www.advelectronicmat.de
Flexible PbZr 0.52 Ti 0.48 O 3 Capacitors with Giant Piezoelectric Response and Dielectric Tunability
Wenxiu Gao, Lu You, Yaojin Wang, Guoliang Yuan,* Ying-Hao Chu, Zhiguo Liu, and Jun-Ming Liu*
DOI: 10.1002/aelm.201600542
but they have slow polarization switching speed, small piezoelectric coefficient, and poor temperature stability.[3–6] As a comparison, oxide ferroelectrics such as PbZr0.52Ti0.48O3 (PZT) possess large polar- ization and fast switching speed, large piezoelectric coefficient, good tempera- ture stability, and, in turn, they have been widely utilized in MEMS and nonvolatile memories.[7–9] Most oxide single crystals or ceramics are hard and brittle which can be break at >4% strain; thus, it is a big challenge for the oxide ferroelectric films to be used in the emerging flexible elec- tronic devices. Oxide ferroelectric films are commonly grown on the hard SrTiO3, LaAlO3, Si, and other rigid substrates with
>0.1 mm thickness and small maximum elastic strain (Smax). Consequently, the fer- roelectric film-based heterostructures also suffer from the brittleness and rigidness features, and could not be flexible similar to organic materials. Furthermore, the piezoelectric coefficient d33 of oxide film on the rigid substrate is much smaller than that of the corresponding ceramics or single crystals, since the hard substrate restrains the electric-field-induced large strain.[2] Gen- erally, most free-standing thin oxide films can be flexible if their rigid substrate can be removed;[10–14] thus, the existing rigid substrate is the main obstacle to the flexibility of submicrom- eter-thick oxide film. In order to obtain flexible PZT films as energy harvesters, nanoactuators, nanosensors, and memories, the ferroelectric oxide films should be deposited on a flexible inorganic substrate or transferred from original rigid substrates to organic flexible ones. In order to fabricate nanogenerators or flexible sensors, Hwang et al. and Park et al. successfully peeled a small piece of PZT film from quarts, Si, and other hard sub- strates through chemical wet etching, mechanical delamina- tion, and then the free-standing PZT film was transferred to the soft organic substrate, such as poly(methylmethacrylate) (PMMA), polyimide, or polyethylene terephthalate.[10–14] How- ever, there are not many reports on large size, cost-effective, and flexible ferroelectric capacitors with oxide films in a simple process until now.[10–14]
In order to obtain large-scale, flexible and high-quality oxide films in a cost-effective and reliable way, oxide films should be grown on a special substrate which is endurable at high tem- perature, high oxygen pressure and can be easily thinned to micrometer- or nanometer-scale thickness. However, it is still Oxide ferroelectric ceramics and single crystals for flexible wearable device
applications significantly suffer from the nature of hardness and brittleness due to their small maximum elastic strain and technological challenge in thinning to micrometer or nanometer scale. Although many oxide films are thin enough to be flexible, their rigid substrates restrain their piezoelectric d33 coefficient and limit their flexible performance. Here, the large-scale PbZr0.52Ti0.48O3 (PZT) films with SrRuO3 (SRO) buffered layer are grown on the 10 µm thick flexible mica substrate. An amplified longitudinal piezoelec- tric d33 of about 1200 pm V−1 is achieved in an SRO/PZT/Pt capacitor due to the direct mechanical coupling between PZT and flexible mica via the d31 piezoelectric tensor. The SRO/PZT/Pt capacitor shows a saturated polariza- tion of ≈60 µC cm−2 and a dielectric tunability of ≈90%. Most importantly, the electric properties do not show obvious deterioration after repeatedly bending 10 000 times at a 2.2 mm radius. The flexible ferroelectric capacitors have potential applications in next-generation wearable devices and micro- electromechanical systems.
Dr. W. X. Gao, Prof. Y. J. Wang, Prof. G. L. Yuan School of Materials Science and Engineering Nanjing University of Science and Technology Nanjing 210094, P. R. China
E-mail: [email protected] Dr. L. You
School of Materials Science and Engineering Nanyang Technological University
Singapore 639798, Singapore Prof. Y.-H. Chu
Department of Materials Science and Engineering National Chiao Tung University
Hsinchu 30010, Taiwan Prof. Z. G. Liu, Prof. J.-M. Liu
National Laboratory of Solid State Microstructures Nanjing University
Nanjing 210093, P. R. China E-mail: [email protected] Capacitors
Wearable devices and micro-electromechanical systems (MEMS) require nanoscale piezoelectric films with high piezo- electricity, good flexibility, and excellent temperature stability so that they can be miniature, soft and superintegration.[1,2]
Compared with the inorganic ferroelectric materials, organic ferroelectric materials such as poly(vinylidene fluoride- trifluoroethylene) (P(VDF-TrFE)) exhibit extraordinary flexibility,
difficult to grow PZT and other oxide films on the flexi ble organic substrates such as PMMA at 500–700 °C because these substrates will decompose above 400 °C. Fortunately, fluorocrystal mica (AlF2O10Si33Mg) is transparent and stable at 700 °C. In particular, it has a layer-stacking structure similar to those of extensively studied 2D materials; so a thin flexible mica layer can be separated from the single-crystal mica by mechanical exfoliation. These unique features suggest the pro- spective application of mica crystals as the substrate to fabricate cost-effective flexible PZT devices in a simple process.[15,16]
In this paper, SrRuO3 (SRO), PZT, and Pt films were grown on 10 µm thick mica to prepare flexible and cost-effective SRO/
PZT/Pt capacitors. An amplified piezoelectric d33 of about 1200 pm V−1 is obtained in such a flexible capacitor. Interest- ingly, the performances of ferroelectric polarization and piezoe- lectric response do not deteriorate obviously after the capacitors were bent repeatedly to 2.2 mm radius for 10 000 times.
The 10 µm thick mica substrates were separated by mechanical exfoliation like the cleavage of graphite (see Figure 1a,b). The flexible ferroelectric capacitor was fabricated by successively depositing the SRO buffered layer, PZT film and platinum electrodes on mica substrate. The SRO/PZT films were grown on mica at 680 °C and 10 Pa oxygen pressure (see Figure 1c).
An average surface roughness of 2.58 nm of the PZT films has been determined by the morphology observed by atomic force microscope (AFM), as shown in Figure 1d. Figure 1e presents the cross-sectional image measured by transmission electron microscope (TEM). It can be seen that a series of PZT columnar
crystals with heights of ≈200 nm and diameters of 60–100 nm are regularly arranged on the mica substrate. The (111) diffrac- tion peak is the strongest among all of the PZT films except for those of mica and Pt in the X-ray diffraction (XRD) (Figure 1f).
The above characterizations confirm the successful growth of a smooth and [111] preferred-oriented PZT film without sec- ondary phases. In order to achieve the transparent capacitors, we also fabricate flexible ferroelectric capacitors using indium tin oxide (ITO) as electrodes. Figure 1g shows the transmit- tance spectrum of mica substrate, mica/ITO/PZT/ITO, and mica/SRO/PZT/Pt capacitors. The results obviously show that the mica substrate and the mica/ITO/PZT/ITO ferroelectric capacitor have an excellent transparent feature, while mica/
SRO/PZT is less transparent due to the light absorption of the SRO buffered layer.
Figure 2a shows the ferroelectric polarization in response to applied electric field (i.e., P–E loop) of the flexible capacitor at various temperatures. It is obvious that the high-quality [111]-preferred texture PZT film has a large saturated polariza- tion (PS) of ≥54.3 µC cm−2 at 30–170 °C. The PZT composition in this study lies at the morphotropic phase boundaries with a high Curie temperature of ≈350 °C, and its film presents the sat- urated polarization versus electric field (P–E) loops at 30–170 °C.
The values of PS decreases from ≈60.5 to ≈54.3 µC cm−2, and the coercive electric field (EC) decreases from ≈63 to ≈50 kV cm−1 with temperature increasing from 30 to 170 °C.[8,9] The satu- rated P–E loop means that the PZT film exhibits low density of charged defects and small leakage current even at 170 °C.
It is noted that the P–E loops of transparent mica/ITO/PZT/ITO capacitors are similar to those of mica/SRO/PZT/Pt (Figure S1 in the Supporting Information).
It is interestingly found that the flexible SRO/PZT/Pt capacitor on 10 µm thick mica has an equivalent piezoelectric d33 value of 1200 pm V−1 compared with tens of pm V−1 for the SRO/PZT/Pt capacitor on the rigid SrTiO3 substrate. The ferroelectric phase and equivalent d33 of the PZT film on these three substrates were measured by piezoresponse force microscopy (PFM) with the conductive tip sitting on the surface of the 100 µm diam- eter Pt top electrode. The PZT film on hard SrTiO3 substrate shows the positive phase angle at a >2 V bias due to the positive piezo- electric effect, while the PZT film on 10 µm thick flexible mica shows negative angle at
>1.8 V bias, which suggests negative piezo- electric effect in Figure 2b. Here the piezoelec- tric d33 values of the PZT film on 10 µm thick flexible mica, 0.1 mm thick hard mica, and SrTiO3 substrates were shown in Figure 2c, and the d33 value of the PZT film on flex- ible mica is as high as about 1200 pm V−1 at a negative voltage bias compared with about 34 pm V−1 for the PZT film on SrTiO3 sub- strate. The piezoelectric amplitude of PZT film on SrTiO3 is restrained by the clamping effect from their rigid substrate (Figure S2 in Figure 1. a) Mechanical peeling of 10 µm substrate from layer-stacking mica crystal. b) AFM
surface of mica. c) PZT films were grown on mica at 680 °C and 10 Pa oxygen pressure. d) AFM surface of PZT film. e) Cross-sectional TEM image of PZT with columnar crystals. f) XRD pattern of mica/SRO/PZT/Pt. g) Transmittances of mica, mica/ITO/PZT/ITO, and mica/SRO/
PZT samples, where the inset shows the photograph of mica/ITO/PZT/ITO.
the Supporting Information); thus, its d33 value is much smaller than that of the PZT single crystal or ceramic. As a compar- ison, Nagarajan et al. prepared 1 × 1 µm2 discrete islands of PbZr0.2Ti0.8O3 to remove the clamping effect of the strain from the hard SrTiO3 substrate; such islands showed a d33 piezoelec- tric coefficient of ≈250 pm V−1 which was approximately tripled the predicted value of 87 pm V−1 for a single domain crystal.[17]
An amplified longitudinal piezoelectric d33 of 1200 pm V−1 is achieved in an SRO/PZT/Pt capacitor on 10 µm thick mica due to the direct mechanical coupling between PZT and flex- ible mica via the d31 piezoelectric tensor.[2] Like piezoelectric bimorphs under AC electric field, the d31 effect of PZT film exerts periodic transverse compression and extension on the flexible mica substrate, which introduces the longitudinal bending deformation of the entire device and finally contributes to a giant displacement perpendicular to the surface. Such a displacement depends on the elastic properties of the film and mica substrate, electrode size, and connection between mica and its rigid base. For example, PZT has positive d33; thus, the electric field parallel with polarization will elongate the PZT thickness. However, due to its negative d31, this induces in- plane compressive strain on mica. Because 10 µm thick mica is thin and flexible, in-plane compressive strain on top surface will cause its downward bending, leading to a net surface depres- sion, as shown in Figure 2d. So the whole sample “shrinks” to achieve a giant negative d33. On the contrary, the electric field antiparallel with polarization will introduce in-plane tensile
strain on mica and an upward bending, which is equivalent to a giant negative d33, too. Surprisingly, there are double hysteresis loops of phase (Figure 2b) and d33 (Figure 2c) for the PZT film on 0.1 mm thick mica. It seems like a crossover between positive d33 and negative d33. In fact, the 0.1 mm thick mica is softer than SrTiO3 since mechanical force may separate a thin piece from the 0.1 mm thick mica with layer-stacking struc- ture. Therefore, the substrate bending effect is still significant in the 0.1 mm thick mica.
The crossover provides additional evidence for the competition between intrinsic posi- tive d33 and the extrinsic negative d33 for PZT film on 0.1 mm thick mica.
Then, the flexibility of the ferroelectric capacitor was validated. It is found that the mica/SRO/PZT retains high-quality mor- phology after it was bent to 1.4 mm radius, which satisfies the demands of wearable devices to certain extend. Figure 3a clearly shows that PZT film was not destroyed by comparing the surface before and during the bending status. It indicates that the tiny bending strain did not change the surface morphology and crystal structure of the fer- roelectric capacitor. Figure S3 in the Sup- porting Information shows the dependence of bending radius on the sample’s thickness at various strains. The thinner the mica/
SRO/PZT/Pt sample is, the more the flex- ible it will be. When an h1-thick oxide film on an h2-thick sub- strate was bent to r radius, the inner and outer circumferences are 2πr and 2π(r + h1+ h2) and the maximum bending strain satisfies δmax≈ (h1+ h2)/r. As PZT film is much thinner than mica, hence the bending strain mainly depends on the radius and thickness of mica in most cases. Besides, the excellent flexibility of 10 µm thick mica also comes from its favorable mechanical properties, such as tensile strength (150–200 MPa) (Table S1 in the Supporting Information).
The ferroelectric polarization and relative dielectric per- mittivity (εr) have a slight variation during mica/SRO/PZT/Pt bending to 1.4 and 2.2 mm radii, respectively. First, the upward and downward polarizations were successively written by the PFM tip with 10 and −10 V on 3 × 3 and 1 × 1 µm2 regions of flat PZT film in sequence, and the two regions were identi- fied in the amplitude images, as shown in Figure 3b, where the surface of the region marked by a red rectangle was shown in Figure 3a. The piezoelectric amplitude of these two regions did not decay after the flat PZT film was bent to 1.4 mm radius (Figure 3c), which reveals negligible depolarization or piezo- electric attenuation arising from the flexoelectric effect during the bending process. Figure 3d shows the P–E loops of flexible mica/SRO/PZT/Pt capacitor under the following conditions:
flat, 2.2 mm radius, and 1.4 mm radius. Here, the PS and Ec values of a 1.4 mm radius capacitor are a little smaller than those of a flat capacitor, because the bending strain slightly decreases the potential barriers among different polarizations.
Figure 2. a) P–E loops of SRO/PZT/Pt cell on 10 µm mica substrate measured at 30, 100, and 170 °C. b) Piezoelectric phase and c) equivalent d33 of SRO/PZT/Pt cells on SrTiO3, 0.1 mm mica and 10 µm mica substrates. d) Schematic diagrams show how the d31-based in-plane extension amplifies vertical amplitude and d33 in the flexible SRO/PZT/Pt capacitor.
Figure 3e shows the curves of electric field versus εr under conditions of flat, 2.2 mm radius, and 1.4 mm radius. The maximum value of εr is ≈1950 at Ec and it becomes ≈187 at 260 kV cm−1 under flat and 2.2 mm radius conditions; thus, a huge dielectric tunability of 90.4% was achieved by the application of a DC bias field at room temperature.[18–20] This dielectric tunability is much larger than that of the PZT film on rigid substrate, mainly because there is less strain to suppress the movements of charged defects and domains under an AC voltage of 0.5 V in flat flexible capacitors.[21,22] In addition, the bending strain can suppress the maximum εr to 1700 in the 2.2 mm radius capacitor, so its dielectric tunability decreases a little to 89%.
In a nut shell, it is promising to apply the flexible PZT devices on the basis of its large dielectric tunability.
The bending at 2.2 mm radius for 10 000 times cannot obviously influence the writing, reading, and keeping of the polari- zation information. Figure 4a illustrates the setup to mechanically bend mica/SRO/PZT/
Pt capacitors. P–E loops before and after these bending are shown in Figure 4b, where the PS of ≈60 µC cm−2 and the remnant
polarization (Pr) of ≈37 µC cm−2 are iden- tified. Both ferroelectric polarization and piezoelectric response do not show obvious decay even after the mica/SRO/PZT/Pt capacitors were bent to 2.2 mm radius for 10 000 times. The stable polarization proves the reliability of the repeating bending pro- cess. Figure 4c,d shows the phase and ampli- tude of the flat PZT film where the upward and downward polarizations have been written in the center 3 × 3 and 1 × 1 µm2 regions by the PFM tip. After the capacitor was bent repeatedly to 2.2 mm radius for 10 000 times, the previous regions were found, and the upward and downward polarizations were kept well in the phase and amplitude images in Figure 4e,f. The bending processes do not change the previous polarization and its piezoelectric amplitude.
Table 1 summarizes the basis properties of several flexible PZT and P(VDF-TrFE)- based memories.[3–5,19,20,23] The mica/SRO/
PZT/Pt capacitors show the bending radius of 1.4 mm and the bending endurance of 10 000 times, which are similar to those of P(VDF-TrFE), i.e., a typical organic ferroelec- tric. Besides, the flat, 1.4 mm radius, and folding mica/SRO/PZT/Pt capacitors show a large PS of ≈60 µC cm−2, while it decreases to ≈38 µC cm−2 after polarization switching
(b) (a)
-300 -150 0 150 300 -60
-30 0 30 60
flat after 104 times
Polarization (µC/cm2 )
Electric Field (kV/cm)
@1kHz
Bent for 104 times
(e) (f) (c) (d)
1 µm 1 µm 1 µm
1 µm
Figure 4. a) Photographs of the flat and curved mica/SRO/PZT/Pt capacitors bent by the mechanical device. b) P–E loops before and after the PZT film was bent to 2.2 mm radius for 10 000 times. The out-of-plane PFM phase and amplitude images c,d) before and (e,f) after the 10 000th bending.
(a) (b) (c)
-300 -150 0 150 300 -60
-30 0 30 60
Electric Field (kV/cm) Relative Permittivity (εr)
Polarization
(
µC/cm2)
r=1.4mm r=2.2mmflat
Electric Field (kV/cm)
-300 -150 0 150 300 0
500 1000 1500
2000 r=1.4mm
r=2.2mm flat 1 µm
Flat
Bent: r=1.4 mm
(d) (e)
1 µm 1 µm
Figure 3. a) Surfaces and their piezoelectric amplitude images b) before and c) after the PZT film was bent to 1.4 mm radius, where the surface morphology comes from the area marked with a red-dashed rectangle, and the two square patterns are due to upward/downward polarization written by 10/−10 V bias before bending. d) P–E loops and e) εr–E loops of the PZT film measured at 1000 Hz in the flat, 2.2 mm radius, and 1.4 mm radius SRO/PZT/Pt capacitors, respectively.
for 1010 cycles (Figure S4 in the Supporting Information). Even so, such PS is still much larger than the initial PS of 8 µC cm−2 for P(VDF-TrFE) film and ≈15 µC cm−2 for Sr2Bi2TaO9 film before fatigue.[4,6,24] Noted that the ferroelectric fatigue of PZT film can be suppressed by using special oxide electrodes instead of metal electrodes.[7–9,25,26] In fact, commercial oxide ferroelec- tric memories have been produced by Fujitsu Co., Texas Instru- ments.[19] In nut shell, the flexible PZT memory is much more mature to be applied in wearable devices and MEMS compared with the flexible P(VDF-TrFE) memory.[3–5,19,20,23]
In summary, flexible, large-scale, high-quality mica/SRO/
PZT/Pt capacitors were prepared in the simple and cost-effec- tive processes. These capacitors show a PS of ≈60 µC cm−2 and a dielectric tunability of ≈90%. Most importantly, an amplified piezoelectric d33 of about 1200 pm V−1 is achieved due to the mechanical coupling between PZT and flexible mica via the d31 piezotensor. After the capacitors were bent repeatedly to 2.2 mm radius, polarization, dielectric tunability and piezoelec- tric response did not show obvious deterioration. These flexible mica/SRO/PZT/Pt capacitors have the merits of high-tempera- ture stability, large-scale, simple fabrication processes; thus, they are promising to be widely used in wearable devices and MEMS.
Experimental Section
The 10 µm thick mica substrates were separated from fluorocrystal mica (AlF2O10Si33Mg, Changchun Taiyuan Co., China) by mechanical exfoliation, and then SRO/PZT/Pt and ITO/PZT/ITO films were grown on mica substrates by pulsed laser deposition (PLD) with a KrF excimer laser (248 nm wavelength). Here SRO/PZT or ITO/PZT films were grown at 680 °C and 13 Pa oxygen pressure with 80 mJ per laser pulse before they were postannealed at 1000 Pa oxygen pressure during cooling with 5 °C min−1. Pt (or ITO) top electrodes with 30–200 µm diameters were grown at 200 °C and 10−4 Pa (or 13 Pa) oxygen pressure on PZT film to prepare capacitors.
The crystal structure of PZT was measured with XRD (Brucker D8) and the cross-sectional image of mica/SRO/PZT was observed by a TEM (FEI Tecnai G2 F20 S-Twin). Then, the transmittance spectra of mica/SRO/PZT/Pt and mica/ITO/PZT/ITO were characterized by a UV–visible spectrometer (Shimadzu Co.). After that, the morphology
and PFM studies were conducted by an AFM (Multimode 8, Bruker Co.).
An AC voltage (VAC) of 2 V at 41 kHz was applied to the conductive tip (Bruker MESP-RC, Co/Cr coating, 35 nm tip radius) to obtain phase and amplitude images. The piezoelectric amplitude with nanometer scale was measured by the other AFM (Asylum Research MFP-3D) with a VAC
of 0.5 V, and then the equivalent d33 coefficient was estimated by the ratio of amplitude/VAC. Polarization versus electric field (P–E) loops and relative permittivity versus electric field (εr–E) loops were measured at 1000 Hz with a ferroelectric tester (Radiant Technologies, USA).
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
The work was supported by the National Natural Science Foundation of China (11234005, 51372111, 51472118, and 51602156) and the Fundamental Research Funds for the Central Universities (30916011104).
Conflict of Interest
The authors declare no conflict of interest.
Keywords
dielectric tunability, ferroelectric memory, flexible capacitors, giant piezoelectric d33
Received: December 16, 2016 Revised: March 19, 2017 Published online:
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Table 1. Properties of several PZT and P(VDF-TrFE) flexible ferroelectric memory devices.
PbZr0.52Ti0.48O3 Capa.
(this work)
PbZr0.48Ti0.52O3 Capa.[19,20]
PbZr0.35Ti0.65O3 Fe-FETa)
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P(VDF-TrFE) Fe-FET[5] P(VDF-TrFE) Fe-FET[4] P(VDF-TrFE) Fe-FET[3]
Substrate Mica Silicon Polyimide Polyimide PMMAb) Polyimide
Method PLD Spin coating Spin coating Spin coating Spin coating Spin coating
Transfer required No Ion etching Yes No No No
Ps [µC cm−2] 60 38 40 – 8.16 –
Pr [µC cm−2] 38 ≈13 30 – 6.77 –
EC [kV cm−1] ≈50 ≈50 – – 744 –
Write/erase [n] 1010 109 1000 120 125 100
Infor. retention >10 years >10 years – – >10 000 s >7000 s
Work temp. [°C] >170 >200 – Room temp. Room temp. 100
Diele. tunability 90.8% at 260 kV cm−1 ≈86% at 500 kV cm−1 – – – –
Radius [mm] 1.4 5 9 5.8 4 0.5
Bending cycles 10 000 at 2.2 mm 1000 200 1000 1000 at 5 mm 1000 at 4 mm
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