High temperature polyimide containing anthracene moiety and its structure, interface, and nonvolatile memory behavior

Electron-Sponge Nature of Polyoxometalates for Next-Generation Electrocatalytic Water Splitting and Nonvolatile Neuromorphic Devices

Designing next-generation molecular devices typically necessitates plentiful oxygen-bearing sites to facilitate multiple-electron transfers. However, the theoretical limits of existing materials for energy conversion and information storage devices make it inevitable to hunt for new competitors. Polyoxometalates (POMs), a unique class of metal-oxide clusters, have been investigated exponentially due to their structural diversity and tunable redox properties. POMs behave as electron-sponges owing to their intrinsic ability of reversible uptake-release of multiple electrons. In this review, numerous POM-frameworks together with desired features of a contender material and inherited properties of POMs are systematically discussed to demonstrate how and why the electron-sponge-like nature of POMs is beneficial to design next-generation water oxidation/reduction electrocatalysts, and neuromorphic nonvolatile resistance-switching random-access memory devices. The aim is to converge the attention of scientists who are working separately on electrocatalysts and memory devices, on a point that, although the application types are different, they all hunt for a material that could exhibit electron-sponge-like feature to realize boosted performances and thus, encouraging the scientists of two completely different fields to explore POMs as imperious contenders to design next-generation nanodevices. Finally, challenges and promising prospects in this research field are also highlighted.

Metal-containing organic compounds for memory and data storage applications

With the upcoming trend of Big Data era, some new types of memory technologies have emerged as substitutes for the traditional Si-based semiconductor memory devices, which are encountering severe scaling down technical obstacles. In particular, the resistance random access memory (RRAM) and magnetic random access memory (MRAM) hold great promise for the in-memory computing, which are regarded as the optimal strategy and pathway to solve the von Neumann bottleneck by high-throughput in situ data processing. As far as the active materials in RRAM and MRAM are concerned, organic semiconducting materials have shown increasing application perspectives in memory devices due to their rich structural diversity and solution processability. With the introduction of metal elements into the backbone of molecules, some new properties and phenomena will emerge accordingly. Consequently, the RRAM and MRAM devices based on metal-containing organic compounds (including the small molecular metal complexes, metallopolymers, metal-organic frameworks (MOFs) and organic-inorganic-hybrid perovskites (OIHPs)) have been widely explored and attracted intense attention. In this review, we highlight the fundamentals of RRAM and MRAM, as well as the research progress of the applications of metal-containing organic compounds in both RRAM and MRAM. Finally, we discuss the challenges and future directions for the research of organic RRAM and MRAM.

Toward Highly Robust Nonvolatile Multilevel Memory by Fine Tuning of the Nanostructural Crystalline Solid-State Order

Organic resistive memory (ORM) offers great promise for next-generation high-density multilevel-cell (MLC) data storage. However, the fine tuning of crystalline order among its active layer still remains challenging, which largely restricts ORM behavior. Here, an exceptional solid-state transition from disordered orientations to highly-uniform orientation within the ORM layer is facilely triggered via molecular strategic tailoring. Two diketopyrrolopyrrole-based small molecular analogues (NI₁ TDPP and NI₂ TDPP) are demonstrated to display different symmetry. The asymmetric NI₁ TDPP shows an irregular solid-state texture, while the centro-symmetric NI₂ TDPP conforms to an ordered out-of-plane single-crystalline pattern that aligns with the foremost charge transportation along the substrate normal, and exhibits excellent MLC memory characteristics. Moreover, this highly oriented pattern guarantees the large-area film uniformity, leading to the twofold increase in the yield of as-fabricated ORM devices. This study reveals that the solid-state crystalline nanostructural order of organic materials can be controlled by reasonable molecular design to actuate high-performance organic electronic circuits.

Chemically Controlled Volatile and Nonvolatile Resistive Memory Characteristics of Novel Oxygen-Based Polymers

Recent advancements in modern microelectronics continuously increase the data storage capacity of modern devices, but they require delicate and costly fabrication processes. As alternatives to conventional inorganic based semiconductors, semiconducting polymers are of academic and industrial interest for their cost-efficiency, power efficiency, and flexible processability. Here, we have synthesized a series of novel oxygen-based polymers through the postmodification reactions of poly(ethylene-alt-maleate) with various oxybenzyl alcohol derivatives. The oxygen-based polymers are thermally stable up to 180 °C, and their nanoscale film devices exhibit reliable, power efficient p-type unipolar volatile and nonvolatile resistive memory characteristics with high ON/OFF current ratios. Additionally, when given a higher number of oxygen atoms in oxyphenyl side groups, the thin film polymer devices demonstrate a wide operational film thickness range. The memory characteristics depend on the oxyphenyl moieties functioning as charge trap sites, where a combination of Schottky emission and trap-limited space charge limited conductions in OFF-state and hopping conduction in ON-state are observed. This study demonstrates the chemical incorporation of oxyphenyl derivatives into polymer dielectrics as a powerful development tool for p-type resistive memory materials.

High-performance non-volatile resistive switching memory based on a polyimide/graphene oxide nanocomposite

Chemical structure of PI-GO, schematic structure of the ITO/PI-GO/Al device and its memory characteristics.

Novel carbazole-based donor-isoindolo[2,1-a]benzimidazol-11-one acceptor polymers for ternary flash memory and light-emission

Three novel donor-acceptor polymers (PCz0, PCz2 and PCz4) based on 9-(9-heptadecanyl)-9H-carbazole and isoindolo[2,1-a]benzimidazol-11-one with fluorine substituents (0, 2 and 4) on the acceptor unit were prepared by Suzuki polymerization. The synthesized polymers were studied by theoretical calculation, and optical and electrochemical characterizations to further investigate the performance of memory storage and light-emission. The memory devices of the three polymers all exhibited obvious ternary flash behavior with total ON/OFF state current ratio around 104 and threshold voltages below 3.0 V, with no other blending or doping. Bright emissions of an electroluminescence device based on PCz0 was obtained at 556 nm, the maximum brightness was 2006 cd m-2 with EQE of 0.21%. The results suggested that polymers with the structure of carbazole-based donor as backbone and isoindolo[2,1-a]benzimidazol-11-one segment as acceptor could be excellent materials for memory storage and light-emitting applications with further investigation and could be used for further design of other new polymer systems.

Preparation of Polyimide/Graphene Oxide Nanocomposite and Its Application to Nonvolatile Resistive Memory Device

2,6-Diaminoanthracene (AnDA)-functionalized graphene oxide (GO) (AnDA-GO) was prepared and used to synthesize a graphene oxide-based polyimide (PI-GO) by the in-situ polymerization method. A PI-GO nanocomposite thin film was prepared and characterized by infrared (IR) spectroscopy, thermogravimetric analysis (TGA) and UV-visible spectroscopy. The PI-GO film was used as a memory layer in the fabrication of a resistive random access memory (RRAM) device with aluminum (Al) top and indium tin oxide (ITO) bottom electrodes. The device showed write-once-read-many-times (WORM) characteristics with a high ON/OFF current ratio (Ion/Ioff = 3.41 × 10⁸). This excellent current ratio was attributed to the high charge trapping ability of GO. In addition, the device had good endurance until the 100th cycle. These results suggest that PI-GO is an attractive candidate for applications in next generation nonvolatile memory.

Thiadizoloquinoxaline-Based N-Heteroacenes as Active Elements for High-Density Data-Storage Device

A novel thiadiazoloquinoxaline (TQ)-based donor-acceptor (D-A)-type N-heteroacene (Py-1-TQ) has been demonstrated for promising applications in organic multilevel resistive memory devices. Compared with its counterparts (Py-0-TQ and Py-2-TQ), which show flash-type binary memory behaviors, Py-1-TQ exhibits excellent nonvolatile write-once-read-many-times-type ternary memory effects with high ON2/ON1/OFF current ratios (10^5.8:10^3.4:1), which can be attributed to the different electron-withdrawing abilities between the pyrazine unit and TQ species that can induce stepwise D-A charge-transfer processes. These results suggest that TQ-based N-heteroacenes can be potentially useful in ultrahigh-density data-storage devices through the rational D-A tuning.

Solvents Effects on Film Morphologies and Memory Behavior of a Perylenediimide-Containing Pendent Polymer

The large polydispersity index of functional pendant polymers has hindered their application in semiconductors. Herein, a novel pendant polymer with perylenediimide (PDI) in the side chains was successfully synthesized through ring-opening metathesis polymerization (ROMP) with a very low polydispersity index. The synthesized polymers were spin-coated on indium tin oxide (ITO) substrate by using a mixture of 1,2-dichlorobenzene (o-DCB) and methanol (MeOH) solvents. The surface morphologies and intermolecular π-π stacking of the fabricated film could be adjusted through tuning of the ratio of o-DCB and MeOH, and thus, the sandwich-structured device of ITO/polymer/aluminum exhibited different electrical behavior. The threshold voltages of the devices decreased as the MeOH content was increased from 0 to 30 % (v/v); however, the device changed from being unrewritable to rewritable if the MeOH content was increased to 40 %; a probable mechanism for this process is discussed. It is hoped that this new idea of synthesizing narrow polydispersity index pendant polymers, and the fabrication of high-quality films through the use of a mixture of solvents could allow high-performance memory devices to be prepared in the future.

Synthesis and non-volatile electrical memory characteristics of triphenylamine-based polyimides with flexibility segments

Two triphenylamine-based polyimides (PI(TPA-PMDA) and PI(TPA-BPDA)) containing a flexibility segments were prepared. The memory device of ITO/PI(TPA-PMDA)/Al exhibited write-once read-many-times (WORM) memory behavior, however, the memory device of PI(TPA-BPDA) demonstrated flash-type memory characteristics.

Organic Electronic Memory Devices

With the rapid development of the electronics industry in recent years, information technology devices, such as personal computers, mobile phones, digital cameras and media players, have become an essential part of our daily life. From both the technological and economic points of view, the development of novel information storage materials and devices has become an emergent issue facing the electronics industry. Due to the advantages of good scalability, flexibility, low cost, ease of processing, 3D-stacking capability and high capacity for data storage, organic-based electrical memory devices have been promising alternatives or supplementary devices to conventional inorganic semiconductor-based memory technology. The basic concepts and historical development of electronic memory devices are first presented. The following section introduces the structures and switching mechanisms of organic electronic memory devices classified as transistors, capacitors and resistors. Subsequently, the progress in the field of organic-based memory materials and devices is systematically summarized and discussed. Finally, the challenges posed to the development of novel organic electronic memory devices are summarized.

Adjustment of ON-state retention ability based on new donor-acceptor imides through structural tailoring for volatile device applications

In this study, two D-A molecules NACANA and CANACA, based on carbazole (CA) donor and naphthalimide (NA) acceptor, with different D-A arrangement (A-D-A and D-A-D) were synthesized. The photophysical and electrochemical properties, microstructure and memory behaviors of both A-D-A and D-A-D molecules were systematically investigated. The fabricated devices ITO/NACANA or CANACA layer/Al with a simple sandwich configuration both exhibited volatile nature after shutting off the external electric field. Interestingly, NACANA showed ON-state retention time of ca. 12 min, longer than that of CANACA (ca. 6 min). The difference in retention ability of the programmed states could be assigned to the difference of the D-A arrangement. This type of retention ability adjustment by varying the arrangement of donor and acceptor segments may provide a guide of structure design for future organic-based specific memory devices with tunable volatile property.

Polymer-based resistive memory materials and devices

Due to the advantages of good scalability, flexibility, low cost, ease of processing, 3D-stacking capability, and large capacity for data storage, polymer-based resistive memories have been a promising alternative or supplementary devices to conventional inorganic semiconductor-based memory technology, and attracted significant scientific interest as a new and promising research field. In this review, we first introduced the general characteristics of the device structures and fabrication, memory effects, switching mechanisms, and effects of electrodes on memory properties associated with polymer-based resistive memory devices. Subsequently, the research progress concerning the use of single polymers or polymer composites as active materials for resistive memory devices has been summarized and discussed. In particular, we consider a rational approach to their design and discuss how to realize the excellent memory devices and understand the memory mechanisms. Finally, the current challenges and several possible future research directions in this field have also been discussed.

Thermally-stable resistive switching with a large ON/OFF ratio achieved in poly(triphenylamine)

Thermally stable poly(triphenylamine) (PTPA) has been used to achieve promising resistive switching memory devices with a large ON/OFF ratio.

Tunable electrical memory characteristics using polyimide:polycyclic aromatic compound blends on flexible substrates

Resistance switching memory devices with the configuration of poly(ethylene naphthalate)(PEN)/Al/polyimide (PI) blend/Al are reported. The active layers of the PI blend films were prepared from different compositions of poly[4,4'-diamino-4″-methyltriphenylamine-hexafluoroisopropylidenediphthalimide] (PI(AMTPA)) and polycyclic aromatic compounds (coronene or N,N-bis[4-(2-octyldodecyloxy)phenyl]-3,4,9,10-perylenetetracarboxylic diimide (PDI-DO)). The additives of large π-conjugated polycyclic compounds can stabilize the charge transfer complex induced by the applied electric field. Thus, the memory device characteristic changes from the volatile to nonvolatile behavior of flash and write-once-read-many times (WORM) as the additive contents increase in both blend systems. The main differences between these two blend systems are the threshold voltage values and the additive content to change the memory behavior. Due to the stronger accepting ability and higher electron affinity of PDI-DO than those of coronene, the PI(AMTPA):PDI-DO blend based memory devices show a smaller threshold voltage and change the memory behavior in a smaller additive content. Besides, the memory devices fabricated on a flexible PEN substrate exhibit an excellent durability upon the bending conditions. These tunable memory performances of the developed PI/polycyclic aromatic compound blends are advantageous for future advanced memory device applications.

Structure and macroscopic tackiness of ultrathin pressure sensitive adhesive films

Ultrathin layers of the statistical copolymer P(nBA-stat-MA) with a majority of n-butyl acrylate (nBA) and a minority of methyl acrylate (MA) are characterized with respect to the film morphology and the mechanical response in a probe tack test. The probed copolymer can be regarded as a model system of a pressure sensitive adhesive (PSA). The films are prepared by spin-coating which enables an easy thickness control via the polymer concentration of the solution. The film thickness is determined with x-ray reflectivity (XRR) and white light interferometry (WLI). Grazing incidence small angle x-ray scattering (GISAXS) provides detailed and statistically significant information about the film morphology. Two types of lateral structures are identified and no strong correlation of these structures with the PSA film thickness is observed. In contrast, prominent parameters of the probe tack test, such as the stress maximum and the tack energy, exhibit an exponential dependence on the film thickness.

Resistive switching induced by metallic filaments formation through poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate)

We report the design and fabrication of Al/poly(3,4-ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/Cu resistive memory devices that utilize the Cu redox reaction and conformational features of PEDOT:PSS to achieve resistive switching. The top Cu electrode acts as the source of the redox ions that are injected through the PEDOT:PSS layer during the forming process. The Cu filament is confirmed directly using the cross-sectional images of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The resultant resistive memory devices can operate over a small voltage range, i.e., the switching-on threshold voltage is less than 1.5 V and the absolute value of the switching-off threshold voltage is less than 1.0 V. The on/off current ratio is as large as 1 × 10(4) and the two different resistance states can be maintained over 10(6) s. Moreover, the devices present good thermal stability that the resistive switching can be observed even at temperature up to 160 °C, at which the oxidation of the Cu top electrode is the failure factor. Furthermore, the cause of failure for Al/PEDOT:PSS/Cu memory devices at higher temperature is confirmed to be the oxidation of Cu top electrode.