Jiuxiang Dai

The edges of nanoribbons have unique electronic and spin states due to the lack of symmetry. It is found that the Young's modulus of single-layer molybdenum disulfide nanoribbons with armchair edges increases with decreasing the width below 3 nm. It means that the bond strength is stiffer at the armchair edge. Density functional theory calculations explain this by buckling at the edge.

Abstract

The physical and chemical properties of nanoribbon edges are important for characterizing nanoribbons and applying them in electronic devices, sensors, and catalysts. The mechanical response of molybdenum disulfide nanoribbons, which is an important issue for their application in thin resonators, is expected to be affected by the edge structure, albeit this result is not yet being reported. In this work, the width-dependent Young's modulus is precisely measured in single-layer molybdenum disulfide nanoribbons with armchair edges using the developed nanomechanical measurement based on a transmission electron microscope. The Young's modulus remains constant at ≈166 GPa above 3 nm width, but is inversely proportional to the width below 3 nm, suggesting a higher bond stiffness for the armchair edges. Supporting the experimental results, the density functional theory calculations show that buckling causes electron transfer from the Mo atoms at the edges to the S atoms on both sides to increase the Coulomb attraction.

Nature Communications, Published online: 12 September 2023; doi:10.1038/s41467-023-41382-8

The control of magnetism by electric field is an important goal for future development of low-power spintronics. Here, the authors demonstrate voltage control of magnetism in van der Waals ferromagnetic/ferroelectric heterostructure devices via the strain-mediated magnetoelectric effect.

Abstract

Two-dimensional transition metal chalcogenides (2D-TMDs) have attracted much attention because of their unique layered structure and physical properties for transistor applications. Mechanically transferred metal contacts on these low-dimensional materials or their homogeneous and heterogeneous multilayers have generated huge interest to avoid deposition damages. In this paper, we show that there are large physical gaps at both the edge contact and surface contact between the transferred electrodes and the 2D materials. A method called laser shock induced superplastic deformation (LSISD) is proposed to tackle this issue and enhance the performance of the transistors. The enhancement mechanism was investigated by molecular dynamics (MD) simulations of the nanoforming process, atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) characterizations of the interfaces, and density functional theory (DFT) modeling. The force effect of laser shock can reduce the contact gap between metals and semiconductors. The electrical performances of the transistors before and after LSISD, along with MD simulations, are used to find the optimal process parameters. In addition, this paper applies the LSISD method to the short-channel MoS₂/graphene vertical transistors to show potential improvement in interface contact and electrical properties. This paper demonstrates the first report on using mechanical force induced by laser shock to enhance metal–semiconductor interfaces and transistor performances.

The synthesis of polypyrrole-encapsulated indium selenide nanoflakes by chemical vapor transport method is reported. The reaction mechanism and the alternating significance of intercalation and conversion reactions and evanescent of alloying reactions are demonstrated. The theoretical calculations manifest that the heterointerface accelerates electron/Na⁺ transfer and reduces the Na⁺ diffusion barrier. The synergy between heterointerface and structural confinement boosts electrochemical properties.

Abstract

Layered indium selenide (InSe) is a new 2D semiconductor material with high carrier mobility, widely adjustable bandgap, and high ductility. However, its ion storage behavior and related electrochemical reaction mechanism are rarely reported. In this study, InSe nanoflakes encapsulated in conductive polypyrrole (InSe@PPy) are designed in consideration of restraining the severe volume change in the electrochemical reaction and increasing conductivity via in situ chemical oxidation polymerization. Density functional theory calculations demonstrate that the construction of heterostructure can generate an internal electric field to accelerate electron transfer via additional driving forces, offering synergistically enhanced structural stability, electrical conductivity, and Na⁺ diffusion process. The resulting InSe@PPy composite shows outstanding electrochemical performance in the sodium ion batteries system, achieving a high reversible capacity of 336.4 mA h g⁻¹ after 500 cycles at 1 A g⁻¹ and a long-term cyclic stability with capacity of 274.4 mA h g⁻¹ after 2800 cycles at 5 A g⁻¹. In particular, the investigation of capacity fluctuation within the first cycling reveals the alternating significance of intercalation and conversion reactions and evanescent alloying reaction. The combined reaction mechanism of insertion, conversion, and alloying of InSe@PPy is revealed by in situ X-ray diffraction, ex situ electrochemical impedance spectroscopy, and transmission electron microscopy.

The interfacing of monolayer MoS₂ with 2DCOF films determines a major enhancement in the charge transport through the 2D semiconductor. Electron doping intensity is amplified by the superlattice's long-range order, while the lattice vibration is suppressed by the high mechanical stiffness yielding a confinement effect. The MoS₂ /2D-COF heterostructures represent a novel strategy to develop high-performance 2D electronics.

Abstract

The coupling of different 2D materials (2DMs) to form van der Waals heterostructures (vdWHs) is a powerful strategy for adjusting the electronic properties of 2D semiconductors, for applications in opto-electronics and quantum computing. 2D molybdenum disulfide (MoS₂) represents an archetypical semiconducting, monolayer thick versatile platform for the generation of hybrid vdWH with tunable charge transport characteristics through its interfacing with molecules and assemblies thereof. However, the physisorption of (macro)molecules on 2D MoS₂ yields hybrids possessing a limited thermal stability, thereby jeopardizing their technological applications. Herein, the rational design and optimized synthesis of 2D covalent organic frameworks (2D-COFs) for the generation of MoS₂/2D-COF vdWHs exhibiting strong interlayer coupling effects are reported. The high crystallinity of the 2D-COF films makes it possible to engineer an ultrastable periodic doping effect on MoS₂, boosting devices’ field-effect mobility at room temperature. Such a performance increase can be attributed to the synergistic effect of the efficient interfacial electron transfer process and the pronounced suppression of MoS₂’s lattice vibration. This proof-of-concept work validates an unprecedented approach for the efficient modulation of the electronic properties of 2D transition metal dichalcogenides toward high-performance (opto)electronics for CMOS digital circuits.

npj 2D Materials and Applications, Published online: 11 September 2023; doi:10.1038/s41699-023-00429-6

Prolonged dephasing time of ensemble of moiré-trapped interlayer excitons in WSe₂-MoSe₂ heterobilayers

Nature Communications, Published online: 11 September 2023; doi:10.1038/s41467-023-41440-1

Effective energy transfer and suppressed dopant aggregation is critical for realizing efficient organic light-emitting diodes. Here, the authors report six host materials capable of exhibiting enhanced thermal stability and film-formation behavior, demonstrating efficiency of over 30% at 1000 cd/m2.

npj 2D Materials and Applications, Published online: 11 September 2023; doi:10.1038/s41699-023-00428-7

Tailoring exciton dynamics in TMDC heterobilayers in the ultranarrow gap-plasmon regime

Light: Science & Applications, Published online: 08 September 2023; doi:10.1038/s41377-023-01268-2

We demonstrate the manipulation of nonlinear polaritons and their prolonged coherence by creating fully deterministic potential wells with the lithographic mesas to trap polaritons in a monolayer WS2 microcavity.

This paper reports a strategy for enhancing triboelectric charge stability by depositing air-stable radicals on a surface. By utilizing AFM probes to precisely induce triboelectric charge on self-assembled monolayers, it is demonstrated that air-stable radicals help triboelectric charge sustain ≈3x longer than without the radicals. This concept opens up the possibility to extend the lifetime of air filtration systems, including masks.

Abstract

This paper demonstrates that air-stable radicals enhance the stability of triboelectric charge on surfaces. While charge on surfaces is often undesirable (e.g., static discharge), improved charge retention can benefit specific applications such as air filtration. Here, it is shown that self-assembled monolayers (SAMs) containing air-stable radicals, 2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), hold the charge longer than those without TEMPO. Charging and retention are monitored by Kelvin Probe Force Microscopy (KPFM) as a function of time. Without the radicals on the surface, charge retention increases with the water contact angle (hydrophobicity), consistent with the understanding that surface water molecules can accelerate charge dissipation. Yet, the most prolonged charge retention is observed in surfaces treated with TEMPO, which are more hydrophilic than untreated control surfaces. The charge retention decreases with reducing radical density by etching the TEMPO-silane with tetrabutylammonium fluoride (TBAF) or scavenging the radicals with ascorbic acid. These results suggest a pathway toward increasing the lifetime of triboelectric charges, which may enhance air filtration, improve tribocharging for patterning charges on surfaces, or boost triboelectric energy harvesting.

A ferroelectric gating photodetector is developed with a ReS₂/WSe₂ vdW heterojunction-channel. This device effectively detects a wide range of light wavelengths, including broadband light exceeding 1300 nm and expandable up to 2700 nm. The successful detection is attributed to the staggered type-II bandgap alignment, which results in an interlayer gap of 0.46 eV. By controlling the polarity of the P(VDF-TrFE) ferroelectric dipole for a specific wavelength, both high photoresponsivity (>6.9 × 10³ A W⁻¹) and low dark current (<0.26 nA) is simultaneously achieved across a broad range of wavelengths.

Abstract

The potential for various future industrial applications has made broadband photodetectors beyond visible light an area of great interest. Although most 2D van-der-Waals (vdW) semiconductors have a relatively large energy bandgap (>1.2 eV), which limits their use in short-wave infrared detection, they have recently been considered as a replacement for ternary alloys in high-performance photodetectors due to their strong light-matter interaction. In this study, a ferroelectric gating ReS₂/WSe₂ vdW heterojunction-channel photodetector is presented that successfully achieves broadband light detection (>1300 nm, expandable up to 2700 nm). The staggered type-II bandgap alignment creates an interlayer gap of 0.46 eV between the valence band maximum (VB_MAX) of WSe₂ and the conduction band minimum (CB_MIN) of ReS₂. Especially, the control of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric dipole polarity for a specific wavelength allows a high photoresponsivity of up to 6.9 × 10³ A W⁻¹ and a low dark current below 0.26 nA under the laser illumination with a wavelength of 405 nm in P-up mode. The achieved high photoresponsivity, low dark current, and full-range near infrared (NIR) detection capability open the door for next-generation photodetectors beyond traditional ternary alloy photodetectors.

Nature Communications, Published online: 08 September 2023; doi:10.1038/s41467-023-41047-6

Here, the authors report the emergence of dark-excitons in transition-metal-dichalcogenide heterostructures that strongly rely on the stacking sequence, i.e., momentum-dark K-Q excitons located exclusively at the top layer of the heterostructure.

npj 2D Materials and Applications, Published online: 09 September 2023; doi:10.1038/s41699-023-00427-8

Ionotronic WS₂ memtransistors for 6-bit storage and neuromorphic adaptation at high temperature

Nature Communications, Published online: 09 September 2023; doi:10.1038/s41467-023-41293-8

Here, the authors theoretically predict the formation of synergistic correlated and topological states in Coulomb-coupled and gate-tunable graphene/insulator heterostructures, proposing a number of promising substrate candidates and a possible explanation for recent experimental observations in graphene/CrOCl heterostructures.

Molecular beam epitaxy is used to realize the wafer-scale growth of uniform 1T-CrTe₂ films and establish the van der Waals integration of Bi₂Te₃/CrTe₂ heterostructures. Endorsed by the intrinsic perpendicular magnetic anisotropy and strong spin-orbit coupling, the Bi₂Te₃/CrTe₂-based crossbar array is fabricated to achieve reliable spin-orbit torque-driven magnetization switching, hence laying out a solid framework for energy-efficient spintronic applications.

Abstract

To harness the intriguing properties of 2D van der Waals (vdW) ferromagnets (FMs) for versatile applications, the key challenge lies in the reliable material synthesis for scalable device production. Here, the epitaxial growth of single-crystalline 1T-CrTe₂ thin films on 2-inch sapphire substrates are demonstrated. Benefiting from the uniform surface energy of the dangling bond-free Al₂O₃(0001) surface, the layer-by-layer vdW growth mode is observed right from the initial growth stage, which warrants precise control of the sample thickness beyond three monolayer and homogeneous surface morphology across the entire wafer. Moreover, the presence of the Coulomb interaction at the CrTe₂/Al₂O₃ interface plays an important role in tailoring the anomalous Hall response, and the structural optimization of the CrTe₂-based spin-orbit torque device leads to a substantial switching power reduction by 54%. The results may lay out a general framework for the design of energy-efficient spintronics based on configurable vdW FMs.

A broadband phototransistor is designed by combining the excellent electrical transportation features of oxide semiconductors with the superior optoelectronic response of InP quantum dots (QDs). The phototransistor array manifests a realistic environmental self-adaptation process on perceiving simple letters. Handwriting pattern recognition accuracy reaches 93% due to the satisfactory weight update linearity, demonstrating its faultless competence for image processing capabilities.

Abstract

The exploration of bionic neuromorphic chips, capable of processing sensory data in a human-like manner, is both a trend and a challenge. There is a strong demand for phototransistors that offer broadband in-sensor adaptability. This study introduces a bioinspired vision sensor based on InP quantum dots (QDs)/InSnZnO hybrid phototransistors. This novel design combines the excellent electrical transportation features of oxide semiconductors with the superior optoelectronic response of InP QDs. The resulting hybrid devices exhibit exceptional gate controllability and a robust visible-light response. These characteristics enable the emulation of multiple functions of the human visual system and the accommodation of varying light intensity environments. Furthermore, the phototransistor array successfully replicates the scotopic and photopic adaptation recognition behaviors of the human retina. Notably, the device demonstrates faultless competency in image processing, achieving an impressive 93% accuracy for digit recognition. These findings contribute to the advancement of bionic neuromorphic chips and offer promising opportunities for future developments in the bioinspired visual system.

This comprehensive article provides an in-depth review of recent advancements in 2D semiconductor-based optoelectronics for artificial vision. The scope includes 2D semiconductor-based photodetectors, optoelectronic memory, artificial synapses, and innovative applications inspired by retinal cells and vision systems. Furthermore, the review addresses critical technical challenges, outlines strategic approaches for development, and explores potential applications in detail.

Abstract

Artificial retina technologies aim to restore visual function by mimicking the natural processes of the eye. These biomimetic devices can convert light into electrical signals that the brain can interpret as visual information, bypassing damaged or non-functional cells of the eye. To be effective, these devices should possess high sensitivity to light, high spatial resolution, biocompatibility, power efficiency, and so on. Recently, 2D semiconductor materials have appeared as a promising candidate for artificial retinal devices, thanks to their excellent optoelectronic properties, ultrathin body, flexible nature, and biocompatibility. Here the recent developments in the field of 2D semiconductors-based optoelectronics for visual function recovery are reviewed and their potential applications are discussed. The photodetector, optoelectronic memory, and artificial synapse mechanisms and devices utilized in artificial systems that are based on 2D semiconductor materials are summarized. Additionally, a range of application scenarios for devices that are inspired by retinal cells and vision systems is explored. Finally, it is concluded with an overview of the critical technical challenges and strategies that must be addressed for the successful development of artificial retina technologies. It also highlights the potential for new applications in other fields, such as robotics and artificial intelligence.

Non-contact Triboelectric Nanogenerator (NC-TENG) uses the electrostatic induction effect to generate power without direct contact, which offers advantages over traditional contact-based TENG, such as a longer lifespan and greater flexibility in design and operation. The recent progress of NC-TENG from fundamental theory to practical applications will be discussed.

Abstract

With the rapid development of science and technology, there is an increasing demand for sustainable energy sources. Although triboelectric nanogenerators (TENGs) can realize self-powered supply by using various weak mechanical energy, the device's robustness and reliability are seriously affected by frequent, direct, and long-term mechanical shocks, as well as negative environmental factors during equipment operation. Therefore, researchers have developed non-contact triboelectric nanogenerators (NC-TENGs) based on the principle of electrostatic induction. This technology enables electricity generation via relative movement of friction layers even when they are not in direct contact, which consequently leads to enhance device robustness, prolonged service life, and broadened operation scenarios. In this review, the recent progress of NC-TENG from fundamental theory to practical applications is systematically summarized. The basic structure and working principle of NC-TENG are first introduced, followed by a discussion of the devices’ structural construction and performance optimization methods. Furthermore, applications of NC-TENG in automatic force position detection system, non-contact human-machine interface, and electrical energy harvesting are reviewed. Finally, a brief prospectus for the future development of NC-TENG is provided.

High-quality 2D ferroelectric perovskite films with mixed spacer cations (BA⁺ and BDA²⁺) are prepared for self-powered photodetectors (PDs). The self-powered ultraviolet (UV) PDs exhibit excellent photoelectric properties, together with excellent reproducibility and stability. After applying pressures to the PD, the maximum responsivities can be modulated by the piezo–phototronic effect with an effective enhancement ratio of 480%.

Abstract

Self-powered photodetectors (PDs) have the advantages of no external power requirement, wireless operation, and long life. Spontaneous ferroelectric polarizations can significantly increase built-in electric field intensity, showing great potential in self-powered photodetection. Moreover, ferroelectrics possess pyroelectric and piezoelectric properties, beneficial for enhancing self-powered PDs. 2D metal halide perovskites (MHPs), which have ferroelectric properties, are suitable for fabricating high-performance self-powered PDs. However, the research on 2D metal halide perovskites ferroelectrics focuses on growing bulk crystals. Herein, 2D ferroelectric perovskite films with mixed spacer cations for self-powered PDs are demonstrated by mixing Ruddlesden–Popper (RP)-type and Dion–Jacobson (DJ)-type perovskite. The (BDA_0.7(BA₂)_0.3)(EA)₂Pb₃Br₁₀ film possesses, overall, the best film qualities with the best crystalline quality, lowest trap density, good phase purity, and obvious ferroelectricity. Based on the ferro–pyro–phototronic effect, the PD at 360 nm exhibits excellent photoelectric properties, with an ultrahigh peak responsivity greater than 93 A W⁻¹ and a detectivity of 2.5 × 10¹⁵ Jones, together with excellent reproducibility and stability. The maximum responsivities can be modulated by piezo–phototronic effect with an effective enhancement ratio of 480%. This work will open up a new route of designing MHP ferroelectric films for high-performance PDs and offers the opportunity to utilize it for various optoelectronics applications.

Nanopatterning design provides extra degrees of freedom to realize tailored material properties and device functionality. This research demonstrates that scalable atomic-scale patterning and controllable resistive switching can be realized in 2D superionic cuprous telluride by using the bonding difference at nonequivalent sites, and proposes a compact model for mimicking the memristor dynamics that can be used for image processing.

Abstract

The ultrathin thickness of 2D layered materials affords the control of their properties through defects, surface modification, and electrostatic fields more efficiently compared with bulk architecture. In particular, patterning design, such as moiré superlattice patterns and spatially periodic dielectric structures, are demonstrated to possess the ability to precisely control the local atomic and electronic environment at large scale, thus providing extra degrees of freedom to realize tailored material properties and device functionality. Here, the scalable atomic-scale patterning in superionic cuprous telluride by using the bonding difference at nonequivalent copper sites is reported. Moreover, benefitting from the natural coupling of ordered and disordered sublattices, controllable piezoelectricity-like multilevel switching and bipolar switching with the designed crystal structure and electrical contact is realized, and their application in image enhancement is demonstrated. This work extends the known classes of patternable crystals and atomic switching devices, and ushers in a frontier for image processing with memristors.

Artificial moiré superlattices have become a fertile playground for emergent quantum phenomena. Modern angle-resolved photoemission spectroscopy (ARPES) can directly visualize electronic structures and thus can provide enlightening insights into fundamental physics in moiré superlattice systems and guides for designing novel devices. Major advances in the ARPES studies of moiré superlattices are reviewed and new experimental directions are discussed.

Abstract

The last decade has witnessed a flourish in 2D materials including graphene and transition metal dichalcogenides (TMDs) as atomic-scale Legos. Artificial moiré superlattices via stacking 2D materials with a twist angle and/or a lattice mismatch have recently become a fertile playground exhibiting a plethora of emergent properties beyond their building blocks. These rich quantum phenomena stem from their nontrivial electronic structures that are effectively tuned by the moiré periodicity. Modern angle-resolved photoemission spectroscopy (ARPES) can directly visualize electronic structures with decent momentum, energy, and spatial resolution, thus can provide enlightening insights into fundamental physics in moiré superlattice systems and guides for designing novel devices. In this review, first, a brief introduction is given on advanced ARPES techniques and basic ideas of band structures in a moiré superlattice system. Then ARPES research results of various moiré superlattice systems are highlighted, including graphene on substrates with small lattice mismatches, twisted graphene/TMD moiré systems, and high-order moiré superlattice systems. Finally, it discusses important questions that remain open, challenges in current experimental investigations, and presents an outlook on this field of research.

Publication date: October 2023

Source: Materials Today, Volume 69

Author(s): Na Zhang, Fakun Wang, Pengyu Li, Yi Liang, Hao Luo, Decai Ouyang, Linbao Luo, Jinsong Wu, Yinghe Zhao, Yuan Li, Tianyou Zhai