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NanoporousPiezo-andFerroelectricThinFilms

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PaulaFerreira

UniversityofAveiro

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MarcGeorgWillinger

FritzHaberInstituteoftheMaxPlanckSociety

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PaulaVilarinho

UniversityofAveiro

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ChristelLaberty-Robert

PierreandMarieCurieUniversity-Paris6

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Nanoporous Piezo- and Ferroelectric Thin FilmsPaula Ferreira,*,† Ru Z. Hou,† Aiying Wu,† Marc-Georg Willinger,† Paula M. Vilarinho,† Jadra Mosa,‡

Christel Laberty-Robert,‡ Cedric Boissiere,‡ David Grosso,‡ and Clement Sanchez‡

†Departamento de Engenharia Ceramica e do Vidro, CICECO, Universidade de Aveiro, Campus Universitario de Santiago, 3810-193Aveiro, Portugal‡Laboratoire de Chimie de la Matiere Condensee de Paris, UMR-7574 CNRS, UPMC Universite Paris 06, College de France, 11Place Marcelin Berthelot, 75231 Paris, France

*S Supporting Information

ABSTRACT: Nanoporous barium titanate and lead titanate thin films (∼100 nmcalculated from ellipsometric data) are prepared starting from sol−gel solutions modifiedwith a commercially available block-copolymer and evaporation-induced self-assemblymethodology. The tuning of the thermal treatment followed by in situ ellipsometryallows the decomposition of the organic components and of the structuring agent leadingto the formation of porous tetragonal crystalline perovskite structures as observed byXRD, HRTEM, SEM, and ellipsoporosimetry. Both nanoporous barium titanate and leadtitanate thin films present local piezoelectric and ferroelectric behavior measured bypiezoresponse force microscopy (PFM), being promising platforms for the preparationof the generation of new multifunctional systems.

■ INTRODUCTIONFerroelectric materials are characterized by a switchablespontaneous polarization, exhibiting piezoelectricity, highnonlinear optical activity, pyroelectricity, and nonlineardielectric behavior.1 These properties are suitable forapplications in the form of bulk or thin films. Ferroelectricfilms are applied in a wide number of commercial applicationsand are still in development for application in nonvolatilememories, microwave electronic components, microdeviceswith pyroelectric and piezoelectric microsensors, and actua-tors.2 Nowadays with the current trend of increasingfunctionality, speed, and portability, there is a real need todecrease the size and weight of devices and materials and at thesame time enhance their performances. The generation ofporosity in the films can be seen as an avenue to achieve lightermaterials, giving the opportunity to further use the voids toincorporate other functionalities in the same area. Somereported works describe the preparation of porous films3−7

mainly for pyroelectric applications highlighting that theporosity would decrease the dielectric permittivity leading toenhancement of the voltage response. Porous lead calciumtitanate films with 490 nm of thickness were prepared by spin-coating.3,4 Films treated with different heating rates result eitherin porous or in dense microstructures with low or highdielectric permittivity. It was verified in the case of the porousfilms that the effective piezoelectric coefficients are increased incomparison with those of dense films probably due to thesmaller constraining effect of the substrate.4 A sol−gel processwas used for the deposition of porous lead zirconium titanate(PZT).5 In this case the porosity was generated by using apolymer and could be controlled by using polymers with

different molecular weights added in different concentration.No information is given in the paper5 concerning the nature ofthe polymer. The films had huge thickness between 0.9 and 1.4μm. Thick films of ca. 600 nm, having crystallite size average of66 nm and variable porosity of PZT, were prepared by additionof organic macromolecular poly(vinyl pyrrolidone) (PVP).6

The dielectric permittivity value decreased from 348 for densefilms to 298 and 48 for 15% and 40% porous films, respectively.Similar work was reported in 20077 using PVP in differentconcentrations to achieve different degrees of porosity anddifferent thickness. The films showed ferroelectric behavior.The films with a high degree of porosity show the lowestdielectric constant. The porosity in these films is completelyrandom, and although some electric properties are measured, itis important to emphasize that the films were generally ratherthick and with large grain size.3−7

In the present work, we report the synthesis and character-ization of ferroelectric thin film matrices (∼100 nm) withaccessible pores. In 2004,8 a noncommercial block copolymerof hydrogenated poly(butadiene-co-ethylene oxide) was used toprepare nanocrystalline multimetallic inorganic mesoporousthin films of SrTiO3, MgTa2O6 and CoxTi(1−x)O(2−x). Themeso-ordering and the demixing of the cations duringcrystallization were avoided through a careful control of thethermodynamic and kinetic parameters during the self-assemblyof the micelles and the heat treatment. So far, this study wasnever followed to prepare other perovskites. Also no properties

Received: October 24, 2011Revised: December 29, 2011Published: December 30, 2011

Article

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were measured in the prepared films. Here, we report the firstexample of the synthesis and the measurement of ferroelectricproperties of porous barium titanate and lead titanate thin films.Barium titanate and lead titanate are important room-temperature lead-free and lead-containing ferroelectric materi-als. By a simple one-pot method using evaporation-inducedself-assembly (EISA),9 we dip-coated modified sol−gelsolutions containing a commercially available amphiphilicpolystyrene−poly(ethylene oxide) (PS−PEO) block copolymertemplate. The optimization of the solutions was a ratherdifficult process since most of the barium and lead sources haverelatively low solubility in the volatile solvents required for theEISA technique. Moreover in the case of the barium ion, it alsohas the disadvantage of easily forming barium chloride orcarbonate, which may lead to cation separation. The effect ofthe temperature on both crystallization of the film andmicrostructure is followed by various techniques such asscanning and transmission electron microscopies (SEM andTEM) and X-ray diffraction (XRD). The local piezoelectricityand ferroelectricity are characterized by piezoresponse forcemicroscopy (PFM). This is the first example of the synthesis ofmultimetallic oxide perovskite thin films achieving tunableporosities and ferroelectric behavior that can find applicationon the preparation of multifunctional composites such asmultiferroic materials.10

■ EXPERIMENTAL SECTIONPreparation of the Porous Thin Films. A solution containing

barium hydroxide monohydrate (Aldrich, purity ≥98.0% w/w) orlead(II) acetate trihydrate (Fluka, purity ≥99.5% w/w) and glacialacetic acid (Merck) was heated to 70 °C for 10 min to allow thedissolution. Afterward 2,4-pentanedione (Fluka, purity 99.3% w/w)and titanium(IV) n-butoxide (Aldrich) were slowly added to thesolution. Separately, the block copolymer poly(styrene-b-ethyleneoxide) [PS(40000)−PEO(45000) from Polymer Source Inc. (P4740-SEO]] was dissolved in tetrahydrofuran (Normapur) and absoluteethanol (Normapur). The block copolymer solution was slowly addedto the precursor solution. The final solution was stirred and heated at70 °C for 15 min for homogenization. Table 1 shows the chemical

composition of the starting solutions used to synthesize BaTiO3 andPbTiO3 thin films. The films were deposited by dip coating(withdrawal rate of 1.6 mm/s) onto 1.5 × 2.5 cm2 sized slides ofsilicon or platinized silicon (Pt/TiO2/SiO2/Si) wafers. The depositionwas done at 8% relative humidity and 30 °C. Subsequently, all thefilms were treated in air up to 350 °C using a heating rate of 1 °C/minand kept at this temperature for 1 h in order to complete the inorganiccondensation of the matrix and to decompose the organic parts of theprecursors and copolymer template. The films were then calcined for 2min at the desired temperatures (625, 700, and 750 °C) to achievecrystallization.

Characterization Techniques. Ellipsometry measurements (ther-mal ellipsometry analysis (TEA) and environmental ellipsometryporosimetry (EEP)) were performed on a UV−vis variable-anglespectroscopic ellipsometer (VASE) from Woollam, and data analyseswere performed with the WVase32 software. Measurements werefitted between 500 and 1000 nm. The optical properties of the Sisubstrate at room temperature and at 828 °C were used in the fitting.Si wafer was used as substrate because it is too difficult to performTEA on a multilayer system. The ellipsometer was fitted with a small,variable humidity flow chamber (SOPRA) flushed with variable ratiosof air and ethanol vapor, tuned by a mass flow controller. Texturalcharacteristics of the film were investigated by EEP using water asadsorbate. Powder X-ray diffraction (XRD) patterns were recorded ina Philips X’Pert MPD X-ray diffractometer using Cu Kα radiation and2° grazing incidence angle. Sherrer equation11 was applied to calculatethe crystallite mean size attending to the fact that instrumental peakwidth is 0.34° (for NIST standard 660a) close to the sample peakchosen for calculation (hkl 111). However, the obtained crystallite sizemight deviate from the real value because we are not considering thestresses induced by the subtract on the film. Transmission electronmicroscopy (TEM) images were taken using a JEOL 2200FS in orderto assess the porosity of the films and the crystallinity of the BaTiO3walls. Samples were prepared by evaporating a drop of ethanolsuspensions containing flakes of scratched films on a holey carbon-coated copper grid. Scanning electron microscopy (SEM) images werecollected using a high-resolution scanning electron microscope (SU-70, Hitachi). Piezoelectric force microscopy (PFM) analyses werecarried out in a modified atomic force microscope in PFM modeNanoscope III, Digital Instruments, using silicon−SPM sensor with Alcoating tips (Nanosensors, l = 125 μm, resonant frequency of 204−497 kHz, force constant of 10−130 N/m). The topographic images ofthe film surface were taken simultaneously with the domain imagesand were collected in noncontact mode. The absolute values of thepiezoelectric coefficient for both strain and field along the polar axis(d33) were not determined. All data concerning the d33 signal is givenin arbitrary units. Since the results were obtained with the samecantilever and identical scanning and acquisition conditions, thecomparison could be made using relative values of local piezoelectriccoefficients for different films. Tip artifacts were excluded by usingfresh tips and calibrated samples to check their resolution. Severalgrains were measured for each sample to ensure that the hysteresisloops were reproducible.

■ RESULTS AND DISCUSSION

Figure 1a,b presents the results obtained by TEA and EEP onthe BaTiO3 thin films. In situ TEA for the as-prepared BaTiO3thin film was performed (using a 10 °C/min ramp) toinvestigate the thermally driven chemical processes suffered bythe film under heating. Figure 1a displays the variation of theindex of refraction and film thickness as a function of thetemperature and results from the fitting of the ellipsometricdata using the optical properties of the Si substrate at roomtemperature and at 828 °C. The refraction index variation canbe used to understand the chemical processes taking placeinside the film such as solvent evaporation, residual carbona-ceous species decomposition, template removal, and evolutionof the porous structure upon barium titanate crystallization.Initially a gradual contraction of the film and a small reductionof the refraction index due to the increase of porosity by releaseof the occluded species (such as solvents, organics decom-position) are observed. The pyrolysis of the block copolymeroccurs in two steps between 150 and 330 °C. Thepoly(ethylene oxide) decomposes around 200 °C, while thestyrene decomposition occurs near 280 °C. The pyrolysis of thepolymer blocks creates porosity, so a sharp decrease of therefraction index and of thickness can be observed. Between 530and 600 °C, the index of refraction increases due to beginning

Table 1. Chemical composition of the Initial Solutions Usedfor the Preparation of BaTiO3 and PbTiO3 Thin Films

Reagents BaTiO3 PbTiO3

P4740−SEO 0.040 g 0.060 gCH3CH2OH 0.750 g (16.3 mmol) 1.125 g (24.5 mmol)C4H8O 0.250 g (3.5 mmol) 0.375 g (5.20 mmol)Ba(OH)2·H2O 0.078 g (0.4 mmol)Pb(CH3COO)2·3H2O 0.228 g (0.617 mmol)CH3COOH 0.500 g (8.3 mmol) 0.375 g (6.25 mmol)Ti(OC4H9)4 0.140 g (0.4 mmol) 0.210 g (0.617 mmol)C5H8O2 0.029 g (0.3 mmol) 0.044 g (0.439 mmol)

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of the crystallization of the barium titanate phase. A secondincrease of the refraction index is observed above 660 °C,which can be related with the sintering process. The filmthicknesses after heat treatment were determined based on theTEA experiments and are presented in Table 2. Figure 2 shows

the SEM and TEM images recorded from BaTiO3 films afterheat treatment at 625 (625-BaTiO3), 700 (700-BaTiO3), and750 °C (750-BaTiO3). As expected, the temperature had asignificant influence on the structural development of the films.While all films present porosity, the organization of the pores

degraded upon thermal treatment above 625 °C due to theinduced crystallization. Films treated at 625 °C are charac-terized by nearly spherical pores with an average diameter of∼30 nm. High-resolution TEM (HRTEM) and selected areadiffraction (SAED) (Figure 1) reveal that up to thistemperature, the films are mainly amorphous, showing justsome small crystalline domains. This is confirmed by thecorresponding X-ray difractogram shown in Figure 3a, wherethe absence of characteristic peaks points to an amorphous film.As expected, the thermal annealing of the samples at 700 °C ledto the formation of crystalline nanoparticles (Figure 2b). Graingrowth further continues when the annealing temperature isincreased to 750 °C (Figure 2c). As a consequence of crystalgrowth and the development of crystalline facets, the pores losetheir organization considerably. X-ray diffraction patternsrecorded for the 700 and 750 °C treated samples (Figure 3a)confirmed the formation of BaTiO3. The asymmetry of theXRD peak in the range between 44−46° 2θ indicates atetragonal structure for the BaTiO3 (for comparison, thereflections corresponding to BaTiO3 in the tetragonal phaseaccording to the JCPDS database card no. 04-012-8129 areshown as solid lines in Figure3a). No other crystalline phasescould be detected. The XRD-determined crystallite sizes of700-BaTiO3 and 750-BaTiO3 films are 16 and 24 nm. Thesevalues were calculated using the Scherrer equation and the peakat 39° 2θ, which is always a single peak, regardless of the cubicor tetragonal crystallographic phases (Table 2).The water adsorption−desorption isotherms using EEP

confirmed the accessibility of the pores of the films (Figure1b shows as example the isotherms of the film treated at 700°C). Furthermore, the porous volume determined using theBruggeman effective medium approximation (EMA) modelestablished a porous volume fraction ranging from 26.4% onthe 625-BaTiO3 film to 30% in the case of the 750-BaTiO3

(Table 2). Although the porosity organization in the 750-

Figure 1. (a) In situ thermal-ellipsometric analysis for a BaTiO3 filmheated at a ramp rate of 10 °C/min. The index of refraction (at 700nm) for the heating cycle is represented by the curve with the symbol(+). The film thickness variation during the heating is represented bythe curve with symbol (×). The initial film thickness was 307 nm. (b)EEP water adsorption−desorption isotherms for the film treated at700 °C.

Table 2. Physical Properties of Porous BaTiO3 Films As aFunction of the Thermal Treatment

temperature,°C

film thickness(nm)a

pore volume(%)b

crystallite size from XRD(nm)c

625 124 26.4700 114 24.6 16750 113 30.0 24

aCalculated from the ellipsometry measurement on a BaTiO3 filmprepared in silicon wafers calcined in air for 2 min at the desiredtemperature. bObtained from the H2O adsorption isothermdetermined using environmental ellipsometric porosimetry. cCalcu-lated using the Scherrer equation and the peak at 39° 2θ, which isalways a single peak either in BaTiO3 cubic or tetragonal crystallo-graphic phases.

Figure 2. TEM, HRTEM, and SAED images of porous BaTiO3 thinfilm network after thermal treatment at (a) 625, (b) 700, and (c) 750°C. The insets in the left column show the aspect of the surface of thefilms as observed by SEM. Insets in the central column show thecorresponding power spectrum and demonstrate the presence oflattice fringes in the images.

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BaTiO3 film is severely modified by the crystal growth (seeSEM and TEM images in Figure 2c), the film remains porous.No significant differences in thickness were observed byellipsometric measurement among the 625-BaTiO3, 700-BaTiO3 and 750-BaTiO3 films (Table 2). The synthesizedfilms exhibit thickness of ∼100 nm. We found that establishingnetworked BaTiO3 crystalline nanoparticles with controlled sizeand homogeneous morphology requires a thermal annealing at700 °C.Lead-based ferroelectric mesostructured films were also

investigated. In order to follow the crystallization and themesostructure, different heat treatments in air have beenperformed.SEM images in Figure 4 display the surface morphology of

PbTiO3 films heat treated at (a) 350 (350-PbTiO3), (b) 600(600-PbTiO3), and (c) 700 °C (700-PbTiO3). Films calcined at350 °C (Figure 4a) show porosity with a quasihexagonal order.From the image contrast of the 350-PbTiO3 film, it can beconcluded that most of the copolymer decomposed since themajority of the pores are empty after treatment at 350 °C.Upon increase of the calcination temperature to 600 °C, thefilms lose their continuous amorphous character, and theformation of nanoparticles with a corresponding degradation ofthe meso-organization can be observed. The pore walls seem tobe formed by aggregates of nanoparticles. Further increase ofthe temperature led to crystal growth; however the film remainsporous. The XRD patterns in Figure 3b confirmed that the

films at 350 °C are amorphous, while the films treated at 600and 700 °C are crystalline. The single crystalline phase isassigned to tetragonal PbTiO3 structure (for comparison,reflections of PbTiO3 in tetragonal phase according to theJCPDS database card no. 04-008-8645 are shown as solid linesin Figure 3b). The heat treatment at 600 °C seems to besufficient to achieve crystallization and a good compromise interms of keeping the nanoporosity.In order to investigate the ferroelectric behavior of the

porous BaTiO3 and PbTiO3 thin films, a modified atomic forcemicroscope (AFM) was used to measure the local piezoelectricresponse of the two perovskite films as a function of the applieddc bias between the platinized substrate and the conductingAFM tip. For comparative reasons, we studied the 700-BaTiO3,750-BaTiO3, and 700-PbTiO3 films. The topographic views,piezoresponse signals (domain images), and local piezoelectrichysteresis loops (piezoloops, measured inside individual grains)of the 700-BaTiO3, 750-BaTiO3, and 700-PbTiO3 films areshown in Figure 5. Topographic images reveal the crystallitestructure of the films with clearly resolved morphologicalfeatures (Figure 5a,d,g). The pores by AFM are not clearlydefined due to the use of contact mode, which is the requiredmode for PFM but not the best suited one for resolving the filmtopography. Clearly, an increase of the nanocrystalline size isverified in BaTiO3 films when the temperature of the treatmentrises from 700 to 750 °C. Both film compositions exhibitpiezoelectric response domain images (Figure 5b,e,h). Differentcontrasts are observed, indicating opposite polarities. Darkregions correspond to domains with polarization orientedtoward the substrate and bright regions to domains withpolarization terminated at the free surface of the film. The

Figure 3. X-ray diffraction patterns of (a) BaTiO3 and (b) PbTiO3thin films thermally treated at different temperatures. For comparison,the solid lines depict the BaTiO3, PbTiO3, and Pt reflections accordingto the JCPDS card nos. 04-012-8129, 04-008-8645, and 04-001-3301,respectively.

Figure 4. SEM images showing the typical morphology of PbTiO3porous thin film network after thermal treatment at: (a) 350, (b) 600,and (c) 700 °C.

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pores can be observed in the topographic images as darkregions and remain dark in the PFM images. The grains arerandomly split into domains of opposite polarities (please seearrows in Figure 5). We observed better defined domains in the750-BaTiO3 film than in the 700-BaTiO3 one. This may beexplained by the higher crystallinity of the film or based on theferroelectric property size dependence.12 It is reported that aprogressive reduction of tetragonal distortion, heat oftransition, Curie temperature, and relative dielectric constantwere observed with a decrease of the physical size of theferroelectric particles. Indeed, the ferroelectricity can com-pletely vanish below a critical size that is very dependent on thesynthesis method of preparation.11

Typical local piezoloops measured in the different filmswhere the tip is located inside individual grains are illustrated inFigure 5c,f,i. The observed hysteresis loops unambiguouslyconfirm the ferroelectric behavior of these films at roomtemperature. The 700-BaTiO3 and 750-BaTiO3 films exhibittilted loops. It is evident that the 750-BaTiO3 loop is betterdefined and more symmetric than that of the 700-BaTiO3 films.This behavior can be related to the increase of the particle sizeor of the crystallinity of the film. It should also be consideredthat the roughness of the surface and the uncertainty of the tipposition may play a role in the loop profile. For the films ofBaTiO3 treated at temperature lower than 700 °C, no suchloops could be recorded probably because the structures werenot tetragonal or the crystallite sizes were too small. The 700-

PbTiO3 film shows a rather squared loop, which is typical forferroelectric materials. The applied voltage necessary to achievethe polarization switching in PbTiO3 is significantly lower thanthe one for BaTiO3. The piezoresponse in the case of thePbTiO3 film is at least 1 order of magnitude higher than theresponse of the BaTiO3. Harnagea et al.

13 reported a study inthin films about the type of information given by thepiezoresponse force microscopy as a quantitative tool. Theauthors observed although tetragonal BaTiO3 and PbTiO3 havesimilar point group symmetry, in the BaTiO3 the out-of-planepiezoelectric response signal does not simply describe the out-of-plane polarization due to the high value of d15 comparedwith d33, while in the case of PbTiO3, the longitudinalpiezoelectric coefficient has a linear relation with the out-of-plane polarization, so a very high out-of-plane piezoelectricresponse signal means a spontaneous polarization perpendic-ular to the film plane.

■ CONCLUSIONS

In summary, this work is the first example of the preparationand characterization of the ferroelectric properties of perovskiteporous functional thin films (thickness ∼100 nm). A veryefficient and simple methodology that allows a fair control ofcomposition and nanostructure has been used to synthesize forthe first time ferroelectric porous thin films of barium titanateand lead titanate. EISA of typical sol−gel solutions containing alarge commercially available block copolymer produced porous

Figure 5. Simultaneously obtained topographic (a, d, g) and piezoresponse images (b, e, h) images and local piezoelectric hysteresis loops of 700-BaTiO3 (c), 750-BaTiO3 (f), and 700-PbTiO3 (i) porous thin films.

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amorphous networks. We found that the thermal processingused to convert the amorphous porous mesostructuredmultimetallic oxide thin films into a crystalline perovskite filmis a key point to achieve both size and shape monodispersity inparticles and pore sizes and in keeping nanoporosity. Puresingle tetragonal phases of barium titanate and lead titanatewere obtained. Piezoresponse force microscopy demonstratedthat porous thin films of BaTiO3 and PbTiO3 show localelectric properties with typical hysteresis piezoresponse loopsand domain images. Hence, the films are piezoelectric andferroelectric. Furthermore, within the present work, we haveestablished that amorphous porous ordered mesostructuredfilms could be obtained, and these are highly promising asplatforms to construct multifunctional ordered distributedcomposite materials with an original architecture if the poresare filled just before crystallization.

■ ASSOCIATED CONTENT*S Supporting InformationPhase loop for the BaTiO3 thin films. This material is availablefree of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Tel: +351 234401419. Fax: +351 234401470. E-mail:pcferreira@ua.pt.

■ ACKNOWLEDGMENTSThe authors thank FCT and FEDER (QREN − COMPETE)for funding the project PTDC/CTM/098130/2008, FCT/CNRS program, and the European Network of ExcellenceFAME as well as the Portuguese network of electronmicroscopy, the RNME, FCT Project REDE/1509/RME/2005. Nathalie Barroca is acknowledged for help with the PFMmeasurements.

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