Novel valley character and tunable quasi-half-valley metal state in Janus monolayer VSiGeP4.

The manipulation and regulation of valley characteristics have aroused widespread interest in emerging information fields and fundamental research. Realizing valley polarization is one crucial issue for spintronic and valleytronic applications, the concepts of a half-valley metal (HVM) and ferrovalley (FV) materials have been put forward. Then, to separate electron and hole carriers, a fresh concept of a quasi-HVM (QHVM) has been proposed, in which only one type of carrier is valley polarized for electron and hole carriers. Based on first-principles calculations, we demonstrate that the Janus monolayer VSiGeP4 has QHVM character. To well regulate the QHVM state, strain engineering is utilized to adjust the electronic and valley traits of monolayer VSiGeP4. In the discussed strain range, monolayer VSiGeP4 always favors the ferromagnetic ground state and out-of-plane magnetization, which ensures the appearance of spontaneous valley polarization. It is found that the QHVM state can be induced in different electronic correlations (U), and the strain can effectively tune the valley, magnetic, and electronic features to maintain the QHVM state under various U values. Our work opens up a new research idea in the design of multifunctional spintronic and valleytronic devices.

High-Performance Azo Cathodes Enabled by N-Heteroatomic Substitution for Zinc Batteries with a Self-Charging Capability.

Redox-active azo compounds are emerging as promising cathode materials due to their multi-electron redox capacity and fast redox response. However, their practical application is often limited by low output voltage and poor thermal stability. Herein, we use a heteroatomic substitution strategy to develop 4,4'-azopyridine. This modification results in a 350 mV increase in reduction potential compared to traditional azobenzene, increasing the energy density at the material level from 187 to 291 Wh kg-1. The introduced heteroatoms not only raise the melting point of azo compounds from 68 °C to 112 °C by forming an intermolecular hydrogen-bond network but also improves electrode kinetics by reducing energy band gaps. Moreover, 4,4'-azopyridine forms metal-ligand complexes with Zn2+ ions, which further self-assemble into a robust superstructure, acting as a molecular conductor to facilitate charge transfer. Consequently, the batteries display a good rate performance (192 mAh g-1 at 20 C) and an ultra-long lifespan of 60,000 cycles. Notably, we disclose that the depleted batteries spontaneously self-charge when exposed to air, marking a significant advancement in the development of self-powered aqueous systems.

Quantum anomalous Hall effect with a high and tunable Chern number in monolayer NdN2.

Due to the presence of dissipationless edge states, the quantum anomalous Hall (QAH) insulator has garnered significant attention for both fundamental research and practical application. However, the majority of QAH insulators suffer from a low Chern number (C = 1), and the Chern number is basically unadjustable, which constrains their potential application in spintronic devices. Here, based on a tight-binding model and first-principles calculations, we propose that two-dimensional (2D) ferromagnetic monolayer NdN2 exhibits a high-Chern-number QAH effect with C = ±3, accompanied by a nontrivial band gap of 97.4 meV. More importantly, by manipulating the magnetization orientation in the xz plane, the Chern number of 2D NdN2 can be further tuned between C = ±3 and C = ±1. When the magnetization vector is confined to the xy plane, the monolayer NdN2 would exhibit either a Dirac half-semimetal or in-plane QAH phase. Moreover, the QAH effect with a higher Chern number C = 9 can be achieved by constructing a multilayer van der Waals heterostructure composed of monolayers NdN2 and BN with alternative stacking order. These findings provide a reliable platform for exploring the novel QAH effect and developing high-performance topological devices.

Electronic-correlation induced sign-reversible Berry phase and quantum anomalous valley Hall effects in Janus monolayer OsClBr.

Topological phase transition can be induced by electronic correlation effects combined with spin-orbit coupling (SOC). Here, based on the first-principles calculations +U approach, the influence of electronic correlation effects and SOC on topological and electronic properties of the Janus monolayer OsClBr is investigated. With intrinsic out-of-plane (OOP) magnetic anisotropy, the Janus monolayer OsClBr exhibits a sequence of states, namely, the ferrovalley (FV) to half-valley-metal (HVM) to quantum anomalous valley Hall effect (QAVHE) to HVM to FV states with increasing U values. The QAVHE is characterized by a chiral edge state linking the conduction and valence bands with a Chern number C = 1, which is closely associated with the band inversion between dx2-y2/dxy and dz2 orbitals, and sign-reversible Berry curvature. The section with larger U values (2.31-2.35 eV) is very essential for determining the new HVM and QAVHE states, and also proves that a strong electron correlation effect exists in the interior of the Janus monolayer OsClBr. When taking into consideration a representative U value (U = 2.5 eV), a valley polarization value of 157 meV can be observed, which can be switched by reversing the magnetization direction of Os atoms. It is noteworthy that the Curie temperature (TC) strongly depends on the electronic correlation effects. Our work provides a comprehensive discussion on the electronic and topological properties of the Janus monolayer OsClBr, and demonstrates that the electronic correlation effects combined with SOC can drive the emergence of QAVHE, which will open up new opportunities for valleytronic, spintronic, and topological nanoelectronic applications.

Possibility of regulating valley-contrasting physics and topological properties by ferroelectricity in functionalized arsenene.

A two-dimensional (2D) multifunctional material, which couples multiple physical properties together, is both fundamentally intriguing and practically appealing. Here, based on first-principles calculations and tight-binding (TB) model analysis, the possibility of regulating the valley-contrasting physics and nontrivial topological properties via ferroelectricity is investigated in monolayer AsCH2OH. Reversible electric polarization is accessible via the rotation operation on the ligand. The broken inversion symmetry and the spin-orbit coupling (SOC) would lead to valley spin splitting, spin-valley coupling and valley-contrasting Berry curvature. More importantly, the reversal of electric polarization can realize the nonvolatile control of valley-dependent properties. Besides, the nontrivial topological state is confirmed in the monolayer AsCH2OH, which is robust against the rotation operation on the ligand. The magnitude of polarization, valley spin splitting and bulk band gap can be effectively modulated by the biaxial strain. The H-terminated SiC is demonstrated to be an appropriate candidate for encapsulating monolayer AsCH2OH, without affecting its exotic properties. These findings provide insights into the fundamental physics for the coupling of the valley-contrasting phenomenon, topological properties and ferroelectricity, and open avenues for exploiting innovative device applications.

Prediction of the ferrovalley property with sizable valley splitting in Janus monolayer GdBrI.

The exploitation of two-dimensional (2D) ferrovalley materials is of great significance in promoting the development of novel information storage devices, which is garnering increasing interest nowadays. However, the currently discovered 2D ferrovalley materials are very limited, and some of them still suffer from the drawback of small valley splitting, which seriously hinders their application in valleytronics. Herein, using first-principles calculations, we predict a promising 2D ferrovalley material, Janus monolayer GdBrI, which harbors sizable valley splitting and the anomalous valley Hall effect (AVHE). Monolayer GdBrI is a stable ferromagnetic semiconductor with an easy magnetization plane and magnetic transition temperature of 264.5 K. When the magnetization orientation is toward the z direction, valley polarization with a large splitting of 120.4 meV is achieved in the valence band due to the synergetic effect between the magnetic exchange interaction and spin-orbit coupling. The valley-contrasting Berry curvature gives rise to the AVHE in the monolayer. The magnitude of valley splitting can be continuously tuned by varying the magnetization orientation, biaxial strain and perpendicular electric field. These findings offer Janus monolayer GdBrI as a potential candidate for spintronic and valleytronic applications.

High excellent hydrogen evolution characteristics and novel mechanism of two-dimensional tetra-phase OsN2 and ReN2.

It is essential to find a kind of electrocatalyst for hydrogen evolution reduction (HER) comparable with a noble metal that has good conductivity and abundant active sites. Based on systematic searches by first-principles calculations, we discovered two-dimensional transition-metal nitrides, tetra-phase OsN2 and ReN2 monolayers, as potential HER electrocatalysts with superior thermodynamic and kinetic stability. They exhibited excellent catalytic activity due to the presence of multiple active sites with a density of 8 × 1015 site per cm2 and an overpotential close to 0. In addition, we also found that the synergistic effect of strain and coverage makes them have a good hydrogen evolution activity. The ΔGH of the OsN2 monolayer at 1% tensile strain under 3/4 hydrogen coverage is 0.02 eV, and that of ReN2 at 1/2 hydrogen coverage could decrease to 0.001 eV. Different from other common transition metal nitrides, we found that the active sites of OsN2 and ReN2 monolayers are both at nitrogen atoms, which could be further understood by the crystal orbital Hamiltonian population analysis between N and metal atoms. All these interesting findings not only provide new excellent candidates but also provide new insights into the mechanism of hydrogen evolution of nitrides.

Janus PtXO (X = S, Se) monolayers: the visible light driven water splitting photocatalysts with high carrier mobilities.

Triggered by the recent experimental synthesis of the Janus PtSSe monolayer, we use the first-principles calculations to predict two new Janus photocatalysts PtXO (X = S, Se), based on the systematic investigations of the structural stabilities, electronic structures, band alignments, catalytic activity and optical absorption. The two Janus structures are found to be mechanically, dynamically and thermodynamically stable, and have suitable band edge positions for the overall water splitting. Owing to the high electron mobility (up to 2164.95 cm2 V-1 s-1) and large disparity between the electron and hole mobilities, together with the indirect band gaps and the intrinsic dipole induced built-in electric fields, the photogenerated electrons/holes can be efficiently separated in PtXO. Moreover, the S/Se vacancy can effectively lower the free energy difference of the HER, making the catalytic reactions occur spontaneously under the potentials of photoexcited electrons and holes. Large optical absorption coefficients (105 cm-1) are also confirmed in the visible light range, and the biaxial tensile strain can further enhance the optical absorption while maintaining the capability of the overall water splitting. Our results not only propose two new Janus materials by demonstrating the possibility of experimental realization, but also indicate that PtXO are peculiar candidates for photocatalytic water splitting.

A novel spin-valley-coupled nodal-ring semimetal in single-layer Ta2C3.

Nodal-ring semimetals with band crossing are the new type of quantum materials that have attracted considerable interest from scholars for research. In general, the spin-orbit coupling (SOC) effect opens a band gap at the Dirac point. Therefore, finding 2D nodal-ring semimetals with resistance to SOC has more challenges. Based on first-principles calculations, we propose here that the two-dimensional (2D) Ta2C3 monolayer is a novel nodal-ring semimetal. In particular, Ta2C3 forms six closed rings in the Brillouin zone (BZ) with SOC, which originates from the dxy,x2-y2 orbitals of Ta and the pz orbitals of C. The nodal-ring bands at the K point in Ta2C3 exhibits characteristics of valley splitting and spin polarization due to the breaking of inversion symmetry and SOC. The masximal spin-splitting at the K point is as large as 268.87 meV and 61.90 meV for the conduction band minimum (CBM) and valence band maximum (VBM), respectively. The massless Dirac fermions in the non-equivalent valley have the opposite Berry curvature and spin moment. Therefore, 2D Ta2C3 is novel spin-valley-coupled nodal-ring semimetal. In addition, we found interesting negative differential resistance effects when calculating its transport properties. Our results not only provide an ideal platform for studying the combination of new physical properties, spintronics and valleytronics, but also open the way for designing low-power and fast-transport electronic devices.

Two-dimensional Weyl semi-half-metallic NiCS3 with a band structure controllable by the direction of magnetization.

Two-dimensional (2D) Weyl semi-half-metals (WSHMs) have attracted tremendous interest for their fascinating properties combining half-metallic ferromagnetism and Weyl fermions. In this work, we present a NiCS3 monolayer as a new 2D WSHM material using systematic first-principles calculations. It has 12 fully spin-polarized Weyl nodal points in one spin channel with a Fermi velocity of 3.18 × 105 m s-1 and a fully gapped band structure in the other spin channel. It exhibits good mechanical and thermodynamic stabilities and the Curie temperature is estimated to be 403 K. The Weyl points are protected by vertical mirror plane symmetry along Γ-K, and each of them remains gapless even under spin-orbit coupling when the direction of spin is perpendicular to the Γ-K line including the Weyl point, which makes it possible to control the opening and closing of Weyl points by applying and rotating external magnetic fields. Our work not only provides a promising 2D WSHM material to explore the fundamental physics of symmetry protected ferromagnetic Weyl fermions, but also reveals a potential mechanism of band engineering of 2D WSHM materials in spintronics.

Two-dimensional van der Waals heterostructure CdO/PtSe2: promising visible light photocatalyst for overall water splitting.

Due to the gradual depletion of fossil fuels and the serious environmental pollution, hydrogen generation by photocatalytic water splitting has been considered as an alternative strategy for clean energy. Herein, using hybrid density functional calculations, we systematically study the structural, electronic and optical properties of van der Waals heterostructure CdO/PtSe2 with different stacking patterns. The heterostructure is found to be dynamically stable, and has type-II band alignment with a large built-in electric field, which is favorable for the efficient spatial separation of photogenerated charge carriers. By revealing the intrinsic interface dipoles dependent photocatalytic mechanisms, we find the band edges of all patterns straddle the water redox levels despite the AB-1 pattern having a bandgap less than 1.23 eV. Moreover, the heterostructure shows globally improved optical absorptions with a large absorption coefficient (105 cm-1) compared to the single layers, demonstrating the enhanced photocatalytic activity. Comparing with widely discussed bilayer systems like graphene/C3N4 and MoS2/C3N4, the CdO/PtSe2 simultaneously has several advantages or peculiarities such as the more favorable absorption of visible light, therefore CdO/PtSe2 is a promising candidate and a unique system for photocatalytic water splitting.

Discovery of a ferroelastic topological insulator in a two-dimensional tetragonal lattice.

Ferroelasticity and band topology are two intriguing yet distinct quantum states of condensed matter materials. Their coexistence in a single two-dimensional (2D) lattice, however, has never been observed. Here, we found that the 2D tetragonal HfC monolayer allowed simultaneous presence of ferroelastic and topological orders. By using first-principles calculations, we found that it could allow a low switching barrier with reversible strain of 17.4%, indicating that the anisotropic properties are achievable experimentally for a 2D tetragonal lattice. More interestingly, the tuning of topological behaviors with strain led to spin-separated and gapless edge states, that is, the quantum spin Hall effect. These findings from the coupling of two quantum orders offer insights into ferroelastic control over topological edge states for achieving multifunctional properties in next-generation 2D nanodevices.

Nontrivial topology and topological phase transition in two-dimensional monolayer Tl.

Topological insulating material with dissipationless edge states is a rising star in spintronics. While most two-dimensional (2D) topological insulators belong to group-IV or -V elements in a honeycomb lattice, herein, we propose a new topological phase in the 2D hexagonal group-III crystal, h-Tl, based on a tight-binding model and density-functional theory calculation. Analysis of band dispersion reveals a Dirac nodal-ring near the Fermi level, which is attributed to px,y/pz band crossing. Upon inclusion of spin-orbit coupling (SOC), h-Tl turns into a quantum spin Hall insulator under 21% biaxial strain, confirmed by integrating spin Berry curvature in the Brillouin zone and spin-polarized edge states. A prominent feature of its electronic properties is that the effect of SOC plays two essential roles of both topological gap opening and band inversion between px,y/pz orbitals, which is the first observed phenomenon in 2D materials. This study extends the scope of 2D elemental topological insulators and presents a platform to design new 2D topotronics materials.

Quantum spin Hall effect in two-dimensional hydrogenated SnPb alloy films.

Using first-principles calculations, we studied the geometric and band structures of 20 possible configurations of buckled hydrogenated SnPb alloy (SnxPb8-xH8) films. The configurations are topological insulators (TIs) when x ≥ 1. When x increases from 1 to 7, the band gap increases from 0.087 eV to 0.98 eV. The topological characteristics are suggested by s-pxy band inversion and confirmed by helical edge states, which are time-reversal symmetry protected. According to spin-orbit coupling (SOC) analysis results, we draw the conclusion that the Pb atoms have greater SOC strength than the Sn atoms, so when the number ratio of Pb and Sn atoms is greater than 1/12 the SOC strength is large enough to trigger the band inversion between the s and pxy orbitals, causing the SnPb alloy film to turn into a topological insulator (TI) from a normal band insulator (NI). We give a simple rule for the topological criterion of hydrogenated SnPb alloy films by comparing the ratio of Pb and Sn atoms. This would provide a useful reference for the design of topological devices based on NI-TI hetero-junctions for experiments.

Quantum spin Hall insulator BiXH (XH = OH, SH) monolayers with a large bulk band gap.

A large bulk band gap is critical for the application of two-dimensional topological insulators (TIs) in spintronic devices operating at room temperature. On the basis of first-principles calculations, we predict BiXH (X = OH, SH) monolayers as TIs with an extraordinarily large bulk gap of 820 meV for BiOH and 850 meV for BiSH, and propose a tight-binding model considering spin-orbit coupling to describe the electronic properties of BiXH. These large gaps are entirely due to the strong spin-orbit interaction related to the pxy orbitals of the Bi atoms of the honeycomb lattice. The orbital filtering mechanism can be used to understand the topological properties of BiXH. The XH groups simply remove one branch of orbitals (pz of Bi) and reduce the trivial 6-band lattice into a 4-band, which is topologically non-trivial. The topological characteristics of BiXH monolayers are confirmed by nonzero topological invariant Z2 and a single pair of gapless helical edge states in the bulk gap. Owing to these features, the BiXH monolayers of the large-gap TIs are an ideal platform to realize many exotic phenomena and fabricate new quantum devices working at room temperature.

Correction: Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect.

Correction for 'Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect' by Min Yuan et al., Phys. Chem. Chem. Phys., 2018, 20, 4308-4316.

Prediction of topological property in TlPBr2 monolayer with appreciable Rashba effect.

A quantum spin Hall (QSH) insulator with high stability, large bulk band gap and tunable topological properties is crucial for both fundamental research and practical application due to the presence of dissipationless edge conducting channels. Recently, chemical functionalization has been proposed as an effective route to realize the QSH effect. Based on first-principles calculations, we predict that a two-dimensional TlP monolayer would convert into a topological insulator with the effect of bromination, accompanied by a large bulk band gap of 76.5 meV, which meets the requirement for room-temperature application. The topological nature is verified by the calculation of Z2 topological invariant and helical edge states. Meanwhile, an appreciable Rashba spin splitting of 77.2 meV can be observed. The bulk band gap can be effectively tuned with external strain and electric field, while the Rashba spin splitting shows a parabolic variation trend under an external electric field. We find that the topological property is available for the TlP film when the coverage rate is more than 0.75. BN and SiC are demonstrated as promising substrates to support the topological nature of TlPBr2 film. Our findings suggest that a TlPBr2 monolayer is an appropriate candidate for hosting the nontrivial topological state and controllable Rashba spin splitting, and shows great potential applications in spintronics.

Prediction of topological crystalline insulators and topological phase transitions in two-dimensional PbTe films.

Topological phases, especially topological crystalline insulators (TCIs), have been intensively explored and observed experimentally in three-dimensional (3D) materials. However, two-dimensional (2D) films are explored much less than 3D TCIs, and even 2D topological insulators. Based on ab initio calculations, here we investigate the electronic and topological properties of 2D PbTe(001) few-layer films. The monolayer and trilayer PbTe are both intrinsic 2D TCIs with a large band gap reaching 0.27 eV, indicating a high possibility for room-temperature observation of quantized conductance. The origin of the TCI phase can be attributed to the px,y-pz band inversion, which is determined by the competition of orbital hybridization and the quantum confinement effect. We also observe a semimetal-TCI-normal insulator transition under biaxial strains, whereas a uniaxial strain leads to Z2 nontrivial states. In particular, the TCI phase of a PbTe monolayer remains when epitaxially grown on a NaI semiconductor substrate. Our findings on the controllable quantum states with sizable band gaps present an ideal platform for realizing future topological quantum devices with ultralow dissipation.

First-principles prediction of a giant-gap quantum spin Hall insulator in Pb thin film.

The quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, QSH insulators currently suffer from requiring extremely high vacuums or low temperatures. Here, using first-principles calculations, we predict cyanogen-decorated plumbene (PbCN) to be a new QSH phase, with a large gap of 0.92 eV, that is robust and tunable under external strain. The band topology mainly stems from s-pxy band inversion related to the lattice symmetry, while the strong spin-orbit coupling (SOC) of the Pb atoms only opens a large gap. When halogen atoms are incorporated into PbCN, the resulting inversion-asymmetric PbFx(CN)1-x can host the QSH effect, accompanied by the presence of a sizable Rashba spin splitting at the top of the valence band. Furthermore, the Te(111)-terminated BaTe surface is proposed to be an ideal substrate for experimental realization of these monolayers, without destroying their nontrivial topology. These findings provide an ideal platform to enrich topological quantum phenomena and expand the potential applications in high-temperature spintronics.

Prediction of flatness-driven quantum spin Hall effect in functionalized germanene and stanene.

Searching for realistic materials able to realize room-temperature quantum spin Hall (QSH) effects is currently a growing field, especially when compatibility with the current group-IV electronics industry is required. Here we predict, through first-principles calculations, a new class of QSH phases in flattened germanene and stanene functionalized with X atoms (f-GeX2 and f-SnX2; X = H, F, Cl, Br, I) with a bulk gap as large as 0.56 eV, that can be tuned efficiently under mechanical strain. More interestingly, different from the normal band order in buckled germanane and stanane, the structural flatness leads to an inverted band order without spin-orbit coupling (SOC), whereas the SOC only opens the band gap. We also find that the characteristics of edge states, such as the Fermi velocity, are enhanced greatly by edge modification. When these films are deposited on a BN substrate, a nontrivial QSH state is preserved with a Dirac cone lying within the nontrivial band gap. These findings provide a promising platform for future realistic applications of the QSH effect at room temperature in two-dimensional group-IV films.