Research Article www.acsami.org
Recoverable Self-Polarization in Lead-Free Bismuth Sodium Titanate Piezoelectric Thin Films Jinyan Zhao,† Wei Ren,*,† Gang Niu,*,† Nan Zhang,*,† Guohua Dong,† Lingyan Wang,† Ming Liu,† Peng Shi,† and Zuo-Guang Ye*,‡,† †
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an 710049, China ‡ Department of Chemistry and 4D Laboratories, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada S Supporting Information *
ABSTRACT: Bismuth sodium titanate, Bi0.5Na0.5TiO3 (BNT), is a promising lead-free ferroelectric material. However, its potential applications have not been fully explored, mainly because of the complex domain structure arising from its intricate phase transitions. A deep and thorough study of its domain structure and polarization switching behavior will greatly help with understanding the polarization nature and with promoting future applications. In this work, we demonstrate that BNT polycrystalline films possess a highly ordered out-ofplane polarization (self-polarization) and randomly oriented inplane polarizations. Interestingly, the inherent nature of polarization in the BNT films does not allow for the nonvolatile domain writing, as the switched polarization spontaneously and rapidly reverses to the initial orientation state once the external poling electric field is removed, making the self-polarization recoverable. Such a stable self-polarization vanishes gradually with temperature increasing over 150 °C but starts to recover to its initial state upon cooling down to 250 °C, and entirely recovers once the temperature is reduced to below 200 °C. Such interesting properties of BNT films are attributed to the combined effects of the free charges at the Pt electrode, (detected) cation vacancies at the oxide/Pt interface and the defects in oxide lattices. Our results make a step closer to fully understand the nature of polarization and related piezoelectricity in BNT. Such films with recoverable self-polarization are of great interest for applications as sensors, actuators, and transducers that can operate particularly under high temperatures and high electric field conditions. KEYWORDS: self-polarization, reversible polarization switching, lead-free piezoelectric films, sodium bismuth titanate, piezoresponse force microscopy
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materials.15 Ferroelectric domain structures significantly influence the electrical properties of thin ferroelectric films.16−20 Many as-deposited ferroelectric films have been reported to have one preferred homogeneous polarization direction, known as self-polarization, which is useful for such applications as pyroelectric infrared detectors and piezoelectric actuators.21−38 However, there have been only a few reports on the domain structure and polarization characteristics of BNT films, especially polycrystalline thin films, and the descriptions of domain structures therein were not clear.39−41 Therefore, research on the polarization distribution and poling behavior in BNT thin films is highly desirable in order to provide an improved fundamental understanding of the essential nature of this material and to achieve devices with high performance. In this work, we have fabricated polycrystalline BNT thin films on Pt/TiO2/SiO2/Si substrates using the sol−gel process.
INTRODUCTION Bismuth sodium titanate, a typical A-site complex perovskite with a high ferroelectric Curie temperature (TC, 320 °C), a large remanent polarization (38 μC cm−2), and a high coercive field (73 kV cm−1), has been considered a very promising replacement of Pb-based ferroelectric materials.1 Since first reported by Smolenskii et al.,2 many reports on the structural, dielectric, piezoelectric, and ferroelectric properties of BNTbased materials have been reported.3−13 However, one of the challenges in developing BNT-based devices is that the samples are difficult to pole at room temperature because of the large coercive field and high leakage conductivity, which originates from various types of defects and vacancies in this material.14 The polarization in a unit cell of a rhombohedral BNT crystal can orient along one of the eight ⟨111⟩ directions. A macroscopic polarization appears below TC when the sample is poled by an external electric field. The local structure of BNT significantly deviates from the average structure, demonstrating bismuth’s adaptability in various polarization directions, which leads to important properties in perovskite functional © XXXX American Chemical Society
Received: March 21, 2017 Accepted: July 31, 2017
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DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Piezoresponse force microscopy (PFM) was utilized to systematically image the local domain structures and poling behavior of the films. It is found that a macroscopically ordered out-of-plane polarization appears naturally in the BNT films without an external electric field. This self-polarization state eliminates the need for, and overcomes the difficulty in, poling the BNT films. Furthermore, in contrast to the common domain-writing behavior in most ferroelectric thin films, the self-polarization in the BNT films is stable and recoverable even when subjected to high electric fields and high temperatures. The polarization is switched by applying an external electric field, but it immediately reverses back to its initial state by removing the field. This recoverable self-polarization provides a new opportunity for the development of as sensors and transducers operating at high electric fields and high temperatures.
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RESULTS Morphology, Phase, and Domain Structures. Figure 1 shows the specular 2θ X-ray diffraction (XRD) results for a 750
Figure 2. (a) 3D topography, (b) 2D topography, and (c−f) corresponding piezoresponse images of the 750 nm thick BNT thin film: (c, e) are OP phase and amplitude images, respectively; (d, f) are IP phase and amplitude images, respectively. Figure 1. XRD pattern of a 750 nm thick BNT thin film. Insets: SEM images with the (a) top-view and (b) cross-sectional view.
component that was directed uniformly toward the bottom of the film by applying electric field (which will be described in the following section). This kind of initial preferred polarization was frequently observed in PbTiO3 (PT) and Pb(Zr,Ti)O3 (PZT) thin films by PFM.25,42 The IP phase image exhibits distinct 180° contrast and the IP amplitude appears bright inside domains and dark at domain walls, suggesting the existence of two kinds of IP components for the spontaneous polarization. Some grains exhibit a multidomain structure and the others present a single-domain state. It was further found that self-polarization exists in all the measured films of 150− 900 nm thick (Figure S2). The results of the sample with a thickness of 750 nm are presented as an example, demonstrating the unusual domain structure and switching behavior observed in all the synthesized films with various thicknesses. Two regions marked in Figure 2b, which are enlarged in Figure 3a, were chosen to study the domain configurations. Two regions, labeled 1 and 2, in the IP piezoresponse phase images indicate the two types of domains. The spontaneous polarization of ferroelectric materials with a rhombohedral structure is along the body diagonal. Three kinds of domain wall orientations with the 109, 71, and 180° domains are usually formed, as schematically shown in Figure 3b, in which the polarization orientations 1 and 2 corresponding to domains 1 and 2 indicate the observed 109° (or 71°) domains in the BNT film. Figure 3c shows the phase hysteresis loops acquired with both the vertical and lateral PFM modes. The loops were
nm thick BNT thin film. The strong and sharp peak at 2θ = 40.2° corresponds to the Pt (111) reflection and the peak at 2θ = 36.2° is related to the substrate reflection. All the other peaks arise from the randomly oriented BNT polycrystalline thin film with a pseudocubic perovskite structure (the rhombohedral perovskite structure of BNT powder is very close to cubic, as shown in Figure S1). The insets of Figure 1 display the surface and cross-sectional morphologies of the film measured by scanning electron microscopy (SEM). The film possesses a dense and well-organized microstructure and homogeneous grains in size of ∼200 nm. Self-Polarization Phenomenon. Three-dimensional (3D) and two-dimensional (2D) atomic force microscopy (AFM) topographies of the BNT thin film are displayed in Figure 2a, b, respectively. The BNT film exhibits smooth surface (roughness: 2.90 nm) and homogeneous grains (size: ∼ 200 nm), in good agreement with the SEM observation. The corresponding outof-plane (OP) and in-plane (IP) PFM phase and amplitude images are shown in Figure 2c−f. There are no obvious variations of the OP component of the polarization within and between the grains. Even when the scan size was expanded to 90 μm, both the OP amplitude and phase images still exhibit remarkable homogeneity across the layer with a weak contrast, indicating the existence of self-polarization. The as-grown polarization of the film was determined to have an ordered OP B
DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. (a) Topography, OP phase and amplitude, IP phase and amplitude of two selected regions from Figure 2, with two domains marked as 1 and 2. (b) Diagram illustrating the 3D domain configuration of domains 1 and 2 shown in (a) in the x−z projection plane with OP direction defined as z axis and IP direction defined as x axis. (c) OP phase hysteresis loop and IP phase hysteresis loops of domain 1 (bottom) and domain 2 (top). (d) OP and IP piezoresponse hysteresis loops. (e) Initial ferroelectric hysteresis loops.
Figure 4. (a) Polarization switching behavior of the 750 nm thick BNT thin film under both positive dc bias and negative dc bias. Each dc bias and zero-field state is kept for 8 min. (b) Schematic of OP polarization switching behavior in the BNT thin film under external dc electric field. PFM tip acts as the top electrode, and the bottom electrode is glued to the PFM sample stage.
measured by fixing the tip on the film surface while applying a triangle voltage ranging from −40 V to +40 V. The OP phase hysteresis loop shows an asymmetric “square” loop, reflecting the existence of a polarization imprint, as described in the PFM image discussion part. Moreover, the OP phase hysteresis loop shows the same orientation across the layer, whereas the IP hysteresis phase loops exhibit a pair of opposite orientations in areas 1 and 2. This is further evidence for a single OP
component of ordered self-polarization and opposite polarization components in the plane of the film. In addition, polarization offset was observed with a systematic vertical drift in the OP piezoresponse hysteresis loop in Figure 3d. The phase and amplitude loops are both asymmetry with an offset of negative bias, indicating a residual bias field pointing downward in the film, which results in a preferable downward polarization. It is noted that the different types of conductive C
DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 5. OP piezoresponse phase images of the 750 nm thick BNT thin film upon both heating and cooling. (a−f) Heating process: (a) 25, (b) 100, (c) 150, (d) 175, (e) 200, and (f) 250 °C. (g−l) Cooling process: (g) 250 °C, where the sample was kept after for 30 min, (h) 200, (i) 175, (j) 150, (k) 100, and (l) 25 °C. For each temperature upon heating or cooling, the top row is the phase image, and the bottom row is the histogram of phase distribution.
self-polarization toward the substrate (i.e., the same as the direction of the external positive voltage). In contrast, polarization switching happens under a negative dc bias. When the applied voltage reaches −20 V, the change of OP PFM phase indicates the switching by 180°. The same OP PFM phase switching is realized under the voltages of −40 and −60 V. Very interestingly, the switched polarization in terms of OP PFM phase immediately reverses back to the initial state upon the removal of dc bias, which is a rather unusual behavior. The detailed switching process is shown in the right column of Figure 4a, with the phase images under a series of negative dc bias from 0 V to −25 V, and then back to 0 V. By increasing the magnitude of the negative dc bias from −5 V to −20 V, the OP phase is gradually switched to the opposite state with a complete reversal of OP phase at −20 V. Further increase of the applied voltage to −25 V cannot alter the phase image anymore. However, the removal of the bias instantly causes the reversal of all the previously switched OP polarizations, as the phase image returns to its initial state before any bias was applied. More details of the switching process are described in Figure S3. The schematic of the switching process of the selfpolarization at different stages of the polarization reversal under a positive or negative external dc electric field, is shown in Figure 4b. The initial polarization state exhibits self-polarization toward the bottom electrode of the film. This self-polarization is reinforced when a positive bias with the same direction as that of self-polarization is loaded on the PFM tip. However, the polarization exhibits a more complex metastable behavior when a series of negative dc bias are loaded. At a low bias, there is no obvious change on the domain structure. As the external
tips used in PFM measurements results in different measured values, which include the response from the ferroelectric films, the surrounding layers and the film-tip interface.43 To avoid the influence from the surrounding layers and electrodes/tip, we measured macroscopic ferroelectric hysteresis loops, as shown in Figure 3e. One stepwise measurement cycle consists of varying the electric field E from 0 kV cm−1 to +1000 kV cm−1 then to −1000 kV cm−1, subsequently to +1000 kV cm−1, and finally back to 0 kV cm−1. The initial polarization P0 = 4.6 μC cm−2 (when E = 0 kV cm−1) increases to Ps+ = 51 μC cm−2 when E augments to 1000 kV cm−1. P(E) continues to decrease when E changes from +1000 to −1000 kV cm−1, and P changes sign from positive to negative at Ec− = −101 kV cm−1. When E reaches −1000 kV/cm, Ps− = −51 μC cm−2. Subsequently P starts increasing again when E changes from −1000 to +1000 kV cm−1, and P changes sign from negative to positive at Ec+ = 84 kV cm−1. From these measurements, we obtained asymmetric values of the coercive field and a remanent polarization of Pr = 15 μC cm−2. The initial nonzero polarization P0 = 4.6 μC cm−2 at E = 0 kV cm−1 indicates the preferred self-polarization in the film. Reversible Polarization Switching by Electric Field. Interestingly, it is found that the domain writing cannot be realized even under a high direct current (dc) bias of 60 V. During the local poling experiments, a series of dc bias voltages were selected and intermittently applied to the film. The measurement procedure and the results are schematically described in Figure 4a. No obvious change is observed in the OP PFM phase of the film under a positive dc bias, indicating that the positive bias is favorable for the stability of the initial D
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internal electric field is smaller than the those reported elsewhere, including numerous epitaxial films.29,30,32,33,35−38 One reason that contributes to such an internal electric field is the ionic vacancies in the film. Auger electron spectroscopy (AES) analysis combined with simultaneous sputter etching, shown in Figure 6, reveals the compositional profile of various
negative dc bias is increased, the polarization switching occurs with enlarged opposite domain areas. When the negative bias is large enough, domain switching is completed in the whole measured area. The switched polarization instantly recovers to its original polarization direction once the dc bias is removed. Thermally Recoverable Self-Polarization. The abovementioned reversibility of domain switching in the BNT film indicates the recoverable nature of the self-polarization with regard to external electric field. Here, we demonstrate that the self-polarization is also recoverable against thermal depoling. Figure 5 shows the local OP piezoresponse phase images of the 750 nm thick BNT thin film at 25, 100, 150, 175, 200, and 250 °C. Both heating and cooling processes are shown. For each temperature upon heating or cooling, the phase image is shown in the upper row, and the histogram of phase distribution is given in the bottom row. At room temperature (Figure 5a), there are no obvious changes of the OP PFM response within and between the grains, confirming the existence of a selfpolarization pointing toward the bottom of the film. When the temperature is increased to 150 °C (Figure 5c), some contrast areas appear, indicating the disruption, decay, and depoling of the self-polarization. Above 200 °C (Figure 5e, f), distinct domains with 180° contrast of phases and random distributions are observed, indicating further degradation of the selfpolarization. Surprisingly, when the film is held at 250 °C for 30 min (Figure 5g), some of the areas, which were switched to the opposite phase during the heating process, return to their original state. A similar effect was reported by Kalinin et al. in the BT thin film, and was explained as potential amplifying directly after the ferroelectric phase transition and subsequently decaying within ∼20−30 min.44 Upon cooling from 250 °C, the initial self-polarization state continues to grow larger (Figure 5h), and the original domain state is fully recovered at 175 °C and remains stable upon further cooling down to room temperature (Figure 5i-5l). After post annealing the BNT films at a temperature higher than TC for 30 min, the self-polarization fully recovers its initial state upon cooling down to room temperature (Figure S4).
Figure 6. Compositional distributions in the 750 nm thick BNT thin film from the film surface to the bottom Pt electrode by AES measurement. The vertical dot-dash lines on the right side indicate the Bi3+, Na+, and Ti4+ end points, respectively, and there is a region of Bi3+ vacancies between the two lines.
elements from the film surface to the bottom Pt electrode (the sputtering rate was about 10.7 nm/min). The Na+, Bi3+ and Ti4+ elements show nonuniform distributions at the BNT/Pt interface. Evidently, the AES signals of Bi3+ disappeared before the Na+ and Ti4+ signals did, suggesting an environment with Bi3+ vacancies that lead to immovable negative charges at the BNT/Pt interface. This environment with Bi3+ vacancies near the interface is independent of thickness, and the AES results for the 150 nm thick film are shown in Figure S5. It should be noted that the factors caused by sputtering such as sputterinduced roughness, knock-on effects, statistical effects, etc., could induce the interface broadening with respect to true profiles.47 And the influence of various diffusion rates of the three elements within the film and the various evaporation rates during ion sputtering of these elements could also contribute to the observed phenomenon to some extent. Although the interface broadening is caused by the measurement itself, the deficiency of Bi3+ at the interface can partly confirm the existence of Bi3+ vacancies at the interface regions. The Bi3+ deficiency was further confirmed by Transmission Electron Microscopy (TEM) results. A scanning TEM (STEM) dark field image in Figure 7a displays the BNT thin film layer, Pt bottom electrode layer and TiO2 layer below the electrode. Figure 7b shows the selective area electron diffraction (SAED) pattern, confirming a polycrystalline perovskite structure in the thin film. A corresponding energy-dispersive spectroscopy (EDS) line profile, as shown in Figure 7c, was taken across the interface of the film and bottom electrode, revealing the distributions of the Bi3+ and O2− ions across the interface. The profile analysis indicates a sharp transition from the film to substrate. The O2− anions show uniform distribution, while the Bi3+ ions are nonuniform at the interface region. The Bi3+/ O2− ratio line drops as the interface is approached, confirming the presence of an ∼40 nm Bi3+-deficient layer at the interface region. Another possible source of the internal electric field is the free electrons from the electrode. In a recent first-principles work, Sai et al. demonstrated that the monodomain was
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DISCUSSION Formation of Self-Polarization. There are two conventional explanations for the polarization orientation in ferroelectric thin films. The first one attributes the polarization distribution to the strain gradient.45 Lattice strain comes from the lattice mismatch and thermal strain comes from the thermal expansion coefficient (TEC) mismatch between the film and substrate. In polycrystalline films, only the thermal strain would be considered. As mentioned for PZT thin films, a large strain gradient seems unlikely to be formed in short distances within 1 μm between the film and the interface.46 Therefore, the strain gradient should not be the dominant factor influencing the polarization in the BNT film. The other explanation is related to an internal electric field, which was reported to exist at the interfaces between the films and the bottom electrodes. 21,24 This internal field is independent of the intrinsic polarization of the film and always points to one direction. The asymmetric piezoresponse (piezoatomistic) shown in Figure 3d and ferroelectric polarizationelectric field hysteresis loops (ferro-macroscopic) shown in Figure 3e confirm the presence of an internal field. It is concluded that the self-polarization in BNT thin films is primarily caused by the internal electric field, although the E
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use of noble metal electrodes (include Pt and Au) is more effective in screening the surface/interface bound charges than the oxide electrodes.50 We believe that the two kinds of charges accumulated at the interface between the BNT film and the Pt electrode, i.e., the Bi3+ vacancies from the BNT film near the interface and the negative free charges from Pt electrode, jointly play a role in promoting the formation of self-polarization. Mechanism of Recoverability of Self-Polarization. In general, domain writing can be realized in most of ferroelectric films, even in those with self-polarization. However, there are still some epitaxial films including BT, (Ba,Sr)TiO3 (BST), Pb(Mg1/3Nb2/3)O3−PbTiO3 (PMN−PT), PZT, and BiFeO3 (BFO) epitaxial films that repolarize after electrical treatment, which is due to the large internal electric field in the films.28−31,36,37,46,51,52 It has also been reported that backswitching happens in sol−gel PZT films with very low internal field, which resulted from the charged defects or a defective layer in the films.46 In this work, we have found an interesting behavior in the BNT films: the poled (written) state switches back to the original self-polarization direction once the applied field is removed. The small internal bias in this BNT film could not provide the force for domain back-switching. This reversible polarization switching in the BNT thin film is possibly not only due to the specific characteristics of this material, such as the universal existence of ion vacancies, especially some vacancies on the perovskite A-site, but also due to the film preparation process: the defect dipoles regime aligns
Figure 7. (a) STEM dark-field image of the BNT thin film. (b) Selective area electron diffraction (SAED) pattern of the polycrystalline film. (c) EDS profile across the film−electrode interface of the marked region in the STEM image and the ratio of Bi to O across the interface.
stabilized by the realistic electrodes in PbTiO3 on different types of substrates. It was found that when a perovskite film is deposited on Pt electrode, negative free charges from Pt electrode naturally tend to screen the depolarization from spontaneous polarization and help form a favorable homogeneous polarization.48,49 Moreover, it has been reported that the
Figure 8. (a) Projection of a typical (111) plane of the ABO3 perovskite structure containing six Ax+ sites and a D6− (O2− negative charge center) site (two Ax+ sites coinciding with the Ti4+ site at the center of the projection plane are not shown) in the nonpolar cubic paraelectric state with no spontaneous polarization Ps. (b) Polar rhombohedral ferroelectric state with Ps along one of the six equivalent polar directions (1, 2, 3, 4, 5, and 6 are equivalent sites). (c, d) Reversible polarization switching between (c) the as-deposited film and (d) the film applied external electric field. (c) There is a built-in internal electric field in the film with a direction toward the bottom of the film. Ps is along one of the directions 1, 2, and 3 (1, 2, and 3 are equivalent and 4, 5, and 6 are equivalent, but these two sets are unequivalent). If there is a Ax+ vacancy in the unit cell, the defect polarization PD (generated from the Ax+ vacancy point defect) must be along one of the 1, 2, or 3 directions, which is consistent with the Ps direction. (d) When an external field against the built-in field is applied on the film, Ps is switched along one of the equivalent 4, 5, or 6 directions. However, PD cannot be switched, which makes Ps reverse to its initial state shown in c upon removal of the external electric field. F
DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Bi3+ vacancies from the BNT film near the interface and negative free charges from the bottom electrode screen the depolarization field and contribute to the formation of an ordered polarization state.
with respect to the self-polarization on cooling through the transition during the repeated thermal annealing processes. Liu et al. reported that Na/Bi ratio of 50:50 usually does not obtained in pure BNT.53 One of the drawbacks of BNT for applications is its high leakage or conductivity.14,54 High conductivity in pure BNT is related to the vaporization of Na2O or Bi2O3 during sintering at high temperatures, which results in A-site cation deficiencies, as well as oxygen vacancies.9,14,55−58 Ren et al. reported a reversible domainswitching mechanism that point defects in typical perovskite ABO3 are arranged by following the symmetry of crystal, inducing a reversible domain switching.59−62 The oxygen vacancies and A-site cation deficiencies in BNT might contribute to the effects we have observed. Here, we focus on A-site cation vacancies in the BNT films as an example to describe the reversible switching of the polarization. This mechanism is depicted in Figure 8. A (111)-oriented grain is presented as an example of the randomly oriented polycrystalline film. The ABO3 perovskite structure is drawn in the {111} plane projection representing the (111)-oriented grains containing six Ax+ sites and a D6− (O2− negative charge center) site. In the rhombohedral ferroelectric phase without any external field, the sites 1, 2, 3, 4, 5, and 6 are equivalent and the spontaneous polarization Ps is along one of the six equivalent directions (Figure 8b) based on the polar asymmetry. There is a built-in internal electric field in our film with a direction toward the bottom of the film, which make sites 1, 2, and 3 the more favorable polarization directions, pointing toward the center of the octahedron with respect to the built-in internal field (Figure 8c). Because the BNT thin films were deposited and thermally annealed layer by layer, if there is an Ax+ vacancy in the unit cell, it tends to be located at one of the 1, 2, and 3 sites following the dipole moment direction upon cooling. The defect dipole PD generated from the point defect (Ax+ vacancy) is favored along the direction of Ps toward the bottom by thermal migration of the point defects. When an external field against the built-in internal field is applied, the spontaneous polarization Ps is switched abruptly to one of the directions 4, 5, or 6, following the direction of external electric field. However, the defect polarization PD cannot be switched in this process, as shown in Figure 8d. Therefore, once the external field is removed, it is this unswitchable defect dipole moment PD that drives spontaneous polarization to switch back, causing the reversible domain switching. This recoverable process is shown in Figure 8c, d. Note that this mechanism should be applied in the opposite direction of the defect site if the anionic defects are predominant in the particular unit cells. Temperature Effect. The domain evolutions upon heating and cooling processes exhibit different characteristics. It is found that there are two characteristic temperatures, TC and Td in BNT. Td is about 170 °C and TC is higher than 250 °C (the highest measurable temperature by PFM).10 It is common that there is an asymmetrical effect in the heating and cooling dynamic processes in relaxor ferroelectric ceramics. During the heating process, increased heat energy induces the incomplete screening of the depolarization field and stimulates the domain motion. Thus, polarization with opposite sign appears to balance the energy of polarization field, point defects field and compensation field from bottom electrode below Td. When the film is kept at 250 °C for 30 min, the energy of the system tends to reach a balanced profile, resulting in back-switching of some areas. During the cooling process, the polarization fully recovers above Td, because with the decrease of heat energy,
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CONCLUSIONS Distinctive characteristics have been observed in polycrystalline BNT thin films deposited on Pt/Ti/SiO2/Si substrates. The out-of-plane polarization of the film as imaged by PFM exhibits a stable self-polarization toward the bottom electrode of the films which is stable against electric-field switching and thermal depoling. The peculiar domain structure and switching behavior observed in the BNT films result from internal electric field arising from the free charges at the Pt surface and the defects at interfaces as well as in the oxides lattices. The negative free charges from the Pt electrode and the Bi3+ vacancies from the BNT film near the interface induce a self-polarization directed toward the bottom of the film. In addition, the point defects (oxygen and ion vacancies in BNT film) within certain unit cells form a defect dipole moment oriented along the spontaneous polarization in the BNT film, creating a restoring force for the recovery of the self-polarization. Such recoverable self-polarization not only overcomes the difficulties encountered in the poling of BNT films which typically exhibit large leakage, but also provides a stable polarization state over a wide temperature range, opening up a promising pathway to design various devices, including sensors and actuators that can operate under extreme conditions such as high temperatures and high electric fields.
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METHODS
Film Fabrication. The perovskite BNT films of 150−900 nm thick were prepared using the sol−gel techinique.63 First, the bismuth nitrate pentahydrate was dissolved in 2-methoxyethanol, the sodium acetate was dissolved in 2-methoxyethanol and acetic acid, and the tetrabutyl titanate was dissolved in 2-methoxyethanol mixed with two equivalents of acetylacetone. Additional 10 mol % Na and 2 mol % Bi were taken into account according to the stoichiometric ratio to compensate for the evaporation of these elements. Tetrabutyl titanate solution was then added to the bismuth nitrate pentahydrate solution drop by drop, followed by adding the sodium acetate solution drop by drop. The resultant mixture was made in 0.4 M concentration and was refluxed at 90 °C for 1 h and aged for more than 24 h before being used to deposit thin films. Further, the solution was deposited on (111) Pt/TiO2/SiO2/Si substrates by spin coating at 3000 rpm for 40 s. Each layer was pyrolyzed at 150 °C for 3 min and at 410 °C for 10 min to remove organics, and then annealed at 700 °C for 3 min for crystallization. These deposit and thermal processes were repeated layer by layer to reach the desired thickness. Au were sputter-deposited onto the thin film surface as top electrodes at room temperature for ferroelectric measurements. Structure and Morphology Characterizations. The crystalline features of the films were decided by X-ray diffraction (XRD, Rigaku D/Max-2400, Tokyo, Japan). The morphologies were characterized by field-emission scanning electron microscopy (FESEM, Quanta 250 FEG, FEI, Japan). Ferroelectric Hysteresis Loops. The polarization hysteresis loops were measured by a ferroelectric measurement system (RT66A, Radiant Technologies, Albuquerque, NM, USA) equipped with a probe station. AES Analysis. Auger electron spectroscopy (AES, PHI 700, Japan) combined with simultaneous sputter etching was used to detect the depth profile of the BNT thin films. The incident angle of Ar+ ion was 30° with an etching rate of 10.7 nm min−1. The compositional signals were collected each 15 s (or 30 s) with an energy resolution of 1‰. G
DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces TEM Measurements. Transmission electron microscopy (TEM, JEOL, ARM200F, Japan) was used to examine the structure and composition of the BNT thin films. The STEM dark-field image, electron diffraction pattern of the polycrystalline film, and STEM/EDS profile across the film-electrode interface were obtained. PFM Measurement. PFM has been a powerful way to characterize the domain structure and switching behavior of piezoresponsive materials.64,65 In this work, PFM measurements were performed in the contact mode by using scanning probe microscope (Bruker, Dimension Icon, Nanoscopy V, USA) equipped with a dc voltage amplifier and a dynamic-heating device. During the PFM measurements, the bottom electrodes were glued to the PFM holder by silver paste, and the film surfaces without electrodes were subjected to PFM tip scanning. All PFM measurements were performed with a diamond coated silicon tip ( f 0: 230−410 kHz, k: 20−80 N m−1) by modulating the system at 45 kHz with peak voltage of 4 V. The ac signal was applied on the bottom electrode to induce piezoresponse-dependent sinusoidal vibrations in BNT films, and a dc voltage was applied on the tip for poling experiments. The induced displacements under the tip due to the converse piezoelectric effect were measured. Threedimensional (3D) PFM was used to examine the stereoscopic domain structure of BNT films both in vertical mode by detecting the bending of the cantilever (proportional to the OP polarization) and in lateral mode by detecting the twisting of the cantilever (proportional to the IP polarization). Piezoresponse hysteresis loops were locally acquired when the PFM tip was stopped at the selected location and the triangular voltage of both polarities were sequentially applied on the tip. Because no calibration was done, the OP displacement and IP displacement are expressed in relative units. The Bruker Dimension heater equipped on the PFM system enables measurement at high temperatures (up to 250 °C), which was used to probe the polarization distribution of BNT thin films from 25 to 250 °C. During the heating and cooling measurements, both the tip and sample were heated and controlled by the thermal applications controller. Note that BNT films with thickness varying from 150 to 900 nm have been prepared and characterized carefully. Given the similar structures, morphologies, and polarization characteristics in all the films, only the results of the 750 nm thick film are discussed in detail in this work.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Natural Science Foundation of China (Grant 51332003, 51202184, 51602247, and 91323303), and the “111 Project” of China (B14040). The PFM capability at XJTU was set up with the support from the “Qian Ren” Program of the Chinese Government. Z.G.Y. also acknowledges the support from the Natural Sciences and Engineering Research Council of Canada (NSERC, Grant 203773). The authors are indebted to Professor Susan TrolierMcKinstry for insightful discussions. The authors thank Alisa Paterson for improving the technical writing.
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(1) Maurya, D.; Zhou, Y.; Wang, Y. J.; Yan, Y. K.; Li, J. F.; Viehland, D.; Priya, S. Giant Strain with Ultra-low Hysteresis and High Temperature Stability in Grain Oriented Lead-free K0.5Bi0.5TiO3BaTiO3-Na0.5Bi0.5TiO3 Piezoelectric Materials. Sci. Rep. 2015, 5, 8595. (2) Smolenskii, G. A.; Isupov, V. A.; Agranovskaya, A. I.; Krainik, N. N. New Ferroelectrics of Complex Composition.4. Sov. Phys.-Solid State 1961, 2, 2651−2654. (3) Li, M.; Pietrowski, M. J.; De Souza, R. A.; Zhang, H. R.; Reaney, I. M.; Cook, S. N.; Kilner, J. A.; Sinclair, D. C. A Family of Oxide Ion Conductors Based on the Ferroelectric Perovskite Na0.5Bi0.5TiO3. Nat. Mater. 2014, 13, 31−35. (4) Levin, I.; Reaney, I. M. Nano- and Mesoscale Structure of Na1/2Bi1/2TiO3: A TEM Perspective. Adv. Funct. Mater. 2012, 22, 3445−3452. (5) Siny, I. G.; Tu, C. S.; Schmidt, V. H. Critical Acoustic Behavior of the Relaxor Ferroelectric Na0.5Bi0.5TiO3 in the Intertransition Region. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 51, 5659−5665. (6) Kreisel, J.; Glazer, A. M.; Bouvier, P.; Lucazeau, G. High-pressure Raman Study of a Relaxor Ferroelectric: The Na 0.5 Bi 0.5 TiO 3 Perovskite. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 174106. (7) Xu, J. B.; Liu, Y.; Withers, R. L.; Brink, F.; Yang, H.; Wang, M. Ferroelectric and Non-linear Dielectric Characteristics of Bi0.5Na0.5TiO3 Thin Films Deposited via a Metallorganic Decomposition Process. J. Appl. Phys. 2008, 104, 116101. (8) Dorcet, V.; Trolliard, G.; Boullay, P. Reinvestigation of Phase Transitions in Na0.5Bi0.5TiO3 by TEM. Part I: First Order Rhombohedral to Orthorhombic Phase Transition. Chem. Mater. 2008, 20, 5061−5073. (9) Yu, T.; Kwok, K. W.; Chan, H. L. W. Preparation and Properties of Sol-gel-derived Bi0.5Na0.5TiO3 Lead-free Ferroelectric Thin Film. Thin Solid Films 2007, 515, 3563−3566. (10) Sung, Y. S.; Kim, J. M.; Cho, J. H.; Song, T. K.; Kim, M. H.; Park, T. G. Effects of Bi Nonstoichiometry in (Bi0.5+xNa) TiO3 Ceramics. Appl. Phys. Lett. 2011, 98, 012902. (11) Jones, G. O.; Kreisel, J.; Jennings, V.; Geday, M. A.; Thomas, P. A.; Glazer, A. M. Investigation of a Peculiar Relaxor Ferroelectric: Na0.5Bi0.5TiO3. Ferroelectrics 2002, 270, 191−196. (12) Jones, G. O.; Thomas, P. A. The Tetragonal Phase of Na0.5Bi0.5TiO3 - A New Variant of the Perovskite Structure. Acta Crystallogr., Sect. B: Struct. Sci. 2000, 56, 426−430. (13) Zhang, M. S.; Scott, J. F.; Zvirgzds, J. A. Raman Spectroscopy of Na0.5Bi0.5TiO3. Ferroelectr., Lett. Sect. 1986, 6, 147−152. (14) Groting, M.; Hayn, S.; Albe, K. Chemical Order and Local Structure of the Lead-free Relaxor Ferroelectric Na1/2Bi1/2TiO3. J. Solid State Chem. 2011, 184, 2041−2046. (15) Keeble, D. S.; Barney, E. R.; Keen, D. A.; Tucker, M. G.; Kreisel, J.; Thomas, P. A. Bifurcated Polarization Rotation in Bismuth-Based Piezoelectrics. Adv. Funct. Mater. 2013, 23, 185−190. (16) Chu, Y. H.; Cruz, M. P.; Yang, C. H.; Martin, L. W.; Yang, P. L.; Zhang, J. X.; Lee, K.; Yu, P.; Chen, L. Q.; Ramesh, R. Domain Control
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b04033. XRD result of BNT powder, PFM images of 150 nm thick and 900 nm thick films, polarization switching behavior of 750 nm thick BNT thin film after post annealing, the detailed switching and back-switching behavior of 750 nm thick BNT thin film, and the AES depth profile of 150 nm thick BNT thin film (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: *E-mail: *E-mail: *E-mail:
REFERENCES
[email protected] (W.R.).
[email protected] (G.N.).
[email protected] (N.Z.).
[email protected] (Z.-G.Y.).
ORCID
Wei Ren: 0000-0001-9749-0699 Gang Niu: 0000-0002-8813-8885 Zuo-Guang Ye: 0000-0003-2378-7304 Author Contributions
All authors contributed to the discussion and writing of the manuscript. The final version was approved by all authors. H
DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces in Mulfiferroic BiFeO3 through Substrate Vicinality. Adv. Mater. 2007, 19, 2662−2666. (17) Lichtensteiger, C.; Weymann, C.; Fernandez-Pena, S.; Paruch, P.; Triscone, J. M. Built-in Voltage in Thin Ferroelectric PbTiO3 Films: the Effect of Electrostatic Boundary Conditions. New J. Phys. 2016, 18, 043030. (18) Catalan, G.; Seidel, J.; Ramesh, R.; Scott, J. F. Domain Wall Nanoelectronics. Rev. Mod. Phys. 2012, 84, 119−156. (19) Thompson, C.; Fong, D. D.; Wang, R. V.; Jiang, F.; Streiffer, S. K.; Latifi, K.; Eastman, J. A.; Fuoss, P. H.; Stephenson, G. B. Imaging and Alignment of Nanoscale 180 Degrees Stripe Domains in Ferroelectric Thin Films. Appl. Phys. Lett. 2008, 93, 182901. (20) Catalan, G.; Bea, H.; Fusil, S.; Bibes, M.; Paruch, P.; Barthelemy, A.; Scott, J. F. Fractal Dimension and Size Scaling of Domains in Thin Films of Multiferroic BiFeO3. Phys. Rev. Lett. 2008, 100, 027602. (21) Yamaka, E.; Watanabe, H.; Kimura, H.; Kanaya, H.; Ohkuma, H. Structural, Ferroelectric, and Pyroelectric Properties of Highly c-axis Oriented Pb1−xCaxTiO3 Thin Film Grown by Radio-frequency Magnetron Sputtering. J. Vac. Sci. Technol., A 1988, 6, 2921−2928. (22) Ohba, Y.; Miyauchi, M.; Tsurumi, T.; Daimon, M. Analysis of Bending Displacement of Lead Zirconate Titanate Thin Film Synthesized by Hydrothermal Method. Jpn. J. Appl. Phys. Part 1 Regul. Pap. Short Notes Rev. Pap. 1993, 32, 4095−4098. (23) Bruchhaus, R.; Pitzer, D.; Primig, R.; Wersing, W.; Xu, Y. Q. Deposition of Self-polarized PZT Films by Planar Multi-target Sputtering. Integr. Ferroelectr. 1997, 14, 141−149. (24) Alguero, M.; Stewart, M.; Cain, M. G.; Ramos, P.; Ricote, J.; Calzada, M. L. Properties of Morphotropic Phase Boundary Pb(Mg1/3Nb2/3)O3-PbTiO3 Films with Submicrometre Range Thickness on Si-based Substrates. J. Phys. D: Appl. Phys. 2010, 43, 205401. (25) Lima, E. C.; Araujo, E. B.; Bdikin, I. K.; Kholkin, A. L. The Selfpolarization Effect in Pb(Zr0.50Ti0.50)O3 Thin Films with no Preferential Orientation. Mater. Res. Bull. 2012, 47, 3548−3551. (26) Pantel, D.; Goetze, S.; Hesse, D.; Alexe, M. Room-Temperature Ferroelectric Resistive Switching in Ultrathin Pb(Zr0.2Ti0.8)O3 Films. ACS Nano 2011, 5, 6032−6038. (27) Jeon, B. C.; Lee, D.; Lee, M. H.; Yang, S. M.; Chae, S. C.; Song, T. K.; Bu, S. D.; Chung, J. S.; Yoon, J. G.; Noh, T. W. Flexoelectric Effect in the Reversal of Self-Polarization and Associated Changes in the Electronic Functional Properties of BiFeO3 Thin Films. Adv. Mater. 2013, 25, 5643−5649. (28) Lee, H.; Kim, T. H.; Patzner, J. J.; Lu, H.; Lee, J.-W.; Zhou, H.; Chang, W.; Mahanthappa, M. K.; Tsymbal, E. Y.; Gruverman, A.; Eom, C.-M. Imprint Control of BaTiO3 Thin Films via Chemically Induced Surface Polarization Pinning. Nano Lett. 2016, 16, 2400−2406. (29) Yoshida, S.; Hanzawa, H.; Wasa, K.; Esashi, M.; Tanaka, S. Highly c-axis-oriented Monocrystalline Pb(Zr,Ti)O3 Thin Films on Si Wafer Prepared by Fast Cooling Immediately after Sputter Deposition. IEEE Trans. Ultrason. Ferroelect. Freq. Control 2014, 61, 1552−1558. (30) Wasa, K.; Adachi, H.; Nishida, K.; Yamamoto, T.; Matsushima, T.; Kanno, I.; Kotera, H. Highly Polarized Sngle-c-domain Singlecrystal Pb(Mn,Nb)O3-PZT Thin Films. IEEE Trans. Ultrason. Ferroelect. Freq. Control 2012, 59, 6−13. (31) Lee, D.; Jeon, B. C.; Baek, S. H.; Yang, S. M.; Shin, Y. J.; Kim, T. H.; Kim, Y. S.; Yoon, J. G.; Eom, C. B.; Noh, T. W. Active Control of Ferroelectric Switching Using Defect-Dipole Engineering. Adv. Mater. 2012, 24, 6490−6495. (32) Zhang, T.; Wasa, K.; Kanno, I.; Zhang, S.-Y. Ferroelectric Properties of Pb(Mn1/3Nb2/3)O3-P (Zr,Ti)O3 Thin Films Epitaxially Grown on (001) MgO Substrates. J. Vac. Sci. Technol., A 2008, 26, 985−990. (33) Wasa, K.; Kanno, I.; Kotera, H.; Yamauchi, N.; Matsushima, T. In Thin Films of PZT-Based Ternary Perovskite Compounds for MEMS. In 2008 IEEE International Ultrasonics Symposium, IEEE: Piscataway, NJ, 2008; pp 213−216. (34) Kanno, I.; Mino, T.; Kuwajima, S.; Suzuki, T.; Kotera, H.; Wasa, K. Piezoelectric Properties of (K,Na)Nb3 Thin Films Deposited on (001) SrRuO3/Pt/MgO Substrates. IEEE Trans. Ultrason. Ferroelect. Freq. Control 2007, 54, 2562−2566.
(35) 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, 4091−4096. (36) Abe, K.; Yanase, N.; Kawakubo, T. Asymmetric Switching of Ferroelectric Polarization in A Heteroepitaxial BaTiO3 Thin Film Capacitor. Jpn. J. Appl. Phys. 2000, 39, 4059−4063. (37) Yanase, N.; Abe, K.; Fukushima, N.; Kawakubo, T. Thickness Dependence of Ferroelectricity in Heteroepitaxial BaTiO3 Thin Film Capacitors. Jpn. J. Appl. Phys. 1999, 38, 5305−5308. (38) Abe, K.; Yanase, N.; Sano, K.; Izuha, M.; Fukushima, N.; Kawakubo, T. Modification of Ferroelectricity in Heteroepitaxial (Ba,Sr)TiO3 Films for Non-volatile Memory Applications. Integr. Ferroelectr. 1998, 21, 197−206. (39) Bousquet, M.; Duclere, J. R.; Gautier, B.; Boulle, A.; Wu, A.; Deputier, S.; Fasquelle, D.; Remondiere, F.; Albertini, D.; Champeaux, C.; Marchet, P.; Guilloux-Viry, M.; Vilarinho, P. Electrical Properties of (110) Epitaxial Lead-free Ferroelectric Na0.5Bi0.5TiO3 Thin Films Grown by Pulsed Laser Deposition: Macroscopic and Nanoscale Data. J. Appl. Phys. 2012, 111, 104106. (40) Remondiere, F.; Wu, A.; Vilarinho, P. M.; Mercurio, J. P. Piezoforce Microscopy Study of Lead-free Perovskite Na0.5Bi0.5TiO3 Thin Films. Appl. Phys. Lett. 2007, 90, 152905. (41) Bousquet, M.; Duclere, J. R.; Champeaux, C.; Boulle, A.; Marchet, P.; Catherinot, A.; Wu, A.; Vilarinho, P. M.; Deputier, S.; Guilloux-Viry, M.; Crunteanu, A.; Gautier, B.; Albertini, D.; Bachelet, C. Macroscopic and Nanoscale Electrical Properties of Pulsed Laser Deposited (100) Epitaxial Lead-free Na0.5Bi0.5TiO3 Thin Films. J. Appl. Phys. 2010, 107, 034102. (42) Yao, Y. P.; Fu, H. X. Charged Vacancies in Ferroelectric PbTiO3: Formation Energies, Optimal Fermi Region, and Influence on Local Polarization. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 064112. (43) Kalinin, S. V.; Bonnell, D. A. Imaging Mechanism of Piezoresponse Force Microscopy of Ferroelectric Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 125408. (44) Kalinin, S. V.; Bonnell, D. A. Temperature Dependence of Polarization and Charge Dynamics on the BaTiO3(100) Surface by Scanning Probe Microscopy. Appl. Phys. Lett. 2001, 78, 1116−1118. (45) Lu, H.; Bark, C.-W.; De Los Ojos, D. E.; Alcala, J.; Eom, C.-B.; Catalan, G.; Gruverman, A. Mechanical Writing of Ferroelectric Polarization. Science 2012, 336, 59−61. (46) Shepard, J. F., Jr; Chu, F.; Kanno, I.; Trolier-McKinstry, S. Characterization and Aging Response of the d31 Piezoelectric Coefficient of Lead Zirconate Titanate Thin Films. J. Appl. Phys. 1999, 85, 6711−6716. (47) Hofmann, S.; Erlewein, J.; Zalar, A. Depth Resolution and Surface Roughness Effects in Sputter Profiling of NiCr Multilayer Sandwich Samples using Auger Electron Spectroscopy. Thin Solid Films 1977, 43, 275−283. (48) Sai, N.; Kolpak, A. M.; Rappe, A. M. Ferroelectricity in Ultrathin Perovskite Films. Phys. Rev. B: Condens. Matter Mater. Phys. 2005, 72, 020101. (49) Pronin, I. P.; Kaptelov, E. Y.; Tarakanov, E. A.; Sorokin, L. M.; Afanasjev, V. P.; Pankrashkin, A. V. Self-polarization and Migratory Polarization in Thin-film Ferroelectric Capacitor. Integr. Ferroelectr. 2002, 49, 285−294. (50) Stengel, M.; Vanderbilt, D.; Spaldin, N. A. Enhancement of Ferroelectricity at Metal-oxide Interfaces. Nat. Mater. 2009, 8, 392− 397. (51) Maria, J.-P.; Hackenberger, W.; Trolier-McKinstry, S. Phase Development and Electrical Property Analysis of Pulsed Laser Deposited Pb(Mg1/3Nb2/3)O3−PbTiO3(70/30) Epitaxial Thin Films. J. Appl. Phys. 1998, 84, 5147−5154. (52) Maria, J.; Hackenberger, W.; Trolier-McKinstry, S. Deposition and Electrical Characterization of Epitaxial Pb(Mg1/3Nb2/3)O3PbTiO3(70/30) Thin Films. MRS Online Proc. Libr. 1997, 493, 421. (53) Liu, Y.; Noren, L.; Studer, A. J.; Withers, R. L.; Guo, Y.; Li, Y.; Yang, H.; Wang, J. Response of Intergrown Microstructure to an I
DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Electric Field and Its Consequences in the Lead-free Piezoelectric Bismuth Sodium Titanate. J. Solid State Chem. 2012, 187, 309−315. (54) Hiruma, Y.; Nagata, H.; Takenaka, T. Thermal Depoling Process and Piezoelectric Properties of Bismuth Sodium Titanate Ceramics. J. Appl. Phys. 2009, 105, 084112. (55) Nagata, H. Electrical Properties and Tracer Diffusion of Oxygen in some Bi-based Lead-free Piezoelectric Ceramics. J. Ceram. Soc. Jpn. 2008, 116, 271−277. (56) Oh, T.; Kim, M. H. Phase Relation and Dielectric Properties in (Bi1/2Na1/2)(1‑x)BaxTiO3 Lead-free Ceramics. Mater. Sci. Eng., B 2006, 132, 239−246. (57) Zhou, Z. H.; Xue, J. M.; Li, W. Z.; Wang, J.; Zhu, H.; Miao, J. M. Ferroelectric and Electrical Behavior of (Na0.5Bi0.5)TiO3 Thin Films. Appl. Phys. Lett. 2004, 85, 804−806. (58) Zhou, Z. H.; Xue, J. M.; Li, W. Z.; Wang, J.; Zhu, H.; Miao, J. M. Leakage Current and Charge Carriers in (Na0.5Bi0.5)TiO3 Thin Film. J. Phys. D: Appl. Phys. 2005, 38, 642−648. (59) Ren, X. B. Large Electric-field-induced Strain in Ferroelectric Crystals by Point-defect-mediated Reversible Domain Switching. Nat. Mater. 2004, 3, 91−94. (60) Zhang, L. X.; Chen, W.; Ren, X. Large Recoverable Electrostrain in Mn-doped (Ba,Sr) TiO3 Ceramics. Appl. Phys. Lett. 2004, 85, 5658−5660. (61) Zhang, L. X.; Liu, W. F.; Chen, W.; Ren, X. B.; Sun, J.; Gurdal, E. A.; Ural, S. O.; Uchino, K. Mn Dopant on the ″Domain Stabilization″ Effect of Aged BaTiO3 and PbTiO3-based Piezoelectrics. Appl. Phys. Lett. 2012, 101, 242903. (62) Wang, D.; Wang, Y. Z.; Zhang, Z.; Ren, X. B. Modeling Abnormal Strain States in Ferroelastic Systems: The Role of Point Defects. Phys. Rev. Lett. 2010, 105, 205702. (63) Ji, H. F.; Ren, W.; Wang, L. Y.; Shi, P.; Chen, X. F.; Wu, X. Q.; Yao, X.; Lau, S. T.; Zhou, Q. F.; Shung, K. K. Structure and Electrical Properties of Na0.5Bi0.5TiO3 Ferroelectric Thick Films Derived From a Polymer Modified Sol-Gel Method. IEEE Trans. Ultrason. Ferroelect. Freq. Control 2011, 58, 2042−2049. (64) Kalinin, S. V.; Rar, A.; Jesse, S. A Decade of Piezoresponse Force Microscopy: Progress, Challenges, and Opportunities. IEEE Trans. Ultrason. Ferroelect. Freq. Control 2006, 53, 2226−2252. (65) Gruverman, A.; Kalinin, S. V. Piezoresponse Force Microscopy and Recent Advances in Nanoscale Studies of Ferroelectrics. J. Mater. Sci. 2006, 41, 107−116.
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DOI: 10.1021/acsami.7b04033 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
