Single Transition-Metal Atom Anchored on a Rhenium Disulfide Monolayer: An Efficient Bifunctional Electrocatalyst for the Oxygen Evolution and Oxygen Reduction Reactions.

Developing efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) bifunctional electrocatalysts is attractive for rechargeable metal-air batteries. Meanwhile, single metal atoms embedded in 2D layered transition metal chalcogenides (TMDs) have become a very promising catalyst. Recently, many attentions have been paid to the 2D ReS2 electrocatalyst due to its unique distorted octahedral 1T' crystal structure and thickness-independent electronic properties. Here, the catalytic activity of different transition metal (TM) atoms embedded in ReS2 using the density functional theory is investigated. The results indicate that TM@ReS2 exhibits outstanding thermal stability, good electrical conductivity, and electron transfer for electrochemical reactions. And the Ir@ReS2 and Pd@ReS2 can be used as OER/ORR bifunctional electrocatalysts with a lower overpotential for OER (ηOER) of 0.44 V and overpotentials for ORR (ηORR) of 0.26 V and 0.27 V, respectively. The excellent catalytic activity is attributed to the optimal adsorption strength for oxygen intermediates coming from the effective modulation of the electronic structure of ReS2 after Ir/Pd doping. The results can help to deeply understand the catalytic activity of TM@ReS2 and develop novel and highly efficient OER/ORR electrocatalysts.

Combined piezoelectricity, valley splitting and Dzyaloshinskii-Moriya interaction in Janus GdXY (X, Y = Cl, Br, I) magnetic semiconductors.

Janus materials, as a family of multifunctional materials with broken mirror symmetry, have played a great role in piezoelectric, valley-related, and Rashba spin-orbit coupling (SOC) applications. Using first-principles calculations, it is predicted that monolayer 2H-GdXY (X, Y = Cl, Br, I) will combine giant piezoelectricity, intrinsic valley splitting and a strong Dzyaloshinskii-Moriya interaction (DMI), resulting from the intrinsic electric polarization, spontaneous spin polarization and strong spin-orbit coupling. Opposite Berry curvatures and unequal Hall conductivities at the K- and K'-valleys of monolayer GdXY are promising for storing information through the anomalous valley Hall effect (AVHE). Through construction of the spin Hamiltonian and micromagnetic model, we obtained the primary magnetic parameters of monolayer GdXY as a function of the biaxial strain. Due to the dimensionless parameter κ having strong tunability, monolayer GdClBr is promising to host isolated skyrmions. The present results are expected to enable the application of Janus materials in piezoelectricity, spin- and valley-tronics and the formation of chiral magnetic structures.

Hexagonal Single-Crystal CoS Nanosheets: Controllable Synthesis and Tunable Oxygen Evolution Performance.

Cobalt-based sulfides with variable valence states and unique physical and chemical properties have shown great potential as oxygen evolution reaction (OER) catalysts for electrochemical water-splitting reactions. However, poor morphological characteristics and a small specific surface area limit its further application. Here, hexagonal single-crystal two-dimensional (2D) CoS nanosheets with different thicknesses are successfully prepared by an atmospheric-pressure chemical vapor deposition method. Because of the advantages of the 2D structure, more exposed catalytic active sites, better reactant adsorption ability, accelerated electron transfer, and enhanced electrical conductivities can be achieved from the thinnest 5 nm CoS nanosheets (CoS-5), significantly improving OER performance. The electrochemical tests manifest that CoS-5 show an overpotential of 290 mV at 10 mA cm-2 and a Tafel slope of 65.6 mV dec-1 in the OER in an alkaline solution, superior to those for other thicknesses of CoS, bulk CoS, and RuO2. For the mechanistic investigation, the lowest charge transfer resistance (Rct) and the highest double-layer capacitance (Cdl) were obtained for CoS-5, demonstrating the faster OER kinetics and the larger active area. Density functional theory calculations further reveal the enhanced density of states around the Fermi level and higher H2O molecule adsorption energy for thinner CoS nanosheets, promoting its intrinsic catalytic activity. Moreover, the two-electrode system with CoS-5 as the anode and Pt/C as the cathode requires only 1.56 V to attain 10 mA cm-2 in the overall water-splitting reaction. We believe that this study will provide a fresh view for thickness-dependent catalytic performance and offers a new material for the study of electronic and energy devices.

Electronic and magnetic properties of phosphorene tuned by Cl and metallic atom co-doping.

Using first-principles calculations based on density functional theory, we studied the electronic and magnetic properties of phosphorene co-doped with Cl and a metal atom (including Sc, Ti, V, Cr, Mn, Fe, Co, and Ni). It is found that Cl atom doping makes it much easier for metallic atoms to dope into phosphorene. Phosphorene co-doped with Cl and V, Cr, Mn, or Fe is magnetic, which is determined by the number of valence electrons. Taking V-Cl and Co-Cl co-doped phosphorene as an example, analyses are carried out on the reasonable selection of the doping sites, which distinctly affect the stability, band gap and magnetic moment. The stability is closely relevant to the electronegativity of impurity atoms. With the biaxial strain ranging from -4% to 4%, the magnetic moment of V-Cl co-doped phosphorene and the band gap of Co-Cl co-doped phosphorene are greatly tunable between 1.757-0.951 µB and 0.687-0.496 eV, which come from the electron transfer from V to the surrounding P atoms and the weakened bond between Co and Cl, respectively. These investigations provide a reference for regulating the electronic structure and magnetic properties of diluted magnetic semiconductors and promote the applications of phosphorene in spintronics and nanodevices.

Electronic properties and transistors of the NbS2-MoS2-NbS2 NR heterostructure.

Based on density function theory and nonequilibrium Green's functions, we construct a NbS2-MoS2-NbS2 NR inplane heterostructure. The effects of channel length, width, chirality and vacancy of the heterostructure on transport properties are systematically investigated. The electron transport of the armchair-edge heterostructure device shows ballistic transport properties, while the zigzag-edge heterostructure device exhibits resonance tunneling transport properties. Further study indicates NbS2-MoS2-NbS2 field effect transistors (FETs) to be excellent ambipolar transistors. The FETs have high performances with current on/off ratio 4.7 × 105 and subthreshold swing 90 mV/decade with channel length m = 16 and width n = 6.  Increases in the channel length sharply reduce the off-state current and enhance the performance of the devices significantly.

Electronic structures and transport properties of a MoS2-NbS2 nanoribbon lateral heterostructure.

Lateral heterostructures built from an armchair MoS2 nanoribbon (AMoS2NR) and an armchair NbS2 nanoribbon (ANbS2NR) were studied based on first-principles calculations and a non-equilibrium Green's function method. It is found that the work function of the AMoS2NR shows substantial oscillation with increasing nanoribbon width, which is different from the work functions of other kinds of nanoribbons. The AMoS2NR-ANbS2NR lateral heterostructure exhibits an anomalous transport gap that is much larger than the bandgap of the AMoS2NR. As a result, a field effect transistor with AMoS2NR as the channel and ANbS2NRs as electrodes has high on-off ratios of 106-107 and a tiny leakage current of the order of 10-8 µA. These results suggest that lateral metal-semiconductor heterostructures of transition metal dichalcogenides may have potential applications in nanodevices with low energy consumption.

Large-Area Synthesis of High-Quality Uniform Few-Layer MoTe2.

The controlled synthesis of large-area, atomically thin molybdenum ditelluride (MoTe2) crystals is crucial for its various applications based on the attractive properties of this emerging material. In this work, we developed a chemical vapor deposition synthesis to produce large-area, uniform, and highly crystalline few-layer 2H and 1T' MoTe2 films. It was found that these two different phases of MoTe2 can be grown depending on the choice of Mo precursor. Because of the highly crystalline structure, the as-grown few-layer 2H MoTe2 films display electronic properties that are comparable to those of mechanically exfoliated MoTe2 flakes. Our growth method paves the way for the large-scale application of MoTe2 in high-performance nanoelectronics and optoelectronics.

Bandgap opening/closing of graphene antidot lattices with zigzag-edged hexagonal holes.

How to predict the bandgap size of graphene antidot lattices (GALs) is a key problem in the field of graphene-based nanoelectronics. Here, we have obtained the universal rules on bandgap opening/closing of GALs with zigzag-edged hexagonal holes (ZH-GALs), as well as the means to control the bandgap size. In the simple case that the electronic property depends on the choice of the supercell, the quantitative relationship between Eg and the density/diameter of antidots is fitted. Turning to complex structures, we reveal that the bandgap opening in ZH-GALs results mainly from the intervalley scattering. In this interpretation, according to their relative position, the antidots can be divided into three categories. A relatively large bandgap appears in ZH-GALs, only when the numbers of the three categories are unequal. This could be explained based on a mechanism similar to diffraction. A formula according to the explanation is provided to estimate the bandgap, which can be used to predict the electronic properties of GALs and guide the design of semiconductor and photoelectronic devices based on GALs.

Observation of Anisotropic Second Harmonic Generation in Two-Dimensional Niobium Diselenide.

The intrinsic anisotropy of NbSe2 provides a favorable prerequisite of second harmonic generation (SHG) and rich possibilities for tailoring its nonlinear optical (NLO) properties. Here we report the highly efficient SHG of mechanically exfoliated NbSe2 flakes. The nonlinear optical response changes with excitation wavelengths, layer thicknesses, and polarizations of the excitation laser. The anisotropic SHG response further exhibits the intrinsic non-centrosymmetric crystal structure and could effectively assign the crystalline orientation of NbSe2 flakes. Interestingly, although NbSe2 flakes with tens of nanometers thickness experience attenuations in SHG performance, more efficient SHG anisotropy ratios were obtained, which are around 4 times higher than that of the 5-layer counterpart. The determined second-order nonlinearities of NbSe2 flakes (monolayer: ∼1.0 × 103 pm/V; 3-layer: ∼73 pm/V) are comparable to those of the commonly reported two-dimensional materials (e.g., MoS2, WSe2, graphene) with the same number of layers and much higher than those of commercial nonlinear optical crystals.

Constructing Slip Stacking Diversity in Van der Waals Homobilayers.

The van der Waals (vdW) interface provides two important degrees of freedom-twist and slip-to tune interlayer structures and inspire unique physics. However, constructing diversified high-quality slip stackings (i.e., lattice orientations between layers are parallel with only interlayer sliding) is more challenging than twisted stackings due to angstrom-scale structural discrepancies between different slip stackings, sparsity of thermodynamically stable candidates and insufficient mechanism understanding. Here, using transition metal dichalcogenide (TMD) homobilayers as a model system, this work theoretically elucidates that vdW materials with low lattice symmetry and weak interlayer coupling allow the creation of multifarious thermodynamically advantageous slip stackings, and experimentally achieves 13 and 9 slip stackings in 1T″-ReS2 and 1T″-ReSe2 bilayers via direct growth, which are systematically revealed by atomic-resolution scanning transmission electron microscopy (STEM), angle-resolved polarization Raman spectroscopy, and second harmonic generation (SHG) measurements. This work also develops modulation strategies to switch the stacking via grain boundaries (GBs) and to expand the slip stacking library from thermodynamic to kinetically favored structures via in situ thermal treatment. Finally, density functional theory (DFT) calculations suggest a prominent dependence of the pressure-induced electronic band structure transition on stacking configurations. These studies unveil a unique vdW epitaxy and offer a viable means for manipulating interlayer atomic registries.

First-principles study of electronic structure, sodium diffusion on 2D TiO2monolayers for sodium-ion battery electrodes.

The search for suitable electrode materials is crucial for the development of high-performance Na-ion batteries (NIBs). In recent years, significant attention has been drawn to two-dimensional (2D) oxides as potential NIB electrode materials. In this study, employing the first-principles density functional theory method, we investigate the thermodynamic and kinetic properties of Na adsorption and diffusion behavior on the 2D TiO2(010) monolayer. Our findings demonstrate that the 2D anatase TiO2(010) monolayer exhibits enhanced thermodynamic stability. Furthermore, the Na atoms preferentially adsorb on the top of oxygen atoms within the TiO2(010) monolayer, and their diffusion along the [100] direction is characterized by a low energy barrier of 0.054 eV. This comprehensive analysis sheds light on the structural stability, preferred adsorption sites, and diffusion paths of Na atoms on the 2D anatase TiO2(010) monolayer, providing valuable insights into the nature of the material's structure and Na ion transport. Moreover, the 2D structure of the TiO2matrix facilitates short Na diffusion lengths and a large electrode/electrolyte interface, thereby demonstrating the potential of this material as an NIB electrode material.

Optical Control of the Localized Surface Plasmon Resonance in a Heterotype and Hollow Gold Nanosheet.

The remote excitation and remote-controlling of the localized surface plasmon resonance (LSPR) in a heterotype and hollow gold nanosheet (HGNS) is studied using FDTD simulations. The heterotype HGNS contains an equilateral and hollow triangle in the center of a special hexagon, which forms a so-called hexagon-triangle (H-T) heterotype HGNS. If we focus the incident-exciting laser on one of the vertexes of the center triangle, the LSPR could be achieved among other remote vertexes of the outer hexagon. The LSPR wavelength and peak intensity depend sensitively on factors such as the polarization of the incident light, the size and symmetry of the H-T heterotype structure, etc. Several groups of the optimized parameters were screened out from numerous FDTD calculations, which help to further obtain some significant polar plots of the polarization-dependent LSPR peak intensity with two-petal, four-petal or six-petal patterns. Remarkably, based on these polar plots, the on-off switching of the LSPR coupled among four HGNS hotspots could be remote-controlled simply via only one polarized light, which shows promise for its potential application in remote-controllable surface-enhanced Raman scattering (SERS), optical interconnects and multi-channel waveguide switches.

Novel Functional Soft Magnetic CoFe2O4/Fe Composites: Preparation, Characterization, and Low Core Loss.

In this study, composites CoFe2O4/Fe were successfully synthesized by in situ oxidation, and their composition, structure, and magnetic properties have been investigated. According to the analysis of X-ray photoelectron spectrometry measure results, the cobalt ferrite insulating layer was completely coated on the surface of Fe powder particles. The evolution of the insulating layer during the annealing process has been discussed, which is correlated to effects on the magnetic properties of the composites CoFe2O4/Fe. The amplitude permeability of the composites reached a maximum of 110, and their frequency stability reached 170 kHz with a relatively low core loss of 253.6 W/kg. Therefore, the composites CoFe2O4/Fe has potential application in the field of integrated inductance and high-frequency motor, which is conducive to energy conservation and carbon reduction.

In situ growth of the CoO nanoneedle array on a 3D nickel foam toward a high-performance glucose sensor.

A glucose sensor with high sensitivity and low detection limit is vital for human beings' health. Herein, a CoO nanoneedle array with an unique electronic structure was successfully constructed by a hydrothermal and subsequent high-temperature calcination process. The optimized CoO-400 nanoneedles exhibit a larger electrochemical active surface area, beneficial electronic structure, favorable lattice distortion, and abundant active sites, which effectively promote electrochemical properties toward glucose sensing. The glucose sensor constructed by CoO-400 nanoneedles shows a high sensitivity of 84.23 mA cm-2 mM-1 and low detection limit of 4.4 × 10-7 M, superior to the results from most previous reports. Moreover, outstanding anti-interference ability, superior long-term stability, good repeatability, and satisfactory reproducibility in glucose detection for CoO-400 nanoneedles are also demonstrated.

Pt single atoms meet metal-organic frameworks to enhance electrocatalytic hydrogen evolution activity.

The electrochemical hydrogen evolution reaction (HER) effectively produces clean, renewable, and sustainable hydrogen; however, the development of efficient electrocatalysts is required to reduce the high energy barrier of the HER. Herein, we report two excellent single-atom (SA)/metal-organic framework (MOF) composite electrocatalysts (PtSA-MIL100(Fe) and PtSA-MIL101(Cr)) for HER. The obtained PtSA-MIL100(Fe) and PtSA-MIL101(Cr) electrocatalysts exhibit overpotentials of 60 and 61 mV at 10 mA cm-2, respectively, which are close to that of commercial Pt/C (38 mV); they exhibit overpotentials of 310 and 288 mV at 200 mA cm-2, respectively, which are comparable to that of commercial Pt/C (270 mV). Theoretical simulations reveal that Pt SAs modulate the electronic structures of the MOFs, leading to the optimization of the binding strength for H* and significant enhancement of the HER activity. This study describes a novel strategy for preparing desirable HER electrocatalysts based on the synergy between SAs and MIL-series MOFs. Using MIL-series MOFs to support SAs could be valuable for future catalyst design.

Structural Symmetry, Spin-Orbit Coupling, and Valley-Related Properties of Monolayer WSi2N4 Family.

Recently prepared layered MoSi2N4 exhibits excellent stability and semiconductor properties, adding building blocks for two-dimensional families. In this research, we present the spin-orbit coupling and valley-related properties of monolayer WSi2N4 family. Better than transition metal dichalcogenides, the structural symmetry of WSi2N4 monolayer can be different by changing the stacking of three parts in the monolayers, resulting in a Rashba spin-orbit field. The vertical and horizontal polarization will lift the degeneration of the in-plane and out-of-plane polarized spin, respectively. The gradient of potential energy and the proportion of d orbitals play dominant roles. The in-plane orbitals contribute to the out-of-plane spin polarization, while the out-of-plane orbitals contribute to the in-plane spin polarization. The characteristics of a Rashba semiconductor can be utilized in spin/valley Hall effects, as well as the regulation of the spin direction of the valley electrons, promoting the manipulation of multiple degrees of freedom of electrons in monolayer materials.

Strain Investigation on Spin-Dependent Transport Properties of γ-Graphyne Nanoribbon Between Gold Electrodes.

Strain engineering has become one of the effective methods to tune the electronic structures of materials, which can be introduced into the molecular junction to induce some unique physical effects. The various γ-graphyne nanoribbons (γ-GYNRs) embedded between gold (Au) electrodes with strain controlling have been designed, involving the calculation of the spin-dependent transport properties by employing the density functional theory. Our calculated results exhibit that the presence of strain has a great effect on transport properties of molecular junctions, which can obviously enhance the coupling between the γ-GYNR and Au electrodes. We find that the current flowing through the strained nanojunction is larger than that of the unstrained one. What is more, the length and strained shape of the γ-GYNR serves as the important factors which affect the transport properties of molecular junctions. Simultaneously, the phenomenon of spin-splitting occurs after introducing strain into nanojunction, implying that strain engineering may be a new means to regulate the electron spin. Our work can provide theoretical basis for designing of high performance graphyne-based devices in the future.

In-Plane Phonon Anisotropy and Anharmonicity in Exfoliated Natural Black Arsenic.

Group-VA two-dimensional layered materials in a puckered honeycomb structure exhibit strong in-plane anisotropy and have emerged as new platforms for novel devices. Here, we report on systematic Raman investigations on exfoliated b-As flakes on the Ag1 and Ag2 polarization dependence on their symmetry, excitation wavelength, and flake thickness. The intensity maximums of both phonons are corrected in the b-As armchair direction under 633 nm excitation regardless of the flake thickness upon considering optical birefringence effects and interference effects. The intensity ratio of Ag1 to Ag2 modes under 532 nm excitation is useful for b-As crystalline orientation identification. Temperature-dependent Raman investigations reveal the linearly anharmonic behaviors of both phonons in the range from 173 to 293 K and a slightly greater first-order temperature coefficient in the zigzag direction. Our findings give deep insight into the in-plane phonon anisotropy and anharmonicity of b-As and provide a step toward future device applications.

Anisotropic Collective Charge Excitations in Quasimetallic 2D Transition-Metal Dichalcogenides.

The quasimetallic 1T' phase 2D transition-metal dichalcogenides (TMDs) consist of 1D zigzag metal chains stacked periodically along a single axis. This gives rise to its prominent physical properties which promises the onset of novel physical phenomena and applications. Here, the in-plane electronic correlations are explored, and new mid-infrared plasmon excitations in 1T' phase monolayer WSe2 and MoS2 are observed using optical spectroscopies. Based on an extensive first-principles study which analyzes the charge dynamics across multiple axes of the atomic-layered systems, the collective charge excitations are found to disperse only along the direction perpendicular to the chains. Further analysis reveals that the interchain long-range coupling is responsible for the coherent 1D charge dynamics and the spin-orbit coupling affects the plasmon frequency. Detailed investigation of these charge collective modes in 2D-chained systems offers opportunities for novel device applications and has implications for the underlying mechanism that governs superconductivity in 2D TMD systems.

Transport properties of T-shaped and crossed junctions based on graphene nanoribbons.

T-shaped and crossed junctions based on graphene nanoribbons (GNRs) were designed and studied in this paper. These junctions were made of shoulders (ZGNRs) joined with stems (AGNRs). We demonstrated the intrinsic transport properties and effective boron (or nitrogen) doping of the junctions by using first-principles quantum transport simulation. Several interesting results were found. (i) The I-V characteristics of the pure-carbon T-shaped junctions were shown to have metallic behavior, and the current of the junction strongly depends on the height of the stem. (ii) The conductances of the devices are found to depend sensitively on their geometric structures and be controlled by selective doping. This feature could make such a quasi-2D carbon-based junction a possible candidate for nanoelectronic devices.