Shifted Morphotropic Phase Boundary in [111]-Oriented Nb-Doped Pb

Aug 6, 2015 - Morphotropic phase boundary (MPB) in Nb-doped Pb(Zr,Ti)O3 thin films grown epitaxially on Nb:SrTiO3 (111) surfaces was studied by invest...
3 downloads 10 Views 5MB Size
Download PDF
Article pubs.acs.org/JPCC

Shifted Morphotropic Phase Boundary in [111]-Oriented Nb-Doped Pb(ZrxTi1−x)O3 Epitaxial Films: Insights into Piezoelectricity and Domain Variation Wei Sun,† Qi Yu,† Jiangyu Li,‡ and Jing-Feng Li*,† †

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 14, 2015 | doi: 10.1021/acs.jpcc.5b05423

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, 100084 Beijing, People’s Republic of China ‡ Department of Mechanical Engineering, University of Washington, Seattle, Washington 98195-2600, United States ABSTRACT: Morphotropic phase boundary (MPB) in Nb-doped Pb(Zr,Ti)O3 thin films grown epitaxially on Nb:SrTiO3 (111) surfaces was studied by investigating their local piezoelectric response and domain structure using piezoresponse force microscopy. A sharp peak of piezoelectric coefficients was observed at Zr/Ti = 40/60, supporting the fact that the substrate constraint shifted the MPB in [111]-epitaxial Pb(Zr,Ti)O3 thin films toward the PbTiO3 side of composition from Zr/Ti = 52/48 for bulk materials. The domains at MPB also show a distinguishing structure with shrinking size and remarkably larger vertical piezoresponse compared to the single-phase samples. This work provided a deeper understanding about the effect of substrate constraint on the phase structures and electrical properties of epitaxial ferroelectric films.

■

compositional intervals in the Nb-doped PZT films (PNZT) after the epitaxial growth. By confirming the precise location of MPB with optimum piezoelectric properties, the in situ ferroelectric domain mapping among various Zr/Ti ratios nearby was performed. The tetragonal-to-rhombohedral phase transition across the shifted MPB could be confirmed by the variation of domain configuration.

INTRODUCTION Pb(Zr,Ti)O3 (PZT) based films have been widely considered for applications in microelectronic and microelectromechanical systems due to their excellent ferroelectric and piezoelectric properties.1−7 Compared with polycrystalline PZT-based films on silicon substrates, the epitaxial ones on single-crystalline substrates possess better properties and also attract more scientific interests. 8−11 In PZT system there exists a morphotropic phase boundary (MPB) locating at Zr/Ti = 52/48, whereby piezoelectric and ferroelectric properties can be significantly enhanced. However, the thin films differentiate from the bulks due to the substrate constraint effect. Several studies indicated that the MPB composition in PZT-based epitaxial films was different from the commonly known Zr/Ti = 52/48 in bulk ceramics or even polycrystalline films due to the strong substrate clamping.12−14 For example, we found the MPB would deviate toward Zr-rich side for [001]-oriented films and toward Ti-rich side for [111]-oriented samples.15−17 Previous studies were mainly based on phase structure analysis by X-ray diffraction (XRD), and there existed difficulties to precisely determine the location and microstructural features of MPB due to the limited difference of lattice parameters between tetragonal and rhombohedral phase in addition to the weak signal intensity from films. Consequently, MPB studies of PZT films are very few compared with their bulk counterparts even though they are equally important. Thin films are ideal for piezoresponse force microscopy (PFM) study, which could provide satisfactory information about ferroelectric domains as well as piezoelectricity at nanometer scale.18−20 In the present study, we carried out piezoelectric characterization with small © 2015 American Chemical Society

■

EXPERIMENTAL METHODS Synthesis. A series of [111]-oriented Pb(ZrxTi1−x)0.98Nb0.02O3 were fabricated by sol−gel approach. Nb doping in PZT films can promote electrical properties without changing PZT phase structure, whereas the [111] orientation was selected because of its high ferroelectricity.17 The Pb source of trihydrate lead acetate [Pb(CH3COO)2· 3H2O] was first dissolved into 2-methoxyethanol (2-MOE) and refluxed. Then zirconium n-propoxide [Zr(OCH(CH3)2)4], titanium isopropoxide [Ti(OCH(CH3)2)4], and niobium ethoxide [Nb(OC2H5)5] were added into the precursor solution as raw materials together with acetylacetone and methanamide as chelating and stabilizing agents, respectively. A total of 10 mol % excess Pb was introduced into all solutions with different Zr/Ti ratios to compensate for Pb loss owing to the volatilization during the subsequent thermal processing. The resulting mixture was stirred, aged, and deposited onto the Received: June 7, 2015 Revised: August 6, 2015 Published: August 6, 2015 19891

DOI: 10.1021/acs.jpcc.5b05423 J. Phys. Chem. C 2015, 119, 19891−19896

Article

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 14, 2015 | doi: 10.1021/acs.jpcc.5b05423

The Journal of Physical Chemistry C

Figure 1. Principles of piezoelectric switching measurement by using PFM: (a) graphic scanning procedure; (b) loading curve of electrical signals with the definitions of the “on” and “off” state; (c) typical piezoresponse hysteresis loop under “off” state; (d) phase-to-voltage chart; (e) piezoresponse curve under “on” state.

the butterfly shaped piezoresponse curve measured under “off” state. The values of Amplitude[0] and Amplitude[max] were recorded, which are divided by AC voltage (2 V) to yield the local effective piezoelectric coefficients d33 and dmax, respectively. The former one represents piezoresponse under zero voltage and the latter one manifests piezoresponse after DC electrical poling. The coercive field (Vc) can be obtained from the fitted phase hysteresis loop in Figure 1d. Moreover, the response under “on” state was also gathered as a narrow butterfly shaped curve shown in Figure 1e. Although the electrostatic effect may interfere with piezoresponse under DC voltage, we got the Amplitude[on], divided by DC voltage (30 V), to estimate the corresponding parameter termed D33. This measurement under “on” state was rarely reported before, which might be used to make comparison with the results under “off” state.

[111]-cut single-crystalline Nb-doped SrTiO3 substrates (Nb:STO) by repeating spin-coating in ambient atmosphere. The lattice parameter of Nb:STO is ∼3.905 Å, a little bit smaller than that of PNZT, which is ∼4.015 Å. Finally the films were subjected to a series of thermal processing for drying, pyrolysis, and crystallization. The detailed parameters used above have been reported elsewhere.17,21 Characterization. The crystallographic structure of the PNZT thin films was examined by X-ray diffraction (XRD, D/ max-2500 and D/max-RB, Rigaku; Tokyo, Japan) using Cu− Kα radiation. The microstructure of film sample was observed using a transmission electron microscope (TEM, G20, Tecnai; OR, USA). For preparing the TEM samples, the PNZT film was coated with platinum and SiO2 with a total thickness of ∼150 nm, then its cross-section was lifted out by focus ion beam (FIB, ZEISS Augriga Focus Ion Beam/Field-Emission Scanning Electron Microscope dual-cross system; Germany). Ferroelectric behaviors were acquired by a ferroelectric testing system (Multiferroic, Radiant Technologies; USA). Pt top electrodes with a diameter of 400 μm were sputter-deposited onto the film surface through a shadow mask. Ferroelectric domain mapping was done using PFM (MFP-3D, Asylum Research; CA, USA). A conductive tip scanned the film surface in contact mode with a modulated AC excitation, whereas the back-contact of sample was grounded and the Nb:STO substrate served as bottom electrode. For the local piezoelectric and ferroelectric switching measurements, an additional sequence of DC voltage was introduced to the films.21,22 Specifically, by introducing the idea of Jesse et al.,23 the scanning procedure was continued point-by-point until the evenly spaced M × N points were examined, as shown in Figure 1a. During each point of testing, a sequence of DC voltage with a maximum value of 30 V was applied to facilitate the ferroelectric switching. Meanwhile, a modulated 2 V-AC voltage was introduced to detect the amplitude and phase of surface vibration. The profile of voltage loadings is shown in Figure 1b. For the “off” state, only an AC voltage performed on film surface, while a DC voltage was performed in the meantime at “on” state. Here we displayed a range of typical resulting curves obtained through the switching measurement. Figure 1c shows

■

RESULTS AND DISCUSSION 1. Microstructure and Morphology. The phase structure and preferred orientation of PNZT films were characterized with the corresponding θ−2θ XRD patterns presented in Figure 2a. All the film samples can be indexed to the perovskite phase with strong [111]/[222] orientation along the out-ofplane dimension, consistent with the cutting direction of Nb:STO substrates. The enlarged high-angle (222) Bragg peaks shift toward the lower angle side clearly as shown in Figure 2b, indicating a gradual lattice expansion of PNZT unit cell with increasing Zr content. The broadening peaks around Zr/Ti = 40/60 may indicate changes in phase structure.16 Typical Phiscan patterns of [111]-oriented films are displayed in Figure 2c by collecting the (100) diffraction signals. Three peaks are separated evenly by 120° intervals in all samples. This symmetry is supposed to inherit from the triangular (111) plane of the SrTiO3, which signifies the 3-fold in-plane ordering within films. Rocking curves were also measured to evaluate the crystallization quality in films, where the full width at halfmaximum (FWHM) value was determined by fitting the obtained curves exemplified in the inset of Figure 2d. The small FWHM values suggest a well-crystallized state in the present PNZT films. It should be noted that the curve shown in Figure 19892

DOI: 10.1021/acs.jpcc.5b05423 J. Phys. Chem. C 2015, 119, 19891−19896

Article

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 14, 2015 | doi: 10.1021/acs.jpcc.5b05423

The Journal of Physical Chemistry C

quasi-atomic smooth surface with uniform crystallization, rather than a rougher island growth mode occasionally reported in films.24 No detectable grain structure can be obtained on the film surface. Figure 3c shows the TEM image of PNZT/ Nb:STO interface, and its high-resolution image is given in Figure 3d. Epitaxial relationship between film and substrate has been confirmed as expected, where the lattice is rigidly matched across the interface area along the [111] direction. The inserted diffraction spots of substrate show the 6-fold symmetry, which is consistent with the crystallographic configuration of [111]cut STO. Then the film also inherits this symmetry with a tiny amorphous ring owing to the Pb volatilization under electron beam. 2. Electrical Properties. As for piezoelectric materials, piezoelectric property is closely connected with the structural information such as phase component, film orientation, and domain configuration.12,15,21 Typical amplitude-to-voltage curves as a function of Zr/Ti ratios, together with the corresponding phase hysteresis curves are illustrated in Figure 4a,b to detect the piezoelectric property in epitaxial films locally. All the samples display complete butterfly shaped piezoresponse curves with 180° domain switching under the external electrical field. The local piezoresponse amplitude is obtained using the DART technique by tracking the resonance of the tip-film surface system.25,26 A much larger voltageinduced displacement was found at Zr/Ti = 40/60. For this composition, typical ferroelectric test results at various frequencies are shown in Figure 4c. It can be derived from hysteresis loops that the remanent polarization Pr is ∼50 μC/ cm2 at high frequency and slightly increases at lower frequency. The coercive filed is ∼600 kV/cm regardless of testing frequency. This value is larger than that reported in polycrystalline films on Si substrates, which can be mainly ascribed to a stronger constraint effect of single-crystalline substrates.27 The loops show a shift along V-axis, which originates from internal bias or aligned charges along the film normal direction.28 To further investigate the ferroelectric behavior, pulsed polarization positive up negative down (PUND) measurements was carried out. The pulsed remanent polarization (ΔP = P* (switched polarization) − P∧ (nonswitched polarization) = ∼2Pr) as a function of electric field at a pulse width of 20 μs is depicted in Figure 4d. ΔP matches well with the 2Pr values obtained in Figure 4c. As the composition-dependent piezoelectric property has been obviously displayed in Figure 4, the Zr/Ti = 40/60 sample manifests a superior piezoresponse. To depict the variation of piezoelectricity more precisely, the d33, dmax, Vc, and D33 values, as defined before, are all presented around Zr/Ti = 40/60 composition with small compositional intervals in Figure 5a,b. For all these data points, ten curves with the largest voltageinduced amplitude were selected from a 2 × 2 μm2 sized mesh of 256 points to calculate the average piezoelectric parameters. Interestingly, both the d33 and dmax under “off” state clearly show their peak values at the Zr/Ti = 40/60 composition with sharp dropping at the neighboring ones, indicating that the MPB in [111]-oriented films has shifted to Zr/Ti = 40/60 composition from the commonly known Zr/Ti = 52/48 in bulk materials. Several works based on phase structure by XRD method have suggested that the tetragonal/rhombohedral phase boundary might move to Ti-rich side.29,30 The large piezoresponse can be ascribed to both the intrinsic lattice distortion and the potential extrinsic domain wall motion under a small voltage. The difference between dmax and d33

Figure 2. (a) XRD full patterns of the [111]-oriented PNZT films on Nb:STO substrates with different Zr/Ti ratios; (b) step-scanning patterns of (222) peaks; (c) Representative Phi-scan profiles; (d) composition-dependent FWHM values with the inset rocking curve collections.

2d has two regions separated by a critical Zr/Ti ratio, which may correspond to a structural transition from the tetragonal to rhombohedral phase. A previous study found that a rhombohedral phase preferred to grow on the triangular STO (111) plane with smaller misfit strain,15 which might be the reason why lower fwhm values were obtained in the Zr-rich rhombohedral region. Surface and cross-sectional observations revealed the epitaxial growth. The topographic image of film surface was depicted under PFM contact mode within a square area of 2 × 2 μm2, as shown in Figure 3a, where the 3D layout is displayed in Figure 3b. A RMS surface roughness value of 0.46 nm indicates a

Figure 3. Morphology observations: (a) surface topography under PFM contact mode with (b) 3D illustration; (c) TEM cross-sectional image of PNZT films on Nb:STO substrate, (d) high-resolution image at the interface region with diffraction spots of film and substrate, respectively (Zr/Ti = 40/60). 19893

DOI: 10.1021/acs.jpcc.5b05423 J. Phys. Chem. C 2015, 119, 19891−19896

Article

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 14, 2015 | doi: 10.1021/acs.jpcc.5b05423

The Journal of Physical Chemistry C

Figure 4. Piezoelectric and ferroelectric response of the PNZT films: (a) typical amplitude-voltage curves; (b) piezoresponse hysteresis loops; (c) a frequency series of ferroelectric hysteresis loops (Zr/Ti = 40/60); (d) PUND measurements for varying voltage (Zr/Ti = 40/60).

optimal position locates at Zr/Ti = 45/55, which deviates slightly from the Zr/Ti = 40/60 MPB composition. This result revealed that the piezoelectric property obtained under “on” state was influenced by electrostatic effect, where the variation of piezoelectric property could be estimated roughly. 3. Domain Mapping and Discussion. Since piezoelectric property is firmly associated with ferroelectric domains in piezoceramics, the MPB composition, which is composed of two phases, would show unique features compared to the pure tetragonal and rhombohedral ones. To facilitate domain observation directly, the vector piezoresponse force microscopy (VPFM) module was introduced to detect the out-of-plane (OP) and in-plane (IP) piezoelectric signals with a sinusoidal AC field of 3 V. The rhombohedral (Zr/Ti = 45/55), MPB (Zr/Ti = 40/60), and tetragonal (Zr/Ti = 25/75) samples were selected, and the amplitude mapping results from both vertical and lateral directions are shown in Figure 6. Substantial differences were found in the respective samples. For the Zr/Ti = 25/75 tetragonal film, both the OP and IP amplitude mapping results show evident contrast with two types of colored regions. These two regions show large and small piezoresponses, respectively. Deeper insights could be achieved when structural analysis was combined. The Zr/Ti = 25/75 composition belongs to a typical tetragonal phase with a relatively large c/a ratio, which facilitates a large displacement polarization of Zr/Ti central atoms along [001] directions.31,32 This spontaneous polarization displays an angle of 54.7° to the [111] film orientation. So both the OP and IP mappings would show relatively large amplitude, as can be seen in the light regions in Figure 6a,d. The inner bias or aligned defects, as shown in our ferroelectric test, exist within films. It would possibly decrease vertical polarization along one certain direction, thus providing obvious contrast in amplitude mapping results in spite of the symmetric configuration of [001] polarization vectors around [111] film orientation. In other words, the low-response areas (the blue region in Figure

Figure 5. Various piezoelectric parameters including (a) d33, Vc and (b) dmax, D33 as a function of Zr/(Zr + Ti) ratio measured by PFM platform.

values can be attributed to the electrical poling effect. Meanwhile, the coercive field would show the lowest value at MPB position in thin films due to the lower energy barrier for domain motions.27 Indeed, the Vc value shows a slight dropping when Zr/Ti ratio is increased to 40/60, as shown in Figure 5a. Moreover, the D33 values measured under the “on” state show a less notable change compared to the d33 and dmax values. The 19894

DOI: 10.1021/acs.jpcc.5b05423 J. Phys. Chem. C 2015, 119, 19891−19896

Article

The Journal of Physical Chemistry C

shifted MPB may not be fixed if the film thickness is changed arbitrarily.

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 14, 2015 | doi: 10.1021/acs.jpcc.5b05423

■

CONCLUSIONS A piezoresponse force microscopic study revealed the substrate constraint effect on piezoelectricity and domain structure of [111]-epitaxial PNZT thin films on Nb:STO substrates. By investigating the piezoelectric responses as a function of Zr/Ti ratio, the morphotropic phase boundary (MPB) is confirmed to shift toward the Ti-rich composition of Zr/Ti = 40/60 from Zr/Ti = 52/48 in bulk materials. The domains at MPB also show a distinguishing structure with shrinking size and remarkably larger vertical piezoresponse compared to the single-phase samples, providing example of complex domain evolution in thin film system. The present results at nanoscale by piezoresponse force microscopy affirmed the shifting of MPB due to the substrate clamping effect in [111]-epitaxial Pb(Zr,Ti)O3 thin films, which should be taken into account for the compositional optimization of high-performance ferroelectric thin films.

Figure 6. Domain mappings of the [111]-oriented PNZT epitaxial films through vector piezoresponse force microscopy. (a−c) Out-ofplane (OP) amplitudes and (d−f) in-plane (IP) amplitudes with three different Zr/Ti ratios of 25/75, 40/60, and 45/55, respectively.

■

6a) possess an opposite polarization compared to the highresponse ones (the yellow region in Figure 6a), and the piezoresponse is lowered by the inner bias. Then we focused on the MPB composition (Zr/Ti = 40/60), where rhombohedral and tetragonal phases would coexist. Figure 6b,e show the PFM mapping results of Zr/Ti = 40/60 films. It is apparent that films show considerably large OP amplitude in most part of the detected areas. This should mainly originate from the extrinsic piezoresponse by domain wall motion under a small voltage, as the MPB region shows a coexisting phase structure with multiple polarization variants and a small coercive field. In contrast, the IP amplitude mapping shows a weak piezoresponse compared to the Zr/Ti = 25/75 samples overall, which reflects the alternation from the tilted [001] polarization in tetragonal phase to the normally aligned [111] polarization in rhombohedrel phase. In addition, the domain size seems to shrink to a certain extent in IP mapping compared to the tetragonal one. This size reduction may be ascribed to the complicated phase coexistence in the MPB composition, analogous to the case in bulk ceramics.33 As for the rhombohedral composition of Zr/Ti = 45/55, the OP and IP amplitudes drop significantly with the relatively smaller signalto-noise ratio, as shown in Figure 6c,f. For the Zr/Ti = 45/55 sample, one of the four ⟨111⟩ rhombohedral polarization variants is consistent with the [111] film orientation, which would show a pure vertical response with no lateral response. The rest of the potential polarization variants have a large angle, 71° to the film orientation, which would mainly contribute to the lateral response. However, the Zr/Ti = 45/55 one is located closely to the MPB composition, where the displacement of central Zr/Ti atoms along the ⟨111⟩ spontaneous polarization directions is supposed to be limited, thus manifesting a poor piezoresponse at both vertical and lateral dimensions. By considering the above results through XRD, piezoelectric, ferroelectric test and domain mapping, the MPB was confirmed to shift to the Zr/Ti = 40/60 composition in [111]-oriented PNZT films. The MPB shifting toward the Ti-rich region compared to that in bulk materials is a result of the misfit strain between the PNZT epitaxial films and the Nb:STO(111) substrate. Meanwhile, as the film thickness is an important parameter to influence the magnitude and distribution of misfit strain, the varied strain would affect the film structure in turn. So it is still worth mentioning that the precise location of the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-1062784845. Fax: +86-10-62771160. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by National Nature Science Foundation of China (Grants Nos. 51332002, 51221291) and the Ministry of Science and Technology of China under the Grant 2015CB654605. The work at the University of Washington was supported by NSF (CMMI 1100339).

■

REFERENCES

(1) Polla, D. L.; Francis, L. F. Processing and Characterization of Piezoelectric Materials and Integration into Microelectromechanical Systems. Annu. Rev. Mater. Sci. 1998, 28, 563−597. (2) Setter, N.; Damjanovic, D.; Eng, L.; Fox, G.; Gevorgian, S.; Hong, S.; Kingon, A.; Kohlstedt, H.; Park, N.; Stephenson, G. Ferroelectric Thin Films: Review of Materials, Properties, and Applications. J. Appl. Phys. 2006, 100, 051606. (3) Jaffe, B. Piezoelectric Properties of Lead Zirconate-Lead Titanate Solid-Solution Ceramics. J. Appl. Phys. 1954, 25, 809. (4) Izyumskaya, N.; Alivov, Y. I.; Cho, S. J.; Morkoç, H.; Lee, H.; Kang, Y. S. Processing, Structure, Properties, and Applications of PZT Thin Films. Crit. Rev. Solid State Mater. Sci. 2007, 32, 111−202. (5) Frantti, J.; Fujioka, Y.; Puretzky, A.; Xie, Y.; Ye, Z. G.; Parish, C.; Glazer, A. M. Phase Transitions and Thermal-Stress-Induced Structural Changes in a Ferroelectric Pb(Zr0.80Ti0.20)O3 Single Crystal. J. Phys.: Condens. Matter 2015, 27, 025901. (6) Hu, B.; Chen, Y.; Yang, A.; Gillete, S.; Fitchorov, T.; Geiler, A.; Daigle, A.; Su, X. D.; Wang, Z.; Viehland, D.; et al. Piezoelectric Properties of Epitaxial Pb(Zr0.525, Ti0.475)O3 Films on Amorphous Magnetic Metal Substrates. J. Appl. Phys. 2012, 111, 07D916. (7) Hu, B.; Chen, Y.; Yang, A.; Gillete, S.; Fitchorov, T.; Geiler, A.; Daigle, A.; Su, X. D.; Wang, Z.; Viehland, D.; et al. Epitaxial Growth of Pb(Zr0.53Ti0.47)O3 Films on Pt Coated Magnetostrictive Amorphous Metallic Substrates toward Next Generation Multiferroic Heterostructures. J. Appl. Phys. 2012, 111, 064104. (8) Ramesh, R.; Schlom, D. G. Orienting Ferroelectric Films. Science 2002, 296, 1975−1976. (9) Vrejoiu, I.; Le Rhun, G.; Pintilie, L.; Hesse, D.; Alexe, M.; Gösele, U. Intrinsic Ferroelectric Properties of Strained Tetragonal 19895

DOI: 10.1021/acs.jpcc.5b05423 J. Phys. Chem. C 2015, 119, 19891−19896

Article

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 1, 2015 | http://pubs.acs.org Publication Date (Web): August 14, 2015 | doi: 10.1021/acs.jpcc.5b05423

The Journal of Physical Chemistry C

(28) Lai, F.; Li, J.-F. Sol-Gel Processing and Characterization of (Na,K)NbO3 lead-Free Ferroelectric Films. Ferroelectrics 2007, 358, 181−187. (29) Kanno, I.; Kotera, H.; Wasa, K.; Matsunaga, T.; Kamada, T.; Takayama, R. Crystallographic Characterization of Epitaxial Pb(Zr,Ti)O3 Films with Different Zr/Ti Ratio Grown by Radio-FrequencyMagnetron Sputtering. J. Appl. Phys. 2003, 93, 4091. (30) Yokoyama, S.; Honda, Y.; Morioka, H.; Okamoto, S.; Funakubo, H.; Iijima, T.; Matsuda, H.; Saito, K.; Yamamoto, T.; Okino, H.; et al. Dependence of Electrical Properties of Epitaxial Pb(Zr,Ti)O3 Thick Films on Crystal Orientation and Zr/(Zr+Ti) Ratio. J. Appl. Phys. 2005, 98, 094106. (31) Lee, H. N.; Nakhmanson, S. M.; Chisholm, M. F.; Christen, H. M.; Rabe, K. M.; Vanderbilt, D. Suppressed Dependence of Polarization on Epitaxial Strain in Highly Polar Ferroelectrics. Phys. Rev. Lett. 2007, 98, 217602. (32) Schönau, K. A.; Schmitt, L. A.; Knapp, M.; Fuess, H.; Eichel, R.A.; Kungl, H.; Hoffmann, M. J. Nanodomain Structure of Pb[Zr1−xTix]O3 at its Morphotropic Phase Boundary: Investigations from Local to Average Structure. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 184117. (33) Garcia-Melendrez, A.; Durkan, C. Reversible Nanoscale Switching of Polytwin Orientation in a Ferroelectric Thin Film Induced by a Local Electric Field. Appl. Phys. Lett. 2013, 103, 092904.

PbZr0.2Ti0.8O3 Obtained on Layer−by−Layer Grown, Defect−Free Single−Crystalline Films. Adv. Mater. 2006, 18, 1657−1661. (10) Brennecka, G. L.; Parish, C. M.; Tuttle, B. A.; Brewer, L. N.; Rodriguez, M. A. Reversibility of the Perovskite-to-Fluorite Phase Transformation in Lead-Based Thin and Ultrathin Films. Adv. Mater. 2008, 20, 1407−1411. (11) Yu, Q.; Li, J.-F.; Zhu, F.-Y.; Li, J. Domain Evolution of Tetragonal Pb(ZrxTi1‑x)O3 Piezoelectric Thin Films on SrTiO3(100) Surfaces: Combined Effects of Misfit Strain and Zr/Ti Ratio. J. Mater. Chem. C 2014, 2, 5836−5841. (12) Li, J.-F.; Zhu, Z.-X.; Lai, F.-P. Thickness-Dependent Phase Transition and Piezoelectric Response in Textured Nb-Doped Pb(Zr0. 52Ti0. 48)O3 Thin Films. J. Phys. Chem. C 2010, 114, 17796− 17801. (13) Pertsev, N. A.; Kukhar, V. G.; Kohlstedt, H.; Waser, R. Phase Diagrams and Physical Properties of Single-Domain Epitaxial Pb(Zr1−xTix)O3 Thin Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2003, 67, 054107. (14) Kukhar, V. G.; Pertsev, N. A.; Kohlstedt, H.; Waser, R. Polarization States of Polydomain Epitaxial Pb(Zr1−xTix)O3 thin Films and Their Dielectric Properties. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 214103. (15) Zhu, Z.-X.; Li, J.-F.; Lai, F.-P.; Zhen, Y.; Lin, Y.-H.; Nan, C.-W.; Li, L.; Li, J. Phase Structure of Epitaxial Pb(Zr,Ti)O3 Thin Films on Nb-Doped SrTiO3 Substrates. Appl. Phys. Lett. 2007, 91, 222910. (16) Zhu, Z.-X.; Li, J.-F.; Liu, Y.; Li, J. Shifting of the Morphotropic Phase Boundary and Superior Piezoelectric Response in Nb-Doped Pb(Zr,Ti)O3 Epitaxial Thin Films. Acta Mater. 2009, 57, 4288−4295. (17) Yu, Q.; Li, J.-F.; Zhu, Z.-X.; Xu, Y.; Wang, Q.-M. Shift of Morphotropic Phase Boundary in High-Performance [111]-Oriented Epitaxial Pb(Zr,Ti)O3 Thin Films. J. Appl. Phys. 2012, 112, 014102. (18) Ahn, C.; Tybell, T.; Antognazza, L.; Char, K.; Hammond, R.; Beasley, M.; Fischer, Ø.; Triscone, J.-M. Local, Nonvolatile Electronic Writing of Epitaxial Pb(Zr0. 52Ti0. 48)O3/SrRuO3 Heterostructures. Science 1997, 276, 1100−1103. (19) Anbusathaiah, V.; Kan, D.; Kartawidjaja, F. C.; Mahjoub, R.; Arredondo, M. A.; Wicks, S.; Takeuchi, I.; Wang, J.; Nagarajan, V. Labile Ferroelastic Nanodomains in Bilayered Ferroelectric Thin Films. Adv. Mater. 2009, 21, 3497−3502. (20) Li, J.; Li, J.-F.; Yu, Q.; Chen, Q. N.; Xie, S. Strain-Based Scanning Probe Microscopies for Functional Materials, Biological Structures, and Electrochemical Systems. Journal of Materiomics 2015, 1, 3−21. (21) Yu, Q.; Li, J.-F.; Sun, W.; Zhu, F.-Y.; Liu, Y.; Chen, Y.; Wang, Z.; Li, J. Orientation-Dependent Piezoelectricity and Domain Characteristics of Tetragonal Pb(Zr0.3,Ti0.7)0.98Nb0.02O3 Thin Films on NbDoped SrTiO3 Substrates. Appl. Phys. Lett. 2014, 104, 012908. (22) Liu, Y.; Zhang, Y.; Chow, M.-J.; Chen, Q. N.; Li, J. Biological Ferroelectricity Uncovered in Aortic Walls by Piezoresponse Force Microscopy. Phys. Rev. Lett. 2012, 108, 078103. (23) Jesse, S.; Baddorf, A. P.; Kalinin, S. V. Switching Spectroscopy Piezoresponse Force Microscopy of Ferroelectric Materials. Appl. Phys. Lett. 2006, 88, 062908. (24) Lange, F. Chemical Solution Routes to Single-Crystal Thin Films. Science 1996, 273, 903−909. (25) Kalinin, S. V.; Rodriguez, B. J.; Jesse, S.; Shin, J.; Baddorf, A. P.; Gupta, P.; Jain, H.; Williams, D. B.; Gruverman, A. Vector Piezoresponse Force Microscopy. Microsc. Microanal. 2006, 12, 206− 20. (26) Jesse, S.; Kalinin, S. V.; Proksch, R.; Baddorf, A. P.; Rodriguez, B. J. The Band Excitation Method in Scanning Probe Microscopy for Rapid Mapping of Energy Dissipation on the Nanoscale. Nanotechnology 2007, 18, 435503. (27) Ruangchalermwong, C.; Li, J.-F.; Zhu, Z.-X.; Muensit, S. Phase Transition and Electrical Properties of Highly [111]-Oriented and Niobium-Modified Pb(ZrxTi1−x)O3 thin Films with Different Zr/Ti Ratios. J. Phys. D: Appl. Phys. 2008, 41, 225302. 19896

DOI: 10.1021/acs.jpcc.5b05423 J. Phys. Chem. C 2015, 119, 19891−19896