the rings areshown by arrows and the indices of the planes are indi-cated within brackets. Table I compares the experimen-tally deduced dhkl values with the standard ASTM datafor gold and gold sulfide phases. The results confirmthe presence of both gold and gold sulfide phases. FromFig. 2(a) the measured diameter of gold nanorod wasfound to be 100 nm. Figure 2(b) gave the diameter to be

93 nm and the shell thickness to be 7 nm. Thus theoverall diameter of the composite nanorods was esti-mated as 107 nm. It is thus evident that after sulfida-tion, the diameter of the nanorod increased by 7%. InFig. 2(d), a few nanorods of gold are shown for thepurpose of determining the aspect ratio, which is in therange 2.8–3.3. Figure 2(e) gives the lattice fringes ob-tained from the gold nanorods. The lattice spacingsmeasured are 0.24 and 0.20 nm, which correspond, re-spectively, to the planes (111) and (200) of Au. InFig. 2(f), we show the lattice fringes obtained from theshell. The spacing measured is 0.29 nm, which corre-sponds to the (111) planes of Au2S. Figure 3 gives theenergy dispersive spectroscopy (EDS) analysis of thegold-gold sulfide core–shell nanorods. The copper andcarbon peaks arise due to the presence of copper grid andthe carbon coating, respectively. The peaks of gold andsulfur are contributed by the gold and gold sulfidephases. Figure 4 shows the voltage–current characteris-tics of gold nanorods within polycarbonate template atthree different temperatures: 306, 273, and 173 K. Theseconfirm the metallic behavior of the nanorods. Tempera-ture coefficient of resistance (TCR) calculated from thesedata (� 0.0039 /K) is in satisfactory agreement with thatof bulk gold (� 0.0.0040 /K).20 Figure 5 shows thevariation of extinction coefficient of sulfide coated goldnanorods as a function of wavelength at different tem-peratures. The temperature variation was controlled by aTCC controller (TCC – 240A, Shimadzu) attached withthe UV-vis spectrophotometer.

It is evident from Fig. 5 that there are two major peaks,one in the range from ∼400 to ∼500 nm and other at 625nm. One minor peak is there at around 520 nm. It isshown below that the peak position which varies withtemperature arises due to the presence of Au2S coated Aunanorods and the peaks at ∼625 and ∼520 nm are due tothe gold nanorods.

Using an extended Mie theory, we first explain thelongitudinal and transverse modes of gold nanorods. Ac-cording to Gans,21,22 for randomly oriented particles inthe dipole approximation, the extinction coefficient � canbe written as

� =2�NV�m

3�2

3� �j

�1�Pj2��2

��1 +1 − Pj

Pj�m�2

+ �22

,

(1)

where N is the number of particles per unit volume, V isthe volume of particle, �m is the dielectric constant of thesurrounding medium, � is the wavelength of the lightbeam, and �1 and �2 are the real and imaginary parts ofthe dielectric function of the particles. The material di-electric function is frequency dependent whereas the �m

can be assumed to be constant. Pj’s are the depolarization

FIG. 1. Schematic diagram of the setup used for the growth of goldnanorods.

K. Chatterjee et al.: Plasmon resonance absorption in sulfide-coated gold nanorods

J. Mater. Res., Vol. 21, No. 1, Jan 2006 35

factors for the three axes X, Y, Z of the rods for whichx � y < z. These are defined as

PZ =1 − e2

e2 � 1

2eln�1 + e

1 − e� − 1� , (2)

PX = Py =1 − PZ

2, (3)

where, e =�1 − �1

R�2

, (4)

R is the aspect ratio of the gold nanorods. Figure 6 showsthe variation of extinction coefficient as a function ofwavelength for the gold nanorods grown here. There aretwo peaks observed. The experimental data were least

FIG. 2. (a) Transmission electron micrograph for gold nanorods. (b) Transmission electron micrograph for sulfide coated gold nanorods.(c) Electron diffraction pattern obtained from (b). (d) Electron micrograph of gold nanorods. (e) Lattice fringes obtained from gold nanorods.(f) Lattice fringe obtained from the gold sulfide shells.

K. Chatterjee et al.: Plasmon resonance absorption in sulfide-coated gold nanorods

J. Mater. Res., Vol. 21, No. 1, Jan 200636

square fitted to Eq. (1) by taking N, �m, and R as adjust-able parameters and substituting the known values for thecomplex dielectric constant of gold.23 The theoretical fitsare also shown in Fig. 6. The extracted parameters areN � 2.1 × 10−11/nm3, �m � 4.1, and R � 3.1. The valueof N is consistent with the pore density (∼10−11 nm3) of

the polycarbonate film. The value of �m (� 4.1) is inagreement with that of chloroform. The aspect ratio of3.1 is consistent with the longitudinal and transversemode of plasmon resonance showing absorption maximaat 640 and 520 nm, respectively.24 A value of aspect ratioof 3.1 implies that the gold wires grown within the poly-carbonate films break up into smaller lengths during thedissolution of the membrane in chloroform. This becamepossible because of the nanowire formation by the inter-connection of gold nanoparticles.

Figure 7 shows the extinction coefficient of goldsulfide coated gold nanorods dispersed in chloroformat 300 K. Here the background absorption has been

TABLE I. Comparison of dhkl values obtained from electron diffrac-tion with standard ASTM data for sulphide coated gold nanorod.

Observeddhkl (nm)

Standard ASTMgold (nm)

Data goldsulphide (nm)

0.289 0.289 (111)0.235 0.2355 (111)0.201 0.2039 (200)0.178 0.178 (220)0.143 0.1442 (220)

FIG. 3 EDS spectrum of gold-gold sulfide core–shell nanorods.

FIG. 4. Voltage current characteristic of gold nanorods at differenttemperatures: (�) 273 K, (×) 173 K, (�) 306 K.

FIG. 5. Optical absorption spectra of Au2S coated gold nanorods as afunction of wavelength taken at different temperatures.

FIG. 6. Theoretical fit of extinction peaks obtained from gold nano-rods measured at 300 K due to longitudinal and transverse modes:experimental–theoretical.

K. Chatterjee et al.: Plasmon resonance absorption in sulfide-coated gold nanorods

J. Mater. Res., Vol. 21, No. 1, Jan 2006 37

subtracted. Peaks 1 and 2 in these curves were fitted bythe procedure outlined as above.

For the theoretical fitting of the extinction maximum(indicated as 3 in Fig. 7) of the sulfide coated gold nano-rods taken at different temperatures from 280 to320 K, we take the following approach. We estimate firstthe effective dielectric function of spherical Au-coreAu2S–shell structure. Then these values are incorporatedin Eqs. (1), (2) and (3) to carry out the theoretical fittingof the experimental results. We consider a hypotheticalsystem consisting of a close-packed structure of goldcore-gold sulfide shell particles with an average radius r.The particles are embedded in an air matrix. The polar-izability for a gold particle with a gold sulfide coatingon it and embedded in a matrix (in this case air) isgiven by25

� = ���s − �h���m� + 2�s�

+ �1 −t

r�3

��m� − �s���h + 2�s�

��m� + 2�s��2�h + �s�

+ 2�1 −t

r�3

��s − �h���m� − �s�� rj

3 ,

(5)

where �s, �h, and �m� are the dielectric permittivities of theshell (here gold sulfide) host medium (here air) and metalcore (gold) respectively, r is particle radius, and t is thethickness of the shell on the particle r. The size-dependent dielectric permitivity �m� in the case of metalcore, for which the composite radius (i.e., inclusive of theshell) is r, can be written as26

�m� = �1 −�p

2

�2� + i��p2

�3 �m

�O+

VF

r − t�� , (6)

where �p is the bulk plasmon frequency, m is the con-ductivity of the metal core with radius (r − t), �O is thefree space permittivity, and VF is the Fermi velocity forgold. The dielectric permittivity for the composite systemcomprising the core-shell nano structure can be evaluatedusing the Clausius Mosotti relation as27

� =1 + 2N��

1 − N��, (7)

where N� is the number of particles per unit volume.Since we have considered a hypothetical close-packedsystem consisting of Au core–Au2S shell particles withan average radius r, the number of such composite par-ticles per unit volume can be taken as (4⁄3�r3)−1 x0.74.Substituting this value in Eq. (7) and using Eqs. (5), (6),and (7), the real and imaginary parts of � for these com-posite particles were calculated. Putting the real andimaginary parts of � in Eq. (1) and keeping all otherparameters unchanged the extinction coefficient � for thesulfide coated gold nanorods were calculated. The theo-retical fit is shown in Fig. 7 for the extinction peak in-dicated as 3. The theoretical fits are in satisfactoryagreement with the experimental data. In this calculationwe have taken VF � 1.4 × 1015 nm/s, �h � 1, and �P �1.36 × 1016rad/s,20 and N, �s, �m, t, and m were taken asthe adjustable parameters. It should be pointed out herethat reliable conductivity values could not be estimatedfrom the voltage–current characteristics as shown inFig. 4. The reason is not all the pores had continuous goldnanowires grown within them. In the electrodepositionprocess used here such wires are formed first only insome of the pores. As soon as metallic connection isestablished between the two electrodes in the system fur-ther growth of metallic phase in the remaining pores willstop. Table II summarizes the values of the extracted

FIG. 7. Theoretical fit of the extinction peaks obtained from goldsulfide coated gold nanorods at 300 K: experimental–theoretical.

TABLE II. Extracted parameters N, �s, �m, t, and m obtained by theoretical fitting for the third peak around 430 nm for different specimens.

Specimenno.

Measurementtemperature (K) N (nm−3) �s �m t (nm) (−1 m−1) R

1 280 2.1 × 10−11 15 ± 0.5 4.1 ± 0.1 7.0 ± 0.5 3.8 × 104 ± 1 × 101 3.1 ± 0.12 300 2.1 × 10−11 15 ± 0.5 4.1 ± 0.1 7.0 ± 0.5 3.76 × 104 ± 1 × 101 3.1 ± 0.13 310 2.1 × 10−11 15 ± 0.5 4.1 ± 0.1 7.0 ± 0.5 3.73 × 104 ± 1 × 101 3.1 ± 0.14 320 2.1 × 10−11 15 ± 0.5 4.1 ± 0.1 7.0 ± 0.5 3.71 × 104 ± 1 × 101 3.1 ± 0.1

K. Chatterjee et al.: Plasmon resonance absorption in sulfide-coated gold nanorods

J. Mater. Res., Vol. 21, No. 1, Jan 200638

parameters. The values of �h � 15 and �m � 4.1 wereobtained by least squares fitting in the case of all tem-peratures. The extracted value of �m is in agreement withthat of chloroform. The value of N as extracted by theleast square fitting is also consistent with the densityestimated from the pore density of the polycarbonatemembrane. The value of r was taken from the pore sizeof the regular template used and the value is identical tothat obtained from Fig. 2(b). The value of shell thickness,extracted from the theoretical fitting is in good agree-ment with Fig. 2(b).

From Fig. 7, it should be evident that there is a smallextinction peak at ∼600 nm, for which we have not ac-counted so far. This is believed to arise due to the pres-ence of gold nanoparticles in the liquid medium. As men-tioned earlier, nanowire formation is brought about bythe interconnection of nanoparticles of metal during theelectrodeposition process.18 For optical studies, the poly-carbonate film was dissolved in chloroform, and in thisprocess it is bound to create some loss of connectivity ofthe gold particles making up the gold wire. We havetheoretically fitted this absorption peak by using Miescattering equation, namely,

� =18�f�m�2

����1 + 2�m�2 + �22�

, (8)

where f is the volume fraction of particles concerned andthe other symbols have their usual meaning. The theo-retical fit to the experimental data is shown in Fig. 8. Thevalue of f extracted is ∼10−3.

In summary, gold nanorods 100 nm in diameter weregrown within a polycarbonate membrane having poresize of 100 nm by an electrodeposition method. The gold

rods showed metallic characteristics. After dissolving thepolycarbonate membrane in chloroform, a gold sulfideshell of ∼7 nm thickness was grown on the gold rods bytreating the latter with thiourea. Detailed optical absorp-tion spectra were investigated. Longitudinal and trans-verse modes as described in the Mie theory gave extinc-tion peaks for gold rods at around 640 and 520 nm,respectively. For the gold rod with a gold sulfidenanoshell, the extinction peak was at ∼430 nm, whichvaried as a function of temperature. The extinction peakswere explained on the basis of extended Mie theory. Apeak around 600 nm was explained as being causedby the presence of nano particles of gold of diameter100 nm. The theoretical fits were found to be in satis-factory agreement with experimental results.

ACKNOWLEDGMENTS

Kuntal Chatterjee and Soumen Basu thank the Councilof Scientific and Industrial Research, New Delhi for aSenior and a Junior Research Fellowship, respectively.D. Chakravorty thanks INSA, New Delhi for a SeniorScientist position. The research was carried out under aNano Science and Technology Initiative Programme ofDST, New Delhi. The authors thank Supriya Chakra-borty for his help with transmission electron microscopywork.

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