Nanoporous Piezo- and Ferroelectric Thin Films - Langmuir (ACS

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Nanoporous Piezo- and Ferroelectric Thin Films Paula Ferreira,*,† Ru Z. Hou,† Aiying Wu,† Marc-Georg Willinger,† Paula M. Vilarinho,† Jadra Mosa,‡ Christel Laberty-Robert,‡ Cédric Boissière,‡ David Grosso,‡ and Clément Sanchez‡ †

Departamento de Engenharia Cerâmica e do Vidro, CICECO, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal ‡ Laboratoire de Chimie de la Matière Condensée de Paris, UMR-7574 CNRS, UPMC Université Paris 06, Collège de France, 11 Place Marcelin Berthelot, 75231 Paris, France S Supporting Information *

ABSTRACT: Nanoporous barium titanate and lead titanate thin films (∼100 nm calculated from ellipsometric data) are prepared starting from sol−gel solutions modified with a commercially available block-copolymer and evaporation-induced self-assembly methodology. The tuning of the thermal treatment followed by in situ ellipsometry allows the decomposition of the organic components and of the structuring agent leading to the formation of porous tetragonal crystalline perovskite structures as observed by XRD, HRTEM, SEM, and ellipsoporosimetry. Both nanoporous barium titanate and lead titanate thin films present local piezoelectric and ferroelectric behavior measured by piezoresponse force microscopy (PFM), being promising platforms for the preparation of the generation of new multifunctional systems.

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INTRODUCTION Ferroelectric materials are characterized by a switchable spontaneous polarization, exhibiting piezoelectricity, high nonlinear optical activity, pyroelectricity, and nonlinear dielectric behavior.1 These properties are suitable for applications in the form of bulk or thin films. Ferroelectric films are applied in a wide number of commercial applications and are still in development for application in nonvolatile memories, microwave electronic components, microdevices with pyroelectric and piezoelectric microsensors, and actuators.2 Nowadays with the current trend of increasing functionality, speed, and portability, there is a real need to decrease the size and weight of devices and materials and at the same time enhance their performances. The generation of porosity in the films can be seen as an avenue to achieve lighter materials, giving the opportunity to further use the voids to incorporate other functionalities in the same area. Some reported works describe the preparation of porous films3−7 mainly for pyroelectric applications highlighting that the porosity would decrease the dielectric permittivity leading to enhancement of the voltage response. Porous lead calcium titanate films with 490 nm of thickness were prepared by spincoating.3,4 Films treated with different heating rates result either in porous or in dense microstructures with low or high dielectric permittivity. It was verified in the case of the porous films that the effective piezoelectric coefficients are increased in comparison with those of dense films probably due to the smaller constraining effect of the substrate.4 A sol−gel process was used for the deposition of porous lead zirconium titanate (PZT).5 In this case the porosity was generated by using a polymer and could be controlled by using polymers with © 2011 American Chemical Society

different molecular weights added in different concentration. No information is given in the paper5 concerning the nature of the polymer. The films had huge thickness between 0.9 and 1.4 μm. Thick films of ca. 600 nm, having crystallite size average of 66 nm and variable porosity of PZT, were prepared by addition of organic macromolecular poly(vinyl pyrrolidone) (PVP).6 The dielectric permittivity value decreased from 348 for dense films to 298 and 48 for 15% and 40% porous films, respectively. Similar work was reported in 20077 using PVP in different concentrations to achieve different degrees of porosity and different thickness. The films showed ferroelectric behavior. The films with a high degree of porosity show the lowest dielectric constant. The porosity in these films is completely random, and although some electric properties are measured, it is important to emphasize that the films were generally rather thick and with large grain size.3−7 In the present work, we report the synthesis and characterization of ferroelectric thin film matrices (∼100 nm) with accessible pores. In 2004,8 a noncommercial block copolymer of hydrogenated poly(butadiene-co-ethylene oxide) was used to prepare nanocrystalline multimetallic inorganic mesoporous thin films of SrTiO3, MgTa2O6 and CoxTi(1−x)O(2−x). The meso-ordering and the demixing of the cations during crystallization were avoided through a careful control of the thermodynamic and kinetic parameters during the self-assembly of the micelles and the heat treatment. So far, this study was never followed to prepare other perovskites. Also no properties Received: October 24, 2011 Revised: December 29, 2011 Published: December 30, 2011 2944

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Characterization Techniques. Ellipsometry measurements (thermal ellipsometry analysis (TEA) and environmental ellipsometry porosimetry (EEP)) were performed on a UV−vis variable-angle spectroscopic ellipsometer (VASE) from Woollam, and data analyses were performed with the WVase32 software. Measurements were fitted between 500 and 1000 nm. The optical properties of the Si substrate at room temperature and at 828 °C were used in the fitting. Si wafer was used as substrate because it is too difficult to perform TEA on a multilayer system. The ellipsometer was fitted with a small, variable humidity flow chamber (SOPRA) flushed with variable ratios of air and ethanol vapor, tuned by a mass flow controller. Textural characteristics of the film were investigated by EEP using water as adsorbate. Powder X-ray diffraction (XRD) patterns were recorded in a Philips X’Pert MPD X-ray diffractometer using Cu Kα radiation and 2° grazing incidence angle. Sherrer equation11 was applied to calculate the crystallite mean size attending to the fact that instrumental peak width is 0.34° (for NIST standard 660a) close to the sample peak chosen for calculation (hkl 111). However, the obtained crystallite size might deviate from the real value because we are not considering the stresses induced by the subtract on the film. Transmission electron microscopy (TEM) images were taken using a JEOL 2200FS in order to assess the porosity of the films and the crystallinity of the BaTiO3 walls. Samples were prepared by evaporating a drop of ethanol suspensions containing flakes of scratched films on a holey carboncoated copper grid. Scanning electron microscopy (SEM) images were collected using a high-resolution scanning electron microscope (SU70, Hitachi). Piezoelectric force microscopy (PFM) analyses were carried out in a modified atomic force microscope in PFM mode Nanoscope III, Digital Instruments, using silicon−SPM sensor with Al coating tips (Nanosensors, l = 125 μm, resonant frequency of 204− 497 kHz, force constant of 10−130 N/m). The topographic images of the film surface were taken simultaneously with the domain images and were collected in noncontact mode. The absolute values of the piezoelectric coefficient for both strain and field along the polar axis (d33) were not determined. All data concerning the d33 signal is given in arbitrary units. Since the results were obtained with the same cantilever and identical scanning and acquisition conditions, the comparison could be made using relative values of local piezoelectric coefficients for different films. Tip artifacts were excluded by using fresh tips and calibrated samples to check their resolution. Several grains were measured for each sample to ensure that the hysteresis loops were reproducible.

were measured in the prepared films. Here, we report the first example of the synthesis and the measurement of ferroelectric properties of porous barium titanate and lead titanate thin films. Barium titanate and lead titanate are important roomtemperature lead-free and lead-containing ferroelectric materials. By a simple one-pot method using evaporation-induced self-assembly (EISA),9 we dip-coated modified sol−gel solutions containing a commercially available amphiphilic polystyrene−poly(ethylene oxide) (PS−PEO) block copolymer template. The optimization of the solutions was a rather difficult process since most of the barium and lead sources have relatively low solubility in the volatile solvents required for the EISA technique. Moreover in the case of the barium ion, it also has the disadvantage of easily forming barium chloride or carbonate, which may lead to cation separation. The effect of the temperature on both crystallization of the film and microstructure is followed by various techniques such as scanning and transmission electron microscopies (SEM and TEM) and X-ray diffraction (XRD). The local piezoelectricity and ferroelectricity are characterized by piezoresponse force microscopy (PFM). This is the first example of the synthesis of multimetallic oxide perovskite thin films achieving tunable porosities and ferroelectric behavior that can find application on the preparation of multifunctional composites such as multiferroic materials.10

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EXPERIMENTAL SECTION

Preparation of the Porous Thin Films. A solution containing barium hydroxide monohydrate (Aldrich, purity ≥98.0% w/w) or lead(II) acetate trihydrate (Fluka, purity ≥99.5% w/w) and glacial acetic acid (Merck) was heated to 70 °C for 10 min to allow the dissolution. Afterward 2,4-pentanedione (Fluka, purity 99.3% w/w) and titanium(IV) n-butoxide (Aldrich) were slowly added to the solution. Separately, the block copolymer poly(styrene-b-ethylene oxide) [PS(40000)−PEO(45000) from Polymer Source Inc. (P4740SEO]] was dissolved in tetrahydrofuran (Normapur) and absolute ethanol (Normapur). The block copolymer solution was slowly added to the precursor solution. The final solution was stirred and heated at 70 °C for 15 min for homogenization. Table 1 shows the chemical

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RESULTS AND DISCUSSION Figure 1a,b presents the results obtained by TEA and EEP on the BaTiO3 thin films. In situ TEA for the as-prepared BaTiO3 thin film was performed (using a 10 °C/min ramp) to investigate the thermally driven chemical processes suffered by the film under heating. Figure 1a displays the variation of the index of refraction and film thickness as a function of the temperature and results from the fitting of the ellipsometric data using the optical properties of the Si substrate at room temperature and at 828 °C. The refraction index variation can be used to understand the chemical processes taking place inside the film such as solvent evaporation, residual carbonaceous species decomposition, template removal, and evolution of the porous structure upon barium titanate crystallization. Initially a gradual contraction of the film and a small reduction of the refraction index due to the increase of porosity by release of the occluded species (such as solvents, organics decomposition) are observed. The pyrolysis of the block copolymer occurs in two steps between 150 and 330 °C. The poly(ethylene oxide) decomposes around 200 °C, while the styrene decomposition occurs near 280 °C. The pyrolysis of the polymer blocks creates porosity, so a sharp decrease of the refraction index and of thickness can be observed. Between 530 and 600 °C, the index of refraction increases due to beginning

Table 1. Chemical composition of the Initial Solutions Used for the Preparation of BaTiO3 and PbTiO3 Thin Films Reagents P4740−SEO CH3CH2OH C4H8O Ba(OH)2·H2O Pb(CH3COO)2·3H2O CH3COOH Ti(OC4H9)4 C5H8O2

BaTiO3 0.040 0.750 0.250 0.078

g g (16.3 mmol) g (3.5 mmol) g (0.4 mmol)

0.500 g (8.3 mmol) 0.140 g (0.4 mmol) 0.029 g (0.3 mmol)

PbTiO3 0.060 g 1.125 g (24.5 mmol) 0.375 g (5.20 mmol) 0.228 0.375 0.210 0.044

g g g g

(0.617 mmol) (6.25 mmol) (0.617 mmol) (0.439 mmol)

composition of the starting solutions used to synthesize BaTiO3 and PbTiO3 thin films. The films were deposited by dip coating (withdrawal rate of 1.6 mm/s) onto 1.5 × 2.5 cm2 sized slides of silicon or platinized silicon (Pt/TiO2/SiO2/Si) wafers. The deposition was done at 8% relative humidity and 30 °C. Subsequently, all the films were treated in air up to 350 °C using a heating rate of 1 °C/min and kept at this temperature for 1 h in order to complete the inorganic condensation of the matrix and to decompose the organic parts of the precursors and copolymer template. The films were then calcined for 2 min at the desired temperatures (625, 700, and 750 °C) to achieve crystallization. 2945

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Figure 2. TEM, HRTEM, and SAED images of porous BaTiO3 thin film 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 the films as observed by SEM. Insets in the central column show the corresponding power spectrum and demonstrate the presence of lattice fringes in the images.

degraded upon thermal treatment above 625 °C due to the induced crystallization. Films treated at 625 °C are characterized by nearly spherical pores with an average diameter of ∼30 nm. High-resolution TEM (HRTEM) and selected area diffraction (SAED) (Figure 1) reveal that up to this temperature, the films are mainly amorphous, showing just some small crystalline domains. This is confirmed by the corresponding X-ray difractogram shown in Figure 3a, where the absence of characteristic peaks points to an amorphous film. As expected, the thermal annealing of the samples at 700 °C led to the formation of crystalline nanoparticles (Figure 2b). Grain growth further continues when the annealing temperature is increased to 750 °C (Figure 2c). As a consequence of crystal growth and the development of crystalline facets, the pores lose their organization considerably. X-ray diffraction patterns recorded for the 700 and 750 °C treated samples (Figure 3a) confirmed the formation of BaTiO3. The asymmetry of the XRD peak in the range between 44−46° 2θ indicates a tetragonal structure for the BaTiO3 (for comparison, the reflections corresponding to BaTiO3 in the tetragonal phase according to the JCPDS database card no. 04-012-8129 are shown as solid lines in Figure3a). No other crystalline phases could be detected. The XRD-determined crystallite sizes of 700-BaTiO3 and 750-BaTiO3 films are 16 and 24 nm. These values were calculated using the Scherrer equation and the peak at 39° 2θ, which is always a single peak, regardless of the cubic or tetragonal crystallographic phases (Table 2). The water adsorption−desorption isotherms using EEP confirmed the accessibility of the pores of the films (Figure 1b shows as example the isotherms of the film treated at 700 °C). Furthermore, the porous volume determined using the Bruggeman effective medium approximation (EMA) model established a porous volume fraction ranging from 26.4% on the 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 film heated at a ramp rate of 10 °C/min. The index of refraction (at 700 nm) for the heating cycle is represented by the curve with the symbol (+). The film thickness variation during the heating is represented by the curve with symbol (×). The initial film thickness was 307 nm. (b) EEP water adsorption−desorption isotherms for the film treated at 700 °C.

of the crystallization of the barium titanate phase. A second increase of the refraction index is observed above 660 °C, which can be related with the sintering process. The film thicknesses after heat treatment were determined based on the TEA experiments and are presented in Table 2. Figure 2 shows Table 2. Physical Properties of Porous BaTiO3 Films As a Function of the Thermal Treatment temperature, °C

film thickness (nm)a

pore volume (%)b

crystallite size from XRD (nm)c

625 700 750

124 114 113

26.4 24.6 30.0

16 24

a

Calculated from the ellipsometry measurement on a BaTiO3 film prepared in silicon wafers calcined in air for 2 min at the desired temperature. b Obtained from the H 2 O adsorption isotherm determined using environmental ellipsometric porosimetry. cCalculated using the Scherrer equation and the peak at 39° 2θ, which is always a single peak either in BaTiO3 cubic or tetragonal crystallographic phases.

the SEM and TEM images recorded from BaTiO3 films after heat treatment at 625 (625-BaTiO3), 700 (700-BaTiO3), and 750 °C (750-BaTiO3). As expected, the temperature had a significant influence on the structural development of the films. While all films present porosity, the organization of the pores 2946

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Figure 4. SEM images showing the typical morphology of PbTiO3 porous thin film network after thermal treatment at: (a) 350, (b) 600, and (c) 700 °C. Figure 3. X-ray diffraction patterns of (a) BaTiO3 and (b) PbTiO3 thin films thermally treated at different temperatures. For comparison, the solid lines depict the BaTiO3, PbTiO3, and Pt reflections according to the JCPDS card nos. 04-012-8129, 04-008-8645, and 04-001-3301, respectively.

films at 350 °C are amorphous, while the films treated at 600 and 700 °C are crystalline. The single crystalline phase is assigned to tetragonal PbTiO3 structure (for comparison, reflections of PbTiO3 in tetragonal phase according to the JCPDS database card no. 04-008-8645 are shown as solid lines in Figure 3b). The heat treatment at 600 °C seems to be sufficient to achieve crystallization and a good compromise in terms of keeping the nanoporosity. In order to investigate the ferroelectric behavior of the porous BaTiO3 and PbTiO3 thin films, a modified atomic force microscope (AFM) was used to measure the local piezoelectric response of the two perovskite films as a function of the applied dc bias between the platinized substrate and the conducting AFM tip. For comparative reasons, we studied the 700-BaTiO3, 750-BaTiO3, and 700-PbTiO3 films. The topographic views, piezoresponse signals (domain images), and local piezoelectric hysteresis loops (piezoloops, measured inside individual grains) of the 700-BaTiO3, 750-BaTiO3, and 700-PbTiO3 films are shown in Figure 5. Topographic images reveal the crystallite structure of the films with clearly resolved morphological features (Figure 5a,d,g). The pores by AFM are not clearly defined due to the use of contact mode, which is the required mode for PFM but not the best suited one for resolving the film topography. Clearly, an increase of the nanocrystalline size is verified in BaTiO3 films when the temperature of the treatment rises from 700 to 750 °C. Both film compositions exhibit piezoelectric response domain images (Figure 5b,e,h). Different contrasts are observed, indicating opposite polarities. Dark regions correspond to domains with polarization oriented toward the substrate and bright regions to domains with polarization terminated at the free surface of the film. The

BaTiO3 film is severely modified by the crystal growth (see SEM and TEM images in Figure 2c), the film remains porous. No significant differences in thickness were observed by ellipsometric measurement among the 625-BaTiO3, 700BaTiO3 and 750-BaTiO3 films (Table 2). The synthesized films exhibit thickness of ∼100 nm. We found that establishing networked BaTiO3 crystalline nanoparticles with controlled size and homogeneous morphology requires a thermal annealing at 700 °C. Lead-based ferroelectric mesostructured films were also investigated. In order to follow the crystallization and the mesostructure, different heat treatments in air have been performed. 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 at 350 °C (Figure 4a) show porosity with a quasihexagonal order. From the image contrast of the 350-PbTiO3 film, it can be concluded that most of the copolymer decomposed since the majority of the pores are empty after treatment at 350 °C. Upon increase of the calcination temperature to 600 °C, the films lose their continuous amorphous character, and the formation of nanoparticles with a corresponding degradation of the meso-organization can be observed. The pore walls seem to be formed by aggregates of nanoparticles. Further increase of the temperature led to crystal growth; however the film remains porous. The XRD patterns in Figure 3b confirmed that the 2947

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

pores can be observed in the topographic images as dark regions and remain dark in the PFM images. The grains are randomly split into domains of opposite polarities (please see arrows in Figure 5). We observed better defined domains in the 750-BaTiO3 film than in the 700-BaTiO3 one. This may be explained by the higher crystallinity of the film or based on the ferroelectric property size dependence.12 It is reported that a progressive reduction of tetragonal distortion, heat of transition, Curie temperature, and relative dielectric constant were observed with a decrease of the physical size of the ferroelectric particles. Indeed, the ferroelectricity can completely vanish below a critical size that is very dependent on the synthesis method of preparation.11 Typical local piezoloops measured in the different films where the tip is located inside individual grains are illustrated in Figure 5c,f,i. The observed hysteresis loops unambiguously confirm the ferroelectric behavior of these films at room temperature. The 700-BaTiO3 and 750-BaTiO3 films exhibit tilted loops. It is evident that the 750-BaTiO3 loop is better defined and more symmetric than that of the 700-BaTiO3 films. This behavior can be related to the increase of the particle size or of the crystallinity of the film. It should also be considered that the roughness of the surface and the uncertainty of the tip position may play a role in the loop profile. For the films of BaTiO3 treated at temperature lower than 700 °C, no such loops could be recorded probably because the structures were not tetragonal or the crystallite sizes were too small. The 700-

PbTiO3 film shows a rather squared loop, which is typical for ferroelectric materials. The applied voltage necessary to achieve the polarization switching in PbTiO3 is significantly lower than the one for BaTiO3. The piezoresponse in the case of the PbTiO3 film is at least 1 order of magnitude higher than the response of the BaTiO3. Harnagea et al.13 reported a study in thin films about the type of information given by the piezoresponse force microscopy as a quantitative tool. The authors observed although tetragonal BaTiO3 and PbTiO3 have similar point group symmetry, in the BaTiO3 the out-of-plane piezoelectric response signal does not simply describe the outof-plane polarization due to the high value of d15 compared with d33, while in the case of PbTiO3, the longitudinal piezoelectric coefficient has a linear relation with the out-ofplane polarization, so a very high out-of-plane piezoelectric response signal means a spontaneous polarization perpendicular to the film plane.

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CONCLUSIONS In summary, this work is the first example of the preparation and characterization of the ferroelectric properties of perovskite porous functional thin films (thickness ∼100 nm). A very efficient and simple methodology that allows a fair control of composition and nanostructure has been used to synthesize for the first time ferroelectric porous thin films of barium titanate and lead titanate. EISA of typical sol−gel solutions containing a large commercially available block copolymer produced porous 2948

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amorphous networks. We found that the thermal processing used to convert the amorphous porous mesostructured multimetallic oxide thin films into a crystalline perovskite film is a key point to achieve both size and shape monodispersity in particles and pore sizes and in keeping nanoporosity. Pure single tetragonal phases of barium titanate and lead titanate were obtained. Piezoresponse force microscopy demonstrated that porous thin films of BaTiO3 and PbTiO3 show local electric properties with typical hysteresis piezoresponse loops and domain images. Hence, the films are piezoelectric and ferroelectric. Furthermore, within the present work, we have established that amorphous porous ordered mesostructured films could be obtained, and these are highly promising as platforms to construct multifunctional ordered distributed composite materials with an original architecture if the pores are filled just before crystallization.

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ASSOCIATED CONTENT

S Supporting Information *

Phase loop for the BaTiO3 thin films. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +351 234401419. Fax: +351 234401470. E-mail: [email protected].

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ACKNOWLEDGMENTS The authors thank FCT and FEDER (QREN − COMPETE) for funding the project PTDC/CTM/098130/2008, FCT/ CNRS program, and the European Network of Excellence FAME as well as the Portuguese network of electron microscopy, the RNME, FCT Project REDE/1509/RME/ 2005. Nathalie Barroca is acknowledged for help with the PFM measurements.

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REFERENCES

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