Metal-containing organic compounds for memory and data storage applications

An enzymolysis-induced energy transfer co-assembled system for spontaneously recoverable supramolecular dynamic memory

The continuing growth of the digital world requires new ways of constructing memory devices to process and store dynamic data, because the current ones suffer from inefficiency, limited reads, and difficulty to manufacture. Here we propose a supramolecular dynamic memory (SDM) strategy based on an enzymolysis-induced energy transfer co-assembly derived from a naphthalene-based cationic monomer and organic dye sulforhodamine 101, enabling the construction of spontaneously recoverable dynamic memory devices. Benefitting from the large exciton migration rate (4.48 × 1015 L mol-1 s-1) between the monomer and sulforhodamine 101, the energy transfer process between the two is effectively achieved. Since alkaline phosphatase can selectively hydrolyze adenosine triphosphate, leading to the disruption of the co-assemblies, an enzyme-mediated time-dependent fluorochromic system is realized. On this basis, a SDM system featuring spontaneous recovery and enabling the memory of dynamic information in optical and electrical modes is successfully constructed. The current study represents a promising step in the nascent development of supramolecular materials for computational systems.

Insight into Facile Ion Diffusion in Resistive Switching Medium toward Low Operating Voltage Memory

The rapid increase in data storage worldwide demands a substantial amount of energy consumption annually. Studies looking at low power consumption accompanied by high-performance memory are essential for next-generation memory. Here, Graphdiyne oxide (GDYO), characterized by facile resistive switching behavior, is systematically reported toward a low switching voltage memristor. The intrinsic large, homogeneous pore-size structure in GDYO facilitates ion diffusion processes, effectively suppressing the operating voltage. The theoretical approach highlights the remarkably low diffusion energy of the Ag ion (0.11 eV) and oxygen functional group (0.6 eV) within three layers of GDYO. The Ag/GDYO/Au memristor exhibits an ultralow operating voltage of 0.25 V with a GDYO thickness of 5 nm; meanwhile, the thicker GDYO of 29 nm presents multilevel memory with an ON/OFF ratio of up to 104. The findings shed light on memory resistive switching behavior, facilitating future improvements in GDYO-based devices toward opto-memristors, artificial synapses, and neuromorphic applications.

High-performance flexible resistive random-access memory based on SnS2 quantum dots with a charge trapping/de-trapping effect

The application of resistive random-access memory (RRAM) in storage and neuromorphic computing has attracted widespread attention. Benefitting from the quantum effect, transition metal dichalcogenides (TMD) quantum dots (QDs) exhibit distinctive optical and electronic properties, which make them promising candidates for emerging RRAM. Here, we show a high-performance forming-free flexible RRAM based on high-quality tin disulfide (SnS2) QDs prepared by a facile liquid phase method. The RRAM device demonstrates high flexibility with a large on/off ratio of ∼106 and a long retention time of over 3 × 104 s. The excellent switching behavior of the memristor is elucidated by a charge trapping/de-trapping mechanism where the SnS2 QDs act as charge trapping centers. This study is of significance for the understanding and development of TMD QD-based flexible memristors.

Photon-Dipole-Spin Interactions in M(TCNE)x/P(VDF-TrFE) Multiferroic Heterostructure Available for Bimodal Control of Multistate Data-Storage

Organic multiferroic heterostructure is one of the most promising structures for the future design of high-density flexible energy-efficient data storage. Here, organic ferromagnetic metal(tetracyanoethylene) (M(TCNE))x/ferroelectric poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) multiferroic heterostructures are fabricated, where the excited state in M(TCNE)x interacted with localized dipole in P(VDF-TrFE) provides a key link for the interfacial coupling. Thus, aligned dipoles in P(VDF-TrFE) by external electric field can affect the magnetization of Fe(TCNE)x effectively to result in a pronounced magnetization-voltage (M-V) hysteresis loop. Moreover, light-induced electron-hole pairs in Fe(TCNE)x with long lifetime effectively interact with the dipoles in P(VDF-TrFE) to lead to an effect in external light control of electric polarization of P(VDF-TrFE). Overall, the organic multiferroic heterostructure provides the possibility of realizing two storage modes, light control of dipole as well as electric field control of spin, which can broaden multifunctional applications of organic multiferroic materials in the area of multistate storage.

A floating-gate field-effect transistor memory device based on organic crystals with a built-in tunneling dielectric by a one-step growth strategy

A floating-gate organic field-effect transistor (FG-OFET) memory device is becoming a promising candidate for emerging non-volatile memory applications due to the advantages of its sophisticated data-storage mechanism and reliable long-term data retention capacity. However, a conventional FG-OFET memory device suffers from complex fabrication technologies and poor mechanical flexibility, which limits its practical applications. Here, we propose a facile one-step liquid-surface drag coating strategy to fabricate a layered stack of 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene (Dif-TES-ADT) crystals and high-quality insulating polymer polystyrene (PS). The liquid surface enhances the spreading area of an organic solution and facilitates the unidirectional growth of organic crystals. In the bilayer-structured blend, the bottom PS polymer and the top Dif-TES-ADT semiconductor serve as a tunneling dielectric and an active memory layer of an FG-OFET memory device, respectively. Consequently, a flexible FG-OFET memory device with a large memory window of 41.4 V, a long retention time of 5000 s, and a high current ON/OFF ratio of 105 could be achieved, showing the best performance ever reported for organic thin film-based FG-OFET memory devices. In addition, multi-level data storage (3 bits per cell) can be achieved by tuning the gate voltage magnitude. Our work not only provides a general strategy for the growth of high-quality organic crystals, but also paves the way towards high-performance flexible memory devices.

Investigating the Impact of Packing and Environmental Factors on the Luminescence of Pt(N^N^N) Chromophores

Four Pt(II)(N^N^N) compounds featuring DMSO coordination at the fourth position were synthesized. Ligands varied in terms of pyridyl central ring (hydrogen/chlorine substituent) and lateral rings (triazoles with CF3 substitution or tetrazoles). Coordination to pyridine yielded tetra-nitrogen coordinated Pt(II) complexes or Pt-functionalized polymers using commercial 4-pyridyl polyvinyl (PV) or dimethylaminopyridine. Luminescence behaviors exhibited remarkable environmental dependence. While some of the molecular compounds (tetrazole derivatives) in solid state displayed quenched luminescence, all the polymers exhibited 3MMLCT emission around 600 nm. Conversely, monomer emission was evident on poly(methyl methacrylate) or polystyrene matrices. DFT calculations were used to analyze the aggregation of the complexes both at the molecular level and coordinated to the PV polymer and their influence on the HOMO-LUMO gaps.

Light-Responsive Supramolecular Liquid-Crystalline Metallacycle for Orthogonal Multimode Photopatterning

The development of multimode photopatterning systems based on supramolecular coordination complexes (SCCs) is considerably attractive in supramolecular chemistry and materials science, because SCCs can serve as promising platforms for the incorporation of multiple functional building blocks. Herein, we report a light-responsive liquid-crystalline metallacycle that is constructed by coordination-driven self-assembly. By exploiting its fascinating liquid crystal features, bright emission properties, and facile photocyclization capability, a unique system with spatially-controlled fluorescence-resonance energy transfer (FRET) is built through the introduction of a photochromic spiropyran derivative, which led to the realization of the first example of a liquid-crystalline metallacycle for orthogonal photopatterning in three-modes, namely holography, fluorescence, and photochromism.

Design and synthesis of a solution-processed redox-active bis(formazanate) zinc complex for resistive switching applications

In this paper, we report the synthesis and characterization of a mononuclear zinc complex (1) containing a redox-active bis(4-antipyrinyl) derivative of the 3-cyanoformazanate ligand. Complex 1 was readily obtained by refluxing zinc acetate with 3-cyano-1,5-(4-antipyrinyl)formazan (LH) in a methanolic solution. Single-crystal X-ray diffraction analysis of complex 1 shows that the formazanate ligands bind to the zinc center in a five-member chelate "open" form via the 1- and 4-positions of the 1,2,4,5-tetraazapentadienyl formazanate backbone leading to the formation of the mononuclear bis(formazanate) zinc complex exhibiting a distorted octahedral geometry. We also report the study of resistive-switching random access memory application of this solution-processable bis(formazanate) Zn(II) complex to facilitate the practical implementation of transition metal complex-based molecular memory devices. The complex demonstrated high conductance switching with a large ON-OFF ratio, good stability, and a long retention time. A trap-controlled space charge limited current mechanism is proposed for the observed resistive switching behavior of the device, wherein the role played by the [ZnIIL2] complex that comprises the extended redox-active conjugated ligand backbone is revealed by corroborating electrochemical studies, spectrochemical experiments, and DFT calculations. In addition to providing significant insights into the molecular design of transition metal complexes for memory applications, this study also presents the utilization of ZnIIL2 towards the realization of non-volatile resistive random access memory (RRAM) devices with inorganic/organic hybrid active layers that are highly cost-effective and sustainable. These devices exhibited multilevel switching and low current operation, both of which are desirable for advanced memory applications.

Porous crystalline materials for memories and neuromorphic computing systems

Porous crystalline materials usually include metal-organic frameworks (MOFs), covalent organic frameworks (COFs), hydrogen-bonded organic frameworks (HOFs) and zeolites, which exhibit exceptional porosity and structural/composition designability, promoting the increasing attention in memory and neuromorphic computing systems in the last decade. From both the perspective of materials and devices, it is crucial to provide a comprehensive and timely summary of the applications of porous crystalline materials in memory and neuromorphic computing systems to guide future research endeavors. Moreover, the utilization of porous crystalline materials in electronics necessitates a shift from powder synthesis to high-quality film preparation to ensure high device performance. This review highlights the strategies for preparing porous crystalline materials films and discusses their advancements in memory and neuromorphic electronics. It also provides a detailed comparative analysis and presents the existing challenges and future research directions, which can attract the experts from various fields (e.g., materials scientists, chemists, and engineers) with the aim of promoting the applications of porous crystalline materials in memory and neuromorphic computing systems.

Efficient ternary WORM memory devices from quinoline-based D-A systems by varying the redox behavior of ferrocene

The design and synthesis of ferrocene-functionalized organic small molecules using quinoline cores are rendered to achieve a ternary write-once-read-many (WORM) memory device. Introducing an electron-withdrawing group into the ferrocene system changes the compounds' photophysical, electrochemical, and memory behavior. The compounds were synthesized with and without an acetylene bridge between the ferrocene unit and quinoline. The electrochemical studies proved the oxidation behavior with a slightly less intense reduction peak of the ferrocene unit, demonstrating that quinolines have more reducing properties than ferrocene with bandgaps ranging from 2.67-2.75 eV. The single crystal analysis of the compounds also revealed good interactive interactions, ensuring good molecular packing. This further leads to a ternary WORM memory with oxidation of the ferrocene units and charge transfer in the compounds. The devices exhibit on/off ratios of 104 and very low threshold voltages of -0.58/-1.02 V with stabilities of 103 s and 100 cycles of all the states through retention and endurance tests.

Carbon Nanodots Memristor: An Emerging Candidate toward Artificial Biosynapse and Human Sensory Perception System

In the era of big data and artificial intelligence (AI), advanced data storage and processing technologies are in urgent demand. The innovative neuromorphic algorithm and hardware based on memristor devices hold a promise to break the von Neumann bottleneck. In recent years, carbon nanodots (CDs) have emerged as a new class of nano-carbon materials, which have attracted widespread attention in the applications of chemical sensors, bioimaging, and memristors. The focus of this review is to summarize the main advances of CDs-based memristors, and their state-of-the-art applications in artificial synapses, neuromorphic computing, and human sensory perception systems. The first step is to systematically introduce the synthetic methods of CDs and their derivatives, providing instructive guidance to prepare high-quality CDs with desired properties. Then, the structure-property relationship and resistive switching mechanism of CDs-based memristors are discussed in depth. The current challenges and prospects of memristor-based artificial synapses and neuromorphic computing are also presented. Moreover, this review outlines some promising application scenarios of CDs-based memristors, including neuromorphic sensors and vision, low-energy quantum computation, and human-machine collaboration.

Dynamical Behavior of Pure Spin Current in Organic Materials

Growing concentration on the novel information processing technology and low-cost, flexible materials make the spintronics and organic materials appealing for the future interdisciplinary investigations. Organic spintronics, in this context, has arisen and witnessed great advances during the past two decades owing to the continuous innovative exploitation of the charge-contained spin polarized current. Albeit with such inspiring facts, charge-absent spin angular momentum flow, namely pure spin currents (PSCs) are less probed in organic functional solids. In this review, the past exploring journey of PSC phenomenon in organic materials are retrospected, including non-magnetic semiconductors and molecular magnets. Starting with the basic concepts and the generation mechanism for PSC, the representative experimental observations of PSC in the organic-based networks are subsequently demonstrated and summarized, by accompanying explicit discussion over the propagating mechanism of net spin itself in the organic media. Finally, future perspectives on PSC in organic materials are illustrated mainly from the material point of view, including single molecule magnets, complexes for the organic ligands framework as well as the lanthanide metal complexes, organic radicals, and the emerging 2D organic magnets.

Switching the memory behaviour from binary to ternary by triggering S62- relaxation in polysulfide-bearing zinc-organic complex molecular memories

The use of crystalline metal-organic complexes with definite structures as multilevel memories can enable explicit structure-property correlations, which is significant for designing the next generation of memories. Here, four Zn-polysulfide complexes with different degrees of conjugation have been fabricated as memory devices. ZnS₆(L)₂-based memories (L = pyridine and 3-methylpyridine) can exhibit only bipolar binary memory performances, but ZnS₆(L)-based memories (L = 2,2'-bipyridine and 1,10-phenanthroline) illustrate non-volatile ternary memory performances with high ON2/ON1/OFF ratios (10^4.22/10^2.27/1 and 10^4.85/10^2.58/1) and ternary yields (74% and 78%). Their ON1 states stem from the packing adjustments of organic ligands upon the injection of carriers, and the ON2 states are a result of the ring-to-chain relaxation of S₆^2- anions. The lower conjugated degrees in ZnS₆(L)₂ result in less compact packing; consequently, the adjacent S₆^2- rings are too long to trigger the S₆^2- relaxation. The deep structure-property correlation in this work provides a new strategy for implementing multilevel memory by triggering polysulfide relaxation based on the conjugated degree regulation of organic ligands.

Selective Dual-Ion Modulation in Solid-State Magnetoelectric Heterojunctions for In-Memory Encryption

Nanoionic technologies are identified as a promising approach to modulating the physical properties of solid-state dielectrics, which have resulted in various emergent nanodevices, such as nanoionic resistive switching devices and magnetoionic devices for memory and computing applications. Previous studies are limited to single-type ion manipulation, and the investigation of multiple-type ion modulation on the coupled magnetoelectric effects, for developing information devices with multiple integrated functionalities, remains elusive. Here, a dual-ion solid-state magnetoelectric heterojunction based on Pt/HfO_{2- x} /NiO_y /Ni with reconfigurable magnetoresistance (MR) characteristics is reported for in-memory encryption. It is shown that the oxygen anions and nickel cations can be selectively driven by voltages with controlled polarity and intensity, which concurrently change the overall electrical resistance and the interfacial magnetic coupling, thus significantly modulate the MR symmetry. Based on this device, a magnetoelectric memory prototype array with in-memory encryption functionality is designed for the secure storage of image and digit information. Along with the advantages including simple structure, multistate encryption, good reversibility, and nonvolatile modulation capability, this proof-of-concept device opens new avenues toward next-generation compact electronics with integrated information functionalities.

Evaluating charge-type of polyelectrolyte as dielectric layer in memristor and synapse emulation

Based on credible advantages, organic materials have received more and more attention in memristor and synapse emulation. In particular, an implementation of the ionic pathway as a dielectric layer is essential for organic materials used as building blocks of memristor and artificial synaptic devices. Herein, we describe an evaluation of the use of positive and negative polyelectrolytes as dielectric layers for a memristor with calcium ion (Ca²⁺) doping. The device based on a negative polyelectrolyte shows the potential to obtain an excellent resistive switching performance and synapse functionality, especially in the transformation behaviours from short-term plasticity (STP) to long-term plasticity (LTP) in both the potentiation and depression processes, which were comparable to the perfomrmance obtained with a positive polyelectrolyte. The mechanism of electrical resistance transition and synaptic function can be attributed to the migration of the doped Ca²⁺ and the ionic functional groups of polyelectrolyte, which result in the formation and vanishing filament-like Ca²⁺ flux.

In-Depth Physical Mechanism Analysis and Wearable Applications of HfOx-Based Flexible Memristors

Since memristors as an emerging nonlinear electronic component have been considered the most promising candidate for integrating nonvolatile memory and advanced computing technology, the in-depth reveal of the memristive mechanism and the realization of hardware fabrication have facilitated their wide applications in next-generation artificial intelligence. Flexible memristors have shown great promising prospects in wearable electronics and artificial electronic skin (e-skin), but in-depth research on the physical mechanism is still lacking. Here, a flexible memristive device with a Ag/HfO_x/Ti/PET crossbar structure was fabricated, and a remarkable analog switching characteristic similar to synaptic behavior was observed. Through detailed data fitting and in-depth physical mechanism analysis, it is confirmed that the analog switching characteristics of the device are mainly caused by carrier tunneling. Furthermore, the memristive properties of the Ag/HfO_x/Ag/PET device can be attributed to the conductive filaments formed by the redox reaction of the active metal Ag. Finally, the interfacial barrier is extracted by the Arrhenius diagram and the energy band diagram, which is drawn to clearly demonstrate the conduction mechanism of charge trapping in the device. Therefore, the HfO_x-based flexible memristor with analog switching behavior and stable memory performance lays the foundation for cutting-edge applications in wearable electronics and smart e-skin.

Metal-Organic Frameworks-Based Memristors: Materials, Devices, and Applications

Facing the explosive growth of data, a number of new micro-nano devices with simple structure, low power consumption, and size scalability have emerged in recent years, such as neuromorphic computing based on memristor. The selection of resistive switching layer materials is extremely important for fabricating of high performance memristors. As an organic-inorganic hybrid material, metal-organic frameworks (MOFs) have the advantages of both inorganic and organic materials, which makes the memristors using it as a resistive switching layer show the characteristics of fast erasing speed, outstanding cycling stability, conspicuous mechanical flexibility, good biocompatibility, etc. Herein, the recent advances of MOFs-based memristors in materials, devices, and applications are summarized, especially the potential applications of MOFs-based memristors in data storage and neuromorphic computing. There also are discussions and analyses of the challenges of the current research to provide valuable insights for the development of MOFs-based memristors.

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.