Write-once-read-many-times Research Articles

Synthesis of tetraphenylphosphonium/polysulfide hybrids and their temperature-induced switchable WORM/flash electrical memory behavior

The development of innovative non-volatile storage materials is essential for the progression of next-generation high-performance storage devices. In this study, a hybrid material (Ph4P)2S7 was synthesized, and single crystal structure analysis demonstrated that the organic tetraphenylphosphonium cation (Ph)4P+ adopts a regular tetrahedral conformation and forms one-dimensional chains through CH···π interactions. The (S7)2- anion, characterized by quasi-bicentric symmetry and a right-handed helical cluster, is confined within the one-dimensional quantum well established by the (Ph)4P+ cation chain in the organic matrix. A device composed of FTO/(Ph4P)2S7/Ag was fabricated through spin-coating, demonstrating remarkable temperature-induced switchable binary write-once-read-many-times (WORM)/Flash electrical memory behavior. At room temperature, the material demonstrated nonvolatile binary WORM-type resistance switching behavior with a switching ratio of 1.95 × 103 and an onset voltage of 1.01 V. Conversely, at 150 °C, it transits into binary Flash-type resistance switching behavior characterized by current ratio of 3.34 × 102 and onset/reset voltages of 1.05/−1.99 V. The mechanism underlying binary resistive switching is identified as spatial charge-limited charge capture. Furthermore, the observed reversible transition between WORM- and Flash- type electrical memory behavior at elevated temperatures can be attributed to lattice expansion of the (Ph)4P+ cations at high temperatures, resulting in denser packing of organic molecules and deeper trapping potential wells for electrons within the active layer. This study provides theoretical insights into developing novel high-performance information storage materials.

CsPbI3 Perovskite Quantum Dot-Based WORM Memory Device with Intrinsic Ternary States.

The migration of mobile ionic halide vacancies is usually considered detrimental to the performance and stability of perovskite optoelectronic devices. Taking advantage of this intrinsic feature, we fabricated a CsPbI3 perovskite quantum dot (PQD)-based write-once-read-many-times (WORM) memory device with a simple sandwich structure that demonstrates intrinsic ternary states with a high ON/OFF ratio of 103:102:1 and a long retention time of 104 s. Through electrochemical impedance spectroscopy, we proved that the resistive switching is achieved by the migration of mobile iodine vacancies (VIs) under an electric field to form conductive filaments (CFs). Using in situ conductive atomic force microscopy, we further revealed that the multilevel property arises from the different activation energies for VIs to migrate at grain boundaries and grain interiors, resulting in two distinct pathways for CFs to grow. Our work highlights the potential of CsPbI3 PQD-based WORM devices, showcasing intrinsic multilevel properties achieved in a simple device structure by rationally controlling the drift of ionic defects.

Synchronous Regulation of Hydrophobic Molecular Architecture and Interface Engineering for Robust WORM‐Type Memristor

AbstractIn the context of human active response to exponentially increasing data volumes, memristors have emerged as a focal point in systematic problem‐solving approaches. Particularly, organic memristors, characterized by excellent scalability, flexibility, and 3D stacking capabilities, have shown its tremendous potential in innovative electronic applications. However, the modulation of molecular assembly and interface interaction at the electrode surface poses a challenging task, despite their crucial role in determining the memristive performance. Herein, a collaborative method involving hydrophobic molecular design and interface engineering is proposed to generate high‐quality nanofilms with optimal photoelectric and electrochemical properties. By subjecting the bottom electrode to O2 plasma treatment and chemical modification using a hydrophobic ionic liquid poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide (PTFSI), the hydrophobic compound DN demonstrates exceptional affinity and stronger intermolecular interactions with the electrode surface. Consequently, the two‐terminal device array comprising Al/DN@PTFSI/ITO exhibits robust non‐volatile write‐once‐read‐many times (WORM) memory behavior, resulting in a tripling production yield from 30% to 92% along with a lower threshold voltage of 1.5 V, higher ON/OFF current ratio of 103 and excellent stability. This study presents a versatile approach to optimize the film assembly and material/electrode interface, thereby accelerating the development of organic memristors toward future applications.

Tweaking the Non-Volatile Write-Once-Read-Many-Times (WORM) Memory using Donor-Acceptor Architecture with Isatin as Core Acceptor.

Organic memory devices have attracted attention because they promise flexible electronics, low manufacturing costs, and compatibility with large-scale integration. A series of new D-A architectures were synthesized employing different donor groups and the isatin moiety as the acceptor through Suzuki-Miyaura coupling reactions. Strong intramolecular interactions were observed in the synthesized compounds, further corroborated by an optimal bandgap. The SEM investigation confirmed good molecular ordering and superior thin film surface coverage. All the compounds demonstrated notable binary Write-Once-Read-Many-Times (WORM) memory behaviour. The threshold switching voltage for these D-A systems ranged from -0.79 to -2.37 V, with the compound having isobutyl substituent showing the lowest threshold voltage and maximum ON/OFF ratio of 102, thus outperforming others. The combined effects of charge transfer and charge trapping are responsible for the resistive switching mechanism prevailing in these systems. The alterations in D-A molecules that affect molecular packing, thin film morphology, and, finally, the memory performance of the active layer are highlighted in this work.

Barium titanate write-once read-many times resistive memory with an ultra-high on/off current ratio of 108

Write-once read-many times (WORM) resistive memories fabricated using perovskite barium titanate (BaTiO3, BTO) resistive switching (RS) layer are demonstrated in this study. The BTO RS layers with thicknesses of 3.2–8.5 nm are fabricated by radio frequency sputtering at room temperature. Annealing processes at 400–600 °C are used to modify the oxygen vacancy content in the BTO layer and thereby control the OFF-state current of an Al/BTO/n+-Si WORM device. The ION/IOFF ratio increases with increasing annealing temperature until 500 °C. A negative effect is observed when the temperature further rises to 600 °C. The optimal Al/BTO/n+-Si device shows an ultra-high ION/IOFF ratio of 108 and a low coefficient of variation of 6% for set voltages. The writing speed is 80 ns and the power consumption is 400 pJ. In addition, retention and stress tests are performed to confirm high data storage reliability of the device. Mechanisms for voltage-polarity independent writing process and the related energy band diagrams are explored.

Hyperbranched polyimide with triazole core structure for nonvolatile resistive switching memory

AbstractHyperbranched polyimides (HBPIs) possess a unique branched structure, which can make the lowest unoccupied molecular orbitals (LUMOs) degenerate and reduce the bandgap between the highest occupied molecular orbital (HOMO) and LUMO energy levels. The triazole group can facilitate the charge transfer process. Thus, a novel hyperbranched polyimide (HBPI‐TZA‐6FDA) has been prepared from new triazole derivative triamine monomer (TZA) and 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride (6FDA) for memory device. The memory device with the structure ITO/HBPI‐TZA‐6FDA/Al exhibits a low‐threshold voltage of −2.2 V and a high ON/OFF current ratio of about 105. Meanwhile, the memory device shows nonvolatile write once read many times (WORM) type memory behavior and excellent stability with an operation time of 104 at a continuous applied voltage of −1 V. Optical, electrochemical experiments, and molecular simulation have been carried out to illustrate the memory performance. The memory performance is governed by the charge transfer between TZA and 6FDA units. The branched structure and triazole unit can reduce the band gap between HOMO and LUMO energy levels effectively, which is favorable for improving charge transfer and reducing threshold voltage. The HBPI‐TZA‐6FDA‐based memory device is favorable for memory device applications due to its relatively low‐power consumption and high‐operation stability.

Barium oxide write-once read-many-times memory with a high resistance window

This study demonstrates write-once-read-many times memory characteristics of BaOx and uses an interfacial AlOx layer to improve the stability of the Al/BaOx/n+-Si memory.

Environment-friendly resistive memory based on natural casein: Role of electrode and bio-material concentration

It is well-known that moisture plays a crucial role on the stability of bio-material-based resistive memory devices. In this work, we report a bio-material-based resistive memory device that does not depend on moisture to attain a stable performance. Sodium caseinate (NaCas) obtained from bovine milk, is used as the active material with silver (Ag) as the top electrode (TE) and indium tin oxide (ITO) as the bottom electrode (BE) for the implementation of the device. The device exhibits Write-Once-Read-Many times (WORM) type current-voltage characteristics with a sufficiently large data retention of >106 s, a cyclic endurance of >6 × 103, and a switching voltage of ∼1.06 V. The use of the less reactive Ag for the TE facilitates the removal of the capacitive nature of the device characteristics. It is found that increasing the NaCas concentration improves the memory performance. The role of the Ag electrode in removing the capacitive nature and possible switching mechanism of the device is suggested.

Top electrode dependence of the write-once-read-many-times resistance switching in BiFeO3 films

The electrode is one of the key factors that influences and controls the resistive switching characteristic of a resistive switching device. In this work, we investigated the write-once-read-many-times (WORM)-resistive switching behavior of BiFeO3 (BFO)-based devices with different top electrodes, including Pt, Ag, Cu, and Al. The WORM-resistive switching behavior has been observed in Pt/BFO/LaNiO3 (LNO), Ag/BFO/LNO, and Cu/BFO/LNO devices. In the initial high resistance state, the Pt/BFO/LNO device shows space-charge-limited current conduction due to the large Schottky barrier height at the Pt/BFO interface, while the Ag/BFO/LNO and Cu/BFO/LNO devices exhibit Schottky emission conduction due to the small barrier height at both top electrode/BFO and BFO/LNO interfaces. In the low resistance state, the metallic conduction of the Pt/BFO/LNO device is a result of the formation of conduction filaments composed of oxygen vacancies, and yet the metallic conduction of Ag/BFO/LNO and Cu/BFO/LNO devices is due to the formation of oxygen vacancies-incorporated metal conduction filaments (Ag and Cu, respectively). The observed hysteresis I-V curve of the Al/BFO/LNO device may be attributed to oxygen vacancies and defects caused by the formation of Al–O bond near the Al/BFO interface. Our results indicate that controlling an electrode is a prominent and feasible way to modulate the performance of resistive switching devices.

Write-Once-Read-Many-Times Memory Characteristics with a Large Memory Window Operating at a Low Voltage by Li-Ion Incorporation from the LiCoOx Ion-Supplying Layer into the InGaZnO Channel of a Thin-Film Transistor

Write-once-read-many-times (WORM) memory characteristics with a large memory window are demonstrated in a thin-film transistor (TFT) composed of an indium-gallium-zinc oxide (IGZO) channel and a lithium-cobalt oxide (LiCoOx) ion-supplying layer in the gate oxide. While the device with a thicker (5 nm) tunneling oxide showing a threshold voltage shift (ΔVT) of about 5 V by electron charging upon positive gate voltage (VGS) sweep to +25 V, the device with a 2 nm-thick tunneling oxide exhibits a large memory window with ΔVT > 20 V by Li-ion migration from LiCoOx to IGZO channel, which can be controlled as multilevel states with respect to the VGS amplitude. Incorporation of Li ions into the IGZO channel acting as p-type dopants reduces carrier concentration in the channel and consequently increases VT. The increased VT and the consequently reduced drain current are not instantly restored back by applying negative VGS, featuring WORM memory characteristics. Although the device undergoes partial retention loss, the retention remains up to about 90% after 100 min of retention time. These results verify WORM memory operations in the IGZO TFTs through gate voltage-driven Li-ion incorporation into the IGZO channel to modify its conductive states instead of using a typical electrical charging route.

Write-once-read-many-times memory device based on Pt/BiFeO3/LaNiO3 heterostructures

Nonvolatile memory devices based on resistive switching effects are promising candidates for next generation high performance nonvolatile data storage. Here, we report the write-once-read-many-times (WORM) resistive switching behavior for the first time in Pt/BiFeO3/LaNiO3 heterostructures. BiFeO3 thin films with (100) preferred orientation have been prepared by RF magnetron sputtering combined with annealing at temperature higher than 600 ℃, and show excellent WORM resistive switching behaviors with data retention for>3600 s and reliable endurance for>1000 cycles. The devices with BiFeO3 annealed at 650 ℃ exhibit extremely high ON/OFF ratio (>3 × 104) and low switching voltage (∼3 V) due to the high quality of BiFeO3 thin films. The observed resistive switching behavior is attributed to the formation of permanent conducting filaments that composed of oxygen vacancies. These results demonstrate that BiFeO3 is promising for novel WORM memory device applications.

Two-Terminal Nonvolatile Write-Once-Read-Many-Times Memory Based on All-Inorganic Halide Perovskite.

Write-once-read-many-times (WORM) memory belonging to an important non-volatile memory type achieves the read-only state after the write operation and is used in the fields of data security storage widely. WORM memory has been developed based on a variety of materials. In recent years, halide perovskites have become the research hotspot material for this memory due to its excellent properties. Here, the all-inorganic CsPbBr3 perovskite thin film was prepared on a FTO substrate by using a two-step method. The prepared CsPbBr3 thin films have the characteristics of densely packed crystal grains and smooth surface. The device, having the FTO/CsPbBr3/Al sandwich structure by evaporating the Al electrode onto the CsPbBr3 thin film, represents the typical WORM behavior, with long data retention time (104 s), a low operation voltage (2.1 V) and a low reading voltage (0.1 V). Additionally, the resistance transition mechanism of the resulting WORM devices was analyzed.

Enhancing the Resistive Switching Behavior of WORM Memory Devices Using D-π-A Based Ester-Flanked Quinolines.

Donor-Acceptor systems are highly appreciated in the field of organic memory devices due to their efficient charge transport within the systems. In this work, we have designed and synthesized a D-π-A system constituting ester-flanked quinolines and functionalized triarylamines (TAA) through a single-step cross-coupling reaction to fabricate memory devices via Write-Once Read-Many times (WORM) non-volatile memory. Structure-property relationships are reconnoitered for these conjugated D-π-A systems through a series of UV, fluorescence, XRD, DFT, and memory characterizations. The UV and CV data show efficient charge transfer with intramolecular charge transfer occurring at 407-417 nm and a short band gap of 2.56-2.65 eV. An enhancement in the resistive switching behavior of the memory devices is observed for the compounds with simple TAA-quinoline and tert-butylphenyl substituted TAA and fluorophenyl substituted quinoline due to balanced charge distribution in the compounds. This enhanced switching induces an on/off ratio of 103 by generating a highly ordered arrangement in the thin films. The HOMO, LUMO levels, and the ESP images together estimate a charge transfer and charge trapping as the plausible mechanism for the solution-processable WORM memory devices. The longer retention time (103 s) and lower threshold voltages (-1.21--2.12 V) of the devices makes them intriguing compounds for memory applications.

Tantalum pentoxide (Ta2O5 and Ta2O5-x)-based memristor for photonic in-memory computing application

Photonic in-memory computing exhibits promising potential to address the inherent limitations of traditional von Neumann architecture. In this study, we demonstrate a tantalum pentoxide (Ta2O5 and Ta2O5−x)-based memristor as a non-volatile memory for photonic in-memory computing functions. The active layer of the memristor on a heavily doped N-Si substrate comprises two films of Ta2O5 and Ta2O5−x with a size of 3 × 3 µm2 of which roughness root-mean-square values are 1.25 nm and 1.59 nm, respectively. A controllable electrical behavior transition from write-once-read-many-times (WORM) memory to resistive random-access memory (RRAM) is achieved by changing the depositional sequence. Benefitting from the visible light response, the in situ photonic Boolean logic operations (“AND/OR”) are achieved in the RRAM device by mixing the light and electric signals, and the power consumption of an “AND” or “OR” operation consumes 4.503 nJ and 4.526 nJ, respectively, proving the superior photonic in-memory computing potential. The basic logic “IMPLICATION” operation is implemented by performing electrical regulation in a circuit with two RRAM devices connected in parallel. Finally, a 5 × 5 RRAM array is developed and thereafter, the array-level logic for image processing applications is realized. The proposed tantalum pentoxide-based memristors possess great potential in constructing efficient in-memory computing architectures.

Regulating WORM/Flash electrical memory behavior of metallopolymers through varying metal centers

To study the influence of metal atoms on the performance of organic polymer memory devices, four new metallopolymers P0, P1, P2 and P3 were designed and synthesized for organic resistive random-access memory (RRAM) applications. Among them, the device based on P1 shows nonvolatile binary write-once-read-many-times (WORM) memory behavior, while the devices based on P2 and P3 show vastly different nonvolatile binary Flash-type memory behavior. The charge transfer (CT) effect between the platinum (Pt) ligand and porphyrin moieties of metallopolymers is determined to be the origin of the electrical switching behavior of P1, which is proved by the electrochemical and DFT calculations. While the reason of the Flash type memory characteristic of P2 and P3 is ascribed to the formation of metal filaments because of the introduction of metal ions (Co and Ni) into the porphyrin ring. It is suggested that the metal ions can form an internal electrode and act as a bridge during the CT process, which contributes to both the CT and back CT, consequently regulating the WORM/Flash conversion. The results indicate the feasibility of modulating the memory behavior of metallopolymers by varying the central metal ions, manifesting the significance of metal complexes in the application of organic memory devices and will attract wide research interest of scientists. The as-fabricated device based on P3 was also successfully applied to memory logic gates and display function, which offers great application prospect as smart sensor in artificial intelligence (AI) network.

Physically Transient, Flexible, and Resistive Random Access Memory Based on Silver Ions and Egg Albumen Composites.

Organic-resistance random access memory has high application potential in the field of next-generation green nonvolatile memory. Because of their biocompatibility and environmental friendliness, natural biomaterials are suitable for the fabrication of biodegradable and physically transient resistive switching memory devices. A flexible memory device with physically transient properties was fabricated with silver ions and egg albumen composites as active layers, which exhibited characteristics of write-once-read-many-times (WORM), and the incorporation of silver ions improved the ON/OFF current ratio of the device. The device can not only complete the logical operations of “AND gate” and “OR gate”, but its active layer film can also be dissolved in deionized water, indicating that it has the characteristics of physical transients. This biocompatible memory device is a strong candidate for a memory element for the construction of transient electronic systems.

Rare earth Nd-doping lead-free double perovskite Cs2AgBiBr6 films with improved resistive memory performance

Lead-free double perovskites have been an excellent candidate for resistive memories due to their mixed electronic-ionic properties, remarkable ambient stability and nontoxicity. However, the studies on resistive memories of rare-earth ion doping lead-free double perovskite are scarce to date. Here, we report the lead-free double perovskite Cs 2 AgBiBr 6 films with tuning rare earth Nd 3+ content by the vacuum sublimation and solution processing. The X-ray diffraction (XRD) pattern and scanning electron microscopy (SEM) confirm that Nd-doped Cs 2 AgBiBr 6 films show high crystallinity and phase purity. We also systematically investigate the resistive memory properties of the resulted Cs 2 AgBiBr 6 films doped by different content Nd 3+ . The resistive memory devices demonstrate a typical write-once-read-many-times (WORM) behavior with low on-set voltage and long retention time. Particularly, the ON/OFF ratio of Nd-doped Cs 2 AgBiBr 6 film is 1000 times higher than that of the undoped Cs 2 AgBiBr 6 film. This study exhibits that Nd-doped Cs 2 AgBiBr 6 film provide a new material platform for lead-free perovskite-based application in future electronics and optoelectronics. • The Cs 2 AgBiBr 6 film doped by different content rare earth Nd 3+ is reported. • The simplest sandwich-like structure device based on doping Cs 2 AgBiBr 6 is designed. • The resistive memory device demonstrates a typical WORM behavior. • The ON/OFF ratio of Nd-doped film is 1000 times higher as compared to undoped film.

Resistive switching behavior of the memristor based on WS2 nanosheets and polyvinylpyrrolidone nanocomposites

Ag/tungsten disulfide (WS2)–polyvinylpyrrolidone (PVP)/Cu memristors based on monolayer WS2 nanosheets and polyvinylpyrrolidone (PVP) nanocomposites were fabricated, and the influence of PVP content on the switching behaviors was investigated. The results indicate that the WS2–PVP based memristors show write-once read-many times (WORM) memory behavior. Remarkable resistive switching results such as a low operating voltage (VSET < 1 V), a high switching ratio (>103), good endurance (>100 cycles), and data retention time (>200 s) are obtained. With the increase in the PVP content, the device VSET gradually increases, and the switching ratio first slightly increases and then remarkably decreases. The double logarithm I–V curves verify that the switching mechanism of the devices is the trap-controlled space charge limited current mechanism, which is explained with the energy band diagram.

Enhanced intra/intermolecular charge transfer for efficient multilevel resistive memory

Adjustment of molecular structure can effectively improve the organic memory device behaviors due to the optimization of intra/intermolecular interaction and molecular packing motif. However, the intrinsic mechanism of how intermolecular interaction and molecular packing motif affect the device performance needs to be further explored. Herein, we synthesized four new small molecules (X-TEBT) by introducing the alkynyl π bridges and different substituents (X = H, CN, tBu, OMe) into the backbone of conjugated donor–acceptor molecules, namely, TEBT, CN-TEBT, tBu-TEBT and OMe-TEBT, for solution-processed organic resistive memory. All memory devices displayed nonvolatile write-once-read-many-times (WORM) behaviors with low threshold voltage (Vth), high production yield, and good thermal stability. The champion device (tBu-TEBT) exhibits ternary WORM property with the highest ON2/ON1/OFF ratio and the lowest Vth1 (1.6 V) and Vth2 (2.4 V). More importantly, the single crystal structure of tBu-TEBT was successfully obtained which helped us analyze the origin of the physical mechanism of resistive switching behavior and establish a common structure–property relationship for high performance organic memory. Its excellent properties enable us to perform logic gates and information display functions which offer possibilities as smart sensor in Internet of Things (IoT) application.

Tunable lanthanum doping in double perovskite films for read-only memory

Rare earth ions have been widely investigated as active dopants in various materials because of their distinct optical, electronic and magnetic properties. Halide perovskite resistive memories have attracted increasingly recent interest for tremendous potential applications in computation and data storage techniques. However, it remains unexplored to implement rare earth doping strategy for tuning perovskite resistive memories. Here, we report read-only memories based on environment-friendly and air-stable lead-free double perovskite Cs2AgBiBr6 films, which is particularly tuned by rare earth La3+ doping. We use the vacuum sublimation and solution processing method to obtain Cs2AgBiBr6 crystals doped by different content La3+, and fabricate resistive memory devices based on doped Cs2AgBiBr6 films. The simplest sandwich-like structure composed of ITO/La-doped-Cs2AgBiBr6/Ag only is designed in cross-bar array architecture with high-integration and simple operation. The resistive memory device of La-doped Cs2AgBiBr6 films demonstrate a typical write-once-read-many-times (WORM) behavior with low onset voltage of 1 V and long retention time of 12000 s. In particular, the ON/OFF ratio of the La-doped Cs2AgBiBr6 film is 100 times higher than that of the undoped Cs2AgBiBr6 film. This study provides a new insight to design and manipulate memory devices based on lead-free halide perovskite materials through doping effect of rare earth ions.