Interface-Modulated Resistive Switching in Mo-Irradiated ReS2 for Neuromorphic Computing

Physical reservoirs based on MoS2-HZO integrated ferroelectric field-effect transistors for reservoir computing systems

Reservoir computing (RC), a variant of recurrent neural networks (RNNs), is well-known for its reduced energy consumption through exclusive focus on training the output weight and its superior performance in handling spatiotemporal information. Implementing these networks in hardware requires devices with superior fading memory behavior. Unlike filament-based two-terminal devices, those relying on ferroelectric switching demonstrate improved voltage reliability, while three-terminal transistors provide additional active control. HfO2-based ferroelectric materials such as Hf0.5Zr0.5O2 (HZO), have garnered attention for their scalability and seamless integration with CMOS technology. This study implements a RC hardware based on MoS2-HZO integrated device structure with enhanced spontaneous polarization field. By adjusting the oxygen vacancy concentration, the devices exhibit consistent responses to both identical and nonidentical voltages, making them suitable for diverse RC applications. The high accuracy of MNIST handwritten digits recognition highlights the rich reservoir states of the traditional RC architecture. Additionally, the impact of masks on RC implementation is assessed, showcasing the device's capability for spatiotemporal signal analysis. This development paves the way for implementing energy-efficient and high-performance computing solutions.

Ag-doped non-imperfection-enabled uniform memristive neuromorphic device based on van der Waals indium phosphorus sulfide

Memristors are considered promising energy-efficient artificial intelligence hardware, which can eliminate the von Neumann bottleneck by parallel in-memory computing. The common imperfection-enabled memristors are plagued with critical variability issues impeding their commercialization. Reported approaches to reduce the variability usually sacrifice other performances, e.g., small on/off ratios and high operation currents. Here, we demonstrate an unconventional Ag-doped nonimperfection diffusion channel-enabled memristor in van der Waals indium phosphorus sulfide, which can combine ultralow variabilities with desirable metrics. We achieve operation voltage, resistance, and on/off ratio variations down to 3.8, 2.3, and 6.9% at their extreme values of 0.2 V, 1011 ohms, and 108, respectively. Meanwhile, the operation current can be pushed from 1 nA to 1 pA at the scalability limit of 6 nm after Ag doping. Fourteen Boolean logic functions and convolutional image processing are successfully implemented by the memristors, manifesting the potential for logic-in-memory devices and efficient non-von Neumann accelerators.

Resistive switching in benzylammonium-based Ruddlesden-Popper layered hybrid perovskites for non-volatile memory and neuromorphic computing

Artificial synapses based on resistive switching have emerged as a promising avenue for brain-inspired computing. Hybrid metal halide perovskites have provided the opportunity to simplify resistive switching device architectures due to their mixed electronic-ionic conduction, yet the instabilities under operating conditions compromise their reliability. We demonstrate reliable resistive switching and synaptic behaviour in layered benzylammonium (BzA) based halide perovskites of (BzA)2PbX4 composition (X = Br, I), showing a transformation of the resistive switching from digital to analog with the change of the halide anion. While (BzA)2PbI4 devices demonstrate gradual set and reset processes with reduced power consumption, the (BzA)2PbBr4 system features a more abrupt switching behaviour. Moreover, the iodide-based system displays excellent retention and endurance, whereas bromide-based devices achieve a superior on/off ratio. The underlying mechanism is attributed to the migration of halide ions and the formation of halide vacancy conductive filaments. As a result, the corresponding devices emulate synaptic characteristics, demonstrating the potential for neuromorphic computing. Such resistive switching and synaptic behaviour highlight (BzA)2PbX4 perovskites as promising candidates for non-volatile memory and neuromorphic computing.

Recent Advances in In-Memory Computing: Exploring Memristor and Memtransistor Arrays with 2D Materials

The conventional computing architecture faces substantial challenges, including high latency and energy consumption between memory and processing units. In response, in-memory computing has emerged as a promising alternative architecture, enabling computing operations within memory arrays to overcome these limitations. Memristive devices have gained significant attention as key components for in-memory computing due to their high-density arrays, rapid response times, and ability to emulate biological synapses. Among these devices, two-dimensional (2D) material-based memristor and memtransistor arrays have emerged as particularly promising candidates for next-generation in-memory computing, thanks to their exceptional performance driven by the unique properties of 2D materials, such as layered structures, mechanical flexibility, and the capability to form heterojunctions. This review delves into the state-of-the-art research on 2D material-based memristive arrays, encompassing critical aspects such as material selection, device performance metrics, array structures, and potential applications. Furthermore, it provides a comprehensive overview of the current challenges and limitations associated with these arrays, along with potential solutions. The primary objective of this review is to serve as a significant milestone in realizing next-generation in-memory computing utilizing 2D materials and bridge the gap from single-device characterization to array-level and system-level implementations of neuromorphic computing, leveraging the potential of 2D material-based memristive devices.

Controllable Resistive Switching in ReS2 /WS2 Heterostructure for Nonvolatile Memory and Synaptic Simulation

Memristors with nonvolatile storage performance and simulated synaptic functions are regarded as one of the critical devices to overcome the bottleneck in traditional von Neumann computer architecture. 2D van der Waals heterostructures have paved a new way for the development of advanced memristors by integrating the intriguing features of different materials and offering additional controllability over their optoelectronic properties. Herein, planar memristors with both electrical and optical tunability based on ReS2 /WS2 van der Waals heterostructure are demonstrated. The devices show unique unipolar nonvolatile behavior with high Roff /Ron ratio of up to 106 , desirable endurance, and retention, which are superior to pure ReS2 and WS2 devices. When decreasing the channel length, the set voltage can be notably reduced while the high Roff /Ron ratios are retained. By introducing electrostatic doping through the gate control, the set voltage can be tailored in a wide range from 4.50 to 0.40 V. Furthermore, biological synaptic functions and plasticity, including spike rate-dependent plasticity and paired-pulse facilitation, are successfully realized. By employing optical illumination, resistive switching can also be modulated, which is dependent on the illumination energy and power. A mechanism related to the interlayer charge transfer controlled by optical excitation is revealed.

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.

Synthesis of a large area ReS2 thin film by CVD for in-depth investigation of resistive switching: effects of metal electrodes, channel width and noise behaviour

The anisotropic crystal structure and layer independent electrical and optical properties of ReS2 make it unique among other two-dimensional materials (2DMs), emphasizing a special need for its synthesis. This work discusses the synthesis and in-depth characterization of a 1 × 1 cm2 large and few layered ReS2 film. Vibrational modes and excitonic peaks observed from the Raman and photoluminescence (PL) spectra corroborated the formation of a ReS2 film with a 1.26 eV bandgap. High resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns inferred the polycrystalline nature of the film, while cross-sectional field emission scanning electron microscopy (FESEM) indicated planar growth with ∼10 nm thickness. The chemical composition of the film analysed through X-ray photoelectron spectroscopy (XPS) indicated the formation of a ReS2 film with a Re : S atomic ratio of 1 : 1.75, indicating a small amount of non-stoichiometric RexSy. Following the basic characterization studies, the ReS2 film was tested for resistive switching (RS) device application in which the effects of different metal electrodes (Pt/Au and Ag/Au) and different channel widths (200, 100, and 50 μm) were studied. The highest memory window equal to 108 was obtained for the Ag/Au electrode while Pt/Au showed a memory window of 102. RS for the former was ascribed to the formation of a conducting filament (CF) because of the migration of Ag+ ions, while defect mediated charge carrier transport led to switching in the Pt/Au electrode. Furthermore, the RHRS/RLRS ratio achieved in this work (108) is also of the highest magnitude reported thus far. Furthermore, a comparison of devices with Ag/Au electrodes but with different channel widths (50, 100 and 200 μm) gave insightful results on the existence of multiple resistance states, device endurance and retention. An inverse relationship between the retention time and the device's channel width was observed, where the device with a 50 μm channel width showed a retention time of 48 hours, and the one with a 200 μm width showed stability only up to 3000 s. Furthermore, low frequency noise measurements were performed to understand the effect of defects in the low resistance state (LRS) and the high resistance state (HRS). The HRS exhibited Lorentzian noise behaviour while the LRS exhibited Lorentzian only at low current bias which converged to 1/f noise at higher current bias.

Optoelectronic Synaptic Memtransistor Based on 2D SnSe/MoS2 van der Waals Heterostructure under UV-Ozone Treatment

Memristive switching devices with electrically and optically invoked synaptic behaviors show great promise in constructing an artificial biological visual system. Through rational design and integration, 2D materials and their van der Waals (vdW) heterostructures can be applied to realize multifunctional optoelectronic devices. Here, a multifunctional optoelectronic synaptic memtransistor based on a SnSe/MoS₂ vdW p-n heterojunction to simulate the human biological visual system is reported. By employing simple mild UV-ozone treatment, the device exhibits reversible resistive switching (RS) behavior with switching ratio up to 10³ . The retina-like selective response to different input light wavelengths is activated, as well as programmable multilevel resistance states and long-term synaptic plasticity. Moreover, memory and logic functions analogous to those found in the visual cortex of the brain are performed by controlling the optical and electrical input signals. This work proposes a feasible strategy to modulate RS in vdW heterostructures for memristive devices, which show significant potential for neuromorphic processing.

Direct Laser Irradiation and Modification of 2D Te for Development of Volatile Memristor

Laser irradiation, as a kind of post-fabrication method for two-dimensional (2D) materials, is a promising way to tune the properties of materials and the performance of corresponding nano-devices. As the memristor has been regarded as an excellent candidate for in-memory devices in next-generation computing system, the application of laser irradiation in developing excellent memristor based on 2D materials should be explored deeply. Here, tellurene (Te) flakes are exposed to a 532 nm laser in the air atmosphere to investigate the evolutions of the surface morphology and atom structures under different irradiation parameters. Laser is capable of thinning the flakes, inducing amorphous structures, oxides and defects, and forming nanostructures by controlling the irradiation power and time. Furthermore, the laser-induced oxides and defects promote the migration of metal ions in Te, resulting in the formation of the conductive filaments, which provides the switching behavers of volatile memristor, opening a route to the development of next-generation nano-devices.