Confinement in Oriented Mesopores Induces Piezoelectric Behavior of

Publication Date (Web): October 16, 2012 ... leading to ultrahigh-aspect-ratio nanowires distributed throughout the templating host, and having up to ...
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Confinement in Oriented Mesopores Induces Piezoelectric Behavior of Polymeric Nanowires Valentina Cauda,*,† Bruno Torre,‡ Andrea Falqui,§ Giancarlo Canavese,† Stefano Stassi,† Thomas Bein,∇ and Marco Pizzi† †

Center for Space Human Robotics@PoliTo, Istituto Italiano di Tecnologia, C.so Trento, 21, 10129 Turin, Italy Nanophysics, Istituto Italiano di Tecnologia, via Morego, 30, 16163 Genoa, Italy § Nanochemistry, Istituto Italiano di Tecnologia, via Morego, 30, 16163 Genoa, Italy ∇ Department of Chemistry and Center of Nanoscience (CeNS), University of Munich, Butenandtstr. 11, 81377 Munich, Germany ‡

S Supporting Information *

ABSTRACT: We report on the preparation and the piezoelectric properties of ultrathin polymeric nanowires in the oriented pores of mesoporous silica, which are embedded in the channels of a supporting anodic alumina membrane. Poly(vinylidene difluoride) [PVDF] and its copolymer, poly(vinylidene difluoride trifluoroethylene) [PVTF], were both confined to two types of columnar silica mesopores of ∼5 and 10 nm in diameter. The extreme spatial confinement induces a preferential orientation of the crystalline domains of the polymer into a ferroelectric phase, leading to ultrahigh-aspect-ratio nanowires distributed throughout the templating host, and having up to 60 μm in length, comparable to the thickness of the hosting alumina. The resulting distributed array of piezoelectric nanowires are isolated from each other by a dielectric matrix, facilitating the handling and electrical contacting. We show, for the first time, that a remarkable piezo-response, in the absence of any poling or stretching, is obtained upon nanoconfinement on the PVDF polymer, which, in contrast, does not show any polarization when in bulk or film form without poling. The piezoelectric behavior was assessed by a piezo evaluation system (PES) and we visualized polar nanowire bundles via piezoresponse force microscopy (PFM). This “nano-structuration” represents a powerful approach, holding promise for applications for nanoactuators or bioinspired ciliated sensors with high sensitivity and resolution. KEYWORDS: ferroelectric polymeric nanowires, confined crystallization, PVDF, mesoporous silica, distributed electromechanical response

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INTRODUCTION The construction of piezoelectric sensors and actuators with nanostructured materials gives access to miniaturized tools with high sensitivity and spatial resolution. One possible approach toward surface distributed piezoelectric devices is the tailoring of chemistry and structure at the molecular level, to design a material intrinsically converting mechanical pressure into an electric signal or vice versa. With these goals in mind, the nanostructuration of a wide range of piezomaterials from ceramic PZT (lead zirconate titanate) to oxides, such as ZnO, and to polymers is under intensive investigation.1 In particular, poly(vinylidene difluoride) (PVDF) and its copolymer, poly(vinylidene difluoride trifluoroethylene) (PVTF), are attractive macromolecules in the context of piezoelectrics, having an intrinsic permanent dipole moment due to the spatial arrangement of the chain segments in the crystalline phase.2 PVDF can exist in five different crystalline forms: the β-phase shows the most pronounced ferroelectric behavior, having the unit cell with two all-trans chains packed with their dipoles pointing in the same direction (TTTT conformation). In contrast, PVTF readily crystallizes into the β-phase, since the trifluoroethylene monomer only shows the TTTT conformation.2,3 Remnant polarization of both polymers can be obtained by orienting the molecular dipoles in the same direction by © 2012 American Chemical Society

subjecting the polymers to mechanical stretching or to an intense electric field at elevated temperature (poling),4 thus inducing its transformation into the β-phase. The ferroelectric phase of PVDF can be also achieved from the melt using epitaxy on KBr,5 ice quenching,6 imprint,7 blending it with small amounts of poly(methyl methacrylate) (PMMA)8 or poly(o-methoxyaniline) (POMA),9 or using a polar solvent.10 When dissolved in a polar solvent, such as N,N′-dimethylformamide (DMF)11 or N,N′-dimethylacetamide (DMA),10 the PVDF undergoes a disruption of the strong interchain forces in the crystalline regions, thus the increased mobility can be used to promote its crystallization into the desired β-phase. Recently, the crystallization of the PVDF polymer was also shown to be affected by confinement.12 Solidifying the polymer from solution or the molten state in a restricted geometry, such as the pores of anodic alumina membranes (AAMs), increases the crystal orientation.13 In this way, arrays of one-dimensional (1D) nanowires supported within AAMs were obtained, showing a high aspect ratio with 150 nm in diameter and several micrometers in length.14 The confined crystallization of Received: August 14, 2012 Revised: September 28, 2012 Published: October 16, 2012 4215

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The piezoresponse was evaluated with atomic force microscopy (AFM) (ASYLUM, Model AFM MFP-3D) by contacting the bottom side of the sample with a sputtered Pt electrode and copper wire, and applying an electric field through the conductive AFM tip (Olympus OMLC-RC800PB double-sided gold-coated cantilever).

PVTF in 30-nm-wide horizontal organosilicate trenches was shown to improve the performance of a ferroelectric transistor.15 Other authors fabricated high-density arrays of PVTF cells by nanoembossing with 100-nm-deep cavities,16 obtaining oriented polymeric nanostructures. Apart from the use of the PVTF polymer for nonvolatile lowvoltage memories,15,16 most of the papers focus on the polymer confinement and the related materials characterization,12,13,17 whereas, to the best of our knowledge, direct measurements of the piezoelectric properties have not yet been reported. Here, we show the preparation of ultralong and nanometersized polymeric nanowires in the oriented pores of mesoporous silica, which are embedded in the channels of a supporting AAM. The extreme polymeric confinement induces a preferential orientation of the crystalline domains into a ferroelectric phase, leading to ultrahigh-aspect-ratio nanowires distributed throughout the templating host. The piezoelectric properties of both polymers confined into the tiny pores of the mesoporous silica were investigated for the first time by both a piezo evaluation system (PES) and piezoresponse force microscopy (PFM). Their unusual electromechanical properties could find an application as nanoactuators or bioinspired ciliated sensors showing high sensitivity and resolution. Significantly, the tuning of the crystal orientation by nanoconfinement into mesoporous templates holds promise for other materials and in other fields as well, such as sol−gel processing of ferroelectric precursors, or the preparation of organic memories, magnetic storage media, multiferroic systems, and other types of nanodevices.

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RESULTS AND DISCUSSION The confined self-assembled synthesis of mesoporous silica (MS) materials in porous AAMs (nominal pore size 100 nm, thickness 60 μm) was recently reported,18,19 showing high versatility regarding pore morphology and pore size. We prepared a columnar arrangement of the mesopores with pore sizes of (i) 5 nm (sample code MS5@AAM) and (ii) 10 nm (MS10@AAM) and proceeded with the wet impregnation of both PVDF and PVTF polymers into the mesopores (see Scheme 1). Scheme 1. Synthetic Pathway for the Preparation of Distributed Ultrathin Nanowires: From the Empty Porous Anodic Alumina Membrane (Back) to the Confined Synthesis of Columnar Mesoporous Silica (in Blue, Middle) to the Filling of the Mesopores with Piezoelectric Polymer (in Red, Front)a

EXPERIMENTAL SECTION

Mesoporous silica with a columnar structure parallel to the AAM channel axis (Anodisc Whatman, nominal pore size = 100 nm, thickness = 60 μm) was synthesized as already reported,18 obtaining mesopores (i) 5 nm in diameter (MS5@AAM) and (ii) 10 nm in diameter (MS10@AAM). Briefly, a silica-precursor solution was dropcast on the empty AAM and evaporation-induced self-assembly (EISA) of the silica mesostructure occurred under controlled humidity and temperature. After the surfactant removal by calcination, the MS@ AAM membranes were polished and evacuated at room temperature (RT) for 5 min in a round flask. Then, 0.7 mL of PVDF in DMA (0.7 wt %) or PVTF in MEK (0.7 wt %) were injected into the flask and after 30 min the samples were dried in air at 403 K for 2 h and then thoroughly washed with the respective solvent and mechanically polished. Reference polymeric films of both PVDF and PVTF were drop-cast from the same solvent solutions at 10 wt % and thermally treated in a similar manner. The samples were characterized by twodimensional (2D) small-angle X-ray scattering (SAXSess, Anton Paar), wide-angle X-ray diffractometry (XRD) (Bruker D8 Discover with Nifiltered Cu Kα radiation), infrared spectroscopy (Bruker Equinox 55 in absorption mode) and N2 sorption measurements (Quantachrome Nova 4000e). The MS@AAM host (in plane view) and the isolated MS filaments (upon alumina dissolution in H3PO4 5 wt %) were imaged by TEM (FEI Titan 80-300 at 300 kV). After dissolution of both mesoporous silica and alumina hosts in NaOH (4 M for 4 h), isolated bundles of polymer nanowires were deposited on a carbon film sitting on a TEM copper grid, then imaged by TEM (JEOL, Model JEM 2200-FS, 200 kV) under cryogenic (T = 93 K on a Gatan 626 sample holder) and low-dose irradiation conditions, in order to reduce the effects of the interaction between electron beam and specimen as much as possible. Piezoelectric hysteresis measurements were recorded simultaneously with sample displacement by a piezo evaluation system (PES, TFAnalyzer 2000HS, Aixacct) coupled to a single-point laser vibrometer (Polytec OVF-505) at 50 Hz, after sputter-coating the bottom and top Pt electrodes.

a

Images are not to scale.

Plane-view TEM images of the empty MS@AAM matrices after calcination (see Figures 1a and 1b) show a well-ordered hexagonal mesostructure with a pore diameter of ∼10 nm for sample MS10@AAM and 5 nm for sample MS5@AAM, respectively. Clear visualization of their columnar and collinear alignment is achieved by dissolving the alumina matrix (see Figures 1c and 1d). These structural features are confirmed by both nitrogen sorption and small-angle X-ray scattering (SAXS) data, as presented in the Supporting Information (see Figures S-1−S-3 and Table S-1). The mesoporous membranes could be loaded with the polymers by impregnating with solutions of PVDF and PVTF, respectively. Subsequently, careful washing and mechanical polishing of the templating AAM membrane was carried out to remove the superficial polymer films deposited on the membrane surface. Here, we demonstrate the formation of ultrathin polymeric nanowires in the collinear pores of mesoporous silica, which are embedded in the 100-nm-wide AAM channels. Upon dissolution of the MS@AAM host templates, polymeric bundles, measuring several micrometers in length and ∼100 nm in diameter, can be isolated, thus showing comparable dimensions of the MS structures in the AAM channels. These bundles are nanostructured into ultrathin wires 4216

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Figure 1. Transmission electron microscopy (TEM) images of columnar mesoporous silica in anodic alumina membranes: (a) plane-view and (c) lateral view after alumina dissolution of sample MS10@AAM; (b) plane-view into alumina channels and (d) lateral view after alumina dissolution of sample MS5@AAM. Bundles of columnar nanostructured polymers are obtained after dissolution of both silica and alumina hosts: (e) PVDF-MS5@AAM, (f) PVTFMS5@AAM, (g) PVDF-MS10@AAM, and (h) PVTF-MS10@AAM.

(see Figures 1e−h), each having a diameter close to the starting mesopore that acts as a structural template. Precise determination of the diameter of the ultrathin polymer wires is difficult, because of the fast degradation of the polymer under the electron beam (see Figure S-4 in the Supporting Information), as similarly reported in the literature and due to the inelastic scattering processes between the high-energy electrons and the organic matter.20 The unprecedented reduction of the diameter of the encapsulated polymer phase leads to ultrahigh-aspect-ratio nanowires, with, at most, 5- and 10-nm diameters, based on the mesopore size, and up to 60 μm in length, comparable to the alumina (AAM) thickness. This nanostructuration of both polymers allows us to study their crystallization and structural− electromechanical property relationships. The wide-angle X-ray scattering (WAXS) pattern of bulk PVDF (prepared as film from the same solvent; see trace 1 in Figure 2a) shows several diffraction peaks belonging to the α

Figure 2. WAXS patterns of the (a) PVDF- and (b) PVTF-templated MS@AAM. [Legend: trace 1, bulk PVDF polymer; trace 2, PVDFMS5@AAM; trace 3, PVDF-MS10@AAM; trace 4, empty template (for MS@AAM samples); trace 5, bulk PVTF polymer; trace 6, PVTFMS5@AAM; and trace 7, PVTF-MS10@AAM.] For clarity, the WAXS spectra are shifted along the y-axis by 1000 units each. (c) Scheme of the molecular orientation of the polymeric chain templated in the mesoporous host. Both the a- and c-axes are in-plane with the alumina surface, thus flat-on the circular mesopore wall, and the b-axis, as well as the polarization axis P, are aligned with the long axis of the nanowires.

nonpolar phase at 17.6°, 18.4°, 19.7°, and 27°, corresponding to the (100), (020), (110) reflections and to the overlapping of (120) and (111) reflections, respectively. Strikingly, both PVDF-MS5@AAM and PVDF-MS10@AAM samples (traces 2 4217

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nanofeatures approaches or is even below the size of the crystalline unit of the material, as shown in the present work. In addition, it is reported that, regardless of the type of polymer, the crystal nucleation and growth is favored in the direction parallel to the flat surface of the hard wall of the template,25 when slippery, repulsive hard walls and high crystallization temperatures are involved. In the present work, the hydrophilic silica surface is expected to show a repulsive effect, with respect to the highly hydrophobic polymers PVDF and PVTF. In addition, the fast growth direction of the polymers is the crystallographic b-axis. Combining these factors, we can conclude that the polymeric crystallites grow preferentially flat-on the templating surface channel, in order to accommodate the confined environment of the nanotemplate (thus, with the b-axis parallel to the long axis of the template).12,24 Therefore, the a- and c-axes should lie perpendicular to the mesopore long axis. As previously assessed,24 the c-axis should be parallel to the plane of the wall surface of the template, thus facilitating the formation of the nuclei flat-on the surface. A scheme of the polymer chain molecular orientation is presented in Figure 2c. Because of the axial symmetry of the templating structure (mesoporous circular walls), here, the labels of both the a- and c-axes represented in this scheme are attributed arbitrarily, and their position is interchangeable. The preferred orientation of the chain c-axis upon nanoconfinement into the mesoporous template also leads to favorable ferroelectric and piezoelectric properties in the direction perpendicular to the template surface. Actually, the dipole moment of both polymers turns on the c-axis upon the application of a vertical electric field perpendicular to the alumina and mesopore channels (as shown in Figure 2c by the red arrow and inset of Figure 3). Therefore, the polarization P is maximized along the long axis of the nanowires, because of the vertical orientation of the polar b-axis. In this configuration, the poling of both ultrathin polymeric nanowires is not required to obtain a good piezoelectric response. To confirm the previous assumptions and conclusively assess the presence of the ferroelectric β-phase and the oriented polarization direction, the electromechanical properties of these structurally oriented and distributed nanowires in the porous MS@AAM hosts were then evaluated by both a piezoevaluation system (PES) and piezoresponse force microscopy (PFM). After the deposition of the top and bottom platinum electrodes, the samples were examined by the PES, an electrical characterization system coupled with a laser vibrometer capable of exploiting the inverse piezoelectric effect. Here, the overall electrical and electromechanical properties of a material are obtained by applying an electric field across the sample (thus, along the nanowires axes) and measuring the resulting ferroelectric polarization hysteresis and mechanical displacement loops (Figure 3). The obtained piezoelectric coefficient (d33) values (see Table 2) are consistent with the literature data for stretched films at similar electric field strengths and frequencies (50 Hz).26 The coercive electric field (Ec) and remnant polarization (Pr) values differ from the literature data because of defects in the samples that cause increased leakage currents. Mainly, only a small percentage of the electrode area comprises the polymeric wires, whereas most of the remaining electrode surface consists of nonpolar silica and alumina. Taking into account the geometrical feature of the samples, we estimated the maximum Pr values (see Table S-2 in the Supporting Information). Differences in d33, Pr, and Ec among

and 3) show a peak at 19.9°, attributed to the overlapping (110) and (200) reflections, and at 40.7°, corresponding to the (111) and (201) reflections, all assigned to the β ferroelectric phase.21 Trace 4 refers to the empty MS@AAM template, only showing a broad bump corresponding to both the amorphous silica and alumina phases. Thus, the extreme PVDF confinement into the MS pores induces a clear preferential orientation of the crystalline units according to the β-phase, the most interesting in terms of piezoelectric properties. These results strongly differ from what was described in the literature,1d,4a,d where a poling or mechanical stretching of the PVDF film is always required to obtain the ferroelectric β-phase. In addition, we exclude the possibility that the use of DMA as a solvent for PVDF could induce the formation of the β-phase, whereas only the α-phase was observed (see above). Moreover, preferential crystallization of PVDF into the β-phase upon confinement was not obtained in the 100−200 nm channels of AAMs in the absence of MS.12 In Figure 2b, the peaks at 19.9°, 40.7°, and additionally at 34.8° assigned to the (001) reflection, are observed for both samples PVTF-MS10@AAM and PVTFMS5@AAM (traces 6 and 7) and for the bulk PVTF (trace 5), since it only possesses the β-phase, as previously discussed. The crystal size of the confined polymers, calculated by the Debye−Scherrer equation, appears to be smaller than the mesopore diameters, which were estimated by the DFT model from nitrogen sorption data (see Table 1 and the Supporting Table 1. Crystal Size of the Polymeric Nanowires versus the Pore Size of Empty MS@AAM sample PVDF-MS5@AAM PVTF-MS5@AAM PVDF-MS10@AAM PVTF-MS10@AAM

crystal size (nm)a

MS pore size (nm)b

± ± ± ±

4.6 4.6 9.8 9.8

4.4 2.5 5.3 4.5

0.2 0.2 0.1 0.3

a

Calculated from the Debye−Scherrer equation. bEstimated with a silica DFT model for nitrogen sorption data of the empty MS@AAM hosts.

Information). In contrast, the PVTF crystal size in bulk is ∼14 nm. These results clearly show that the polymers are confined in the mesoporous structure and that control over the crystal size and phase can be achieved upon nanotemplate confinement. The SAXS patterns of the polymer-loaded host templates show a reduction of the reflection intensities of the columnar mesopores (see Figure S-5 in the Supporting Information), with respect to the empty templates. This reduction is due to the decrease in scattering contrast between the silica walls and the polymer-impregnated mesopores,22 again confirming the successful filling of the mesopores with both polymers. Fourier transform infrared (FTIR) spectra on both PVDF and PVTF ultrathin nanowires (see Figure S-6 in the Supporting Information) show the B1 band at 1403 cm−1, corresponding to the wagging vibration of CH2 of the βphase,23 whereas other bands are masked by the huge bump arising at 1200 cm−1, attributed to both silica and alumina hosts. Concerning the crystallization of polymers in confined geometries, experimental studies reported in the literature12,16,24 are in good agreement with molecular simulations.25 They demonstrate that nanogeometries lead to preferential orientation of semicrystalline polymers, when the size of the 4218

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Figure 3. Polarization hysteresis loops (solid lines) and piezoelectric displacement (dotted lines) from the piezo evaluation system (PES) at 50 Hz as a function of the electric field applied to the polymer-templated samples. (a) PVDF-MS5@AAM, (b) PVTF-MS5@AAM, (c) PVDF-MS10@AAM, and (d) PVTF-MS10@AAM. Inset: Scheme of the measurement setup: polymeric nanowires in MS supported into porous AAM, sandwiched between the bottom and top Pt electrodes.

Table 2. Electromechanical Values for the Polymeric Nanowires Confined into the MS@AAM Hosts sample PVDF-MS5@ AAM PVTF-MS5@ AAM PVDF-MS10@ AAM PVTF-MS10@ AAM

Pr (μC cm−2)

Ec (MV cm−1)

d33-PES (pm V−1)

|d33|-PFM (pm V−1)

0.012

0.153

−16.7 ± 3.8

19.2 ± 3.0

0.743

0.031

−10.2 ± 1.3

12.7 ± 1.5

0.017

0.077

−7.8 ± 1.2

8.9 ± 0.7

0.146

0.074

−16.5 ± 2.5

22.3 ± 1.1

the four samples were attributed mostly to their heterogeneity rather than to morphological or structural features. Nevertheless, these measurements show the presence of polar domains, because of the nanoconfinement of the polymers and, thus, its crystalline preferential orientation, although there is an absence of any poling or mechanical stretching. An accurate evaluation of the electromechanical response of the distributed nanowires in the MS@AAM hosts was also performed by PFM,27 in this case, before and after mechanical polishing. After polishing, the inner polymer phase was directly accessed by the AFM tip in contact mode (Figure 4), and both the resulting topographic and calculated |d33| maps clearly resemble the hard template structure, albeit at a lower accuracy for the |d33| maps due to tip curvature convolution. The tip

Figure 4. Piezoresponse force microscopy (PFM) maps of the distributed array of nanowires: PVDF-MS10@AAM (top) and PVTFMS5@AAM (bottom). Panels (a) and (c) show the topography height (nm); panels (b) and (d) show the modulus of the piezoelectric coefficient d33 (|d33|, pm V−1).

curvature radius (30 nm) is greater than the nanowire diameters.28 The measurements were performed by detecting both the amplitude and the phase in contact mode (see eq S-4 4219

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in the Supporting Information) at low voltage excitation (Vac = 1 V, see the Supporting Information), corresponding to an electric field far below the coercive field of the polymeric nanowires. Under this condition, thus without poling, the oriented nanowires show parallel and antiparallel polarizations among each other. From the amplitude signal, we obtained the absolute value of d33 piezoelectric constant (|d33|, see Figures 4b and 4d). The phase signal could not provide any information about the d33 sign, because of the different polar orientation of the nanowires, as mentioned previously. The different bright regions of the calculated |d33| map correspond to different degrees of piezoelectric polarization, as well as nonpolar regions of the surface (in darkness). Specifically, we consider two textures in Figures 4a and 4b. We attribute the upper part of both maps to a continuous and thin polymeric film covering the surface (intentionally left unpolished), showing a more homogeneous electromechanical response mediated by the surface polymer. In contrast, the lower semicircular area corresponds to a better polished surface, where the piezoresponse of single polymeric bundles can be observed. By measuring the samples before polishing, a high accuracy of the |d33| with high spatial resolution of the piezoelectric domains was achieved (see Figure S-8 in the Supporting Information), attributed to the polymeric film averaging the piezoresponse of the single nanowires. However, the texture of individual nanowires could not be resolved. As a reference, a nonpiezoelectric surface (the empty MS@ AAM host) shows a homogeneously colored |d33| map (Figure S-9 in the Supporting Information). The absolute values of d33 obtained from the PFM measurements (Table 2) after a Gaussian fitting (Figure S-10 in the Supporting Information) are consistent with those obtained by PES and reported in the literature2,3 for both PVDF and PVTF films after poling (∼10−20 pm V−1). This shows that, in our case, piezoelectric properties were obtained upon nanoconfinement and without prepoling, achieving a distributed array of ultrathin nanowires that holds promise for future piezoelectric applications.

voltage memories,15,16 nanotip-based protein biosensors,31 or stem cell differentiating guides through highly resolved mechanical or electric stimuli.32

CONCLUSIONS Summarizing, we report on a successful wet-impregnation and hard-templating method for the confinement of piezopolymeric nanowires into a hierarchical host created from columnar mesoporous silica embedded in anodic alumina membranes. These ultrathin nanowires made from poly(vinylidene difluoride) (PVDF) and its copolymer poly(vinylidene difluoride trifluoroethylene) (PVTF) are 5 and 10 nm in diameter and up to 60 μm in length. This unprecedented downscaling of the polymeric nanostructures and their confinement in columnar mesopores induces their oriented crystallization into the desired ferroelectric β-phase, with a crystal size smaller than the diameter of the mesoporous host and a convenient orientation of the polarization axis. For the first time, a clear piezoelectric behavior of PVDF is obtained, despite the absence of any prepoling, mechanical stretching, or other conventional strategies. In the resulting distributed array of piezoelectric nanowires, the latter are isolated from each other by a dielectric matrix, facilitating the handling and electrical contacting of the nanowires. Therefore, these polymeric nanoarrays could be a promising platform for addressing various applications, including not only nanoactuators29 and nanowire-based largearea bioinspired ciliated sensors,30 but also nonvolatile low-

ABBREVIATIONS MS, mesoporous silica; AAM, anodic alumina membranes; PVDF, poly(vinylidene difluoride); PVTF, poly(vinylidene difluoride trifluoroethylene); TEM, transmission electron microscopy; WAXS, wide-angle X-ray scattering; SAXS, smallangle X-ray scattering; XRD, X-ray diffraction; DFT, density functional theory; FTIR, Fourier transform infrared spectroscopy; PES, piezo-evaluation system; PFM, piezoresponse force microscopy; AFM, atomic force microscopy; Pr, remnant polarization; Ec, coercive electric field; d33, piezoelectric coefficient; DMA, dimethylacetamide; MEK, methyl ethyl ketone

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ASSOCIATED CONTENT

* Supporting Information S

Detailed synthesis procedures, characterization results including small-angle X-ray scattering (SAXS), nitrogen sorption measurements, additional transmission electron microscopy (TEM) images, infrared spectroscopy data, additional electromechanical measurements, and current−voltage curves from piezo evaluation system (PES), piezoresponse maps, and |d33| Gaussian fit from piezoresponse force microscopy (PFM). Correspondence and requests for materials should be addressed to V.C. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +39.011.0903436. Fax: +39.011.0903401. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. V.C. carried out the synthesis of the samples, and the SAXS and WAXS characterizations. A.F. carried out the TEM characterization, B.T. the piezoresponze force microscopy, G.C. and S.S. performed the measurements at the piezo evaluation system. V.C., M.P., and T.B. conceived the project and supervised the experiments. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the experimental assistance of Tina Reuter and Dr. Steffen Schmidt of the University of Munich, and of Francesco Canepa for graphics design.

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REFERENCES

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dx.doi.org/10.1021/cm302594s | Chem. Mater. 2012, 24, 4215−4221