Flexible Nonvolatile Memory Research Articles

Organic Ferroelectrics for Regulation of Electronic and Ionic Transport Toward Neuromorphic Applications

AbstractOrganic ferroelectrics (OFEs) have been of significant research interest not only for nonvolatile memory applications but also for their unique material characteristics such as mechanical softness, biocompatibility, facile processibility, and chemically tailorable functionalities that inorganic counterparts are hard to achieve. Despite these promising merits, the utilization of OFEs has mainly focused on simply demonstrating flexible nonvolatile memories wherein modulation of electronic conductance is of interest. Recent studies indicate that the applicability of OFEs can be further extensive, particularly when combined with electronic, ionic, and mixed electronic‐ionic conducting media. Herein, we discuss that OFEs can be employed for the regulation of electronic as well as ionic charges, and lead to unique device behaviors. First, we comprehensively introduce organic ferroelectric materials classified with their structures and compositions. Next, we discuss recent studies where organic ferroelectricity has been incorporated with electronic, ionic, or mixed transport system to resolve issues in devices and endow multifunctionality, which are promising for neuromorphic computing and sensory memory systems. Finally, insight into the research direction of OFEs is provided, and what hurdles shall be overcome for real‐world applications.

Ni3C nanosheets and PVA nanocomposite based memristor for low-cost and flexible non-volatile memory devices

Two-dimensional (2D) MXenes have gained extensive attention in the field of memristors due to their high ion mobility, chemical stability and tunable electrical and surface properties. Moreover, MXenes have tunable interlayer bonding which enables enhancement in memristor performance. In this work, we synthesize a novel nanocomposite of Ni3C nanosheets and polyvinyl alcohol (PVA) and fabricate a flexible memristor with Ag/Ni3C-PVA/ITO/PET configuration for non-volatile memory applications. Ni3C nanosheets were synthesized from Ni-MAX (Ni3AlC2) through a solvothermal etching and exfoliation process. The nanosheet like structure of Ni3C is confirmed through Scanning Electron Microscopy (SEM) and Transmission Electron microscopy (TEM). Characterization techniques like X-ray diffraction (XRD), Raman spectroscopy and Fourier Transform Infrared Spectroscopy (FTIR) reveal the crystalline phases, surface functional groups and chemical bonds present in Ni3C nanosheets and Ni3C-PVA nanocomposite. The memristor was fabricated through a facile low cost solution processed based fabrication technique. The Ag/Ni3C-PVA/ITO/PET memristor exhibits bipolar non-volatile switching characteristics. The device with optimised mass ratio of 4:3 (Ni3C: PVA) shows a decent off/on ratio of 2.09 × 102, long retention time of 104 s, and an endurance of 102 DC cycles. The SET voltage of the as-fabricated device is 2.8 V and the RESET voltage is −5.33 V. The flexible memristor exhibits outstanding mechanical robustness over 500 bending cycles. The high charge carrier mobility of Ni3C results in lower switching voltage and the dielectric property of PVA improves data retention and off/on ratio of the memristor. The resistive switching behaviour is explained through conductive metallic bridge mechanism. The conduction mechanism follows Ohmic conduction in ON state and trap controlled space charge limited current (TCSCLC) mechanism in OFF state. The results demonstrate that the nanocomposite with its superior resistive switching effects is suitable for cost-effective, high-performance, flexible non-volatile memory devices.

Non-volatile memory characteristics of flexible ferroelectric field-effect transistors consisting of Sm-doped BiFeO3 thin films

This study investigates the impact of Sm doping on the structural, electrical, and functional properties of BiFeO3 (BFO) thin films. The aim is to improve their non-volatile memory characteristics for use in flexible ferroelectric field-effect transistors (FeFETs). We fabricated Sm-doped BFO (SBFO) thin films on flexible mica substrates with Sm concentrations ranging from 0 to 15 %. The FeFET device structures, which include MoS2 monolayer channels and SBFO thin films with Pt bottom electrodes, were analyzed. The SBFO thin films displayed a (111) preferential orientation due to the influence of underlying Pt electrodes. Notably, the SBFO thin film doped with a 10 % Sm concentration showed optimal enhancements, including a reduction in leakage current density and an increase in remanent polarization to 55.8 μC/cm2, compared to undoped thin film (35.5 μC/cm2). Furthermore, the thin film exhibited superior fatigue resistance for up to 1012 switching cycles and maintained stable values under significant bending stress for ferroelectric polarizations. On the other hand, SBFO thin film with 15 % Sm doping displayed relaxor antiferroelectric behavior, highlighting the importance of doping concentration. Therefore, for non-volatile memory application reliability, the FeFET device featuring a 10 % Sm-doped SBFO thin film not only enhanced the memory window but also showed superior fatigue characteristics over 1012 switching cycles, compared to its undoped counterpart. These findings demonstrate the effect of Sm doping in customizing the properties of BFO thin films for advanced, flexible non-volatile memory devices, offering a novel insight to optimize ferroelectric field-effect transistor performance.

Multifunctionality in Electrically Poled PMN–PT/Ni–Mn–In Multiferroic Heterostructure for Flexible Magnetic Field Sensing and Nonvolatile Memory Applications

Multifunctionality in Electrically Poled PMN–PT/Ni–Mn–In Multiferroic Heterostructure for Flexible Magnetic Field Sensing and Nonvolatile Memory Applications

Highly Flexible Non-Volatile Resistive Memory Devices Based on ZnO Nanoparticle/Graphene Heterostructures Embedded in Poly(methyl methacrylate)

Highly Flexible Non-Volatile Resistive Memory Devices Based on ZnO Nanoparticle/Graphene Heterostructures Embedded in Poly(methyl methacrylate)

Flexible SnO2–MoS2 based memristive device exhibiting stable and enhanced memory phenomenon

Flexible non-volatile memory devices have been gaining interest in expanding the digital data storage world. Due to the burgeoning advancement in the healthcare industry, the Internet of Things, and wearable electronics, the demand for ultra-thin, low-power, and flexible memory is increasing. Further, the advancement of synthesis procedures for two-dimensional nanomaterials having better optical, electrical, and mechanical strength with flexibility has fuelled the flexible memory device area, as commonly used flash memory is approaching its physical limit. In this context, the present work reports the flexible resistive switching memory device based on pure molybdenum disulfide (MoS2) and tin oxide (SnO2) based nanocomposite powder synthesized using the simple hydrothermal process. The nanocomposite formation was characterized using x-ray diffraction and Raman spectroscopic techniques. The memory device was fabricated by spin-coating the pure MoS2, pure SnO2, and MoS2–SnO2 nanocomposite film over the ITO-PET flexible substrate. The device was completed by thermally evaporating the thin Al layer through a shadow mask. It was found that MoS2–SnO2 based memory devices exhibited improved switching performance having a higher I ON/I OFF ratio, and lower switching voltage in comparison to pure MoS2 and pure SnO2-based devices (I ON/I OFF ratio ∼100, V = 0.5 V). Furthermore, to check the stability and cyclic performance of the fabricated device, the retention and endurance test was also performed, and the MoS2–SnO2 device retained the HRS and LRS states up to 2 × 103s and showed stable performance up to 100 switching cycles without much degradation, respectively. It should be mentioned that the presently proposed ReRAM device based on SnO2 and MoS2 with flexible and low-power features had excellent potential for use in the wearable device industry.

Flexible magnetoelectric sensor and nonvolatile memory based on magnetization-graded Ni/FSMA/PMN-PT multiferroic heterostructure

Flexible multiferroic heterostructures are promising to unveil technological developments in wearable magnetic field sensing, nonvolatile memory, soft robotics, and portable energy harvesters. Here, we report an enhanced and a zero-biased magnetoelectric (ME) effect in flexible, cost-effective, and room temperature sensitive Ni/FSMA/PMN-PT magnetization-graded ME heterostructure. Flexible Ni foil with −q (piezomagnetic coefficient) and the ferromagnetic shape memory alloy (FSMA; Ni-Mn-In) layer with +q offers the desired q-grading. The temperature-dependent dielectric behavior shows an anomaly in the martensite transformation regime of the FSMA layer. The Ni/FSMA/PMN-PT ME heterostructure exhibits noteworthy ME output of ∼3.7 V/cm Oe, significantly higher than Ni/PMN-PT (∼1 V/cm Oe). The q-grading-induced bending moment impedes the asymmetry-related flexural strain and strengthens the ME interaction. The zero-bias ME output of ∼0.4 V/cm Oe is ascribed to the interaction between q-grading-induced transverse magnetization and AC magnetic field. Ni/Ni-Mn-In/PMN-PT ME heterostructure displays excellent magnetic field sensing parameters: correlation coefficient, sensitivity, inaccuracy, and hysteresis of 0.99916, ∼0.74 mV/Oe, 1.5% full-scale output (FSO), and 1.8% FSO, respectively. The reversible and repeatable nonvolatile switching of the ME coefficient obtained with positive and negative electric fields is useful for next-generation memory devices. The flexible ME heterostructure shows no degradation in performance up to 1500 bending cycles. Such Ni/FSMA/PMN-PT based ME heterostructures are propitious for multifunctional flexible magnetic field sensors and nonvolatile memory applications.

Fabrication of metal/oxide/fluorographene/oxide/silicon capacitors and their charge trapping properties

Graphene is well-known as a material with extremely high carrier mobility, although its electrical properties can be changed into those of a highly insulating thin film by a fluorination treatment (fluorographene). Taking advantage of this property, a flexible nonvolatile memory using fluorographene as a charge trapping layer has been proposed. However, the mechanism of charge trapping on fluorographene is not yet fully understood. In this paper, we fabricated fluorographene and analyzed it by X-ray photoelectron spectroscopy and Raman spectroscopy. Furthermore, metal–insulator–semiconductor (MIS) capacitors inserted with fluorographene were fabricated and evaluated electrically. As a result, a large flat-band voltage shift was observed even though the fluorine concentration (C–F bonding ratio) was saturated at around 14%. Furthermore, the charge centroid of the MIS capacitor with fluorographene coincides with the location of the fluorographene. These results indicate that fluorographene can capture injected charges and is a candidate for use in nonvolatile memory with two-dimensional materials.

Rubrene/CuInP2S6 organic/inorganic heterojunctions for ferroelectric organic transistor memory devices

Ferroelectric organic field-effect transistor (Fe-OFET) memory devices exhibit low power consumption, low cost and good flexibility, which are indispensable to next generation flexible and wearable memory devices. However, up until the present, the ferroelectric materials used in Fe-OFETs have been limited to ferroelectric polymers, i.e. poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], and suffer from large surface roughness and high-risk of depolarization. This work integrates a new two-dimensional CuInP2S6 crystal into Fe-OFET, which can introduce excellent ferroelectric property due to its single crystalline smoothness and noncentrosymmetric crystal structure. Meanwhile, the high-mobility rubrene crystal is selected as the channel material to construct rubrene/CuInP2S6 organic/inorganic van der Waals heterojunctions. Associated with synergetic effect of single-crystalline nature of the heterojunction and atomically flat/defect-free interface, the memory devices possess a high programming speed of 0.5 s and long retention time of 104 s with a current ratio larger than 103. Furthermore, the performance of flexible devices is also demonstrated, including a programming/erase cycle of 1000 with a current on/off ratio approaching 103, data retention time of 104 s with a current on/off ratio of 103 and a current on/off ratio maintained at 102 even after 100 bending cycles under a bending radius of 25 mm, which are qualified for memory buffer usage or short-term data storage especially when they are integrated with other flexible electronics. The proposed configuration of memory transistors using organic/inorganic heterojunctions provides a rational design of device structure toward flexible Fe-OFET non-volatile memory applications.

High-performance flexible pentacene transistor memory with PTCDI-C13 as N-type buffer layer

Flexible organic field-effect transistor nonvolatile memories (OFET-NVMs) with polymer electrets have aroused great attention for its important role in next-generation flexible data storage devices application. However, the OFET-NVMs to date still hardly reach the requirements for practical applications. In air environment, the positively charged defects formed in pentacene near the interface with polymer, result in unsatisfied high programming/erasing (P/E) voltages. Here, we propose an OFET memory structure, in which an n-type semiconductor N, N′-Bis(3-pentyl) perylene-3, 4, 9, 10-bis(dicarboximide) (PTCDI-C13) is inserted between pentacene and poly(4-vinyl phenol. Based on the electrostatic induction effect, electrons are induced on the surface of PTCDI-C13 due to the electrostatic field generated by the positive charges at the interface of pentacene/polymer, and compensate part of the positive charges at the interface, resulting in the reduction of the height of hole-barrier. The flexible memory device with PTCDI-C13 exhibits a memory window of larger than 7 V at P/E voltages (±20 V), fast switching speeds (0.5 ms), good P/E endurance (>400 cycles), large field-effect mobility (0.026 cm2 V−1 s−1), and long retention time (over 104 s).

Advances in flexible non-volatile resistive switching memory based on organic poly (3, 4-ethylenedioxythio phene): Poly (styrenesulfonate) film

In this work, the resistive switching memory has been investigated in conducting polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS) with aluminium (Al) and indium tin oxide (ITO) acting as electrodes. Current-voltage (I-V) characteristics of the fabricated Al/PEDOT:PSS/ITO flexible Polyethylene terephthalate substrate device reveal a non-volatile resistive switching behaviour. The memory device demonstrates resistive switching non-volatile memory with ION/IOFF current ratio between the high resistance state (HRS) and low resistance state (LRS) up to 104. Both the HRS and LRS of the memory devices are stable, showing no significant change in value of ION/IOFF current ratio, which is maintained at 104 in magnitude under continuous read-out testing for 100 cycles at 90º bending angle, and also retention time of 33 min that confirms the device durability and non-volatility. The charge transport mechanism of the fabricated non-volatile organic devices is governed by space-charge limited conduction. It is proposed that the reduction and oxidation of PEDOT:PSS film might be the switching mechanism. Interestingly, the performance of the memory device revealed no detectable degradation and remained identical even after bending the device from 70 to 160° angles, representing excellent flexibility. Besides, the lowest power and the critical compliant current for resistive switching are determined. The power density for 180° (without bending), 160°, 90° and 70° bending angles is ∼ 30 Wm−2. It demonstrates that for a small range of compliance current, our device showed more power capacity with lower voltage consumption, which makes it a potential candidate for low power consumption flexible non-volatile memory devices.

Flexible Threshold-Type Switching Devices with Low Threshold and High Stability Based on Silkworm Hemolymph.

In this paper, a floating-gate flexible nonvolatile memory is reported that is composed of natural biological materials, namely, silkworm hemolymph, graphene quantum dots as the floating-gate layer, and polymethyl methacrylate (PMMA) as the insulating layer. The device has a high ON/OFF current ratio (4.76 × 106), a low setting voltage (<−1.75 V), and good durability and retention ability. The device has two storage characteristics, namely, Flash and WORM, which can be effectively and accurately controlled by adjusting the limiting current during device setting. The resistance switching characteristics are the result of the formation and fracture of conductive filaments. The floating-gate flexible bioresistive random access memory prepared in this paper provides a new idea for the development of multifunctional and biocompatible flexible memory.

Resistive Switching in Biocompatible Glucose Films Doped With Honey for Highly Flexible Nonvolatile Memory Application

A mixture of honey and glucose (GH) as a new natural biomaterial for resistive random access memory (RRAM) switching layer on polyethylene terephthalate (PET) substrate is presented. The flexible substrate allows the devices to be bent and used for a wider range of applications. These devices with the GH film exhibit good reproducibility and reliability over more than 300 cycles. Moreover, the ON/OFF ratio of these devices is over 105, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {SET}}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {RESET}}$ </tex-math></inline-formula> are lower than that of the device with a pure glucose film. The nonvolatile nature of the devices is confirmed by measuring retention for 104 s. This switching behavior is attributed to ohmic conduction and trap-controlled space charge-limited current (TC-SCLC) mechanisms that confirm filament formation and breakage at high-resistance state (HRS) and low-resistance state (LRS), respectively.

High-speed, low-voltage programmable/erasable flexible 2-bit organic transistor nonvolatile memory with a monolayer buffered ferroelectric terpolymer insulator

Achieving multi-bit data storage in one transistor is a promising strategy to further multiply the storage density of the nonvolatile memories (NVMs). Low-voltage programming/erasing (P/E) operation is a prerequisite for the commercialization of the multi-bit NVMs. And, the fast P/E switching is also a desirable figure of merit for the practical NVMs. Here, we develop a route to achieve a high-speed, low-voltage P/E flexible organic transistor-based NVM, by processing a monolayer buffered ferroelectric terpolymer insulator. The physical mechanisms for achieving the high-speed, low-voltage P/E properties in the organic transistor-based NVMs are investigated. As a result, high-performance flexible 2-bit NVMs are achieved, with the low P/E voltage of ±15 V, fast P/E switching capability of 50 ns, high mobility up to 7.4 cm2 V−1 s−1, high stable retention time up to 10 years, reliable endurance over 200 cycles, good mechanical bending durability, and atmosphere stability.

Molecular Compounds in Spintronic Devices: An Intricate Marriage of Chemistry and Physics.

Spintronics, a flourishing new field of microelectronics, uses the electron spin for reading and writing information in modern computers and other spintronic devices with a low power consumption and high reliability. In a quest to increase the productivity of such devices, the use of molecular materials as a spacer layer allowed them to perform equally well or even better than conventional all-inorganic heterostructures from metals, alloys, or inorganic semiconductors. In this review, we survey various classes of chemical compounds that have already been tested for this purpose─from organic compounds and coordination complexes to organic-inorganic hybrid materials─since the creation of the first molecule-based spintronic device in 2002. Although each class has its advantages, drawbacks, and applications in molecular spintronics, together they allowed nonchemists to gain insights into spin-related effects and to propose new concepts in the design and fabrication of highly efficient spintronic devices. Other molecular compounds that chemistry could offer in great numbers may soon emerge as suitable spacers or even electrodes in flexible magnetic field sensors, nonvolatile memories, and multifunctional spintronic devices.

Optically Encodable and Erasable Multilevel Nonvolatile Flexible Memory Device Based on Metal-Organic Frameworks.

Multilevel and flexible nonvolatile memory (NVM) is a promising candidate for data storage in next-generation devices but its high bias and low mobility of conducting channels are often its drawbacks. In this study, we demonstrate a low bias of smaller than 0.1 V and a high-mobility graphene layer as a conducting channel for flexible optoelectronic NVM based on a composite thin film of indium-based MOF-derived InCl3 and 4,4-oxydiphthalic anhydride (odpta), Na[In3(odpt)2(OH)2(H2O)2](H2O)4, and reduced graphene oxide (rGO). The optoelectronic NVM device can be encoded and erased optically by ultraviolet (UV) light and visible light, respectively. Our device also achieves memory states over 192 (6-bit storage) distinct levels, which can emerge as mass data storage. It also shows an excellent endurance of write-erase cycles under irradiation with a laser of varying wavelengths, the mechanical stability of more than 1000 bending cycles, and stable retention for longer than 10 000 s. These results open an alternative route for developing low bias and innovative optoelectronic technologies.

How to process P(VDF-TrFE) thin films for controlling short circuits in flexible non-volatile memories

Severe electrical shorting is one of the biggest challenges for the development of flexible non-volatile ferroelectric memories with low operation voltage. In this work, short circuits in flexible ferroelectric capacitors based on copolymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)], were explicitly investigated. It was found that processing conditions of P(VDF-TrFE) films have huge effects on electrical shorting conditions. Both deciding factors such as drying temperatures, solvent evaporation rates, and solution concentrations, and plausible factors such as post-drying thermal annealing, were clearly identified. These deciding factors were found to affect short circuits by altering the film porosity, surface roughness, and defect conditions in P(VDF-TrFE) thin films. The detailed correlation of these factors with film porosity were established and explained within the framework of liquid-liquid phase separation. The correlation with the surface roughness and defect conditions were explained with Marangoni convection. The net effects of these factors on electrical shorting conditions are rather complicated because a favorable factor with regards to film porosity may be detrimental in terms of surface roughness and/or defects. So these factors have to been carefully tailored to control short circuits and generate high yields in P(VDF-TrFE) thin film memory devices. Comparative study shows that drying at 80 °C in open air is the optimal condition for processing P(VDF-TrFE) thin films from PGMEA solution. Under this optimal drying condition, a yield of 76% was achieved from large-area devices of about 200 μm × 200 μm with thin P(VDF-TrFE) films of about 200 nm processed with roll-2-roll compatible techniques on plastic substrates. An effective two-step approach to manufacturing P(VDF-TrFE) layers for ferroelectric capacitors with low operation voltage but high yields, is presented. The key information gained from this work on ferroelectric capacitors could be generalized to another type of ferroelectric memories, namely ferroelectric field effect transistors. So it provides researchers and engineers a general guideline on how to control short circuits by processing P(VDF-TrFE) thin films for flexible non-volatile memories.

Flexible and Transparent Electrode Based on Ag-Nanowire Embedded Colorless Poly(amide-imide).

Graphene oxide-cysteamine-silver nanoparticle (GCA)/silver nanowire (AgNW)/GCA/colorless poly(amide-imide) (cPAI) structures based on cPAI substrates with polyimide and polyamide syntheses were fabricated to study their characteristics. A layer of electrodes was constructed using a sandwich structure—such as GCA/AgNW/GCA—with cPAI used as a substrate to increase the heat resistance and improve their mechanical properties. Furthermore, to overcome the disadvantages of AgNWs—such as their high surface roughness and weak adhesion between the substrate and electrode layers—electrodes with embedded structures were fabricated using a peel-off process. Through bending, tapping, and durability tests, it was confirmed that these multilayer electrodes exhibited better mechanical durability than conventional AgNW electrodes. Resistive random-access memory based on GCA/AgNW/GCA/cPAI electrodes was fabricated, and its applicability to nonvolatile memory was confirmed. The memory device had an ON/OFF current ratio of ~104@0.5 V, exhibiting write-once-read-many time characteristics, maintaining these memory characteristics for up to 300 sweep cycles. These findings suggest that GCA/AgNW/GCA/cPAI electrodes could be used as flexible and transparent electrodes for next-generation flexible nonvolatile memories.

Solution-processed flexible nonvolatile organic field-effect transistor memory using polymer electret

Flexible organic field-effect-transistor (OFET) memory is one of the promising candidates for next-generation wearable nonvolatile data storage due to its low price, solution-processability, light-weight, mechanically flexibility, and tunable energy level via molecular tailoring. In this paper, we report flexible nonvolatile OFET memory devices fabricated with solution-processed polystyrene-brush electret and organic semiconductor blends of p-channel 6, 13-bis-(triisopropylsilylethynyl)pentacene (TIPS-PEN) and n-channel poly-{[N,N′-bis(2- octyldodecyl)-naphthalene-1,4,5,8-bis-(dicarboximide)-2,6-diyl]-alt-5,5′-(2,2′-bithiophene)} (P-(NDI2OD-2T); N2200). Fabricated flexible OFET memory devices exhibited high memory window (30 V) and ON/OFF current ratio (memory ratio) over 103. Furthermore, we obtained reliable memory ratio (~103) over retention time of 108 s, 100 times of repeated programming/erasing cycles, and 1000 times of bending tests at a radius of 3 mm.

Sinter-free inkjet-printed PEDOT:PSS/WO3/PEDOT:PSS flexible valency change memory

With the rapid proliferation of consumer electronics in our day to day lives, there is an ever increasing demand for flexible electronic devices which are low cost, easy to fabricate and deliver reliable performance. In this work, we report fabrication of symmetric poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/WO3/PEDOT:PSS memory cells by inkjet printing on transparent, flexible polyethylene naphthalate (PEN) substrates. The cells show resistive switching behavior with two stable resistance states. The current conduction across the interface between the PEDOT:PSS electrode and dielectric WO3 is determined by localized filamentary features which is space charge limited in the high resistance state, facilitated through the migration of oxygen vacancies at low voltages (<2 V) whereas Schottky emission dominates the current conduction for higher voltages (>2 V). The cells show good retentivity and endurance (>6000 cycles). The cells are entirely sinter free and no electroforming is required to activate them. These characteristics make them suitable for the next generation of flexible non-volatile memory devices.