2011_Optical Properties of Silver Nanoparticles

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Optical properties of silver nanoparticles

thermally grown in a mesostructured hybrid

silica film

Y. Battie,1

N. Destouches,1,*

F. Chassagneux,2

D. Jamon,3

L. Bois,2

N. Moncoffre,

4and N. Toulhoat

4,5

1Universit de Lyon, Universit Jean Monnet, CNRS UMR 5516 Laboratoire Hubert Curien, 18 rue Pr. Benoit

Lauras, 42000 Saint Etienne, France2Universit de Lyon, Universit Claude Bernard Lyon 1, CNRS UMR 5615 Laboratoire Multimatriaux et interfaces,

43 bd 11 novembre 1918, 69622, Villeurbanne, France3Universit de Lyon, Universit Jean Monnet, CNRS EA 3523, Laboratoire Tlcom Claude Chappe, 25 rue du

Docteur Rmy Annino, 42000 Saint Etienne, France4Universit de Lyon, Universit Claude Bernard Lyon1, CNRS/IN2P3, IPNL, 4 rue E. Fermi, 69622 Villeurbanne

cedex, France5CEA/DEN, Centre de Saclay, 91191 Gif sur Yvette cedex, France

*[email protected]

Abstract: The composition, the structure and the optical properties of

mesostructured hybrid silica films elaborated by sol-gel routine are studied

versus the temperature of the post treatment by comparing ellipsometricmeasurements, atomic force microscopy and electronic microscopy

characterizations, X-ray diffraction and thermal analysis. This paper shows

that the refractive index variation is a combination between the structure

contraction, the solvent evaporation, the silica wall condensation and the

pyrolysis of the copolymer. We also investigate the optical properties of

thermally grown silver nanoparticles in the mesotructured films and we

demonstrate that these properties depend on the optical properties of the

host matrix and on the silver concentration profile in the film.

2011 Optical Society of America

OCIS codes: (310.6860) Thin films, optical properties; (160.4236) Nanomaterials; (120.2130)

Ellipsometry and polarimetry.

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#146172 - $15.00 USD Received 19 Apr 2011; revised 17 Aug 2011; accepted 17 Aug 2011; published 24 Aug 2011

(C) 2011 OSA 1 September 2011 / Vol. 1, No. 5 / OPTICAL MATERIALS EXPRESS 1019


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1. Introduction

Because of various applications in microelectronics [1,2], in catalysis [3], or for optical

coatings [4,5], chemical sensors [6] or host matrices for metallic and semiconductor

nanoparticles growth [79], the synthesis of porous materials has become an important

research field in the last few years. Sol-gel routine is used to elaborate mesoporous silica thin

films with 2D or 3D hexagonal and cubic mesostructures [10]. These films are prepared by

surfactant template methods in which an organic micellar mesophase self-organizes in the

silica network during the deposition [11]. The formed mesostructure depends essentially on

the surfactant nature and on the silica/surfactant ratio. The mesoporous silica films areobtained after removing the surfactant by calcination. Mesoporous thin films synthesized from

non-ionic triblock co-polymer surfactants are interesting materials as they have high degree of

ordering [12,13]. Moreover, this type of surfactant is both low cost and environmentally

benign [14,15]. The refractive index of mesoporous silica films mainly depends on the film

porosity and can be tuned by varying the molecular weight of the triblock copolymer [16]. A

low refractive index film can then be obtained with a high molecular weight copolymer like

Pluronic F127 (PEO106-PPO70-PEO106). Muro and associates have recently reported an

increase of the porosity and a refractive index decrease, when raising the F127 concentration

in a silica sol [17].

The use of such mesoporous films as host matrices for in situ growth of metal

nanoparticles has been investigated for the control of the surface plasmon resonance (SPR).

The latter leads to an absorption band in the UV/visible spectrum whose position and shape

depend on the nanoparticles size and shape, the refractive index of the host matrix [18,19] and

the interparticle distance [20,21]. We have recently demonstrated that mesostructured silica

films doped with a silver salt could be used to control the growth of silver nanoparticles

thanks to an in situ chemical [22] or photo-chemical [23] reduction of silver ions. Mesoporous

titanium dioxide films were also used to limit the growth of silver nanoparticles to the pore

size under laser illumination [24]. Unlike the two previous reduction methods, the thermal

growth of silver nanoparticles is known to generate greater nanoparticle sizes than the pore

size [25].

Although spectroscopic ellipsometry is known to be an efficient tool to follow the

chemical [26,27] or the physical [2729] growth of silver nanoparticles on a substrate, to date,

no work was reported on a complete spectroscopic ellipsometric characterization of the optical

properties of silver nanoparticles embedded in a mesostructured silica film. Such

nanocomposite films may present singular properties like anisotropy or graded index that need

cross investigations to attest the results.

To our knowledge, this paper reports the first complete ellipsometric characterisation ofmesoporous materials loaded with thermally grown silver nanoparticles. We firstly

characterize the changes in mesostructured silica thin films without silver during calcination

of F127 copolymer. The mesostructure, the compression in the normal direction, the chemical

composition of the films are measured as well as their refractive index. The modelling of the




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ellipsometric parameters is correlated to the different results obtained on the films. Secondly,

we demonstrate the capabilities of spectroscopic ellipsometry to investigate the optical

properties of mesostructured silica films containing thermally grown silver nanoparticles. Our

interpretations are also based on the characterization of the opto-geometrical matrix

parameters and the silver location in the film thickness. This kind of material offers a great

interest for the achievement of optical components based on low refractive index or on SPR.

2. Synthesis

All reactants are used as received from Sigma Aldrich Inc.. In our typical synthesis, 4 g of

tetraetoxysilane (TEOS) are mixed with 1.76 g of hydrochloride acid HCl (0.055M). This

solution is stirred 30 min. Then, 1.14 g of the tribloc copolymer F127 dissolved in 18 mg of

ethanol are added to the initial solution. The sol is stirred 15 min. Finally, 1.7 g of silver

nitrate are added in the sol and the sol is mixed during 15 min. This addition of silver nitrate is

omitted for elaborating undoped films. Each film is deposited by dip-coating with a 3.5

cm/min speed rate on a clean glass substrate or on a thermanox substrate, for TEM

measurements, in a clean room class 1000 and under a yellow light in order to prevent the

photoreduction of silver salt when it is added. The humidity rate and the temperature are

respectively fixed at 50 5% and 21 1C in order to insure a better repeatability of the

synthesis. Each film is transparent, crack-free and of good optical quality. Films are then

directly heated 30 min in air at a temperature comprised between 100C and 400C.

3. Characterisation

The mesostructure of the film is characterized by X-ray diffraction with a Philips Xpert Pro

diffractometer equipped with a monochromator and using the Cu K radiation. Transmission

electron microscopy (TEM) pictures are recorded with a TOPCON EM002B transmission

electrons microscope operating at 200 kV. The samples for TEM observations of film top

views were prepared by scrapping the films. For TEM observations of the film cross sections,

slices were cut by ultramicrotomy in films deposited on a thermanox substrate. In both cases,

pieces of film were collected on a holey comb cabon TEM grid. The topography of the film

surface is obtained thanks to a Philipps FEI NovaNanoSEM scanning electron microscope

(SEM) operating at 15 kV or by using a 5500 atomic force microscope (AFM) from Agilent

technologies in taping mode. Absorption spectra are recorded with a UV/Visible Perkin Elmer

lambda900 spectrophotometer. The spectroscopic ellipsometric measurements were carried

out using a UVISEL Jobin Yvon ellipsometer. The incident angle is fixed at 60. Thewavelength range used is 300 nm-800 nm. The glass substrate back surface of each sample

characterized by ellipsometry is roughened to eliminate back reflection during ellipsometric

measurements. The film thickness is measured with a DekTak mechanical profilometer.

The porosity of the mesoporous thin films is deduced from the variation of their refractive

index versus the relative pressure of water vapour using a SOPRA ES4G ellipsometer. The

wavelength and the incidence angle are 632.8 nm and 70, respectively. Thermogravitometric

analysis (TGA) data and thermal differential analysis (TDA) data are recorded with a Pyrus

Perkin Elmer analyzer, in ambient air, with a heating rate of 10C/min from room temperature

to 450C. The infrared spectra are measured in transmission thanks to a FT-NIR Perkin Elmer

Spectrum One spectrophotometer. The wave number ranges from 1900 cm1

to 4000 cm1, it

is limited by the absorption of the glass substrate.

4. Results and discussions

4.1 Undoped films

TEM images of the untreated and undoped films clearly show that the film is made of

juxtaposed domains with different nanoorganizations and orientations (Fig. 1a). According to

previous works [16,30,31], copolymers with high ethylene oxide/propylene oxide (EO/PO)




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ratio (>1.5) such as F127 are known to form preferentially cubic micellar structures. Many

domains observed on our films have the same symmetry corresponding to the cubic Im 3m

phase oriented along the axis , or , and in each case, the lattice parameter

in a plane parallel to the surface plane is a// = 14.8 nm. This organization comes from the

phase separation between the inorganic silica matrix and the organic F127 tribloc copolymer

[11], which forms micelles that appear brighter than the silica walls on the SEM and TEM

pictures. The average micelles diameter is estimated to 7.4 nm 0.3nm from TEMcharacterizations.

a

200nm 200nm

b

100nm

ca

200nm

a

200nm 200nm

b

200nm

b

100nm

c

100nm

c

Fig. 1. (a) TEM pictures of an untreated mesostructured thin film and SEM pictures of an

untreated film surface (b) before and (c) after annealing at 400C.

The mesostructure is also clearly observed on the SEM pictures of the untreated film

surface (Fig. 1b). The micelles diameter and the lattice parameter of the mesostructure

estimated in the surface plane from Fig. 1b to within the SEM resolution are similar to that

determined from the TEM characterizations. After heat treatment at 400C these parameters

are kept constant in the surface plane (Fig. 1c), which is consistent with results previously

reported in the literature [12,32]. The latter also report a film shrinkage in the direction

perpendicular to the film surface. In order to characterize the lattice parameter in that normal

direction, XRD measurements were carried out on untreated and annealed films from 100C

to 400C (Fig. 2). The XRD patterns of all films show oscillations resulting from interferences

between the incident wave and the reflected wave on the substrate. The pseudo-period of these

oscillations can be used to roughly estimate the film thickness that is 160 nm 46 nm at

400C. The diffraction angle attributed to diffraction on the {110} reticular plane of a phase

mIm 3 is identified on all XRD patterns. It indicates that the periodic structure is conserved

after the thermal treatment. Kitazawa et al. [16] have shown that a thermal post-treatment can

induce structural modifications when the copolymer used has a low molecular weight, which,

according to Losurdo et al. [33] is likely to be due to the generation of thicker silica walls

during the phase separation in such a case. The XRD diffraction angles and the calculated

lattice parameters a for each temperature are summarized in Table 1. The lattice parameter a

of an untreated film is 13.4 nm. It differs by about 10% from a// measured in the surface plane.

This difference is caused by the anisotropic shrinkage of the mesostructure in the direction

normal to the substrate plane that occurs during the drying step and that transforms spherical

micelles into oblate ellipsoidal micelles [12,25].




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Fig. 2. XRD measurements of mesostructured thin films annealed at different temperatures.

According to the XRD patterns, the annealing at 100C has no significant effect on the

film mesostructure whereas the anisotropic shrinkage increases at higher temperature with a

lattice parameter a decreasing by about 30% after a heat treatment at 400C. This contraction

factor is similar to that observed by S. Besson et al. [12] and Kho et al. [13]. Thus, the thermal

post treatment induces an important shrinkage along the normal direction to the substrate thatis linked to the thermal condensation of the silica network [34].

Table 1. Lattice Parameter a in the Direction Perpendicular to the Film Surface

Calculated from the XRD Diffraction Angle Measured after Heating the Films at

Different Temperatures

Temperature (C) Angle () a (nm)20 0.93 13.4

100 0.9 13.8200 1.05 11.8300 1.25 9.9400 1.29 9.6

The film topography was measured by AFM on an untreated sample and samples annealed

at 100C, 200C, 300C or 400C (Fig. 3). A fine roughness of low amplitude that is likely to

correspond to the film mesostructure appears on the films surface from 200C. However, theradius of curvature of the tip apex being of the order of magnitude of the pores radius after

pyrolysis of the copolymer micelles, the AFM tip cannot give a faithful description of

mesopores even after the thermal treatment at 400C. Without heat treatment or after

annealing at 100C, the film surface is smoother at the nanometer scale and the high spatial

frequencies are not observed anymore. In these cases the pores are filled with the copolymer

and the topography signal does not distinguish the organic phase from the inorganic one.

These AFM characterizations seem to indicate that the copolymer micelles located at the film-

air interface degrade under heat treatment from 200C. Such degradation does not occur when

measuring the TDA-TGA spectra of the only F127 copolymer, in the form of powder (see

later), but may be due to a surface effect as the copolymer is in the form of micelles in the

silica film.




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Fig. 3. AFM pictures of the surface of (a) an untreated thin film, and of films annealed at (b)

100C, (c) 200C, (d) 300C and (e) 400C.

The film porosity was also estimated from ellipsoporosimetry measurements after heat

treatment at 400C. Ellipsoporosimetry combines ellipsometry and water vapour adsorption

and desorption and allows estimating the film porosity and the pore diameter from the

variations of the refractive index with relative pressure [35]. The hysteresis loop of the insert

in Fig. 4a, obtained after adsorption and desorption of water corresponds to a type IV

adsorption desorption isotherm and is characteristic of a porous sample. According to the

modelling described in [35] the porosity and the pore diameter of this film can be estimated

from the hysteresis loop to 31% and 3-4 nm, respectively (Fig. 4b). A similar porosity is

reported in the literature on a mesoporous thin film after calcination of the same copolymer

[16]. The pore diameter estimated from ellipsoporosimetry is smaller than that estimated from

SEM characterizations on the same sample (Fig. 1c). This difference results from the

compression of the mesostructure during calcination that flattens the pores and reduces their

average volume and to the fact that our modelling assumes the presence of spherical pores

[12].




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Fig. 4. (a) Ellipsoporosimetric measurements on a film annealed at 400C and (b) pores radiusdistribution.

The changes in the film chemical composition with increasing temperature are evaluated

by infrared spectroscopy in the light of the thermal analysis (DTA-TGA) of pure F127

triblock copolymer (Fig. 5a). DTA and TGA curves recorded under ambient atmosphere

exhibit a large exothermic peak centred at 390C and a significant weight loss occurring from

350C. Therefore, the copolymer template can be completely removed by calcination at

350C or more [36]. The comparison of the infrared transmission spectra (Fig. 5b) of

untreated films and annealed films at 400C shows firstly the full disappearance of two

absorption bands located at 2875 cm1

and 2913 cm1

and attributed to the stretching

vibrations of the alkyl groups, and secondly the decrease of the absorption band centred at

about 3350 cm1 and due to the stretching vibration of the alcohol or silanol groups. The lastband is also shifted toward 3600 cm

1after heat treatment. This shift can be attributed to the

modification of the OH vibrations frequency during the condensation of the silica walls. The

persistence of the band indicates that silanol groups remain in the film and that the

condensation of the silica network is incomplete. In addition, the disappearance of the two

bands located around 2900 cm1

results from the evaporation of residual solvent, the

condensation of the silica network and the pyrolysis of F127 that is supposed to be complete

according to DTA-TGA results. IR spectra show that no change occur when annealing the

film for 1h instead of 30 min at 400C. They confirm that the chemical composition of the

film is stabilized after a 30 min-long heat treatment at 400C.




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Fig. 5. (a) DTA/TGA of pure F127 tribloc copolymer and (b) IR transmission spectra of thefilm as deposited and after annealing at 400C for 30 min and 1h.

The changes in the chemical composition during the thermal treatment significantly affect

the opto-geometrical properties of the film. Spectroscopic ellipsometry [37] was used to

measure the film thickness and refractive index after different heat treatments. The results

(Fig. 6) show that the mesostructured film behaves as an isotropic homogeneous layer after

annealing up to 300C and as an anisotropic homogeneous layer at higher temperatures. For

the isotropic films the refractive index is estimated by fitting the results with a Lorentz

dispersion law [38]:

( ) ( )( )1/2

2 2 21 1 / ,s t tn = + (1)

where s is the permittivity of the film at high wavelength, and and t are the frequency of

light and the resonance frequency, respectively. In the case of anisotropic films, the

birefringence is assumed to be uniaxial with an optical axis perpendicular to the film surface.

The ordinary no and extraordinary ne refractive indexes are both modelled with Eq. (1).

Whatever the calcination temperature the films are transparent in the visible range and the

extinction coefficient is supposed to be null. The estimation of the refractive index relies on

the minimization of a mean square error (MSE) function giving the distance between the

experimental and the simulated ellipsometric parameters. The use of a Levenberg Marquardt

algorithm allows optimizing three parameters simultaneously, s, t, and the film thickness d

[39]. The optimized results shown in Fig. 6 correspond to MSE smaller than 0.7 for the

isotropic films and smaller than 0.2 for the anisotropic ones. This low MSE values confirm

that considering our films, made of self-assembled copolymer micelles in a silica network, ashomogeneous films is coherent. Such films do not scatter light, the micelles are much smaller

than the visible wavelengths and the dielectric contrast inside the film is not high.




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1.25

1.3

1.35

1.4

1.45

1.5

1.55

1.6

1.65

300 400 500 600 700 800

Wavelength (nm)

Refractiveindexo

fthematri

untreated 100C

200C 250C

300C 350C Ordinary

350C Extraordinary 400C Ordinary

400C Extraordinary 200C with silver

Fig. 6. Refractive index dispersion at various temperatures during calcination of F127.

The estimated refractive index of the film without annealing or after annealing up to

100C (Fig. 6), 1.480 at 500 nm wavelength, is very close to that of silica [40]. This refractiveindex abruptly decreases with the annealing temperature between 100C and 200C where it

falls to 1.399 at 500nm, then it slightly decreases down to 1.382 for a temperature reaching

300C and falls again abruptly between 300C and 350C. The lower refractive index values

are obtained at 400C where n0 = 1.285 and ne = 1.270, corresponding to a negative

birefringence. At the same time, during the temperature increase the film thickness

progressively decreases by 37%, as also observed by profilometric measurements (Table 2).

This shrinkage is in agreement with the contraction of 30% of the film structure previously

observed by XRD.

Table 2. Film Thickness Estimated from Ellipsometric and Profilometric Measurements

after Heating the Films at Different Temperatures

Temperature (C)

Thickness estimated by

ellipsometry (nm)

Thickness estimated by

profilometry (nm)20 250 243

100 234 226

200 205 198

250 186 191

300 182 177

350 163 168

400 156 151

The first fall in refractive index between 100C and 200C is most probably due to the

evaporation of residual solvent that creates microporosity in the film. It must also be related to

the fact that nanoscale porosity appears on the surface topography (Fig. 3c) from 200C. It

could then also correspond to the formation of mesopores near the top surface of the film. The

second fall observed between 300C and 350C is attributed to the pyrolysis of the copolymer(Fig. 5a). The latter leads to the formation of a mesoporous silica film whose refractive index

depends on the air volume ratio. The observed birefringence from such temperature was also

reported by Peiris et al. [41] who concluded that higher the surfactant concentration in the sol

greater the birefringence.




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4.2 Silver containing films

The mesostructured silica films doped with silver nitrate underwent the same heat treatment

than the undoped films, which led to the formation of silver nanoparticles on top of the

copolymer degradation. The initially colourless films take a yellowish color characteristic of

the surface plasmon resonance of silver nanoparticles [25] from 120C. This temperature is

the same as that reported for the thermal growth of silver nanoparticles in a polystyrene film

[42]. The SPR band observed on the absorbance spectra of Fig. 7 has growing amplitude withincreasing temperature up to 200C, which results from the rise of the nanoparticles

concentration and size. Then, from 200C to 400C the amplitude of the SPR band decreases

so far as to disappear. The nanoparticles are indeed likely to be reoxidized at such high

temperatures as already reported in silica films [43]. It can be noted here that the absorbance

level, given by log (1-A-R), A being the absorption coefficient and R the reflection

coefficient of the film, after calcination at 400C is smaller than that of the untreated film.

After calcinations at 400C, the film is mesoporous and has a low effective index. It behaves

like an antireflection coating, with lower R and then lower absorbance than the initial film.

Such a film is transparent and colourless and cannot be coloured again by annealing at 200C

or at any temperature below 550C.

300 400 500 600 700 800

0.0

0.2

0.4

Absorbance(a.u.)

Wavelength (nm)

Untreated

120C

200C

250C

300C

400C

Fig. 7. Absorbance spectra of mesostructured silica films doped with silver salt after thermal

reduction at temperatures ranging from room temperature to 400C.

The formation of silver nanoparticles between 120C and 200C as well as their

disappearance at 400C are confirmed by TEM characterizations of the films (Fig. 8). High-

resolution TEM images (insert in Fig. 8a) show that the nanoparticles are crystallised in the

typical fcc symmetry of bulk silver. TEM pictures of the film cross section after annealing at

160C (Fig. 8a) show that the silver-nanoparticle distribution is not homogeneous in the film

depth. Even if silver nanoparticles grow within the whole film, they mostly concentrate at the

film-substrate interface. Near this interface, their size exceeds the copolymer micelle size with

a mean diameter of 11 nm with a standard deviation of 7 nm. Therefore, we cannot conclude

as we did previously when using a chemical reduction [22], or an optical reduction [23]

process that the silica walls of the mesostructured films act as molds for the nanoparticles

growth.




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Fig. 8. TEM pictures of mesostructured silica films doped with silver salt and annealed (a) at160C or (b) at 400C during 30min.

Silver diffusion towards the sample substrate was better evidenced by Rutherford

Backscattering Spectrometry (RBS) measurements. The experimental conditions are presented

in a previous paper [22]. Before heat treatment, silver (in the form of silver nitrate) is

distributed in the whole film thickness with however higher concentration near the film-air

interface. After annealing at 200C, the concentration gradient is completely reversed with a

higher concentration near the film-substrate interface consistent with the TEM observations

showing silver nanoparticles concentrated near the substrate. RBS data processing (Fig. 9b)

also evidence the diffusion of a part of silver inside the substrate. After annealing at 400C,

silver has completely disappeared from the film to diffuse in the substrate where it is diluted.Such observations were also reported on silver containing silica films annealed at similar

temperatures and were attributed to an interdiffusion between Ag in the film and Na in the

glass substrate [43].

Fig. 9. (a) RBS spectra and (b) calculated distributions of silver in the film depth for differentannealing temperatures. The films are deposited on glass substrates.

Removing Na from the substrate by using quartz instead of glass substrates, we indeed

thickness (Fig. 10) and that only a low decrease of the SPR band occurs after annealing at

400C (not shown). Therefore, interdiffusion between Ag and Na is likely to occur in our film

also. The latter may explain the stability of the colourless film (on glass) annealed at 400C

since sodium likely increases the stability of silver oxide at high temperature [43].




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Fig. 10. (a) RBS spectra and (b) calculated distributions of silver in the film depth for differentannealing temperatures. The films are deposited on silica substrates.

Ellipsometric measurements were performed on a sample annealed at 200C. Because of

the presence of silver nanoparticles, the film effective index was modelled using the Maxwell

Garnett theory [44]. In this effective medium approximation the film refractive index is

calculated from the refractive indexes of silver nanoparticles and of the hybrid silica matrix as

a function of filling factor q. As previously, Eq. (1) is used to describe the refractive index of

the hybrid matrix including silica and copolymer and Eq. (2), below, describes the

nanoparticle refractive index according to the Drude model [45]:

( )2 / ,D p i = (2)

where

represents the contribution of the interband transitions in the visible range,

is thefree-electrons relaxation rate, p the plasmon frequency and the light frequency. Moreover,

in the light of TEM (Fig. 8a) and RBS (Fig. 9) characterizations, the film is assumed to be

made of two sub-layers of unknown thickness and filling factor. Therefore the minimization

of the MSE function relies here on the optimization of 9 parameters: two thicknesses and

filling factors, s and t for the dielectric matrix and the three parameters of the Drude model

, and p. The refractive indexes obtained after fitting the ellipsometric data are shown in

Fig. 6 and Fig. 11 and correspond to a MSE of 0.04. The optimized fit leads to a 88 nm-thick

top layer composed of 0.2% of silver and a 8 nm-thick bottom layer composed of 11.5% of

silver. The total film thickness deduced from ellipsometry measurements is in accordance

with that estimated from profilometric measurements on the same sample (90 14 nm). Since

TEM characterizations on the cross-section need to deposit the films on polymer substrates

the film thickness changes in that case and cannot be compared with the other results. The

bottom layer thickness corresponds to the height of the nanoparticles located at the film-

substrate interface according to TEM characterizations (Fig. 8a).

The refractive index of the host matrix reported on Fig. 6 is slightly higher than the

refractive index of an undoped film treated at the same temperature. This little difference can

be due to the presence of silver nitrate in the sol, which modified the gelification conditions of

the sol by increasing its viscosity [43] and consequently its refractive index. Moreover, the




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growth of silver nanoparticles in the film can strained the silica walls and consequently can

slightly increase their refractive index.

The complex effective indexes of both layers are plotted on the Fig. 11. Due to the low

concentration of silver nanoparticles in the top layer, the real part of its effective refractive

index is relatively similar to the refractive index of the host matrix excepted near the

resonance. The real part of the effective refractive index of the bottom layer has a sigmoidal

shape that varies by 0.5 between near UV and red leading to a strong dispersion of light. The

SPR band of silver nanoparticles (imaginary part of the effective refractive index) strongly

differs in amplitude between the two sub-layers due to the large difference in silver content

and is centred at 448 nm and 465 nm in the top and bottom layer, respectively. As the matrix

refractive index is identical for both sub-layers, the redshift of the SPR band of the bottom

layer is attributed to higher silver-nanoparticles mean diameter in keeping with TEM

observations (Fig. 8a).

Fig. 11. Dispersion laws of the complex effective index of (a) the top layer and (b) the bottomlayer of the silver containing films after annealing at 200C.

Finally, the recovering of the film effective refractive index from ellipsometric

measurements when the film is nanostructured and leads to involve many parameters in the

modelling is not a priori easy and sure. But, the combination of characterization techniques

allowed us to establish an appropriate modelling that gives coherent results. The latter may be

useful for developing optical devices based on silver nanoparticles

5. Conclusion

The optical properties of mesostructured silica thin films were directly correlated to their

composition and structural properties. Under increasing temperature, the film refractive index

decreases from 1.47 to 1.27 and becomes anisotropic. Used as a host matrix for the silver

nanoparticles growth the film exhibits two different complex refractive index values




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depending on the depth. Silver is indeed largely concentrated in the film depth as confirmed

by RBS measurements.

Acknowledgments

This work is supported by the region Rhne Alpes and ANR in the framework of JCJC 10

1002 project (UPCOLOR). We thank S. Reynaud and F. Vocanson from Hubert Curien

Laboratory (LHC), Saint-Etienne France, for the AFM characterizations and the IR spectra,

respectively. We also thank Aziz Boukenter from LHC for fruitful discussions.



2011_Optical Properties of Silver Nanoparticles

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