All-Inorganic Flexible Ba0.67Sr0.33TiO3 Thin Films with Excellent

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Functional Inorganic Materials and Devices

All-inorganic flexible Ba0.67Sr0.33TiO3 thin films with excellent dielectric properties over a wide range of frequencies Dong Gao, Zhengwei Tan, Zhen Fan, Min Guo, Zhipeng Hou, Deyang Chen, Minghui Qin, Min Zeng, Guofu Zhou, Xingsen Gao, Xubing Lu, and Jun-Ming Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08712 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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ACS Applied Materials & Interfaces

All-inorganic flexible Ba0.67Sr0.33TiO3 thin films with excellent dielectric properties over a wide range of frequencies Dong Gao†,‡,#, Zhengwei Tan†,‡,#, Zhen Fan†,

‡,*,

Min Guo†, Zhipeng Hou†, Deyang

Chen†, Minghui Qin†, Min Zeng†, Guofu Zhou‡, ⊥ , Xingsen Gao†, Xubing Lu†,‡,*, & Jun-Ming Liu†,∇ †

Institute for Advanced Materials, South China Academy of Advanced

Optoelectronics, South China Normal University, Guangzhou 510006, China. ‡

Guangdong Provincial Key Laboratory of Optical Information Materials and

Technology, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China. ⊥ National

Center for International Research on Green Optoelectronics, South China

Normal University, Guangzhou 510006, China. ∇Laboratory

of Solid State Microstructures and Innovation Center of Advanced

Microstructures, Nanjing University, Nanjing 210093, China. #These

authors contributed equally to this work.

*Corresponding author.

KEYWORDS: Flexible capacitor; (Ba,Sr)TiO3; Mica; Dielectricity; Microwave frequencies

1

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ABSTRACT: With rapid advances in flexible electronics and communication devices, flexible dielectric capacitors exhibiting high permittivity, low loss, and large electric-field tunability over a wide frequency range have attracted increasing attention. Here, a large-scale Ba0.67Sr0.33TiO3 (BST) dielectric thin film sandwiched between SrRuO3 (SRO) bottom electrode and Pt top electrode is fabricated on a flexible mica substrate. The mica/SRO/BST/Pt capacitor exhibits a dielectric constant (εr’) of over 1200, a loss tangent [tan(δ)] as low as 0.16, and a tunability of 67% at low frequencies around 10 kHz. Simultaneously, the capacitor can retain an εr’ of 540 and a tan(δ) of 0.07 at microwave frequencies, e.g., 18.6 GHz. Moreover, even when the capacitor is bent to a small radius of 5 mm or undergoes 12000 bending cycles (at 5 mm radius), almost no deterioration in εr’, tan(δ), and tunability is observed. The excellent dielectricity and mechanical flexibility and durability endow the mica/SRO/BST/Pt capacitor with huge potential for flexible electronic and microwave applications.

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1. INTRODUCTION The new era of Internet of Things (IoT) promises a widespread use of flexible electronic devices, such as paper-like displays,1-3 bendable smart phones4,5 and electronic skins.6-8 Notably, almost all these flexible devices need the integration of a key component, namely, dielectric capacitors, in order to function properly. Therefore, flexible, lightweight, and scalable dielectric capacitors have been subjected to extensive research over the past few years. So far a variety of flexible capacitors based on polymers,9-11 inorganic/polymer nanocomposites,12 and inorganic/flexible substrate heterostructures13-16 have been well developed, and their low-frequency (frequency below one megahertz) dielectric properties, particularly energy density and charge/discharge efficiency, were greatly concerned. However, the dielectric properties in the microwave region (frequency of the order of gigahertz) remain almost unexplored for these flexible capacitors. Indeed, the capacitors exhibiting tunable dielectric constant at microwave frequencies can find wide applications in the field of communication, such as waveguide phase shifter,17 frequency agile filters18 and tunable oscillators.19 Hence, developing flexible capacitors with good dielectric properties over a wide frequency range (extending to the microwave region) is quite appealing. While polymer-based capacitors are flexible enough, they typically exhibit limited thermal and operational stabilities, as well as poor compatibility with conventional inorganic electronic circuits. These drawbacks may be circumvented in all-inorganic flexible capacitors, which consist of the stack of inorganic dielectric layers on flexible substrates. For the inorganic dielectric layers, barium strontium titanate (BaxSr1−xTiO3; BST) appears to be the best choice, because it is among the lead-free materials with the highest dielectric constant (εr’) at room temperature.20 More specifically, the BST possesses a distribution of εr’ ranging from several hundred to more than one thousand depending on the Ba/Sr ratio. The maximum εr’ at room temperature is achieved when the molar faction of Ba approaches 0.7, because BST at this composition has a Curie temperature around room temperature.21 At the 3



Curie temperature, the transition between ferroelectric and paraelectric phases occurs, giving rise to a very high dielectric constant. Besides, the BST also has low dielectric loss [tan(δ)], high breakdown voltage, and large electric-field tunability of εr’, which makes BST a promising candidate dielectric for dynamic random access memory (DRAM) and tunable microwave devices.22-24 However, the integration of BST on flexible substrates is still challenging because the growth of high-quality BST thin films typically requires high temperatures (>600 °C), which limits the selection of flexible substrates. Commercially available flexible polymer substrates, e.g., polyimide (PI),25 polyester (PET),26 polydimethylsiloxane (PDMS),27 and polyethylene naphthalate (PEN),28 have relatively low melting temperatures (TM < 350 °C). Some inorganic substrates can be flexible when made ultrathin, e.g., ultrathin glass and silicon (~50 μm), but they are fragile and expensive.29,30 Alternatively, one may first grow thin films on rigid substrates and then transfer the free-standing films onto flexible substrates.14,31 However, this “grow-transfer” technique involves a tedious multistep process, and it can hardly produce large-scale films. Recently, mica emerges as a popular flexible substrate for the van der Waals growth of two-dimensional materials and oxide thin films.32 The mica has mechanical flexibility, chemical inertness, high thermal stability (TM > 700 °C), and high transparency. Additionally, the mica owns a layered structure, enabling it to be exfoliated down to ~10 μm with an atomically smooth cleavage surface and enhanced flexibility. Therefore, the mica has been widely used as a flexible substrate for the development of all-inorganic flexible electronics with novel functionalities. For example, Jiang et al.33 developed flexible ferroelectric memories consisting of lead zirconium titanate (PZT) thin films on mica substrates, which exhibited a large polarization of 60 μC/cm2 and almost no polarization degradation after being bent to 2.5 mm radius for 1000 cycles. In addition, Yang et al.13 prepared flexible mica/SrRuO3 (SRO)/BaTi0.95Co0.05O3 (BTCO)/Au resistive switching memories with an ON/OFF ratio of over 50, which could withstand a bending radius down to 1.4 mm and 10000 bending cycles. Because BST has a similar perovskite structure as PZT and BTCO, it is therefore expected that the high-quality 4


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BST films with desired dielectric properties [high εr’, low tan(δ), and large tunability] can be well integrated with the flexible mica substrates. In this work, polycrystalline BST dielectric thin films sandwiched between SRO bottom electrodes and Pt top electrodes have been successfully fabricated on mica substrates (Figure 1a). The composition of Ba/Sr = 0.67/0.33 is chosen for BST, because this composition is near the optimal one which shows the highest room-temperature εr’, as introduced earlier. The mica/SRO/BST/Pt capacitors exhibit excellent dielectric properties in both low-frequency and microwave regions [εr’ = 1200 and tan(δ) = 0.16 @ 10 kHz, and εr’ = 540 and tan(δ) = 0.07 @ 18.6 GHz], and their dielectric performances show almost no deterioration after being bent to a small radius of 5 mm or repeatedly bending for 12000 cycles (at 5 mm radius). The mechanisms for the performance degradation are also investigated.

2. EXPERIMENTAL PROCEDURE 2.1 Fabrication of thin films A flexible mica substrate of 15 × 15 mm2 in area and ~10 μm in thickness was prepared by stripping a thin layer from a thick fluorophlogopite mica plate [KMg3(AlSi3O10)F2] (Changchun Taiyuan Co.) in clean water with the aid of a polyimide tape. Then, SRO/BST/Pt heterostructures were sequentially grown on the mica substrate using pulsed laser deposition (PLD) (Shenyang Kecheng Vacuum Tech Co.) with a KrF excimer laser (λ = 248 nm) operated at a laser energy of 90 mJ and a repetition rate of 5 Hz. Both SRO and BST films were grown at 680 °C and under 15 Pa oxygen pressure. After it, the SRO/BST films were cooled to room temperature at a rate of 5 °C/min under 104 Pa oxygen pressure. Then, Pt top electrodes with a diameter of 200 µm were grown on the SRO/BST films at room temperature and in high vacuum (10−4 Pa) through a shadow mask. 2.2 Characterizations The transmission spectra were measured using the UV-vis spectroscopy (UV1800Pc). The crystal structures were characterized by X-ray diffraction (XRD) (Brucker D8). The characterizations of surface morphology and piezoresponse force microscopy 5



(PFM) were performed using a commercial atomic force microscope (AFM) (Cypher Asylum Research). The cross section of the sample was characterized by the transmission electron microscopy (TEM) (Tecnai G2-F20). The TEM sample was prepared following the steps: cutting, grinding, polishing, and ion milling. The polarization-electric field (P-E) hysteresis loops were measured with a ferroelectric workstation (Radiant Precision Multiferroic). The DC current-voltage (I-V) characteristics were measured with a source meter (Keithley 6430). The dielectric properties in the frequency range of 500 to 106 Hz were measured using an inductance–capacitance–resistance (LCR) meter (Agilent E4980A) with an AC voltage of 0.1 V. The microwave dielectric measurements were made with a network analyzer (Agilent N5244A) in a shielded resonant cavity (see Figure S1 and Ref. 34 for details). The tests under bending were performed when the flexible capacitor was taped on homemade semi-circular iron moulds with different radii. The repeated bending cycles were applied by a homemade uniaxial stretching machine.

3. RESULTS AND DISCUSSION Figure 1a shows the photograph of an actual mica/SRO/BST/Pt flexible capacitor with a lateral size of 15×15 mm2, where the mica substrate has been peeled off to ~10 μm thick. The capacitor is capable of being bent to mm-scale radii, demonstrating the good mechanical flexibility. In addition, the mica/SRO/BST film is seen to be semitransparent by eyes, while the mica substrate and the mica/BST film are quite transparent (Figure 1b). The transparences of mica, mica/BST, and mica/SRO/BST samples were comparatively studied using the UV-vis spectroscopy. As shown in Figure 1b, the transmittances of both mica substrate and mica/BST film are above 50% in the wavelength range of 400-900 nm. By contrast, the transmittance of the mica/SRO/BST film drops to below 20% in the same wavelength range. These results suggest that the BST layer itself exhibits good transparency, and it may be used in flexible screens if it is able to be grown on a transparent electrode like indium-tin-oxide (ITO). 6


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Figure 1. (a) Schematic diagram of the mica/SRO/BST/Pt flexible capacitor (left panel) and photograph of the capacitor under bending (right panel). (b) Photographs (upper panel) and transmission spectra (bottom panel) of mica substrate, mica/BST film, and mica/SRO/BST film. (c) XRD θ-2θ scan patterns of mica/SRO/BST film, mica/SRO film and mica substrate. (d) AFM topography image of the BST film grown on the SRO-buffered mica substrate. (e) Cross-sectional TEM image of the SRO/BST heterostructure (the mica substrate was unintentionally removed during the preparation of TEM sample).

Figure 1c displays the XRD θ-2θ scan pattern (2θ = 20-50°) of a mica/SRO/BST bilayer film together with two control samples, i.e., a mica/SRO film and a mica substrate. The characteristic (00l) peaks from the mica substrate can be easily identified. Besides, the (001), (110) and (111) peaks from BST and SRO are also observed, suggesting the polycrystalline nature of BST and SRO films. While the 7



(001) and (110) peaks from BST and SRO may overlap, the presence of (111) peak only in the BST film unambiguously confirms that a perovskite phase of BST (either ferroelectric or paraelectric or a mixture of both) exists. The average lattice constants of BST are estimated to be: a = 3.95 Å and c = 3.96 Å, consistent with those reported previously.35,36 Figure 1d shows a typical topography image for a 3×3 μm2 area of the mica/SRO/BST film. The film surface exhibits a granular feature, and it is relatively flat with a roughness of ~3.9 nm. As can be seen from the cross-sectional TEM image (Figure 1e), the BST layer is tightly adhered to the SRO layer, and their interface is sharp. In addition, the BST layer exhibits densely packed columnar grains, which are ~300 nm in height and ~50 nm in diameter. These results demonstrate the good crystalline qualities of the BST and SRO films grown on the mica substrate.

Figure 2. (a) Typical polarization-electric field (P-E) hysteresis loop of the mica/SRO/BST/Pt capacitor (frequency: 10 kHz) with different applied voltages. PFM (b) amplitude and (c) phase images of the BST film after box-in-box writing, where the outer box (2×2 μm2) was written by -8 V while the inner box (1×1 μm2) 8


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was written by +8 V. (d) PFM amplitude and phase hysteresis loops.

For BST at the composition of Ba/Sr = 0.67/0.33, the ferroelectric and paraelectric phases may coexist. The ferroelectric properties of the BST film grown on the mica substrate were therefore studied via the P-E hysteresis loop measurement and PFM. Figure 2a shows typical P-E hysteresis loops measured at 10 kHz with different applied voltages for the mica/SRO/BST/Pt capacitor. The loops are quite slim with noticeable nonlinearity. As the applied voltage increases, the loop nonlinearity increases, indicating the gradual saturation of BST’s polarization. At the applied voltage of 6 V, a small remanent polarization (Pr) of ~2 μC/cm2 and a coercive field (Ec) of ~20 kV/cm can be observed. Similar loops with comparable Pr and Ec values were observed for the BST films grown on rigid substrates, which indeed suggests the mixed ferroelectric-paraelectric behavior.37,38 Figure 2b,c presents the PFM amplitude and phase images of the mica/SRO/BST film after a box-in-box writing. The outer 2×2 μm2 region was first written with a tip bias of -8 V, followed by another tip bias of +8 V applied to the inner 1×1 μm2. As seen from the phase image (Figure 2c), only a partial of the -8 V-written region shows white color, while the as-grown and +8 V-written regions mainly exhibit brown color. This phase contrast indicates that there are some switchable domains existing in the BST film. However, due to the coexistence of the paraelectric phase, a full switching of domains cannot be realized in the -8 V-written region. Additionally, as seen from Figure 2d, the PFM phase loop shows a sharp ~180° switching while the amplitude loop shows a butterfly-like shape. These PFM results demonstrate that the BST film grown on the mica substrate contains a ferroelectric phase and thus it can exhibit ferroelectric switching behavior.

9



Figure 3. (a) Photography of the mica/SRO/BST/Pt capacitor pasted onto a mould in the flex-out bending mode. (b) Dielectric constant (εr’) and (c) loss tangent [tan(δ)] as a function of frequency, and (d) dielectric constant versus electric field (εr’-E) characteristics (frequency: 10 kHz) in the bending states with different radii. The bending radii are indicated in Panel c, and “flat-1”, “flat-2”, and “flat-3” denote the initial flat state, second flat state after being bent to R = 5 mm, and third flat state after being bent to R = 2 mm. Inset in Panel b illustrates that a smaller bending radius corresponds to a larger extent of bending.

Now let us focus on the dielectric properties of the flexible mica/SRO/BST/Pt capacitor. Figure 3b,c shows the typical εr’ and tan(δ) as a function of frequency in flat and bending states. Only the data in the frequency range of 500 Hz to 1×106 Hz are shown, because the dielectric behaviors at frequencies below 500 Hz and above 1×106 Hz are significantly influenced by space charge polarization and stray inductance, respectively.39,40 In the initial flat state, the εr’ decreases monotonously with frequency 10


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(Figure 3b), consistent with the fact that fewer dipoles are able to follow the AC field with higher frequencies. At the frequency of 10 kHz, the εr’ can reach up to ~1210 while the tan(δ) is as low as ~0.16 (Figure 3b,c). These dielectric properties are as good as those of BST and other dielectric films grown on rigid substrates,38,41 which can be attributed to the good quality of our BST film. The growth of high-quality BST film on the mica substrate is enabled by the SRO buffer layer which is flat and highly conductive (Figure S2). If the SRO buffer layer is replaced with the Pt buffer layer, the BST film exhibits much poorer electrical properties (Figure S3). After the measurement in the initial flat state, the capacitor was sequentially bent to the radii (R) of 20, 15, 10, and 5 mm in the flex-out bending mode, and the corresponding dielectric spectra were measured. As shown in Figure 3b,c, the εr’ and tan(δ) in these bending states remain almost unchanged compared with those in the initial flat state. After the bending with R = 5 mm, the capacitor was flattened again and the εr’ and tan(δ) can be fully recovered. These results demonstrate the good mechanical flexibility of our mica/SRO/BST/Pt capacitor. However, when the capacitor was further bent to R = 2 mm, the spectrum of εr’ shifts downward while that of tan(δ) shifts upward (purple lines in Figure 3b,c). More specifically, at the frequency of 10 kHz, the εr’ drops to ~930 and the tanδ increases to ~0.47. The degradation of dielectric properties seems to be persistent, because the εr’ and tan(δ) cannot be recovered after re-flattening the capacitor (orange lines in Figure 3b,c). Therefore, it is deduced that the BST film undergoes permanent damage after being bent to R = 2 mm. Besides the εr’ and tan(δ) spectra, the characteristics of εr’ (@ 10 kHz) versus applied electric field (E) were also measured for the flexible BST capacitor in the flat and bending states. As shown in Figure 3d, in the initial flat state, the εr’ at E ≈ 0 exhibits a maximum value of ~1210, but it decreases nonlinearly to ~420 as E increases to 133 kV/cm. The decrease of εr’ with increasing E is due to the polarization gradually getting saturated at high electric fields, typical for nonlinear dielectrics like BST. In addition, the observation of two εr’ maxima located very close to E = 0 implies the mixed ferroelectric-paraelectric behavior in the BST film, 11



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consistent with the results of P-E loops.42,43 The ratio of electric field-induced change in εr’ can be defined as a tunability: Tunability (%) =

𝜀,𝑟(0) ― 𝜀,𝑟(𝐸) 𝜀,𝑟(0)

,

(1)

where εr’(0) and εr’(E) are the εr’ values at zero DC field and E (here, E = 133 kV/cm), respectively. The tunability of our mica/SRO/BST/Pt capacitor is calculated to be ~67%, which is comparable to those achieved in BST and other dielectric films grown on rigid substrates.44,45 Figure 3d also illustrates that the εr’-E characteristics almost do not change even when the capacitor is bent to R = 5 mm. However, when a further bending with R = 2 mm is performed, the εr’-E curve exhibits a noticeable downward shift and the value of εr’(0) - εr’(E) decreases. These changes are persisted after re-flattening the capacitor, indicating that the capacitor undergoes permanent degradations of both zero-DC field dielectric properties and tunable dielectric performance after being bent to R = 2 mm.

Figure 4. (a) Schematic diagram of the set-up for measuring the bending endurance of the mica/SRO/BST/Pt capacitor in the flex-out bending mode. The two ends of the 12


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sample was clamped by two holders. One holder was fixed while another one was movable and its movement was controlled by a computer. (b) εr’ and (c) tan(δ) as a function of frequency, and (d) εr’-E characteristics (frequency: 10 kHz) after different bending cycles. The bending cycles are indicated in Panel c.

In order to demonstrate that the flexible mica/SRO/BST/Pt capacitor is sufficiently reliable to fulfil the requirement of flexible electronic applications, the capacitor was bent (R = 5 mm)/flattened repeatedly, and its dielectric properties were measured after various bending cycles. Figure 4a displays the schematic diagram of an automatic apparatus which was used to perform the multiple bends for our flexible capacitor. As shown in Figure 4b-d, the εr’ and tan(δ) spectra as well as the εr’-E characteristics remain roughly identical after being bent for 12000 cycles, suggesting that the dielectric properties of the flexible BST capacitor are immune to up to 12000 bending cycles. As the bending cycle increases to 16000, the dielectric properties degrade slightly, and the degradation becomes more significant after another 4000 bending cycles. Nevertheless, an εr’ of ~940, a tan(δ) of ~1.75 and a tunability of ~63% can be retained after the total bending cycles of 20000. For our flexible mica/SRO/BST/Pt capacitor, good mechanical flexibility and durability are observed not only in the flex-out bending mode, but also in the flex-in bending mode. As shown in Figure S4 and S5, flex-in bending with R = 5 mm or bending repeatedly for 12000 cycles does not degrade the dielectric performance. However, further decreasing R or increasing bending cycle leads to the degradation. These results are similar to those obtained in the flex-out bending mode, suggesting that the degradation of dielectric performance in the mica/SRO/BST/Pt capacitor depends on the magnitude of strain (or accumulated strains) rather than the type of strain (tensile or compressive).

13



Figure 5. (a) Schematic illustration of the shielded resonant cavity method for the measurement of microwave dielectric properties. (b) εr’ and tan(δ) as a function of bending cycle (frequency: 18.6 GHz).

Figure 5a depicts a schematic setup for measuring microwave dielectric properties, which mainly consists of a shielded resonant cavity and a network analyzer. The εr’ and tan(δ) at 18.6 GHz of the mica/SRO/BST/Pt capacitor after various bending cycles are shown in Figure 5b. In the initial flat state, the εr’ and tan(δ) are ~540 and ~0.07, respectively. Such a value of εr’ at a microwave frequency is indeed quite high, comparable to those obtained in rigid BST capacitors.46,47 Moreover, the εr’ almost does not change after the capacitor is bent for 10000 cycles (@ R = 5 mm), but it decreases to ~460 after 20000 bending cycles. Nevertheless, the tan(δ) remains below 0.1 even after 20000 bending cycles. Therefore, similar to those at low frequencies, the dielectric properties at microwave frequencies of our mica/SRO/BST/Pt capacitor are also very stable against mechanical bending. To show the up-to-date research progress in flexible capacitors, the comprehensive dielectric and mechanical performances of some representative micaand polymer-based capacitors are compared, as shown in Table 1. Notably, our mica/SRO/BST/Pt capacitor emerges as the first flexible capacitor ever reported which can exhibit an εr’ higher than 500 and simultaneously a tan(δ) lower than 0.1 at a microwave frequency. In addition, the mica/SRO/BST/Pt capacitor can also exhibit an εr’ of over 1200 and a tan(δ) of below 0.2 at low frequencies (e.g., 10 kHz), which are comparable to or even superior than those of other capacitors. Moreover, the 14


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tunability of ~67% achieved in our mica/SRO/BST/Pt capacitor is the highest among all the capacitors listed in Table 1. Last but not the least, the mica/SRO/BST/Pt capacitor can be bent to R = 5 mm and endure 12000 bending cycles (@ R = 5 mm), which are among the best mechanical properties of the reported capacitors. In a word, our mica/SRO/BST/Pt capacitor have excellent dielectricity in a wide frequency range as well as good mechanical flexibility and durability. Table 1. Comparison of dielectric properties between our flexible BST capacitor and other flexible and rigid capacitors. Mica/BST

Mica/PZT

Mica/BLT

Mica/BZT

PVDF/BCZT

Silicon/PZT

LaAlO3/BSTa

Ag/nanopaper

εr’

1200

540

1950

460

500

80

90

540

700

1660

1057

@ f (Hz)

10k

18.6G

1k

1M

—

10k

10k

10k

1.1G

10k

10G

tan(δ)

0.16

0.07

—

—

—

0.005

0.025

—

0.25

0.05

0.172

@ f (Hz)

10k

18.6G

—

—

—

10k

10k

—

1.1G

10k

10G

Tunabilityb

67%

11%

43%

42%

—

—

18%

—

56%

—

Rmin (mm)

2

1.4

2.5

1.4

4

—

5

5

—

—

Nbending

15000

10000

1000

10000

10000

—

1000

1000

—

—

@ R (mm)

5 mm

2.2 mm

5 mm

1.4 mm

4.5 mm

—

5 mm

—

—

—

Reference

This work

Ref. 48

Ref. 33

Ref. 49

Ref. 50

Ref. 51

Ref. 44

Ref. 54

Ref. 52

Ref. 53

a

This is a reference rigid capacitor.

b

A benchmark for the evaluation of tunability was adopted: only the values of εr’(0)

and εr’(E = 133 kV/cm) were used in Eq. (1) to calculate the tunability for all the capacitors. Below

we

proceed

to

analyze

the

degradation

mechanism

of

the

mica/SRO/BST/Pt capacitor in the flex-out bending tests (because the results of flex-out and flex-in bending tests are similar, only the former are analyzed in detail to reveal the mechanism). Two capacitors, the one after being bent to R = 2 mm (denoted as “Cap-1”) and the other after undertaking 20000 bending cycles at R = 5 mm (denoted as “Cap-2”), were systematically compared with their respective initial flat states, in terms of morphology, impedance, and DC conductivity. First, the AFM topography images of Cap-1 and Cap-2 after the bending tests show the presence of microcracks (Figure S6). The microcracks often act as leakage paths,55,56 which may 15



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be the origin for the increase in dielectric loss. Next, the complex impedance data of Cap-1 and Cap-2 were analyzed to gain deeper insights into the degradation of dielectric properties, by using the Nyquist plot of imaginary component of impedance (Z’’) versus real component of impedance (Z’). Typically, the Nyquist plot would be composed of two semicircles, each semicircle representing a distinct process whose time constant is sufficiently separated from the other. The semicircle at higher frequencies represents the bulk (grains) contribution, while that at lower frequencies represents grain boundary and film/electrode interface contributions.57 As shown in Figure 6, only one semicircular arc is observed in each Nyquist plot for Cap-1 and Cap-2 in initial flat state and after bending. This semicircular arc can be assigned to the BST bulk, allowing for the fact that the BST bulk mainly contributes to the dielectric response in the frequency range of 500 to 1×105 Hz.58 Therefore, the mica/SRO/BST/Pt capacitor may be modelled using an equivalent circuit, which combines a series resistance (Rs) and a RC circuit consisting of a constant phase element (CPE) and a parallel resistance (Rp). The Rs represents the contact resistance between the BST film and the electrode. The RC circuit represents the BST bulk, where the leakage contribution is reflected by Rp and the non-ideal Debye-like dielectric behavior is described by CPE.59 The impedance of a CPE is defined by ∗ (𝜔) = [𝐴(𝑗𝜔)𝑛] 𝑍CPE

―1

,

(2)

where A is the pseudocapacitance and n is a quantity between 0 and 1 describing the deviation from an ideal capacitor. For n = 1, the CPE becomes an ideal capacitor with a capacitance equaling A; whereas, for n = 0, the CPE represents an ideal resistor with a resistance equaling 1/A.

16


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Figure 6. Nyquist plots of complex impedances for (a) Cap-1 and (b) Cap-2. The equivalent circuit is indicated in Panel a.

As shown in Figure 6, the complex impedance data of both Cap-1 and Cap-2 can be well fitted using the equivalent circuit model. The fitting parameters are summarized in Table 2. For Cap-1, the values of CPE-related parameters (A and n) remain almost unchanged after being bent to R = 2 mm, whereas the Rp decreases dramatically from 1.1×107 to 2.9×105 Ω and the Rs increases from 1.1×103 to 7.0×103 Ω. The small changes in CPE-related parameters suggest that the BST bulk is nearly unaffected by the bending. The decrease in Rp is well correlated with the microcracks formed after a large extent of bending, which increases the leakage current. The enhancement in Rs may be due to the film/electrode contact becomes poor owing to the bending. Likewise, for Cap-2, the variation trends of CPE-related parameters, Rp, and Rs are similar to those in Cap-1 (Table 2). It is therefore deduced that the cyclical bending leads to similar degradation behavior as the large extent of bending, i.e., the formation of microcracks increasing the leakage current and the deterioration of film/electrode contact. Table 2. Parameters obtained after fitting the complex impedance data using the equivalent circuit model.

Cap-1

State

Rs

Rp

A

n

Initial

1.1×103

1.1×107

3.7×10-10

0.94

R = 2 mm

7.0×103

2.9×105

3.7×10-10

0.92

17



Cap-2

Page 18 of 31

Initial

6.9×102

5.8×106

3.0×10-10

0.96

20000 cycles

3.0×103

3.1×105

3.2×10-10

0.94

The enhanced leakage current due to microcracks is responsible not only for the degradation of zero-DC field dielectric properties (as discussed above), but also for the degradation of tunable dielectric performance. It is known that the tunable dielectric constant results from the polarization increasing nonlinearly with the electric field. For the capacitor after bending, the enhanced leakage current can reduce the effective field used for inducing the polarization. Therefore, the dielectric constant variation [εr’(0) - εr’(E)] is smaller in the capacitors after bending than in those before bending (Figure 3d and 4d). To further understand the enhanced leakage current arising from microcracks, the DC current-voltage (I-V) characteristics were measured for Cap-1 and Cap-2. As shown in Figure 7a, the leakage current in Cap-1 increases significantly, from ~149.8 μA (@ 6 V) in the initial flat state to ~873.7 μA (@ 6 V) after the bending with R = 2 mm. Similar enhancement in leakage current is observed in Cap-2 which undertakes 20000 bending cycles (Figure 7d).

Figure 7. Typical current-voltage (I-V) characteristics of (a) Cap-1 and (d) Cap-2. Replots of I-V characteristics (voltage range: 0 to -3 V) for (b) Cap-1 and (e) Cap-2 in 18


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Log-Log scale. Replots of I-V characteristics (voltage range: -3 to -6 V) for (c) Cap-1 and (f) Cap-2 as Ln(I/V2) versus 1/V . The dotted lines are the fitting lines.

The conduction mechanisms at low and intermediate voltages (|V| < 3 V) were first analyzed. The I-V characteristics in this voltage range are replotted in Log-Log scale for both Cap-1 and Cap-2, as shown in Figure 7b,e and Figure S7a,c. The Log(I)-Log(V) curves show three characteristic linear regions, in accordance with a space-charge-limited current (SCLC) model. Region (i) at |V| < ~0.4 V exhibits a slope of ~1, indicating typical Ohmic conduction behavior: 𝐽=

𝑞𝑛𝑒𝜇𝑉 𝑑

,

(3)

where J is the leakage current density, q is the charge of electron, ne is the electron density, μ is the mobility of the electrons (0.001 cm2/Vs for BST60), V is the applied voltage, and d is the sample thickness. The ne of Cap-1 in the initial state is calculated to be 5.6×1014 cm-3, which increases to 2.2×1015 cm-3 in the bending state with R = 2 mm. For Cap-2, the ne increases from 1.1×1014 cm-3 in the initial state to 2.1×1015 cm-3 after 20000 bending cycles. The increase in ne may be caused by the defects in the microcracks contributing more free electrons. In Region (ii) at ~0.4 V < |V| < ~2 V, the number of injected electrons is greater than that of thermally stimulated free electrons, thus leading to the SCLC behavior with a slope of ~2. The current density from SCLC follows the modified Child’s law, which can be expressed as:61 9

𝑉2

𝐽 = 8𝜀,𝑟𝜀0𝜃𝜇𝑑3,

(4)

where ε0 is the permittivity of free space, and θ is the ratio of free-to-trapped electrons. As the applied voltage enters Region (iii) (~2 V < |V| < 3 V), the Log(I)-Log(V) slope abruptly increases to much greater than 2, indicating a trap filling process. The voltage where the slope increases abruptly is called the trap-filled limit voltage (VTFL), which can be used to calculate the trap density (Nt):62

19



𝑉𝑇𝐹𝐿

9

𝑁𝑡 = 8𝑞𝜀,𝑟𝜀0

𝑑2

,

Page 20 of 31

(5)

where an average value of VTFL is taken if the VTFL values in positive and negative voltage regions are different. For Cap-1, the average Nt value increases from 3.9×1017 cm-3 in initial flat state to 4.4×1017 cm-3 after being bent to R = 2 mm. Similarly, the Cap-2 also exhibits an increase in average Nt value, from 4.0×1017 cm-3 in initial flat state to 4.4×1017 cm-3 after 20000 bending cycles. The increase in Nt can also be attributed to the formation of microcracks after bending, because the dangling bonds in microcracks may create a large number of trap states.55,56 At high voltages (|V| > 3 V), the Fowler-Nordheim (FN) tunneling often occurs, manifesting itself as the tunneling of electrons through the triangular-shaped interface potential barrier (Φt) into the conduction band of the dielectric. The current for the FN tunneling is given by: 2

𝐼 = 𝐵𝐸 exp

(

―8𝜋 2𝑚effΦ3/2 t 3ℎ𝑞𝐸

),

(6)

where B is a constant, E is the electric field (i.e., V/d), meff is the effective electron mass,63 and h is the Planck constant. According to Eq. (6), a linear relationship between Ln(I/V2) versus 1/V exists for the FN tunneling. To check it, we have plotted the I-V curves of Cap-1 and Cap-2 in the voltage range of 3 V < |V| < 6 V as Ln(I/V2) versus 1/V, as presented in Figure 7c,f and Figure S7b,d. The Ln(I/V2)-1/V curves exhibit good linearity at high voltages, indicating that the FN tunneling is the dominant conduction mechanism. Through the fittings, the Φt values of both Cap-1 and Cap-2 are found to decrease after bending, which may be attributed to the microcracks that can lower the local potential barrier for electron tunneling. From the above analyses, it is inferred that suppressing the formation of microcracks is critical to address the bending-induced degradation of dielectric properties. A viable approach may be reducing the thickness of the mica substrate. This is because as the mica thickness becomes smaller, the bending-induced strain on the BST film is reduced (note: the strain can be estimated with dsub/2R, where dsub and R are the mica thickness and bending radius, respectively, and the thicknesses of BST and SRO layers are neglected because they are too small compared with dsub). The 20


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reduced strain may give rise to a smaller amount of microcracks formed during the bending.

4. CONCLUSIONS In summary, large-scale and high-quality SRO/BST/Pt capacitors were fabricated on flexible mica substrates through PLD. The polycrystalline BST film shows an average grain size of ~50 nm, a roughness of ~3.9 nm, and a high transparency of ~50% in the 400-900 nm wavelength range. The BST film also exhibits ferroelectricity with a small polarization of ~2 μC/cm2. Then, we focused on the investigation on the dielectric properties of flexible mica/SRO/BST/Pt capacitor in both low-frequency and microwave regions. The capacitor exhibits an εr’ exceeding 1200, a tan(δ) as low as ~0.16, and a tunability of 67% at low frequencies (e.g., 10 kHz). Meanwhile, a high εr’ of ~540 and a low tan(δ) of ~0.07 are retained at microwave frequencies (e.g., 18.6 GHz). More importantly, the capacitor can endure a small bending radius of 5 mm and 12000 bending cycles (@ R = 5 mm) with almost no decays in εr’, tan(δ), and tunability. Further decreasing bending radius or increasing bending cycle leads to the dielectric performance degradation, which may be caused by the formation of microcracks increasing the leakage current. Nevertheless, the dielectricity of the BST bulk (grains) is sufficiently robust against the bending. Given the excellent dielectric and mechanical properties, the flexible mica/SRO/BST/Pt capacitor is promising for wide applications in flexible electronic and microwave devices.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge on the ACS Publications website at (to be inserted by the publisher). Details of the method for measuring microwave dielectric properties, morphology and electrical conductivity of the SRO film, characterizations of the BST film grown on the Pt-buffered mica substrate, dielectric properties in the flex-in bending mode, 21



observation of microcracks using AFM, and fittings of I-V characteristics in the positive voltage region.

AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors would like to thank National Key Research Program of China (No. 2016YFA0201002), State Key Program for Basic Researches of China (No. 2015CB921202), National Natural Science Foundation of China (Nos. 51602110, 11674108, 51431006, and 51561135014). Guangdong Innovative Research Team Program (No. 2013C102), Science and Technology Project of Guangdong Province (Nos. 2016B090918083, 2017B030301007, and 2015B090927006), Natural Science Foundation of Guangdong Province (No. 2016A030308019), and Science and Technology Project of Shenzhen Municipal Science and Technology Innovation Committee (GQYCZZ20150721150406). X.L. and Z.F. acknowledge the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme 2016 and 2018, respectively. D.G. and Z.T. acknowledge the support from the Innovation Project of Graduate School of South China Normal University.

REFERENCES (1) Koo, J. H.; Kim, D. C.; Shim, H. J.; Kim, T.-H.; Kim, D.-H., Flexible and Stretchable Smart Display: Materials, Fabrication, Device Design, and System 22


Page 22 of 31

Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Integration. Adv. Funct. Mater. 2018, 28 (35), 1801834. (2) Lee, H. E.; Kim, S.; Ko, J.; Yeom, H.-I.; Byun, C.-W.; Lee, S. H.; Joe, D. J.; Im, T.-H.; Park, S.-H. K.; Lee, K. J., Skin-Like Oxide Thin-Film Transistors for Transparent Displays. Adv. Funct. Mater. 2016, 26 (34), 6170-6178. (3) Gelinck, G. H.; Huitema, H. E.; van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; van der Putten, J. B.; Geuns, T. C.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B. H.; Meijer, E. J.; Benito, E. M.; Touwslager, F. J.; Marsman, A. W.; van Rens, B. J.; de Leeuw, D. M., Flexible Active-Matrix Displays and Shift Registers based on Solution-Processed Organic Transistors. Nat. Mater. 2004, 3 (2), 106-110. (4) HUAWEI, HUAWEI Mate X. https://consumer.huawei.com/cn/phones/mate-x/, 2019 (accessed 11 May 2019) (5) ROYOLE, FlexPai. http://www.royole.com/cn, 2019 (accessed 11 May 2019) (6) Wang, S.; Xu, J.; Wang, W.; Wang, G. N.; Rastak, R.; Molina-Lopez, F.; Chung, J. W.; Niu, S.; Feig, V. R.; Lopez, J.; Lei, T.; Kwon, S. K.; Kim, Y.; Foudeh, A. M.; Ehrlich, A.; Gasperini, A.; Yun, Y.; Murmann, B.; Tok, J. B.; Bao, Z., Skin Electronics from Scalable Fabrication of An Intrinsically Stretchable Transistor Array. Nature 2018, 555 (7694), 83-88. (7) Son, D.; Kang, J.; Vardoulis, O.; Kim, Y.; Matsuhisa, N.; Oh, J. Y.; To, J. W.; Mun, J.; Katsumata, T.; Liu, Y.; McGuire, A. F.; Krason, M.; Molina-Lopez, F.; Ham, J.; Kraft, U.; Lee, Y.; Yun, Y.; Tok, J. B.; Bao, Z., An Integrated Self-Healable Electronic Skin System Fabricated Via Dynamic Reconstruction of A Nanostructured Conducting Network. Nat. Nanotechnol. 2018, 13 (11), 1057-1065. (8) Oh, J. Y.; Bao, Z., Second Skin Enabled by Advanced Electronics. Adv. Sci. 2019, 6 (11), 1900186.

23



(9) Li, Q.; Chen, L.; Gadinski, M. R.; Zhang, S.; Zhang, G.; Li, U.; Iagodkine, E.; Haque, A.; Chen, L. Q.; Jackson, N.; Wang, Q., Flexible High-Temperature Dielectric Materials from Polymer Nanocomposites. Nature 2015, 523 (7562), 576-579. (10) Ji, S.; Jang, J.; Cho, E.; Kim, S. H.; Kang, E. S.; Kim, J.; Kim, H. K.; Kong, H.; Kim, S. K.; Kim, J. Y.; Park, J. U., High Dielectric Performances of Flexible and Transparent Cellulose Hybrid Films Controlled by Multidimensional Metal Nanostructures. Adv. Mater. 2017, 29 (24), 1700538. (11) Shown, I.; Ganguly, A.; Chen, L.-C.; Chen, K.-H., Conducting Polymer-Based Flexible Supercapacitor. Energy Sci. & Eng. 2015, 3 (1), 2-26. (12) Huang, X.; Jiang, P., Core-Shell Structured High-k Polymer Nanocomposites for Energy Storage and Dielectric Applications. Adv. Mater. 2015, 27 (3), 546-554. (13) Yang, Y.; Yuan, G.; Yan, Z.; Wang, Y.; Lu, X.; Liu, J. M., Flexible, Semitransparent, and Inorganic Resistive Memory based on BaTi0.95Co0.05O3 Film. Adv. Mater. 2017, 29 (26), 1700425. (14) Bakaul, S. R.; Serrao, C. R.; Lee, O.; Lu, Z.; Yadav, A.; Carraro, C.; Maboudian, R.; Ramesh, R.; Salahuddin, S., High Speed Epitaxial Perovskite Memory on Flexible Substrates. Adv. Mater. 2017, 29 (11), 1605699. (15) Liu, W.; Liu, M.; Ma, R.; Zhang, R.; Zhang, W.; Yu, D.; Wang, Q.; Wang, J.; Wang, H., Mechanical Strain-Tunable Microwave Magnetism in Flexible CuFe2O4 Epitaxial Thin Film for Wearable Sensors. Adv. Funct. Mater. 2018, 28 (10), 1705928. (16) Tu, N.; Jiang, J.; Chen, Q.; Liao, J.; Liu, W.; Yang, Q.; Jiang, L.; Zhou, Y. Flexible Ferroelectric Capacitors Based on Bi3.15Nd0.85Ti3O12/muscovite structure. Smart Mater. Struct. 2019, 28 (5), 054002. (17) Arndt, F.; Frye, A.; Wellnitz, M.; Wirsing, R. J., Double Dielectric-Slab-Filled 24


Page 24 of 31

Page 25 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Waveguide Phase Shifter. IEEE T. Microw. Theory 1985, 33 (5), 373-381. (18) Zhang, X. Y.; Xue, Q.; Chan, C. H.; Hu, B.-J., Low-Loss Frequency-Agile Bandpass Filters with Controllable Bandwidth and Suppressed Second Harmonic. IEEE T. Microw. Theory 2010, 58 (6), 1557-1564. (19) Miranda, F. A.; Subramanyam, G.; Van Keuls, F. W.; Romanofsky, R. R., A K-Band

(HTS,

Gold)/Ferroelectric

Thin

Film/Dielectric

Diplexer

for

A

Discriminator-Locked Tunable Oscillator. IEEE T. Appl. Supercon. 1999, 9 (2), 3581-3584. (20) Takemura, K., Sakuma, T., & Miyasaka, Y. (1994). High Dielectric Constant (Ba, Sr)TiO3 Thin Films Prepared on RuO2/Sapphire. Appl. Phys. Lett. 1994, 64 (22), 2967-2969. (21) Jona, F.; Shirane, G. Ferroelectric Crystal; Dover Publication: New York, 1993. (22) Song, L.; Chen, Y.; Wang, G.; Yang, L.; Li, T.; Gao, F.; Dong, X., Dielectric Properties of La/Mn Codoped Ba0.63Sr0.37TiO3 Thin Films Prepared by RF Magnetron Sputtering. Ceram. Int. 2014, 40 (8), 12573-12577. (23) Liu, H.; Avrutin, V.; Zhu, C.; Özgür, Ü.; Yang, J.; Lu, C.; Morkoç, H., Enhanced Microwave Dielectric Tunability of Ba0.5Sr0.5TiO3 Thin Films Grown with Reduced Strain on DyScO3 Substrates by Three-Step Technique. J. Appl. Phys. 2013, 113 (4), 044108. (24) Dietz, G. W.; Schumacher, M.; Waser, R.; Streiffer, S. K.; Basceri, C.; Kingon, A. I., Leakage Currents in Ba0.7Sr0.3TiO3 Thin Films for Ultrahigh-Density Dynamic Random Access Memories. J. Appl. Phys. 1997, 82 (5), 2359-2364. (25) Xie, S.-H.; Zhu, B.-K.; Wei, X.-Z.; Xu, Z.-K.; Xu, Y.-Y., Polyimide/BaTiO3 Composites with Controllable Dielectric Properties. Compos. Part A: Appl. S. 2005, 36 (8), 1152-1157. 25



(26) Kane, M. G.; Campi, J.; Hammond, M. S.; Cuomo, F. P.; Greening, B.; Sheraw, C. D.; Nichols, J. A.; Gundlach, D. J.; Huang, J. R.; Kuo, C. C., Analog and Digital Circuits Using Organic Thin-Film Transistors on Polyester Substrates. IEEE Electr. Device L. 2000, 21 (11), 534-536. (27) Dufour, R.; Harnois, M.; Coffinier, Y.; Thomy, V.; Boukherroub, R.; Senez, V., Engineering Sticky Superomniphobic Surfaces on Transparent and Flexible PDMS substrate. Langmuir 2010, 26 (22), 17242-17247. (28) Kim, E.; Cho, H.; Kim, K.; Koh, T. W.; Chung, J.; Lee, J.; Park, Y.; Yoo, S., A Facile Route to Efficient, Low-Cost Flexible Organic Light-Emitting Diodes: Utilizing the High Refractive Index and Built-in Scattering Properties of Industrial-Grade PEN Substrates. Adv. Mater. 2015, 27 (9), 1624-1631. (29) Auch, M. D. J.; Soo, O. K.; Ewald, G.; Soo-Jin, C., Ultrathin Glass for Flexible OLED Application. Thin Solid Films 2002, 417 (1), 47-50. (30) Shahrjerdi, D.; Bedell, S. W., Extremely Flexible Nanoscale Ultrathin Body Silicon Integrated Circuits on Plastic. Nano Lett. 2013, 13 (1), 315-320. (31) Shen, L.; Wu, L.; Sheng, Q.; Ma, C.; Zhang, Y.; Lu, L.; Ma, J.; Ma, J.; Bian, J.; Yang, Y.; Chen, A.; Lu, X.; Liu, M.; Wang, H.; Jia, C.-L., Epitaxial Lift-Off of Centimeter-Scaled Spinel Ferrite Oxide Thin Films for Flexible Electronics. Adv. Mater. 2017, 29 (33), 1702411. (32) Chu, Y.-H., Van der Waals Oxide Heteroepitaxy. npj Quantum Materials 2017, 2 (1), 67. (33) Jiang, J.; Bitla, Y.; Huang, C.-W.; Do, T. H.; Liu, H.-J.; Hsieh, Y.-H.; Ma, C.-H.; Jang, C.-Y.; Lai, Y.-H.; Chiu, P.-W.; Wu, W.-W.; Chen, Y.-C.; Zhou, Y.-C.; Chu, Y.-H., Flexible Ferroelectric Element based on Van der Waals Heteroepitaxy. Sci. Adv. 2017, 3 (6), e1700121.

26


Page 26 of 31

Page 27 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Wang, Y.; Zhang, C. C.; Wang, J. Z.; He, B.; Han, Y.Q., Permittivity Measurement of Microwave Dielectric Ceramics Using Shielded-Cavity Method. Chin. J. Sci. Instrum. 2017, 38 (10), 152-159 (35) Yustanti, E.; Hafizah, M. A. E.; Manaf, A., Synthesis of Strontium Substituted Barium Titanate Nanoparticles by Mechanical Alloying and High Power Ultrasonication Destruction. AIP Conf. Proc. 2016, 1725 (1), 020102. (36) Gim, Y.; Hudson, T.; Fan, Y.; Kwon, C.; Findikoglu, A. T.; Gibbons, B. J.; Park, B. H.; Jia, Q. X., Microstructure and Dielectric Properties of Ba1−xSrxTiO3 Films Grown on LaAlO3 Substrates. Appl. Phys. Lett. 2000, 77 (8), 1200-1202. (37) Hu, W.; Yang, C.; Zhang, W.; Qiu, Y., Dielectric Characteristics of Sol-Gel-Derived BST/BSLaT/BST Multilayer. J. Sol-Gel Sci. Technol. 2005, 36 (3), 249-255. (38) Pontes, F. M.; Leite, E. R.; Pontes, D. S. L.; Longo, E.; Santos, E. M. S.; Mergulhão, S.; Pizani, P. S.; Lanciotti, F.; Boschi, T. M.; Varela, J. A., Ferroelectric and Optical Properties of Ba0.8Sr0.2TiO3 Thin Film. J. Appl. Phys. 2002, 91 (9), 5972-5978. (39) Zhu, X. H.; Chan, H. L. W.; Choy, C. L.; Wong, K. H., Influence of Ambient Oxygen Pressure on The Preferred Orientation, Microstructures, and Dielectric Properties of (Ba1-xSrx)TiO3 Thin Films with Compositionally Graded Structures. Appl. Phys. A 2005, 80 (3), 591-595. (40) Baniecki, J. D.; Laibowitz, R. B.; Shaw, T. M.; Duncombe, P. R.; Neumayer, D. A.; Kotecki, D. E.; Shen, H.; Ma, Q. Y., Dielectric Relaxation of Ba0.7Sr0.3TiO3 Thin Films from 1 mHz to 20 GHz. Appl. Phys. Lett. 1998, 72 (4), 498-500. (41) Zafar, S.; Jones, R. E.; Chu, P.; White, B.; Jiang, B.; Taylor, D.; Zurcher, P.; Gillepsie, S., Investigation of Bulk and Interfacial Properties of Ba0.5Sr0.5TiO3 Thin Film Capacitors. Appl. Phys. Lett. 1998, 72 (22), 2820-2822. 27



(42) Tyunina, M.; Levoska, J.; Jaakola, I., Polarization Relaxation in Thin-Film Relaxors Compared to that in Ferroelectrics. Phys. Rev. B 2006, 74 (10), 104112. (43) Dawber, M.; Rabe, K. M.; Scott, J. F., Physics of Thin-Film Ferroelectric Oxides. Rev. Mod. Phys. 2005, 77 (4), 1083-1130. (44) Lu, S. G.; Zhu, X. H.; Mak, C. L.; Wong, K. H.; Chan, H. L. W.; Choy, C. L., High Tunability in Compositionally Graded Epitaxial Barium Strontium Titanate Thin Films by Pulsed-Laser Deposition. Appl. Phys. Lett. 2003, 82 (17), 2877-2879. (45) Park, B. H.; Gim, Y.; Fan, Y.; Jia, Q. X.; Lu, P., High Nonlinearity of Ba0.6Sr0.4TiO3 Films Heteroepitaxially Grown on MgO Substrates. Appl. Phys. Lett. 2000, 77 (16), 2587-2589. (46) Liu, H.; Avrutin, V.; Zhu, C.; Özgür, Ü.; Yang, J.; Lu, C.; Morkoç, H. Enhanced microwave dielectric tunability of Ba0.5Sr0.5TiO3 thin films grown with reduced strain on DyScO3 substrates by three-step technique. J. Appl. Phys. 2013, 113 (4), 044108. (47) Kim, W. J.; Chang, W.; Qadri, S. B.; Pond, J. M.; Kirchoefer, S. W.; Chrisey, D. B.; Horwitz, J. S., Microwave Properties of Tetragonally Distorted (Ba0.5Sr0.5)TiO3 Thin Films. Appl. Phys. Lett. 2000, 76 (9), 1185-1187. (48) Gao, W.; You, L.; Wang, Y.; Yuan, G.; Chu, Y.-H.; Liu, Z.; Liu, J.-M., Flexible PbZr0.52Ti0.48O3 Capacitors with Giant Piezoelectric Response and Dielectric Tunability. Adv. Electron. Mater. 2017, 3 (8), 1600542. (49) Su, L.; Lu, X.; Chen, L.; Wang, Y.; Yuan, G.; Liu, J. M., Flexible, Fatigue-Free, and Large-Scale Bi3.25La0.75Ti3O12 Ferroelectric Memories. ACS Appl. Mater. Interfaces 2018, 10 (25), 21428-21433. (50) Liang, Z.; Liu, M.; Shen, L.; Lu, L.; Ma, C.; Lu, X.; Lou, X.; Jia, C. L., All-Inorganic Flexible Embedded Thin-Film Capacitors for Dielectric Energy Storage with High Performance. ACS Appl. Mater. Interfaces 2019, 11 (5), 5247-5255. 28


Page 28 of 31

Page 29 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(51) Luo, B.; Wang, X.; Wang, Y.; Li, L., Fabrication, Characterization, Properties and Theoretical Analysis of Ceramic/PVDF Composite Flexible Films with High Dielectric Constant and Low Dielectric Loss. J. Mater. Chem. A 2014, 2 (2), 510-519. (52) Ghoneim, M. T.; Zidan, M. A.; Alnassar, M. Y.; Hanna, A. N.; Kosel, J.; Salama, K. N.; Hussain, M. M., Thin PZT-Based Ferroelectric Capacitors on Flexible Silicon for Nonvolatile Memory Applications. Adv. Electron. Mater. 2015, 1 (6), 1500045. (53) Inui, T.; Koga, H.; Nogi, M.; Komoda, N.; Suganuma, K., A Miniaturized Flexible Antenna Printed on A High Dielectric Constant Nanopaper Composite. Adv. Mater. 2015, 27 (6), 1112-1116. (54) Wang, H.; Bian, Y.; Shen, B.; Zhai, J., Comparison of Microwave Dielectric Properties of Ba0.6Sr0.4TiO3 Thin Films Grown on (100) LaAlO3 and (100) MgO Single-Crystal Substrates. J. Electron. Mater. 2013, 42 (6), 988-992. (55) Huhtinen, H.; Palonen, H.; Paturi, P., The Growth Rate and Temperature Induced Microcracks in YBCO Films Pulsed Laser Deposited on MgO Substrates. IEEE T. Appl. Supercon. 2013, 23 (3), 7200104-7200104. (56) Qin, W. F.; Xiong, J.; Zhu, J.; Tang, J. L.; Jie, W. J.; Wei, X. H.; Zhang, Y.; Li, Y. R., High Tunabilty Ba0.6Sr0.4TiO3 Thin Films Fabricated on Pt–Si Substrates with La0.5Sr0.5CoO3 Buffer Layer. J. Mater. Sci.: Mater. Electron. 2007, 19 (5), 429-433. (57) Czekaj, D.; Lisińska-Czekaj, A.; Orkisz, T.; Orkisz, J.; Smalarz, G., Impedance Spectroscopic Studies of Sol–Gel Derived Barium Strontium Titanate Thin Films. J. Eur. Ceram. Soc. 2010, 30 (2), 465-470. (58) Garcia, D.; Guo, R.; Bhalla, A. S., Field Dependence of Dielectric Properties of BST Single Crystals. Ferroelectrics Lett. 2000, 27 (5-6), 137-146. (59) Jorcin, J.-B.; Orazem, M. E.; Pébère, N.; Tribollet, B. CPE analysis by local 29



Page 30 of 31

electrochemical impedance spectroscopy. Electrochim. Acta 2006, 51 (8-9), 1473-1479. (60) Zafar, S.; Jones, R. E.; Jiang, B.; White, B.; Kaushik, V.; Gillespie, S., The Electronic Conduction Mechanism in Barium Strontium Titanate Thin Films. Appl. Phys. Lett. 1998, 73 (24), 3533-3535. (61) Chang,

S.-T.;

Lee,

J.

Y.-m.,

Electrical

Conduction

Mechanism

in

High-Dielectric-Constant (Ba0.5,Sr0.5)TiO3 Thin Films. Appl. Phys. Lett. 2002, 80 (4), 655-657. (62) Peng, C. J.; Krupanidhi, S. B., Structures and Electrical Properties of Barium Strontium Titanate Thin Films Grown by Multi-Ion-Beam Reactive Sputtering Technique. J. Mater. Res. 2011, 10 (03), 708-726. (63) Pantel, D.; Alexe, M. Electroresistance effects in ferroelectric tunnel barriers. Phys. Rev. B 2010, 82 (13),134105.

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All-Inorganic Flexible Ba0.67Sr0.33TiO3 Thin Films with Excellent

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