Journal of Non-Crystalline Solids 356 (2010) 1689–1695

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Journal of Non-Crystalline Solids

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Second harmonic generation in SiO2 sol–gel films functionalized withEthyl-[4-(4-nitro-phenylazo)-phenyl]-(2-oxiranylmethoxy-ethyl)-amine(ENPMA) molecules

Alfredo Franco a,⁎, Giovanna Brusatin a, Massimo Guglielmi a, Glauco Stracci b, Fabio De Matteis b,Mauro Casalboni b, Heiner Detert c, Bernd Grimm d, Sigurd Schrader d

a Dipartimento di Ingegneria Meccanica, Settore Materiali, Università degli studi di Padova, Via Marzolo 9, 35131, Padova, Italyb Dipartimento di Fisica, Università degli studi di Roma – Tor Vergata – and INSTM, Via della Ricerca Scientifica 1, I-00133 Roma, Italyc Institute of Organic Chemistry, Johannes-Gutenberg University Mainz, Duesbergweg 10-14, D55099 Mainz, Germanyd Engineering Physics, University of Applied Sciences Wildau, Bahnhofstrasse, 15745 Wildau, Germany

⁎ Corresponding author. Tel.: +52 55 56225103; fax:E-mail address: alfredofranco@fisica.unam.mx (A. Fr

0022-3093/$ – see front matter © 2010 Elsevier B.V. Aldoi:10.1016/j.jnoncrysol.2010.06.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 February 2010Received in revised form 25 May 2010Available online 12 July 2010

Keywords:Second harmonic generation;Sol–gel films;Corona poling;Chromophores

High concentrations of Ethyl-[4-(4-nitro-phenylazo)-phenyl]-(2-oxiranylmethoxy-ethyl)-amine (ENPMA)chromophores were linked covalently to two different kinds of SiO2 matrices prepared by the sol–gelmethod. The matrices differed in their reticulation level, determined by the organometallic precursor usedduring the synthesis. The materials were deposited as films by spin-coating. The chromophores in the filmswere oriented in a non-centrosymmetric way by a modified Corona poling set-up, and the nonresonantsecond order non-linear optical coefficient d33(0) of the films was determined by means of the Maker fringetechnique. It was found that there is an optimum concentration of chromophores for attaining the highestd33(0) value in these materials. Their maximum d33(0) value was found in the films with the least reticulatedmatrix, that value was equal to 5.2±0.5 pm/V. Besides, the d33(0) value was more stable in time in the lessreticulated materials.

+52 55 56225011.anco).

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Sol–gel films containing second order non-linear optical chromo-phores are materials with potential technological applications aselectro-optic devices [1–3]. However, these materials have someproblems to overcomewhen the chromophores are oriented by classicalcorona technique, mainly: (1) the restricted area with electro-opticproperties which is obtained by a classical single point corona electricfield, (2) the lowelectro-optic coefficient values and (3) their poor long-term stability. These features are intimately related to the number ofchromophores oriented in such away that thematerials exhibit a lack ofinversion symmetry [4,5].

The Ethyl-[4-(4-nitro-phenylazo)-phenyl]-(2-oxiranylmethoxy-ethyl)-amine (ENPMA) chromophores, used in the present work, havethe most of the properties of the so well known N-Ethyl-N-(2-hydroxyethyl)-4-(4-nitro-phenylazo)aniline (commonly called DisperseRed1orDR1) chromophore,mainlywith respect to thepermanentdipolemoment and the second order hyperpolarizability, but with one extraadvantage: the ENPMA chromophores have an epoxy ring in one of theirextremes, it allows a non-conventional way to link covalently this kind of

chromophores to a sol–gel inorganic matrix, which gives place tomaterialswithahighchromophore content and in consequencematerialswith the possibility to exhibit high electro-optic coefficients.

In order to avoid the dipole–dipole chromophore aggregation usuallypresent in materials with high loads of dipolar chromophores; in thiswork a carbazole molecule was used as a good chromophore spacer [6].

Introduction of chromophores into SiO2 sol–gel materials has theadvantage that their matrices reveal a high rigidity and high glasstransition temperature in contrast tomany polymers commonly used forelectro-optic purposes, e.g. polymethylmetacrylate (PMMA). Thus, inprinciple, the inorganic matrices synthesized via the sol–gel methodallow a better stability of the chromophore orientation, which should bereflected in a better stability of thematerial's electro-optic coefficient [7].

In this work the materials were designed in such a way that theyhave a soft matrix which allows an easy achievement of a non-centrosymmetric average orientation of chromophores at the highestpossible level during the first stage of the corona poling process. Afterheating the material, as part of the corona poling process, the sol–gelmatrix reticulates even more, restricting in this way the mobility ofchromophores inside the material. This leads to an enhanced long-term orientational stability of chromophores.

Two kinds of sol–gel matrices were studied. They differ in thenumber of alcoxide precursor reactive branches. The resulting matrixshould be more rigid as more branches react.

1690 A. Franco et al. / Journal of Non-Crystalline Solids 356 (2010) 1689–1695

The chromophores in the material were oriented in a non-centrosymmetric way by means of a modified Corona poling set-up.This novel Corona poling set-up produces large electro-optic activeareas in the material. The quantitative determination of their d33(0)non-linear optical coefficient was carried out in-situ during the polingprocess by means of second harmonic generation (SHG) measure-ments, using the Maker fringe technique. Some recent results aboutpromising poled materials considered for electro-optic applicationshave d33(0) values in the region between 1.2 and 16.6 pm/V [5,8–12],thus the values obtained in our materials are high enough to beconsidered useful for potential electro-optic applications.

2. Experimental techniques and materials

2.1. ENPMA chromophore synthesis and characterization

Disperse red 1 was purchased from Aldrich. Disperse red (3.14 g,0.01 mol) was dissolved in benzene (100 mL), Aliquat 336 (0,8 g),powdered KOH (1.20 g, 0.018 mol) and epichlorohydrin (2.20 g,0.024 mol) were added. After stirring for 5 days at ambient temper-ature, the mixture was filtered, the solvent and residual epichlorohy-drine were removed in vacuo and the residue was dissolved intoluene/ethyl acetate (10/1). Purification by column chromatographyon silica gel using toluene/ethyl acetate (10/1). as eluent. Rf 0.24Yield: 3,03 g (82%) m.p. 95 °C; MS (FD): 370,2 [M+.], HNMR (CDCl3)δ=1.24 (t, 3H), 2.60 (dd, 1H), 2.82 (dd, 1H), 3.22 (m, 1 H), 3.44–3.58(m, 3H), 3.79 (ddd2H), 6.80 (d, 2H), 7.85–7.93 (2xd, 4H), 8.33 (d, 2H);Elemental analysis(C19H22N4O4, 370.176) calc. 61,59 C 5.99 H 15,13 N;found 61,92 C 5.79 H 14,88N (Fig. 1).

The ENPMAmolecule has a very rigid quasi cylindrical shape, with alarge permanent dipole moment aligned almost along the mainmolecular axis. The large dipole moment is due to the presence of astrong acceptor group at one of the terminal positions of the moleculeand the presence of a conjugated bonds system along the molecule,which is responsible for its good microscopic second order non-linearoptical properties. In fact, the ENPMA chromophore has practically thesame properties of the very well known Disperse Red 1 (DR1)chromophore, but with an epoxy ring at one of its ends, in such a waythat the ENPMA chromophore can be linked to the matrix in a differentway than the usual procedure followed for the covalent linkage of theDR1 chromophores to a SiO2 sol–gel matrix.

2.2. Materials synthesis and characterization

Two kinds of SiO2 sol–gel matrices were functionalized with a highconcentration of ENPMA chromophores. These two matrices differ bytheir degree of reticulation.

One of the matrices was made using tetraethyl ortosilicate (TEOS)as alkoxysilane precursor. The other one was made using phenyl-trimethyl ortosilicate (PhTMS) as alkoxysilane precursor.

The main difference between the two precursors is that TEOS hasfour branches which can take part in sol–gel reactions but the PhTMShas only three of them. Thus, the films produced from PhTMS can haveat most three covalent bonds for each Si atom to thematrix, comparedto four covalent bonds for each Si atom in case of matrices producedfrom TEOS. Hence, films synthesized from TEOS are more reticulatedthan those synthesized from PhTMS.

Fig. 1. Synthesis of Ethyl-[4-(-nitro-phenylazo]-(2-oxira

The synthesis of the sol–gel matrices followed the steps,schematically depicted in Fig. 2:

(1) Pre-hydrolysis of TEOS (PhTMS) and (3-Glycidyloxypropyl)trimethoxysilane (GPTMS). GPTMS and TEOS (PhTMS) weremixed and stirred in a solvent, in our case methanol (MeOH).Somewaterwas addeddropwise. Everything is kept under refluxduring 4 h at 80 °C. If the precursor was TEOS, the resulting sol isnamed as GT. If the precursor was PhTMS, the sol is named asGPhT.

(2) In another batch ENPMA chromophores and Carbazole spacermolecules were dissolved together in 2-Methoxyethanol.

(3) (3-Aminopropyl)trimethoxysilane (APTMS) was added to thechromophore–spacer mixture and subsequently the mixturewas stirred by a magnet stirrer.

(4) Finally, the GT (GPhT)was added to themixture of step (3). It isleft at 40 °C under magnetic stirring over night. Related to theused precursor the resulting sols are named as GTA or GPhTA.

For preparationof theGT (GPhT) sol, the [TEOS (PhTMS):GPTMS:H2O:MeOH] molar ratios between the reactants were [1:2.33:6.66:18.33].

For preparation of the GTA (GPhTA) sols containing an ENPMAmolar concentration, with respect to the SiO2 moles, equal to 20%, 30%and 40%, the molar ratios [ENPMA:APTMS:GT(GPhT):CbOH:Methox-yethanol] (with CbOH–1-Ethanolylcarbazole) were: [1:1:4:2.9:373],[1:1:2.33:2.9:373], and [1:1:1.5:2.9:373] respectively. In these casesthe ENPMA:APTMS molar ratio is always 1:1, which means that thereis the same quantity of amine groups and chromophores, thus inprinciple all the chromophores are linked to the matrix and there areno amine groups remaining after linkage.

However, another set of films was prepared with an exceedingquantity of amino groups, in order to promote a second matrixreticulation during poling of the samples, between the APTMS aminogroups and the epoxy rings of the GT (GPhT) sol. In these cases thecriteria was tomaintain the same number of moles between the APTMSand the GT (GPhT). Thus, the preparation of this second set of GTA(GPhTA) sols containinganENPMAmolar concentration,with respect tothe SiO2 moles, equal to 20%, 30% and 40%, the [ENPMA:APTMS:GT(GPhT):CbOH:Methoxyethanol] molar ratios were: [1:2.5:2.5:2.9:373],[1:1.66:1.66:2.9:373], and [1:1.25:1.25:2.9:373], respectively.

The GPTMS is a precursor with four active branches, three of themcan be covalently linked to the rest of the inorganic matrix throughsol–gel reactions, but the other one can covalently be linked throughthe reaction of its epoxy group with an amine group. The epoxyfunctionality can favorably be used during heat application; it meansthat after heat treatment (which ideally occurs during corona poling)the films can become even more reticulated.

The ENPMA chromophores can be covalently linked to the inorganicmatrix through opening of their epoxy ring, just as described for GPTMSin thepreviousparagraph. TheENPMAepoxy ring reacts underpresenceof an amine group. In these materials, the amine group is provided bythe APTMS precursor. The APTMS molecule is a precursor with fouractive branches; three of them can be covalently linked to the rest of theinorganic matrix through sol–gel reactions, but the other one has anaminegroup. This amine group can reactwith the epoxy rings present inthe ENPMA chromophores and in the GPTMS precursor molecules.

The GPTMS and the APTMS have an extra function in the material;they avoid a possible non desirable protonation of the chromophores,

nylmethoxy-ethyl)-amine (ENPMA) chromophores.

Fig. 2. Scheme of the materials synthesis. (a) Prehydrolization of the precursors. Obtained products: GT and GPhT. (b) Linkage of ENPMA to APTMS. Obtained product: ENPMA–APTMS. (c) Sol formation. Obtained products: GTA and GPhTA. Common solvent: Methoxyethanol.

1691A. Franco et al. / Journal of Non-Crystalline Solids 356 (2010) 1689–1695

preserving optical properties of the chromophores. The film deposi-tion was done by spin-coating onto BK7 soda lime glasses. The glasseswere cleaned with acetone and distilled water and dried undernitrogen flow.

The film deposition was made after filtering with a 0.2 μm pore sizesyringe filter; the films were directly deposited by spin-coating ontoBK7 substrates at 1000 rpm for 30 s in a clean room, where the relativehumidity was equal to 30% and the temperature equal to 20 °C. Afterdeposition, thefilmswereheated at a rate equal to 1 °C/min and cured at100 °C for 150 min.

Thickness and **spectral dispersion measurements of the sampleswere performed with a Wvase (Woollam Co., Inc.) spectroscopicellipsometer with a wavelength resolution of ±0.03 nm. Therefractive index was obtained fitting the experimental ellipsometricdata to a Cauchy model in the spectral region far apart from anyabsorption resonance wavelength [13] and to a Tauc–Lorentz modelin the region of absorbance [14].

The deposition of the films was carried out within a short time (fewweeks) after the GT and GPhT sol preparation, because aging of the solscauses an increase in the number of reacted groups in the precursors, asit is possible to see from the infrared absorption spectra of the films (seeFig. 3). The increase in the number of reacted groups in the precursorsimplies an increase in the number of covalent bonds in the matrix, i.e.

these films becomemore reticulated. Infrared absorption spectra in therange 4500–400 cm−1 were recorded by Fourier transform infraredspectroscopy (FTIR) using a Jasco 620 Spectrometer with a resolution of±2 cm−1.

From these FTIR spectra it is not clear whether there existsubstantially different covalent links in GT and GPhT sols, but as wedescribe later, the non-linear optical responses of GTA and GPhTAfilms are quite different. It means too, that the local environmentaround chromophores and the matrix reticulation are the uniqueparameters corresponding to the inorganic part of the films which candefine the efficiency with which the chromophores orient in a non-centrosymmetric way.

2.3. Corona poling and second harmonic generation set-ups

The experimental set-up used for chromophore orientation is amodified version of the conventional single tip corona poling set-up.An intense electric field is produced across the films when the electriccharges, created by ionization of the atmosphere surrounding somethin wires, deposit onto the film surface. The ionization of theatmosphere gas is caused by the strong electric field created at a sharpneedle, a wire or a grid placed at a high electric potential (of the order

Fig. 3. FTIR spectra for the GT and GPhT sols showing their ageing effect. The spectra arenormalized with respect to the largest peak.

1692 A. Franco et al. / Journal of Non-Crystalline Solids 356 (2010) 1689–1695

of kilovolts) with respect to a metallic plate which works as a groundelectrode and, at the same time, as a sample holder and as a heater.

The corona system used in this work (Fig. 4) consists of eightvertical thin metallic tips, all of them disposed over the sample at adistance of 4 cm. The system of eight tips at a same electrical potentialrotates as a whole in order to have a very homogeneous poled area inthe films.

Fig. 4. Scheme of the modified Corona poling set-up. V: voltage source. A: Amperemeter.W: angular speed of the tips.w: fundamental frequency. 2w: second harmonic frequency.

The electrical current between the electrodes is controlledexternally in order to have a constant electric field across the sample.All the system is electrically isolated and immersed in a nitrogenatmosphere in order to control the humidity and, consequently, tolimit the flow of electric charges towards the sample.

This system allows the existence of large homogeneous areas ofnon-centrosymmetric material without damage, which has beenverified measuring the d33(0) coefficient in several different places ofthe sample. This is an important feature since the manufacture ofelectro-optic devices requires large and homogenous poled areas (ofthe order of several cm2).

The determination of the d33(0) parameter was carried out bymeans of Maker fringe measurements. The optical set-up for theoptical measurements uses a Q-switched Nd-YAG laser at 1064 nm,this laser is coupled to a solid-state Raman-shifter which shifts thebeam wavelength to 1368 nm. The intensity and the polarization ofthe laser beam were controlled using a half-wave plate and a Glan–Taylor polarizer. A 10/90 beam-splitter divides the beam between areference and a measurement line.

The insertion of a combination of mirrors allows the performanceof in-situ second harmonic generation measurements during thepoling procedure. Fig. 5 shows a scheme of the SHG set-up.

The second order non-linear macroscopic coefficient d33(0) wascalculated considering the intensity of the second harmonic beam oflight I2ω

pgenerated by the poled materials, and using the intensity of

the second harmonic beam of light generated by a y-cut quartz slab asa reference (quartz coefficient: d11=0.30 pm/V at 1064 nm) [15]. Forp-polarized light the SHG intensity is:

Ip2ω = Fpd2eff t4ωT2ωp

2I2ωsin2½ΨðφÞ�ðn2

ω−n22ωÞ2

; ð1Þ

where Fp is a geometrical factor, tω and T2ω are the transmission Fresnelcoefficients for the fundamental and second harmonic beam of light,respectively, p is a form factor, Iω is the intensity of the fundamentalbeam, nω and n2ω the refractive indices at the fundamental and at thesecond harmonic frequencies, deff is an effective SHG coefficient whichcan be written as [16]:

deff = d33 sin2φω sinφ2ω + d31 cos

2φω sinφ2ω + 2d15 sinφω cosφω cosφ2ω;

ð2Þ

Fig. 5. Scheme of the second harmonic generation (SHG) set-up. BS: 10/90 beam-splitter, P: linear polarizer, λ/2: half-wave plate, Q: Y-cut quartz reference, L: lens, M:kinematic mirror, M-PhT: monochromator (resolution of 0.2 nm)with photomultiplier,OD: neutral density filter, S: sample holder, m: dichroic mirror, F0: highpass-filter at850 nm, F1: lowpass-filter at 750 nm, F2: highpass-filter at 420 nm.

Fig. 6. Plot of refractive index vs. wavelength for films made with (a) GTA or (b) GPhTAmatrices functionalized with ENPMA. The decrease of the refractive index after poling isdue to the lack of Carbazole moieties in the film after poling.

1693A. Franco et al. / Journal of Non-Crystalline Solids 356 (2010) 1689–1695

with φ the refraction angle andΨ(φ) a function given by:

Ψ φð Þ = 2πLλðnω cosφω−n2ω cosφ2ωÞ; ð3Þ

where L is the thickness of thefilm and λ is the free-spacewavelength ofthe fundamental beam.

Far from optical resonance, Kleinman symmetry can be assumedyielding the identity d31=d15. Moreover, it has been shown that forpoled samples one has the approximate relation d31≈d

33

�3 [16].

Following the calculation of d33(0) usually reported in literature[17] the error has been estimated at 10%.

Additionally the non-centrosymmetric orientation of the chromo-phores inside thefilmswasmonitored byUV–Vis spectroscopy, throughthe calculation of the second order parameter (A2). This parameter wascalculated by means of the following equation [18–20]:

A2 = 1−OD⊥OD0

; ð4Þ

where OD⊥ and OD0 are the optical densities of a film, measured withnon-polarized light at normal incidence, after and before the polingprocess, respectively. The UV–Vis spectra were measured with aPerkin-Elmer Lambda 19 spectrophotometer, with a wavelengthresolution equal to 0.05 nm and an optical absorbance relativeuncertainty equal to 1%.

3. Results

The films had a typical thickness of 520.1±1.7 nm just afterdeposition. The thicknesses of the films reduce after the corona polingprocess. The samples made with a GTA matrix reduce their thicknessby ca. 30%, and those made from GPhTA by ca. 10%.

The refractive index dispersions of the samples with ENPMA:SiO2:APTMS molar ratios equal to 0.3:1:0.3 are shown in Fig. 6 for thespectral range of 450–1700 nm. At the fundamental wavelength(1368.00±0.03 nm) the extraordinary refractive indices are 1.519±0.004 for GTA and 1.515±0.003 for GPhTA samples. At the secondharmonic wavelength (684.00±0.03 nm) the extraordinary refrac-tive indices are 1.554±0.008 and 1.544±0.005 for GTA and GPhTA,respectively. Both of these wavelengths are far apart from anyabsorption resonance wavelength of the ENPMA non-linearchromophores.

The characteristic UV–Vis spectra of the films with ENPMA:SiO2:APTMS molar ratios equal to 0.3:1:0.3, before and after their poling,are shown in Fig. 7.

The calculation of the second order parameter (A2), just after thepoling of each material, gives the following values: 0.10±0.01 for theGTA films and 0.15±0.01 for the GPhTA ones.

On the other hand, the d33(0) coefficient was determined using thefilm thickness and refractive index obtained by ellipsometricmeasurements for each of the samples.

The maximum value obtained for the d33(0) coefficient was equalto 5.2±0.5 pm/V for the GPhTA matrix, and 4.8±0.5 pm/V for theGTAmatrix. These values were determined 2.5 h after poling at 120 °Cwith a voltage between the electrodes equal to 11.00±0.01 kV.

The stability of the d33(0) value was followed as function of timeafter the poling process for each kind of sample at room temperatureand under darkness. The results show, as expected, that the d33(0)coefficient becomes smaller with time due to random thermalagitation of the chromophores. A plot of the d33(0) coefficient asfunction of time is reported in Fig. 8.

Finally, in case of samples with an ENPMA molar concentration inthe SiO2 matrix equal to 20% or 40% the d33(0) value was smaller thanthat for samples with a concentration equal to 30%, as can be seen inTable 1.

4. Discussion

The films with a very large reticulation do not provide enoughmobility to the chromophores, necessary for their adequate non-centrosymmetric orientation under the appliance of a large electrostaticfield, like that obtained bymeans of the corona technique. From the FTIRspectra of Fig. 3 it is possible to see that both, the GT and the GPhT,exhibit a similar behavior: the strong increase in time of the shoulderband around 1132 cm−1, corresponding to Si–O–Si bonds, and thestrong decrease of the peaks around 910 cm−1 and 3395 cm−1,corresponding to silanol groups, indicate a notable reticulation sixmonths after sol preparation. From these FTIR spectra it is not clearwhether there exist substantially different covalent links inGT andGPhTsols, but as we describe later, the non-linear optical responses of GTAand GPhTA films are quite different. It means too, that the localenvironment around chromophores and the matrix reticulation are theunique parameters corresponding to the inorganic part of the filmswhich candefine theefficiencywithwhich the chromophoresorient in anon-centrosymmetric way.

In Fig. 6 it is possible to see the behavior of the ordinary andextraordinary indices of refraction just after the corona poling process;the difference between the indices of refraction at the fundamental andat the second harmonic wavelength for the extraordinary refractiveindices is always around 0.03. These values are important for thedetermination of the d33(0) values through the Eqs. (1)–(3). The

Fig. 7. UV–Vis spectra of films with ENPMA:SiO2:APTMS molar ratios equal to 0.3:1:0.3,before and after poling for films made with (a) GTA or (b) GPhTA matrices functionalizedwith ENPMA.

Table 1Second order non-linear coefficient as a function of the chromophore concentration.

Chromophoreconcentration

20% 30% 40% 20% 30% 40%

GTA GTA GTA GPhTA GPhTA GPhTA

d33(0) (pm/V) 3.8±0.3 4.8±0.5 4.1±0.4 4.1±0.4 5.2±0.5 2.4±0.2

1694 A. Franco et al. / Journal of Non-Crystalline Solids 356 (2010) 1689–1695

refractive indices dispersions reveal that some of the chromophoresremain orientated along the preferential direction established by thecorona field. A quantitative measure of the percentage of aligned

Fig. 8. Normalized second harmonic coefficient as function of time. The straight linesare drawn as guide to the eyes.

chromophores in a preferential direction is donebymeansof theUV–Visoptical absorption spectra shown in Fig. 7 through the determination ofthe A2 parameter. In fact, the A2 parameter obtained values imply thatthe GPhTA matrix allows the orientation of a larger number ofchromophores than the GTA matrix does. This orientation is inagreement with the anisotropy detected in the samples by theellipsometry and by second harmonic generation signal measurements.

From Fig. 7 it is possible to see too that the main peak position ispositioned at 494 nm for the GTAmatrices and at 493 nm for the GPhTones. After the poling process, the peaks become lower and they shifttowards shorter wavelengths; actually the shift is 12 nm for bothkinds of matrices. In all the cases the peak width is always the same.The hypochromic shift as well as the change in the wavelength of themain peak in the UV–Vis spectra is due to the electric field generatedacross the film by the electric charges deposited on the surface of thematerial during the Corona poling process [18,19].

Also from the UV–Vis spectra it is possible to distinguish thepresence of the carbazole molecules in the films just before the polingprocess. The carbazole molecules exhibit some characteristic peaks inthe UV region of the spectra, thus it is possible to check the lack ofCarbazole molecules in the UV–Vis spectra after the poling process.These spacermoleculeswerenot covalently linked, and leave thematrixupon heating. Therefore, they are not present inside the material at theendof the heating step of the Coronapoling process. It is consistentwiththe fact that, as can be seen in Fig. 6, before poling the refractive indicesare higher than those measured after poling, because the carbazolemoieties tend to increase the index of refraction in somematerials [21].

From the plot shown in Fig. 8 one can conclude that the GPhTAmatrices show higher time stability of the second order non-linearoptical coefficient compared to the materials made with GTA. Theresults reveal that the chromophores loose their non-centrosymmet-ric orientation in the GTAmatrix faster than in the GPhTA one, despitethe fact that the former one is more reticulated than the latter one.The GPhTA matrix maintains phenyl groups attached to the matrixeven after the corona poling process. These phenyl groups could act aschromophore spacers too, hindering the dipole–dipole interactionsbetween the chromophores and, consequently, providing a highernon-linear optical coefficient [22].

It means that there are two main results which show that GPhTAmatrix improves the optical responses of the GTA ones, these tworesults show that these matrices act in a different way on the hostedchromophores. Summarizing, these couple of results consist of a 10%larger value of the d33(0) coefficient for the GPhTA matrix and a 25%larger value of the d33(0) coefficient for the GPhTA matrix 60 daysafter the poling, which means a significant improved stability withrespect to the films made with the GTA matrix.

It is evident fromTable1, that there exists anoptimumconcentrationof chromophores in the films, presumably at that concentration wherethere is an optimum balance between a large chromophore concentra-tion and a weak chromophore–chromophore interaction exists. Lowerconcentrations of chromophores imply less quantity of moleculescontributing to SHG, instead, larger concentrations of chromophoresimply stronger chromophore–chromophore interaction hindering theirnon-centrosymmetric alignment [23].

As a final comment, in the case of sampleswith a large concentrationof APTMS, their d33(0) value is quite small. Larger concentrations ofamine groups imply a larger reticulation of the matrices when heated.

1695A. Franco et al. / Journal of Non-Crystalline Solids 356 (2010) 1689–1695

Presumably, this reticulation is attained during the poling process,before the electrostatic orientation of the chromophores is completed,reducing their finally achieved non-centrosymmetric alignment.

5. Conclusions

SiO2 sol–gel films containing high concentrations of covalently boundENPMAmoleculeswere successfully synthesized. Thesefilmswere of twodifferent types differing in their reticulation level and chemicalcomposition. The materials were poled by means of a novel coronapoling set-up which provides large, uniformly poled areas. Lessreticulated materials made of GPhTA and ENPMA reached a 10% largerd33(0) value than that obtained for the GTA:ENPMA films. Theexperimentally determined d33(0) value of the GPhTA-ENPMA filmswas5.24 pm/V. 60 days after poling, thismaterialwhich is less reticulatedshowed a 25% larger d33(0) value than that of the GTA:ENPMA films. Thismeans that the GPhTA-ENPMA films aremore stable, due to the presenceof phenyl groups in thematrix. TheGPhTA-ENPMAfilms are anewtypeofmaterials which can be considered for the fabrication of sol–gel filmelectro-optical devices for information and communication technologies,due to their d33(0) coefficient values and their stability.

Acknowledgements

The authors gratefully acknowledge the FIRB Italian project RBNE033KMA “Molecular compounds and hybrid nanostructured materials withresonant andnon resonant optical properties for photonic devices” for thefinancial support.

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