p s sapplications and materials science

a

statu

s

soli

di

www.pss-a.comph

ysi

ca

REPRINT

4!05"#( )-,/*5# -')-(',/($)2 #5$(' %').0 6)!"#(1$,'!*,!'($./,3 %)#6 050)-5',/*#($506 &/0* )+/6( ,3/0 2/#7$

-B 407%20!" 9" &B :+%.* A.= :5C</" &B 45?CC<,A?@<#'" 6B (<8$5)#?#'" ?CA 1B 3B 65;?5=.#>,9

%

'XZ2C^X JK9C3_X3 DC dZIC2Q_;&g_XZC2 '_CZQ_>_g&2c dZ2Q_QKQX DC '_CZg_& DC S&QC3_&\C2 DC +&3gC\XZ& 8d'S.+`'Jd'7

&ZD 'CZQ3C D4dZIC2Q_;&g_XZ2 CZ R&ZXg_BZg_& _ R&ZXQCgZX\X;_&c 'XZ2C^X JK9C3_X3 DC dZIC2Q_;&g_XZC2 '_CZQ_>_g&2 8'dRM`'Jd'7c

'&[9K2 F.+c )A:YO +C\\&QC33&c J9&_Z

#

'XZ2C^X JK9C3_X3 DC dZIC2Q_;&g_XZC2 '_CZQ_>_g&2c dZ2Q_QKQX DC '_CZg_& DC S&QC3_&\C2 DC +&3gC\XZ& 8d'S.+`'Jd'7c

'&[9K2 F.+c )A:YO +C\\&QC33&c J9&_Z

$

R&Q_XZ&\ fC\\CZ_g LC2C&3ga #XKZD&Q_XZc HaCX3CQ_g&\ &ZD Na/2_g&\ 'aC[_2Q3/ dZ2Q_QKQCc 0&2_\CX2 WXZ2Q&ZQ_ZXK .IC6 <Ac

::VO= .QaCZ2c !3CCgC

&

R&Q_XZ&\ dZ2Q_QKQC >X3 U&2C32c N\&2[& &ZD L&D_&Q_XZ2 Na/2_g2c N6P6 +XE S! =<c 11:M= +Kga&3C2Qc LX[&Z_&

LCgC_ICD :O S&3ga M))Ac 3CI_2CD M1 bKZC M))Ac &ggC9QCD O) bKZC M))A

NK"\_2aCD XZ\_ZC M: bK\/ M))A

#!$" V:6)=6g9c V:6<V6%>c VA6O16N2c 1A6VV6@Gc 1A6V16+>c A:6:V6S]

!X\D Z&ZX9&3Q_g\C2 XZ ,_Zg XE_DC Qa_Z >_\[2 2K3>&gC2

a&IC "CCZ 2/ZQaC2_,CD "/ [C&Z2 X> QGX 2QC9 9K\2CD \&2C3

DC9X2_Q_XZ6 (_Zg &ZD ;X\D Q&3;CQ2 GC3C 2K"2C5KCZQ\/ 2K"[_Q`

QCD QX 9K\2C2 ;CZC3&QCD "/ &Z F0 W3#$ 8! ? M<A Z[c

""'!(

" M) Z2c # ? M f,7 CEg_[C3c &2 GC\\ &2 & >3C5KCZg/ Q3_`

9\CD RDe*.! 8! ? O== Z[c " ! :) Z27 \&2C3 2XK3gC26 HaC X"`

Q&_ZCD 2Q3KgQK3C2 2K3>&gC [X39aX\X;/c g3/2Q&\\_ZC 5K&\_Q/c &ZD

gaC[_g&\ gX[9X2_Q_XZ DC9Qa 93X>_\C GC3C _ZIC2Q_;&QCD "/

&gXK2Q_g 8D/Z&[_g7 [XDC &QX[_g >X3gC [_g3X2gX9/c -`3&/ D_>`

>3&gQ_XZc &ZD G&IC\CZ;Qa D_29C32_IC -`3&/ 29CgQ3X2gX9/6 HaC

gX33C29XZD_Z; &"2X39Q_XZ &ZD 3C>\CgQ_XZ 29CgQ3& GC3C 3C`

gX3DCD G_Qa QaC &_D X> & DXK"\C "C&[ 29CgQ3X9aXQX[CQC36 HaC

3C2K\Q2 93XICD QaC 9X22_"_\_Q/ QX Q&_\X3 QaC ;X\DT,_Zg XE_DC

Z&ZX2Q3KgQK3C2 X9Q_g&\ 93X9C3Q_C2 Qa3XK;a QaC 93X9C3 gaX_gC X>

QaC ;3XGQa 9&3&[CQC32c _6C6 ZK["C3 X> \&2C3 9K\2C2 K2CD >X3

QaC &"\&Q_XZ X> QaC ;X\D Q&3;CQ2 &2 GC\\ &2 Z&QK3C X> QaC 2K"`

2Q3&QC [&QC3_&\6 HaC QKZ&"\C X9Q_g&\ >C&QK3C2 _Z QaC I_2_"\C`

ZC&3 _Z>3&3CD 29CgQ3&\ 3C;_XZ &\\XG >X3 QaC DC2_;Z X> ;X\DT,_Zg

XE_DC Z&ZX2Q3KgQK3C2 G_Qa 93C`DC>_ZCD X9Q_g&\ ga&3&gQC3_2Q_g26

phys. stat. sol. (a) 205, No. 8, 1978–1982 (2008) / DOI 10.1002/pssa.200778874

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

p s sapplications and materials science

a

statu

s

soli

di

www.pss-a.comph

ysi

ca

phys. stat. sol. (a) 205, No. 8, 1978–1982 (2008) / DOI 10.1002/pssa.200778874

Tunable optical properties of laser grown double-structures with gold nanoparticlesand zinc oxide thin films

E. György*, 1, 4, A. Pérez del Pino2, A. Giannoudakos3, M. Kompitsas3, and I. N. Mihailescu4

1 Consejo Superior de Investigaciones Cientificas, Instituto de Ciencia de Materiales de Barcelona (ICMAB-CSIC)

and Centre d’Investigacions en Nanociència i Nanotecnologia, Consejo Superior de Investigaciones Cientificas (CIN2-CSIC),

Campus UAB, 08193 Bellaterra, Spain 2 Consejo Superior de Investigaciones Cientificas, Instituto de Ciencia de Materiales de Barcelona (ICMAB-CSIC),

Campus UAB, 08193 Bellaterra, Spain 3 National Hellenic Research Foundation, Theoretical and Physical Chemistry Institute, Vasileos Konstantinou Ave. 48,

11635 Athens, Greece 4 National Institute for Lasers, Plasma and Radiations Physics, P.O. Box MG 54, 77125 Bucharest, Romania

Received 13 March 2008, revised 27 June 2008, accepted 30 June 2008

Published online 21 July 2008

PACS 61.05.cp, 61.46.Df, 68.37.Ps, 78.66.–w, 78.67.Bf, 81.16.Mk

* Corresponding author: e-mail egyorgy@icmab.es

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Preparation and characterization of noble metal nanoparticles/oxide matrix composites at-tracted increasing interest in recent years due to the large number of potential applications, from nanoscale electron-ics and optics to nanobiology systems and nanomedicine [1–4]. All these technological fields valorize the drasti-cally different physical and chemical properties of noble metal nanoparticles as compared to those of the corre-sponding bulk materials. The identification of direct corre-spondence between synthesis parameters of nanoparticles on the surface or embedded into oxide matrices as well as their functional properties concentrates therefore signifi-cant attention [5–8].

As known, ZnO is an appropriate candidate for optical applications due to its high transparency and low reflection within the visible – IR spectral range [9, 10]. Due to the absence of any contribution in the absorption/reflection spectra in the visible-IR spectral range, it represents also an ideal raw material for the growth of the Au nanoparti-cles with the purpose to investigate their optical character-istics. This work is a development of our studies on ZnO thin films and Au/ZnO nanostructures deposition on (001)Si and (001)SiO2 substrates [11–13]. Investigations of the lo-cal electric properties of the Au covered ZnO surfaces re-vealed the metallic character of the Au nanoparticles [13].

Gold nanoparticles on zinc oxide thin films surfaces

have been synthesized by means of two step pulsed laser

deposition. Zinc and gold targets were subsequently submit-

ted to pulses generated by an UV KrF* (λ = 248 nm,

τFWHM ≅ 20 ns, ν = 2 Hz) excimer, as well as a frequency tri-

pled Nd:YAG (λ = 355 nm, τ ∼ 10 ns) laser sources. The ob-

tained structures surface morphology, crystalline quality, and

chemical composition depth profile were investigated by

acoustic (dynamic) mode atomic force microscopy, X-ray dif-

fraction, and wavelength dispersive X-ray spectroscopy. The

corresponding absorption and reflection spectra were re-

corded with the aid of a double beam spectrophotometer. The

results proved the possibility to tailor the gold/zinc oxide

nanostructures optical properties through the proper choice of

the growth parameters, i.e. number of laser pulses used for

the ablation of the gold targets as well as nature of the sub-

strate material. The tunable optical features in the visible-

near infrared spectral region allow for the design of gold/zinc

oxide nanostructures with pre-defined optical characteristics.

phys. stat. sol. (a) 205, No. 8 (2008) 1979

www.pss-a.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

This feature justifies the use of the obtained nanostructures in advanced technological applications. Moreover, these previous studies permitted us to identify the optimum ex-perimental conditions that allow for the growth of ZnO films with c-axis oriented crystal structure. This article focus on the correlation between the opti-cal properties and surface morphology of the laser grown Au/ZnO nanostructures as well as the synthesis process pa-rameters. We mention that we could not identify in the lit-erature similar studies on laser deposited Au nanoparticles. The main purpose of this paper is to investigate whether the optical properties of the laser grown Au/ZnO nano-structures can be controlled and therefore pre-defined through their surface morphology, determined in turn by the experimental process parameters. 2 Materials and methods Both the ZnO thin films and Au nanoparticles were grown by pulsed laser deposi-tion. A pulsed Quantel Mo. YG851 Nd:YAG (λ = 355 nm, τFWHM ∼ 10 ns, ν = 10 Hz) laser source was used for the multipulse irradiation of the Zn and Au targets. The laser fluence incident on the targets’ surface was set at 6.6 J/cm2. Prior to each experiment the irradiation chamber was pumped down to a residual pressure of 6 × 10–4 Pa residual pressure. For the synthesis of each ZnO thin film 70000 subsequent laser pulses were applied to the Zn targets in 20 Pa oxygen pressure. The Au nanoparticles were grown in a second step in vacuum by the irradiation of the Au tar-gets with up to 9000 laser pulses. Both targets were mounted on a vacuum-compatible computer controlled XY table which allows for the uni-form irradiation of a 10 × 10 mm2 surface area. This large scanned area avoids the significant changes in the targets’ surface morphologies caused by the action onto the same irradiation site of a large number of laser pulses. (001)Si and (001)SiO2 were chosen as substrate materials. The substrates were positioned parallel to the targets at a dis-tance of 40 mm and were maintained during the deposition process at a temperature of 300 °C. The targets and substrates were carefully cleaned with acetone in ultrasonic bath before introducing them into the irradiation chamber. Additional target cleaning was per-formed by a preliminary ablation step which proved to be essential for removing the last contaminants and impurities. During this process a shutter was interposed between the targets and the substrate, parallel to them. The surface morphology of the deposited ZnO thin films and Au/ZnO nanostructures was studied by atomic force microscopy (AFM) in acoustic (dynamic) mode, with a PicoSPM apparatus from Molecular Imaging. The crys-talline status was investigated by X-ray diffraction (XRD) in θ–2θ configuration with a RIGAKU diffractometer (Cu K

α, λ = 1.5418 Å radiation). The wavelength disper-

sive X-ray spectroscopy (WDX) studies were performed with a Cameca Camebax SX-50 equipment (15 kV, 100 nA, 1 µm diameter spot area, 2 s integrating time per point). The optical absorbance and reflectivity measure-

ments were carried out with a double beam spectropho-tometer (ThermoSpectronic) within the (200–1000) nm wavelength range. 3 Results and discussions Under visual inspection the reference ZnO films were completely transparent. Their thickness, measured by wavelength dispersive X-ray spectroscopy, was about 80 nm. The films’ average trans-mittance in the visible-IR spectral range when grown on quartz substrate exceeded 90%. On the other hand, the cyan blue-like color of the ZnO thin films covered with Au was a first indication for the formation of Au clusters [14].

The Au/ZnO nanostructures color further changed to golden with the gradual increase of the Au coverage. Figure 1 shows typical AFM images and the corre-sponding surface local height histograms of reference ZnO films grown on (001)Si (Fig. 1a, c) and (001)SiO2 quartz (Fig. 1b, d) substrates. As can be observed, both the diam-eters and heights of the ZnO grains grown on quartz sub-strate are larger as compared to those on Si. Previous results showed that the crystallization of ZnO films is enhanced and the average size of ZnO particles is superior on quartz as compared to Si, independent on the deposition technique [15, 16]. Indeed, the mean grains diameter and surface local height are estimated to be about 20 nm and 7 nm for the laser deposited ZnO films on Si, but about 60 nm and 30 nm for those deposited on quartz substrate. The further evolution of the surface morphology during the second irradiation step conducted for the growth of Au nanoparticles on the ZnO films deposited on Si substrate is presented in Fig. 2. The observed 3D growth of Au on the ZnO thin films surfaces could be associated with the reduced reactivity of

Figure 1 AFM images and the corresponding surface local

height histograms of reference ZnO thin films deposited on (a, c)

(001)Si and (b, d) (001)SiO2 quartz substrates.

1980 E. György et al.: Tunable optical properties of laser grown double-structures

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

ph

ysic

ap s sstat

us

solid

i a

Figure 2 AFM images and the corresponding surface profiles as

well as local height histograms of ZnO thin films on (001)Si sub-

strates with Au coverage obtained with (a, c, e) 4200 and (b, d, f)

9000 laser pulses incident on the Au targets.

Au to oxygen as well as metal adatom migration and island nucleation [17, 18]. With the increase of the number of applied laser pulses incident on the Au targets, i.e. with the increase of the Au coverage, the nanoparticles (grains) in plane diameter in-creases, accompanied by the decrease of their local heights. The average particles diameter and height reach around 60 nm and 2.5 nm in case of the largest Au coverage (Fig. 2b, d). Moreover, the grains density decreases with the increase of the Au coverage proving, together with the decrease of their relative height, the partial coalescence of the Au nanoparticles during their growth. Indeed, as re-ported in Ref. [19] the increase of the coupling between the nanoparticles leads to a gradual change of the reflected color from green to a metallic gold, as occurred in our ex-periments. In Fig. 3 we show the diffractograms of a reference ZnO thin film (curve a) and Au/ZnO nanostructures ob-tained with 9000 (curve b) laser pulses incident on the Au targets. The diffractogram of the reference thin film is composed by the lines of hexagonal phase ZnO at 34.4° and 36.2° corresponding to the (002) and (101) lattice plane reflections (curve a), as referred in the JCPDS 36-1451 file [20]. The intense line at 34.4° indicates that the films are composed by crystallites with the c-axis

Figure 3 X-ray diffractograms of thin films deposited on

(001)Si substrates: (a) reference ZnO film, and (b) ZnO film with

Au coverage obtained with 9000 laser pulses incident on the Au

target.

grown preferentially perpendicular to the plane of the sub-strate surface. The diffractogram of the Au/ZnO nanostruc-tures contains besides those already mentioned an addi-tional line at 38.2° attributed to the (111) lattice plane re-flection of cubic phase Au (curve b) as referred in the 04-0784 file [20], proving the growth of Au crystallites along the (111) preferred orientation. The typical optical reflection spectra of the uncovered ZnO thin film and the Au/ZnO nanostructures are given in Fig. 4. The first minimum at around 370 nm of the reflec-tion spectra correspond to the energy band gap of ZnO at 3.3 eV, followed by a sharp maxima peaking at around 380 nm. As well known from the theory of optical transi-tions in semiconductors, a peak in the reflectance spectra is appearing in the vicinity of a band edge [21]. With the in-crease of the Au coverage the optical characteristics of the Au/ZnO nanostructures gradually change in the visible-IR spectral range from transparent (Fig. 4a) to a highly re-flecting metallic like behavior (Fig. 4e). In addition, a re-flection minimum centered at around 520 nm appears in the Au/ZnO nanostructures spectra. The intensity of this band increases with the increase of the number of laser

Figure 4 Reflection spectra of thin films deposited on (001)Si

substrates: (a) reference ZnO film, and ZnO films with Au cover-

age obtained with (b) 900, (c) 1200, (d) 4200, and (e) 9000 laser

pulses incident on the Au targets.

phys. stat. sol. (a) 205, No. 8 (2008) 1981

www.pss-a.com © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original

Paper

Figure 5 AFM images and the corresponding surface profiles as

well as local height histograms of ZnO thin films deposited on

(001)SiO2 quartz substrates with Au coverage obtained with (a, c,

e) 4200, and (b, d, f) 9000 laser pulses incident on the Au targets.

pulses incident on the Au target, i.e. the increase of the Au coverage. The main reflection minimum around 520 nm is asso-ciated with the surface plasmon resonance (SPR) absorp-tion band of Au nanoparticles [22, 23]. As known, the in-cident photons of this wavelength are converted to surface plasmons as a result of interaction with the free electron distribution of gold nanoparticles surface, leaving a gap in the reflected light intensity [23]. The much weaker band at longer, around 800 nm wavelength (Fig. 4e) could be assigned to the contribution of larger aggregates, composed by Au nanoparticles [5]. As reported, the size dependent feature of SPR absorp-tion can allow for the production of noble metal nanoparti-cles containing composite systems with tunable optical properties [8, 16, 23–27]. The main geometrical param-eters that induce changes in the surface plasmon resonance band are the Au nanoparticles (i) in-plane diameters [24, 25], (ii) aspect ratio [26], (iii) height [27], (iv) shape [16], or (v) volume fraction in case of nanocermet (nanoparticles embedded in ceramic matrix) thin films [8]. As a general rule it was found that both the increase of nanoparticles diameter and/or the decrease of their height, i.e. the rise of aspect ratio, cause the same changes in the reflection/absorption spectra of the composite systems.

This is the shift of the surface plasmon resonance absorp-tion band towards higher wavelengths. However, the as-pect ratio of the Au clusters grown on the surface of ZnO films deposited on quartz substrates are similar to the structures deposited on Si (Fig. 5). The average particles diameter and height for the high-est Au coverage reach around 160 nm and 7 nm, respec-tively. Nevertheless, the absorption band of the Au/ZnO structures deposited on quartz substrates are red-shifted to around 650 nm, as compared to the structures deposited on Si (Fig. 6). It was reported that the most relevant geometrical pa-rameter on the SPR wavelength in case of Au particles is their in-plane diameter [6]. This could explain the observed red-shift of the SPR absorption band of Au clusters grown on the surface of ZnO films deposited on quartz, as com-pared to those deposited on Si. The absorption spectrum of the structure deposited at the highest Au coverage (Fig. 6d) contains a broad band in the visible-IR spectral region. The broadening of the absorption band could be due to the increase of particles diameter with the increase of the Au coverage until their partial coalescence. As a result, the optical properties converge towards those characteristic to bulk material [22, 28]. Our results indicate that the Au nanoparticles morphol-ogy is determined by the base ZnO thin films surface to-pography as well as the degree of the Au corevage, i.e. number of laser pulses applied for the ablation of the Au targets (see Figs. 2 and 5). The Au deposition rate was es-timated by wavelength dispersive X-ray spectroscopy (WDX) and was found to be around 0.02 Å/laser pulse. This low value allows for the accurate control of the amount of Au deposited on the ZnO films surface and thus, of the nanoparticles morphology. The thickness of the un-derlying ZnO films was estimated from the WDX data at about 80 nm. The established dependence between the Au nanoparticles morphology and SPR absorption permits the continuous tuning of the optical properties of the Au/ZnO nanostructures in the visible spectral range.

Figure 6 Absorption spectra of thin films deposited on (001)SiO2

quartz substrates: (a) reference ZnO film, and ZnO films with Au

coverage obtained with (b) 900, (c) 4200, and (d) 9000 laser

pulses incident on the Au targets.

1982 E. György et al.: Tunable optical properties of laser grown double-structures

© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-a.com

ph

ysic

ap s sstat

us

solid

i a

4 Conclusions We investigated the interrelations between the optical properties and surface morphology of Au nanoparticles/ZnO thin films nanostructures depos-ited on Si and quartz substrates. Both the ZnO films and Au nanoparticles were grown by pulsed laser deposi- tion. A frequency tripled Nd:YAG laser (λ = 355 nm, τFWHM ∼ 10 ns, ν = 10 Hz) was used for the two steps irra-diation of the Zn and Au targets. The deposition of the ZnO films was performed in a low pressure oxygen atmos-phere, while the Au nanoparticles were grown in vacuum. We have demonstrated for the first time the possibility of the size controlled growth by pulsed laser deposition of Au/ZnO nanostructure systems with predetermined optical properties. Our results demonstrated that through the con-trol of growth parameters (e.g. number of laser pulses used for the irradiation of the Au targets, which determine the amount of the Au coverage and the morphology of the Au nanoparticles) we can achieve the continuous tuning of the SPR peak. The ability to predict and modify in a controlled manner the optical properties of a nanocomposite system is useful in many key technological applications from biol-ogy to engineering of photonic and opto-electronic devices, as well as in chemo- and biosensing.

Acknowledgements The authors acknowledge with

thanks the financial support from NATO (PST.CLG 980464)

and Romanian National University Research Council (Grant

863/2006).

References

[1] G. Cao, Nanostructures & Nanomaterials, Synthesis, Prop-

erties & Applications (Imperial College Press, London,

2004).

[2] H. S. Nalwa, Nanostructured Materials and Nanotechnology

(Academic Press, San Diego, 2002).

[3] D. Roy and J. Fendler, Adv. Mater. 16, 479 (2004).

[4] Z. Liu, H. Song, L. Yu, and L. Yang, Appl. Phys. Lett. 86,

113109 (2005).

[5] R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz, and

J. G. Zheng, Science 294, 1901 (2001).

[6] R. Gupta, M. J. Dyer, and W. A. Weimer, J. Appl. Phys. 92,

5264 (2002).

[7] P. T. Miclea, A. S. Susha, Z. Liang, F. Caruso, C. M. So-

tomayor Torres, and S. G. Romanov, Appl. Phys. Lett. 84,

3960 (2004).

[8] S. Hazra, A. Gibaud, and C. Sella, Appl. Phys. Lett. 85, 395

(2004).

[9] D. R. Lide, Handbook of Chemistry and Physics, 71st ed.

(CRC, Boca Raton, FL, 1991).

[10] D. P. Norton, Y. W. Heo, M. P. Ivill, K. Ip, S. J. Pearton,

M. F. Chisholm, and T. Steiner, ZnO: growth, doping &

processing, Materials Today, June (2004), p. 34.

[11] E. György, J. Santiso, A. Giannoudakos, M. Kompitsas,

I. N. Mihailescu, and D. Pantelica, Appl. Surf. Sci. 248, 147

(2005).

[12] E. György, J. Santiso, A. Figueras, A. Giannoudakos,

M. Kompitsas, I. N. Mihailescu, and C. Ducu, Appl. Surf.

Sci. 252, 4429 (2006).

[13] E. György, J. Santiso, A. Figueras, A. Giannoudakos,

M. Kompitsas, and I. N. Mihailescu, J. Appl. Phys. 98,

084302 (2005).

[14] D. Barreca, A. Gasparotto, E. Tondello, G. Bruno, and

M. Losurdo, J. Appl. Phys. 96, 1655 (2004).

[15] M. Okoshi, K. Higashikawa, and M. Hanabusa, Appl. Surf.

Sci. 154/155, 424 (2000).

[16] S. Chakrabarti, D. Ganguli, and S. Chaudhuri, Mater. Lett.

58, 3952 (2004).

[17] U. Diebold, J. M. Pan, and T. E. Madey, Phys. Rev. B 47,

3868 (1993).

[18] J. Yoshihara, S. C. Parker, and C. T. Campbell, Surf. Sci.

439, 153 (1999).

[19] P. Mulvaney, L. M. Liz-Marzan, M. Giersig, and T. Ung,

J. Mater. Chem. 10, 1259 (2000).

[20] Powder Diffraction File, JCPDS International Centre for

Diffraction Data, Pennsylvania, USA, 1986.

[21] F. Wooten, Optical Properties of Solids (Academic, New

York, 1972).

[22] U. Kreibig and M. Vollmer, Optical Properties of Metal

Clusters (Springer, Berlin, 1995).

[23] Y. Sun and Y. Xia, J. Am. Chem. Soc. 126, 3892 (2002).

[24] U. Pal, J. Garcia-Serrano, G. Casarrubias-Segura, N. Koshi-

zaki, T. Sasaki, and S. Terahuchi, Sol. Energy Mater. Sol.

Cells 81, 339 (2004).

[25] D. M. Schaadt, B. Feng, and E. T. Yu, Appl. Phys. Lett. 86,

063106 (2005).

[26] S. Link, M. B. Mohamed, and M. A. El-Sayed, J. Phys.

Chem. B 103, 3073 (1999).

[27] T. R. Jensen, M. D. Malinsky, C. L. Haynes, and R. P. Van

Duyne, J. Phys. Chem. B 104, 10549 (2000).

[28] S. V. Roth, H. Walter, M. Burghammer, C. Riekel,

B. Lengeler, C. Schroeder, M. Kuhlmann, T. Walther,

A. Sehrbrock, R. Domnick, and P. Muller-Buschbaum, Appl.

Phys. Lett. 88, 021910 (2006).