Multifunctional polyimides for resistive switching memory devices based on flexible transparent polyimide–silver nanowires hybrid electrodes

With the quick development of flexible memory electronics, multifunctional organic materials have been the necessary for fabricating electronics. In this work, the highly transparent and flexible electrode was successfully prepared by coating the high‐performance silver nanowires (AgNWs) onto the colorless polyimide (PI) substrate. The prepared flexible PI–AgNWs electrodes exhibited a relatively low sheet resistance of 15 Ω/sq with the high transparency of 68% at the wavelength of 400 nm. A novel kind of polyimide TPC6FPI was successfully synthesized and characterized with the excellent thermostability and high glass transition temperature (Tg) above 250°C. Furthermore, a kind of flexible transparent PI–AgNWs/TPC6FPI/Al resistive memory device was prepared and exhibited good SRAM switching behavior with the threshold voltage of around 2.1 V and the ON/OFF current ratio of ˜104, which indicated that multifunctional PI‐based memory device showed the potential to the wearable devices.


INTRODUCTION
Nowadays, flexible electronics are widely used in our daily life because of their better convenience and providing us more bright horizons. 1,2 With the rapid growth of digital information over the past decades, the commercial optical and magnetic storage technologies have nearly reached their upper theoretical limit, which cannot meet the abundant demands of memory devices with high density, fast response, and flexible portable data storage. [3][4][5] Compared to the traditional memory devices, resistive random-access memory (RRAM) devices with a top electrode/organic active layer/bottom electrode sandwiched structure, 6 play an important role in the next-generation digital memory devices because of their high performance including high reliability, 7 fast operation, 8 excellent scalability 9 and durable flexibility, 10 and draw much attention as the most promising materials for the resistor, capacitor and transistor and so forth. [11][12][13] However, the performances of flexible RRAM devices are still confronted with many challenges such as the serious degradation of reliability during continued deformation.
To meet the increasing requirement of flexible RRAM devices, polymers as the active layer are more favorable than other organic materials due to their low cost, intrinsic flexibility and large area fabricating capability. [14][15][16] Polymer memory materials can store electronic data through the conversion between the low and high current states between two electrodes by the applied voltage, which can be assigned as "0" and "1" or "OFF" state and "ON" state. [17][18][19][20][21] In recent years, functional polyimides (PIs) were regarded as the most promising polymeric electronic memory materials due to their excellent thermal resistance, 22 chemical stability, 23 mechanical durability, 24 and flexibility. 25 More importantly, PIs with the special donor-acceptor (D-A) type molecular structures exhibiting stronger intramolecular and intermolecular charge transfer (CT) effects are more promising for the flexible memory devices. [26][27][28][29] Up to now, a large number of PIs exhibit various kinds of memory properties, including the volatile memory properties (dynamic random access memory 30 (DRAM), static random access memory 31 (SRAM)) and the non-volatile memory properties (FLASH, 32 write once read many 33 (WORM)). However, most reported PI-based memory devices are rigid and non-wearable, which limit their potential applications in flexible electronics. 34 In order to reach the demand of wearable intelligent electronic memory devices, advances in electrodes are becoming more and more essential to enhance their properties of high conductivity, flexibility, and transmittance. The polymer-based electrodes have attracted great attention by investigating indium tin oxide (ITO), 35 carbon nanotubes (CNTs), 36 graphene, 37 and metallic nanowires 38 and so forth. Especially, silver nanowires (AgNWs) have been considered as the most potential candidates in the future. [39][40][41] However, the poorer adhesion performance of AgNWs and the lower glass transition temperature (T g ) of flexible substrates are further restricted to the development of the polymer-AgNWs electrodes.
Hence, in this work, we designed and synthesized a kind of colorless PI with a high glass transition temperature and transmittance properties, which was chosen to prepare flexible PI-AgNWs hybrid electrodes. Then, a novel kind of polyimide memory material (TPC6FPI) was prepared by the polycondensation of the diamine 4,4 ′ -((4-(diphenylamino)phenyl)methylene) dianiline (TPCDA) and the dihydride 2,2-bis(3,4-anhydrodicarboxyphenyl)hexafluoropropane (6FDA), which was used for the flexible resistive memory device with a sandwich-like structure. The memory device was fabricated by casting TPC6FPI as organic active layer between the bottom PI-AgNWs hybrid electrode and the top Al electrode. Furthermore, the properties of TPC6FPI-based flexible memory devices exhibiting SRAM type memory behavior were studied and evaluated systematically. This work aimed to provide an effective and reliable method for the performance optimization of multifunctional flexible PI based memory devices.

Synthesis of colorless PI
As shown in Figure 1A, the colorless polyimide TFBPI used in this work was synthesized according to a traditional two-step by thermal imidization as follows: 1.0410 g (2 mmol) of 4,4 ′ -[(isopropylidene)bis(p-phenyleneoxy)]diphthalic dianhydride (BPADA) and 0.6404 g (2 mmol) of 2,2'-Bis-trifluoromethyl-biphenyl-4,4 ′ -diamine (TFMB) were added into a solution of N,N-dimethylformamide (DMF) (20 wt% solid content) under a nitrogen (N 2 ) flow to react over 12 h to form polyamide acid (PAA) at room temperature. The resulting PAA solution was coating on a glass substrate and was heated to form TFBPI thin film by raising the temperature slowly to 300 • C to achieve the thermal imidization. After cooling to

Preparation of AgNWs
AgNWs were synthesized by the modified polyol process: 2 g of polyvinylpyrrolidone (PVP) was added into ethylene glycol and glycerol mixed solution (50 ml) and stirred at 130 • C to dissolve under a N 2 flow. After cooling to the room temperature, 0.1 g of silver nitrate (AgNO 3 ) and sodium bromide (NaBr) 0.5 ml were added into the reactor and stirred for 0.5 h. Finally, the mixed solution was moved into vacuum oven for an oxidation reaction over 6 h at 150 • C. The resulting AgNWs were obtained and cleared by ethanol with 5 times to remove the residual PVP impurities and Ag nanoparticles.

Preparation of PI-AgNWs hybrid electrodes
First, the AgNWs were transferred from ethanol to DMF by using a solvent exchange method. And the weight fraction of the AgNWs in DMF solution with 1.0 mg/ml was modulated through thermogravimetry analysis. Then, the AgNWs solutions were coated on the TFBPI substrates, and performed thermal annealing at 200 • C for 0.5 h under inert condition to reduce the electrical resistance of the PI-AgNWs hybrid electrodes.

Synthesis of TPC6FPI memory materials
The diamine TPCDA was synthesized according to our reported work 16 with high efficiency, and the detail synthesis process (Scheme S1), structure characterization ( Figure S1) and measurement method were detailed described in Supporting Information. TPC6FPI was prepared by the chemical imidization as follows: 0.2221 g (0.5 mmol) of 6FDA and 0.2206 g (0.5 mmol) of TPCDA were added into DMF solution (20 wt% solid content) to stirred for 12 h to form PAA solution. Then, acetic anhydride (1 ml) and triethylamine (0.5 ml) were added and the imidization reaction of polyimide proceeded over 48 h in the room temperature. The resulting polyimide solution was poured into 250 ml of ethanol obtaining a light-yellow precipitate.

Fabrication of PI-AgNWs/TPC6FPI/Al memory device
The flexible electronic memory device was fabricated on AgNWs-coated colorless PI with the configuration of TFBPI-AgNWs/TPC6FPI/Al. First, the active layer was fabricated by spreading a 50 mg/ml TPC6FPI solution in DMF onto the TFBPI-AgNWs substrate using a spin coater set at 2000 rpm. Second, the spin-coated films were thermally treated at 150 • C for 1 h under vacuum to remove the remaining solvent. The thickness of the spin-coated TPC6FPI film was about 40 nm, which was suitable as the active layer. Finally, the sandwich device was fabricated by vacuum evaporation of a thin aluminum (Al) layer (ca. 200 nm) through a shadow mask onto the polyimide surface as the top electrode.

Properties of the colorless TFBPI
The colorless TFBPI used as substrate materials in this work were shown in Figure 1A and characterized by FT-IR in Figure S2. The FT-IR characteristic peaks around 1780, 1715, and 1370 cm −1 , were attributed to the imide groups. There were no peaks of amino groups at 3500 cm −1 appeared, which indicated TFBPI was imidized completely. The TFBPI exhibited excellent solubility in the usual organic solvents, such as DMF, DMSO and NMP, indicating that TFBPI could afford an easy blending method with AgNWs. The TFBPI showed excellent thermal stabilities, and the diagrams of the thermogravimetric (TGA) and differential scanning calorimetry (DSC) thermal analysis were depicted in Figures S3 and  S4, respectively. Meanwhile, the TFBPI exhibited excellent mechanical properties with the ultimate tensile strength and elongation at break around 140 MPa and 5%, respectively, as shown in Figure S5. The 5 wt% decomposition temperatures (T d5% ) and T g values of TFBPI were 510 and 260 • C, indicating that the PI exhibited highly thermal stability. The optical properties of TFBPI were investigated by ultraviolet-visible (UV-Vis) absorption and transmittance. The optical transmittance of TFBPI thin film at 400 nm is 76% and the ultraviolet-visible cutoff wavelength of 80% is 432 nm. And the maximum absorption was 306 nm and the edge absorption was 341 nm, which indicated that the optical gap (band gap) was 3.64 eV by the equation: E gap = 1240 edge eV. This change suggested that the charge transfer effect depressed to result in a colorless TFBPI.

Properties of the TFBPI-AgNWs hybrid based transparent electrode
The scheme of the procedure of the TFBPI-AgNWs hybrid electrodes were shown in Figure 1B. The AgNWs were prepared with a modified polyol method by using the ethylene glycol and glycerol as the reductant and solvent, the NaBr used as the oxygen scavenger and PVP used as the capping agent. The obtained PI-based flexible electrodes with different amounts of AgNWs were summarized in Figure 1C. With the increasing concentrations of AgNWs from 0.1 to 2.0 mg/ml, the transmittance of PI-based flexible electrodes was cut down from 88% to 42% at 800 nm. At the same time, the sheet resistance of flexible electrodes was decreased quickly from ∼10 9 to 15 Ω/sq at the concentrations of 1.0 mg/ml, which was comparable to the commercial ITO electrodes to meet the performance on flexible substrates for the resistive memory applications. As we all knew, it was rare difficult for flexible PI-AgNWs electrodes to show the high transparency with low resistance simultaneously because of the contradiction relationship between transmittance and conductivity. Therefore, in this work, 1.0 mg/ml of AgNWs in DMF solution was chosen to prepare flexible TFBPI-AgNWs hybrid based transparent electrode. Compared to the TFBPI thin film, the transmittance of the TFBPI-AgNWs hybrids at 400 nm was down to 67%, and the maximum transmittance was still beyond 70%, as shown in Figure 1D. The inset image of TFBPI thin film and TFBPI-AgNW flexible electrode suggested the synthesized polyimide exhibited excellent transparency. And the XRD patterns of TFBPI-AgNW flexible electrode was similar with the AgNWs except for the wide dispersion peak of polyimide TFBPI around 22 • (shown in Figure S6), which indicated that the structure of AgNWs was not changed by thermal annealing with high temperature. The morphology of the AgNWs was characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM), and these images were depicted in Figure 2. The obtained AgNWs had an average diameter of 50 nm with an average length around 30 μm, suggesting that the average aspect ratio of these AgNWs was around 600, which was high enough for the flexible transparent electrodes. And the selected area electron diffraction (SAED) image of AgNW indicated that the prepared electrode showed highly ordered lattice and interface. As shown in Figure S7, the {100} facet with periodic fringes spaced about 0.23 nm from one side region. And the interplanar spacing (D hkl ) values of AgNWs was calculated about 0.25 nm by Bragg's Law. Therefore, the TFBPI-AgNW flexible electrode was prepared with high conductivity and transparency, which exhibited the potential application for the flexible electronics.

Properties of TPC6FPI memory materials
The polymer TPC6FPI was synthesized by chemical imidization, as shown in Figure 3A and Figure S8. TPC6FPI exhibited good solubility in a variety of polar solvent such as DMF, DMAc, DMSO and NMP, and the intrinsic viscosities of TPC6FPI was of 0.82 dl/g. The high solubility was favorable to the introduction of bulky side-chain triphenylamine group in the polymer structure. The T d5% and T g values of TPC6FPI were 489 and 252 • C, respectively, as shown in Figure 3B,C. The optical spectra of the TPC6FPI in thin film state and in DMF solution were shown in Figure 3D-F. TPC6FPI exhibited the absorption peak maxima at 266 nm in solution which was the assignable to the π-π* transition. And the middle absorption peak around 300 was belong to n-π* intramolecular CT effect, which even showed the yellow color due to stronger the intermolecular CT effect in thin film state. Thus, the optical band gap of TPC6FPI was estimated to be 3.35 eV with the initial absorption wavelength at around 370 nm. The emission spectra of TPC6FPI in DMF solution was obtained with the maximum excitation wavelength of 393 nm. TPC6FPI exhibited intense emission peaks at 433 nm, which was consistent with the fact of triphenylamine derivatives possessing efficient fluorescence chromophores.

Memory device characteristics of PI-AgNWs/TPC6FPI/Al
The memory effects of TPC6FPI were demonstrated by the current-voltage (I-V) characteristics of a flexible PI-AgNWs/TPC6FPI/Al device. The PI-based memory device can store data based on the high ("ON" and low "OFF") bistable conductive state responses to the external applied voltages. 8,12,23 Figure 4A exhibited the typical I-V curves of the  Figure 4B, first, the PI-based memory device was the OFF state ("0" signal) with a low current from ∼10 −11 to ∼10 −7 A by the voltage increasing from 0 V (sweep 1). When the applied voltage of the device raised further, the current increased sharply at a threshold voltage of about 2.1 V, which indicated that the transition from the OFF state to the ON state ("1" signal) occurred with the current raising to ∼10 −6 to ∼10 −5 A. The electronic transition from the OFF state to the ON state was served as the "writing" process. Furthermore, the flexible device maintained the ON state under the positive voltage scan (sweep 2) from 4 V to 0, almost keeping the current curves on conductive state. After the applied voltage was turned off about 1 min, the device relaxed to the OFF state again. The memory device could be reset from the initial OFF state to ON state by the application of a reverse sweep (sweep 3) at −2.4 V. the following sweep from −4 to 0 V (sweep 4) was still stable at high conductivity. Therefore, the flexible PI-based memory device which kept the OFF and ON state was randomly accessible and volatile, suggesting that the resistive switching behavior was regarded as the SRAM characteristic. The I-V characteristics were also found to show good reproducibility, except for the slight variations in switching threshold voltages associated with the sheet resistance of flexible PI-AgNWs electrodes, as shown in Figures S9 and S10. In addition, the flexible memory device exhibited an ON/OFF current ratio as high as 10 4 at 1 V, and the no obvious degradation in current was observed between the ON and OFF states over 2 h a period ( Figure 4C).
Furthermore, according to the reported various conduction models, the charge transport mechanism of the memory device could be obtained from the I-V curves in OFF and ON states. As shown in Figure 5A, the OFF state for the PI-AgNWs/TPC6FPI/Al device obviously exhibited the space charge limited current model 6 (SCLC, from 0 to 2.0 V), which was descript by the equation as follow: I = 9A 0 V 2 ∕8d 3 , where d was thickness of active layer, was the carrier mobility of polymer, 0 was the absolute dielectric constant. On the other hand, as shown in Figure 5B, the current in the ON state was almost linear in dependence on applied voltage just as the ohmic model, 23 which indicated that the ON state the charge transport dominated similarly as metallic conduction.
The above results suggested that the bipolar ON and OFF switching behaviors of TPC6FPI film could be simulated well according to proper theoretical model. Geometry optimization of TPC6FPI was realized by DFT with dispersion-corrected density functional theory (DFT-D3) at the B3LYP-D3/def2-SVP level, and their excitation energies were calculated at the CAM-B3LYP/def2-SVP level with TD-DFT method. [42][43][44][45] The detail progress of calculation was descripted in the Supporting Information. As shown in Figure 5C, the highest occupied molecular orbital (HOMO) and HOMO + 4 mainly located on the triphenylamine unit, which was the donor cell of the structure of TPC6FPI. While the lowest unoccupied molecular orbital (LUMO) and the LUMO + 2 distributed on the phthalimide unit completely, which was the acceptor cell of the structure of TPC6FPI. It was noted that the switching behavior between the OFF state and ON state correspond to electronic transition from the ground state to the excited state. The electron was easily excited from the HOMO to LUMO + 4 by the applied bias reaches at the threshold voltage. However, the excited state tended to relax to LUMO by internal conversion leading to the forming the CT effect. The CT complex was beneficial to stable the ON state. After the applied voltage was turned off, the CT complex was slowly returned to the initial state. This progress was descripted in Figure 5D by the hole-electron analysis to fully characterize electron excitations. It was noted that blue and green isosurfaces represent hole and electron distributions, respectively. 46 As shown in Figure 5E. The electrostatic potential (ESP) surfaces showed that TPC6FPI consisted mainly of the positive ESP region (red color), 47 with special independent negative regions (blue color) mainly arising from the sp 2 hybridized O atoms in the phthalimide unit. These negative ESP regions suggested that concentrated local electron densities near the phthalimide group possessed a certain electron-withdrawing ability, suggesting that electrons would be transferred from the triphenylamine moiety to the phthalimide moieties under excitation. 48 Therefore, the separation of hole and electron was easy to form a charge transfer excited state, which was the characteristic of the structure of polyimide. In other word, the strong charge transfer effect of polyimides was the essence of resistive switching memory device.

CONCLUSION
In conclusion, introducing the prepared the AgNWs on to colorless PI through a facile solution casting method was demonstrated as a viable and effective way to prepare flexible transparent PI-AgNWs electrodes in this work. The resulting flexible PI-AgNWs hybrid electrodes exhibited high transparency and excellent electroconductivity with the sheet resistance of 15 Ω/sq. In addition, the multifunctional polyimide TPC6FPI was synthesized and proved as a novel kind of memory materials. The PI-AgNWs/TPC6FPI/Al resistive memory device was successfully prepared and exhibited excellent SRAM switch behavior, which indicating that the flexible transparency PI-AgNWs hybrid electrodes afforded the potential to resistive switching memory devices and served as the electrodes for wearable devices.