Resistive Switching of the Tetraindolyl Derivative in Ultrathin Films: A Potential Candidate for Nonvolatile Memory Applications

Coexistence of WORM and RRAM Resistive Switching in Coumarin Derivatives: A Comprehensive Performance Analysis

Over the past few years, organic small molecules (OSM) having a π-conjugated heteroatomic aromatic backbone along with terminal donor-acceptor (D-A) groups have emerged as one of the most promising materials for organic resistive switching (ORS) devices. In this research, the resistive switching (RS) properties of two rationally synthesized coumarin derivatives, 7-(2-(benzylamino)ethoxy)-4-methyl-2H-chromen-2-one (CAMN1) and 7-(2-(4-methoxyphenylamino)ethoxy)-4-methyl-2H-chromen-2-one (CAMN2), have been exhaustively studied. The CAMN1-based ORS device exhibited WORM RS behavior with an excellent device yield of 97.22%, while the CAMN2-based device showed both WORM as well as RRAM RS behavior depending on the compliance current (CC) with a perfect device yield of 100%. Both devices exhibited superior read endurance on the order of 104 as well as a retention time of at least 3 × 104 s with a very good memory window of the order of 104 or more. Moreover, both devices exhibited superior long-term physical and thermal stability. The cyclability of the CAMN2-based device in the RRAM mode of operation was found to be 116 cycles. DFT-based calculations as well as absorption spectroscopic studies reveal the role of the intra/intermolecular charge transfer (CT) in the RS behavior of both the devices. Moreover, the presence of the methoxy (-OCH3) group in the CAMN2 molecule has been identified as the key reason behind the observed difference in the RS behaviors of the two molecules.

Electrosynthesis of Ru (II)-Polypyridyl Oligomeric Films on ITO Electrode for Two Terminal Non-Volatile Memory Devices and Neuromorphic Computing

Molecular electronics exhibiting resistive-switching memory features hold great promise for the next generation of digital technology. In this work, electrosynthesis of ruthenium polypyridyl nanoscale oligomeric films is demonstrated on an indium tin oxide (ITO) electrode followed by an ITO top contact deposition yielding large-scale (junction area = 0.7 × 0.7 cm2) two terminal molecular junctions. The molecular junctions exhibit non-volatile resistive switching at a relatively lower operational voltage, ±1 V, high ON/OFF electrical current ratio (≈103), low-energy consumption (SET/RESET = 27.94/14400 nJ), good cyclic stability (>300 cycles), and switching speed (SET/RESET = 25 ms/20 ms). A computational study suggests that accessible frontier molecular orbitals of metal-complex to the Fermi level of ITO electrodes facilitate charge transport at a relatively lower bias followed by a filamentformation. An extensive analysis is performed of the performance of binary neural networks exploiting the current-voltage features of the devices as binary synaptic weights and exploring their potential for neuromorphic logic-in-memory implementation of IMPLICATION (IMPLY) operation which can realize universal gates. The comprehensive analysis indicates that the proposed redox-active complex-based memory device may be a promising candidate for high-density data storage, energy-efficient implementation of neuromorphic networks with software-level accuracy, and logic-in-memory implementations.

Fabrication and Photovoltaic Conversion Performances of Imidazolyl and Fumaric Acid Composite Langmuir-Blodgett Membranes

Novel organic small molecule structures have received increasing attention in the preparation of multifunctional thin film materials and have become the subject of research in many cutting-edge directions. In this work, imidazolyl and transbutylene glycolic acid molecules and dye molecules were designed and prepared as composite films by supramolecular self-assembly in the LB technique, and their morphological features and spectral properties were analyzed. The results showed that the prepared LB films presented different aggregation states. In addition, their photoelectrochemical properties, on ITO sheets were tested, all of which showed good optoelectronic properties, and their binding energies, structure optimization, and electrostatic potentials were theoretically calculated by DFT simulations, which proved that the prepared films have good optoelectronic properties, and have the potential to become optoelectronic multifunctional ultrathin film devices.

Design of Triphenylamine-based D-π-A Systems for Efficient Ternary WORM Memory Devices

The impact of various substituent moieties on the molecular framework of a conjugated D-A system in resistive switching memory property has been scrutinized through an array of novel D-π-A molecules. The synthesized molecules with triphenylamine (TPA) as the electron donor and dicyanovinylindanone (IC) as the electron acceptor demonstrated substantial non-volatile WORM (Write-Once Read-Many) memory behaviour with appreciable ON/OFF current ratios up to 105 and a lowest recorded threshold voltage of -0.80 V. The well-balanced combination of these potent electron donating and accepting units culminated in exceptional intramolecular charge transfer interactions and minimal band gap values (1.82-2.31 eV) for the molecules, as demonstrated by photophysical and electrochemical investigations. These factors, coupled with the thin-film morphological studies, corroborated the superior performance of the fabricated devices. A longer retention time of 2000s and an endurance of 100 cycles mark the substantial stability of the memory devices. Moreover, conversion from binary to ternary WORM memory was achieved by the effective tuning of the electronic properties of the D-A systems by various substituent moieties. Molecular simulation studies revealed that the resistive switching phenomenon arises from a synergistic interplay of charge transfer and charge trapping processes within these D-A systems.

Modified Donor End Caps for Binary-to-Ternary WORM Memory Conversion in N-Heteroaromatic Systems

A series of novel D-π-A type organic small molecules have been designed, synthesized, and demonstrated for non-volatile resistive switching WORM memory application. The electron-deficient phenazine and quinoxaline units were coupled with various functionalized triphenylamine end caps to explore the structure-property correlations. The photophysical investigations displayed considerable intramolecular charge transfer, and the electrochemical analysis revealed an optimum band gap of 2.44 to 2.83 eV. These factors and the thin film morphological studies suggest the feasibility of the compounds as better resistive memory devices. All the compounds indicated potent non-volatile resistive switching memory capabilities with ON/OFF ratios ranging from 103 to 104, and the lowest threshold voltage recorded stands at -0.74 V. A longer retention time of 103 s marks the substantial stability of the devices. The phenazine-based compounds outperformed the others in terms of memory performance. Exceptionally, the compound with -CHO substituted triphenylamine exhibited ternary memory performance owing to its multiple traps. The resistive switching mechanism for the devices was validated using density functional theory calculations, which revealed that the integrated effect of charge transfer and charge trapping contributes significantly to the resistive switching phenomena.

Noncytotoxic WORM Memory Using Lysozyme with Ultrahigh Stability for Transient and Sustainable Electronics Applications

Biocompatibility and transient nature of electronic devices have been the matter of attention in recent times due to their immense potential for sustainable solutions toward hazardous e-wastes. In order to fulfill the requirement of high-density data-storage devices due to explosive growth in digital data, a resistive switching (RS)-based memory device could be the promising alternative to the present Si-based electronics. In this research work, we employed a biocompatible enzymatic protein lysozyme (Lyso) as the active layer to design a RS memory device having a device structure Au/Lyso/ITO. Interestingly the device showed transient, WORM memory behavior. It has been observed that the WORM memory performance of the device was very good with high memory window (2.78 × 102), data retention (up to 300 min), device yield (∼73.8%), read cyclability, as well as very high device stability (experimentally >700 days, extrapolated to 3000 days). Bias-induced charge trapping followed by conducting filament formation was the key behind such switching behavior. Transient behavior analysis showed that electronic as well as optical behaviors completely disappeared after 10 s dissolution of the device in luke warm water. Cytotoxicity of the as-prepared device was tested by challenging two environmentally derived bacteria, S. aureus and P. aeruginosa, and was found to have no biocidal effects. Hence, the device would cause no harm to the microbial flora when it is discarded. As a whole, this work suggests that Lyso-based WORM memory device could play a key role for the design of transient WORM memory device for sustainable electronic applications.

Effect of long chain fatty acids on the memory switching behavior of tetraindolyl derivatives

Non-volatile memory devices using organic materials have attracted much attention due to their excellent scalability, fast switching speed, low power consumption, low cost etc. Here, we report both volatile as well as non-volatile resistive switching behavior of p-di[3,3'-bis(2-methylindolyl)methane]benzene (Indole2) and its mixture with stearic acid (SA). Previously, we have reported the bipolar resistive switching (BRS) behavior using 1,4-bis(di(1H-indol-3-yl)methyl)benzene (Indole1) molecules under ambient conditions [Langmuir 37 (2021) 4449-4459] and complementary resistive switching (CRS) behavior when the device was exposed to 353 K or higher temperature [Langmuir 38 (2022) 9229-9238]. However, the present study revealed that when the H of -NH group of Indole1 is replaced by -CH3, the resultant Indole2 molecule-based device showed volatile threshold switching behaviour. On the other hand, when Indole2 is mixed with SA at a particular mole fraction, dynamic evolution of an Au/Indole2-SA/ITO device from volatile to non-volatile switching occurred with very good device stability (>285 days), memory window (6.69 × 102), endurance (210 times), data retention (6.8 × 104 s) and device yield of the order of 78.5%. Trap controlled SCLC as well as electric field driven conduction was the key behind the observed switching behaviour of the devices. In the active layer, trap centers due to the SA network may be responsible for non-volatile characteristics of the device. Observed non-volatile switching may be a potential candidate for write once read many (WORM) memory applications in future.

Resistive random access memory: introduction to device mechanism, materials and application to neuromorphic computing

The modern-day computing technologies are continuously undergoing a rapid changing landscape; thus, the demands of new memory types are growing that will be fast, energy efficient and durable. The limited scaling capabilities of the conventional memory technologies are pushing the limits of data-intense applications beyond the scope of silicon-based complementary metal oxide semiconductors (CMOS). Resistive random access memory (RRAM) is one of the most suitable emerging memory technologies candidates that have demonstrated potential to replace state-of-the-art integrated electronic devices for advanced computing and digital and analog circuit applications including neuromorphic networks. RRAM has grown in prominence in the recent years due to its simple structure, long retention, high operating speed, ultra-low-power operation capabilities, ability to scale to lower dimensions without affecting the device performance and the possibility of three-dimensional integration for high-density applications. Over the past few years, research has shown RRAM as one of the most suitable candidates for designing efficient, intelligent and secure computing system in the post-CMOS era. In this manuscript, the journey and the device engineering of RRAM with a special focus on the resistive switching mechanism are detailed. This review also focuses on the RRAM based on two-dimensional (2D) materials, as 2D materials offer unique electrical, chemical, mechanical and physical properties owing to their ultrathin, flexible and multilayer structure. Finally, the applications of RRAM in the field of neuromorphic computing are presented.

Complementary Resistive Switching Behavior in Tetraindolyl Derivative-Based Memory Devices

Complementary resistive switching (CRS) devices are more advantageous compared to bipolar resistive switching (BRS) devices for memory applications as they can minimize the sneak path problem observed in the case of BRS having a crossbar array structure. Here, we report the CRS behavior of 1,4-bis(di(1H-indol-3-yl)methyl)benzene (Indole1) molecules. Our earlier study revealed that Au/Indole1/Indium tin oxide (ITO) devices showed BRS under ambient conditions. However, the present investigations revealed that when the device is exposed to 353 K or higher temperatures, dynamic evolution of the Au/Indole1/ITO device from BRS to CRS occurred with a very good memory window (∼10³), data retention (5.1 × 10³ s), stability (50 days), and device yield (∼ 60%). This work explores the application possibility of indole derivatives toward future ultradense resistive random access memory.

Interaction of a Phospholipid and a Coagulating Protein: Potential Candidate for Bioelectronic Applications

In the present communication, we have investigated the interaction between a biomembrane component 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and a coagulating protein protamine sulfate (PS) using the Langmuir-Blodgett (LB) technique. The π-A isotherm, π-t characteristics, and analysis of isotherm curves suggested that PS strongly interacted with DOPC, affecting the fluidity of the DOPC layer. Electrical characterization indicates that PS as well as the PS-DOPC film showed resistive switching behavior suitable for Write Once Read Many (WORM) memory application. Trap-controlled space charge-limited conduction (SCLC) was the key mechanism behind such observed switching. The presence of DOPC affected the SCLC process, leading to lowering of threshold voltage (V _Th), which is advantageous in terms of lower power consumption.

Ternary Flash Memory with a Carbazole-Based Conjugated Copolymer: WS2 Composites as Active Layers

For nonvolatile memory devices, the design and synthesis of their substrate materials are very important. Due to the versatility and large-area fabrication of the low-temperature spin coating process, organic/inorganic nanomaterials as active layers of memory devices have been deeply studied. Inorganic nanoparticles can engage in interactions with polymers via external voltage. WS₂ NPs have a large specific surface area and good conductivity. They can be used as the charge trap center in the active layer, which is conducive to the charge transfer in the active layer. Poly[2,7-9-(9-heptadecanyl)-9H-carbazole-co-benzo[4,5] imidazole[2,1-α] isoindol-11-one] (PIIO) was synthesized via the Suzuki coupling reaction. ITO/PIIO/Al and ITO/PIIO:WS₂ NP/Al devices were prepared by the spin coating method and vacuum evaporation technology. All devices showed tristable switching behavior. The influence of the WS₂ mass fraction on memory performance was studied. The device composite with 6 wt % WS₂ NPs showed the best storage features. The OFF/ON1/ON2 current ratio was 1: 1.11 × 10¹: 2.03 × 10⁴, and the threshold voltage V_th1/V_th2 was -0.60 V/-1.05 V. The device is steady for 12,000 s in three states-high-resistance state (HRS), intermediate state (IRS), and low-resistance state (LRS). After reading 3500 times, the switch-state current displayed no obvious attenuation. This work shows that the polymer and its composites have broad prospects in next-generation nonvolatile storage.