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The Dzyaloshinskii–Moriya interaction (DMI), which only exists in noncentrosymmetric systems, plays an important role in the formation of exotic chiral magnetic states.

By obtaining the real magnetic anisotropy energy (MAE) under different strains, when 1.5% ε < 1.8%, due to spontaneous valley polarization, intrinsic out-of-plane (OOP) magnetic anisotropy, and a chiral edge state, the VQAHE can reliably appear between two HVM states.

The combination of ML-CGT and Sc2CO2 provides a way for the design of new non-volatile FE memory devices.

Based on first-principles calculations, we study the hydrogen evolution reaction (HER) on novel 2D monolayers namely OsN2 and ReN2, and indicate their excellent catalytic performance.

A new two-dimensional metal-boride, namely, a NiB2 monolayer, is proposed to possess an anisotropic nodal loop state that is protected by nonsymmorphic glide mirror symmetry.

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.

Two-dimensional (2D) nodal-loop semimetal (NLSM) materials have attracted much attention for their high-speed and low-consumption transporting properties as well as their fantastic symmetry protection mechanisms. In this paper, using systematic first-principles calculations, we present an excellent NLSM candidate, a 2D AlSb monolayer, in which the conduction and valence bands cross with each other forming fascinating multiple nodal-loop (NL) states. The NLSM properties of the AlSb monolayer are protected by its glide mirror symmetry, which was confirmed using a symmetry-constrained six-band tight-binding model. The transport properties of the AlSb monolayer under in-plane uniaxial strains are also studied, based on a non-equilibrium Green's function method. It is found that both compressive and tensile strains from -10% to 10% improve the transporting properties of AlSb, and it is interesting to see that flexure configurations are energetically favored when compressive uniaxial strains are applied. Our studies not only provide a novel 2D NLSM candidate with a new symmetry protection mechanism, but also raise the novel possibility for the detection of out-of-plane flexure in 2D semimetal materials.

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.

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 (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.

The Dirac cone structure provides massless Fermions, ultrahigh carrier mobility, and many other novel features that are significant for massless and dissipationless quantum devices. The anisotropic...

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.

Monolayers of transition metal ReX2 and ReSX (X=S, Se) have been proposed as new electronic materials for nanoscale devices. In this paper, there are three structures: ReS2, Janus ReSSe, and ReSe2. Based on the first-principles theory, we analyzed the structures, electronic properties, and Fermi speed. Remarkably, we studied the stability of structures of ReS2, Janus ReSSe, and ReSe2 monolayers under biaxial tensile and compressive strain by density functional approach. It is worth noting that when the strain changes, not only the band gap changes but also the band gap properties (direct and indirect) also change. The bond gaps decrease with the increase of tensile strain and compressive strain; Moreover, when the strain is greater than 0, the bond angle decreases as the strain increases, and when the strain is less than 0, the bond angle increases as the strain increases.

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.

The quantum spin Hall effect, the valley Hall effect and ferroelectricity are all extensively studied yet distinct properties of insulators and it would be very interesting to see if they could coexist in certain two-dimensional (2D) materials. We present a family of fluorinated methyl-functionalized bismuthene (Bi2C2H6−xFx) films to achieve the 2D ferroelectric valley topological insulator (FEVTI) state, due to strong spin–orbit coupling (SOC) and non-centrosymmetric layer structures. Bi2C2H6−xFx films have topological non-trivial band gaps up to 1.08 eV, which are robust against the coverage and distribution of fluorine atoms, and the strongest out-of-plane ferroelectric polarization is up to 0.25 × 10−10 C m−1. Moreover, some Bi2C2H6−xFx configurations exhibit an extra valley degree of freedom by the valley splitting at the K and K′ points in the Brillouin zone due to the symmetry breaking, resulting in the coexistence of quantum spin and valley Hall states. These phenomena could also be found in other similarly decorated 2D materials, therefore offering unique quantum material platforms for discovering novel physics and exploring innovative applications.

Recent experimental success in the realization of two-dimensional (2D) magnetism has invigorated the search for new 2D magnetic materials with a large magnetocrystalline anisotropy, high Curie temperature, and high carrier mobility. Using first-principles calculations, here we predict a novel class of single-spin Dirac fermion states in a 2D Ta2S3 monolayer, characterized by a band structure with a large gap in one spin channel and a Dirac cone in the other with carrier mobility comparable to that of graphene. Ta2S3 is dynamically and thermodynamically stable under ambient conditions, and possesses a large out-of-plane magnetic anisotropy energy and a high Curie temperature (TC = 445 K) predicted from the spin-wave theory. When the spin and orbital degrees of freedom are allowed to couple, the Ta2S3 monolayer becomes a Chern insulator with a fully spin-polarized half-metallic edge state. An effective four-band tight-binding model is constructed to clarify the origin of a semi-Dirac cone in a spin-up channel and nontrivial band topology, which can be well maintained on a semiconducting substrate. The combination of these unique single-spin Dirac fermion and quantum anomalous Hall states renders the 2D Ta2S3 lattice a promising platform for applications in topologically high fidelity data storage and energy-efficient spintronic devices.

The two-dimensional (2D) materials with nodal line band crossing have been attracting great research interest. However, it remains a challenge to find high-stable nodal line structure in 2D systems. Herein, based on the first-principles calculations and theoretical analysis, we propose that monolayer B6O possesses symmetry protected Dirac nodal line (DNL) state, with its Fermi velocity of 106 m/s in the same order of magnitude as that of graphene. The origin of DNL fermions is induced by coexistence of time-reversal symmetry and inversion symmetry. A two-band tight-binding model is further given to understand the mechanism of DNL. Considering its robustness against spin–orbit coupling (SOC) and high structural stability, these results suggest monolayer B6O as a new platform for realizing future high-speed low-dissipation devices.

Half-Dirac semimetals (HDSs), which possess 100% spin-polarizations for Dirac materials, are highly desirable for exploring various topological phases of matter as low-dimensionality opens unprecedented opportunities for manipulating the quantum state of low-cost electronic nanodevices. The search for high-temperature HDSs is still a current hotspot and yet challenging experimentally. Herein based on first-principles calculations, we propose the realization of Half Dirac semimetals (HDS) in two-dimensional (2D) Kagome transition-metal nitride Cd

The nodal ring material has recently attracted wide attention due to its singular properties and potential applications in spintronics. Here, two-dimensional Zn3C6S6 is calculated and discussed by using first-principle calculations. We found that two-dimensional Zn3C6S6 can generate a nodal ring at 10% compressive strain, and the existence of the ring is proved by a partial charge density map. And as the compressive strain increases, the nodal ring does not disappear. At the same time, the stability of the electron-orbit coupling to the nodal ring is applied. Our findings indicate that the two-dimensional Zn3C6S6 is promising in new electronic and spintronic applications.

Based on first-principles calculations, the BrHfN monolayer has a direct band gap of 3.19 eV, and the edge position can be adjusted by applying pressure. The BrHfN monolayer meets the energy band structure requirements of photocatalytic water splitting. In addition, the calculation of cohesive energy shows the feasibility of the experiment. Ab initio molecular dynamics simulations at room temperature and phonon dispersion curves show the stability of the structure. Young's modulus can be as high as 158 N m−1, and Poisson's ratio shows high anisotropy. High absorption coefficients, appropriate band edge positions and strong ability for carrier separation and transfer make it a promising candidate for photocatalytic water splitting. Additionally, the band structure and optical absorption of monolayer BrHfN can be effectively tuned by strain effect. Our research provides valuable insight for finding more potential functional two-dimensional semiconductors applied in optoelectronics, solar energy conversion, and photocatalytic water splitting.

Rashba spin-orbit coupling (SOC) in topological insulators (TIs) is a very interesting phenomenon and has received extensive attention in two-dimensional (2D) materials. However, the coexistence of Rashba SOC and band topology, especially for materials with a square lattice, is still lacking. Here, by using first-principles calculations, we propose for the first time a SeTe monolayer as a 2D candidate with these novel properties. We find that the square lattice exhibits anisotropic band dispersions near the Fermi level and a Rashba effect related to large SOC and inversion asymmetry, which leads to a Dirac semimetal state. Another prominent feature is that SeTe can achieve a topological state under a tensile strain of only 1%, characterized by the Z2 invariant and helical edge states. Our findings demonstrate that SeTe is a promising material for novel electronic and spintronics applications.

Two-dimensional (2D) large-gap quantum spin Hall (QSH) insulator are imperative for achieving dissipationless transport devices. The bottleneck preventing applications from QSH phase, however, is lack of feasible 2D films experimentally. By using first-principles calculations, here we propose a new 2D topological phase in the bilayer plumbene. Its geometric stability is confirmed by phonon spectrum simulation and ab initio molecular dynamics simulation. We find that it can turn into a nontrivial QSH phase under tensile strain and its nontrivial gap is highly tunable by external strains. The origin behind QSH effect can be identified by s–px,y band inversion, topological invariant Z2, and helical edge states within the bulk gap. Also, a heterostructure composed of bilayer plumbene deposited on semiconducting MoS2 substrate remains topologically nontrivial properties with a sizable gap. These findings provide a promising platform to design a large-gap QSH insulator in Pb film, which show potential applications in spintronics devices.

Based on the first-principles, we survey the structure, electronic and optical properties of ReS2/bi-layer, The result indicate that ReS2 is a direct band-gap semiconductor. And we calculate the optical properties of the ReS2 and bi-layer. Remarkably, the band gap opened in the ReS2 and bi-layer can be effectively regulated through external E-field and biaxial strain. These results provide a way to design photoelectric device and high-performance FETs at room temperature.

Two-dimensional (2D) nodal-loop (NL) semimetals have attracted tremendous attention for their abundant physics and potential device applications, whereas the realization of gapless NL semimetals robust against spin-orbit coupling (SOC) remains a big challenge. Recently, breakthroughs have been made with the realization of gapless NL semimetals in 2D half-metallic materials, where NLs were protected by a horizontal mirror plane symmetry. Here we first propose an alternative nonsymmorphic horizontal glide mirror plane symmetry which could protect the NLs in 2D materials. On the basis of comprehensive first-principles calculations and symmetry analysis, we found that the glide mirror symmetry together with intrinsic out-of-plane spin polarization can protect the NL against SOC in a half-metallic semimetal, namely, the MnNF monolayer. Moreover, we predict that the MnNF monolayer has strong anisotropic characteristics, tunable band structure by changing the magnetization direction, and 100% spin-polarized transport properties. Our work not only provides a novel 2D half-metallic semimetal with strong anisotropy but also broadens the scope of 2D nodal-loop materials.

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.

The prospect of a Dirac half-metal (DHM) and the realization of the quantum anomalous Hall effect (QAHE) on a honeycomb lattice without external fields are a great challenge in experiments due to the structural complexities of two-dimensional (2D) crystals. Here, based on density-functional theory calculations, we propose an ideal candidate material for realizing these exotic quantum states in a 2D honeycomb metal–halogen lattice, single-layer PdCl3. We find that the ground state of PdCl3 is a 100% spin-polarized DHM with a ferromagnetic Curie temperature TC = 528 K predicted from Monte Carlo simulations. Upon including spin–orbit coupling (SOC), PdCl3 reveals the QAHE due to the splitting of the manifold of Pd |dxz〉 and |dyz〉 bands near the Fermi level, which is characterized by the nontrivial Chern number (C = −1) and chiral edge states. In particular, the origin of the topological properties of the PdCl3 honeycomb lattice is explained by the tight-binding model. The sensitivity of nontrivial topology to the cooperative effect of the electron correlation of Pd-4d electrons and SOC is demonstrated: when increasing the on-site Coulomb repulsion U, a sizable nontrivial band gap Eg = 68.6 meV is obtained. Additionally, we explore the mechanical and dynamical stability, as well as strain response of PdCl3 for possible epitaxial growth conditions in experiments. The coexistence of a high temperature DHM and the QAHE in PdCl3 presents a promising platform for the emerging area of spintronics devices with dissipationless edge states.

A Dirac half-metal material, which has a gapped band structure in one spin channel but Dirac cones in the other, combines two intriguing properties of 100% spin polarization and massless Dirac fermions and has recently started to attract increasing attention. In this work, using first-principles calculations we predict that the disodium carbide (Na2C) monolayer is an intrinsic 2p Dirac half-metal material with 12 fully spin-polarized and symmetry-protected Dirac cones, and a slightly gapped (53 meV) spin-polarized nodal line coexisting in one spin channel, leaving the other spin channel insulated with a gap of 1.9 eV. There are two kinds of Dirac cones in Na2C, protected by different crystalline symmetries, both of which are robust against biaxial strains (±5%) and spin–orbit coupling effects, with Fermi velocities of up to 5.2 × 105 m s−1. Ferromagnetism is mainly contributed to by the unpaired 2p electrons in the carbon, with a Curie temperature estimated to be 382 K, and the origin of the 2p magnetism could be explained by the superexchange mechanism between C2− anions with the Na+ cation as a bridge. Our results not only indicate a promising candidate for high-speed spintronic devices, but also reveal the hidden mechanism of the origin of symmetric protection and ferromagnetic exchange interactions in a Dirac semi-metal, which would provide a feasible strategy for the design of Dirac materials.

Nodal-ring materials with a spin-polarized feature have attracted intensive interest recently due to their exotic properties and potential applications in spintronics. However, such a type of two-dimensional (2D) lattice is rather rare and difficult to realize experimentally. Here, we identify the first 2D Honeycomb-Kagome (HK) lattice, Mn-Cyanogen, as a new single-spin nodal-ring material by using first-principles calculations. Mn-Cyanogen shows gapless and semiconducting properties in spin-up and spin-down orientations, respectively, indicating a spin-gapless semiconductor nature. Remarkably, a spin-polarized nodal ring induced by px,y/pz band inversion is captured from the 3D band structure, which is irrelevant to spin-orbit coupling. The origin of the single-spin nodal-ring can be further clarified by the effective tight-binding (TB) model. These results open a new avenue to achieving spin-polarized nodal-ring materials with promising applications in spintronic devices.

Exploring experimentally feasible two-dimensional (2D) films with high carrier mobility, anisotropic electronic properties and tunable band structure is one key to developing next-generation nanodevices. We performed first-principles calculations to explore the geometric and electronic properties of IZrP monolayer. Moreover, the direct band gap of the IZrP monolayer can be effectively tuned by applying mechanical strain, while it turns into a indirect band gap under a tensile strain of 2.0%. The carrier mobility can reach 1.68 × 105 cm2V-1S-1 in the a direction, and it has anisotropic electron transport characteristics. Under a bias voltage of 2.0 V, the current along the a direction can reach 5.5 × 10-6 A. Considering that the IZrP monolayer is both dynamically and thermodynamically stable at a temperature of 300 K, these findings expand the potential applications of the emerging field of 2D materials in nanoelectronics.

Using density functional theory, we systematically studied the stability, electronic structure and optical properties of two-dimensional ternary nitrides XNY (X ​= ​Ti, Zr, Hf; Y F, Cl, Br). Through phonon spectrum calculations and ab initio molecular dynamics simulations, we found that XNY is dynamic and thermodynamically stable. The calculated results of cohesive energy indicate the possibility of synthesis experimentally. XNY is a direct band gap semiconductor, and the band gap is larger than the 1.23 ​eV required for water splitting. In addition, XNY has anisotropic mechanical and optical properties. In addition, XNY has absorption of visible light, and the band-edge arrangement of ZrNF and HfNCl is feasible for water photo oxidation and photo reduction. These comprehensive properties make 2D ternary nitride XNY a candidate material for direction-dependent optoelectronic devices, and it is expected to be applied in the field of photo catalysis.

Single-layer AgBiP2Se6 and AgInP2Se6 are extremely stable semiconductors. Their optical absorption peak happens to fall in the visible light region (1.6–3.2 eV), so they have excellent solar energy conversion potential. In addition, uniaxial and biaxial strains of 0.90–1.10 can realize the transition between indirect band gap and direct band gap and the phenomenon of optical red shift. In addition, compared with compressive strain, applying tensile strain will cause the dielectric function spectrum to respond in a lower energy direction. The present findings could provide a helpful reference to design photoelectronic materials with Ag(Bi,In)P2Se6 by strain engineering.

Knowledge about chemical functionalization is of fundamental importance to design novel two-dimensional topological insulators. Despite theoretical predictions of quantum spin Hall effect (QSH) insulator via chemical functionalization, it is quite challenging to obtain a high-quality sample, in which the toxicity is also an important factor that cannot be ignored. Herein, using first-principles calculations, we predict an intrinsic QSH effect in amidogen-functionalized Bi/Sb(111) films (SbNH2 and BiNH2), characterized by nontrivial Z2 invariant and helical edge states. The bulk gaps derived from px,y orbitals reaches up to 0.39 and 0.83 eV for SbNH2 and BiNH2 films, respectively. The topological properties are robust against strain engineering, electric field, and rotation angle of amidogen, accompanied with sizable bulk gaps. Besides, the topological phases are preserved with different arrangements of amidogen. The H-terminated SiC(111) is verified as a good candidate substrate for supporting the films w...

Based on first-principles calculations, the electronic properties of Cadmium Sulfide nanoribbons (CdSNRs) have been studied with both zigzag (Z-CdSNRs) and armchair shaped edges (A-CdSNRs). For Z-CdSNRs, the structures with both edges decorated by H or F atoms show half-metallic or semiconducting properties, respectively. Only S-dominated edge decorated by H/F atoms, Z-CdSNRs show metallic properties. Only Cd-dominated edge is hydrogenated, Z-CdSNRs can be observed from a metallic to half metallic transition. But instead of fluorinated, the structures show the metallic properties. However, either edge or both edges are hydrogenated or fluorinated, A-CdSNRs exhibit semiconducting properties, and their band gap decreases monotonically with increasing ribbons width (n). When a stress is applied on the half-hydrogenated A-CdSNRs, their band gap displays a steady decrease trend. Moreover, A-CdSNRs are more stable than Z-CdSNRs, while the hydrogenated systems are more stable than fluorinated systems. The results show that different structures of CdSNRs decorated with the different edges can play different nature as semiconducting – half-metallic – metallic properties. The research has important theoretical significance for the electronic design and assembly of CdSNRs structures, and provides a new perspective for the potential application of CdSNRs in nanoelectronics.

The quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices due to their robust gapless edge states inside insulating bulk gap. However, the currently discussed QSH insulators usually suffer from ultrahigh vacuum or low temperature due to the small bulk gap, which limits their practical applications. Searching for large-gap QSH insulators is highly desirable. Here, the tunable QSH state of a Bi(110) films with a black phosphorus (BP) structure, which is robust against structural deformation and electric field, is explored by first-principles calculations. It is found that the two-monolayer BP-Bi(110) film obtains a tunable large bulk gap by strain engineering and its QSH effect shows a favorable robustness within a wide range of combinations of in-plane and out-of-plane strains, although a single in-plane compression or out-of-plane extension may restrict the topological phase due to the self-doping effect. More interestingly, in view of biaxial strain, two competing physics on band topology induced by bonding-antibonding and p

Exploring experimentally feasible two-dimensional (2D) films with high carrier mobility and tunable band structure is one key to developing next-generation nanodevices. Motivated by successful synthesis of the layered GaGeTe in previous experiments, we performed first-principles calculations to explore the geometric and electronic properties of GaGeTe few-layers, with a focus on the monolayer. The interlayer interaction in layered GaGeTe was found to be rather weak, with a cleavage energy of 0.53 J m−2, suggesting that the exfoliation of bulk crystal is a viable means for the preparation of GaGeTe few-layers. When thinned from bulk to few-layers, a transition from semimetal to semiconductor occurs with a band gap in the range of 0–0.74 eV, which is promising for the photovoltaic emitting devices. Moreover, the indirect band gap of the GaGeTe monolayer can be effectively tuned by applying mechanical strain and decreases with increasing tensile strain, while it increases and turns into a direct band gap under a compressive strain of 2.0%. Anisotropic carrier mobility can be observed along zigzag and armchair directions of the GaGeTe monolayer, and particularly for the former, a maximum of 7.83 × 104 cm2 V−1 s−1 was observed. Considering that the GaGeTe monolayer is both dynamically and thermodynamically stable at a temperature of 600 K, these findings expand the potential applications of the emerging field of 2D germanene-based crystals in nanoelectronics.

The quantum spin Hall (QSH) effect may promote revolutionary device development due to dissipationless propagation of spin currents. The bottleneck preventing applications from the QSH effect, however, is a lack of large nontrivial bulk gap and highly stable two-dimensional (2D) films. In this work, we design a novel 2D honeycomb lattice, namely a SnCH3 monolayer, using comprehensive density-functional theory (DFT) computations. The structural stability is confirmed using a phonon spectrum and molecular dynamics simulations. Interestingly, its nontrivial bulk gap can reach up to 0.34 eV, which is further tunable via external strain. The nontrivial topology stems mainly from band inversion between the s–px,y orbitals, demonstrated by the nonzero topological invariant Z2 and a single pair of gapless helical edge states located in the bulk gap. The effects of a growth substrate on the QSH effect are also checked by hydrogen bonding on a single side in stanene, showing the robustness of the observed QSH phase. Considering its compatibility with the current electronics industry, these findings present an efficient platform to enrich topological phenomena and expand potential applications of 2D stanene at high temperature.

Bismuth (Bi) and antimony (Sb) have similar electronic structures but distinct topological properties in their two-dimensional (2D) films, due to their different spin–orbit coupling strengths. An Sb/Bi hetero-junction is a good candidate for the formation of normal band insulator (NI)–topological insulator (TI) boundary states where dissipationless spin currents exist. However, the topological properties of 2D Bi/Sb alloy films, forming at the boundaries of their hetero-junctions, have not been well studied yet. Here, first-principles calculations are performed to study the geometric and band structures of buckled and puckered SbBi alloy 2D films. The transition point between the TI and NI phases is at x = 5 in buckled BixSb8−x and at x = 3 in puckered BixSb4−x. The topological transition of buckled SbBi can be explained using the well-known band inversion mechanism between p orbits, caused by the variation of either components or lattice parameters. According to the analysis on the SOC strength, we propose an experiential ratio rule for the topological phase transition point of SbBi, by simply comparing the number of Bi and Sb atoms, which will be useful in the designing of topological materials and devices for experiment.