Jiuxiang Dai

Novel and phase‐pure layered iron diselenide (FeSe₂) nanocrystals, confirmed by X‐ray diffraction and atomic resolution scanning transmission electron microscopy, are epitaxially grown on mica by sublimed‐salt‐assisted chemical vapor deposition. They exhibit metallic behavior with high electrical conductivity and a phase transition at ≈11 K.

Abstract

Layered iron chalcogenides (FeX, X = S, Se, Te) provide excellent platforms to study intertwined phase transitions, superconductivity, and magnetism. However, layered iron dichalcogenides (FeX₂, X = S, Se, Te) are rarely reported and their intrinsic properties are still unknown. Here, phase‐pure layered iron diselenide (FeSe₂) nanocrystals are epitaxially grown on mica by the sublimed‐salt‐assisted chemical vapor deposition method at atmospheric pressure. The layered atomic structure of FeSe₂ is confirmed by X‐ray diffraction and atomic‐resolution scanning transmission electron microscopy. Electrical transport shows that the layered FeSe₂ is a metal with high conductivity and a phase transition at ≈11 K. The phase transition manifests itself as a kink in the temperature‐dependent resistivity, as well as anomalous magnetoresistance (MR) appearing around the phase‐transition temperature. The MR changes from negative to positive, accompanied by large hysteresis near the phase‐transition temperature upon cooling. The negative MR and hysteresis might originate from magnetic field suppression scattering of spin fluctuations and competition of magnetic interactions induced by the phase transition, respectively. Layered iron dichalcogenide will be potential candidate to explore novel quantum phenomena and other applications.

A chemical vapor deposition method using mixed molten salts is established for vapor–liquid–solid growth of Re- and V-doped transition metal dichalcogenide (TMDC) monolayers. Tunable semiconductor to metal transition is observed in the as-grown Re- and V-doped TMDC monolayers. Using heavily V-doped WSe₂ as van der Waals contact, the performance of WSe₂-based field-effect transistors is improved by 1–3 orders of magnitude compared to Au and Pd.

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with unique electrical properties are fascinating materials used for future electronics. However, the strong Fermi level pinning effect at the interface of TMDCs and metal electrodes always leads to high contact resistance, which seriously hinders their application in 2D electronics. One effective way to overcome this is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW) contacts. Alternatively, using highly conductive doped TMDCs will have a profound impact on the contact engineering of 2D electronics. Here, a novel chemical vapor deposition (CVD) using mixed molten salts is established for vapor–liquid–solid growth of high-quality rhenium (Re) and vanadium (V) doped TMDC monolayers with high controllability and reproducibility. A tunable semiconductor to metal transition is observed in the Re- and V-doped TMDCs. Electrical conductivity increases up to a factor of 10⁸ in the degenerate V-doped WS₂ and WSe₂. Using V-doped WSe₂ as vdW contact, the on-state current and on/off ratio of WSe₂-based field-effect transistors have been substantially improved (from ≈10^–8 to 10^–5 A; ≈10⁴ to 10⁸), compared to metal contacts. Future studies on lateral contacts and interconnects using doped TMDCs will pave the way for 2D integrated circuits and flexible electronics.

CrSBr is an air‐stable, intrinsically magnetic, van der Waals semiconductor with an electronic bandgap ∆_E = 1.5 ± 0.2 eV and photoluminescence peak centered at 1.25 ± 0.07 eV. Magnetometry and magnetotransport measurements demonstrate that CrSBr exhibits intraplanar ferromagnetic ordering and interplanar antiferromagnetic ordering below T _N ≈ 132 ± 1 K, producing a large intrinsic negative magnetoresistance.

Abstract

The recent discovery of magnetism within the family of exfoliatable van der Waals (vdW) compounds has attracted considerable interest in these materials for both fundamental research and technological applications. However, current vdW magnets are limited by their extreme sensitivity to air, low ordering temperatures, and poor charge transport properties. Here the magnetic and electronic properties of CrSBr are reported, an air‐stable vdW antiferromagnetic semiconductor that readily cleaves perpendicular to the stacking axis. Below its Néel temperature, T _N = 132 ± 1 K, CrSBr adopts an A‐type antiferromagnetic structure with each individual layer ferromagnetically ordered internally and the layers coupled antiferromagnetically along the stacking direction. Scanning tunneling spectroscopy and photoluminescence (PL) reveal that the electronic gap is Δ_E = 1.5 ± 0.2 eV with a corresponding PL peak centered at 1.25 ± 0.07 eV. Using magnetotransport measurements, strong coupling between magnetic order and transport properties in CrSBr is demonstrated, leading to a large negative magnetoresistance response that is unique among vdW materials. These findings establish CrSBr as a promising material platform for increasing the applicability of vdW magnets to the field of spin‐based electronics.

Author(s): Maxim Bykov, Timofey Fedotenko, Stella Chariton, Dominique Laniel, Konstantin Glazyrin, Michael Hanfland, Jesse S. Smith, Vitali B. Prakapenka, Mohammad F. Mahmood, Alexander F. Goncharov, Alena V. Ponomareva, Ferenc Tasnádi, Alexei I. Abrikosov, Talha Bin Masood, Ingrid Hotz, Alexander N. Rudenko, Mikhail I. Katsnelson, Natalia Dubrovinskaia, Leonid Dubrovinsky, and Igor A. Abrikosov

The layered compound BeN4, berillontirene, is a Dirac semimetal with linear dispersion in the vicinity of the Fermi energy and two Dirac points coinciding with the Fermi energy, similar to graphene.

[Phys. Rev. Lett. 126, 175501] Published Mon Apr 26, 2021

An MXene/BP-based self-powered smart sensor system can be created by integrating a flexible pressure sensor with direct-laser-writing micro-supercapacitors and solar cells. The smart sensor system exhibits a detection capability for the state of the human heart under physiological conditions. The MXene materials design and system integration developed in this work offers a general platform for next-generation self-powered electronics.

Abstract

Accurate and continuous detection of physiological signals without the need for an external power supply is a key technology for realizing wearable electronics as next-generation biomedical devices. Herein, it is shown that a MXene/black phosphorus (BP)-based self-powered smart sensor system can be designed by integrating a flexible pressure sensor with direct-laser-writing micro-supercapacitors and solar cells. Using a layer-by-layer (LbL) self-assembly process to form a periodic interleaving MXene/BP lamellar structure results in a high energy-storage capacity in a direct-laser-writing micro-supercapacitor to drive the operation of sensors and compensate the intermittency of light illumination. Meanwhile, with MXene/BP as the sensitive layer in a flexible pressure sensor, the pressure sensitivity of the device can be improved to 77.61 kPa^–1 at an optimized elastic modulus of 0.45 MPa. Furthermore, the smart sensor system with fast response time (10.9 ms) shows a real-time detection capability for the state of the human heart under physiological conditions. It is believed that the proposed study based on the design and integration of MXene materials will provide a general platform for next-generation self-powered electronics.

Bismuth-based binary compounds, including Bi 2 Se 3 and Bi 2 Te 3 , have attracted increasing attention as well-known topological insulators. On the other hand, bismuth-based ternary compounds exhibit diverse properties, such as, ultrahigh carrier mobility, and strong Rashba spin splitting. Moreover, they boast of superior photocatalytic properties, implying great potential to be used in a wide range of applications. The unique structure and properties of two-dimensional (2D) materials, especially the extraordinary electronic and optical properties of 2D Bi 2 O 2 Se, have given rise to significant research interests for the exploration of 2D bismuth-based ternary compounds. In this review, we will comprehensively discuss the properties of three important families of bismuth-based ternary compounds, including Bi 2 O 2 X (X = S, Se, Te), BiTeX (X = Cl, Br, I), and BiOX (X = Cl, Br, I). In particular, we have p...

Rechargeable batteries can effectively mitigate the increasing crisis associated with clean energy storage technologies. The alkali metal-ion based rechargeable batteries require a low diffusion barrier, a low average open-circuit voltage (OCV), and a high storage capacity for their superior performance. Using comprehensive first-principle calculations, we demonstrate that calcium carbide monolayer (Ca 2 C-ML) MXene meets all the aforementioned criteria and is a superior anode material for lithium (Li), sodium (Na), and potassium (K) metal-ion batteries. By first-principles calculations, the structural and electronic properties of Ca 2 C-ML and its extensive ion battery applications are studied. The adsorption properties of Li, Na, and K alkali ions on the Ca 2 C-ML sheet confirm excellent charge transfer and electrical conductivity. The ultra-low diffusion barriers of 0.027, 0.059, and 0.028 eV for Li, Na, and K alkali ions, respectively, indicate the...

Nature Materials, Published online: 26 April 2021; doi:10.1038/s41563-021-00968-7

The polarization of photoluminescence is found to depend on spin orientation in a van der Waals antiferromagnet.

Nature Nanotechnology, Published online: 18 January 2021; doi:10.1038/s41565-020-00835-7

During a liquid-to-solid phase transition, a Bi–Ga alloy forms ordered nanostructured patterns on its surface.

Nature Nanotechnology, Published online: 19 April 2021; doi:10.1038/s41565-021-00885-5

Antiferromagnets are interesting materials for fast spintronics applications, but control of the antiferromagnetic order has been limited to bulk materials so far. Now, uniaxial strain is shown to align the Néel vector in MnPSe3 down to the monolayer limit.

High quality, all inorganic perovskite crystalline grain can be grown by a solvent-free chemical vapor deposition method. Carefully controlling the growth conditions allows for single crystalline grain growth on a large scale. The single grain exhibits excellent opto-electronic properties which makes it an ideal candidate for devices like photo and radiation detectors.

Abstract

The chemical vapor deposition (CVD) method is a dry approach that can produce high quality crystals and thin films at large scale which can be easily adapted by industry. In this work, CVD technology is employed to grow high quality, large size all-inorganic cesium lead bromide perovskite crystalline film for the first time. The obtained films have millimeter size crystalline domains with high phase purity. The growth kinetics are examined in detail by optical microscopy and X-ray diffraction. The deposition rate and growth temperature are found to be the key parameters allowing to achieve large scale crystal growth. The large crystalline grains exhibit exceptional optical properties including negligible Stokes shift and uniform photoluminescence over a large scale. This suggests a high degree of crystallinity free from internal strain or defects. A lateral diode within one large crystalline grain is further fabricated and significant photo-generated voltage and short circuit current are observed, suggesting highly efficient carrier transport and collections without scattering within the grain. This demonstration suggests that the CVD grown all-inorganic perovskite thin films enable a promising fabrication route suitable for photovoltaic or photo-detector applications.

The growth mechanism of Li on Ti₃C₂T _x is studied via experiments and ab initio calculations, which predict that Li⁺ initially formed hcp layers on the Ti₃C₂T _x surface followed by plating of bcc-Li. Large formation energy and small migration barrier of Li⁺ result in planar Li deposition on Ti₃C₂T _x . Thus, the dense and planar Li metal electrode formed on Ti₃C₂T _x exhibit a stable charge-discharge performance.

Abstract

The propensity of Li to form irregular and nonplanar electrodeposits has become a fundamental barrier for fabricating Li metal batteries. Here, a planar, dendrite-free Li metal growth on 2D Ti₃C₂T _x MXene is reported. Ab initio calculations suggest that Li forms a hexagonal close-packed (hcp) layer on the surface of Ti₃C₂T _x via ionic bonding and the lattice confinement. The ionic bonding weakens gradually after a few monolayers, resulting in a nanometers-thin transition region of hcp-Li. Above this transition region, the deposition is dominated by plating of body-centered cubic (bcc) Li via metallic bonding. Formation of a dense and planar Li metal anode with preferential growth along the (110) facet is explained by the lattice matching between Ti₃C₂T _x and hcp-Li and then with bcc-Li, as well as preferred thermodynamic factors including the large dendrite formation energy and small migration barrier for Li. The prepared Li metal anode shows stable cycling in a wide current density range from 0.5 to 10.0 mA cm^–2. The LiFePO₄‖Li full cell fabricated with this Li metal anode exhibits only 9.5% capacity fading after 500 charge–discharge cycles at 1 C rate.

Ultrathin 2D heterojunction constructed by ZnIn₂S₄ and g-C₃N₄ successfully accelerates photocatalytic hydrogen evolution under visible light due to NS bonds forming at the interface of heterojunction affording charge transferring tunnel.

Abstract

Designation of high-efficiency water splitting photocatalyst is still a challenge in converting solar energy into chemical fuels. Heterojunction can inhibit recombination of carriers which is considered to be a reliable strategy to improve photocatalytic performance on water splitting. In this work, a “face-to-face” 2D tight heterostructure is constructed by growing ZnIn₂S₄ nanosheets on g-C₃N₄ nanosheets. Due to the ultrathin 2D structure and large amounts of NS bonds forming at the interface of heterojunction affording charge transferring tunnel, the synthesized 2D heterojunction photocatalysts successfully accelerate the carrier migration rate and decrease recombination probability of photogenerated electrons and holes. As a result, the optimized ZISCN-50 sample has excellent photocatalytic H₂ production activity (10.92 mmol h⁻¹ g⁻¹) under visible light, which is ≈5.2 times of ZnIn₂S₄ (2.09 mmol h⁻¹ g⁻¹) and 136.5 times of pure g-C₃N₄ nanosheets (0.08 mmol h⁻¹ g⁻¹). Cycle experiments show that the composite material has excellent stability and recyclability. This work provides fresh insights into atomic-level structure and interface design in order to synthesize high-efficiency 2D/2D heterojunction photocatalysts.

A facile strategy to fabricate large scale and high performance CVD-grown polycrystalline MoTe₂ transistors with a 1T'/2H vertical homojunction structure by combining a spatial and phase controlled MoTe₂ patterns scalable synthesis and a scalable universal transfer method with water-soluble PVP/PVA bilayer mediator is demonstrated.

Abstract

2D transition metal dichalcogenides (TMDs) have emerged as an ideal alternative to silicon in advanced electronics. Especially, MoTe₂ attracts peculiar attention since it offers a unique opportunity of resolving critical electrical contacts. Currently, although MoTe₂-based coplanar semiconductor-metal circuitry realized by epitaxial growth and chemical assembly has been demonstrated, while still suffers from the requirement of extremely accurate synthesis process control. Here, a facile strategy is demonstrated to fabricate large scale and high performance MoTe₂ transistors with a 1T'/2H vertical homojunction structure by combining a spatial and phase controlled MoTe₂ scalable synthesis and a scalable universal transfer method with water-soluble poly-vinylpyrrolidone and poly-(vinyl alcohol) bilayer mediator. Both high quality 1T'- and 2H- MoTe₂ with controlled dimensions can be scalable synthesized via a shadow mask assisted chemical vapor deposition method. These as-synthesized MoTe₂ patterns can be successfully transferred to a wide range of substrates at a high yield >80% with well-retained properties to construct transistors with a complex vertical 1T'/2H-MoTe₂/HfAlO₂ structure. The devices exhibit an on/off current ratio surpassing 10⁴ and a typical mobility of ≈29 cm² V⁻¹ s⁻¹. The developed scaled strategy of combining both scalable MoTe₂ synthesis and transfer offers a feasible way for potential MoTe₂-based large-scale electronics.

Nature Materials, Published online: 15 April 2021; doi:10.1038/s41563-021-00971-y

A general method for the synthesis of high-purity crystals of metastable 1T′-phase transition metal dichalcogenides is reported, providing a source of phase-engineered materials that can be used to systematically explore their intrinsic properties.

Nature Materials, Published online: 22 April 2021; doi:10.1038/s41563-021-00984-7

The compositional dependence of magnetic, superconducting and topological surface states on an iron-based superconductor is reported.

Nature Communications, Published online: 13 April 2021; doi:10.1038/s41467-021-22486-5

In order to develop perovskite nanocrystals as a single-photon source, there is a need to understand the complex exciton photo-physics. Here, the authors employ resonant and near-resonant excitation technique to study single CsPbI3 nanocrystal that allows them to probe the continuous and size-quantised acoustic-phonon modes.

Nature Communications, Published online: 21 April 2021; doi:10.1038/s41467-021-22681-4

Precisely regulating Pt catalytic sites is important and challenging. Herein the authors engineer the clustering of single atomic Te vacancies in atomically thin PtTe2 to optimize the electronic structure, adsorption energy, and catalytic performance of atomically defined Pt sites.

Multivalent nanosheet artificial antibodies with high selectivity and strong binding affinity to pathogenic bacteria are created by spontaneous assembly of various tripeptide recognition phases on transition metal dichalcogenide nanosheets. The luminescent antibody mimics can detect pathogenic bacteria at a single-cell level from human serum and urine and effectively inactivate them on wounds of infected mice for healing.

Abstract

Antibodies are widely used as recognition elements in sensing and therapy, but they suffer from poor stability, long discovery time, and high cost. Herein, a facile approach to create antibody mimics with flexible recognition phases and luminescent rigid scaffolds for the selective recognition, detection, and inactivation of pathogenic bacteria is reported. Tripeptides with a nitriloacetate-Cu group are spontaneously assembled on transition metal dichalcogenide (TMD) nanosheets via coordination bonding, providing a diversity of TMD-tripeptide assembly (TPA) antibody mimics. TMD-TPA antibody mimics can selectively recognize various pathogenic bacteria with nanomolar affinities. The bacterial binding sites for TMD-TPA are identified by experiments and molecular dynamics simulations, revealing that the dynamic and multivalent interactions of artificial antibodies play a crucial role for their recognition selectivity and affinity. The artificial antibodies allow the rapid and selective detection of pathogenic bacteria at single copy in human serum and urine, and their effective inactivation for therapy of infected mice. This work demonstrates the potential of TMD-TPA antibody mimics as an alternative to natural antibodies for sensing and therapy.

The realization of Yu–Shiba–Rusinov (YSR) states in graphene, a non-superconducting 2D material, and without the participation of magnetic atoms, is shown. Graphene is made superconducting by the proximity effect with Pb islands. These scanning tunneling microscopy experiments reveal the very special nature of these YSR states and constitute an unequivocal demonstration of carbon magnetism at graphene grain boundaries.

Abstract

When magnetic atoms are inserted inside a superconductor, the superconducting order is locally depleted as a result of the antagonistic nature of magnetism and superconductivity. Thereby, distinctive spectral features, known as Yu–Shiba–Rusinov states, appear inside the superconducting gap. The search for Yu–Shiba–Rusinov states in different materials is intense, as they can be used as building blocks to promote Majorana modes suitable for topological quantum computing. Here, the first observation of Yu–Shiba–Rusinov states in graphene, a non-superconducting 2D material, and without the participation of magnetic atoms, is reported. Superconductivity in graphene is induced by proximity effect brought by adsorbing nanometer-scale superconducting Pb islands. Using scanning tunneling microscopy and spectroscopy the superconducting proximity gap is measured in graphene, and Yu–Shiba–Rusinov states are visualized in graphene grain boundaries. The results reveal the very special nature of those Yu–Shiba–Rusinov states, which extends more than 20 nm away from the grain boundaries. These observations provide the long-sought experimental confirmation that graphene grain boundaries host local magnetic moments and constitute the first observation of Yu–Shiba–Rusinov states in a chemically pure system.

2D phase transition materials, including transition metal dichalcogenides (TMDs) and metal phosphorous trichalcogenides (MPTs), attract researchers' interests for their unique combination of large surface area, direct bandgap, intrigue chemical, and mechanical properties. Great efforts are devoted to preparing 2D TMD and MPT materials by engineering their intrinsic structures at the atomic scale, aiming to achieve active catalyst toward catalytic purposes.

Abstract

2D layered materials turn out to be the most attractive hotspot in materials for their unique physical and chemical properties. A special class of 2D layered material refers to materials exhibiting phase transition based on environment variables. Among these materials, transition metal dichalcogenides (TMDs) act as a promising alternative for their unique combination of atomic-scale thickness, direct bandgap, significant spin–orbit coupling and prominent electronic and mechanical properties, enabling them to be applied for fundamental studies as catalyst materials. Metal phosphorous trichalcogenides (MPTs), as another potential catalytic 2D phase transition material, have been employed for their unusual intercalation behavior and electrochemical properties, which act as a secondary electrode in lithium batteries. The preparation of 2D TMD and MPT materials has been extensively conducted by engineering their intrinsic structures at the atomic scale. In this study, advanced synthesis methods of preparing 2D TMD and MPT materials are tested, and their properties are investigated, with stress placed on their phase transition. The surge of this type of report is associated with water-splitting catalysis and other catalytic purposes. This study aims to be a guideline to explore the mentioned 2D TMD and MPT materials for their catalytic applications.