Polymer memories: Bistable electrical switching and device performance

An ultrasmall organic synapse for neuromorphic computing

High-performance organic neuromorphic devices with miniaturized device size and computing capability are essential elements for developing brain-inspired humanoid intelligence technique. However, due to the structural inhomogeneity of most organic materials, downscaling of such devices to nanoscale and their high-density integration into compact matrices with reliable device performance remain challenging at the moment. Herein, based on the design of a semicrystalline polymer PBFCL10 with ordered structure to regulate dense and uniform formation of conductive nanofilaments, we realize an organic synapse with the smallest device dimension of 50 nm and highest integration size of 1 Kb reported thus far. The as-fabricated PBFCL10 synapses can switch between 32 conductance states linearly with a high cycle-to-cycle uniformity of 98.89% and device-to-device uniformity of 99.71%, which are the best results of organic devices. A mixed-signal neuromorphic hardware system based on the organic neuromatrix and FPGA controller is implemented to execute spiking-plasticity-related algorithm for decision-making tasks.

Triphenylamine-Based Helical Polymer for Flexible Memristors

Flexible nonvolatile memristors have potential applications in wearable devices. In this work, a helical polymer, poly (N, N-diphenylanline isocyanide) (PPIC), was synthesized as the active layer, and flexible electronic devices with an Al/PPIC/ITO architecture were prepared on a polyethylene terephthalate (PET) substrate. The device showed typical nonvolatile rewritable memristor characteristics. The high-molecular-weight helical structure stabilized the active layer under different bending degrees, bending times, and number of bending cycles. The memristor was further employed to simulate the information transmission capability of neural fibers, providing new perspectives for the development of flexible wearable memristors and biomimetic neural synapses. This demonstration highlights the promising possibilities for the advancement of artificial intelligence skin and intelligent flexible robots in the future.

Nonvolatile Ternary Memristor Based on Fluorene-Benzimidazole Copolymer/Au NP Composites

A donor-acceptor type polymer of poly [2,7-(9,9-dioctyl)-fluorene-alt-7H-benzimidazo-[2,1-a]benzo[de]isoquinolin-7-one] (PF-BBO) based on benzimidazole groups was synthesized. This material was incorporated into data storage devices that exhibited good data storage characteristics. In order to improve the storage properties of the device, Au NPs were compounded in this material. We observed an increase in the ratio of switching current for the device with the PF-BBO/Au NP composite as the active layer. The device comprising 8 wt% Au NPs demonstrated optimal storage performance with a switching current ratio of 1:3.4 × 10²:1.0 × 10⁵ and a threshold voltage of -0.40 V/-0.85 V, respectively. The number of cycle times of this device was over 3000, which indicates excellent stability. Thus, the devices containing PF-BBO/Au NP composite as active materials offer a new dimension for future application prospects of high-density data storage.

Zinc phthalocyanine conjugated cellulose nanocrystals for memory device applications

We present the electrical properties of zinc phthalocyanine covalently conjugated to cellulose nanocrystals (CNC@ZnPc). Thin films of CNC@ZnPc sandwiched between two gold electrodes showed pronounced hysteresis in their current-voltage characteristics. The layered metal-organic-metal sandwich devices exhibit distinct high and low conductive states when bias is applied, which can be used to store information. Density functional theory results confirmed wave function overlap between CNC and ZnPc in CNC@ZnPc, and helped visualize the lowest (lowest unoccupied molecular orbital) and highest molecular orbitals (highest occupied molecular orbital) in CNC@ZnPc. These results pave the way forward for all-organic electronic devices based on low cost, earth abundant CNCs and metallophthalocyanines.

From Memristive Materials to Neural Networks

The information technologies have been increasing exponentially following Moore's law over the past decades. This has fundamentally changed the ways of work and life. However, further improving data process efficiency is facing great challenges because of physical and architectural limitations. More powerful computational methodologies are crucial to fulfill the technology gap in the post-Moore's law period. The memristor exhibits promising prospects in information storage, high-performance computing, and artificial intelligence. Since the memristor was theoretically predicted by L. O. Chua in 1971 and experimentally confirmed by HP Laboratories in 2008, it has attracted great attention from worldwide researchers. The intrinsic properties of memristors, such as simple structure, low power consumption, compatibility with the complementary metal oxide-semiconductor (CMOS) process, and dual functionalities of the data storage and computation, demonstrate great prospects in many applications. In this review, we cover the memristor-relevant computing technologies, from basic materials to in-memory computing and future prospects. First, the materials and mechanisms in the memristor are discussed. Then, we present the development of the memristor in the domains of the synapse simulating, in-memory logic computing, deep neural networks (DNNs) and spiking neural networks (SNNs). Finally, the existent technology challenges and outlook of the state-of-art applications are proposed.

Nanoscale "Noise-Source Switching" during the Optoelectronic Switching of Phase-Separated Polymer Nanocomposites

A method is developed to directly map nanoscale "noise-source switching" phenomena during the optoelectronic switching of phase-separated polymer nanocomposites of tetrathiafulvalene (TTF) and phenyl-C₆₁ -butyric acid methyl ester (PCBM) molecules dispersed in a polystyrene (PS) matrix. In the method, electrical current and noise maps of the nanocomposite film are recorded using a conducting nanoprobe, enabling the mapping of a conductivity and a noise-source density. The results provide evidence for a repeated modulation in noise sources, a "noise-source switching," in each stage of a switching cycle. Interestingly, when the nanocomposite is "set" by a high bias, insulating PS-rich phases shows a drastic decrease in a noise-source density which becomes lower than that of conducting TTF-PCBM-rich phases. This can be attributed to a trap filling by charge carriers generated from a TTF (donor)-PCBM (acceptor) complex. In addition, when the film is exposed to UV, an optical switching occurs due to chemical reactions which lead to irreversible changes on the noise-source density and conductivity. The method provides a new insight on noise-source activities during the optoelectronic switching of polymer nanocomposites and thus can be a powerful tool for basic noise research and applications in organic memory devices.

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.

The Effect of Random and Block Copolymerization with Pendent Carbozole Donors and Naphthalimide Acceptors on Multilevel Memory Performance

Polymeric materials have been widely used in the fabrication of data-storage devices, owing to their unique advantages and defined conduction mechanisms. To date, the most-functional polymers that have been reported for memory devices were synthesized through random copolymerization, whilst there have been no reports regarding the memory effect of block polymers. Herein, we synthesized a random copolymer (PMCz₈ -co-PMBNa₂ ) and its corresponding block copolymer (PMCz₈ -b-PMBNa₂ ) to study the effect of the method of polymerization on the memory properties of the corresponding devices. Interestingly, both devices (ITO/PMCz₈ -co-PMBNa₂ /Al and ITO/PMCz₈ -b-PMBNa₂ /Al) exhibited ternary memory performance, with threshold voltages of -1.7 V/-3.3 V and -2.7 V/-3.8 V, respectively. However, based on comprehensive measurements, the memory properties of PMCz₈ -co-PMBNa₂ and PMCz₈ -b-PMBNa₂ were found to be owing to the operation of different conduction mechanisms, which resulted from different molecular stacking in the film state. Therefore, we expect that this work will be helpful for improving our understanding of the conduction mechanisms in polymer-based data-storage devices.

Highly uniform supramolecular nano-films derived from carbazole-containing perylene diimide via surface-supported self-assembly and their electrically bistable memory behavior

For electrical memory applications, two perylenetetracarboxylic diimide (PDI) derivatives were synthesized and their surface-supported self-assembly behaviors were investigated.

Asymmetric isomerization: an efficient strategy to tune the electrical resistive memory behaviors of functional polyimides containing N-phenylcarbazole moieties

Three isomeric polyimides with exactly the same chemical components and altered spatial configuration possess vastly different memory behaviors.

Electrode buffer layers producing high performance nonvolatile organic write-once-read-many-times memory devices

CuI and Bphen buffer layers result in decreased switch threshold voltage and an increased ON/OFF ratio of an organic WORM memory device.

A WORM type polymer electrical memory based on polyethersulfone with carbazole derivatives

A series of high-performance polyethersulfone, which had pendent carbazole moieties (Cz-PES 1–3), have been designed and successfully synthesized for an application in a write-once read-many type memory device as the active polymer layer. The memory performance can be tuned by changing the substituent in the Cz derivatives units. Cz-PES 3 with excellent thermal properties ( Tg = 185°C and Td = 378°C) exhibits the best memory performance. For Cz-PES 3-based device indium tin oxide/Cz-PES 3/aluminum, the turn-on voltage is 2.5 V and the ON/OFF current ratio is higher than 106. Moreover, the data can be maintained for longer than 3 × 105 s once written and can be read for more than 450 cycles under a reading voltage of 1.0 V at ambient conditions. Thus Cz-PES 3 can serve as an excellent memory material in the data storage field of next generation.

Controlling carrier trapping and relaxation with a dipole field in an organic field-effect device

A novel polyimide electret using as the gate dielectric layer and charge trapping layer of n-type organic transistors was synthesized to improve the memory effect and electrical stability.

Nonvolatile electrical switching behavior and mechanism of functional polyimides bearing a pyrrole unit: influence of different side groups

Influence of side groups to the nonvolatile electrical switching behaviors and its mechanism of polyimides bearing pyrrole unit were systematically studied.

Design and synthesis of hyperbranched polyimide containing multi-triphenylamine moieties for memory devices

A hyperbranched polyimide (HBPI) was synthesized from a designed triamine monomer and the HBPI based memory device presented SRAM (static random access memory) behavior with a low switch voltage and high ON/OFF current ratio.

Adjusting the Proportion of Electron-Withdrawing Groups in a Graft Functional Polymer for Multilevel Memory Performance

A polymer containing aldehyde active groups (PVB) was synthesized by atom transfer radical polymerization (ATRP), acting as a polymer precursor to graft a functional moiety via nucleophilic addition reaction. DHI (2-(1,5-dimethyl-hexyl)-6-hydrazino-benzo[de]isoquinoline-1,3-dione) and NPH (nitrophenyl hydrazine) groups, which contain naphthalimides that act as narrow traps and nitro groups that act as deep traps, were anchored onto the PVB at different ratios. A series of graft polymers were obtained and named PVB-DHI, PVB-DHI4 -NPH, PVB-DHI-NPH4 , and PVB-NPH. The chemical composition of the polymers was analyzed by (1) H-NMR spectroscopy and X-ray photoelectron spectroscopy (XPS). Memory devices were prepared from the polymers, and I-V characteristics were measured to determine the performance. By adjusting the ratio of different electron acceptors (DHI and NPH) to 4:1, ternary memory behavior was achieved. The relationship between memory behavior of PVB-DHIx NPHy and acceptor groups as well as their conduction mechanism were studied in detail.

Synthesis and Morphology of Two Carbazole-Pyrazoline-Containing Polymer Systems and Their Electrical Memory Performance

A new atom-transfer radical polymerization (ATRP) initiator 4-[1-(2-dodecyl-1,3-dioxo-2,3-dihydro-1H-benzo[de]isoquinolin-6-yl)-3-(4-nitrophenyl)-4,5-dihydro-1H-pyrazol-5-yl]phenyl 2-bromo-2-methylpropanoate (IN) as an electron acceptor (A) and a monomer 2-(9H-carbazole-9-yl)-ethyl methacrylate (MCz) as an electron donor (D) were simultaneously introduced into two different D-A polymer systems by using the end-functionalizing or blending method. The mass percentage of IN in the end-functionalized polymer PMCz-IN and the mixed polymer composite PMCz+IN were both controlled at approximately 1.0 wt %. The optical, electrochemical, and surface morphology properties of the two polymeric films prepared by means of spin-coating technology were comparatively investigated. Sandwich devices based on PMCz-IN and PMCz+IN demonstrated nonvolatile write-once-read-many-times memory (WORM) and volatile static random access memory (SRAM) characteristics, respectively, which were further verified by the Kelvin probe force microscopy (KPFM) measurements. The proposed memory mechanism could be attributed to the formation of a stable charge-transfer (CT) complex for PMCz-IN and an unstable CT complex for PMCz+IN. Furthermore, the different distribution of IN in the two polymeric films might be the main reason for the stability of the CT complex.

Writing nanopatterns with electrochemical oxidation on redox responsive organometallic multilayers by AFM

Nanoelectrochemical patterning of redox responsive organometallic poly(ferrocenylsilane) (PFS) multilayers is demonstrated by electrochemical dip pen lithography (EDPN). Local electrochemical oxidation and Joule heating of PFS multilayers from the tip are considered as relevant mechanisms related to structure generation. The influence of applied bias potential, tip velocity, and multilayer thickness on the pattern height and width were investigated.

Different interactions between a metal electrode and an organic layer and their different electrical bistability performances

Different electrical bistability performances were obtained by tuning metal electrodes.

Synthesis of imidazole derivatives and study of the ON-based different memory performances

We report the synthesis of two imidazole-based small molecules with different planarity of terminal aromatic rings and their application in memory devices with a sandwich configuration. The optical, electric, and the on-based device performances were systematically investigated. Surprisingly, the device based on BT-PMZ exhibited volatile static random access memory (SRAM) behavior, whereas that based on BT-BMZ showed nonvolatile write-once-read-many-times (WORM) behavior. Further studies on the film morphology and the molecular electronic structure were carried out to investigate the underlying mechanism for the large difference in their performance. Moreover, the performance of the device that incorporates a LiF buffer layer (5 nm) embedded at the interface between the BT-BMZ active layer and the Al top electrode as well as that of the device with a cold-deposited top electrode of mercury droplet was further investigated. At that point a dramatic change in memory performance of the devices from the WORM to SRAM type was observed. The intrinsic volatile SRAM performance for the two molecules results from the moderate electron-withdrawing strength of the acceptor moieties and thus weak trapping of the charge carriers.

Metal-free, single-polymer device exhibits resistive memory effect

All-polymer, write-once-read-many times resistive memory devices have been fabricated on flexible substrates using a single polymer, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). Spin-cast or inkjet-printed films of solvent-modified PEDOT:PSS are used as electrodes, while the unmodified or as-is PEDOT:PSS is used as the semiconducting active layer. The all-polymer devices exhibit an irreversible but stable transition from a low resistance state (ON) to a high resistance state (OFF) at low voltages caused by an electric-field-induced morphological rearrangement of PEDOT and PSS at the electrode interface. However, in the metal-PEDOT:PSS-metal devices, we have shown a metal filament formation switching the device from an initial high resistance state (OFF) to the low resistance state (ON). The all-PEDOT:PSS memory device has low write voltages (<3 V), high ON/OFF ratio (>10(3)), good retention characteristics (>10,000 s), and stability in ambient storage (>3 months).