Nanoskiving of van der Waals Materials toward Edge/Basal Plane Contact Heterojunctions for High-Performance Photodetection.

The unique features of edges in van der Waals materials may lead to edge-basal plane contacts that could provide new opportunities for electronic and optoelectronic devices. However, few studies have addressed edge/basal plane contact heterojunctions owing to the formidable challenges in integrating edges with the basal plane to form a heterojunction. Here, taking the example of black phosphorus (BP)/ReS2, a heterojunction with contact between the edge and the basal plane was successfully achieved by the introduction of a nanoskiving technique to fabricate BP edges with controlled orientation, followed by the dry transfer of a ReS2 flake. The deformation of BP during the nanoskiving process was clearly revealed, where interlayer slipping in the BP determined the formation of the edges. The edge/basal plane contact heterojunctions based on BP/ReS2 exhibited a reverse-rectifying behavior upon contact, and a high rectifying current was attributed to direct tunneling and Fowler-Nordheim tunneling in low and high bias regimes, respectively. As a photodetector, the heterojunction diode demonstrated an impressive responsivity of 65 A/W, a rapid response time (<10 ms), and polarization-sensitive detection under 532 nm illumination without gate biasing.

Intercorrelated Ferroelectricity and Bulk Photovoltaic Effect in Two-Dimensional Sn2P2S6 Semiconductor for Polarization-Sensitive Photodetection.

A two-dimensional (2D) ferroelectric semiconductor, which is coupled with photosensitivity and room-temperature ferroelectricity, provides the possibility of coordinated conductance modulation by both electric field and light illumination and is promising for triggering the revolution of optoelectronics for monolithic multifunctional integration. Here, we report that semiconducting Sn2P2S6 crystals can be achieved in a 2D morphology using a chemical vapor transport approach with the assistant of space confinement and experimentally demonstrate the robust ferroelectricity in atomic-thin Sn2P2S6 nanosheet at room temperature. The intercorrelated programming of ferroelectric order along out-of-plane (OOP) and in-plane (IP) directions enables a tunable bulk photovoltaic (BPV) effect through multidirectional electrical control. By combining the capability of anisotropic in-plane optical absorption, a highly integrated Sn2P2S6 optoelectronic device vertically sandwiched with graphene electrodes yields the polarization-dependent open-circuit photovoltage with a dichroic ratio of 2.0 under 405 nm light illumination. The reintroduction of ferroelectric Sn2P2S6 to the 2D asymmetric semiconductor family provides possibilities to hardware implement of the self-powered polarization-sensitive photodetection and spotlights the promising applications for next-generation photovoltaic devices.

Low-Potential Iodide Oxidation Enables Dual-Atom CoFe─N─C Catalysts for Ultra-Stable and High-Energy-Efficiency Zn-Air Batteries.

The low energy efficiency and limited cycling life of rechargeable Zn-air batteries (ZABs) arising from the sluggish oxygen reduction/evolution reactions (ORR/OERs) severely hinder their commercial deployment. Herein, a zeolitic imidazolate framework (ZIF)-derived strategy associated with subsequent thermal fixing treatment is proposed to fabricate dual-atom CoFe─N─C nanorods (Co1 Fe1 ─N─C NRs) containing atomically dispersed bimetallic Co/Fe sites, which can promote the energy efficiency and cyclability of ZABs simultaneously by introducing the low-potential oxidation redox reactions. Compared to the mono-metallic nanorods, Co1 Fe1 ─N─C NRs exhibit remarkable ORR performance including a positive half-wave potential of 0.933 V versus reversible hydrogen electrode (RHE) in alkaline electrolyte. Surprisingly, after introducing the potassium iodide (KI) additive, the oxidation overpotential of Co1 Fe1 ─N─C NRs to reach 10 mA cm-2 can be significantly reduced by 395 mV compared to the conventional destructive OER. Theoretical calculations show that the markedly decreased overpotential of iodide oxidation can be ascribed to the synergistic effects of neighboring Co─Fe diatomic sites as the unique adsorption sites. Overall, aqueous ZABs assembled with Co1 Fe1 ─N─C NRs and KI as the air-cathode catalyst and electrolyte additive, respectively, can deliver a low charging voltage of 1.76 V and ultralong cycling stability of over 230 h with a high energy efficiency of ≈68%.

Defect Emission and Its Dipole Orientation in Layered Ternary Znln2 S4 Semiconductor.

Defect engineering is promising to tailor the physical properties of 2D semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors are not clarified. Here, the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2 S4 is thoroughly studied via versatile spectroscopic tools in combination with theoretical calculations. It is demonstrated that the Zn-In antistructural defects induce the formation of a series of donor and acceptor energy levels and sulfur vacancies induce donor energy levels, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair in Znln2 S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef ). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2 S4 is directly revealed by back focal plane imagining, where it presents obviously in-plane dipole orientation within a dozen-layer thickness of Znln2 S4 . These unique features of defects in Znln2 S4 including extrinsic absorption, rich recombination paths, gate tunability, and in-plane dipole orientation are definitely a benefit to the advanced orientation-functional optoelectronic applications.

Formulating N-Doped Carbon Hollow Nanospheres with Highly Accessible Through-Pores to Isolate Fe Single-Atoms for Efficient Oxygen Reduction.

It is challenging yet promising to design highly accessible N-doped carbon skeletons to fully expose the active sites inside single-atom catalysts. Herein, mesoporous N-doped carbon hollow spheres with regulatable through-pore size can be formulated by a simple sequential synthesis procedure, in which the condensed SiO2 is acted as removable dual-templates to produce both hollow interiors and through-pores, meanwhile, the co-condensed polydopamine shell is served as N-doped carbon precursor. After that, Fe─N─C hollow spheres (HSs) with highly accessible active sites can be obtained after rationally implanting Fe single-atoms. Microstructural analysis and X-ray absorption fine structure analysis reveal that high-density Fe─N4 active sites together with tiny Fe clusters are uniformly distributed on the mesoporous carbon skeleton with abundant through-pores. Benefitted from the highly accessible Fe─N4 active sites arising from the unique through-pore architecture, the Fe─N─C HSs demonstrate excellent oxygen reduction reaction (ORR) performance in alkaline media with a half-wave potential up to 0.90 V versus RHE and remarkable stability, both exceeding the commercial Pt/C. When employing Fe─N─C HSs as the air-cathode catalysts, the assembled Zn-air batteries deliver a high peak power density of 204 mW cm-2 and stable discharging voltage plateau over 140 h.

Neuro-inspired optical sensor array for high-accuracy static image recognition and dynamic trace extraction.

Neuro-inspired vision systems hold great promise to address the growing demands of mass data processing for edge computing, a distributed framework that brings computation and data storage closer to the sources of data. In addition to the capability of static image sensing and processing, the hardware implementation of a neuro-inspired vision system also requires the fulfilment of detecting and recognizing moving targets. Here, we demonstrated a neuro-inspired optical sensor based on two-dimensional NbS2/MoS2 hybrid films, which featured remarkable photo-induced conductance plasticity and low electrical energy consumption. A neuro-inspired optical sensor array with 10 × 10 NbS2/MoS2 phototransistors enabled highly integrated functions of sensing, memory, and contrast enhancement capabilities for static images, which benefits convolutional neural network (CNN) with a high image recognition accuracy. More importantly, in-sensor trajectory registration of moving light spots was experimentally implemented such that the post-processing could yield a high restoration accuracy. Our neuro-inspired optical sensor array could provide a fascinating platform for the implementation of high-performance artificial vision systems.

Revisiting the Hetero-Interface of Electrolyte/2D Materials in an Electric Double Layer Device.

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 (Li2 Al2 SiP2 TiO13 ) 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.

Two-dimensional semiconducting SnP2Se6 with giant second-harmonic-generation for monolithic on-chip electronic-photonic integration.

Two-dimensional (2D) layered semiconductors with nonlinear optical (NLO) properties hold great promise to address the growing demand of multifunction integration in electronic-photonic integrated circuits (EPICs). However, electronic-photonic co-design with 2D NLO semiconductors for on-chip telecommunication is limited by their essential shortcomings in terms of unsatisfactory optoelectronic properties, odd-even layer-dependent NLO activity and low NLO susceptibility in telecom band. Here we report the synthesis of 2D SnP2Se6, a van der Waals NLO semiconductor exhibiting strong odd-even layer-independent second harmonic generation (SHG) activity at 1550 nm and pronounced photosensitivity under visible light. The combination of 2D SnP2Se6 with a SiN photonic platform enables the chip-level multifunction integration for EPICs. The hybrid device not only features efficient on-chip SHG process for optical modulation, but also allows the telecom-band photodetection relying on the upconversion of wavelength from 1560 to 780 nm. Our finding offers alternative opportunities for the collaborative design of EPICs.

Synaptic plasticity realized by selective oxidation of TiS3 nanosheet for neuromorphic devices.

Memristive devices operating analogous to biology synapses demonstrate great potential for neuromorphic applications. Here, we reported the space-confined vapor synthesis of ultrathin titanium trisulfide (TiS3) nanosheets, and subsequent laser manufacturing of a TiS3-TiOx-TiS3 in-plane heterojunction for memristor applications. Due to the flux-controlled migration and aggregation of oxygen vacancies, the two-terminal memristor demonstrates reliable "analog" switching behaviors, in which the channel conductance can be incrementally adjusted by tuning the duration and sequence of programming voltage. The device allows the emulation of basic synaptic functions, featuring excellent linearity and symmetry in conductance change during long-term potentiation/depression processes. The small asymmetric ratio of 0.15 enables it to be integrated into a neural network for the pattern recognition task with a high accuracy of 90%. The results demonstrate the great potential of TiS3-based synaptic devices for neuromorphic applications.

Self-Sacrificing Template Synthesis of Carbon Nanosheets Assembled Hollow Spheres with Abundant Active Fe-N4 O1 Moieties for Electrocatalytic Oxygen Reduction.

Single-atom Fe-N-C (Fe1 -N-C) materials represent the benchmarked electrocatalysts for oxygen reduction reaction (ORR). However, single Fe atoms in the carbon skeletons cannot be fully utilized due to the mass transfer limitation, severely restricting their intrinsic ORR properties. Herein, a self-sacrificing template strategy is developed to fabricate ultrathin nanosheets assembled Fe1 -N-C hollow microspheres (denoted as Fe1 /N-HCMs) by rational carbonization of Fe3+ chelating polydopamine coated melamine cyanuric acid complex. The shell of Fe1 /N-HCMs is constructed by ultrathin nanosheets with thickness of only 2 nm, which is supposed to be an ideal platform to isolate and fully expose single metal atoms. Benefiting from unique hierarchical hollow architecture with highly open porous structure, 2 nm-thick ultrathin nanosheet subunits and abundant Fe-N4 O1 active sites revealed by X-ray absorption fine structure analysis, the Fe1 /N-HCMs exhibit high ORR performance with a positive half-wave potential of 0.88 V versus the reversible hydrogen electrode and robust stability. When served as air-cathode catalysts with ultralow loading mass of 0.25 mg cm-2 , Fe1 /N-HCMs based Zn-air batteries present a maximum power density of 187 mW cm-2 and discharge specific capacity of 806 mA h gZn -1 in primary Zn-air batteries, all exceeding those of commercial Pt/C.

A polarization-sensitive photothermoelectric photodetector based on mixed-dimensional SWCNT-MoS2 heterostructures.

Mixed-dimensional van der Waals (vdW) integration has been demonstrated to be effective for the modulation of the physical properties of homogeneous materials. Herein, we reported the enhancement of photothermal conversion and decrease of thermal conductivity in metallic single-walled carbon nanotube (SWCNT) films with the integration of chemical vapor deposition-grown monolayer MoS2 films. The induced temperature gradient in SWCNT-MoS2 hybrid films drives carrier diffusion to generate photocurrent via the photothermoelectric (PTE) effect, and a self-powered photodetector working in the visible band range from 405 to 785 nm was demonstrated. The maximum responsivity of the device increases by 6 times compared to that of the SWCNT counterpart. More importantly, the mixed-dimensional device exhibits polarization-dependent photogeneration, showing a large anisotropy ratio of 1.55. This work paves a way for developing high-performance, polarization-sensitive photodetectors by mixed-dimensional integration.

Isolating Fe Atoms in N-Doped Carbon Hollow Nanorods through a ZIF-Phase-Transition Strategy for Efficient Oxygen Reduction.

Transition metal-nitrogen-carbon (TM-N-C) catalysts have been intensely investigated to tackle the sluggish oxygen reduction reactions (ORRs), but insufficient accessibility of the active sites limits their performance. Here, by using solid ZIF-L nanorods as self-sacrifice templates, a ZIF-phase-transition strategy is developed to fabricate ZIF-8 hollow nanorods with open cavities, which can be subsequently converted to atomically dispersed Fe-N-C hollow nanorods (denoted as Fe1 -N-C HNRs) through rational carbonization and following fixation of iron atoms. The microstructure observation and X-ray absorption fine structure analysis confirm abundant Fe-N4 active sites are evenly distributed in the carbon skeleton. Thanks to the highly accessible Fe-N4 active sites provided by the highly porous and open carbon hollow architecture, the Fe1 -N-C HNRs exhibit superior ORR activity and stability in alkaline and acidic electrolytes with very positive half-wave potentials of 0.91 and 0.8 V versus RHE, respectively, both of which surpass those of commercial Pt/C. Remarkably, the dynamic current density (JK ) of Fe1 -N-C HNRs at 0.85 V versus RHE in alkaline media delivers a record value of 148 mA cm-2 , 21 times higher than that of Pt/C. The assembled Zn-air battery using Fe1 -N-C HNRs as cathode catalyst exhibits a high peak power density of 208 mW cm-2 .

A medium-entropy transition metal oxide cathode for high-capacity lithium metal batteries.

The limited capacity of the positive electrode active material in non-aqueous rechargeable lithium-based batteries acts as a stumbling block for developing high-energy storage devices. Although lithium transition metal oxides are high-capacity electrochemical active materials, the structural instability at high cell voltages (e.g., >4.3 V) detrimentally affects the battery performance. Here, to circumvent this issue, we propose a Li1.46Ni0.32Mn1.2O4-x (0 < x < 4) material capable of forming a medium-entropy state spinel phase with partial cation disordering after initial delithiation. Via physicochemical measurements and theoretical calculations, we demonstrate the structural disorder in delithiated Li1.46Ni0.32Mn1.2O4-x, the direct shuttling of Li ions from octahedral sites to the spinel structure and the charge-compensation Mn3+/Mn4+ cationic redox mechanism after the initial delithiation. When tested in a coin cell configuration in combination with a Li metal anode and a LiPF6-based non-aqueous electrolyte, the Li1.46Ni0.32Mn1.2O4-x-based positive electrode enables a discharge capacity of 314.1 mA h g-1 at 100 mA g-1 with an average cell discharge voltage of about 3.2 V at 25 ± 5 °C, which results in a calculated initial specific energy of 999.3 Wh kg-1 (based on mass of positive electrode's active material).

Transition Metal (Co, Ni, Fe, Cu) Single-Atom Catalysts Anchored on 3D Nitrogen-Doped Porous Carbon Nanosheets as Efficient Oxygen Reduction Electrocatalysts for Zn-Air Battery.

Exploring highly active and cost-efficient single-atom catalysts (SACs) for oxygen reduction reaction (ORR) is critical for the large-scale application of Zn-air battery. Herein, density functional theory (DFT) calculations predict that the intrinsic ORR activity of the active metal of SACs follows the trend of Co > Fe > Ni ≈ Cu, in which Co SACs possess the best ORR activity due to its optimized spin density. Guided by DFT calculations, four kinds of transition metal single atoms embedded in 3D porous nitrogen-doped carbon nanosheets (MSAs@PNCN, M = Co, Ni, Fe, Cu) are synthesized via a facile NaCl-template assisted strategy. The resulting MSAs@PNCN displays ORR activity trend in lines with the theoretical predictions, and the Co SAs@PNCN exhibits the best ORR activity (E1/2  = 0.851 V), being comparable to that of Pt/C under alkaline conditions. X-ray absorption fine structure (XAFS) spectra verify the atomically dispersed Co-N4 sites are the catalytically active sites. The highly active CoN4 sites and the unique 3D porous structure contribute to the outstanding ORR performance of Co SAs@PNCN. Furthermore, the Co SAs@PNCN catalyst is employed as cathode in Zn-air battery, which can deliver a large power density of 220 mW cm-2 and maintain robust cycling stability over 530 cycles.

Encapsulating atomic molybdenum into hierarchical nitrogen-doped carbon nanoboxes for efficient oxygen reduction.

Construction of single-atom catalysts (SACs) with maximally exposed active sites remains a challenging task mainly because of the lack of suitable host matrices. In this study, hierarchical N-doped carbon nanoboxes composed of ultrathin nanosheets with dispersed atomic Mo (denoted as hierarchical SA-Mo-C nanoboxes) were fabricated via a template-engaged multistep synthesis process. Comprehensive characterizations, including X-ray absorption fine structure analysis, reveal the formation of Mo-N4 atomic sites uniformly anchored on the hierarchical carbon nanoboxes. The prepared catalysts offer structural and morphological advantages, including ultrathin nanosheet units, unique hollow structures and abundant active Mo-N4 species, that result in excellent activity with a half-wave potential of 0.86 V vs. RHE and superb stability for the oxygen reduction reaction in 0.1 M KOH; thus, the catalysts are promising air-cathode catalysts for Zn-air batteries with a high peak power density of 157.6 mW cm-2.

Bifunctional WC-Supported RuO2 Nanoparticles for Robust Water Splitting in Acidic Media.

We report the strong catalyst-support interaction in WC-supported RuO2 nanoparticles (RuO2 -WC NPs) anchored on carbon nanosheets with low loading of Ru (4.11 wt.%), which significantly promotes the oxygen evolution reaction activity with a η10 of 347 mV and a mass activity of 1430 A gRu -1 , eight-fold higher than that of commercial RuO2 (176 A gRu -1 ). Theoretical calculations demonstrate that the strong catalyst-support interaction between RuO2 and the WC support could optimize the surrounding electronic structure of Ru sites to reduce the reaction barrier. Considering the likewise excellent catalytic ability for hydrogen production, an acidic overall water splitting (OWS) electrolyzer with a good stability constructed by bifunctional RuO2 -WC NPs only requires a cell voltage of 1.66 V to afford 10 mA cm-2 . The unique 0D/2D nanoarchitectures rationally combining a WC support with precious metal oxides provides a promising strategy to tradeoff the high catalytic activity and low cost for acidic OWS applications.

2D Indium Phosphorus Sulfide (In2 P3 S9 ): An Emerging van der Waals High-k Dielectrics.

2D van der Waals (vdW) semiconductors hold great potentials for more-than-Moore field-effect transistors (FETs), and the efficient utilization of their theoretical performance requires compatible high-k dielectrics to guarantee the high gate coupling efficiency. The deposition of traditional high-k dielectric oxide films on 2D materials usually generates interface concerns, thereby causing the carrier scattering and degeneration of device performance. Here, utilizing a space-confined epitaxy growth approach, the authors successfully obtained air-stable ultrathin indium phosphorus sulfide (In2 P3 S9 ) nanosheets, the thickness of which can be scaled down to monolayer limit (≈0.69 nm) due to its layered structure. 2D In2 P3 S9 exhibits excellent insulating properties, with a high dielectric constant (≈24) and large breakdown voltage (≈8.1 MV cm-1 ) at room temperature. Serving as gate insulator, ultrathin In2 P3 S9 nanosheet can be integrated into MoS2 FETs with high-quality dielectric/semiconductor interface, thus providing a competitive electrical performance of device with subthreshold swings (SS) down to 88 mV dec-1 and a high ON/OFF ratio of 105 . This study proves an important strategy to prepare 2D vdW high-k dielectrics, and greatly facilitates the ongoing research of 2D materials for functional electronics.

Mechanical Anisotropy in Two-Dimensional Selenium Atomic Layers.

Two-dimensional (2D) trigonal selenium (t-Se) has become a new member in 2D semiconducting nanomaterial families. It is composed of well-aligned one-dimensional Se atomic chains bonded via van der Waals (vdW) interaction. The contribution of this unique anisotropic nanostructure to its mechanical properties has not been explored. Here, for the first time, we combine experimental and theoretical analyses to study the anisotropic mechanical properties of individual 2D t-Se nanosheets. It was found that its fracture strength and Young's modulus parallel to the atomic chain direction are much higher than along the transverse direction, which was attributed to the weak vdW interaction between Se atomic chains as compared to the covalent bonding within individual chains. Additionally, two distinctive fracture modes along two orthogonal loading directions were identified. This work provides important insights into the understanding of anisotropic mechanical behaviors of 2D semiconducting t-Se and opens new possibilities for future applications.

2D-1D mixed-dimensional heterostructures: progress, device applications and perspectives.

Two-dimensional (2D) materials have attracted broad interests and been extensively exploited for a variety of functional applications. Moreover, one-dimensional (1D) atomic crystals can also be integrated into 2D templates to create mixed-dimensional heterostructures, and the versatility of combinations provides 2D-1D heterostructures plenty of intriguing physical properties, making them promising candidate to construct novel electronic and optoelectronic nanodevices. In this review, we first briefly present an introduction of relevant fabrication methods and structural configurations for 2D-1D heterostructures integration. We then discuss the emerged intriguing physics, including high optical absorption, efficient carrier separation, fast charge transfer and plasmon-exciton interconversion. Their potential applications such as electronic/optoelectronic devices, photonic devices, spintronic devices and gas sensors, are also discussed. Finally, we provide a brief perspective for the future opportunities and challenges in this emerging field.

Topotactic Growth of Free-Standing Two-Dimensional Perovskite Niobates with Low Symmetry Phase.

Here, we report a novel topotactic method to grow 2D free-standing perovskite using KNbO3 (KN) as a model system. Perovskite KN with monoclinic phase, distorted by as large as ∼6 degrees compared with orthorhombic KN, is obtained from 2D KNbO2 after oxygen-assisted annealing at relatively low temperature (530 °C). Piezoresponse force microscopy (PFM) measurements confirm that the 2D KN sheets show strong spontaneous polarization (Ps) along [101̅]pc direction and a weak in-plane polarization, which is consistent with theoretical predictions. Thickness-dependent stripe domains, with increased surface displacement and PFM phase changes, are observed along the monoclinic tilt direction, indicating the preserved strain in KN induces the variation of nanoscale ferroelectric properties. 2D perovskite KN with low symmetry phase stable at room temperature will provide new opportunities in the exploration of nanoscale information storage devices and better understanding of ferroelectric/ferroelastic phenomena in 2D perovskite oxides.