A R T I C L E

Journ

al of

Materials

Ch

emistry

ww

w.rsc.o

rg/m

aterials

Controlling the crystallinity and nonlinear optical properties of

transparent TiO2–PMMA nanohybrids

Akhmad Herman Yuwono,a Binghai Liu,a Junmin Xue,a John Wang,*a

Hendry Izaac Elim,b Wei Ji,b Ying Lic and Timothy John Whitec

aDepartment of Materials Science, Faculty of Science, National University of Singapore,

Singapore 119260. E-mail: maswangj@nus.edu.sgbDepartment of Physics, Faculty of Science, National University of Singapore, Singapore

117542cInstitute of Environmental Science and Engineering, Nanyang Technological University,

Singapore 637723

Received 8th March 2004, Accepted 19th July 2004

First published as an Advance Article on the web 20th August 2004

Titania–polymer nanohybrid thin films represent a new class of potential materials for optoelectronic

applications. While most such nanohybrid thin films lack control in crystallinity, we report in this paper

transparent nanohybrids of titania-polymethyl methacrylate (TiO2–PMMA) thin films having a remarkably

enhanced nanocrystallinity. Post-treatments with water vapor at relatively low temperatures were applied on

these thin films, following in situ sol–gel polymerization. They promoted rearrangement of flexible Ti–O–Ti

bonds leading to enhanced crystallization of the TiO2 phase. The degree of TiO2 crystallinity in the resulting

nanohybrid films was studied by using XRD, FTIR, UV–Vis spectroscopies and HRTEM. Both linear and

nonlinear optical responses increase with the enhancement of TiO2 crystallinity in the nanohybrids. The highest

two-photon absorption coefficient (b) and nonlinear refractive index (n2) were observed for the nanohybrid thin

films with highest TiO2 crystallinity, as confirmed by open and closed aperture Z-scan techniques using 250 fs

laser pulses at 800 nm, having a value of 2260 cm GW21 and 6.2 6 1022 cm2 GW21, respectively.

Introduction

Over the past decade, there has been a rapid surge in theresearch interest in organic–inorganic nanohybrid materials.One of the main reasons for this development is the widevariety of controllable optical, mechanical and electricalproperties that can be obtained by tailoring the organic andinorganic moieities into nanosized domains. Accordingly, newoptoelectronic materials based on these nanohybrids havebeen extensively reported, such as those for optical coatings,1

optical switches,2 high refractive index devices,3 contactlenses,4 optical waveguides5 and nonlinear optical devices.6

Among the various processing techniques under developmentfor these nanohybrids, the in situ sol–gel process is well knownto be versatile as it enables control of the organic–inorganicinteraction at various molecular, nanometer and micrometerscales.7,8 For example, a metal alkoxide can be mixed withorganic monomers to be polymerized, while it undergoessimultaneous hydrolysis and condensation, resulting in ahomogeneous polymer–metal oxide nanohybrid material.

Poly(methyl methacrylate) or PMMA has been recognized asan excellent optical polymer for use in optical fibers, opticaldisks and lenses.9 However, the refractive index (n0) of PMMAis limited to 1.49. On the other hand, titanium oxide (TiO2,titania) is well-known as an inorganic material having a highrefractive index, i.e. y2.45 (anatase) and y2.70 (rutile).Therefore, an incorporation of titania into PMMA at a nano-meter scale has been considered for the preparation of highrefractive index polymers. Accordingly, titania–poly(methylmethacrylate) (TiO2–PMMA) nanohybrids have been studiedpreviously by several researchers. Zhang et al.10 synthesizedTiO2–PMMA nanohybrids using a chelating ligand as acoupling agent. However, the hybrid thin films were notdeeply investigated for their properties. A further study on thepreparation and optical properties of TiO2–PMMA thin films

was performed by Chen et al.,11,12 who synthesized thin filmTiO2–PMMA nanocomposites by an in situ sol–gel processusing trialkoxysilane-capped TiO2–PMMA combined withspin coating and multi-step annealing process. The refractiveindices of the films thus prepared were reported to be in therange 1.505–1.867, provided the loading of titanium alkoxide inthe precursor solution was at least 90 wt%. However, accordingto the results of their field emission scanning electron micro-scopy (FE-SEM) and infrared spectra (IR), the hybrid thinfilms were still considered amorphous, indicating an incompletecondensation reaction of the hydrolyzed product of thealkoxide precursor (Ti–OH group). They attributed this tothe insufficiently high curing temperature (150 uC), as a tem-perature of higher than 400 uC is required for formation ofcrystalline TiO2. Increasing the curing temperature of TiO2–PMMA nanohybrid thin films, however, is limited by thethermal stability of the acrylic moiety. Hence, in their sub-sequent studies,13 they replaced this polymer with pyromelliticdianhydride (PMDA), which is a highly thermally stable poly-mer, in order to increase the curing temperature up to 300 uC.However from the FE-SEM study, there were still no signifi-cant inorganic domains shown in the hybrid films thusprepared. Therefore the incomplete condensation of Ti–OHgroups can not be merely because of the insufficient curingtemperature.

Owing to its potential applications in several technologicallydemanding areas, concern on the formation and densificationof TiO2 thin films at low temperatures has led to investigationsby many researchers.14–26 According to the model proposed byBrinker et al.19 and Langlet et al.,21 the largely amorphousnature of TiO2 films could be due to the high functionality oftitanium alkoxide favoring the fast development of a stiffTi–O–Ti network, which in turn hinders the condensation anddensification during drying. For the case of nanohybrids, it isunderstood that the condensation and densification becomeD

OI:

10

.10

39

/b4

03

53

0e

2 9 7 8 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7 T h i s j o u r n a l i s � T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online / Journal Homepage / Table of Contents for this issue

even more difficult due to an additional hindrance effect fromthe polymerization of organic monomers taking place at thesame time. In accordance with this understanding, a previousstudy by Matsuda et al.14,15 suggested that structural changesof sol–gel films can be induced by treatment in a high humidityenvironment at temperatures above 100 uC. Further investiga-tion by Imai et al.16,17 confirmed that exposure of sol–gelderived TiO2 films to water vapor induced rearrangement of theTi–O–Ti network, leading to formation of an anatase phase atrelatively low temperature (180 uC). It is thus of interest in thepresent study to investigate whether an appropriate watervapor treatment can be applied and promote crystallization ofTiO2 in TiO2–PMMA nanohybrid thin films. Our previousstudies show that the TiO2–PMMA hybrid is promising as anonlinear optical material, showing a unique optical behaviorwhere the two-photon absorption coefficient (b) and thenonlinear refractive index (n2) are dependent on titanialoading.27,28 The observed optical nonlinearity had a veryfast recovery time of y1.5 ps, suggesting a strong potential ofthis material for optical switching devices. Therefore in thisstudy, the effects of enhanced TiO2 crystallinity of nano-particles in PMMA thin films on the optical properties are alsoinvestigated.

Experimental

Materials

The starting materials in this work were titanium isopropoxide(Ti–iP, 98%, Acros), ethyl alcohol (Et–OH, 95%, Merck),ethylene glycol (EG, 99.5%, Riedel de Haen), ethylene glycolmonomethyl ether (EGME, 99.5%, Merck), de-ionized water,hydrochloric acid (HCl, 36%, Ajax), methyl methacrylate(MMA, 99%, Acros), 3-(trimethoxysilyl)propyl methacrylate(MSMA, 98%, Acros), tetrahydrofuran (THF, 99%, Acros)and benzoyl peroxide (BPO, 98%, Acros).

Preparation of nanohybrid TiO2–PMMA

The synthesis route for the nanohybrid TiO2–PMMA ismodified from the technique reported in our previouspapers.27,28 In addition to ethyl alcohol (Et–OH), ethyleneglycol monomethyl ether (EGME) and ethylene glycol (EG)were used to dissolve the inorganic precursor. This is aimed atavoiding the fast evaporation of the solvents during the spincoating process, as often encountered in our previous prepara-tions with ethyl alcohol for certain nanohybrid compositions(i.e. 60 wt% of titanium alkoxide and 40 wt% of MMA 1

MSMA). Due to the evaporation problem, there was atendency for the separation of the constituents involved,which resulted in rough and opaque thin films.

In detail, titanium isopropoxide (Ti–iP) was first mixed withan appropriate amount of Et–OH and EGME and stirred for30 minutes. A mixture of de-ionized water containing HCl andEGME was added to the transparent solution with stirring topromote hydrolysis. EG was then added slowly to the solutionand stirred for 1 h. A stable solution was then obtained bycontrolling the Ti–iP concentration at 0.4 M, with a water-to-alkoxide ratio (rw) of 2 and a pH value of 1.35. On the otherhand, the organic precursor was prepared following theprocedure reported in the literature,12 whereby the monomers,MMA and MSMA, and the initiator BPO in THF, were addedinto a reaction flask and polymerized at 60 uC for 1 h. Finally,the homogeneous mixture of inorganic precursors was addeddropwise over 30 min into the partially polymerized monomerswith rigorous stirring to avoid local inhomogeneities. Thereaction was allowed to proceed at 60 uC for another 2 h. In thisstudy, the weight ratio between the inorganic and organicprecursors in the reaction mixture was fixed at 60 : 40. Theresultant transparent nanohybrid thin films are termed T60.

Thin film formation was realized by spin coating the solutionon quartz substrates and silicon wafers at 3000 rpm for 20 s.For comparison purposes, coated thin films were divided intotwo groups, i.e. those treated with the conventional annealingprocess and those treated with water vapor. In the first group,the thin film was dried at 60 uC for 5 min (sample T60-A) andanother thin film, following drying at 60 uC, was furtherannealed in a dry atmosphere at 150 uC for 24 h (sample T60-B). In the second group, after drying at 60 uC, the samples wereexposed to water vapor using a teflon-lined stainless steelautoclave. A specially designed stand was placed inside theautoclave in order to prevent samples from direct contact withliquid water. The autoclave containing the thin film samplesand about 25 ml of purified water was kept at 110 uC (sampleT60-C) and at 150 uC (sample T60-D) for 24 h, respectively.Another sample was also prepared by following a ratherdifferent route, i.e. drying at 60 uC for 5 min and annealing at110 uC for 15 min, prior to the water vapor treatment at 150 uCfor 24 h (sample T60-E).

Characterization

Infrared spectra of the nanohybrid thin films were recorded atroom temperature in the range 4000–400 cm21 using a Bio-Radmodel QS-300 spectrometer, which has a resolution of¡8 cm21. The samples for FTIR were prepared by spincoating the precursor solution onto a KBr pellet, followed byannealing and water vapor treatment procedures as describedabove. Thermal analysis was performed by thermogravimetricanalysis (TGA Dupont model 2950) under a nitrogen flow at aheating rate of 20 uC min21. The sample was prepared bypouring the precursor solution into a petri-dish to form a thickfilm. The thick film was dried at room temperature for oneweek and then at 60 uC for 3 days. Finally it was pulled outfrom the dish with a razor blade and used for TGA analysis.X-ray diffraction (XRD) measurements on the thin filmscoated on silicon wafers were performed on Bruker AXS h–2hdiffractometer using Cu Ka radiation (1.5406 A) operated at40 kV, 40 mA with a step size of 0.02u and a time step of 20 s.The incidence angle between the beam and film plane was fixedat 1.5u. The microstructure of the nanohybrid thin films wasexamined by using a high resolution transmission electronmicroscope (HRTEM, JEOL–3010), while the surface morpho-logy of the coated films was also probed using an atomic forcemicroscope (Nanoscope Inc., model DI 5000 AFM) with atapping mode. The linear absorption spectra of nanohybridfilms on quartz substrates were measured using a UV–Visspectrophotometer (UV-1601, Shimadzu, with a resolution of¡0.3 nm) in the wavelength range 800–200 nm. The thicknessand linear refractive index, n0 (at l ~ 632.8 nm) were deter-mined simultaneously using Filmetrics F20 Film MeasurementSystem. The optical nonlinearity of the nanohybrid thin filmswas studied by the Z-scan technique, by which each sample wasmounted on a translation stage that moved the sample alongthe Z-axis with respect to the focus of the lens. This techniquewas developed originally by Sheik-Bahae et al.29 and is con-sidered to be a sensitive and reliable method to determine thesign and magnitude of b and n2.

Results and discussion

According to our preliminary observation using an opticalmicroscope, the spin-coated thin films prepared using EGMEand EG as additional solvents for the inorganic precursor weresmooth and transparent. This suggested that the evaporationrate of these solvents during spin coating is much slower thanthat of ethyl alcohol on its own, resulting in a good integrationof the hybrid mixture of 60 wt% of TiO2 and 40 wt% ofMMA 1 MSMA. Based on further investigation using SEM,no cracks were observed in the samples after drying, annealing

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7 2 9 7 9

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

and/or water vapor treatment. This will be discussed further inconnection with the results of AFM studies.

Fig. 1 shows the FTIR spectra for the nanohybrid thinfilms prepared via different treatment routes over the range4000–400 cm21. A broad absorption centered in the range3400–3500 cm21 was observed for all thin films and is assignedto the hydroxyl groups of Ti–OH.12 The stretching vibrationbands of CLO and C–H bonds in the PMMA segments areshown at 1737 and 2950 cm21, respectively.12 The intensities ofthese two bands are found to be stronger on the post-treatedthin films (T60-B, C, D and E) than the as-dried one (T60-A).On the other hand, the intensity of the CLC band at 1650 cm21

decreases. These results indicate that polymerization of MMAmonomer had taken place during the heating process at tem-peratures higher than 110 uC.30 The band in the range 1003–1258 cm21 is assigned to the stretching vibration of the C–O–Cbond, which was observed for all thin films. However, the Si–Cbond at 1277 cm21 appeared more obviously for the post-treated thin films, which originated from the coupling agentMSMA between the organic and inorganic moieties.

A strong and broad absorption band ranging from 900 to400 cm21 clearly shows for all post-treated thin films, exceptthe sample annealed in a dry atmosphere at 150 uC (T60-B).This peak, centered at y650 cm21, is accounted for by vibra-tions of Ti–O–Ti groups and is regarded as the characteristicpeak for TiO2.31 It can be seen that a less intense peak of thisband also exists for samples T60-A and T60-B at 650 cm21. Itsintensity is more pronounced with thin film T60-B than forT60-A, suggesting the onset of formation of TiO2 after dry-annealing at 150 uC. An additional band at y453 cm21 is alsopresent for both samples and can be assigned to the Ti–Ostretching vibration.23

There is a possibility that several peaks corresponding toPMMA such as CLO and C–H bonds at 1737 and 2950 cm21

overlap with those for organic solvents, i.e. EG and EGME.However, the previous study by Matsuda et al.14,15 confirmedthat such organic solvents effectively leached out from theTiO2–SiO2 films immersed in the hot water, at a relativelylow temperature and after a short time (i.e. 97 uC; 1 min).Therefore, the water vapor treatment in this study effectively

removed these solvents, and therefore, the remaining organicsubstances belong to the PMMA matrix only. This is furtherconfirmed by the results from TGA and AFM studies.

It is of interest to compare the water vapor treated samples(T60-C, D and E). Firstly, it was observed that sample T60-Eexhibited a relatively lower intensity of the Ti–OH band in therange 3400–3500 cm21 and a higher intensity of the Ti–O–Tiband at y650 cm21, as compared to T60-C and T60-D, whichwill be further discussed in connection with the XRD resultsbelow. Secondly, the intensities of C–H, CLO and C–O–C at2950, 1737 and 1150 cm21 for T60-C and T60-D decrease asthe temperature of water vapor treatment increases, i.e. from110 to 150 uC. This is due to the leaching out of some organicsegments in PMMA by water molecules, together with removalof EG and EGME. However, it was not the case for T60-E,where the intensities of those peaks were maintained and werecomparable to those of T60-B, which was conventionallyannealed without the involvement of any water vapor.

Fig. 2 shows XRD patterns of the nanohybrid thin filmsprepared by the conventional annealing and water vaportreatment. It is clear that the conventionally annealed sampleT60-B exhibits a weak peak at 2h ~ 55.08u corresponding tothe (211) crystal plane of the anatase TiO2 phase. The lowintensity of this peak indicates that the film was still largelyamorphous, as expected. By contrast, the water vapor treatedsample T60-C demonstrated enhanced crystallization, which isrepresented by a slightly higher peak intensity of the samecrystal plane, and the appearance of additional diffractionpeaks at 2h ~ 25.35u and 48.12u, corresponding to (101) and(200) crystal planes. A further increase in the intensity of thesepeaks is demonstrated by T60-D. A much more enhanced TiO2

crystallization is obviously shown by T60-E, where these peaksbecome much more intense and are accompanied by an addi-tional peak at 2h ~ 38.63u, corresponding to the (112) crystalplane.

The FTIR and XRD results presented above stronglysuggest that the crystallinity of TiO2 in TiO2–PMMA nano-hybrids has been significantly promoted by water vapor treat-ment at relatively low temperatures. Based on the Imaimodel,16,17 this is due to the cleavage of strained Ti–O–Tibonds in nanohybrids by water molecules. This results in theformation of flexible Ti–OH and thus more stable Ti–O–Ti

Fig. 1 FTIR spectra of TiO2–PMMA nanohybrid thin films: T60-A,T60-B, T60-C, T60-D and T60-E.

Fig. 2 XRD patterns of TiO2–PMMA nanohybrid thin films: T60-B,T60-C, T60-D and T60-E.

2 9 8 0 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

bonds, which in turn rearrange and crystallize into TiO2

nanoparticles. Comparing T60-D and T60-C, it is understoodthat the water vapor treatment at 150 uC led to a higher degreeof crystallinity than that at 110 uC. This is due to the highervapor pressure involved, inducing a greater number of bondcleavages in the former. This is supported by the XRD patternsof these two samples (Fig. 2), where the diffraction peak at2h ~ 25.35u for T60-D, for example, is somewhat sharper thanthat of T60-C. This is also in agreement with the observation byWang et al.32 on the hydrothermal processing of nanocrystal-line titania, where the particle size grew from 6 to 10 and finallyto 28 nm when the processing temperature was varied from 80to 180 to 240 uC.

It is of interest to note that an additional annealing stepat 110 uC, prior to water vapor treatment at 150 uC (sampleT60-E), can further enhance TiO2 crystallinity, as comparedto those directly exposed to water vapor at 110 and 150 uC(samples T60-C and D). From the FTIR analysis, it is shownthat the intensity of the OH band of T60-E is lower than theothers, while the intensity of Ti–O–Ti band is higher. Itsuggests that more Ti–OH groups had been converted to formflexible Ti–O–Ti and thus TiO2 crystals. A possible explanationfor this is related to the formation of TiO2 nuclei by the initialannealing process at 110 uC. As observed with sample T60-B inFig. 1, the occurrence of these nuclei is represented by thestretching vibration of Ti–O bonds at 453 cm21 and a small butsharp peak of Ti–O–Ti at 650 cm21. Subsequent water vaportreatment at 150 uC thus involved these nuclei when the flexibleTi–O–Ti bonds were rearranged, leading to formation of TiO2

nanocrystallites. Such involvement of nuclei was not possible inthe case of the films with direct water vapor treatment at 110 uC(T60-C) or at even higher temperatures, i.e. 150 uC (T60-D).

Fig. 3 illustrates the TGA curve of the nanohybrid sample ata heating rate of 20 uC min21 in a nitrogen flow. The weightloss occurs in four stages, namely below 150 uC, between 150and 335 uC, between 335 and 425 uC, and from 425 to 700 uC.Below 150 uC, the weight loss is due to evaporation of waterand thermal decomposition of the remnants of organicsolvents. Between 150 and 335 uC, it is attributed to theelimination and oxidation of organic compounds, i.e. loss ofcarbon, hydrogen and oxygen in EGME, EG and PMMA.Between 335 and 425 uC, it is ascribed to further combustion ofthe remaining organic residues. This is followed by a slowdownin the weight loss rate from 425 to 700 uC, beyond which therewas no appreciable weight loss.

From the TGA results shown above, it can be seen that the

decomposition reactions in the nanohybrids took place ataround 275 uC, which is due to destruction of the PMMAsegments because of the low bond dissociation energy of theC–C bond in the main chains. This is very close to the TGAresult obtained by Chen et al.,12 whose nanohybrid contained asimilar portion of titanium alkoxide precursor (60 wt%), andhad a decomposition temperature of 281 uC. For furtherconfirmation, an experiment was performed on pure PMMA,for which a decomposition temperature of 270 uC was observedin our study. This supports what is shown by the FTIR results;that PMMA can withstand the water vapor treatment up to150 uC, which is lower than its decomposition temperature, andwas able to maintain its integrity with the inorganic moiety.On the other hand, the water vapor treatment up to 150 uCsuccessfully removed other organic substances of EGME andEG, which were used as solvents. Indeed, there were possiblysome mild segments in PMMA that were leached out, togetherwith both solvents under the hydrothermal conditions.

Figs. 4(a–d) are the bright-field HRTEM micrographs ofnanohybrid thin film samples showing the presence of TiO2

nanoparticles in a largely amorphous PMMA matrix. Thecorresponding size distribution of TiO2 crystallites in eachsample is given in the inset. It is seen from these figures that theaverage crystallite sizes of TiO2 nanoparticles in these samplesare rather different. The average crystallite size and standarddeviation are 3.98 ¡ 0.97, 4.95 ¡ 1.66, 7.65 ¡ 1.55 and 9.09 ¡1.51 nm for T60-B, T60-C, T60-D and T60-E, respectively.By contrast to the narrow crystallite size distribution for theconventionally annealed sample T60-B, the standard devia-tions of the nanohybrids treated with water vapor aresomewhat larger, indicating an apparent crystallite growth.

There are other characteristic differences among thesenanohybrid samples. Firstly, in terms of particle morphology,TiO2 in T60-B (Fig. 4(a)) occurs as nearly spherical nano-particles, which are rather uniformly dispersed in the PMMAmatrix, although they apparently lack crystallinity. By con-trast, T60-C (Fig. 4(b)) shows a tendency for crystalliteaggregation, together with a clear enhancement in crystallinity.This explains the observation that it exhibits a relatively wideparticle size distribution. A similar behavior is seen for T60-Dand T60-E (Figs. 4(c and d)), where the average crystallite sizeis much enlarged. Secondly, TiO2 nanoparticles in T60-Bdemonstrate little clear lattice fringe. On the other hand, T60-Cshows a clear lattice fringe in some of the TiO2 nanoparticles.The lattice fringe becomes more obvious for T60-D and T60-E,indicating the further established crystallinity of TiO2 particles.The d-spacing of lattice fringe (Fig. 4(d)) is 0.352 nm, which isin good agreement with the d-value of the (101) crystal plane inanatase titania. It is also seen that there is an increase in theaverage particle size of TiO2 in PMMA. This is consistent withwhat is shown by the XRD traces in Fig. 2, where the peakintensity and width of (101), (112) and (200) vary dramaticallyupon water vapor treatment. For example, the full width at halfmaximum (FWHM) of (101) for T60-C, T60-D and T60-E was0.0316, 0.0195 and 0.0170 rad, respectively. The line broad-ening is expected to be due to the nanocrystalline nature ofTiO2 particles and it consistently decreases with the growthof crystallite size. Therefore by applying Scherrer’s equationto these values, the average crystallite sizes of TiO2 werecalculated to be 4.85, 7.87 and 8.99 nm for T60-C, T60-D andT60-E, respectively. The calculated value for T60-D is slightlylarger than that observed using HRTEM. It still falls into therespective standard deviation range. For T60-B, the averagecrystallite size was not calculated due to its amorphous nature.Although it exhibits a weak peak at 2h ~ 55.08u correspondingto the (211) crystal plane, calculation using Scherrer’s equationwas not possible due to the much broadened peak.

Figs. 5(a–h) show the AFM images for the four nanohybridthin films. The images obtained at large scale (8–10 mm) (asshown in Figs. 5(a, c, e and g)) confirmed what is observed by

Fig. 3 TGA curve of T60 gel sample at a heating rate of 20 uC min21

in a nitrogen flow.

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7 2 9 8 1

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

SEM, that no severe cracks were formed with thin films.Based on further studies on selected areas (1 mm) as shown inFigs. 5(b, d, f and h), it is observed that they all demonstratenear-spherical particle morphology with average sizes in therange of 40 to 80 nm corresponding to the typical morphologyof PMMA aggregates. It is also observed that some porousregions had been formed on these thin films as a consequence ofthe removal of organic solvents and organic residues during theannealing and water vapor treatment. The average roughness(Ra) for T60-B, T60-C, T60-D and T60-E is 1.73, 2.22, 2.64 and2.13 nm, while the mean square roughness (Rq) is 2.16, 2.82,3.31 and 2.70 nm, respectively. It can be seen from these AFMstudies that by applying the direct water vapor treatments(T60-C and T60-D), a slightly rougher surface resulted. Incontrast, the sample that had been annealed prior to watervapor treatment (T60-E) is comparable to the one thatwas conventionally annealed (T60-B). This confirms whatwas indicated by FTIR, that a better integrity of PMMA wasretained in T60-E, supported by the stretching vibrationintensity of C–H, CLO and C–O–C bands.

The linear absorption spectra shown in Fig. 6 demonstratethat all four nanohybrid thin films are highly transparent in thevisible region. It can also be seen that the samples treated withwater vapor (T60-C, D and E) exhibit a higher absorbance thanthat of the conventionally annealed sample (T60-B). The onsetof absorbance, as a result of the excitation of electrons from the

valence band to the conduction band of TiO2, is observed atabout 350 nm for T60-B. For T60-C, D and E, however, theabsorption edge is rather ‘‘red shifted’’ to higher wavelengths.The differences in absorption wavelength among the nanohy-brids indicate the difference in band gap as affected by thevariation in crystallinity of the TiO2 phase. It is known thatthe relationship between the absorption coefficient a and theoptical band gap, Eg, for fine particles obeys the followingclassical Tauc expression:33

(a0hn) ~ A(hn 2 Eg) (1)

where A, a0 and hn are the edge-width parameter, linearabsorption coefficient and incident photon energy, respectively.The band gap energy, Eg, can be thus determined from a Taucplot of (a0hn) versus hn. Fig. 7 shows the extrapolation of thelinear parts of the curves to the energy axis, estimating aband gap energy of 3.48 ¡ 0.02, 3.27 ¡ 0.01, 3.23 ¡ 0.01 and3.22 ¡ 0.02 eV for T60-B, C, D and E, respectively. Thenanohybrid thin films treated with water vapor demonstratedan Eg comparable to that of pure anatase TiO2 thin films,which is in the range 3.20–3.23 eV.25,26 This agrees with theobservations that TiO2 nanoparticles in the water vapor treatednanohybrids are well crystallized on the basis of XRD andHRTEM studies. On the other hand, the conventionallyannealed nanohybrid was confirmed to be rather amorphous.

Fig. 4 High resolution transmission electron microscopy (HRTEM) images of TiO2–PMMA nanohybrid thin films: (a) T60-B, (b) T60-C, (c) T60-D and (d) T60-E.

2 9 8 2 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

Indeed, the dependence of band gap energy on crystallinity wasreported by Gao et al.26 for titania thin films derived fromaqueous peroxotitanate solution and annealed at varioustemperatures up to 700 uC.

The linear refractive index (n0) of the nanohybrids measuredat 632.8 nm is 1.643, 1.651, 1.672 and 1.781 for T60-B, C, D andE, respectively. They show that n0 for T60-C and T60-D is onlyslightly higher than that of the conventionally annealed sample.

Fig. 5 Atomic force microscopy (AFM) images of TiO2–PMMA nanohybrid thin films: (a, b) T60-B, (c, d) T60-C, (e, f) T60-D and (g, h) T60-E.

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7 2 9 8 3

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

However, a much more remarkable increase in n0 is shown byT60-E. In an agreement with the XRD, FTIR and UV–Visresults, this is attributed to the higher crystallinity of the TiO2

phase. The effects of crystallinity and densification of TiO2 onthe refractive index has been reported elsewhere.25

Fig. 8 shows the results of nonlinear optical measurement forT60 obtained from (a) open aperture, and (b) closed apertureZ-scan with 250 fs laser pulses at 800 nm, where the laser pulseswere delivered by a mode locked Ti–sapphire laser with arepetition rate of 4.0 MHz and input irradiance of 0.23 GWcm22. The minimum beam waist v0 of the focused laser beamwas measured to be y7 mm. The open aperture set-up refers tothe configuration where the entire laser beam transmitted bythe sample is collected and detected. This enables the

observation of nonlinear variation of the absorption coefficient(b) by assuming the total absorption effects as:

a ~ a0 1 bI (2)

where a0 is the linear absorption coefficient and I is the lightintensity.29 The normalized transmission Tn(z) for an openaperture Z-scan is described as follows:

Tn(z)~C 1zz2

�z2

0

� �

ffiffiffipp

bI0Leff

ð?

{?

ln (1zq0e{t2

)dt (3)

where I0 is the on-axis instantaneous intensity of the laser beam

Fig. 6 UV–Vis spectra of TiO2–PMMA nanohybrid thin films: T60-B,T60-C, T60-D and T60-E.

Fig. 7 Estimation of the band gap energy, Eg, for TiO2–PMMAnanohybrid thin films: T60-B, T60-C, T60-D and T60-E.

Fig. 8 The results of Z-scan measurement for TiO2–PMMA nanohy-brid thin films: (a) open aperture and (b) closed aperture, performedwith 800 nm, 250 fs laser pulses at a repetition rate of 4 MHz. The inputirradiance used was 0.23 GW cm22 and the beam waist v0 was y7 mm.The solid lines are the best fit using the Z-scan theory.29

2 9 8 4 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

at focus, C is a normalization constant, Leff ~ [1 2 exp(2a0L)]/a0 is the effective thickness, L is the sample thickness, and z0 isthe diffraction length of the laser beam, defined by z0 ~ pv2

0/l,

where l denotes the laser wavelength and q0~bI0Leff

1zz2=z20

. In order

to exclude the linear transmission of the sample, it is necessaryto normalize the transmission Tn(z). The nonlinear absorptioncoefficient (b) is determined by fitting eqn. (3) to theexperimental open aperture z-scan data of Tn(z). As shownin Fig. 8(a), the open aperture Z-scan curves are nearlysymmetrical and have a minimum at z ~ 0 indicating that bis positive, which is consistent with the result of our previousmeasurement.27,28 This confirms that the origin of the observedoptical nonlinearity for nanohybrids is due to two-photonabsorption. It takes place when TiO2 nanoparticles of lowerenergy absorb the energy of two photons to excite to the higherenergy state. The b values extracted from the best fitting were885, 1060, 1320 and 2260 cm GW21 for T60-B, C, D and E,respectively. The measurements on each sample were repeatedwith a lower repetition rate of 4 kHz. The result shows nosignificant difference, indicating that the laser-induced thermallensing effects can be negligible. Moreover, measurementswere conducted at different input irradiances, ranging from0.23 to 5 GW cm22. It was observed that the results areindependent of the laser irradiance, which implies the observednonlinearities are of a pure third-order process.

In contrast to open aperture Z scan, closed aperture refers tothe configuration where the transmitted light is partiallyblocked by a definite aperture. This provides information onthe nonlinear variation of the refractive index (n2) through thefollowing relationship:

n ~ n0 1 n2I (4)

where n0 is the linear refractive index of the sample. Thenonlinear refractive index (n2) was obtained by dividing thedata of a closed aperture Z-scan by that of an open apertureZ-scan, in which both Z-scans are performed at the sameincident intensity. By measuring the resultant curve of thedifference between the peak and the valley of the normalizedtransmission (DTp2n), the nonlinear refractive index n2 wascalculated by the following equation:

n2~DTp{v l=2pð Þ

0:406I0(1{S)0:25Leff

(5)

where S ~ 1 2 exp(22r2a/v2

a) is the linear aperture transmissionwith ra and va being the aperture and the beam radii, respectively.Similar to the open aperture results, the positive nonlinearityis demonstrated by closed aperture Z-scans in Fig. 8(b), wherethe normalized transmission exhibits a pre-focal transmissionminimum (valley), followed by a post-focal transmissionmaximum (peak). The best fitting of n2 provides values of1.3 6 1022, 1.6 6 1022, 1.8 6 1022 and 6.2 6 1022 cm2 GW21

for nanohybrid thin films T60-B, C, D and E, respectively. Theestimated error in both open and closed aperture measurementsis ¡5%, which originates mainly from the fluctuation of laserenergy.

The imaginary and real parts of the third-order nonlinearoptical susceptibility, Im x(3) and Re x(3), of the nanohybridscan be calculated using the following relationships:29

Imx(3)~n2

0e0cl

2pb (6)

Re x(3) ~ 2n20e0cn2 (7)

where c is the speed of the light and e0 is the vacuum dielectricpermittivity. The calculated Im x(3) for nanohybrid thin filmsT60-B, C, D and E are 0.58 6 1029, 0.70 6 1029, 0.90 6 1029

and 1.74 6 1029 esu, respectively, while the Re x(3) are

0.88 6 1029, 1.10 6 1029, 1.27 6 1029 and 4.97 6 1029 esu,respectively. Consistent with our previous results, the imagi-nary part of the third-order susceptibility is smaller than thereal part.

The absolute third-order susceptibility x(3) can be calculatedby using the following equation:

x(3) ~ [(Re x(3))2 1 (Im x(3))2] (8)

It provides the values of 1.05 6 1029, 1.30 6 1029, 1.56 61029

and 5.27 6 1029 esu for T60-B, C, D and E, respectively. Thesevalues are comparable to that obtained for a conjugatedporphyrin polymer,34 which is one of the largest one-photonoff-resonant third-order susceptibilities to date among thosestudied, for which a value of 2.08 ¡ 0.43 6 1029 esu wasmeasured using a degenerate four-wave mixing (DFWM)technique at 1064 nm. Further comparison with bulk TiO2

35

and TiO2–SiO2 nanocomposites36 shows that the valuesobtained in the present study are two orders of magnitudehigher than those of inorganic compositions. Such a significantenhancement in the observed nonlinear optical responses isdue to the resonance with exciton transition below the band-gap energy.28 The exciton energy, Eexc of TiO2 nanoparticlescan be estimated by using a modified Brus formula37 as follows:

Eexc~EgzRyp2a2

B

R2{3:6

aB

R

� �(9)

where Eg is the band gap energy of anatase TiO2 (3.2 eV), Ry isthe Rydberg energy unit, aB is the exciton Bohr radius in bulkTiO2 (which is 0.8 nm for electron and hole effective masses ofme ~ 10m0 and mh ~ 0.8m0),37 and R is the particle radius. Inorder to calculate the exciton energy for each nanohybrid, Rwas determined as half of the crystallite size measured fromHRTEM studies. The standard deviation of R for each samplewas taken into account in order to predict the effect ofnanoparticle size distribution on the exciton energy and theresulting optical properties. The calculation using eqn. 9provided an estimated exciton energy, Eexc of 3.284 ¡ 0.147,3.215 ¡ 0.132, 3.114 ¡ 0.01 and 3.109 ¡ 0.001 eV for T60-B,C, D and E, respectively. Fig. 9 plots Eexc for these nanohybridsshowing the different position of their exciton energies ascompared to the two-photon energy of laser pulses at thewavelength 800 nm used in this study, which was 3.105 eV. It isthus of interest to observe that the respective x(3) values ofnanohybrids increase following the sequence given by thecloseness between their exciton energy and the two-photonlaser energy. This indicates the extent of the resulting resonanceleading to the nonlinear optical response. It is understandabletherefore why T60-E exhibits a much higher x(3) value amongthese nanohybrids. In addition, it can be seen from the inset inFig. 9 that the standard deviation of this sample is very narrow,suggesting the distribution of statistically very similar excitonenergy levels which contribute to the highest resonance.

A different approach to analyze the observed nonlinearitycan also be performed in terms of the dielectric confinementeffect of TiO2 nanoparticles in the nanohybrids. This effect isdue to surface polarization between TiO2 nanoparticles havinga higher refractive index and the PMMA matrix possessing alower refractive index.38,39 Strong electrical charge interactionbetween them results in an electric dipole layer at the nano-particle surface. This effect accelerates the separation of excitedcharges and enhances the electric field inside the nanoparticles.Wang40 and Gan41 proposed that such enhancement in thedielectric confinement effect of nanohybrids can be performedby either raising the refractive index of the semiconductor orlowering the refractive index of surrounding medium (matrix).In this study, the open and closed aperture Z-scan results showthat the nonlinear absorption increases in accordance with theenhancement in crystallinity of TiO2 and thus refractive index

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7 2 9 8 5

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

in nanohybrids, following the sequential increase shown bysamples T60-B to T60-E. TiO2 nanoparticles in T60-Eexhibited a much enhanced crystallinity and thus a higherrefractive index, leading to the observed nonlinear opticalresponses. Furthermore, as confirmed by the FTIR studies, theintegrity of the PMMA matrix was maintained, providing amedium having lower refractive index surrounding the highrefractive index TiO2 nanoparticles. The cumulative effectstherefore result in an enhancement in the dielectric confinementeffect responsible for nonlinear optical responses in T60-E.Indeed, the nonlinear responses can also be affected by thepopulation/concentration of TiO2 nanoparticles. In order toassess the effects of both the dielectric confinement and nano-particle concentration in a quantitative manner, the followingmodel42 is adopted:

xð3Þnanohybrid ~ (1 2 p)x

ð3Þm 1 f4px

ð3Þdot (10)

where xð3Þnanohybrid, x

ð3Þm and x

ð3Þdot are the third order nonlinear

susceptibility of the nanohybrid, matrix and quantum dots, p isthe filling factor (or volume fraction) and f is the local fieldcorrection factor, resulting from the dielectric confinement

effect, f ~3n2

m

2n2m{n2

dot

. Here, nm is the refractive index of the matrix

and ndot is that of quantum dots, which can be substituted withthat of the bulk constituent. Furthermore the value of x

ð3Þdot in

eqn. 10 can be substituted with the expression: xð3Þdot ~ ax

ð3Þbulk

where xð3Þbulk is the third-order nonlinear susceptibility of the

bulk constituent and a is a scaling factor. For this calculationpurpose, the volume fraction (p) of TiO2 nanoparticles wascalculated from the results of HRTEM studies, where anappropriate sampling technique was performed. The p valuecalculated for T60-B, C, D and E was 0.23, 0.30, 0.33 and0.43, respectively. The third-order nonlinear susceptibilityof the PMMA matrix, x

ð3Þm is taken to be 3 6 10214 esu.43

Unfortunately, the experimental xð3Þdot value for TiO2 nano-

particles is not available from the literature. By taking intoaccount the enhancement in optical nonlinearity of semi-conductor dots due to the quantum confinement effect, thescaling factor a is assumed to be around 10. This predicts a x(3)

value for TiO2 nanoparticles to be one order of magnitudehigher than its bulk (anatase) value, which is 2.4 6 10212 esu.35

Furthermore, the value of f for each nanohybrid was calculatedusing the n0 data obtained from linear refractive indexmeasurements. The calculation using eqn. 10 thus providesestimated x(3) values of 1.16 6 1029, 1.60 6 1029, 2.07 6 1029

and 7.55 6 1029 esu for T60-B, C, D and E, respectively. Thevalue for T60-E is demonstrated to be much larger than theothers, which results from a much higher TiO2 nanoparticleconcentration, in addition to the contribution from the localfield effect. These results are consistent with those obtainedexperimentally, although there exists some discrepancy whichcan be accounted for by the error in estimation of TiO2

nanoparticle concentration. Therefore the model has confirmedthat the dielectric confinement effect and variation in con-centration of TiO2 nanoparticles play important roles in theobserved third-order nonlinearity in this study.

Conclusions

Transparent nanohybrid thin films of TiO2–PMMA with aremarkably enhanced nanocrystallinity have been prepared viaan in situ sol–gel polymerization route, assisted by subsequentthermal and water vapor treatments. Post-treatment by watervapor at 110 and 150 uC led to a higher degree of crystallinityfor TiO2 than the conventional annealing at 150 uC. This isrelated to the cleavage of stiff Ti–O–Ti bonds in the nano-hybrids by water molecules, which effectively increases thenumber of flexible Ti–OH groups and rearranges Ti–O–Tibonds promoting crystallization of TiO2 in the presence ofPMMA. A further significant enhancement in crystallinitywas observed when an additional annealing at 110 uC wasintroduced before the water vapor treatment at 150 uC.

HRTEM studies reveal that the TiO2 nanocrystallites inPMMA are 5–9 nm in size, which is also confirmed by phaseanalysis using XRD. A significant enhancement in linearrefractive index, n0, up to 1.780 was measured, as a result of theincrease in crystallinity of TiO2. The nanohybrid thin films arehighly transparent in the visible region, with estimated bandgap energies, Eg, close to that of anatase TiO2 (y3.20 eV).Controlling the crystallinity of TiO2 in the nanohybridsstrongly affects the nonlinear optical responses. A two-photonabsorption coefficient (b) as high as 2260 cm GW21 and anonlinear refractive index (n2) as high as 6.2 6 1022 cm2 GW21

were demonstrated for the nanohybrid exhibiting the enhancedcrystallinity.

References

1 A. Ershad-Langroudi, C. Mai, G. Vigier and R. Vassoile, J. Appl.Polym. Sci., 1997, 65, 2387.

2 G. Cartenuto, Y. S. Her and E. Matijevic, Ind. Eng. Chem. Res.,1996, 35, 2929.

3 B. Wang, G. L. Wilkes, J. C. Hedrick, S. C. Liptak andJ. E. McGrath, Macromolecules, 1991, 24, 3449.

4 G. Phillip and H. Schmidt, J. Non-Cryst. Solids, 1984, 63, 283.5 M. Yoshida and P. N. Prasad, Chem. Mater., 1996, 8, 235.6 H. Jiang and A. K. Kakkar, Adv. Mater., 1998, 10, 1093.7 L. L. Hench and J. K. West, Chem. Rev., 1990, 90, 33.8 B. M. Novak, Adv. Mater., 1993, 5, 422.9 H. A. Hornak, Ed., Polymers for Lightwave and Integrated Optics,

Marcel Dekker, New York, 1992.10 J. Zhang, S. Luo and L. Gui, J. Mater. Sci., 1997, 32, 1469.11 W. C. Chen, S. J. Lee, L. H. Lee and J. L. Lin, J. Mater. Chem.,

1999, 9, 2999.12 L. H. Lee and W. C. Chen, Chem. Mater., 2001, 13, 1137.

Fig. 9 Plots of estimated exciton energy, Eexc, and calculated x(3) ofTiO2–PMMA nanohybrid thin films: T60-B, T60-C, T60-D and T60-E.

2 9 8 6 J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online

13 C. C. Chang and W. C. Chen, J. Polym. Sci., Part A: Polym.Chem., 2001, 39, 3419.

14 A. Matsuda, Y. Kotani, T. Kogure, M. Tatsumisago andT. Minami, J. Am. Ceram. Soc., 2000, 83(1), 229.

15 Y. Kotani, A. Matsuda, T. Kogure, M. Tatsumisago andT. Minami, Chem. Mater., 2001, 13, 2144.

16 H. Imai, H. Moromoto, A. Tominaga and H. Hirashima,J. Sol–Gel Sci. Technol., 1997, 10, 45.

17 H. Imai and H. Hirashima, J. Am. Ceram. Soc., 1999, 82(9), 2301.18 N. Uekawa, J. Kajiwara, K. Miyatake, K. Kakegawa and

Y. Sasaki, Chem. Lett., 1999, 2000, 382.19 C. J. Brinker and A. J. Hurd, J. Phys. III, 1994, 4, 1231.20 M. Burgos and M. Langlet, J. Sol–Gel Sci. Technol., 1999, 16, 267.21 M. Langlet, M. Burgos, C. Couthier, C. Jimenez, C. Morant and

M. Manso, J. Sol–Gel Sci. Technol., 2001, 22, 139.22 K. I. Gnanasekar, V. Subramanian, J. Robinson, J. C. Jiang,

F. E. Posey and B. Rambabu, J. Mater. Res., 2002, 17, 1507.23 Y. Djaoued, S. Badilescu, P. V. Ashrit, D. Bersani, P. P. Lottici

and R. Bruning, J. Sol–Gel Sci. Technol., 2002, 24, 247.24 M. Langlet, A. Kim, M. Audier and J. M. Hermann, J. Sol–Gel

Sci. Technol., 2002, 25, 223.25 Z. Wang, U. Helmersson and P. O. Kall, Thin Solid Films, 2002,

405, 50.26 Y. Gao, Y. Masuda, Z. Peng, T. Yonezawa and K. Koumoto,

J. Mater. Chem., 2003, 13, 608.27 A. H. Yuwono, J. M. Xue, J. Wang, H. I. Elim, W. Ji, Y. Li and

T. J. White, J. Mater. Chem., 2003, 13, 1475.28 H. I. Elim, W. Ji, A. H. Yuwono, J. M. Xue and J. Wang, Appl.

Phys. Lett., 2003, 82(16), 2691.

29 M. Sheik-Bahae, A. A. Said, T. Wei, D. J. Hagan andE. W. Van Stryland, IEEE J. Quantum Electron., 1990, 26, 760.

30 W. C. Chen and S. J. Lee, Polym. J., 2000, 32, 67.31 S. X. Wang, M. T. Wang, Y. Lei and L. D. Zhang, J. Mater. Sci.

Lett., 1999, 18, 2009.32 C. C. Wang and J. Y. Ying, Chem. Mater., 1999, 11, 3113.33 J. Tauc, R. Grigorovich and A. Vancu, Phys. Status Solidi, 1966,

15, 627.34 S. M. Kuebler, R. G. Denning and H. L. Anderson, J. Am. Chem.

Soc., 2000, 122, 339.35 T. Hashimoto, T. Yoko and S. Sakka, Bull. Chem. Soc. Jpn., 1994,

67, 653.36 Q. F. Zhou, Q. Q. Zhang, J. X. Zhang, L. Y. Zhang and X. Yao,

Mater. Lett., 1997, 31, 39.37 S. Monticone, R. Tufeu, A. V. Kanaev, E. Scolan and C. Sanchez,

Appl. Surf. Sci., 2000, 162(163), 565.38 X. C. Ai, H. Fei, Y. Yang, L. Han, R. Nie, Y. Zhang, C. Zhao,

L. Xiao, T. Li, J. Zhao and J. Yu, J. Luminesc., 1994, 60(61), 364.39 X. Wu, R. Wang, B. Zou, P. Wu, L. Wang, J. Xu and W. Huang,

Appl. Phys. Lett., 1997, 71, 2097.40 Y. Wang, Acc. Chem. Res., 1991, 24, 133.41 F. Gan, J. Sol–Gel Sci. Technol., 1998, 13, 559.42 F. D’Amore, S. M. Pietralunga, P. Lorusso, M. Martinelli,

A. Zappetini, E. Dal Bo, F. Tassone, P. Tognini and M. Travagnin,Phys. Status Solidi C, 2004, 1, 1001.

43 F. D’Amore, M. Lanata, S. M. Pietralunga, M. C. Gallazzi andG. Zerbi, Opt. Mater., 2004, 24, 661.

J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 7 8 – 2 9 8 7 2 9 8 7

Publ

ishe

d on

20

Aug

ust 2

004.

Dow

nloa

ded

by M

ount

Alli

son

Uni

vers

ity o

n 21

/05/

2013

03:

03:0

0.

View Article Online