Developing fatigue-resistant ferroelectrics using interlayer sliding switching.

Ferroelectric materials have switchable electrical polarization that is appealing for high density non-volatile memories. However, inevitable fatigue hinders practical applications of these materials. Fatigue-free ferroelectric switching could dramatically improve the endurance of devices. We report a fatigue-free ferroelectric system based on the sliding ferroelectricity of bilayer 3R-MoS2. The memory performance of this ferroelectric device does not show the "wake-up effect" at low cycles or a substantial "fatigue effect" after 106 switching cycles under different pulse widths. The total stress time of device under an electric field is up to 105 s, which is long relative to other devices. Our theoretical calculation uncovers that the fatigue-free feature of sliding ferroelectricity is due to the immobile charge defects in sliding ferroelectricity.

The Integration of Two-Dimensional Materials and Ferroelectrics for Device Applications.

In recent years, there has been growing interest in functional devices based on two-dimensional (2D) materials, which possess exotic physical properties. With an ultrathin thickness, the optoelectrical and electrical properties of 2D materials can be effectively tuned by an external field, which has stimulated considerable scientific activities. Ferroelectric fields with a nonvolatile and electrically switchable feature have exhibited enormous potential in controlling the electronic and optoelectronic properties of 2D materials, leading to an extremely fertile area of research. Here, we review the 2D materials and relevant devices integrated with ferroelectricity. This review starts to introduce the background about the concerned themes, namely 2D materials and ferroelectrics, and then presents the fundamental mechanisms, tuning strategies, as well as recent progress of the ferroelectric effect on the optical and electrical properties of 2D materials. Subsequently, the latest developments of 2D material-based electronic and optoelectronic devices integrated with ferroelectricity are summarized. Finally, the future outlook and challenges of this exciting field are suggested.

Controllable resistive switching behaviors in heteroepitaxial LaNiO3/Nb:SrTiO3Schottky junctions through oxygen vacancies engineering.

Perovskite oxide-based memristors have been extensively investigated for the application of non-volatile memories, and the oxygen vacancies associated with Schottky barrier changing are considered as the origin of the memristive behaviors. However, due to the difference of device fabrication progress, various resistive switching (RS) behaviors have been observed even in one device, deteriorating the stability and reproducibility of devices. Precisely controlling the oxygen vacancies distribution and shedding light on the behind physic mechanism of these RS behaviors, are highly desired to help improve the performance and stability of such Schottky junction-based memristors. In this work, the epitaxial LaNiO3(LNO)/Nb:SrTiO3(NSTO) is adopted to explore the influence of oxygen vacancy profiles on these abundant RS phenomena. It demonstrates that the migration of oxygen vacancy in LNO films plays a key role in memristive behaviors. When the effect of oxygen vacancies at the LNO/NSTO interface is negligible, improving the oxygen vacancies concentration in LNO film could facilitate resistance on/off ratio of HRS and LRS, and the corresponding conducting mechanisms attributes to the thermionic emission and tunneling-assisted thermionic emission, respectively. Moreover, it is found that reasonably increasing the oxygen vacancies at LNO/NSTO interface makes trap-assisted tunneling possible, also providing an effective way to improve the performance of the device. The results in this work have clearly elucidated the relationship between oxygen vacancy profile and RS behaviors, and give physical insights into the strategies for improving the device performance of Schottky junction-based memristors.

High-Performance Sliding Ferroelectric Transistor Based on Schottky Barrier Tuning.

Sliding ferroelectricity associated with interlayer translation is an excellent candidate for ferroelectric device miniaturization. However, the weak polarization gives rise to the poor performance of sliding ferroelectric transistors with a low on/off ratio and a narrow memory window, which restricts its practical application. To address the issue, we propose a facile strategy by regulating the Schottky barrier in sliding ferroelectric semiconductor transistors based on γ-InSe, in which a high performance with a large on/off ratio (106) and a wide memory window (4.5 V) was ultimately acquired. Additionally, the memory window of the device can be further modulated by electrostatic doping or light excitation. These results open up new ways for designing novel ferroelectric devices based on emerging sliding ferroelectricity.

Recent progress in the synthesis of novel two-dimensional van der Waals materials.

The last decade has witnessed the significant progress of physical fundamental research and great success of practical application in two-dimensional (2D) van der Waals (vdW) materials since the discovery of graphene in 2004. To date, vdW materials is still a vibrant and fast-expanding field, where tremendous reports have been published covering topics from cutting-edge quantum technology to urgent green energy, and so on. Here, we briefly review the emerging hot physical topics and intriguing materials, such as 2D topological materials, piezoelectric materials, ferroelectric materials, magnetic materials and twistronic heterostructures. Then, various vdW material synthetic strategies are discussed in detail, concerning the growth mechanisms, preparation conditions and typical examples. Finally, prospects and further opportunities in the booming field of 2D materials are addressed.

Solid-Ionic Memory in a van der Waals Heterostructure.

Defect states dominate the performance of low-dimensional nanoelectronics, which deteriorate the serviceability of devices in most cases. But in recent years, some intriguing functionalities are discovered by defect engineering. In this work, we demonstrate a bifunctional memory device of a MoS2/BiFeO3/SrTiO3 van der Waals heterostructure, which can be programmed and erased by solely one kind of external stimuli (light or electrical-gate pulse) via engineering of oxygen-vacancy-based solid-ionic gating. The device shows multibit electrical memory capability (>22 bits) with a large linearly tunable dynamic range of 7.1 × 106 (137 dB). Furthermore, the device can be programmed by green- and red-light illuminations and then erased by UV light pulses. Besides, the photoresponse under red-light illumination reaches a high photoresponsivity (6.7 × 104 A/W) and photodetectivity (2.12 × 1013 Jones). These results highlighted solid-ionic memory for building up multifunctional electronic and optoelectronic devices.

Mimicking Neuroplasticity via Ion Migration in van der Waals Layered Copper Indium Thiophosphate.

Artificial synaptic devices are the essential components of neuromorphic computing systems, which are capable of parallel information storage and processing with high area and energy efficiencies, showing high promise in future storage systems and in-memory computing. Analogous to the diffusion of neurotransmitter between neurons, ion-migration-based synaptic devices are becoming promising for mimicking synaptic plasticity, though the precise control of ion migration is still challenging. Due to the unique 2D nature and highly anisotropic ionic transport properties, van der Waals layered materials are attractive for synaptic device applications. Here, utilizing the high conductivity from Cu+ -ion migration, a two-terminal artificial synaptic device based on layered copper indium thiophosphate is studied. By controlling the migration of Cu+ ions with an electric field, the device mimics various neuroplasticity functions, such as short-term plasticity, long-term plasticity, and spike-time-dependent plasticity. The Pavlovian conditioning and activity-dependent synaptic plasticity involved neural functions are also successfully emulated. These results show a promising opportunity to modulate ion migration in 2D materials through field-driven ionic processes, making the demonstrated synaptic device an intriguing candidate for future low-power neuromorphic applications.

Photoinduced Hysteresis of Graphene Field-Effect Transistors Due to Hydrogen-Complexed Defects in Silicon Dioxide.

Photoinduced hysteresis (PIH) of graphene field-effect transistors (G-FETs) has attracted attention because of its potential in developing photoelectronic or nonvolatile memory devices. In this work, we focused on the role of SiO2 dielectric layer on PIH, where G-FETs have only a SiO2 dielectric layer. Adsorbates are effectively removed before the PIH test. The effects of laser wavelength, laser power density, and temperature on the PIH are systematically investigated. The PIH is significantly enhanced by increasing the hydrogen flow in a hydrogen-atmosphere device thermal annealing. This strongly suggests proton-related defects that play a key role. The pure electronic process for PIH is further ruled out by the significant dependence of the doping rate on the temperature. A mechanism of PIH based on proton generation after hole trapping at [O3≡Si-H] is proposed. The proposed mechanism is well-supported by our experimental data: (1) the observed threshold photon energy for PIH is between 2.76 and 2.34 eV, which is close to the energy barrier for [O3≡Si-H], releasing a proton. (2) No obvious carrier mobility degradation after the PIH process suggests that the bulk defects in SiO2 are the major contributors rather than graphene/SiO2 interface defects. (3) The dependence of the doping rate on the temperature and the laser power density matches a theoretical model based on the random hopping of H+. The results in this work are also valuable for the study of degradation of other oxide dielectric materials in various field-effect transistors.