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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.

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.

Nodal-line semimetals have brought a hot-spot of research due to its novel properties and great potential application in spin electronics. It is more challenging to find 2D nodal-line semimetals that can resist the spin-orbit coupling (SOC) effect. Here, we predict that 2D tetragonal ZnB is the nodal-line semimetal with great transport properties. There are two crossing bands centered on S point at Fermi surface without SOC, which are mainly composed of p xy orbitals of Zn and B atoms and the p z orbitals of B atom. Therefore, the system presents nodal line centered on S point in its Brillouin zone (BZ). And the nodal line is protected by the horizontal mirror symmetry Mz. We further examined robustness of nodal line under biaxial strain by applying up to -4% in-plane compressive strain and 5% tensile strain on ZnB monolayer, respectively. The transmission along a direction is significantly stronger than that along b direction in the conductive channel. The current in a direction is as high as 26.63 μA at 0.8 V and that in b direction reaches 8.68 μA at 0.8 V. It is interesting that transport characteristics of ZnB are negative differential resistance (NDR) effect after 0.8 V along a (b) direction. The results provide ideal platform for research of fundamental physics of 2D nodal-line fermions and nanoscale spintronics as well as design of new quantum devices.

The quantum anomalous Hall (QAH) effect has attracted enormous attention since it can induce topologically protected conducting edge states in an intrinsic insulating material. For practical quantum applications, the main obstacle is the non-existent room temperature QAH systems, especially with both large topological band gap and robust ferromagnetic order. Here, according to first-principles calculations, we predict the realization of the room temperature QAH effect in a two-dimensional (2D) honeycomb lattice, RuCS3 with a non-zero Chern number of C = 1. Especially, the nontrivial topology band gap reaches up to 336 meV for RuCS3. Moreover, we find that RuCS3 has a large magnetic anisotropy energy (2.065 meV) and high Curie temperature (696 K). We further find that the non-trivial topological properties are robust against the biaxial strain. The robust topological and magnetic properties make RuCS3 have great applications in room temperature spintronics and nanoelectronics.

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.

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.

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.

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.

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.

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...

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.

We study the geometric and electronic properties of germanene adsorbed with several small organic molecules (SOMs) by using first-principles calculations. The adsorption energies are found in the range from −0.459 to −0.231 eV, higher than those of SOMs adsorbed on graphene and silicene, indicating strong interactions between organic molecules and germanene. These lead to a large enhancement of band gap of germanene with sizable values of 3.9–81.9 meV due to the sublattice symmetry breaking. Noticeably, the characteristics of Dirac cone with low effective mass and high electron mobility are preserved. These findings provide a possible way to design germanene based optoelectronic devices.

Based on tight-binding model and first-principles calculations, we systematically investigate the geometric, electronic, and magnetic properties of hydrogenated stanene. The results indicate that the half-hydrogenation breaks the π-bondings of stanene, leaving π electrons in unsaturated Sn atoms localized and unpaired, which makes it transform into half-metal (HM) with room-temperature ferromagnetism. Especially, the magnetism of hydrogenated stanene can be effectively tuned by different rates of coverage for hydrogen atoms. While for the case of full-hydrogenated stanene, two different configurations exhibit the nature of semiconductor and semimetal, respectively, which is dependent on the arrangement of hydrogen atoms. We also find that the band gaps of stanane bilayer and monolayer can be effectively modulated by external electric field and strain. These findings demonstrate that hydrogenation is an efficient way to tune the electronic properties of stanene, and it provides a new perspective for the potential application in nanoelectronics.

Searching for two-dimensional (2D) group IV films with high structural stability and large-gaps is crucial for the realization of a dissipationless transport edge state using the quantum spin Hall effect (QSHE). Based on first-principles calculations, we predict that 2D germanene decorated with ethynyl-derivatives (GeC2X; X = H, F, Cl, Br, I) can be a topological insulator (TI) with a large band-gap for room-temperature applications. Both GeC2I and GeC2Br films are intrinsic TIs with a gap reaching up to 180 meV over a wide range, while GeC2H, GeC2F, and GeC2Cl transform from trivial to nontrivial phases under tensile strain. This topological characteristic can be confirmed by s–pxy band inversion, topological invariant Z2, and time-reversal symmetry protected helical edge states. Notably, the characteristic properties of edge states, such as the Fermi velocity and edge shape, can be tuned by edge modifications. Furthermore, we demonstrate that the h-BN sheet is an ideal substrate for the experimental realization of GeC2X, maintaining their nontrivial topology. Considering their higher thermo-stability, these GeC2X films may be good QSHE platforms for topological electronic device design and fabrication in spintronics.

The control of spin without a magnetic field is one of the challenges in developing spintronic devices. Here, based on first-principles calculations, we predict a new kind of ferromagnetic half-metal (HM) with a Curie temperature of 244 K in a two-dimensional (2D) germanene van der Waals heterostructure (HTS). Its electronic band structures and magnetic properties can be tuned with respect to external strain and electric field. More interestingly, a transition from HM to bipolar-magnetic-semiconductor (BMS) to spin-gapless-semiconductor (SGS) in a HTS can be realized by adjusting the interlayer spacing. These findings provide a promising platform for 2D germanene materials, which hold great potential for application in nanoelectronic and spintronic devices.

Band topology and Rashba spin splitting (RSS) are two extensively explored exotic properties in condensed matter physics. However, the coexistence has rarely been reported in simplest stoichiometric films so far. Here, by using first-principles calculations, we demonstrate that a series of inversion-asymmetric group-IV XYH2 monolayers (X, Y = Si, Ge, Sn, Pb) allow for the simultaneous presence of topological order and large RSS that derives from their peculiarly atomic structure. The topological bulk gaps and RSS energies of PbSnH2, PbGeH2, and PbSiH2 are tunable over a wide range of strains (−8 to 8%), even the maximum value can be enhanced to 0.68 eV and 0.24 eV under achievable strain, but another three configurations transform from trivial to nontrivial phases under tensile strain. Furthermore, we find that the Te(111)-terminated BaTe surface is an ideal substrate for the growth of these monolayers, without destroying their intrinsic band topology. Our findings provide a possible route to future applications of inversion-asymmetric topological insulators in spintronic devices.

The new quantum materials known as inversion-asymmetry topological insulators (IATIs) have recently drawn intense attention because these structures possess distinct properties from those of inversion-symmetry insulators. However, the reported IATIs are rare in monolayer structures thus far. On the basis of first-principles calculations, we predict that the two-dimensional I-decorated HgSe monolayer (HgSeI2) is a new IATI with a large gap of 59.1 meV, as well as large Rashba spin splitting (RSS) of 37.2 meV, which derives from the polarity of atoms. The topological characteristic is confirmed by the s–pxy band inversion, topological invariant Z2, and time-reversal symmetry-protected helical edge states. The topologically nontrivial band gap and RSS of HgSeI2 can be effectively modulated with a wide range of strain (−12 to 6%) and external electric field (−10 to 10 V nm−1), and the maximum band gap can be improved to 156 meV under compressive strain, while other F-, Cl-, and Br-decorated cases can transform from trivial to nontrivial TIs with respect to appropriate strain. These novel IATIs with controlled band gap and RSS provide excellent platforms to realize topological spintronic devices based on inversion-asymmetry films.

A great obstacle for the practical applications of the quantum anomalous Hall (QAH) effect is the lack of suitable two-dimensional (2D) materials with a sizable nontrivial band gap, high Curie temperature, and high carrier mobility. Based on first-principles calculations, here, we propose the realizations of these intriguing properties in asymmetry-functionalized 2D SnHN and SnOH lattices. Spin-polarized band structures reveal that SnOH monolayer exhibits a spin gapless semiconductor (SGS) feature, whereas SnNH is converted to SGS under compressive strain. The Curie temperature of SnOH reaches 266 K, as predicted by Monte Carlo simulation, and it is comparable to the room temperature. When the spin and orbital degrees of freedom are allowed to couple, both systems become large-gap QAH insulators with fully spin-polarized half-metallic edge states and higher Fermi velocity of 4.9 × 105 m s-1. These results pave a new way for designing topological field transistors in group-IV honeycomb lattices.

One-dimensional (1D) nanowires (NWs) with robust half-metallicity are a rising star in spintronics. Herein, we theoretically investigate the magnetic and electronic properties of 3d transition-metal tribromide NWs, i.e. TMBr3 (TM = Sc, Ti, V, Cr, Co, and Cu). These systems represent repeated TMBr3 motifs with octahedral configuration, and are expected to be synthesized in a nanotube using an established method. Among these NWs, both VBr3 and CuBr3 NWs exhibit a ferromagnetic (FM) ground state, accompanied by sizable magnetocrystalline anisotropic energy, which is dominated by the superexchange coupling between the TM atoms. Strikingly, a half-metallic nature with a magnetic moment of 4.0μB per unit cell is predicted for the VBr3 NW. By combining with a tight-binding model, we demonstrate that the origin behind the half-metallicity is the half-filled e2 orbitals of the V atoms. The Curie temperature is evaluated to be up to 80 K using Monte Carlo simulations, which is comparable to the temperature of liquid nitrogen. We also find that the half-metallic behavior shows a favorable tolerance to the longitudinal elongation of the wire (∼10%). Additionally, a transition from FM semiconductor to half-metal can be realized in the CuBr3 NW through carrier doping. The coexistence of intrinsic high-temperature FM ordering and half-metallicity endows 1D TMBr3 NWs with great promise for spintronic and photoelectron device applications.

The electronic structure and optical properties of Mo, S vacancy and V doping in MoS 2 monolayer will be investigated through first-principles calculations based on the density functional theory. The results indicate that the MoS 2 with Mo, S vacancy and V doping (Mo 14 VS 32 , Mo 15 VS 31 and Mo 14 VS 31 ) will gain the property of magnetic semiconductor with the magnetic moment of 1 μB, 1 μB and 0.95 μB, respectively. The optical properties of these V-doped and vacancy defect structures all reflect the phenomenon of red shift. The absorption edge of pure monolayer molybdenum disulfide is 0.8 eV, whereas the absorption edges of Mo 14 VS 32 , Mo 15 VS 31 and Mo 14 VS 31 become 0 eV, 0.2 eV and 0.16 eV, respectively. As a potential material, MoS 2 is widely used in many fields such as the production of optoelectronic devices, military devices and civil devices.

We perform first-principles calculations to study the energetics and electronic properties of silicene and MoSe2 heterobilayers (Si@MoSe2 HBLs). It is found that the silicene is bound to MoSe2 substrate with a binding energy of −0.56 eV per silicon atom, indicating a weak interaction between two layers. The nearly linear band dispersion character of silicene with a sizable band gap is obtained in Si@MoSe2, due to the variation of on-site energy induced by MoSe2 substrate. Remarkably, the band gaps and electron effective mass (EEM) of HBLs can effectively be tuned by interlayer spacing, external electric field, and strains. These findings indicate that Si@MoSe2 HBLs are promising candidates for high-performance silicene-based FET channel operating at room temperature, in which both finite band gap and high carrier mobility are obtained.

The geometry, electronic structure, and optical properties of Bi and N co-doped SnO2 are investigated by first-principles calculations. The calculated results show that the N and Bi atoms can be introduced to intrinsic SnO2 with reasonable formation energy (8.95–9.61 eV/cell) at different sites. Interestingly, the BiSn15O31N presents the character of indirect gap semiconductor with n-type conductivity. Increasing the doping concentration of N or Bi, BiSn15O32−x N x (x = 2,3) behaves like a hole-rich semiconductor, while BiySn16−y O31N (y = 2,3) possesses the characteristic of metal. Moreover, the band gap of doped structures becomes smaller than intrinsic SnO2 due to the emergence of energy bands contributing from doping elements near the Fermi level. The absorption intensity is enhanced in UV region, and the optical absorption edge shows red-shift phenomenon for all the doped systems. Our results on Bi,N co-doped SnO2 display the improved capacity of absorption and broadened absorption region. These findings can be utilized in light sensor and solar cell.

The quantum anomalous Hall (QAH) effect has attracted extensive attention due to time-reversal symmetry broken by a staggered magnetic flux emerging from ferromagnetic ordering and spin-orbit coupling. However, the experimental observations of the QAH effect are still challenging due to its small nontrivial bulk gap. Here, based on density functional theory and Berry curvature calculations, we propose the realization of intrinsic QAH effect in two-dimensional hexagonal metal-oxide lattice, $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$, which is characterized by the nonzero Chern number $(C=1)$ and chiral edge states. Spin-polarized calculations indicate that it exhibits a Dirac half-metal feature with temperature as large as ${T}_{C}=392\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ using spin-wave theory. When the spin-orbit coupling is switched on, $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ becomes a QAH insulator. Notably, the nontrivial topology is robust against biaxial strain with its band gap reaching up to ${E}_{g}=75\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$, which is far beyond room temperature. A tight-binding model is further constructed to understand the origin of nontrivially electronic properties. Our findings on the Dirac half-metal and room-temperature QAH effect in the $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ lattice can serve as an ideal platform for developing future topotronics devices.

Topological phases, especially topological crystalline insulators (TCIs), have been intensively explored observed experimentally in three-dimensional (3D) materials. However, the two-dimensional (2D) films are explored much less than 3D TCI, and even 2D topological insulators. Based on ab initio calculations, here we investigate the electronic and topological properties of 2D PbTe(001) few-layers. 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 TCI phase can be attributed to the p band inversion,which is determined by the competitions of orbital hybridization and quantum confinement. We also observe a semimetal-TCI-normal insulator transition under biaxial strains, whereas a uniaxial strains lead to Z2 nontrivial states. Especially, the TCI phase of PbTe monolayer remains when epitaxial grow on 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., 7 pages; 7 figures

Adequately understanding band inversion mechanism, one of the significant representations of topological phase, has substantial implications for design and regulation of topological insulators (TIs). Here, by identifying an unconventional band inversion, we propose an intrinsic quantum spin Hall (QSH) effect in iodinated group-V binary (ABI2) monolayers with a bulk gap as large as 0.409 eV, guaranteeing its viable application at room temperature. The nontrivial topological characters, which can be established by explicit demonstration of Z2 invariant and gapless helical edge states, are derived from the band inversion of antibonding states of px,y orbitals at the K point. Furthermore, the topological properties are tunable under strain engineering and external electric field, which supplies a route to manipulate the spin/charge conductance of edge states. These findings not only provide a new platform to better understand the underlying origin of QSH effect in functionalized group-V films, but also are highly desirable to design large-gap QSH insulators for practical applications in spintronics.

Searching for two-dimensional (2D) realistic materials able to realize room-temperature quantum spin Hall (QSH) effects is currently a growing field. Here, we through ab initio calculations to identify arsenene oxide, AsO, as an excellent candidate, which demonstrates high stability, flexibility, and tunable spin-orbit coupling (SOC) gaps. In contrast to known pristine or functionalized arsenene, the maximum nontrivial band gap of AsO reaches 89 meV, and can be further enhanced to 130 meV under biaxial strain. By sandwiching 2D AsO between BN sheets, we propose a quantum well in which the band topology of AsO is preserved with a sizeable band gap. Considering that AsO having fully oxidized surfaces are naturally stable against surface oxidization and degradation, this functionality provides a viable strategy for designing topological quantum devices operating at room temperature., Comment: 8 pages, 5 figures

The quantum anomalous Hall (QAH) phase is a novel topological state of matter characterized by a nonzero quantized Hall conductivity without an external magnetic field. The realizations of QAH effect, however, are experimentally challengeable. Based on ab initio calculations, here we propose an intrinsic QAH phase in DCA Kagome lattice. The nontrivial topology in Kagome bands are confirmed by the nonzero chern number, quantized Hall conductivity, and gapless chiral edge states of Mn-DCA lattice. A tight-binding (TB) model is further constructed to clarify the origin of QAH effect. Furthermore, its Curie temperature, estimated to be ~ 253 K using Monte-Carlo simulation, is comparable with room temperature and higher than most of two-dimensional ferromagnetic thin films. Our findings present a reliable material platform for the observation of QAH effect in covalent-organic frameworks., Comment: 9 pages,4 figures

Opening up a band gap without lowering high carrier mobility and finding a suitable substrate material are a challenge for designing silicon-based nanodevices. Using density functional theory calculations incorporating vdW corrections, we find that the semiconducting silicane monolayer is free of dangling bonds, providing an ideal substrate for silicene to sit on. The nearly linear band dispersion character of silicene with a sizable band gap (44–61 meV) opening is obtained in all heterobilayers (HBLs). We also find that the effective masses of electrons and holes near the Dirac point (ranging from 0.033 to 0.045m0) are very small in HBLs, and thus high carrier mobility (105cm2 V–1 s–1) of silicene is expected. These characteristics of HBLs can be flexibly modulated by applying bias voltage or strain, suitable for the high-performance FET channel operating at room temperature.

We perform first-principles calculations to study the geometric, energetics and electronic properties of graphene supported on BC3 monolayer. The results show that overall graphene interacts weakly with BC3 monolayer via van der Waals interaction. The energy gap of graphene can be up to ∼0.162 eV in graphene/BC3 heterobilayers (G/BC3 HBLs), which is large enough for the gap opening at room temperature. We also find that the interlayer spacing and in-plane strain can tune the band gap of G/BC3 HBLs effectively. Interestingly, the characteristics of a Dirac cone with a nearly linear band dispersion relationship of graphene can be preserved, accompanied by a small electron effective mass, and thus the higher carrier mobility is still expected. These findings provide a possible way to design effective FETs out of graphene on a BC3 substrate.

In this theoretical work, we perform first-principles calculation to study the geometric, electronic, and magnetic properties of the two-dimensional germanene and silicene hybrid sheet (GeSiHS) saturated with brominate atoms (Br). Although the pristine GeSiHS is semi-metallic, half-saturation on only a single side of Si atoms with Br results in the localized and unpaired electrons in unsaturated Ge atoms, showing long-range ferromagnetic (FM) properties with a high Curie temperature. Interestingly, half-brominated GeSiHS exhibits half-metallic character with 100% spin-polarized currents at the Fermi level. These findings provide the possibility of GeSiHS as promising building blocks for spintronic devices.

On the basis of detailed studies of structural and electronic properties with first-principles calculations, we have carefully analyzed enhanced H2 splitting catalyzed by the early transition metals that substitutionally doped in the top layer and the subsurface of an ideal flat Al surface and that at the edge site of a stepped surface. The 3d orbitals facilitating Kubas interaction significantly reduce the activation energy of H2 splitting catalyzed by a transition metal doped in the top surface. The catalyst doped in the subsurface could not develop Kubas interaction with H2 because of the screening from the charge distributed on the top surface, whose role could be understood by combining the structural deformation induced by the doping, the attraction of the dopant to the electrons distributed around Al atoms in the top layer, and the d orbital attendance in the reaction. For the sake of recycling perspectives of the doped catalyst, the diffusion of the dissociated H atoms has also been studied. Thus,...

By applying the nonequilibrium Greenʼs function formalism combined with density functional theory, we have investigated the electronic transport properties of two nitrogen-doped armchair graphene nanoribbon-based junctions M1 and M2. In the left part of M1 and M2, nitrogen atoms are doped at two edges of the nanoribbon. In the right part, nitrogen atoms are doped at one edge and at the center for M1 and M2, respectively. Obvious rectifying and negative differential resistance behaviors are found, which are strongly dependent on the doping position. The maximum rectification and peak-to-valley ratios are up to the order of 10 4 in M2.

We perform first-principles study on the electronic and magnetic properties of X-doped (X=Be, B, C, N) graphene-like ZnO nanosheet (NS). When one oxygen is substituted by X atom in ZnONS, X-induced spin polarizations led to transition from the semiconducting to half-metallic properties, with magnetic moments of 2.0, 1.0, 2.0, and 1.0 μ{sub B} per Be, B, C, and N dopant, respectively. The local magnetic moments are found to equal to unpaired electrons in the 2p spin states of the doping X atoms. While two oxygen atoms are substituted by X in ZnONS, the formation energy analysis indicates that X ions have a clear clustering tendency. Depending on distance between two X dopants, the ferromagnetic, antiferromagnetic or nonmagnetic states are all found in X-doped ZnONSs. More interestingly, for C and N doped cases, the half-metallic properties are robust independent on the doping concentrations, while Be or B doped systems would result in half-metallic to magnetic state transition as the doping concentrations increase. - Graphical abstract: Structure of ZnO NS employed to define various configurations of X-doped, as well as spin-density distribution of one Be-doped system. Highlights: ► X-induced spin polarizations result in half-metallicity. ► The local moments equal to unpaired electronsmore » in 2p spin states of X atom. ► The FM, AFM, and NM states are all found in X-doped ZnONSs. ► The half-metallicity in C and N doped cases are robust.« less

Group III-V films are of great importance for their potential application in spintronics and quantum computing. Search for two-dimensional III-V films with a nontrivial large-gap are quite crucial for the realization of dissipationless transport edge channels using quantum spin Hall (QSH) effects. Here we use first-principles calculations to predict a class of large-gap QSH insulators in functionalized TlSb monolayers (TlSbX2; (X = H, F, Cl, Br, I)), with sizable bulk gaps as large as 0.22~0.40 eV. The QSH state is identified by Z2 topological invariant together with helical edge states induced by spin-orbit coupling (SOC). Noticeably, the inverted band gap in the nontrivial states can be effectively tuned by the electric field and strain. Additionally, these films on BN substrate also maintain a nontrivial QSH state, which harbors a Dirac cone lying within the band gap. These findings may shed new light in future design and fabrication of QSH insulators based on two-dimensional honeycomb lattices in spintronics., 20 pages, 6 figures

Quantum spin Hall (QSH) insulators feature edge states that topologically protected from backscattering. However, the major obstacles to application for QSH effect are the lack of suitable QSH insulators with a large bulk gap. Based on first-principles calculations, we predict a class of large-gap QSH insulators in ethynyl-derivative functionalized stanene (SnC2X; X = H, F, Cl, Br, I), allowing for viable applications at room temperature. Noticeably, the SnC2Cl, SnC2Br and SnC2I are QSH insulators with a bulk gap of ~0.2 eV, while the SnC2H and SnC2F can be transformed into QSH insulator under the tensile strains. A single pair of topologically protected helical edge states is established for the edge of these systems with the Dirac point locating at the bulk gap and their QSH states are confirmed with topological invariant Z2 = 1. The films on BN substrate also maintain a nontrivial large-gap QSH effect, which harbors a Dirac cone lying within the band gap. These findings may shed new light in future design and fabrication of large-gap QSH insulators based on two-dimensional honeycomb lattices in spintronics.

Using first-principles density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of (8,0), (9,0) and (13,0) zigzag single-walled carbon nanotube junctions with one undoped and one nitrogen-doped zigzag carbon nanotube electrode. Our results show that the transport properties are strongly dependent on the magnitude of energy gap of carbon nanotube. Large rectifying behavior can be obtained in the junction with large energy gap. The observed rectifying behavior are explained in terms of the evolution of the transmission spectra and energy band structures with applied bias voltage combined with molecular projected self-consistent Hamiltonian eigenstates analysis.

Using first-principles density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of heteronanotubes by joining a zigzag (6,0) carbon nanotube and a zigzag (6,0) boron nitride nanotube with different atomic compositions and joint configurations. Our results show that the atomic composition and joint configuration affect strongly the electronic transport properties. Obvious negative differential resistance behavior and large rectifying behavior are obtained in the heterostructure with certain composition and joint configuration. Moreover, tube length and tube radius can affect strongly the observed NDR and rectifying behaviors. The observed negative differential resistance and rectifying behaviors are explained in terms of the evolution of the transmission spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analysis.

By applying the nonequilibrium Green function formalism combined with density functional theory, we have investigated the electronic transport properties of the C60 dimer and its endohedral complex Li@C60 dimer. Our results show that the doping of Li atoms significantly changes the transport properties of the C60 dimer. Negative differential resistance is found in such systems. Especially, the doping of Li atoms can lead to a much larger negative differential resistance at much lower bias, and it is quite evident from the plot of differential conductance versus bias. The negative differential resistance behavior is understood in terms of the evolution of the transmission spectrum and projected density of states spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analyses.