Redox-active ferrocene-containing iridium(III) complex for non-volatile flash memory

We demonstrated unipolar organic bistable memory devices with 8 x 8 cross-bar array type structure. The active material for the organic non-volatile memory devices is poly(styrene-co-styrenesulfonic acid Na) (PSSANa). From the electrical measurements of the PSSANa organic memory devices, we observed rewritable unipolar switching behaviors with a stable endurance and narrow cumulative probability. Also the PSSANa memory devices exhibited a uniform cell-to-cell switching with a high ON/OFF ratio of approximately 10(5) and good retention time of approximately 10(4) seconds without significant degradation.

In recent years, polymer systems have attracted significant research interest for memory device applications due to their tunable electrical properties through molecular design and good processibility. The representative classes of polymer materials used for fabrication on memories are conjugated polymers, non-conjugated polymers (functional polyimide systems or polymers with specific pendent donor or acceptor moiety) and polymer nanocomposites (metal nanoparticle or fullerene embedded). However, the effects of polymer structure on the memory characteristics have not been fully explored yet. In this thesis, the homopolymers pendant with donors or acceptors and related copolymers containing of both donor and accepter units were synthesized and evaluated on memory devices. In chapter 2, the homopolymers with pendant para-substituted dicyano- triphenylamine (D, as donor) or three different oxadiazole derivatives (2-phenyl-5-(4-vinylphenyl)-1,3,4-oxadiazole (A1), 2-(4-vinylbiphenyl)-5-(4-phenyl)- 1,3,4-oxadiazole (A2), and 2-(4-vinylbiphenyl)-5-(4-ethoxyphenyl)-1,3,4-oxadiazole (A3), as electron acceptor) and also related random copolymers consisting of pendant donor and acceptor moieties (the ratio of donor:acceptor is 8:2, 5:5 and 2:8, respectively) were synthesized by Nitroxide-Mediated Living Free Radical Polymerization (NMP) and characterized by 1H NMR spectrum and element analysis (EA). The para-substituted dicyano groups on triphenylamine improve the stability without the dimerization. The side-chain acceptor A2 shows more conjugated length relative to A1 by introducing another benzene ring, and A3 system further enhances the electronic density by adding terminal electron donating moiety (-OEt). The two thin film absorbance peaks of copolymers at 353 nm and 308 ~ 332 nm belong to the corresponding homopolymers (PD at 355 nm, PA1 at 308 nm, PA2 at 330 nm, PA3 at 332 nm, respectively) and the relative absorbance of two peaks vary with donor and acceptor ratios. The band gaps calculated from absorption edge are in the range of 3.11 ~ 3.20 eV and increase slightly with the larger acceptor content. The HOMO and LUMO energy levels of the random copolymers with different donor and acceptor ratios obtained from cyclic voltammetry (CV) are almost the same with the corresponding HOMO level of donor (-5.63 eV) and LUMO levels of acceptor (-2.63 ~ 2.81 eV), respectively, mainly due to poor hybridization of adjacent donor and acceptor which could be considered as an isolated system. The electronic properties can be well-tuned through different donor-acceptor structures and ratios of the copolymers systems. In chapter 3, the memory device of the random copolymers and homopoymers are fabricated and measured in ambient atmosphere. Different solvents (N,N-dimethyl formamide (DMF) or chlorobenzene (CB)) are chosen for polymer thin film processing. The bistable memory behavior is conducted by a simple sandwich device configuration consisted of spin-coated polymer film between indium-tin oxide (ITO) bottom electrode and aluminum (A1) top electrode. All the polymers exhibit nonvolatile flash type of switching behavior. The memory devices have low turn-on threshold voltage below 1 V and exhibits long retention time of 11000 seconds. The mechanism of the switching behavior is based on filamentary conduction with space charge limited current (SCLC) theory. As the polymer films prepare from DMF, the OFF state current during erasing scan increases from 10-5 A to 10-2 A after several WRER cycles. These phenomena are further obviously demonstrated in the case of the larger acceptor contents of the random copolymers (5:5 and 2:8 donor:acceptor ratio) and homopolymers pendant with acceptor due to the presence of the binding heteroatom (N) on acceptor with Al which contributes to filament conductions. Introducing electron donating ethoxy substituent (A3) which enhance the binding ability of N atom of oxadiazole group makes the memories difficult to turn off and also keep them on the high conductance since the stable filament channel are formed. The memory can still restore to the initial OFF state if the erasing process scan more negatively which also means the enhanced threshold voltage (from -3 V to -5 V) as the acceptor ratio increases. The polymer surface prepared from the CB solution is smooth and the current elevation of OFF state during the erasing switch could not be found. The study demonstrates that pendent donor/acceptor structures and ratio of copolymer and processing condition on the switching characteristics of memory device are fully explored.

Carrier transport mechanism in aluminum nanoparticle embedded AlQ 3 structures for organic bistable memory devices

Polymer electrets for organic nonvolatile memory devices: Recent advances

Recently, films created by incorporating metallic nanoparticles into organic or polymeric materials have demonstrated electrical bistability, as well as the memory effect, when subjected to an electrical bias. Organic and polymeric digital memory devices based on this bistable electronic behavior have emerged as a viable technology in the field of organic electronics. These devices exhibit fast response speeds and can form multiple‐layer stacking structures, demonstrating that organic memory devices possess a high potential to become flexible, ultrafast, and ultrahigh‐density memory devices. This behavior is believed to be related to charge storage in the organic or polymer film, where devices are able to exhibit two different states of conductivity often separated by several orders of magnitude. By defining the two states as “1” and “0”, it is now possible to create digital memory devices with this technology. This article reviews electrically bistable devices developed in our laboratory. Our research has stimulated strong interest in this area worldwide. The research by other laboratories is reviewed as well.

Organic low voltage rewritable memory device based on PEDOT:PSS/f-MWCNTs thin film

In recent years, organic resistive memory devices in which active organic materials possess at least two stable resistance states have been extensively investigated for their promising memory potential. From the perspective of device fabrication, their advantages include simple device structures, low fabrication costs, and printability. Furthermore, their exceptional electrical performances such as a nondestructive reading process, nonvolatility, a high ON/OFF ratio, and a fast switching speed meet the requirements for viable memory technologies. Full understanding of the underlying physics behind the interesting phenomena is still challenging. However, many studies have provided useful insights into scientific and technical issues surrounding organic resistive memory. This Feature Article begins with a summary on general characteristics of the materials, device structures, and switching mechanisms used in organic resistive devices. Strategies for performance enhancement, integration, and advanced architectures in these devices are also presented, which may open a way toward practically applicable organic memory devices.

Over the past few years, extremely high charge-storage capacities of glucose-based oligo- or polysaccharides have been discovered that enables such materials to hold a great potential in electronics applications due to their abundant and renewable materials. Typically, the introduction of an α-glucan-containing polysaccharide into the dielectric layer of a transistor-type memory device causes excellent irreversible electron-capture properties; hence, a transistor device embedded with oligosaccharide maltoheptaose domains expresses write-once-read-many (WORM)-type behavior, which confirms that green materials can be used for advanced electronics. However, their irreversible WORM-type characteristics also restrict the application α-glucan-containing polysaccharides in high-performance electronic devices. Recent advances in oligosaccharide-based block copolymers through the inherent immiscibility of different polymer segments can enhance not only the formation of self-assembled nanostructures but also the switching of memory properties. Furthermore, supramolecular structures composed of oligosaccharide-based block copolymers and conjugated moieties can be developed into high-performance nonvolatile organic field-effect transistor (OFET) memory devices. This mini review presents an overview of the recent literature in oligosaccharide maltoheptaose-based block copolymers and their promising applications in organic memory devices.

The D-A structured small-molecule-based resistive random-access memory (ReRAM) device has been well-researched in the last decade, and the switching mechanism was mainly induced by the intramolecular/intermolecular charge transfer processes from the donors to the acceptors. However, in the previous work, some small molecules with pristine electron acceptors in the backbone could still show the typical memory behaviors, of which the switching mechanism is still ambiguous. In this work, two 1,2,4,5-tetrazine based n-type small-molecular isomers, 2-DPTZ and 4-DPTZ, with the same electron acceptor, 1,2,4,5-tetrazine and pyridine, are chosen to investigate the isomeric effects on molecular packing, switching mechanism, and memory performance. Because of the abundant nitrogen atoms with a localized lone pair of electrons in the sp2 orbital, 2-DPTZ and 4-DPTZ compounds could self-assemble into a long-range ordered molecular packing through intermolecular C-H...N interactions, affording effective transporting tunnels for charge-carrier transport. As expected, the sandwich-structured ITO/2-DPTZ or 4-DPTZ/Al memory devices both showed binary memory characteristics, with 2-DPTZ based memory devices showing the write once read many times (WORM) memory behavior and 4-DPTZ based memory devices having the negative differential resistance (NDR) memory performance. These distinct ReRAM properties arose from the different morphologies of 2-DPTZ and 4-DPTZ films that were induced by the different packing styles between the adjacent molecules, as confirmed by X-ray diffraction (XRD) and tapping-mode atomic force microscopy (AFM) height images. Most importantly, the switching mechanism was thought to be attributed to the injected electrons that reduced the neutral molecules of 2-DPTZ and 4-DPTZ to their corresponding anion radicals. Thus, this present work helps us better understand the conducting mechanism of small molecules with pristine electron acceptors in the backbone and provides a supplementary guideline for designing multilevel small molecules to match the structure-stacking-property relationship.

Promising applications of organic materials in next-generation electronic and optoelectronic devices have driven extensive efforts to form various composites. Electronic and optoelectronic devices fabricated utilizing organic materials, such as organic thin film transistors, organic light-emitting diodes, and organic solar cells, have been extensively studied. However, relatively little work has been done on organic memory devices fabricated utilizing hybrid composites. Among the various kinds of composites, hybrid composites based on fullerene C60 and poly(N-vinylcarbazole) (PVK) organic materials have become particularly attractive because of their potential applications in organic electronic devices. Even though some works concerning the organic memory devices consisting of organic/metal/organic structures formed by using evaporation methods, very few studies concerning the memory effect of the organic/inorganic hybrid structures fabricated by using spin-coating techniques have been reported. This paper reports data for the memory effects of the effect of C60 existence on the flat-band voltage shift of nonvolatile polymer memory devices with the PVK and the C60 nanocomposites active layers was described.

Memristive devices have shown promising applications in various fields (e.g., non‐volatile memory, reconfigurable switches, and bio‐inspired neuromorphic computing). Despite the significant advances in this field, the exploration of nanomaterials with tunable composition, thickness, dimension, and conductivity for memristive devices is still in great necessity. Here, the transformation of the ultrathin film polydopamine (PDA) obtained at air/water interface from amorphous to quasi‐ordered structure is revealed. Owing to the unique feature, the self‐assembled PDA film at air/water interface is firstly employed as the active material in constructing write‐once‐read‐many‐times (WORM) memory devices. With an increase in the applied electric field, the device shows a switching from high resistance state (HRS, OFF state) to low resistance state (LRS, ON state) with a high ON/OFF current ratio of >103. Furthermore, the abundant amidogen and phenolic hydroxyl group enable the in situ Ag nanoparticle decoration on the PDA film, which can be further used in the construction of anisotropic volatile memristors with a high ON/OFF current ratio (>104), exceptional stability and good integrability.

A solution processed two terminal organic bistable memory device was fabricated utilizing films of polymethyl methacrylate PMMA/ZnO/PMMA on top of ITO coated glass. Electrical characterization of the device structure showed that the two terminal device exhibited favorable switching characteristics with an ON/OFF ratio greater than 1 x 10(4) when the voltage was swept between - 2 V and +3 V. The device maintained its state after removal of the bias voltage. The device did not show degradation after a 1-h retention test at 120 degrees C. The memory functionality was consistent even after fifty cycles of operation. The charge transport switching mechanism is discussed on the basis of carrier transport mechanism and our analysis of the data shows that the charge carrier trans- port mechanism of the device during the writing process can be explained by thermionic emission (TE) and space-charge-limited-current (SCLC) mechanism models while erasing process could be explained by the FN tunneling mechanism. This demonstration provides a class of memory devices with the potential for low-cost, low-power consumption applications, such as a digital memory cell.

Ferroelectrics are polar substances of either solid (crystalline or polymeric) or liquid crystals, in which inverting the external electric field can reverse the spontaneously generated electric polarization. The bistable hysteresis of ferroelectric materials offers the possibility to develop electrically switchable data storage devices. Organic non-volatile memory devices based on ferroelectricity are a promising approach towards the development of low-cost memory technology. In addition, ferroelectric memory devices generally possess the advantages of long data or operating endurance, short switching time, and low-voltage operation. In this chapter, we discuss the latest developments of ferroelectric data storage memory devices based on three main device configurations, including capacitors, field-effect transistors, and diodes. Key materials and process issues for optimizing memory performance in each device architecture and thus realizing organic ferroelectric memory are discussed. The effects of the polymer orientation, interfacial engineering, device structure, and processing parameters on the memory switching characteristics are explored systematically.

The thin insulator films for organic memory device were made by the plasma polymerization method using the styrene monomer which was not the wet process but the dry process. For the formation of stable plasma, we make an effort for controlling the monomer with bubbler and circulator system. The thickness of plasma polymerized styrene insulator layer was 430 nm, the thickness of the Au memory layer was 7 nm thickness of plasma polymerized styrene tunneling layer was 30, 60 nm, the thickness of pentacene active layer was 40 nm, the thickness of source and drain electrodes were 50 nm. The I-V characteristics of fabricated memory device got the hysteresis voltage of 45 V at 40/-40 V double sweep measuring conditions. If it compared with the results of previous paper which was the organic memory with the plasma polymerized MMA insulation thin film, this result was greater than 18 V, the improving ratio is 60%. From the paper, styrene indicated a good charge trapping characteristics better than MMA. In the future, we expect to make the organic memory device with plasma polymerized styrene as the memory thin film.

A novel approach for using conjugated rod-coil materials as a floating gate in the fabrication of nonvolatile photonic transistor memory devices, consisting of n-type Sol-PDI and p-type C10-DNTT, is presented. Sol-PDI and C10-DNTT are used as dual functions of charge-trapping (conjugated rod) and tunneling (insulating coil), while n-type BPE-PDI and p-type DNTT are employed as the corresponding transporting layers. By using the same conjugated rod in the memory layer and transporting channel with a self-assembled structure, both n-type and p-type memory devices exhibit a fast response, a high current contrast between "Photo-On" and "Electrical-Off" bistable states over 105 , and an extremely low programing driving force of 0.1 V. The fabricated photon-driven memory devices exhibit a quick response to different wavelengths of light and a broadband light response that highlight their promising potential for light-recorder and synaptic device applications.