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Transparent, Flexible, Fatigue-Free, Optical-Read, and Nonvolatile Ferroelectric Memories

Huan Gao,

^†,∇

Yuxi Yang,

^†,∇

Yaojin Wang,

^†

Lang Chen,

^‡

Junling Wang,

^§,∥

Guoliang Yuan,*

^,†

and Jun-Ming Liu*

^,⊥,#

†School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

‡Department of Physics and^§Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology, Shenzhen 518055, China

∥School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore

⊥Laboratory of Solid State Microstructure, Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

#Institute for Advanced Materials, South China Normal University, Guangzhou 510006, China

*^S Supporting Information

ABSTRACT: Perovskite oxide films are widely used in various commercial industries. However, they are usually prepared at high temperature and in oxygen ambience, detrimental to most transparent and flexible substrates and bottom conductive electrodes such as indium tin oxide (ITO). It remains challenging to integrate perovskite oxides into transparent and flexible electronics. Here, the 1.2 wt % Ag-doped ITO (Ag-ITO) grown on a mica substrate is employed as the bottom electrode, which can withstand high temperature and repeated bending, and then we achieve the transparent,flexible, fatigue-free, and optical-read ferroelectric nonvolatile memories based on the mica/Ag-ITO/Bi_3.25La_0.75Ti₃O₁₂/ITO structures. The as-prepared memories show ∼80% transmittance for visible lights and fatigue-free performance after

more than 10⁸writing/erasing cycles. These performances are stable after repeated bending down to 3 mm in a curvature radius.

More importantly, the “1/0” state of the memory can be read out by the photovoltaic current rather than destructive polarization switching, an emergent functionality for many applications. This work substantially promotes the applications of perovskite oxidefilms in transparent andflexible electronics, including wearable devices.

KEYWORDS: inorganic memories, transparent,flexible, fatigue-free, optical-read

1. INTRODUCTION

Transparent and flexible electronics are applied to emergent applications ranging from smart windows and head-up displays to wearable sensor arrays.¹⁻³In such devices, each component, including substrates, functionalfilms, and conductive electro- des, should be transparent, flexible, lightweight, and stable.

One big class of functionalfilms is ferroelectrics due to their multiple functionalities. To prepare the transparent andflexible sensors and memories,^3,4 polyimide, polyethylene terephtha- late, and other polymers are often chosen asflexible substrates, and ferroelectric polymers such as poly(vanylidenefluoride-co- trifluoroethylene) (i.e., P(VDF-TrFE)) films are commonly used as functional layers. Meanwhile, the most concerned transparent conductive electrodes include Al₂O₃-doped ZnO (AZO),^5,6 conductive polymers,^2,7−9 carbon nanotubes,^10,11 graphene,¹¹⁻¹³metal grids,¹⁴nanowire meshes,^8,14−16ultrathin metalfilms,^8,17in addition to indium tin oxide (90 wt % SnO₂· 10 wt % In₂O₃, ITO), which is most widely used in industrial production.^18,19 Indeed, transparent and flexible transistors, memories, and sensors have been prepared on organic substrates at or near room temperature. However, most

organic sensors and memories show poor comprehensive performances compared with the commercial inorganic memories nowadays.

It is well known that some perovskite oxide films (POFs) exhibit much better functionalities than polymers, and in fact, many POFs have been widely used in capacitors, gyroscopes, sensors, actuators, microgenerators, and memories. However, the integration of them intoflexible and transparent electronics has been a long-standing dream because of severe challenges.

The major problems include the brittleness and harsh processing conditions of POFs. We take the Bi_3.25La_0.75Ti₃O₁₂ (BLT) film as an example. BLT has a large saturation polarization (P_s> 20μC/cm²), a short switching time (T_switch

∼nanoseconds), and a good fatigue resistance, all of which are much better than those of the P(VDF-TrFE)film (P_s∼9μC/

cm², T_switch ∼ microseconds, and prone to fatigue).^4,20,21 However, the deposition of high-quality BLT and other POF

Received: August 7, 2019 Accepted: September 4, 2019 Published: September 4, 2019

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films often requires a deposition temperature (T_d) of over 500

°C and high-oxygen ambience (∼10 Pa) where organic materials usually fully decompose.

Recently, flexible generators, sensors, and memories have been prepared indeed using POFs as functionalfilms, allowing us to design POF-based transparent and flexible elec- tronics.²²⁻²⁶ Based on this, it is particularly attractive to use fluorophlogopite mica (i.e., KMg₃(AlSi₃O₁₀)F₂) as a flexible substrate. Mica has a layer-stacking structure similar to other two-dimensional materials. The thinflexible mica layers can be separated from bulk mica single crystals through mechanical exfoliation.^23,27−30 More importantly, mica is transparent, thermostable, and oxidation resistant. Also, it can survive without damage in the harsh environment of∼700°C and at a

>1 Pa oxygen pressure. In fact, a number of ferroelectricfilms such as Pb(Zr,Ti)O₃, BaTiO₃, and BiFeO₃ films with conductive electrodes like SrRuO₃, LaSr_1/3Mn_2/3O₃, Au, and Pt have been deposited on single-crystal mica substrates of <10 μm in thickness to achieve POF-based flexible elec- tronics.^23,27,28

The bottom-electrode/POF/top-electrode sandwich struc- ture is widely used in many commercial memories and sensors, and their high transparency and good flexibility are highly expected. There are many transparent,flexible, and conductive films such as ITOs, polymers, graphenes, nanowires, and ultrathin metals, which can be grown on the POF surface near room temperature for top electrodes.⁷⁻¹⁷ However, these conductive films as bottom electrodes will be damaged seriously and/or become highly resistant when the POF film is grown on bottom electrodes at 500−700 °C and high oxygen ambience later.²⁰⁻²³Although the ITOfilm grown near 300 °C has high conductive and high transparency simultaneously, it becomes very brittle too due to the abnormal grain growth and weakened cohesion among grains during the POF growth.^18,19It is thus crucial to develop bothflexible and transparent conductive bottom electrodes and substrates that can withstand high temperature and oxygen ambience during the POF growth. Here, major problem on bottom electrodes remains unsolved. First, there are a number of issues when SrRuO₃, LaSr_1/3Mn_2/3O₃, Au, and Pt are used as transparent bottom electrodes. ITO-based electrodes should be considered a priority over others. Second, to overcome the brittleness of ITOfilms after the high-T_dannealing, a proper modification of ITO by chemical doping would be highly desirable and appreciated.

Here, we synthesize the 1.2 wt % Ag-doped ITO (Ag-ITO) film, which is stable enough even after anneal during POF growth at ∼700 °C and a >1 Pa oxygen pressure. It exhibits much better performance than pure ITO as bottom electrodes in terms of transparency,flexibility, thermostability, and fatigue resistance. Furthermore, we demonstrate the excellent performance of ferroelectric memories based on the stacked heterostructures: mica (substrate)/Ag-ITO (bottom elec- trode)/BLT (ferroelectric layer)/ITO (top electrode). They are not only transparent andflexible but also a fatigue-free and nonvolatile-state material. More importantly, the high trans- parency allows an additional functionality, that is, the nondestructive optical read of the data storage.

2. EXPERIMENTAL SECTION

Each 50μm thick mica substrate was separated from the (001) single- crystalfluorophlogopite mica (Changchun Taiyuan Co., China) by mechanical exfoliation. Then, the pure ITO (i.e., 700°C ITO) and

Ag-ITO conductivefilms as bottom electrodes were prepared with a pulsed laser deposition system (PLD, 248 nm wavelength) at a 1 Pa oxygen pressure and 700°C condition. The 700°C-ITO film was grown by sputtering the ITO target for 8000 pulses with a laser beam.

Also, the 1-Ag-ITO was grown by sputtering the Ag target for 600 pulses and then the ITO for 8000 pulses. The 2-, 3-, 4-, 5-, and 6-Ag- ITOfilms were grown by sputtering the Ag target for 375, 1000, 2000, 4000, and 8000 pulses, respectively, and then sputtering the ITO target for 2000 pulses alternatively for 4 times. After that, the BLT film was grown on the mica/2-Ag-ITO by sputtering the Bi_3.25La_0.75Ti₃O₁₂ceramic target at 700 °C and at the 3 Pa oxygen pressure with a PLD system,^20,21and then they were cooled to room temperature at a rate of 5°C/min and at a 1000 Pa oxygen pressure.

For comparison, the AZOfilm as a bottom electrode and the 100−

300 nm BLTfilm as a functional layer were also grown on the mica substrate at 700−730°C and 1−3 Pa oxygen pressure with a 110 mJ per laser pulse. Furthermore, the ITO (i.e., 300°C ITO) conductive film as top electrodes with a 50−200μm diameter was grown on the BLTfilm at 300°C and 1 Pa oxygen pressure.^18,19Finally, each mica substrate was reduced to a ∼10 μm thickness by mechanical exfoliation before the further electric and mechanical measurements.

Crystal structures of these samples were measured with X-ray diffraction (XRD, Bruker D8). The cross section of mica/Ag-ITO/

BLT was observed by TEM (FEI Tecnai G2 F20 S-Twin). The distributions and the concentration of Ag/In/Sn elements of the Ag- ITOfilm were characterized by the HAADF and EDX of TEM. Then, the binding energies of each element were characterized by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). The surface morphologies, conductive atomic force microscopy (C-AFM) images, and piezoelectric force microscopy (PFM) images of the BLTfilm were observed by an atomic force microscope (AFM, Multimode 8, Bruker Co.) at 10 kHz. After that, transmittance was measured by a UV−vis spectrometer (Shimadzu Co.). Furthermore, the relative permittivity versus electricfield (εr−E) loops, the polarization versus electric field (P−E) loops, the ferroelectric fatigue curves, and the polarization retention versus time curves were measured by a ferroelectric tester (Radiant Technologies, USA). Finally, photo- voltaic current of memories was measured by a multimeter (Keithley 2635A) when its 300 °C-ITO top electrodes were illuminated by ultraviolet lights.

3. RESULTS AND DISCUSSION

We first check the crystal structure and the microstructure properties of the Ag-doped ITO (Ag-ITO) conductivefilms. It is observed that the films remain pure cubic phase at low doping level, and then Ag particles precipitate at grain boundaries as the doping level is high. The X-ray diffraction (XRD) patterns (Figure S1) indicate that the pure 700 °C- ITOfilm and the 1-, 2-, 3-, and 4-Ag-ITOfilms all prefer the

<222> orientation.¹⁹ The 700 °C-ITO and 2-Ag-ITO films have smooth surface morphologies with roughnessR_avalues of 3 and 2.7 nm as shown in Figure 1a,b. Furthermore, the homogenous conductivity suggests that there are no Ag particles in the 2-Ag-ITOfilm (Figure 1d). On the contrary, a number of small Ag particles appearing at the grain boundaries are observable in the 3-Ag-ITO film (Figure 1c,e) and the other films with more Ag doping (Figure S2). The conductance of the Ag particle is over 100 times better than that of the Ag-ITO grain (Figure 1f). This suggests that the Ag doping level inside the grains is oversaturated, and the excessive Ag is segregated at the Ag-ITO grain boundaries.

As expected, more and more Ag particles or slices appear with increasing Ag doping level, as identified in the morphology and conductance images for the 4-, 5-, and 6-Ag-ITOfilms (Figure S2).

Certainly, it would be necessary to determine the real Ag doping level in thesefilms. The characteristic binding energies ACS Applied Materials & Interfaces

of Ag⁺ ions in the 2-Ag-ITO film, as probed by the X-ray photoelectric spectroscopy (XPS), are∼367.85 and 374.0 eV, only slightly lower than 368.48 and 374.5 eV of the pure Ag film (Figure S3).³¹Indeed, no Ag-rich phase is detectable on the surface and cross section of the 2-Ag-ITO film (Figure 1b,g). Homogeneous distribution of Ag⁺can be confirmed by the high-angle annular dark-field image (HAADF) of trans- mission electron microscopy (TEM) shown in Figure 1h.

More importantly, the atomic rates of In, Sn, Ag, and O in the 2-Ag-ITO film, characterized by the energy-dispersive spec- troscopy (EDX), are 34.4, 3.87, 0.60, and 61.13 at % (Figure 1i), which allow us to estimate the Ag⁺ doping level of 1.2 wt

%. The 2-Ag-ITO film is stable at ∼700 °C, although the single-phase Ag may volatilize gradually at ∼700 °C. These results are different from the previous reports on (Ag/ITO)n multilayers grown near room temperature in which Ag is difficult to diffuse and react with the ITOfilm.³²

The 2-Ag-ITO film shows a high transmittance of visible lights. As shown inFigure 2a, the minimum transmittance for 400−800 nm incident lights is 82% for the 700 °C-ITOfilm, 83% for the 1-Ag-ITO film, 83.1% for the 2-Ag-ITO film, 83.5% for the 3-Ag-ITOfilm, and 81.5% for the 4-Ag-ITOfilm.

The Ag doping does not decrease the transmittance of the 1-, 2-, and 3-Ag-ITOfilms in visible light; however, the stronger absorbance of Ag particles at grain boundaries slightly decreases the transmittance of the 4-Ag-ITOfilm.

It is revealed that the Ag doping has a remarkable impact on the mechanical and electric properties of the ITO films. We characterize the electric properties and flexibility of the pure 700°C-ITO, 1-, 2-, 3-, and 4-Ag-ITOfilms deposited on the mica substrates. In theflat state, they have the sheet resistance (R_sh) from 15 to 30Ω/Sq. Repeated bending of thefilms to 3 mm in the curvature radius and back will break the 700°C- ITOfilm and make theR_shto soar over 10⁹Ω/sq (Figure 2b).

This rupture is mainly due to a high density of cracks

introduced by the high-temperature anneal (Figure S4) and the delamination of the 700 °C-ITO film from the mica substrate during bending.^18,19,32 The rapid increase in the tendency ofR_shis weakened in the 1-Ag-ITOfilm, and its R_sh increases gradually from 20 to 1632Ω/sq with bending cycling up to 1520 cycles. In particular, for the 2-Ag-ITO film, R_sh slightly changes after the bending of up to 10,000 cycles. As expected, the surface of the originalflat 2-Ag-ITOfilm is the same as that of after bending to a 3 mm radius (Figure S5a,b).

For the cases with higher Ag-doping levels, for example, the 3- Ag-ITOfilm, many nanometer-scale Ag particles embedded at the grain boundaries as a result of the excessive Ag doping can be identified. However, the measuredR_shis nearly overlapped with that of the 2-Ag-ITOfilm. Since the Ag particles are so small in size, they act as bonding glues between the grains, as reported in previous studies.³²Unfortunately, the excessive Ag particles become large and the intergrain adhesion is reduced for the 4-Ag-ITOfilm. This is the reason why theR_shincreases from∼20 to 58 Ω/sq after the 500 bending cycles.

The 1.2 wt % Ag represents a proper doping level so that cracks were compressed, and thus, the adhesion between the neighboring grains and between thefilm and underlying mica substrate can be maximally enhanced, improving theflexibility of the 2-Ag-ITOfilm.³²Although ITOfilm has a very lowR_sh and high transmittance, it can break at very low tensile strains.^17,19According to the TEM images (Figure S4), there are many cracks at the mica/ITO interface, which is the essential factor for the ITO film separating from the mica substrate during the repeated bending, while none cracks are observed at the mica/3-Ag-ITO interface. That is to say, the 1.2 wt % Ag doping suppresses the cracks, enhances the adhesion of crystal grains, andfinally increases theflexibility of the 3-Ag-ITOfilm.

It is noted that theR_shvalues of 700°C-ITO, 1-, 2-, 3-, and 4-Ag-ITOfilms show only weak dependence of the annealing or characteristic temperature, especially below 300°C (Figure 2c,d). Since each thermal treatment that lasts for 30 min is just Figure 1.Microstructures of 700°C-ITO and Ag-ITOfilms. Surface

morphologies of (a) 700°C-ITO, (b) 2-Ag-ITO, and (c) 3-Ag-ITO films. Conductive images of (d) 2-Ag-ITO and (e) 3-Ag-ITOfilms.

(f) Current along the red line across an Ag particle of 3-Ag-ITOfilm.

(g) TEM image of 2-Ag-ITOfilm. (h) HAADF mapping of In and Ag elements of 2-Ag-ITOfilm. (i) EDX elementary line scan profiles of 2- Ag-ITOfilm.

Figure 2.Transparent andflexible properties of 700°C-ITO and Ag- ITOfilms. (a) Transmittance, (b) dependence ofR_shon the bending cycles, (c) dependence ofR_shon the annealing temperature, and (d) dependence ofR_sh/R_sh0on the characteristic temperature where the inset of (a) shows the picture of mica/2-Ag-ITO andR_sh0is defined as R_shat 25°C.

ACS Applied Materials & Interfaces

carried out at a temperature (≤640 °C) far below thefilms’ growth temperature of 700°C, it is not enough to obviously change the films’ microstructure (Figure S5c,d), mechanical, and electric properties. Although the commercial ITO film grown near room temperature on a polyethylene terephthalate substrate (i.e., PET/ITO) can show a high transmittance of over 80% and a lowR_shof∼25Ω/sq at room temperature, it cannot be used above 200°C due to the poor thermal stability of the PET substrate and PET/ITO interface.

To this stage, we can compare the experimental data on thermostability, antioxidation, transparency,flexibility, andR_sh values of some well-known conductive films, reported in literature, as listed inTable S1.5−8,10−19,23,33For example, the 2 wt % Al₂O₃-doped ZnO (AZO) film deposited at reduced ambience exhibited the low R_sh, high transparency, and flexibility.³³ Unfortunately, it will become high resistant (>5000 Ω/sq) during the subsequent oxide deposition at high temperature and high oxygen pressure where most oxygen vacancies in the AZO film are annihilated, and then the corresponding free electrons in conductance band are reduced.^6,33 The data comparison in Table S1 suggests that the 2-Ag-ITOfilm exhibits the best performance as the bottom electrode for the transparent and flexible perovskite oxide electronics.

Given the good endurance of the 2-Ag-ITOfilm against high temperature and oxygen pressure during the POF growth, we prepare theflexible and transparent inorganic memories using the mica/2-Ag-ITO (143 nm)/BLT (380 nm)/300 °C-ITO (143 nm) heterostructures (Figure S4c). The ITOfilm as top electrodes is prepared at 300°C after the BLT film growth, and thus, it is conductive, flexible, and transparent.^18,19 Concerning the mechanical flexibility, it is understandable that the thinner the mica is, the more flexible the memory is.^29,30 The theoretical maximum strain (σmax) for a 10 μm thick sample is 0.5% at a 1.4 mm bending radius, which is too small to destroy the BLT-based memories (Figure S6). The as- grown flat BLT film has the [117] preferred orientation (Figure S1) and rod-like grains along one of the hexagonal crystallographic directions (Figure S7).^20,21The composition and crystal structure of the BLT film grown by the PLD method should be consistent with its ceramic target according to previous studies.^20,21,27 The morphology of the BLT film remains similar for the flat and bent states (Figure S7), and thus, the memories are flexible enough for most wearable devices.

For ferroelectric random access memories (FeRAMs), the upward/downward polarization (i.e., +P/−P) state stores the

“1/0” information, and the writing/erasing cycle can be realized via the polarization switching.20,21,23,25

The larger the remnant polarization (P_r) and the faster the polarization switching, the better the memory will be. The as-prepared mica/2-Ag-ITO/BLT/300 °C-ITO memories show the saturated polarization versus electricfield (P−E) loops at the flat and bent states as shown inFigure 3a. At theflat and bent states with 2.2 mm in a curvature radius, the memories show a largeP_Sof∼20μC/cm², aP_rof∼10μC/cm², and a coercive field E_C of ∼70 kV/cm at room temperature.^20,21 BothP−E loops and dielectric permittivity versus electric field curves (Figure S8) do not change obviously after the memories were bent to a 3 mm curvature radius for 10,000 cycles.

Furthermore, these saturated loops are also observed over the temperature window of 20 to 150 °C, at the ac signal frequency of as high as 100 kHz or under light illumination

(Figure S9). As the temperature increases to up to 150°C, the measuredP_Shas a slight decrease from 20 to 19μC/cm², and the E_C decreases from ∼70 to ∼60 kV/cm, noting that the Curie temperature is 350°C. Besides, the frequency depend- ence of polarization is also weak, and the 266−650nm laser illumination does not obviously change the ferroelectricP−E loops (Figure S9).

Theflexible and transparent mica/2-Ag-ITO/BLT/300°C- ITO memories are fatigue free during the 10⁸ consecutive writing/erasing cycles. The saturatedP−Eloop and nonvolatile polarization (P_NV ∼ 2P_r) show a negligible change after the

∼10⁸constant writing/erasing cycles at 1 MHz, compared with the origin ones (Figure 3b,c). The intrinsic fatigue-free characteristic of the BLTfilm is mainly because charge defects are difficult to diffuse among different crystal cells that composed of (Bi₂O₂)²⁺ and (B_1.25La_0.75Ti₃O₁₀)²⁻ layers.^20,21 Besides, the polarization states of our mica/2-Ag-ITO/BLT/

300°C-ITO memory do not change after 10⁶s (i.e., 11.6 days) (Figure 3c).^26,27As it is known, the fast writing/erasing speed and the maximum writing/erasing cycles are the two most essential factors for all type of memories.^20,21,25 Figure 3d shows the writing/erasing speed and cycles of some organic memories, Si-based flash memories, inorganic resistance random-access memories (ReRAM), and inorganic FeRAM.20,23,25,26,28,34−46 The flexible memories on the basis of the organic PVDF, CH₃NH₃PbI₃, and P3HT:PEO films together with the MoS₂ film commonly endure for less than 10⁵ writing/erasing cycles and have a slow writing/erasing speed of 10⁻¹−10⁻³s compared with that of the commercial Figure 3.Storage performances of the mica/2-Ag-ITO/BLT/300°C- ITO memory. (a)P−Eloops of the memory with theflat state, the 2.2 mm bending state, and that after the 3 mm radius bending for 10,000 cycles. (b) OriginalP−Eloop and that after∼10⁸polarization switching cycles. (c) Dependence of nonvolatile polarization (P_NV∼ 2P_r) on writing/erasing cycles. (d) The best writing/erasing speed and cycles of rigid memories, transparent (Trans) memories, and transparent andflexible (Trans and Flex) memories, where BLT (star solid, triangle up open), Fe3O4(circle dotted), PZT (box), BTCO (triangle down open), BFO (diamond open), P3HT:PEO (triangle left-pointing open), CH3NH3PbI3(square dotted), HfO2(plus sign), MoS2 (times sign), PVDF (pentagon open)-based memories are shown.20,23,25,26,28,34−46

ACS Applied Materials & Interfaces

flash memories.^3,47−51Our mica/2-Ag-ITO/BLT/300°C-ITO FeRAM not only endures >10⁸ writing/erasing cycles with a fast speed of∼10⁻⁸s but also isflexible and transparent.

For mica/2-Ag-ITO/BLT/300 °C-ITO memories, the +P and −P states (i.e., 1 and 0 of memory cell) can be nondestructively optical read by photovoltaic current. In conventional FeRAM devices, the reading is destructive by applying a bias to the memory and detecting the polarization switching current, followed by a rewrite step to recover the initial information. Therefore, nondestructive read is highly desirable and is a novel functionality of the present FeRAM devices.^52,53The original memories show a zero short-circuit photovoltaic current (I_SC) under an ultraviolet laser beam illumination due to the absence of macroscopic polarization (Figure 4a). When a−Pstate is induced by an 8 V external

bias, theI_SCincreases to −0.43 nA. After a−8 V bias induces the +Pstate, theI_SCchanges to 0.35 nA. The−Pand +Pstates can be identified by detecting the corresponding I_SC under illumination (Figure 4b,c). The dependence of I_SC on polarization is explained according to the schematic energy- band diagrams as shown in the inset of Figure 4d.⁵² The Schottky junctions at 2-Ag-ITO/BLT and BLT/300 °C-ITO interfaces generated two different internal electricfields (E_bi).

The polarization switching changes the directions of the depolarizationfield (E_dp), two Schottky barriers and theirE_bi. Ultraviolet light can excite electron−hole pairs in the BLTfilm with a 3.5 eV band gap, and E_bi and E_dp can separate the electron−hole pair and then induce photovoltaic current being dependent on polarization.⁵² The nondestructive read by photovoltaic current is reliable enough to be used in the full- inorganic ferroelectric memories. Figure 4d shows the dependence of I_SC on the writing/erasing cycles of up to 1940 for memories where theI_SCvalues are stable enough to identify both−Pand +Pstates. These studies suggest that the

−Pand +Pstates can be nondestructively read usingI_SCunder ultraviolet light illumination in our flexible and transparent memories.

Nowadays, a transparent and flexible memory is the most important to extend the functions of intelligent screens, smart windows, head-up displays, and other flexible electronics.⁵¹ Similar to the transparency of Ag-ITO bottom electrodes and 300°C-ITO top electrodes, both the mica substrate and BLT film are transparent for visible lights due to their large band gaps (E_g) of 6.2 and 3.5 eV. Accordingly, the mica/2-Ag-ITO/

BLT/300°C-ITO memories show the transmittance of∼80%

for visible lights (Figure 5a,b). As shown inFigure 5c,d, these

memories can attach on a contact lens through the van der Walls force at their interface, and then they can be coated on an artificial eye. As a result, the updating contact lenses not only help us to watch images clearly but also record them in a simple way.

A commercial memory should satisfy a lot of conditions before it can be widely used in industries, such as the fast writing/erasing speed, millions of writing/easing cycles, the high memory density, the low power dissipation, the good uniformity, and stability of each memory cell, the compatibility with semiconductor technology, a high cost performance, and etc. Flash memory already owns the vast majority of the market of information storage since its comprehensive performances have been best until now. Although these have been extensive studies on many transparent andflexible memories on the basis of organic or two-dimensional materials, their electric perform- ances such as writing/erasing speeds and cycles are still poor compared with theflash memory and the POF-based FeRAM (Figure 3d).⁴⁷⁻⁵¹ FeRAM has excellent comprehensive performances and already holds an important market share of commercial information storage,^25,26 although the growth processes of POFs are not well compatible with the current semiconductor technology. For our transparent and flexible BLT memory, the writing/erasing speed is smaller than 50 ns, maximum writing/erasing cycles are over 10⁸times, and each memory cell is stable even at 150 °C or under light illumination (Figure S9), according to the previous studies and our experiments (Table 1).3,26,47−51,54,55Therefore, these Figure 4.Information nondestructively readout through photovoltaic

current. (a) Current−voltage curves of the mica/2-Ag-ITO/BLT/300

°C-ITO memories in the darkness or under the illumination of ultraviolet lights. (b)I_SCof the BLTfilm with either +Por−Pin the darkness and under illumination, respectively. (c) real-time I_SC evolution after +P/−P is written by−8/+8 V. (d) Dependence of I_SCon writing/erasing cycles where the inset shows the energy-band diagrams.

Figure 5.Application of the transparent andflexible memories. (a) Sketch of mica/2-Ag-ITO/BLT/300 °C-ITO memories, (b) trans- mittance of mica and memories, optical maps of (c) the contact lens with memory, and (d) them on an artificial eye.

ACS Applied Materials & Interfaces

memories are promising to be used as an important part of the transparent andflexible electronics.

4. CONCLUSIONS

In summary, we have prepared the 1.2 wt % Ag-doped ITO films at ∼700 °C and high oxygen pressure, which are necessary during most POF growths by the pulsed laser deposition. Also, the as-grownfilm is not only conductive but also transparent andflexible, which is much better than that of the brittle pure 700°C-ITOfilm. It is believed that the 1.2 wt

% Ag⁺doping enhances the adhesion of Ag-ITO crystal grains andfinally increases theflexibility of the Ag-ITO film. As an example of Ag-ITO applications, we prepared the 2-Ag-ITO/

BLT/300°C-ITO ferroelectric memories on the flexible mica substrate. These memories not only keep the good comprehensive performances (e.g., fast writing/erasing speed, fatigue-free, and etc) mentioned above but also add three most important functions, that is, high transparency, goodflexibility, and nondestructive optical read through photovoltaic current.

This study develops a new way to prepare the transparent and flexible electronics with traditional perovskite oxidefilms.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acsami.9b14095.

Transparent and flexible conductive films; XRD, XPS, and strains of ITO and BLT films; AFM and C-AFM images of 700°C-ITO and Ag-ITOfilms; TEM images of 700 °C-ITO, Ag-ITO-2, and BLT films; AFM and PFM images of the BLTfilm;εr−Ecurves; andelectric stability of mica/2-Ag-ITO/BLT/300 °C-ITO memo- ries (PDF)

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AUTHOR INFORMATION Corresponding Authors

*E-mail:yuanguoliang@njust.edu.cn. (G.Y.).

*E-mail:liujm@nju.edu.cn. (J.-M.L.).

ORCID

Lang Chen: 0000-0002-8762-892X

Guoliang Yuan:0000-0002-0147-7893

Jun-Ming Liu:0000-0001-8988-8429

Author Contributions

∇H.G. and Y.Y. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to thefinal version of the manuscript.

Notes

The authors declare no competingfinancial interest.

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ACKNOWLEDGMENTS

The work is supported by the National Natural Science Foundation of China (51790492, 51431006, and 61874055) and the National Key Research Program of China (2016YFA0300101).

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material P3HT:Au Si MoS₂ MoS₂ PVDF:P3HT PVDF PZT PZT BLT

substrate PEN polymide PET PET polymide PET polymide silicon mica

electrode Au, Au Au/Cr, Cu Al, Au Al, Al Au, Al graph, Au/Cr graph, Pt Pt, Pt Ag/ITO, ITO

etch and transfer no yes yes yes no yes yes yes no

bending radius 5.0 mm 3.5 mm 15 mm 3.0 mm 5.8 mm 1.0 mm 9.0 mm 5.0 mm 3 mm

bending cycles 1000 1000 500 100 1000 1000 10,000

W/R speed >1 ms <50 ns <50 ns <50 ns

W/R cycles 100 120 10³ >10⁹ >10⁷

optical read no no no no no no no no yes

work temp ∼20°C ∼20°C ∼20°C ∼20°C ∼20°C ∼20°C ∼20°C ≤200°C ≤150°C

transmittance >90% ∼80%

aW/R: write/erase, PEN: polyethylene naphthalate, P3HT: poly(3-hexylthiophene), PET: poly(ethylene terephthalate), and graph: graphene.

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