Self-Polarization in Epitaxial Fully Matched Lead-Free Bismuth

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Self-Polarization in Epitaxial Fully Matched Lead-Free Bismuth Sodium Titanate Based Ferroelectric Thin Films Jinyan Zhao,† Gang Niu,*,† Wei Ren,*,† Lingyan Wang,† Guohua Dong,† Nan Zhang,† Ming Liu,† and Zuo-Guang Ye*,†,‡ †

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Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education & International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China ‡ Department of Chemistry and 4D LABS, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada S Supporting Information *

ABSTRACT: The Bi0.5Na0.5TiO3-based ferroelectric is one of the most promising candidates for environment-friendly lead-free ferroelectric/piezoelectric materials for various applications such as actuators and micro-electromechanical systems. The understanding and tailoring of the ferro-(piezo-)electric properties of thin films, however, are strongly hindered by the formation of the defects such as dislocations, ion vacancies in the film, as well as by the complexity of the interface between the film and the substrate. An ideal system for the study of the polarization behavior in the ferro-(piezo-)electric film would be a fully matched system. In this work, monocrystalline 0.89Bi0.5Na0.5TiO3−0.11BaTiO3 thin films were epitaxially grown on (001)-oriented Nbdoped SrTiO3 substrates using a sol−gel technique. The films were almost fully lattice- and thermally matched with the substrate, thus avoiding the impact of dislocations and thermal stress. The films were self-poled by a built-in electric field, originating from the sedimentation of heavier atoms during the film preparation. As a consequence, an upward self-polarization was introduced into the films, giving rise to asymmetric phase hysteresis loops and domain switching current responses. These results highlight the importance of the interface complexity for the self-polarization of piezoelectric thin films, even for fully matched films, which will therefore facilitate the control of properties of piezoelectric films and their applications for various functional devices. KEYWORDS: lead-free, thin film, lattice-matched, thermally matched, epitaxial growth, sol−gel

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INTRODUCTION Ferroelectric thin films exhibit a switchable spontaneous polarization, which can be used in memories, multilayer capacitors, sensors, and actuators.1 In these applications, ferroelectric films generally need to be poled by an external electric field to display the macroscopic polarization. In some peculiar cases, as-deposited ferroelectric films exhibit the macroscopic polarization without being applied external electric field, namely they have a self-polarization.2 The formation of self-polarization is beneficial for some ferroelectric films which are difficult to be poled by an external electric field and greatly advantageous for some device applications. Self-polarization has been reported in a number of ferroelectric films, especially in epitaxial films, such as PbTiO3,3,4 Pb(Zr,Ti)O3,5,6 BaTiO3,7 and BiFeO3.8,9 This phenomenon was mainly attributed to a built-in electric field or strain gradients. The former could originate from charged defects,10 the film/substrate interface effect,11,12 and so on, whereas the latter could arise from the mismatch of © XXXX American Chemical Society

the lattice parameters and thermal expansion coefficients between the film and the substrate.13,14 Nevertheless, it is still a challenge to determine the true origin of self-polarization in films and the reports on how self-polarization was induced by various above-mentioned factors are not consistent. Therefore, an ideal way to clarify the mechanism of the self-polarization is to fabricate and to study fully matched films. The solid solutions of (1 − x)Bi0.5Na0.5TiO3−xBaTiO3 (BNT−100·xBT) are one of the most promising candidates for lead-free piezoelectric materials, which have been extensively investigated.15−17 The BNT−100·xBT solution has a morphotropic phase boundary (MPB) at x = 0.06− 0.07, where the dielectric and piezoelectric properties reach their peak values.18,19 Different from conventional MPBs, Jo et Received: February 6, 2018 Accepted: May 29, 2018

A

DOI: 10.1021/acsami.8b02239 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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splitting of (002) peaks is not evident but broadening can be obviously observed in BNT−11BT thin film. Considering the possibility of the analysis using TOPAS software, the BNT− 11BT powder obtained from the gel solution was also characterized using the XRD method, and the data were analyzed by Rietveld refinements, as shown in Figure S1 (Supporting Information). The pattern can be well-refined using the mixed model of an R phase and a T phase. Because of the small distortion of the R phase and the T phase compared with the cubic phase of BNT−11BT, the (002) peaks of both ceramic and film samples only broaden rather than splitting. Figure S2 shows the θ−2θ XRD patterns of BNT-based thin films with different compositions and BNT−11BT thin films with different thicknesses. They also confirm that the overlapped peaks in Figure 1a,b are due to the close lattice parameters between the BNT−11BT film and the Nb:STO substrate. Figure 1c shows the reciprocal space mapping (RSM) of the (103) reflection of the heterostructure. It can be observed that the primary axis of pseudocubic lattice coincides with that of Nb:STO and the spot of the BNT−11BT film is in close vicinity to that of Nb:STO, resulting in superimposed spots. These results indicate that the BNT−11BT thin film was indeed epitaxially grown on the substrate, in a fully latticematched way. To examine the microscopic characteristics of the monocrystalline BNT−11BT thin film epitaxially grown on the Nb:STO substrate, atomic force microscopy (AFM), and transmission electron microscopy (TEM) measurements were performed. A surface AFM image of the film is presented in Figure 2a. The film exhibits a dense and smooth surface with a

al. reported a novel structural phase boundary in the BNT−BT system at x = 0.11, i.e., BNT−11BT. BNT−11BT exhibits the coexistence of a rhombohedral (R) phase and a tetragonal (T) phase, and its space group symmetry changes across the phase boundary which separates the R3m and P4mm phases.20 It is interesting to note that the bulk lattice parameter of BNT− 11BT is 0.3910 nm,20 which is almost the same (only +0.1% lattice mismatch) as that of SrTiO3 (STO, with a cubic symmetry and a lattice parameter of 0.3905 nm), a commonly used substrate for the deposition of ferroelectric films. Furthermore, the TEC of BNT−11BT is 11 ppm/K, which is the same as that of STO (11 ppm/K) as well. As a consequence, lattice strain and thermal strain can be negligible when BNT−11BT films are deposited on STO substrates, and defects such as dislocations which are related to the strain relaxation in mismatched systems rarely exist in these latticematched films. Moreover, the cracks induced by the thermal mismatch during the film deposition can also be avoided in this thermally matched system. In this way, BNT−11BT/STO is an ideal system for the control and the analysis of the polarization of ferroelectric thin films. In this work, a fully lattice- and thermal-matching system was prepared by a sol−gel technique. Sol−gel is a low-cost method permitting easy adjustment of the composition of films. The crystal structure, microstructural characteristics, macroscopic switching behavior, and domain morphology of (001)-oriented monocrystalline BNT−11BT films on Nb:STO substrates have been examined. It is found that the upward self-polarization observed in this film is related to the atom sedimentation during the layer-by-layer film deposition process.

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RESULTS A specular θ−2θ X-ray diffraction (XRD) pattern is shown in Figure 1a. It can be seen that the thin film exhibits a single-

Figure 1. (a) Specular θ−2θ XRD pattern, (b) enlarged (002) peaks, and (c) RSM of the monocrystalline BNT−11BT thin film deposited on the Nb:STO substrate.

Figure 2. Morphology of the monocrystalline BNT−11BT thin film on the Nb:STO substrate: (a) AFM image revealing the surface properties of the BNT−11BT thin film; (b) low-magnification TEM cross-sectional image of the 370 nm-thick BNT−11BT thin film; (c) HAADF-STEM image with a higher magnification on the interface region of BNT−11BT film and Nb:STO substrate as marked in (b) by a red square; (d) schematic image of the sedimentation of heavier elements on each deposited layer.

phase structure oriented parallel to the (00l) plane, with the pattern coinciding exactly with that of the Nb:STO substrate. Figure 1b shows an enlarged figure of BNT−11BT/STO (002) peaks, in which the diffraction peaks of the epitaxial film and the substrate can hardly be distinguished because of their very close lattice parameters. It was reported that BNT−11BT exhibits the coexistence of R and T phases.20 In Figure 1b, the

root-mean-square roughness Ra of only 1.60 nm. A lowmagnification TEM cross-sectional image of the BNT−11BT film is displayed in Figure 2b, from which the film thickness can be extracted as 370 nm. A high-angle annular dark field scanning TEM (HAADF-STEM) image with a higher magnification on the interface region (marked by a red square in Figure 2b) is shown in Figure 2c. Not only the interface B

DOI: 10.1021/acsami.8b02239 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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peaks in both powder (Figure S2) and film (Figure 1) samples. Figure 3b reveals an atomic-resolution HAADF-STEM image focused on the interface of the BNT−11BT/Nb:STO system, illustrating an atomic registry at the interface area. The lattice planes of the thin film are well-aligned with those of Nb:STO, and no dislocation can be observed. The lattice distance of the film (dBNT−11BT) obtained in the high-resolution atomic image is exactly equal to that of the substrate (dSTO). The top inset enlarged image of the green-squared region in the film bulk in Figure 3b shows more atomic details of the film. It can be seen that the A-site atom column in the film shows an irregular contrast without a periodic feature. This indicates a random distribution of the three possible kinds of elements (Bi, Ba, and Na) at A-site in the unit cells. Furthermore, in HAADF-STEM images, heavier atoms show brighter spots. Interestingly, in the bottom inset image of Figure 3b, slight irregularities of atomic dimension also exist at the bottom of the film closed to the substrate surface. This is probably due to the enrichment of Bi and Ba at the interface. To further prove the enrichment of Bi/Ba elements at the interfaces observed in the HAADF-STEM images, a powerful elemental analysis technique, auger electron spectroscopy (AES) analysis, combined with simultaneous sputter etching was utilized. Figure 4a reveals the compositional profile of various elements from the film surface to the bottom electrode. The BNT−11BT film, Nb:STO substrate, and interface regions are observed clearly in the profile. The Na, Bi, and Ba elements show nonuniform distributions in the film. Because the factors caused by sputtering such as film roughness, knock-on effects, statistical effects, and so on could affect the distribution with respect to true profiles, and the influence increases with increasing thickness. Therefore, the profile close to the film surface is more reliable. As a consequence, the Bi/O, Ba/O, and Ti/O ratios in the marked square region close to the film surface in Figure 4a are shown in Figure 4b. A horizontal line of the Ti/O ratio is observed while the Bi/O and Ba/O lines exhibit two maximum peaks in this region, which is related to the two interface layers in the film. The phenomenon is in accordance with the contrast observed in the HAADF-STEM image, indicating the enrichment of Bi/Ba elements at the bottom of each deposited film layer. To investigate the ferroelectric properties of the fully matched films and to possibly correlate the structural properties of the films with their functionalities, electrical measurements were performed on BNT−11BT/Nb:STO heterostructures using the positive-up-negative-down (PUND) method;23,24 the results are shown in Figure 5. Figure 5a depicts the pulse sequences with 2 μs duration symbolized by P, U, N, and D for measuring the positive and negative switching current. The first positive pulse (P) was applied to measure both the switching and nonswitching responses of the polarization, and subsequent by the second positive pulse (U) was applied to measure the nonswitching response. The positive switching response was obtained from the difference between the values of P and U. Similarly, the negative switching response was obtained from the difference between the values of the first negative pulse (N) and the second negative pulse (D). As a result, the difference between the positive and negative switching currents was observed, as shown in Figure 5b. In Figure 5b, the red squares and blue circles show positive and negative switching currents, respectively, while the black solid lines reveal the fitting data. The positive switching current isw+ = iP − iU, and it reaches a maximum of ∼5 A/cm2 at ∼400 ns. Meanwhile, negative

between the film and substrate but also the interfaces between the deposited layers are legible in the image. Considering that the intensity is directly proportional to the atomic number in DF-TEM images,21 the brighter lines in Figure 2c (marked by the arrows) are due to the enrichment of heavier elements at the interfaces. A similar phenomenon was observed in BNT polycrystalline thin films where lower contrast at the interface is due to the deficiency of Bi at the interface between the film and substrate.22 We note that there are some dark areas of several nanometers that appear in the image; this is due to either the defects induced by the ion beam thinning process during TEM lamella preparation or the existence of porosity defects in the BNT−11BT thin film introduced during the sol−gel film preparation process. A schematic figure of the layered distribution of heavier atoms in the BNT−BT/Nb:STO heterostructure is presented in Figure 2d. When the first film layer was deposited on the virgin surface of the substrate, a small number of the Bi and Ba elements with higher atomic weight settled at the film−substrate interface. Subsequently, the settling of Bi and Ba atoms happened when each film layer was deposited, resulting the enrichment of these elements at the bottom of each film layer. To further investigate the crystallinity and the interface details of the BNT−11BT film on the Nb:STO substrate, more TEM measurements and analysis were carried out. Figure 3a

Figure 3. TEM analysis of the BNT−11BT/Nb:STO heterostructure: (a) SAED image of BNT−11BT film along the [100] zone axis; (b) atomic-resolution HAADF-STEM image showing the atom registry at the interface area.

shows the select area electron diffraction (SAED) patterns obtained from an electron beam focused on the film along the [100] zone axis (see Figure S3 for more details). The crystalline interplanar distance calculated from the SAED patterns is consistent with the XRD results. The ordered spots with quadruple symmetry in the image confirm the monocrystalline feature of the film. The distortion of the spots in the patterns, for example (002̅,013), demonstrates the coexistence of the R and T phases in the film, which is in agreement with the expectation of the MPB feature20 and the broadening of (002) C

DOI: 10.1021/acsami.8b02239 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Compositional profile of the BNT−11BT thin film from the film surface to the Nb:STO substrate by AES measurement. (b) Bi/O, Ba/ O, and Ti/O ratio lines in the region close to film surface marked in (a) as dash square.

Figure 5. (a) Sequential pulses to measure the switching and nonswitching currents for the positive and negative polarities. (b) Asymmetric switching current responses in the BNT−11BT thin film. The experimental data were plotted using dots, and the solid lines indicate the exponential fitting of the switching current.

Figure 6. (a) Asymmetric phase hysteresis loop and butterfly-like amplitude loop shifted to the positive direction at room temperature. Out-of-plane PFM phase (b) and amplitude (d) images switched by +20 and −20 V tip dc bias; the light and dark areas refer to the upward and downward polarization in the phase image. (c) Phase value of the area marked by blue dash line in the phase image (b). (e) Amplitude value of the area marked by blue dash line in the amplitude image (d).

switching current isw− = iN − iD, and it reaches a maximum of ∼4 A/cm2 at ∼400 ns, smaller than that of isw+. This asymmetric switching behavior and the higher positive switching current confirm the existence of a built-in electric field in the film, pointing to the film surface. To understand the built-in electrical field in the films, piezoresponse force microscopy (PFM) measurements were carried out to further explore the polarization behavior; the results are shown in Figure 6. Figure 6a shows the classical phase hysteresis loop and butterfly-like amplitude loop obtained in BNT−11BT thin film. The piezoelectric coefficient was calculated in this work as shown in Figure S5; however, the d33 value can be hardly reliable even though the complicated calibration process was done.25,26 Both the asymmetric loops

shift to the positive direction horizontally, confirming the existence of a built-in upward electric field. The macroscopic P−E hysteresis loop shown in Figure S6 also confirms the existence of a built-in upward electric field. Figure 6b shows the out-of-plane phase image switched by the Vdc obtained in the BNT−11BT thin film. The 5 × 5 μm square was poled by a +20 V bias, and the inner square of 2 × 2 μm was poled by a −20 V bias. The distribution of the phase value in the area (marked by the blue dash line) is shown in Figure 6c. A positive dc bias on the PFM tip induced the downward polarization and a negative dc bias on the tip induced the upward polarization. The phase of the as-grown film exhibits the same orientation D

DOI: 10.1021/acsami.8b02239 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. (a) Out-of-plane PFM phase image with a favorable polarization direction (self-polarization) at room temperature. (b) Out-of-plane phase image exhibiting self-polarization at 75 °C. (c) Out-of-plane phase image without polarization information at 100 °C. (d) Schematic image depicting the built-in electric field and spontaneous polarization in the BNT−11BT thin film. The sedimentation of heavier elements on each deposited layer induces the adsorption of water vapor on the films surface in the form of hydroxyls, resulting in an upward built-in electric field and leading to the upward self-polarization in the film.

the fully matched epitaxy was realized by carefully controlling the film composition. The defects such as threading dislocations and misfit dislocations induced by strain stress and cracks induced by thermal stress were excluded. The polarization orientation and piezoresponse of the BNT−11BT thin films with different thicknesses (Figure S4) also confirm that the self-polarization exists in the films with thickness from 60 to 370 nm, which differs from the polarization evolution with thickness in epitaxial (001) BaTiO3 thin films affected by epitaxial strain.13 The interface effect plays an important role in the polarization distribution in thin films. It was reported that the insertion of ultrathin dielectric spacers at the metal− ferroelectric interface was realized to control the polarization distribution in the PbTiO3 thin film.27 Therefore, we believe that the built-in electric field induced by the interface effect is the major origin of the self-polarization in this strain-free film. The upward built-in electric field was confirmed by PUND (Figure 5) and PFM measurements (Figures 6 and 7). According to the detailed TEM (Figures 2 and 3) and AES (Figure 4) observations, it is induced by the enrichment of Ba and/or Bi elements at the interfaces. Such a mechanism was explained by the schematic diagram in Figure 7d. Because the BNT−11BT thin film was deposited from the solution and thermal-treated layer by layer, heavier elements (Bi and Ba) tended to settle to the bottom of each layer during the deposition process. The film was exposed to the atmosphere, and thereby, a small amount of water vapors was adsorbed onto the film surface. To achieve the charge balance, the enrichment of heavier electropositive elements (Bi and Ba) at the interfaces induces hydroxyl groups (OH−, electronegative) adsorbed on the film surface,7 generating a stable built-in electric field pointing to the film surface and leading to an upward selfpolarization along the direction of the built-in electric field. In this scenario, the upward polarization state is stable and independent on the thickness of the film. The polarization can be switched by an external electric field. Heating the film to high temperature leads to the desorption of OH− group, resulting in the disappearance of the self-polarization (Figure 7a−c). The temperature of 100 °C is not enough to induce the redistribution of A-site ions in perovskite lattices. By cooling the film, the OH− group is absorbed on the film surface to

with negative tip dc bias, indicating the upward self-polarization in the film, which is in accordance with the offset observed in the hysteresis loops. Figure 6d shows the out-of-plane amplitude image switched by the Vdc obtained in BNT− 11BT thin film. The distribution of amplitude value in the area (marked by the blue dash line) is shown in Figure 6e. The amplitude exhibits extensive response inside both the switched domain and the as-grown domain and drops to zero at the domain walls, corresponding to the phase image. The amplitude of the downward domain was observed smaller than that of upward domain because of the unequal contribution of the cantilever bending to the measurement information. To reveal more details of the self-polarization found in the BNT−11BT film, the temperature dependent polarization evolution was explored by PFM equipped with a high temperature controller. The results are shown in Figure 7. As shown in Figure 7a, the out-of-plane PFM phase of the asgrown film exhibits a homogenous distribution with a preferable orientation, i.e., a self-polarization, which is due to the self-poling effect. As the temperature increased, the selfpolarization maintained till 75 °C (Figure 7b) and vanished at 100 °C (Figure 7c). The self-polarization then recovered when the temperature was decreased to room temperature (the pattern is similar as Figure 7a).

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DISCUSSION Let us now consider the origins of the observed selfpolarization in the fully matched BNT−11BT film on the Nb:STO substrate. It is well-known that a self-polarization forms in the film when a built-in electric field is large enough to screen the depoling field. The built-in electric field results from the charged defects in the film bulk and (or) at the interface. There are two kinds of factors, which would contribute to this effect, including the nonuniform distribution of elements in the film and the strain gradients related to lattice strain/thermal strain. In many epitaxial thin film systems, thermal strain and/ or lattice misfit strain play a significant role in determining their polarization behavior.14,29−31 For instance, the self-polarization direction could be determined by controlling the strain evolution in BFO thin films.28 In our BNT−11BT thin films, E

DOI: 10.1021/acsami.8b02239 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Bruker) operated at low frequency. An ac bias (frequency: 45 kHz, amplitude: 3 V) was applied to the sample stage to simulate the ferroelectric film, and the conductive tip was used as the top electrode. A dc bias Vdc was applied to the tip during scanning the selected square to switch the polarization.

satisfy the charge balance, resulting in the recovery of the selfpolarization.

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CONCLUSIONS In conclusion, epitaxial and fully matched BNT−11BT thin films with upward self-polarization were grown on (001)oriented Nb:STO substrates by the sol−gel technique. XRD and TEM measurements revealed that the pseudocubic BNT− 11BT film was highly (001)-oriented without undesirable phases or orientations because of the lattice-matched and thermally matched epitaxial growth. The self-polarization was observed by PFM in the (001)-oriented BNT−11BT thin film at room temperature, and it vanished when temperature was increased to 100 °C. The upward polarization was confirmed by writing domains under both positive and negative biases in the film. The asymmetric phase hysteresis loop and domain switching currents indicate the existence of the internal electric field, which can be attributed to the inhomogeneous distribution of the chemical elements, induced by the sedimentation of heavier atoms at the interfaces during the film deposition. These results, which exclude the strain effects thanks to the fully matched system, highlight the significant impact of the interface on the self-polarization characteristics of lead-free ferroelectric films and point out the importance of the interface tailoring for their potential applications of sensors and actuators.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b02239. XRD results of the BNT−11BT powder, BNT-based (001)-oriented thin films, and (001) BNT−11BT thin films with different thicknesses; TEM results of (001) BNT−11BT thin film; PFM results of (001) BNT− 11BT thin films with different thicknesses; and the P−E hysteresis loops of the (001) BNT−11BT thin film (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected] (G.N.). *Email: [email protected] (W.R.). *Email: [email protected] (Z.-G.Y.). ORCID

Gang Niu: 0000-0002-8813-8885 Wei Ren: 0000-0001-9749-0699 Zuo-Guang Ye: 0000-0003-2378-7304

METHODS

Author Contributions

Film Fabrication. The BNT−11BT thin films were prepared by the sol−gel method. Sodium acetate, bismuth acetate, and barium acetate according to the stoichiometry were dissolved in the ethylene glycol monomethyl ether and glacial acetic acid. Butyl titanate was dissolved in the ethylene glycol monomethyl ether, and acetylacetone was used to avoid the hydrolysis of butyl titanate. The BNT−11BT solution with a concentration of 0.4 mol/L was obtained by mixing the two precursors and stirring at 80 °C for 1 h. To compensate the evaporation of metallic elements during the film deposition, 10 mol % excess Na and 2 mol % excess Bi were added to the precursor solution.22,32 The BNT−11BT solution was deposited on (001)oriented Nb:STO substrates by spin-coating and thermally treated in a rapid thermal annealing furnace at 150 °C for 3 min, 420 °C for 10 min, and 750 °C for 3 min. The thin film with designed thickness was obtained by repeating the deposition and thermal treatment processes. The thickness of the BNT−11BT thin film increases with increasing deposited layers. The structural and electrical properties of BNT− 11BT thin film with thickness of 370 nm (six deposited layers, 6L) are presented in this work. The films with different thicknesses (one deposited layer, 1L; three deposited layers, 3L) are also presented in the Supporting Information. Characterizations. Specular out-of-plane θ−2θ XRD scans and RSM measurements were performed to characterize the crystal structure and the film orientation by using an Xpert pro MRD Modern Globe diffractometer. TEM (JEOL, ARM200F) was employed to investigate the microscopic characteristics. The lowmagnification TEM cross-sectional images, HAADF-STEM images, atomic-resolution HAADF-STEM images, and SAED patterns were obtained. AES (PHI 700, Japan) analysis combined with simultaneous sputter etching was used to obtain the depth profile of the film. The sputtering rate was about 11.2 nm/min. The Au top electrode with a diameter of 200 μm was prepared by the lift-off process and the sputtering method for the macroscopic electrical measurement. The response characteristics of the domain switching currents as a function of time were measured by a home-built system using a signal source Agilent 81150A and an oscilloscope LeCroy HDO4104. The domain structure and the phase hysteresis loop were imaged by PFM (Dimension Icon, Bruker, U.S.A.) using a conductive tip (SCM-PIT,

All authors contributed to the discussion and writing of the manuscript. The final version was approved by all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (grant nos. 51332003, 51202184, 91323303, 51602247), the Natural Science Fundamental Research Project of Shaanxi Province of China (no. 2017JQ6003), the “111 Project” of China (B14040), the Fundamental Research Funds for the Central Universities, and the National Science and Engineering Research Council of Canada (NSERC, grant no. 203773). Ming Wu and Ruyi Zhang are acknowledged for their contribution on the PUND and XRD measurements and analysis. The authors thank Owen Liang for improving the technical writing.

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REFERENCES

(1) Khan, A.; Abas, Z.; Kim, H. S.; Oh, I.-K. Piezoelectric Thin Films: An Integrated Review of Transducers and Energy Harvesting. Smart Mater. Struct. 2016, 25, 053002. (2) Lichtensteiger, C.; Zubko, P.; Stengel, M.; Aguado-Puente, P.; Triscone, J. M.; Ghosez, P.; Junquera, J. Ferroelectricity in Ultrathin Film Capacitors; Wiley: New York, 2012; Chapter 12. (3) 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. (4) Fong, D. D.; Kolpak, A. M.; Eastman, J. A.; Streiffer, S. K.; Fuoss, P. H.; Stephenson, G. B.; Thompson, C.; Kim, D. M.; Choi, K. J.; Eom, C. B.; Grinberg, I.; Rappe, A. M. Stabilization of Monodomain Polarization in Ultrathin PbTiO3 Films. Phys. Rev. Lett. 2006, 96, 127601. F

DOI: 10.1021/acsami.8b02239 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.8b02239 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX