TiO2 nanorods/PMMA copolymer-based nanocomposites: highly homogeneous linear and nonlinear optical material

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TiO2 nanorods/PMMA copolymer-based nanocomposites: highly homogeneous linear and

nonlinear optical material

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2008 Nanotechnology 19 205705

(http://iopscience.iop.org/0957-4484/19/20/205705)

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IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 19 (2008) 205705 (8pp) doi:10.1088/0957-4484/19/20/205705

TiO2 nanorods/PMMA copolymer-basednanocomposites: highly homogeneouslinear and nonlinear optical materialC Sciancalepore1, T Cassano2, M L Curri3, D Mecerreyes4,A Valentini2, A Agostiano1,3, R Tommasi5 and M Striccoli3

1 Dipartimento di Chimica, Universita di Bari, Via Orabona 4, I-70126 Bari, Italy2 Dipartimento Interateneo di Fisica and INFN, Universita di Bari, Via Amendola 173, I-70126Bari, Italy3 CNR-IPCF Sede di Bari, Via Orabona 4, I-70126 Bari, Italy4 CIDETEC, Centro de Tecnologıas Electroquımica, Parque Tecnologico de San Sebastian,Paseo Miramon 19, E-20009 Donostia-San Sebastian, Spain5 Dipartimento di Biochimica Medica, Biologia Medica e Fisica Medica, Universita di Bari,Piazza G Cesare 11, Policlinico, I-70124 Bari, Italy

E-mail: [email protected] and [email protected]

Received 28 January 2008, in final form 4 March 2008Published 15 April 2008Online at stacks.iop.org/Nano/19/205705

AbstractOriginal nanocomposites have been obtained by direct incorporation of pre-synthesized oleicacid capped TiO2 nanorods into properly functionalized poly(methyl methacrylate) copolymers,carrying carboxylic acid groups on the repeating polymer unit. The presence of carboxylicgroups on the alkyl chain of the host functionalized copolymer allows an highly homogeneousdispersion of the nanorods in the organic matrix. The prepared TiO2/PMMA-co-MAnanocomposites show high optical transparency in the visible region, even at high TiO2 nanorodcontent, and tunable linear refractive index depending on the nanoparticle concentration.Finally measurements of nonlinear optical properties of TiO2 polymer nanocompositesdemonstrate a negligible two-photon absorption and a negative value of nonlinear refractiveindex, highlighting the potential of the nanocomposite for efficient optical devices operating inthe visible region.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Polymer nanocomposites, consisting of semiconductingcolloidal nanocrystals (NCs) embedded in polymer matrices,are original materials able to join the structural flexibilityand convenient processing of the polymers with high carriermobility, bandgap tunability, and thermal and mechanicalstability of the inorganic components. Such novel materialshave been explored in many application fields, including linearand nonlinear optical devices, light-emitting diodes, opticalswitches, waveguides, sensors, and hard transparent coatingsas protective layers [1–6].

One of the most crucial points for the fabrication of sucha class of nanocomposites relies on the ability to control

the dispersion of the nanoparticles in the host matrix. Infact, nanoscale particles typically possess a strong tendency toaggregate, which might be detrimental for retaining their size-dependent properties. In nanocomposites, nanofillers must befinely dispersed in polymers so that the heterogeneous natureof the material should be evident only for sampling on ananometric scale. For this reason a critical challenge in thedesign of these inorganic–organic systems is to control themixing between the two dissimilar phases and several chemicalapproaches have been applied to overcome this problem [7].To obtain dispersed inorganic fillers in a polymeric matrixtwo general methods are usually followed: (i) ‘in situ’synthesis of nanoparticles in the polymer matrix [8, 9]or ‘in situ’ polymerization of an organic matrix around

0957-4484/08/205705+08$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 205705 C Sciancalepore et al

inorganic nanoparticles [10] and (ii) incorporation of ‘ex situ’synthesized nanoparticles in pre-made organic polymers withthe use of a common solvent [11, 12]. The former mayproduce undesired species in the matrix, coming either fromproducts of the nanoparticle synthesis process or from thepolymerization steps [13]. In addition, a large dispersion in thedimensions and a partial deterioration of the NC properties,as a consequence of the increased density of surface defects,are among the typical drawbacks of such an approach. On theother hand, methods based on mixing of synthesized inorganicNCs with pre-formed polymers have been reported to inducethe formation of strongly connected aggregates, due to thehigh specific surface energy of NCs [14]. Nevertheless theprocess involving the blending of pre-made colloidal NCsinto a polymer matrix provides a remarkable advantage, asit allows for full synthetic control of both the inorganic andorganic moieties. A governing factor, aiding the uniformdispersion of colloidal nanoparticles in the polymer, deals withthe control of the interaction between the polymer matrix andorganic agent capping the NC surface, derived directly fromthe NC synthesis. The NC surface ligands can be properlyexchanged or modified [15] to improve the compatibility withthe polymer, or alternatively a tailored functionalization of thepolymer chains with suitable chemical groups can be carriedout [16, 17]. The latter approach has been used in this work.

Here the preparation of an original nanocompositebased on the incorporation of pre-synthesized oleic acid(OLEA) capped TiO2 crystalline nanorods (NRs) intoproperly functionalized, optically transparent, poly(methylmethacrylate) (PMMA) copolymers, carrying carboxylic acidgroups on the repeating polymer unit (namely poly(methylmethacrylate-co-methacrylic acid) (PMMA-co-MA)) has beenperformed and the result of its optical and morphologicalcharacterizations are presented. Indeed, in the past, the roleof poly(acrylic acid) derivatives in stabilizing the TiO2 NCsurface has been successfully investigated in fields rangingfrom photocatalysts [18] to the supramolecular organization ofNCs [19].

In this work the influence of the acrylic groups on the NCdistribution in the polymers and on the optical properties ofthe nanocomposites has been investigated. The spectroscopicand structural properties arising in the nanocomposite arecompared with those of the TiO2 NR modified PMMAhomopolymer. The TiO2/PMMA-co-MA nanocompositeshows highly homogeneous dispersion of the NRs in theorganic matrix, higher optical transparency in the visibleoptical region, even at high TiO2 NR content, and a linearrefractive index which is tunable with the NR concentration.Finally, nonlinear optical (NLO) properties of TiO2 polymernanocomposites have been evaluated by means of the Z -scan technique [20]. In particular, a negligible two-photonabsorption (TPA) coefficient and a negative value of nonlinearrefractive index, which induces self-defocusing effects, havebeen measured in the visible region, highlighting the potentialof the nanocomposite for efficient optical devices [21].

2. Materials and methods

All chemicals were of the highest purity available andused as received without further purification or distillation.Titanium tetraisopropoxide (Ti(OPri)4 or TTIP, 99.999%),trimethylamino-N-oxide dihydrate ((CH3)3NO·2H2O orTMAO, 98%, water solution) and oleic acid (C18H33CO2H orOLEA, 90%) were purchased from Aldrich. All solvents wereof analytical grade and purchased from Aldrich.

Absorption measurements were carried out by using anUV/vis/NIR spectrophotometer Cary 5000 (Varian). Theoptical measurements were performed at room temperatureon samples obtained directly from synthesis without any sizesorting treatments. Low resolution transmission electronmicroscopy (TEM) images were recorded using a JEOLTEM 1011 microscope operating at an accelerating voltage of100 kV. Non-contact mode scanning atomic force microscopy(AFM) images of nanocomposite thin films were recordedwith a PSIA Xe-100 scanning probe microscope, using a non-contact cantilever. Film refractive indexes have been obtainedby means of ellipsometric measurements at a fixed wavelengthof 632.8 nm, using a Horiba Jobin–Yvon ellipsometer.

The Z -scan technique was employed to investigate thethird-order NLO properties of the TiO2 nanorods using thesecond harmonic (hν = 2.33 eV) of a Q-switched andmode-locked Nd:YAG laser emitting τ = 25 ps pulses at10 Hz repetition rate. Measurements to evaluate both the TPAcoefficient and the nonlinear refractive index were performedat room temperature on a 1 mm thick quartz cuvette filled witha 1 M solution of TiO2 NRs in PMMA70-co-MA30 in CHCl3.

2.1. Synthesis of TiO2 colloidal nanorods

In this work, organic capped anatase TiO2 NRs weresynthesized by hydrolysis of titanium tetraisopropoxide (TTIP)using technical oleic acid (OLEA) as surfactant at lowtemperatures (80–100 ◦C) [22]. Hydrolysis of TTIP wascarried out by an excess of aqueous base solution with respectto TTIP. Quaternary ammonium hydroxide (trimethylamino-N oxide dihydrate or anhydrous, (CH3)3NO or TMAO) wasemployed as catalysts for polycondensation in order to ensurea crystalline product. TiO2 NRs were obtained when a TMAOaqueous base solution was rapidly injected in a single portioninto OLEA:TTIP mixtures (fast hydrolysis method). Theresulting OLEA-coated TiO2 nanoparticles were then easilyand homogeneously dispersed in chloroform and opticallyclear, highly concentrated solutions were obtained, stable overmonths without any further growth or irreversible aggregation.NRs with 30 nm of average length and 6 nm of diameter wereobserved by TEM analysis, carried out on samples depositedon a carbon-coated 400-mesh copper grid.

2.2. Synthesis of PMMA-co-MA copolymers

In a typical procedure, an acetone solution of methylmethacrylate and acrylic acid was prepared and added to aflask. Next, azobisisobutylonitrile (AIBN) was added. Thereaction was carried out at 65 ◦C for 6–7 h. After cooling thepolymer was isolated by precipitation in hexane, recovered by

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Figure 1. Absorption spectra of TiO2 NRs PMMA70-co-MA30-basedTiO2 nanocomposite solutions in CHCl3 at different TiO2 NRcontent: (A) 3.8 wt%, (B) 7.4 wt%, (C) 13.8 wt%, (D) 24 wt%,(E) 44 wt%, (F) 61.5 wt%; inset: absorption spectrum of TiO2 NRsin CHCl3.

filtration and dried under vacuum. Copolymer compositionwas calculated by H1 NMR, indicating a value of 30% forthe percentage of methacrylic acid functional groups in thecopolymer. The molecular weight was measured by gel-permeation chromatography (GPC); a value of 6400 g mol−1

and polydispersity of Mw/Mn = 1.9 were obtained.

2.3. Nanocomposite preparation

Nanocomposite films with dispersed titania nanoparticles inpolymer were fabricated by adding powdered PMMA70-co-MA30 to a chloroform solution of TiO2 NRs at increasingnanoparticle concentration (from 0.025 to 1 M). A polymerconcentration of 0.05 g ml−1 was selected to achieve opticallyclear nanocomposite solutions. The mixture was gently stirreduntil complete dissolution of the polymer in the solvent. Thefinal fraction of the nanocomposite composed of nanorodsranges from 3.8 wt% for an NR concentration of 0.025 M to61.5 wt% for an NR concentration of 1 M. Nanocompositethin films samples for AFM measurements were prepared bydepositing by spin-coating (using a EC101DT Photo ResistSpinners, Headway Research Inc.) at 3000 pm for 30 s theNC–polymer mixture onto properly cleaned quartz substrates.

3. Results and discussions

The effect of the incorporation of TiO2 NRs in PMMA70-co-MA30 was investigated by UV–visible absorption spectroscopy.The absorption spectra of the PMMA70-co-MA30-based TiO2

nanocomposite solutions at different TiO2 loading contents areshown in figure 1. The incorporation in PMMA70-co-MA30

does not significantly perturb the TiO2 NR absorption features.The spectra highlight a strong absorption in the UV region,which is characteristic of the nanosized TiO2 (as can benoticed from the inset in figure 1), being the copolymeroptically transparent in the same spectral region. Raising

Figure 2. (αhν)1/2 and (αhν)2 versus excitation forPMMA70-co-MA30-based TiO2 nanocomposite solution.

the TiO2 NR content in the matrix, the absorption increaseand, for high nanoparticle loading, a concomitant tail can beclearly observed in the visible region of the absorption curve.Such a tail can be attributed to scattering losses induced bythe large number of inorganic nanoparticles in the polymer.However, these absorption modifications follow a Lambert–Beer behavior. Indeed, by multiplying the absorbance curveof a dilute solution for the corresponding dilution factor,it is possible to reproduce the absorption curve of all theconcentrated samples. Such evidence demonstrates that nonanoparticle aggregation takes place, even at the highest TiO2

NR loading factor in the copolymer matrix [23].Bulk TiO2 is a indirect semiconductor, i.e. the minimum of

the lowest conduction band is shifted relative to the maximumof the highest valence band in k space, and the lowest-energyinterband transition requires a change in both energy andmomentum of carriers. In this case a two-step process isnecessary for the optical transition because a photon cannotprovide the required change in momentum and the transitionmust then be accompanied by a phonon exchange. Whenphonon energy can be neglected, the absorption coefficient α

near the absorption edge for indirect interband transitions isgiven by equation [24, 25]:

(αhν)1/2 = Bi(hν − Eg,ind) (1)

where Bi is an absorption constant for indirect transitions, hν

is the photon energy and Eg,ind is the indirect bandgap of thesemiconductor. α can be determined for each wavelength bymeans of the relation [26]

α = 2, 303 ∗ ρ ∗ 103 ∗ A/ l ∗ c ∗ M (2)

where A is the absorbance of a colloidal suspension ofstabilized TiO2 colloids, ρ = 3.89 g cm−3 is the densityof anatase TiO2, l is the optical path length, c is the molarconcentration of TiO2 (from 0.025 to 1 M) and M is themolecular weight.

The absorption onset of TiO2 NRs in polymer solutionscan be obtained by plotting (αhν)1/2 versus hν (figure 2,grey line, on the left). By intersecting the straight line fitting

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of the linear portion of the obtained curve with the energyaxis, the absorption onset for TiO2 NRs in PMMA70-co-MA30-based nanocomposite was estimated at ∼3.2 eV for all theinvestigated samples. This value can be associated with theindirect transition of anatase �3 → Xlb, predicted to occur at3.19 eV in TiO2 bulk [23, 27, 28] and supports the conclusionthat TiO2 NRs embedded in PMMA70-co-MA30 are not in aquantum confinement regime [28].

At higher energies, a further contribution can be identifiedin the absorption spectrum of figure 1, which can be ascribedto a direct transition.

For direct interband transitions, the following equationholds [24, 25]:

(αhν)2 = Bd(hν − Eg,dir) (3)

where Bd is an absorption constant for direct transitions andEg,dir is the direct bandgap of the semiconductor. By plottingthe experimental absorption data as (αhν)2 versus hν (figure 2,black line), the intersection of the straight line fitting the linearportion of the curve with the energy axis lies at 3.6 eV, whichhas been attributed to the energy of the first allowed directtransition (X2b → X1b) [23, 29].

In order to test the influence of the polymer chemistry onthe nanocomposite optical properties, the TiO2 NR solutionshave been incorporated in the pure PMMA homopolymer,synthesized by following the same procedure described above,without any functional groups. The absorption spectra,measured in PMMA homopolymer, for the correspondingTiO2 NRs loading, highlight a lower optical transparency withrespect to PMMA70-co-MA30-based nanocomposite solutions(data not reported here) that can be reasonably ascribed to anon-uniform dispersion of TiO2 NRs in a polymer matrix [30].

The above results have been confirmed by absorptionmeasurements performed on nanocomposite thin filmsdeposited on a quartz substrate. Figure 3(a) showsthe absorption spectrum of a TiO2/PMMA homopolymerblend thin film (44 wt% of TiO2) compared with thatof a PMMA70-co-MA30-based nanocomposite (44 wt% ofTiO2): the increase in the absorption baseline value forthe NRs/PMMA homopolymer sample can be ascribed tostrong NR aggregations in the organic matrix and concomitantscattering processes, due to the low compatibility between thePMMA homopolymer and the OLEA capped NR surface.

The optical micrographs of the thin film surfaces(figures 3(b) and (c)) reveal a rather uniform morphology in thefilm formed by the TiO2/PMMA70-co-MA30 nanocomposite,while a rough surface can be observed in the TiO2/PMMAhomopolymer deposited films, thus supporting the aggregationhypothesis suggested by the spectroscopic data. Such anindication does also point out that a clear improvement of thethin film optical and morphological properties is obtained byusing PMMA copolymer functionalized with acrylic acid, evenat high TiO2 NR content.

The morphological characteristics of the NC-basedcomposite films were also investigated by AFM in theno-contact mode. The comparison of the topographicimages of TiO2 NRs PMMA homopolymer (figure 4(a))and of TiO2 NRs PMMA70-co-MA30 composite thin film

Figure 3. (a) Absorption spectra of TiO2 NRs 44 wt%/PMMAhomopolymer (dotted line) and TiO2 NRs44 wt%/PMMA70-co-MA30 (solid line) thin films and opticalmicroscope images of TiO2NRs/PMMA70-co-MA30 (b) and TiO2

NRs/PMMA homopolymer thin film (c), respectively.

(figure 4(b)), both with the same TiO2 NR content at44 wt%, indicates a higher roughness (Ra = 1.594 nm)for the homopolymer base composite film with respectto the TiO2 NRs PMMA70-co-MA30 thin film (Ra =0.784 nm). Moreover, in the PMMA homopolymer-basednanocomposite the presence of fractures with depths of severalnanometers shows the occurrence of partial phase separationphenomena at the micrometric level. Poor quality andwidespread inhomogeneity of the PMMA homopolymer-basednanocomposite film could be attributed to the formation oflarge TiO2 NR aggregates in the polymer matrix, in accordancewith the absorption measurements.

In contrast, the TiO2 NR/PMMA70-co-MA30 compositeshows homogeneous and regular coating, with standard heightsin the range of 3–5 nm. In addition AFM topographic measure-ments performed on TiO2/PMMA copolymer nanocompositethin films at different NR load percentage (data not reportedhere) demonstrated that the roughness does not vary signif-icantly by changing the TiO2 NR content in the copolymermatrix.

In the nanoscale range, a morphological investigationon the prepared nanocomposites has been accomplished byanalyzing TEM images of the PMMA homopolymer andfunctionalized PMMA copolymer-based nanocomposites. Theanalysis of microscopy images clearly shows uniform NRdispersion in the case of PMMA copolymer nanocomposites(figure 5(b)) while large and irregular NR clusters are found inthe case of PMMA homopolymer-based thin films (figure 5(a)).The occurrence of extended NR aggregation phenomena in the

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Figure 4. 5 μm × 5 μm AFM plan view images of TiO2 NR composites, at 44 wt%, in (a) PMMA homopolymer and (b) PMMA70-co-MA30.

Figure 5. TEM images of TiO2 NRs (44%wt) incorporated in(a) PMMA homopolymer and (b) PMMA70-co-MA30.

PMMA homopolymer and the uniform dispersion when NRsare embedded in the functionalized copolymer are in goodagreement with AFM and optical results.

From the morphological and structural measurementsit can be deduced that TiO2 NRs, when incorporated inPMMA homopolymer, suffer low compatibility with the hostenvironment, because the PMMA methyl-carboxylate groupsdo not interact with oleic acid alkylic chains present on theTiO2 surface. Thus mutual hydrophobic interactions betweenoleic acid alkylic chains prevail, inducing local nanocrystalaggregation [9].

Conversely, interactions between methacrylic functionali-ties on the polymer chains and the TiO2 surface are extremelyeffective and oleic acid ligands can be effectively replaced, oralternatively the methacrylic carboxylic groups can intercalatebetween the fatty acid native ligand chain. As a consequence astrong nanocrystal stabilization and uniform dispersion in thematrix are obtained.

It is well known that the oleic acid ligands bound toa NR surface can be removed and exchanged effectivelywith molecules able to bind more strongly to NCs, suchas alkylphosphonic acids [22] or other carboxylic acids(e.g. metacrylic acid) [31]. Carboxylate groups can becoordinated to a titania surface in three possible structures [32].In the first case, carboxylate is bound to one TiIV center in achelating bidentate mode (i). The carboxylate could also bebound to one TiIV in a monodentate (ester-like linkage) mode(ii), and finally, the carboxyl group could bind with each of itsoxygen atoms to two TiIV atoms yielding the bridging bidentatemode (iii). The most favorable absorption for the carboxylicgroup is a dissociative process with its two oxygen atomsbonding to the surface titanium, with the proton (H+) bondingto the bridging oxygen sites, forming a hydroxyl group [33].When NRs are embedded in a functionalized copolymer, apartial replacement of native ligands with functional polymerchains takes place. In fact, oleic acid ligands can be easilyremoved [22] and substituted with acrylic acid that has acarboxyl group for coordination to the TiO2 NR surface [31].The stabilization of the colloidal system can be explainedby the formation of a protective layer of adsorbed polymer,forming mainly a train-type structure [34].

In addition, NR solutions, repeatedly washed in orderto remove most of the native surfactant, have been addedto functionalized PMMA-based copolymer in order to clarifythe role played by the OLEA ligand on the NR surfacein the dispersion process. After the washing procedures,the nanoparticles precipitate in the copolymer matrix, thusshowing the key function of the oleic molecules on thedispersion mechanism. This behavior can be ascribed to thecooperative effect of the multivalence ligands on free sites onNC surfaces, which induces the NC dispersion.

In previous papers it has been demonstrated thatthe incorporation of high refractive index nanofillers inan organic matrix can change the linear refractive indexof the nanocomposite materials [30, 35–40]. Moreover,polymers with high refractive index are very appealing inoptics and photonics due to their high processability forfabrication of low cost integrated optical devices and theirability to reduce reflection losses at interfaces, increasinglight output. Therefore, the high value of the refractiveindex for bulk TiO2, together with both the homogeneous

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Figure 6. Refractive index n of TiO2 NRs/PMMA70-co-MA30-basednanocomposites for different loading percentages.

dispersion of the nanoparticles in the copolymer matrix andthe optical transparency of the nanocomposite thin filmsinduce the expectation of interesting optical properties for theprepared materials. The measured values of the refractiveindex, obtained for PMMA70-co-MA30-based nanocompositetransparent thin films, are reported in figure 6 for different TiO2

NR content. The refractive index appears to be proportional tothe content of TiO2 NRs in the polymer host matrix and can beeasily modulated by modifying the nanoparticle loading, whilethe optical transparency of the spin-coated films is preservedeven at high nanorod concentration.

4. Nonlinear optical measurements

Flexible materials that exhibit optical nonlinearities are at theforefront of research owing to their potential applications inphotonics-based technologies. The device-oriented stringentrequirements of such materials include high optical damagethreshold, and high mechanical and thermal stability, jointlywith transparency in the vis/IR region for low opticallosses. TiO2 NRs/PMMA70-co-MA30 nanocomposites possessall these qualities, suggesting it could be employed inNLO devices. Single-beam Z -scan nonlinear opticalmeasurements [20, 41] were performed using the secondharmonic (hν = 2.33 eV) of a Q-switched and mode-lockedNd:YAG laser emitting ∼25 ps pulses at 10 Hz repetition rate.Experiments were carried out at room temperature on a 1 mmthick quartz cuvette filled with a 1 M solution of TiO2 NRs inPMMA70-co-MA30 in CHCl3.

Open-and closed-aperture Z -scan measurements wereperformed at different excitation intensities (in the range20–80 GW cm−2 at the focus position) to evaluate thetwo-photon absorption (TPA) coefficient α2 and nonlinearrefractive index n2, respectively.

Experiments performed in pure solvent confirmed that thenonlinear refractive index of chloroform is positive (n2,CHCl3 =1 × 10−15 cm2 W−1) and that TPA can be disregarded athν = 2.33 eV. Open-aperture curves obtained for the solutionare nearly flat, indicating a negligible TPA (α2 < 3 ×1011 cm W−1) at the investigated wavelength. In contrast,

Figure 7. Closed-aperture Z -scan measurements (dots) andtheoretical fittings (solid lines) at hν = 2.33 eV on TiO2 NRs inPMMA70-co-MA30 solution (loading percentage 61.5%) at differentexcitation intensities. The curves have been shifted for clarity and thevertical bar represents the scale of the normalized transmittancechanges.

the closed-aperture curves are characterized by a peak–valleyshape, which is typical of negative n2 (figure 7). Tothe best of our knowledge, this is the first time that anegative n2 is measured in this class of materials. Fittedexperimental data using the theory developed by Sheik-Bahaeand collaborators [20] provide n2 = −6 × 10−15 cm2 W−1 forthe TiO2 NR composite material at all excitation intensities.An intensity-independent estimate of n2 demonstrates that thelatter has a pure third-order origin.

Optical nonlinearities in bulk, indirect bandgap semicon-ductors have been little investigated and very few papers arereported in the literature. From the theoretical point of view,in these semiconductors the nonlinear refractive index is pre-dicted to be always positive in the transparency region [42],and experiments performed in bulk Si support the theoreticalresults [43].

Our finding of negative n2 is therefore unexpected andrather surprising as nonlinearities of thermal origin (that mightbe responsible for a negative refractive nonlinearity) can beexcluded on the basis of low repetition rate and picosecondtime duration of laser pulses. In addition, our sample is quitecomplex and cannot be straightforwardly assimilated into bulksemiconductors. However, it is possible to tentatively explainour experimental results by considering that (i) in TiO2 thedirect bandgap (Eg,dir ∼ 3.6 eV) is not far from the indirect one(Eg,ind ∼ 3.2 eV) and that (ii) usually the optical nonlinearitiesassociated with direct transitions are larger in magnitude thatthose associated with indirect transitions.

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Bulk, direct bandgap semiconductors have been muchmore investigated in the literature, following varioustheoretical approaches. One of the most reliable estimatesof nonlinear refraction in these semiconductors has beenobtained by considering all the contributions deriving fromthe Kramers–Kronig transform of TPA processes, Ramantransitions, linear Stark effect and quadratic Stark effect [44].From the calculated dispersion curve it results that, for hν ∼0.7Eg,dir, the nonlinear refractive index sign changes frompositive to negative, in agreement with several experimentalresults [44, 45]. In our experimental conditions a negativen2 is expected, principally due to the quadratic Stark effect,which overcomes both the positive nonlinearity of the solventand the positive nonlinearity associated with indirect transition.This end result is paramount for many applications requiringa negative nonlinearity associated with negligible two-photonabsorption. As an example, in fiber optics when high-intensity laser beams are used, α2 ∼ 0 ensures that two-photon absorption losses are negligible. At the same time anegative nonlinear refractive index, thanks to the deriving self-defocusing action, is desired to prevent catastrophic effectsdue to self-focusing that, in contrast, would be induced bypositive n2.

5. Conclusions

In this paper the preparation and the optical and morphologicalcharacterization of a new nanocomposite based on theincorporation of OLEA capped TiO2 NRs into properlyfunctionalized poly(methyl methacrylate) copolymers withcarboxylic acid groups are presented. We demonstrate thatan highly homogeneous dispersion of the NRs in the organicmatrix can be obtained, in the presence of functional groups,with chemical affinity with the NR surfaces on the alkyl chainof the host copolymer. The UV–vis absorption spectra ofnanocomposites, measured for different TiO2 load content,show optical transparency of the nanocomposites in the visiblerange and two electronic transitions, respectively indirect anddirect, have been identified. In addition the tunability of thelinear refractive index of the nanocomposites with the NRcontent, the negative value of the nonlinear refractive index andan almost negligible nonlinear absorption have been measured.

These peculiar characteristics propose the investigatednanocomposites as suitable materials for photonic applicationsand the illustrated preparation method opens the way towardsinnovative materials with easily controllable linear andnonlinear optical properties.

Acknowledgments

The partial support of the EC-funded project NaPa (contractno. NMP4-CT-2003-500120), of MIUR SINERGY (FIRBRBNE03S7XZ) Italian National Project and University ofBari (PRIN COFIN 2006) are gratefully acknowledged. TheCNISM consortium is also acknowledged for grant financialsupport. The content of this work is the sole responsibility ofthe authors. The TEM measurements have been carried out atthe NNL Laboratories in Lecce, Italy.

References

[1] Gorelikov I and Kumacheva E 2004 Chem. Mater. 16 4122[2] Caseri W 2000 Macromol. Rapid Commun. 21 705[3] Judeinstein P and Sanchez C 1996 J. Mater. Chem. 6 511[4] Komarneni S 1992 J. Mater. Chem. 2 1219[5] Godovski D Yu 1995 Adv. Polym. Sci. 119 79[6] Beecroft L L and Ober C K 1997 Chem. Mater. 9 1302[7] Kickelbick G 2003 Prog. Polym. Sci. 28 83[8] Selvan S T, Spatz J P, Klok H A and Moller M 1998

Adv. Mater. 10 132[9] Chen H-J, Jian P-C, Chen J-H, Wang L and Chiu W-Y 2007

Ceram. Int. 33 643[10] Pang L, Shen Y, Tetz K and Fainman Y 2005 Opt. Express

13 44[11] Tamborra M, Striccoli M, Comparelli R, Curri M L,

Petrella A and Agostiano A 2004 Nanotechnology 15 S240[12] Tamborra M, Comparelli R, Curri M L, Striccoli M,

Petrella A and Agostiano A 2005 Proc. SPIE 5838 236–44[13] Corbierre M K, Cameron N S, Sutton M, Laaziri K and

Lennox R B 2005 Langmuir 21 6063[14] Caseri W 2000 Macromol. Rapid Commun. 21 705[15] Potapova I, Mruk R, Prehl S, Zentel R, Basche T and

Mews A 2003 J. Am. Chem. Soc. 125 320[16] Wang X S, Dykstra T E, Salvador M R, Manners I,

Scholes G D and Winnik M A 2004 J. Am. Chem. Soc.126 7784

[17] Tamborra M, Striccoli M, Curri M L, Alducin J A,Mecerreyes D, Pomposo J A, Kehagias N, Reboud V,Sotomayor Torres C M and Agostiano A 2007 Small 3 822

[18] Titheridge D J, Barteau M A and Idriss H 2001 Langmuir17 2120

[19] Wu C-S 2004 J. Appl. Polym. Sci. 92 1749[20] Sheik-Bahae M, Said A A, Wei T-H, Hagan D J and

van Stryland E W 1990 IEEE J. Quantum Electron. 26 760[21] Sutherland Richard L 1996 Handbook of Nonlinear Optics

(New York: Dekker)[22] Cozzoli P D, Kornowski A and Weller H 2003 J. Am. Chem.

Soc. 125 14539[23] Serpone N, Lawless D and Khairutdinov R 1995 J. Phys. Chem.

99 16646[24] Pankove J I 1971 Optical Processes in Semiconductors

(New York: Dover) chapter 3[25] Moser E and Pearson W B 1960 Progress in Semiconductor

vol 5 , ed A F Gibson (New York: Wiley) p 53[26] Delgass W N, Haller G L, Kellerman R and Lundsford J H

1979 Spectroscopy in Heterogeneous Catalysis(New York: Academic) p 128

[27] Coutinho P J G and Barbosa M T C M 2006 J. Fluoresc.16 387

[28] Monticone S, Tufeu R, Kanaev A V, Scolan E andSanchez C 2000 Appl. Surf. Sci. 162/163 565

[29] Daude N, Gout C and Jouanin C 1977 Phys. Rev. B 15 3229[30] Convertino A, Leo G, Tamborra M, Sciancalepore C,

Striccoli M, Curri M L and Agostiano A 2007 SensorsActuators B 126 138

[31] Khaled S M, Sui R, Charpentier P A and Rizkalla A S 2007Langmuir 23 3988

[32] Rotzinger F P, Kesselman-Truttmann J M, Hug S J,Shklover V and Graetzel M 2004 J. Phys. Chem. B108 5004

[33] Foster S and Nieminen R M 2004 J. Chem. Phys. 121 9039[34] Liufu S, Xiao H and Li Y 2005 J. Colloid Interface Sci.

281 155[35] Yoshida M and Prasad P N 1996 Chem. Mater. 8 235

7

Nanotechnology 19 (2008) 205705 C Sciancalepore et al

[36] Lu C, Cui Z, Wang Y, Li Z, Guan C, Yang B and Shen J 2003J. Mater. Chem. 13 2189

[37] Chau J L H, Lin Y-M, Li A-K, Su W-F, Chang K-S,Hsu S L-C and Li T-L 2007 Mater. Lett. 61 2908

[38] Chen Q, Lin M R, Lee J E, Zhang Q M and Yin S 2006Appl. Phys. Lett. 89 141121

[39] Lu C, Cui Z, Li Z, Yang B and Shen J 2003 J. Mater. Chem.13 526

[40] Lu C, Guan C, Liu Y, Cheng Y and Yang B 2005 Chem. Mater.17 2448

[41] Cassano T, Tommasi R, Babudri F, Cardone A, Farinola G Mand Naso F 2002 Opt. Lett. 27 2176

[42] Dinu M 2003 IEEE J. Quantum Electron. 39 1468[43] Lin Q, Zhang J, Piredda G, Boyd R W, Fauchet P M and

Agrawal G P 2007 Appl. Phys. Lett. 91 021111[44] Sheik-Bahae M, Hutchings D C, Hagan D J and

van Stryland E W 1991 IEEE J. Quantum Electron.27 1296

[45] Balu M, Hales J, Hagan D J and van Stryland E W 2005Opt. Express 13 359

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