As novel topological states, the higher-order topological insulators have attracted great attentions in the past years. However, their realizations in realistic materials, in particular in two-dimensional systems, remains the big challenge due to the lack of adequate candidates. Here, based on the first-principle calculation, we identify the currently existing bilayer alpha 5-phase borophenes as the two-dimensional second-order topological insulators, related to the C2-rotational symmetry. The formation of interlayer B-B covalent bonds, stabilizing the bilayer borophenes and opening the large direct bulk gaps (similar to 0.55 - 0.62 eV) at Fermi level, plays the key roles. The second-order topology is characterized by the bulk quantized quadrupole momentum. Our results enriches the candidates for the second-order topological insulators, and also provide a way to study topological states in borophenes.

Realizing the rational control of two-dimensional valleytronics is appealing for potential applications in valleytronic devices. Using density functional theory method, we systematically study the magnetoelectronic coupling properties in ferrovalley/ferroelectricity van der Waals (vdW) heterostructure, ScI2/Ga2STe2. Interestingly, an on/off transition of the valley nature is found for this multiferroic ScI2/Ga2STe2 heterostructure. Under the Ga2STe2-P up arrow polarization state, the heterostructure shows a ferromagnetic (FM) semiconducting feature with valley polarization, while it turns out to be FM metal when the ferroelectric (FE) polarization is switched to Ga2STe2-P down arrow; therefore, a switchable valleytronic device can be realized by altering the FE polarization direction. Furthermore, it is found that such switchable valleytronic behaviors of the ScI2/Ga2STe2 heterostructure can be modulated by applying biaxial strains for both FE polarization directions. The research of nonvolatile electrical control of valley splitting in the two-dimensional ScI2/Ga2STe2 multiferroic heterostructure provides a way for designing all-in-one valleytronic devices.

Borophene with diverse geometries and rich electronic properties has attracted great research interest over the past few years due to its multicenter bonding characteristics derived from the electron deficiency of boron. However, the members of borophene as well as their stability mechanism have not been fully explored yet. In this work, we explored the stabilities of various free-standing (beta(12)-, alpha(1)-, beta(1)-, alpha-, alpha(4)-, and alpha(5)-phase) borophenes with single/double vacancies (SVs/DVs) and adatom defects using density functional theory methods. Our results show that the most stable configurations of the single-vacant borophene favor the one with the A site vacancy and form the elongated hexagon in defected beta(12) borophene and hexagonal vacancies in other phase borophenes, respectively. The structures of borophene with DVs favor the ones with two fused hexagonal rings. All of the vacant borophenes are found to be experimentally feasible with low formation energies (E-f_vs) for the lowest-energy SVs/DVs around -1.11 to 1.49 eV. Among them, the E-f_vs of three single-vacant (alpha-, alpha(4)-, and alpha(5)-) phase borophenes and two double-vacant (alpha(4)- and alpha(5)-) phase borophenes are negative, showing that they are more stable than their pristine ones. Besides, the beta(12)-phase borophene is energetically favorable to adsorb the B adatom. Detailed analysis shows that the stability of the defective borophene is sensitive to the ratio of hexagons in the systems. Moreover, the ultrahigh stability of the vacant alpha-, alpha(4)-, and alpha(5)-phase borophene can also be derived from the minimization of the imbalance ratio of the sigma/pi orbital occupation. This study is significant for evaluation of stability in defected borophene and very useful to understand the influence of defects in two-dimensional boron.

Quantum anomalous Hall (QAH) insulators with tunable Chern numbers have excellent application prospects in spintronics. Based on the eight-band tight-binding (TB) model, we realized a Chern number tunable QAH phase and valley-polarization quantum anomalous Hall (VP-QAH) state in the A3B3C2 lattice. Using density functional theory calculations, the monolayer Ti3S3Te2, a candidate for the TB model, was predicted to be a robust ferromagnetic Weyl semimetal protected by C2x rotation symmetry. When the spin-orbital-coupling effect was included, the Weyl point was gapped, resulting in a QAH phase with a Chern number C = 1. Specifically, the monolayer Ti3S3Te2 transitioned into a high-Chern-number QAH insulator with C = -2 under 4% or larger compressive strains. Furthermore, breaking the C2xT rotation symmetry by applying an external electric field led to the VP-QAH state. Our work provides a promising candidate for the QAH state with a tunable Chern number and VP-QAH state, making it suitable for use in spintronic devices.

Developing two-dimensional valleytronic materials with spin-valley coupling attracts great interest in the field of valleytronics and spintronics devices. Here, starting from a tight-binding model based on a colorful triangle lattice, we suggest that the valley polarization can be realized in monolayer Ti3X3Y2 (X = S, Se; Y = S, Se, Te) by using first-principles calculations. Our results show that the proposed monolayer exhibits intrinsic ferrovalley with large valley polarization, derived from the time-reversal and C2 symmetry broken, which is beneficial for observing the anomalous valley hall effect in these systems. Furthermore, strain engineering can induce the ferrovalley semiconductor to half-valley metal transition in monolayer Ti3X3Y2. Our work not only provides a platform for two-dimensional (2D) valleytronic research but also promises the fundamental research of coupling physics in 2D lattices.

The integration of two-dimensional heterostructure materials remains a fundamental way for the manipulation of spintronics in practical applications. Here, we predicted the transform of stripy antiferromagnetic (AFM) CoI2 and MnBr2 monolayers to interlayer AFM CoI2/MnBr2 heterostructure with intralayer ferromagnetic orders by using density functional theory. Interestingly, the CoI2/MnBr2 heterostructure exhibits a typical bipolar magnetic semiconducting state with type-I band alignments. Moreover, the half-metal/semiconductor transition and spin-up/spin-down polarization switching in CoI2/MnBr2 heterostructure can be effectively triggered by electron/hole doping. Our study provides the potential of AFM spintronics for information storage and processing.

The hydrogen evolution reaction (HER) in alkaline solutions does not access H* directly, resulting in a slower reaction kinetics compared to that in acidic solutions. Here, we report a Cr-doped Ni3N/Pt heterostructure that provides additional active sites to produce H* via the water-cleaving step, in addition to the intrinsic active Pt site for H* absorption. It is demonstrated that the Cr dopant can modulate the charge redistribution between Ni3N and Pt interface, lowering the energy barrier of both the Volmer step and the following Heyrovsky step. As a result, the prepared Cr-Ni3N/Pt catalyst achieves an extreme low overpotential of 20 mV to deliver a current density of 10 mA/cm2 under alkaline conditions, which is significantly better than the commercial Pt/C catalyst (45 mV). Density Functional Theory (DFT) further reveals that the Cr-modified Ni3N/Pt interface undergoes electronic orbital hybridization, enhancing the water adsorption and dissociation processes on the Ni sites. This work presents the feasibility of the electronic structure modulation in low-platinum catalysts, which provides an effective strategy for the design of electrocatalysts used in multi-step reactions.

Magnetic semiconductors, which are characterized by strong magnetic coupling and semiconducting features, have the potential to be applied in spintronic devices. Here, a family of TM3X5 (TM = V, Fe, Co, Ni; X = O, S, Se) monolayers were predicted to be robust magnetic semiconductors based on first-principles calculations. Our findings show that the V3Se5 monolayer exhibits ferromagnetic (FM) semiconductor character, while Fe3S5, Co3O5, and Ni3O5 monolayers are found to be antiferromagnetic (AFM) semiconductors. Among them, the FM order in the V3Se5 monolayer is ultraly stable with a high Curie temperature of 500 K, which can be enhanced to 900 K under 2% tensile strain. Moreover, the electronic and magnetic properties of these TM3X5 monolayers are sensitive to carrier doping. The Ni3O5 monolayer can be transformed to an AFM half-metal or an AFM bipolar magnetic semiconductor under electron or hole doping, respectively. Such properties endow TM3X5 monolayers with tremendous potential for practical applications in the realm of nanoscale spintronic devices.

The valley-polarized quantum anomalous Hall (VP-QAH) materials, which combine valley polarization and the quantum anomalous Hall (QAH) effect, are of both fundamental and technological importance due to their potential applications for nanoscale devices. Here, we explored the stability, magnetic, and topological properties of two-dimensional MnTiX2 (X = F/Cl/Br) monolayers based on first-principles calculations. Our results show that all the structures have robust antiferromagnetic orders with large magnetic anisotropic energies and high transition temperatures (480–670 K). In the absence of spin–orbital coupling (SOC), the MnTiX2 monolayers represent antiferromagnetic quadratic crossing semimetals. In the presence of SOC, the quadratic crossing points of the systems are opened with sizable bandgaps (> 0.5 eV), and they are transformed to be QAH insulators with |C| = 1. Moreover, the MnTiF2 monolayer is found to be a spontaneous VP-QAH semiconductor due to the time reversal symmetry and inversion symmetry being broken. These insights provide an ideal platform for achieving VP-QAH materials for dissipationless transport and quantum computing.

Two-dimensional transition metal borocarbides with intrinsic magnetism have garnered significant research attention due to their potential applications in spintronics. Using density functional theory calculations, we designed a type of transition metal borocarbides with two distinct configurations, TMBC-Is and TMBC-IIs (TM = V–Co), and explored their electronic and magnetic properties. Our results demonstrate that all the studied systems exhibit both thermal and kinetic stability. Notably, four systems of MnBC-I/MnBC-II and FeBC-I/FeBC-II are robust ferromagnetic (FM) half metals (HMs) with Curie temperatures of 145, 180, 108, and 315 K. Expect FeBC-II monolayer, FM to antiferromagnetic transition occurs for three other FM HMs under 8%–10% compressive strains, while FM HM to FM semiconductor transition is found for MnBC-II monolayer under 8% tensile strain. These findings provide a promising way to design two-dimensional FM HMs, which hold potential applications in spintronics.

Borophene with rich polymorphs has largely enriched the family of two-dimensional materials and attracted great research interest. Recently, the evolution of bilayer borophene (BLB) structures has aroused curiosity about the exploration of more members of BLB and their stability mechanism. Herein, we predicted a number of stable BLBs built by borophene monolayers using density functional theory calculations. Interestingly, four BLB polymorphs, such as alpha(5)-BLB, beta(8)-BLB, alpha(4)-BLB, and beta(9)-BLB, are identified as semiconductors with band gaps of 0.81, 0.34, 0.04, and 0.02 eV, respectively. In addition, four other types of BLBs, i.e., alpha(3)-BLB, beta(1)-BLB, beta(6)-BLB, and beta(12)-BLB, are found to be stable metals. It is revealed that the interlayer B-B pillars play a crucial role in balancing the distribution of surplus electronic density in BLBs and are responsible for their super stabilities than those of borophene monolayers. Besides, the strong interlayer covalent interaction plays an important role in the semiconducting properties of these BLBs. Our results are helpful to understand the stability mechanism of BLB structures and expand the family of bilayer borophenes.

Developing electrocatalysts for the N2 reduction reaction (NRR) with high activity, high selectivity, and low cost is urgently required to enhance the NH3 yield rate. Based on first-principles calculations, we predict a series of new transition metal boride TMB6 (TM = Ti, V, Cr, Mn, Fe, and Co) monolayers and investigate their magnetoelectronic and electrocatalytic properties. The results reveal that VB6 and CoB6 favor ferromagnetic coupling, while TiB6, CrB6, MnB6, and FeB6 display antiferromagnetic ordering. Furthermore, TiB6 exhibits a high Néel temperature of 344 K and a large magnetic anisotropy energy of 614 μeV per Ti atom. Most interestingly, TiB6 and VB6 exhibit superior NRR catalytic activity with a limiting potential of -0.50 and -0.19 V, respectively, and favorable NRR selectivity over the HER. Finally, the structural stability of TMB6 monolayers has been confirmed by a set of phonon dispersion, molecular dynamics, and elastic constant calculations. Our results highlight the use of the newly designed two-dimensional (2D) TM borides as promising candidates for spintronic devices and nitrogen fixation applications.

Creating low dimensional ferromagnetic (FM) semiconductors or half metals with strong FM orders is promising to meet the requirement for next-generation spintronics. However, most of the demonstrated FM semiconductors or half metals suffer from low Curie temperatures (TCs). Here, by first-principles calculations, we predict that the two-dimensional (2D) M3XSe4 (M = V, Cr; X = S, Te) monolayers are a type of intrinsic 2D ferromagnets with thermodynamical stability. Our results show that V3XSe4 (X = S, Te) monolayers are FM semiconductors with indirect bandgaps of 0.60 and 0.50 eV, respectively. Particularly, both structures are revealed to have high TCs of 387 and 770 K and suppress the application limit of room-temperature. In addition, Cr3XSe4 (X = S, Te) monolayers are FM half metals with 100% spin-polarized currents. Moreover, the electronic and magnetic properties of these M3XSe4 monolayers can be modulated by biaxial strains. V3TeSe4 monolayer can be tuned to be room temperature direct bandgap semiconductor under biaxial 1% tensile strain, and TC of V3SSe4 can be largely enhanced under compressive strains. Our results suggest that M3XSe4 monolayers are promising candidates for spintronic devices.

The intercalation of various atoms or molecules has become one promising way to manipulate the electronic and magnetic properties of layered materials. Using density functional calculations, we explored the 3d transition metal (TM) intercalated alpha-borophene/black phosphorus (alpha-B/BP) heterostructure, TM@(alpha-B/BP) (TM = Sc-Ni), on their structure, electronic and magnetic properties. Our results demonstrate that TM@(alpha-B/BP)s can be ferromagnetic (FM), antiferromagnetic (AFM) and nonmagnetic depending on the choice of TM atoms, and most systems have large magnetic anisotropic energy. Particularly, Ti@(alpha-B/BP) is AFM semiconductor with N & eacute;el temperature of 470 K, which is much higher than room temperature. Moreover, the electronic and magnetic properties of TM@(alpha-B/BP)s can be further altered by the TM intercalation concentration. Our results provide a feasible way to design promising candidates for applications in electronic and information storage devices.

The search of 2D magnetic materials with quantum anomalous Hall (QAH) effect is a fascinating topic in the field of condensed matter physics, which holds great promise for topological quantum computing. This work proposes that Kagome-Honeycomb-Triangle lattice Cr3O4Cl monolayer represents a class of ferromagnetic Weyl half-metal with high Curie temperature. Upon considering the spin-orbit coupling (SOC) effect, the Weyl point is broken and a topologically nontrivial gap of 101.96 meV is opened. Under biaxial tensile strain, the Cr3O4Cl monolayer is transformed from QAH metal to QAH insulator with Chern number C = 1. Remarkably, a Chern number phase transition from C = 1 to C = 3 can be achieved via manipulating the hole concentration of 1.05 holes per unit cell. This work proposes a platform for realizing 2D high Chern number QAH effect for practical applications.

Developing 2D materials with intrinsic magnetism is an essential issue owing to their wide potential applications in spintronics. Here, a type of transition metal tetraborides, TMB4, (TM = Ti-Co) monolayers were systematically investigated by employing first principles calculations. Our results showed that the predicted structures hold buckled motif, ultrahigh stability and large magnetic anisotropy energy. It is revealed that CrB4, MnB4, and FeB4 monolayers are robust ferromagnetism, differently, TiB4, VB4, and CoB4 monolayers are stable anti-ferromagnetism. Based on the Monte Carlo simulation, the critical temperatures of these magnetic systems are around 177-755 K. Most importantly, CoB4 monolayer is confirmed to be an antiferromagnetic semiconductor with out-of-plane magnetic easy axis and out-of-plane piezoelectricity with large piezoelectric stress coefficients. Our study suggests that the 2D TMB(4)s hold promising applications for spintronic devices and piezoelectric devices.

The development of effective and non-precious electrocatalyts for hydrogen evolution reaction (HER) has attracted massive research interests. Herein, we report a density functional theory (DFT) investigation on the activation and optimization of Molybdenum disulfide (MoS2) monolayer as efficient HER electrocatalysts by cobalt-nonmetal atom (X = B, C, N, P, Se) codoping. Our results show that three CoX-MoS2 (X = C, N, and Se) catalysts display enhanced HER performance with |DGH|s in the range of 0.12-0.23 eV. Careful electronic structure analysis manifests that the favorable H adsorption process on the MoS2 basal plane is induced by suitable in-gap states upon codoping. Furthermore, appropriate biaxial strain can help optimize the HER performance of these co-doped systems, e.g, the DGHs of CoC@MoS2, CoN@MoS2, and CoSe@MoS2 reaches 0.0 eV, -0.04 eV, and -0.01 eV at 1.86% tensile strain, 5% compressive strain, and 4% compressive strain, respectively. Our work offers a highly promising catalyst for HER and guides the atomic design of more efficient non-noble electrocatalysts.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Two dimensional (2D) van der Waals (vdW) heterostructures have recently emerged as attractive candidates in spintronic devices. Here, a field-effect spin filter based on the 2D antiferromagnetic (AFM)/ferromagnetic (FM) vdW heterostructure, MnPS3/MnCl3, is proposed. The ground state of the MnPS3/MnCl3 heterostructure is predicted to be a type-I AFM semiconductor with AFM Néel ordering in MnPS3 and MnCl3 monolayers. Interestingly, an AFM–FM transition is found for the MnPS3/MnCl3 heterostructure when reducing the interlayer distance to the range of d < 2.0 Å. Moreover, the MnPS3/MnCl3 heterostructure undergoes semiconductor, metal, and half-metal transitions with the interlayer distance decreasing. Moreover, a semiconductor–half-metal transition can also be observed for MnPS3/MnCl3 by applying an external electric field. Our study offers a potential way to design spintronic devices by 2D layered magnetic vdW heterostructure.

To realize the quantum anomalous Hall (QAH) effect in two-dimensional (2D) intrinsic magnetic materials, which combines insulating bulk states and metallic edge channel states, is still challenging in experiment. Here, based on first-principles calculations, we predicted two stable kagome-latticed QAH insulators: Cr3Se4 and Fe3S4 monolayers, with the Chern number C = 1. It is found that both structures exhibit a large magnetic anisotropy energy and sizable band gaps, and a topological phase transition from C = -1 to C = 1 occurs when the magnetization orientation changes from the z-axis to the -z-axis. Remarkably, the non-trivial topological properties are robust against biaxial strains of up to ±6%. Furthermore, a variable high Chern number of C = 2 or C = 3 can be observed by stacking two or three layers of the QAH monolayer with an MoS2 insulator. Our results signify that such layered kagome materials can be promising platforms for exploring novel QAH physics.

The development of quantum anomalous hall (QAH) insulator with high transition temperature is the key to realize practical applications in future quantum technology and spintronics. Here, we predicted two stable two-dimensional kagome structures, Ti3Te4 and Cr3Te4, and found that both of them are intrinsic QAH insulators, using density functional theory calculations. In the absence of spin–orbit coupling (SOC), both systems display ferromagnetism (FM) Weyl semimetal states. Remarkably, Ti3Te4 monolayer is revealed to be a robust ferromagnetic half metal with high Curie temperature (TC) of 403 K. When the SOC effect occurs, it spontaneously creates QAH states with large nontrivial bandgap and chiral edge states. As a result, the Ti(Cr)3Te4 monolayer is changed to be QAH insulators with Chern number C = ±1 by rotation of magnetization orientation. In addition, the phase change from FM QAH insulator to antiferromagnetic insulator can be manipulated by applying external strains. Moreover, a high-Chern number phase (C = 2) arises by building Ti3Te4/MoS2/Ti3Te4 heterostructure. Meanwhile, the topological phase transition can be well recurred by using a spinless three-band tight-binding (TB) model. The findings present ways to realize potential QAH insulators with high transition temperatures.