Jiuxiang Dai

The synthesis of multiclass 2D ultrathin metal halides is realized through a simple droplet method, which possesses the advantages of simple operation, low growth temperature, substrate independence, low cost, and fabrication feasibility for heterostructures and devices.

Abstract

2D metal halides have attracted increasing research attention in recent years; however, it is still challenging to synthesize them via liquid-phase methods. Here it is demonstrated that a droplet method is simple and efficient for the synthesis of multiclass 2D metal halides, including trivalent (BiI₃, SbI₃), divalent (SnI₂, GeI₂), and monovalent (CuI) ones. In particular, 2D SbI₃ is first experimentally achieved, of which the thinnest thickness is ≈6 nm. The nucleation and growth of these metal halide nanosheets are mainly determined by the supersaturation of precursor solutions that are dynamically varying during the solution evaporation. After solution drying, the nanosheets can fall on the surface of many different substrates, which further enables the feasible fabrication of related heterostructures and devices. With SbI₃/WSe₂ being a good demonstration, the photoluminescence intensity and photo responsivity of WSe₂ is obviously enhanced after interfacing with SbI₃. The work opens a new pathway for 2D metal halides toward widespread investigation and applications.

Nature, Published online: 28 June 2023; doi:10.1038/s41586-023-06072-x

Monolayer tungsten ditelluride (WTe₂) can be both a quantum spin Hall insulator and gated into a superconducting state. Josephson weak-links are fabricated using local gates on hBN-encapsulated WTe₂ and the 2D nature of the superconducting leads is demonstrated. The fabrication provides a first step toward realizing versatile all-in-one gate configurable topological Josephson weak-links.

Abstract

Systems combining superconductors with topological insulators offer a platform for the study of Majorana bound states and a possible route to realize fault tolerant topological quantum computation. Among the systems being considered in this field, monolayers of tungsten ditelluride (WTe₂) have a rare combination of properties. Notably, it has been demonstrated to be a quantum spin Hall insulator (QSHI) and can easily be gated into a superconducting state. Measurements on gate-defined Josephson weak-link devices fabricated using monolayer WTe₂ are reported. It is found that consideration of the 2D superconducting leads are critical in the interpretation of magnetic interference in the resulting junctions. The reported fabrication procedures suggest a facile way to produce further devices from this technically challenging material and the results mark the first step toward realizing versatile all-in-one topological Josephson weak-links using monolayer WTe₂.

A strategy is proposed to activate the transverse flexoelectricity with a widely influenced range through the flexural suspended films. Sizable-area (2500 times larger than the tip-surface contact area) domain switching is successfully realized in hundreds of nanometer-thick (up to 550 nm) van der Waals ferroelectric CuInP₂S₆, driving by a single-point ultralow tip-force with the surface intact.

Abstract

Deterministic control of ferroelectric domain is critical in the ferroelectric functional electronics. Ferroelectric polarization can be manipulated mechanically with a nano-tip through flexoelectricity. However, it usually occurs in a very localized area in ultrathin films, with possible permanent surface damage caused by a large tip-force. Here it is demonstrated that the deliberate engineering of transverse flexoelectricity offers a powerful tool for improving the mechanical domain switching. Sizable-area domain switching under an ultralow tip-force can be realized in suspended van der Waals ferroelectrics with the surface intact, due to the enhanced transverse flexoelectric field. The film thickness range for domain switching in suspended ferroelectrics is significantly improved by an order of magnitude to hundreds of nanometers, being far beyond the limited range of the substrate-supported ones. The experimental results and phase-field simulations further reveal the crucial role of the transverse flexoelectricity in the domain manipulation. This large-scale mechanical manipulation of ferroelectric domain provides opportunities for the flexoelectricity-based domain controls in emerging low-dimensional ferroelectrics and related devices.

2D vdW ferroelectric materials reveal great potential applications in electronics, optoelectronics, and spintronics, owing to robust spontaneous polarization, widespread bandgap tunability, inert surfaces, and silicon-based technology compatibility. This review covers their recent advances in ferroelectricity origin and practical applications, especially in artificial intelligence. The hot topic of sliding ferroelectrics in physical origins and novel properties is also focused on.

Abstract

In recent years, an increasing number of 2D van der Waals (vdW) materials are theory-predicted or laboratory-validated to possess in-plane (IP) and/or out-of-plane (OOP) spontaneous ferroelectric polarization. Due to their dangling-bond-free surfaces, interlayer charge coupling, robust polarization, tunable energy band structures, and compatibility with silicon-based technologies, vdW ferroelectric materials exhibit great promise in ferroelectric memories, neuromorphic computing, nanogenerators, photovoltaic devices, spintronic devices, and so on. Here, the very recent advances in the field of vdW ferroelectrics (FEs) are reviewed. First, theories of ferroelectricity are briefly discussed. Then, a comprehensive summary of the non-stacking vdW ferroelectric materials is provided based on their crystal structures and the emerging sliding ferroelectrics. In addition, their potential applications in various branches/frontier fields are enumerated, with a focus on artificial intelligence. Finally, the challenges and development prospects of vdW ferroelectrics are discussed.

2D PdTe₂/WSe₂ van der Waals (vdWs) field effect transistor (FET) device with broadband photo-response is constructed by using the ideal bandgap and excellent optoelectronic properties of PdTe₂ nanoflakes and few-layer p-type WSe₂. Benefiting from strong Schottky barrier and carrier-depletion of WSe₂ layer modulated by gate voltage, an extremely low dark current of ≈1.2 pA and high light on/off ratio of ≈10⁶ is achieved.

Abstract

Due to its unique band structure and topological properties, the 2D topological semimetal exhibits potential applications in photoelectric detection, polarization sensitive imaging, and Schottky barrier diodes. However, its inherent large dark current hinders the further improvement of the performance of the semimetal-based photodetectors. In this study, a van der Waals (vdWs) field effect transistor (FET) composed of semimetal PdTe₂ and transition metal dichalcogenides (TMDs) WSe₂ is fabricated, which exhibits high sensitivity photoelectric detection performance in a wide band from visible light (405 nm) to mid-infrared (5 µm). The dark current and the noise level in the device are greatly suppressed by the effective control of the gate. Benefiting from the extremely low dark current (1.2 pA), the device achieves an optical on/off ratio up to 10⁶, a high detectivity of 9.79 × 10¹³ Jones and a rapid response speed (219 and 45 µs). This research demonstrates the latent capacity of the 2D topological semimetal/TMDs vdWs FET for broadband, high-performance, and miniaturized photodetection.

Quasi-1D BiSeI nanowires with high purity and high quality are synthesized via chemical vapor deposition on mica substrate. The high crystallinity and inherent structural anisotropy of BiSeI nanowire offer its detector board photoresponse with high responsivity, fast response rate, and prominent linear dichroism. This work opens new prospects for building novel multifunctional optoelectronic devices.

Abstract

Bismuth chalcohalides (BiSeI and BiSI), a class of superior light absorbers, have recently garnered great attention owing to their promise in constructing next-generation optoelectronic devices. However, to date, the photodetection application of bismuth chalcohalides is still limited due to the challenge in controllable preparation. Herein, the synthesis of large-scale quasi-1D BiSeI nanowires via chemical vapor deposition growth is reported. By precisely tuning the growth temperature and the Se supply, it can effectively control the growth thermodynamics and kinetics of BiSeI crystal, and thus achieve high purity quasi-1D BiSeI nanowires with high crystal quality, uniform diameter, and tunable domain length. Theory and optical characterizations of the quasi-1D BiSeI nanowires reveal an indirect bandgap of 1.57 eV with prominent optical linear dichroism. As a result, the quasi-1D BiSeI nanowire-based photodetector demonstrates a broadband photoresponse (400–800 nm) with high responsivity of 5880 mA W⁻¹, fast response speed of 0.11 ms and superior air stability. More importantly, the photodetector displays strong polarization sensitivity (anisotropic ratio = 1.77) under the 532 nm light irradiation. This work will provide important guides to the synthesis of other quais-1D metal chalcohalides and shed light on their potential in constructing novel multifunctional optoelectronic devices.

The ion dynamics and electric field distribution at the electrolyte/2D material interface and their influence on monolayer transition metal dichalcogenides (TMDs) properties are clarified by Kelvin probe force microscope and optical techniques experimentally as well as theoretically in this study. Potential distribution, electric field-induced junctions, and gate-dependent photonic dynamics in TMD electric double layer (EDL) transistors are revealed.

Abstract

Electric double layer (EDL) devices based on 2D materials have made great achievements for versatile electronic and opto-electronic applications; however, the ion dynamics and electric field distribution of the EDL at the electrolyte/2D material interface and their influence on the physical properties of 2D materials have not been clearly clarified. In this work, by using Kelvin probe force microscope and steady/transient optical techniques, the character of the EDL and its influence on the optical properties of monolayer transition metal dichalcogenides (TMDs) are probed. The potential drop, unscreened EDL potential distribution, and accumulated carriers at the electrolyte/TMD interface are revealed, which can be explained by nonlinear Thomas–Fermi theory. By monitoring the potential distribution along the channel, the evolution of the electric field-induced lateral junction in the TMD EDL transistor is accessed, giving rise to the better exploration of EDL device physics. More importantly, EDL gate-dependent carrier recombination and exciton–exciton annihilation in monolayer TMDs on lithium-ion solid state electrolyte (Li₂Al₂SiP₂TiO₁₃) are evaluated for the first time, benefiting from the understanding of the interaction between ions, carriers, and excitons. The work will deepen the understanding of the EDL for the exploitation of functional device applications.

Polarized light-emitting diodes (LEDs) based on 2D monolayer semiconductors are demonstrated at room temperature by using single Ag nanowires as both the electrical contact and the plasmonic resonant nanocavity. The polarized 2D LEDs show pronounced linear polarization dependency with a degree of polarization as high as 63% and work with multiple types of monolayers, e.g., WSe₂, MoSe₂, and WS₂.

Abstract

Transition metal dichalcogenide (TMD)-based 2D monolayer semiconductors, with the direct bandgap and the large exciton binding energy, are widely studied to develop miniaturized optoelectronic devices, e.g., nanoscale light-emitting diodes (LEDs). However, in terms of polarization control, it is still quite challenging to realize polarized electroluminescence (EL) from TMD monolayers, especially at room temperature. Here, by using Ag nanowire top electrode, polarized LEDs are demonstrated based on 2D monolayer semiconductors (WSe₂, MoSe₂, and WS₂) at room temperature with a degree of polarization (DoP) ranging from 50% to 63%. The highly anisotropic EL emission comes from the 2D/Ag interface via the electron/hole injection and recombination process, where the EL emission is also enhanced by the polarization-dependent plasmonic resonance of the Ag nanowire. These findings introduce new insights into the design of polarized 2D LED devices at room temperature and may promote the development of miniaturized 2D optoelectronic devices.

Quantum-dot light-emitting diodes (QLEDs) show a promising application in displays. However, challenges in QLED industrialization remain, including unclear device mechanisms, low performance of blue QLEDs, and immature color patterning. This paper reviews recent advancements addressing these challenges, covering an overview of device mechanisms and degradation, progress in blue QLEDs, and color patterning.

Abstract

Light-emitting diodes (LEDs) based on colloidal quantum-dots (QDs) such as CdSe, InP, and ZnSeTe feature a unique advantage of narrow emission linewidth of ≈20 nm, which can produce highly accurate colors, making them a highly promising technology for the realization of displays with Rec. 2020 color gamut. With the rapid development in the past decades, the performances of red and green QLEDs have been remarkably improved, and their efficiency and lifetime can almost meet industrial requirements. However, the industrialization of QLED displays still faces many challenges; for example, (1) the device mechanisms including the charge injection/transport/leakage, exciton quenching, and device degradation are still unclear, which fundamentally limit QLED performance improvement; (2) the blue performances including the efficiency, chromaticity, and stability are relatively low, which are still far from the requirements of practical applications; (3) the color patterning processes including the ink-jet printing, transfer printing, and photolithography are still immature, which restrict the manufacturing of high resolution full-color QLED displays. Here, the recent advancements attempting to address the above challenges of QLED displays are specifically reviewed. After a brief overview of QLED development history, device structure/principle, and performances, the main focus is to investigate the recent discoveries on device mechanisms with an emphasis on device degradation. Then recent progress is introduced in blue QLEDs and color patterning. Finally, the opportunities, challenges, solutions, and future research directions of QLED displays are summarized.

Herein, the potential applications of 2D-HfTe₂ nanosheets in nonlinear optics and photonics are discussed. A mode-locked laser with 724 fs pulsed width is realized using a side-polished fiber-based HfTe₂-saturable absorber, at a second-lowest mode-locking threshold pump power among 2D material-based SAs. Finally, the HfTe₂-SA is used to generate a highly stable single-frequency fiber laser illustrating its promising ultranarrow photonic applications.

Abstract

2D semi-metallic hafnium ditelluride material is used in several applications such as solar steam generation, gas sensing, and catalysis owing to its strong near-infrared absorbance, high sensitivity, and distinctive electronic structure. The zero-bandgap characteristics, along with the thermal and dynamic stability of 2D-HfTe_2, make it a desirable choice for developing long-wavelength-range photonics devices. Herein, the HfTe₂-nanosheets are prepared using the liquid-phase exfoliation method, and their superior nonlinear optical properties are demonstrated by the obtained modulation depth of 11.9% (800 nm) and 6.35% (1560 nm), respectively. In addition, the observed transition from saturable to reverse saturable absorption indicates adaptability of the prepared material in nonlinear optics. By utilizing a side polished fiber-based HfTe₂-saturable absorber (SA) inside an Er-doped fiber laser cavity, a mode-locked laser with 724 fs pulse width and 56.63 dB signal-to-noise ratio (SNR) is realized for the first time. The generated laser with this SA has the second lowest mode-locking pump threshold (18.35 mW), among the other 2D material based-SAs, thus paving the way for future laser development with improved efficiency and reduced thermal impact. Finally, employing this HfTe₂-SA, a highly stable single-frequency fiber laser (SNR ≈ 74.56 dB; linewidth ≈ 1.268 kHz) is generated for the first time, indicating its promising ultranarrow photonic application.

npj 2D Materials and Applications, Published online: 26 June 2023; doi:10.1038/s41699-023-00400-5

npj 2D Materials and Applications, Published online: 26 June 2023; doi:10.1038/s41699-023-00410-3

Reservoir computing (RC) provides an alternative brain-inspired framework for fast learning with low training cost. Memristor exhibits nonlinear characteristics and retains a memory of previous inputs, which is suitable for implementing physical RC systems. This review focuses on the recent progress in memristor-implemented RC systems and their neuromorphic applications. In particular, it outlines how to modulate the dynamic memory behaviour of the memristor from a material perspective.

Abstract

The booming development of artificial intelligence (AI) requires faster physical processing units as well as more efficient algorithms. Recently, reservoir computing (RC) has emerged as an alternative brain-inspired framework for fast learning with low training cost, since only the weights associated with the output layers should be trained. Physical RC becomes one of the leading paradigms for computation using high-dimensional, nonlinear, dynamic substrates. Among them, memristor appears to be a simple, adaptable, and efficient framework for constructing physical RC since they exhibit nonlinear features and memory behavior, while memristor-implemented artificial neural networks display increasing popularity towards neuromorphic computing. In this review, the memristor-implemented RC systems from the following aspects: architectures, materials, and applications are summarized. It starts with an introduction to the RC structures that can be simulated with memristor blocks. Specific interest then focuses on the dynamic memory behaviors of memristors based on various material systems, optimizing the understanding of the relationship between the relaxation behaviors and materials, which provides guidance and references for building RC systems coped with on-demand application scenarios. Furthermore, recent advances in the application of memristor-based physical RC systems are surveyed. In the end, the further prospects of memristor-implemented RC system in a material view are envisaged.

The precise modulation of diffusion of metal ion/atoms and their reduction/oxidation probability holds promise to overcome the speed, size, and energy issues of present-day computer. Here, this work shows that the diffusion metal ion can be modulated by the defects inside the switching medium and confines metal filaments in a precise, 1D channel.

Abstract

The neuromorphic and in-memory computing using memristors are promising for the building of the next generation computing systems. However, the diffusion dynamics of metal ions/atoms inside the switching medium impose variability in conducting filament (CF) formation, thus limiting their use in von-Neumann architecture. The precise modulation on the diffusion of metal ions/atoms and their reduction/oxidation probability holds promise to overcome the speed, size, and energy issues of present-day computers. Here, this study shows that the diffusion of metal ions can be modulated by defects inside the switching medium and confines metal filaments in a precise 1D channel. This filament confinement by the defect engineering leads to an anomalous switching mechanism with two interchangeable modes: unipolar threshold and bipolar modes. The variation between two modes can be modulated by controlling defects in the structures, leading to a uniform switching with low SET/RESET voltage variations of 17.3% and −17.6%, respectively. Moreover, the convolutional neural network is implemented to emulate synaptic plasticity and image recognition to achieve recognition accuracy of 87% due to a highly linear weight update, demonstrating its potential for in-memory computing.

By van der Waals (vdW) composite (graphene/MoS₂ etc.) nanopowder-to-heterojunction conversion under graphene edge-oxygen incorporation, a coexistent structure of nanoscale homojunctions and heterojunctions is formed on the grinding balls. Tunable, wide-temperature (−200 to 300 °C) and macroscale superlubricity (0.005) is achieved on diamond-like carbon surfaces by nanoscale vdW heterojunction-to-homojunction transformation unlike traditional incommensurability-to-commensurability theory.

Abstract

Achieving macroscale superlubricity of van der Waals (vdW) nanopowders is particularly challenging, due to the difficulty in forming ordered junctions before friction and the friction-induced complex contact restructuration among multiple nanometer-sized junctions. Here, a facile way is reported to achieve vdW nanopowder-to-heterojunction conversion by graphene edge-oxygen (GEO) incorporation. The GEO effectively weakens the out-of-plane edge–edge and in-plane plane–edge states of the vdW nanopowder, leading to a coexistent structure of nanoscale homojunctions and heterojunctions on the grinding balls. When sliding on diamond-like carbon surfaces, the ball-supported structure governs macroscale superlubricity by heterojunction-to-homojunction transformation among the countless nanoscale junctions. Furthermore, the transformation guides the tunable design of superlubricity, achieving superlubricity (µ ≈ 0.005) at wide ranges of load, velocity, and temperature (−200 to 300 °C). Atomistic simulations reveal the GEO-enhanced conversion of vdW nanopowder to heterojunctions and demonstrate the heterojunction-to-homojunction transformation superlubricity mechanism. The findings are of significance for the macroscopic scale-up and engineering application of structural superlubricity.