Organic Resistive Random Access Memory Research Articles

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.

Molecular-Shape-Controlled Binary to Ternary Resistive Random-Access Memory Switching of N-Containing Heteroaromatic Semiconductors.

In organic resistive random-access memory (ReRAM) devices, deeply understanding how to control the performance of π-conjugated semiconductors through molecular-shape-engineering is important and highly desirable. Herein, we design a family of N-containing heteroaromatic semiconductors with molecular shapes moving from mono-branched 1Q to di-branched 2Q and tri-branched 3Q. We find that this molecular-shape engineering can induce reliable binary to ternary ReRAM switching, affording a highly enhanced device yield that satisfies the practical requirement. The density functional theory calculation and experimental evidence suggest that the increased multiple paired electroactive nitrogen sites from mono-branched 1Q to tri-branched 3Q are responsible for the multilevel resistance switching, offering stable bidentate coordination with the active metal atoms. This study sheds light on the prospect of N-containing heteroaromatic semiconductors for promising ultrahigh-density data-storage ReRAM application.

Enhancement of resistive switching behavior of organic resistive random access memory devices through UV-Ozone treatment

We fabricated organic resistive random-access memory (RRAM) devices using a low-cost solution-process method. All the processes were performed at temperatures below 135 °C under ambient atmospheric conditions. The RRAM resistive switching layer was formed from a polymer-fullerene bulk heterojunction using poly(3-hexylthiophene-2,5-diyl) (P3HT) and (6,6)-phenyl C61 butyric acid methyl ester (PCBM). The fabricated organic RRAM device exhibited typical nonvolatile bipolar resistive switching behavior with an ON/OFF ratio of ∼40, but it provided a low endurance of 27 cycles. Therefore, for enhanced stability, simple UV–Ozone (UVO) treatment was applied to the P3HT:PCBM organic bulk heterojunction layer. The organic RRAM device with UVO treatment exhibited an enhanced performance with an ON/OFF ratio of ∼400 and an endurance of 47 cycles. In addition, complementary resistive switching behavior was observed. The conduction mechanisms of the organic RRAM device were investigated by fitting the measured I–V data to numerical equations, and Schottky emission and Ohmic conduction were the main conduction mechanisms for the high-resistance and low-resistance states for the RRAM device with or without UVO treatment.

Unraveling the Role of Polydopamines in Resistive Switching in Al/Polydopamine/Al Structure for Organic Resistive Random-Access Memory.

In an era of rapidly evolving artificial intelligence and 5G communications technologies, massive data storage and processing are required for the real-time operation of digital processors in conventional wearable devices. However, classical von-Neumann architecture computers are limited by bottleneck-related issues. As a solution, resistive random-access memory (RRAM) devices are being considered as next generation in-memory computing devices. Among various materials, a polydopamine (PDA) is an attractive candidate for the fabrication of wearable and flexible RRAM devices. Herein, an aluminum/PDA/aluminum structure is proposed to investigate the influence of the PDA layer on resistive switching. The resistance-switching characteristics of an Al/PDA/Al structure are investigated by changing the PDA’s coating time and an on/off ratio of 2.48 × 103 is recorded. X-ray photoelectron spectroscopy reveals the presence of an Al2O3 layer in Al/PDA/Al structure, and the contents of oxygen vacancies are changed according to PDA coating time. Conductive filaments in the PDA/Al structure are confirmed by conductive atomic-force microscopy. As an application, a flexible Al/PDA/Al structure is fabricated using polyethylene terephthalate substrate and its operation is successfully confirmed. These results describe the resistive-switching characteristics, including oxygen vacancies, of Al/PDA/Al structures and provide new ways of understanding the resistive-switching mechanism of PDA-based RRAM devices.

Fully solution-processed organic RRAM device with highly stable butterfly-shaped hysteresis

Resistive random access memory (RRAM) is considered a promising next-generation memory device owing to its low energy consumption and ultra-high speed. In this study, we demonstrated a vacuum-free and low-cost solution to fabricate a highly stable nonvolatile RRAM device. Organic materials such P3HT and PEDOT:PSS were used to form the active layer and top electrode, respectively. The fabricated organic RRAM has a structure of ITO/P3HT/PEDOT:PSS, and exhibits well-controlled and highly stabilized nonvolatile butterfly shape resistive switching behavior, with an endurance of up to 1000 repeated cycling tests. Schottky emission, hopping, and direct tunneling were evaluated for switching mechanism. The geometrical structure was analyzed using the field emission scanning electron microscopy image, and the switching mechanisms were confirmed through the ultraviolet photoelectron spectroscopy measurement and energy band diagram analysis. Our results indicate that the fabricated solution-processed organic RRAM devices have the potential for use in nonvolatile memory devices.

Electroforming-free flexible organic resistive random access memory based on a nanocomposite of poly(3-hexylthiophene-2,5-diyl) and orange dye with a low threshold voltage

The charge trapping characteristics of an organic nanocomposite based on two polymers, poly(3-hexylthiophene-2,5-diyl) (P3HT) and orange dye (OD), were investigated by fabricating a sandwiched structure on a flexible polyethylene terephthalate substrate. The fabricated flexible organic device with a configuration of silver (Ag)/P3HT–OD/Ag displayed highly stable results consistent with a bipolar non-volatile resistive random access memory (RRAM) device using simple and controllable fabrication technology. It was observed that by the addition of OD, low threshold voltages (V th) of V set = 1.5 V and V reset = −1.5 V were achieved owing to the values of work function and energy bandgap of the materials used. Electrical rewritable effects of the fabricated memory device were tested for 102 voltage sweeps and 104s without any considerable degradation of its bistable resistive states. The switching mechanism was considered to be space charge limited current and this was verified by plotting the double logarithmic I–V curve. This is the first RRAM device based on P3HT polymer whose mechanical robustness has been tested for its mechanical robustness by bending it for 25 bending cycles at a bending diameter ranging from 15 cm to 5 cm without any considerable change in its properties. This finding offers important guidelines for reproducing next-generation flexible organic nanocomposite-based memory devices that are simple to fabricate and have low operating voltages and highly stable memory behavior.

Tip-enhanced electric field-driven efficient charge injection and transport in organic material-based resistive memories

Organic materials show promise as switching layers for resistive random access memory (RRAM). However, practical application has been limited by inefficient charge injection and transport in typical organic materials. This study proves that local enhancement of electric fields through structured electrodes can improve charge injection and transport in organic RRAM. Specifically, pyramid-structured electrodes with an extremely sharp tip are introduced into RRAM with a polyimide (PI) switching layer. The electric field in the pyramid-structured RRAM can be significantly enhanced only at the tip, thereby facilitating charge injection at the electrode/PI interface and charge transport through the PI switching layer. Indeed, the resulting RRAM exhibits low and reliable operating voltages (SET: 1.76 V ± 0.41 V / RESET: −0.49 V ± 0.15 V) compared with conventional PI-based RRAMs with planar electrodes. The conductive path formed in the tip region is observed directly using conductive atomic force microscopy, demonstrating that resistive switching occurs by tip-enhanced electric fields. Also, the charge transport mechanism follows the space charge-limiting current model modified by the Poole–Frenkel effect. These results provide an effective strategy to control the charge concentration, injection, and transport in organic RRAM, for realization of low-cost, large-area, shape-deformable data storage devices.

Performance improvement of organic resistive memories by exploiting synergistic layered nanohybrid dispersed polymer composites

Functionalized layered nanohybrid materials have been thoroughly explored in numerous practical applications including photocatalysis, supercapacitors, and sensors, and have inherent potential to excel in the field of organic resistive random access memory devices as well. Herein, we report an efficient, facile, and cost-effective in situ synthesis route to prepare a layered nanohybrid by grafting cadmium sulfide (CdS) nanorods with exfoliated graphene oxide (GOs) sheets via the one-step hydrothermal functionalization technique. Such a nanohybrid material displays advantageous characteristics possessed by them due to the formation of organic–inorganic heterojunctions at their interface. Sandwiched devices, with a configuration of ITO/reduced graphene oxide–CdS/poly-methyl methacrylate (PMMA)/Al, are fabricated by dispersing different wt. % of the layered nanohybrid in a poly-methyl methacrylate (PMMA) matrix. Detailed current density vs voltage (J–V) studies exhibit excellent bipolar resistive switching characteristics reaching a very low set voltage of ∼−0.48V, high current ION/IOFF ratios of ∼106, and ultralow set/reset power densities of ∼10−8/10−6W/cm2 for devices containing 0.4 wt. % nanohybrid nanofillers. Partial reduction of GOs and the formation of heterojunctions during the synthesis process undoubtedly boost the resistive memory performances resulting from the synergistic effect in layered nanohybrid materials. The fabricated devices have been subjected to various analyses in order to test their stability, repeatability, and reliability, all of which have been found to display encouraging results.

Photo-tunable organic resistive random access memory based on PVP/N-doped carbon dot nanocomposites for encrypted image storage

A photo-tunable and flexible organic RRAM device based on poly(4-vinylphenol) (PVP) and N-doped carbon quantum dot nanocomposites for encrypted image storage.

Fluorenone/carbazole based bipolar small molecules for non-volatile memory devices

Developing new-style non-volatile memory is an important part to solve von Neumann bottleneck. Among numerous studies of memory device, resistive random-access memory (RRAM) is widely researched for their simple geometrical structure and high integration density. In recent decades, organic RRAM catches many researchers’ attention for their superiorities such as outstanding flexibility, simple solution processability, low-cost fabrication and tunable functionalities by changing the chemical groups. Herein, a series of organic small molecules 2,7-di(9H-carbazol-9-yl)-9H-fluoren-9-one (FLOCZ) and 2,7-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)-9H-fluoren-9-one (FLOBCZ) are synthesized and their band structures and electronic states are systematically studied with the assistance of density function theory (DFT). Then, using simple solution-processing approach, FLOCZ/FLOBCZ based RRAMs are fabricated and their memory characteristics are studied. The as-fabricated Ag/FLOBCZ/ITO RRAM exhibits non-volatile memory effect with a memory window up to 7 × 103 and data retention time of more than 2 × 104 s, which can meet the requirement of data storage. Meanwhile, by controlling compliance current on the RRAM precisely, multi-level data storage can be implemented experimentally. Our work extends the applications of organic small molecule and promotes the evolution of organic RRAM.

Effects of the Molecular Chain Length of Polyimide on the Characteristics of Organic Resistive Random Access Memories

We developed organic resistive random access memory (ReRAM) devices using spin-coated polyimide (PI) as the resistive layer. In this article, the effect of the chain length of the PI macromolecules on the electrical performance of ReRAM devices was studied. The chain length of the PI macromolecule was controlled by repeating the addition step of 4, $4^\prime $ -diaminodioxydianiline in a polyamic acid precursor. A long molecular chain of the PI film could be obtained by increasing the addition times. The electrical properties of the PI-based ReRAM devices revealed that the leakage current in the high-resistance state and memory window can be improved by increasing the molecular chain length of the PI film. A model is proposed to explain the effects of the chain length on the conduction mechanism in the PI film. Moreover, PI films with long molecular chains exhibited an increased retention time.

Integration of biocompatible organic resistive memory and photoresistor for wearable image sensing application

The integration of multiple functional devices to achieve complex functions has become an essential requirement for future wearable biomedical electronic devices and systems. In this paper, we present a flexible multi-functional device composed of a biocompatible organic polymer resistive random-access memory (RRAM) and a photoresistor for wearable image sensing application. The resistive layer of organic polymer RRAM is composed by polychloro-para-xylylene (parylene-C), which is a flexible, transparent, biocompatibility and chemical stability polymer material. What is more, parylene-C is quite safe to be used within human body as it is a Food and Drug Administration (FDA)-approved material. This organic RRAM shows stable switching characteristics, low operation voltages (3.25 V for set voltage and $-$0.55 V for reset voltage), low static power consumption, high storage window and good retention properties ($>$10$^4$ s). A multi-functional device that can detect the light intensity of incident light and simultaneously store the information in the memory devices for wearable image sensing application was proposed and fabricated by integrating the organic resistive memory and a photoresistor. The threshold of incident light intensity can be easily adjust by changing the external voltage. This device is promising for building wearable electronic systems with various multiple functionalities.

Ferroelectric-mediated filamentary resistive switching in P(VDF-TrFE)/ZnO nanocomposite films.

In ferroelectric (FE) polymer-semiconducting polymer blend based organic resistive random access memory devices (ReRAM), the carriers are injected into the semiconductor region of the blend because of the polarization originated internal electric field in the FE polymer. A higher concentration of semiconducting polymer in the FE polymer-semiconducting polymer blends usually generate a high leakage current and degrades the FE characteristics of the FE polymer resulting in a high OFF current and consequently a low ON/OFF ratio. In order to achieve a high ON/OFF ratio in the FE polymer/semiconducting polymer blends, the FE properties of the FE polymer should be preserved. In this study, organic ReRAMs based on ferroelectric poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and ZnO nanoparticle (NPs) blends exhibiting bipolar resistive switching and a high ON/OFF ratio were realized using a low-cost solution process. Unlike conventional ferroelectric polymer-semiconducting polymer blend systems where FE characteristics are suppressed in ReRAMs, our Au/P(VDF-TrFE)_ZnO NPs/n++Si devices retain the FE characteristics of the P(VDF-TrFE) polymers. Our devices switch between bi-stable resistance states via the ferroelectric-assisted filamentary conduction mechanism. Based on ex situ transmission electron microscopy and elemental mapping analyses, we found that the resistive switching occurs through the formation of conduction paths consisting of Zn-rich/F-deficient regions. The device fabricated at a blend ratio of 20 wt% ZnO NPs in P(VDF-TrFE) matrix exhibited optimal stable resistive switching behavior with an ON/OFF ratio of up to 2 × 107 and a retention time of 104 s.

Graphdiyne for multilevel flexible organic resistive random access memory devices

A novel carbon material graphdiyne and thermally deposited Al–Al2O3core–shell NPs were employed to realize flexible multilevel RRAM.

A flexible organic resistance memory device for wearable biomedical applications

Parylene is a Food and Drug Administration (FDA)-approved material which can be safely used within the human body and it is also offers chemically inert and flexible merits. Here, we present a flexible parylene-based organic resistive random access memory (RRAM) device suitable for wearable biomedical application. The proposed device is fabricated through standard lithography and pattern processes at room temperature, exhibiting the feasibility of integration with CMOS circuits. This organic RRAM device offers a high storage window (>104), superior retention ability and immunity to disturbing. In addition, brilliant mechanical and electrical stabilities of this device are demonstrated when under harsh bending (bending cycle >500, bending radius <10 mm). Finally, the underlying mechanism for resistance switching of this kind of device is discussed, and metallic conducting filament formation and annihilation related to oxidization/redox of Al and Al anions migrating in the parylene layer can be attributed to resistance switching in this device. These advantages reveal the significant potential of parylene-based flexible RRAM devices for wearable biomedical applications.

Direct Observation of a Carbon Filament in Water-Resistant Organic Memory.

The memory for the Internet of Things (IoT) requires versatile characteristics such as flexibility, wearability, and stability in outdoor environments. Resistive random access memory (RRAM) to harness a simple structure and organic material with good flexibility can be an attractive candidate for IoT memory. However, its solution-oriented process and unclear switching mechanism are critical problems. Here we demonstrate iCVD polymer-intercalated RRAM (i-RRAM). i-RRAM exhibits robust flexibility and versatile wearability on any substrate. Stable operation of i-RRAM, even in water, is demonstrated, which is the first experimental presentation of water-resistant organic memory without any waterproof protection package. Moreover, the direct observation of a carbon filament is also reported for the first time using transmission electron microscopy, which puts an end to the controversy surrounding the switching mechanism. Therefore, reproducibility is feasible through comprehensive modeling. Furthermore, a carbon filament is superior to a metal filament in terms of the design window and selection of the electrode material. These results suggest an alternative to solve the critical issues of organic RRAM and an optimized memory type suitable for the IoT era.

Record Low-Power Organic RRAM With Sub-20-nA Reset Current

In this letter, organic resistive random access memory (RRAM) devices based on double-layer polychloro-para-xylylene (parylene-C) are fabricated, which show stable bipolar resistive switching behavior, excellent data retention, and high scalability. Moreover, extremely low reset current of sub-20 nA and set current of 0.15 μA are obtained with adequate switching margin for the first time in the field of organic RRAM, almost 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> times lower than that of the single-layer parylene-C cells, exhibiting great potentials for future low-power applications. Possible mechanism for the ultralow operating current of double-layer device is discussed.

Congenital malformations of the hand and forearm

Resistive random access memory (RRAM) is considered a promising next-generation memory device owing to its low energy consumption and ultra-high speed. In this study, we demonstrated a vacuum-free and low-cost solution to fabricate a highly stable nonvolatile RRAM device. Organic materials such P3HT and PEDOT:PSS were used to form the active layer and top electrode, respectively. The fabricated organic RRAM has a structure of ITO/P3HT/PEDOT:PSS, and exhibits well-controlled and highly stabilized nonvolatile butterfly shape resistive switching behavior, with an endurance of up to 1000 repeated cycling tests. Schottky emission, hopping, and direct tunneling were evaluated for switching mechanism. The geometrical structure was analyzed using the field emission scanning electron microscopy image, and the switching mechanisms were confirmed through the ultraviolet photoelectron spectroscopy measurement and energy band diagram analysis. Our results indicate that the fabricated solution-processed organic RRAM devices have the potential for use in nonvolatile memory devices.

Homme, société, évolution

Resistive random access memory (RRAM) is considered a promising next-generation memory device owing to its low energy consumption and ultra-high speed. In this study, we demonstrated a vacuum-free and low-cost solution to fabricate a highly stable nonvolatile RRAM device. Organic materials such P3HT and PEDOT:PSS were used to form the active layer and top electrode, respectively. The fabricated organic RRAM has a structure of ITO/P3HT/PEDOT:PSS, and exhibits well-controlled and highly stabilized nonvolatile butterfly shape resistive switching behavior, with an endurance of up to 1000 repeated cycling tests. Schottky emission, hopping, and direct tunneling were evaluated for switching mechanism. The geometrical structure was analyzed using the field emission scanning electron microscopy image, and the switching mechanisms were confirmed through the ultraviolet photoelectron spectroscopy measurement and energy band diagram analysis. Our results indicate that the fabricated solution-processed organic RRAM devices have the potential for use in nonvolatile memory devices.

Queensland branch

Resistive random access memory (RRAM) is considered a promising next-generation memory device owing to its low energy consumption and ultra-high speed. In this study, we demonstrated a vacuum-free and low-cost solution to fabricate a highly stable nonvolatile RRAM device. Organic materials such P3HT and PEDOT:PSS were used to form the active layer and top electrode, respectively. The fabricated organic RRAM has a structure of ITO/P3HT/PEDOT:PSS, and exhibits well-controlled and highly stabilized nonvolatile butterfly shape resistive switching behavior, with an endurance of up to 1000 repeated cycling tests. Schottky emission, hopping, and direct tunneling were evaluated for switching mechanism. The geometrical structure was analyzed using the field emission scanning electron microscopy image, and the switching mechanisms were confirmed through the ultraviolet photoelectron spectroscopy measurement and energy band diagram analysis. Our results indicate that the fabricated solution-processed organic RRAM devices have the potential for use in nonvolatile memory devices.