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High-performance and stable spintronic devices have garnered considerable attention in recent years. Based on first-principle and Monte Carlo calculations, we demonstrate that under reasonable carrier doping, H-VSe2 exhibits 100% spin polarization, a magnetic anisotropy energy of 581 μeV, a tunable easy-axis, and a Curie temperature of 330 K. Moreover, Dzyaloshinskii–Moriya interactions in H-VSe2 and magnetic hysteresis loops are determined theoretically for the first time, which provides more precise and comprehensive descriptions of its magnetic properties under finite temperatures and external magnetic field. This work suggests that H-VSe2 is a powerful candidate for spintronic devices, and it provides solid theoretical support for future experiments. [ABSTRACT FROM AUTHOR]

Cobalt oxides are known for their excellent heat transfer properties. The main component of cobalt oxides is the CoO2 monolayer, which exhibits high-temperature superconductivity caused by strong electron–phonon coupling (EPC). We here systematically investigate the structural stability, electronic structure, and magnetism of the CoO2 monolayer using first-principles and Monte Carlo simulations. On this basis, we further study the changes in the spin energy gap, magnetic axis direction, and other properties of the CoO2 monolayer with the changes in carrier concentration. By appropriately doping the CoO2 monolayer with holes, the magnetic axis direction of the CoO2 monolayer can be reversed, thereby enhancing its potential application in the field of spin electronic devices. Monte Carlo simulation is used to study the regulation of different factors on the magnetothermal properties of the CoO2 monolayer. Through the analysis of physical parameters such as Curie temperature (TC) and bandgap, we find that the appropriate carrier concentration and magnetic field can not only regulate the magnetothermal properties of materials but also further improve the efficiency of materials in low-temperature environments. [ABSTRACT FROM AUTHOR]

Investigating high-performance and stable spintronics devices has been a research hotspot in recent years. In this paper, we employed first-principles methods and Monte Carlo (MC) simulations to explore the structure, electronic, and magnetic properties of monolayer NbSe2, as well as its behavior under carrier concentration modulation. The research on the electronic structure reveals that by introducing an appropriate amount of holes, the material can undergo a transition from metal to a half-metal state, achieving 100% high spin polarization. Investigation of magnetic crystalline anisotropy shows that the magnetic crystal anisotropy energy of 1210 μeV in out-of-plane is beneficial to maintain ferromagnetic order at high temperatures. In addition, doping with suitable carriers can effectively enhance or strengthen the ferromagnetic coupling in NbSe2 so that the magnetization easy axis is shifted. This reveals the potential application prospects of NbSe2 in electronically controlled spintronic devices. Analysis of the Fermi surface shows that both holes and electron doping increase the Fermi velocity of the material. The effect of hole doping is particularly significant, indicating its potential application in Fermi velocity engineering. Under the theoretical framework of the extended two-dimensional Ising model, based on MC simulation, the Curie temperature (TC) of NbSe2 is predicted to be 162 K. The effects of carrier concentration and the magnetic field on the magnetic and thermal properties of monolayer NbSe2 are simulated. The results show that appropriately increasing the hole doping concentration and magnetic field is conducive to obtaining ferromagnetic half-metallic materials with TC higher than room temperature, which provides theoretical support for experimental preparation. [ABSTRACT FROM AUTHOR]

Motivated by the recent experimental synthesis of two-dimensional (2D) NbOI2 which possesses a moderate bandgap and outstanding absorption of sunlight, using the first-principles calculations, we conduct a thorough study of the geometric configuration, electronic structures, and photocatalytic properties for NbOX2 (X = Cl, Br, I) monolayers. These NbOX2 monolayers have been demonstrated to be dynamically, thermally, and mechanically stable. The significant anisotropic mechanical properties of NbOX2 monolayers are reflected by the calculated Young's modulus and Poisson's ratio. Our results indicate that these NbOX2 materials unfold semiconductor characters with indirect bandgaps of 1.886, 1.909, and 1.813 eV, respectively. Among these monolayers, it is found that the NbOBr2 system exhibits a favorable photocatalytic activity in an acidic condition (pH = 0), and the NbOI2 monolayer can act as a potential photocatalyst for spontaneous photocatalytic water splitting under a neutral environment (pH = 7). Furthermore, the response of bandgap and band edge positions of NbOX2 monolayers to the exerting in-plane strain (–6% to 6%) are investigated. These NbOX2 monolayers also show strong light absorption from the visible to ultraviolet region and anisotropic high carrier transport. Particularly, the high solar-to-hydrogen efficiency of the NbOCl2 (1% tensile strain), NbOBr2, and NbOI2 monolayers are predicted to be 14.11% (pH = 0), 16.34% (pH = 0), and 17.05% (pH = 7), respectively. Therefore, we expect the NbOX2 monolayers to be promising candidates for highly efficient photocatalytic water splitting. [ABSTRACT FROM AUTHOR]

The investigation and development of high thermoelectric value materials has become a research hotspot in recent years. In this work, based on the density functional theory on the Perdew–Burke–Ernzerhof (GGA-PBE) level, the thermoelectric properties of transition metal halides CdBr, Janus Cd2BrI, and CdI monolayers have been systematically investigated using Boltzmann transport theory. The calculation of the electronic band structure shows that these three materials have indirect band gap semiconductor properties. For carrier transport, the electron mobilities for CdBr, Janus Cd2BrI, and CdI monolayers are found to be 74, 16, 21 cm2 s−1 V−1 for p-type doping and 116, 102, 78 cm2 s−1 V−1 for n-type doping. Regarding their phonon transport, the CdBr, Cd2BrI, and CdI monolayers all have very low lattice thermal conductivity (4.78, 2.46, and 1.65 W m−1 K−1, respectively) that decreases with increasing temperature, which is favorable for obtaining large zT values. The electrical transport results show that the performance of p-type doping is better than that of n-type doping. At 300 K, the Seebeck coefficients of p-type doping for the CdBr, Cd2BrI, and CdI monolayers are 217.72, 246.43, and 226.24 μV K−1, respectively. In addition, we predict that the zT values of the CdBr, Cd2BrI, and CdI monolayers are 0.62, 1.64, and 0.87 for p-type doping at 300 K respectively. The zT values increase with the increase of temperature. In particular, the Janus Cd2BrI monolayer has a zT value of 3.03 at 600 K. These results suggest that all these materials can be good candidates for thermoelectric materials. [ABSTRACT FROM AUTHOR]

Bismuth chalcogenide and its derivatives have been attracting attention in various fields as semiconductors or topological insulators. Inspired by the high piezoelectric properties of Janus Bi2TeSeS monolayer and the excellent optical absorption properties of the Bi2X3 (X = Te, Se, S) monolayers, we theoretically predicted four new-type two-dimensional (2D) monolayers Janus Bi2X2Y (X = Te, Se; Y = Te, Se, S) using the first principles combined with density functional theory (DFT). The thermal, dynamic, and mechanical stabilities of Janus Bi2X2Y monolayers were confirmed based on ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constants calculations. Their elastic properties, band structures, piezoelectric, and optical properties were systematically investigated. It was found that Janus Bi2X2Y monolayers have a typical Mexican hat-shaped valence band edge structure and, therefore, have a ring-shaped flat band edge, which results in their indirect band gaps. The results show that Janus Bi2X2Y monolayers are semiconductors with moderate band gaps (0.62–0.98 eV at the HSE + SOC level). After considering the electron–phonon renormalization (EPR), the band gaps are reduced by less than 5% at 0 K under the zero-point renormalization (ZPR) and further reduced by approximately 10% at 300 K. Besides, Janus Bi2X2Y monolayers also exhibit excellent optical absorption properties in the blue-UV light region, with the peak values at the order of 8 × 105 cm−1. Particularly, the Janus Bi2Te2S monolayer was found to exhibit a piezoelectric strain coefficient d11 of up to 20.30 pm V−1, which is higher than that of most of the 2D materials. Our results indicate that Janus Bi2X2Y monolayers could be promising candidates in solar cells, optical absorption, and optoelectronic devices; especially, a Janus Bi2Te2S monolayer can also be an excellent piezoelectric material with great prospects in the fields of mechanical and electrical energy conversion. [ABSTRACT FROM AUTHOR]

A superior piezoelectric coefficient and diminutive lattice thermal conductivity are advantageous for the application of a two-dimensional semiconductor in piezoelectric and thermoelectric devices, whereas an imperfect piezoelectric coefficient and large lattice thermal conductivity limit the practical application of the material. In this study, we investigate how the equibiaxial strain regulates the electronic structure, and mechanical, piezoelectric, and thermal transport properties. Tensile strain can deduce the bandgap of the monolayer CrX2 (X = S, Se, Te), whereas compressive strain has an opposite effect. Additionally, the transition from a semiconductor to a metal state and the transition between direct and indirect band gaps will occur at appropriate strain values, so the electronic structure can be effectively regulated. The reason is the different sensitivities of the energy corresponding to K and Γ on the valence band to the strain due to the changes in different orbital overlaps. The tensile strain can effectively improve the flexibility of monolayers CrX2, which provides a possibility for the application of flexible electronic devices. Furthermore, the tensile strain can improve the piezoelectric strain coefficient of monolayers CrX2. Using Slacks formulation, we calculate the lattice thermal conductivity, and the tensile biaxial strain can reduce the lattice thermal conductivity. Our research provides a strategy to enhance the piezoelectric and flexible electronic applications and decrease the lattice thermal conductivity, which can benefit the thermoelectric applications. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) materials have been one of the most popular objects in the research field of thermoelectric (TE) materials and have attracted substantial attention in recent years. Inspired by the synthesized 2H-MoSSe and numerous theoretical studies, we systematically investigated the electronic, thermal, and TE properties of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers by using first-principles calculations. The phonon dispersion curves and AIMD simulations confirm the thermodynamic stabilities. Moreover, Janus 2H-MXTe were evaluated as indirect band-gap semiconductors with band gaps ranging from 0.56 to 0.90 eV using the HSE06 + SOC method. To evaluate the TE performance, firstly, we calculated the temperature-dependent carrier relaxation time with acoustic phonon scattering τac, impurity scattering τimp, and polarized scattering τpol. Secondly, the calculation of lattice thermal conductivity (κl) shows that these monolayers possess relatively poor κl with values of 3.4–5.4 W mK−1 at 300 K, which is caused by the low phonon lifetime and group velocity. After computing the electronic transport properties, we found that the n-type doped Janus 2H-MXTe monolayers exhibit a high Seebeck coefficient exceeding 200 μV K−1 at 300 K, resulting in a high TE power factor. Eventually, combining the electrical and thermal conductivities, the optimal dimensionless figure of merit (zT) at 300 K (900 K) can be obtained, which is 0.94 (3.63), 0.51 (2.57), 0.64 (2.72), and 0.50 (1.98) for n-type doping of ZrSeTe, HfSeTe, ZeSTe, and HfSTe monolayers. Particularly, the ZrSeTe monolayer shows the best TE performance with the maximal zT value. These results indicate the excellent application potential of Janus 2H-MXTe (M = Zr and Hf; X = S and Se) monolayers in TE materials. [ABSTRACT FROM AUTHOR]

Inspired by the interesting and novel properties exhibited by Janus transition metal dichalcogenides (TMDs) and two-dimensional pentagonal structures, we here investigated the structural stability, mechanical, electronic, photocatalytic, and optical properties for a class of two-dimensional (2D) pentagonal Janus TMDs, namely penta-MSeTe (M = Ni, Pd, Pt) monolayers, by using density functional theory (DFT) combined with Hubbard's correction (U). Our results showed that these monolayers exhibit good structural stability, appropriate band structures for photocatalysts, high visible light absorption, and good photocatalytic applicability. The calculated electronic properties reveal that the penta-MSeTe are semiconductors with a bandgap range of 2.06–2.39 eV, and their band edge positions meet the requirements for water-splitting photocatalysts in various environments (pH = 0–13). We used stress engineering to seek higher solar-to-hydrogen (STH) efficiency in acidic (pH = 0), neutral (pH = 7) and alkaline (pH = 13) environments for penta-MSeTe from 0% to +8% biaxial and uniaxial strains. Our results showed that penta-PdSeTe stretched 8% along the y direction and demonstrates an STH efficiency of up to 29.71% when pH = 0, which breaks the theoretical limit of the conventional photocatalytic model. We also calculated the optical properties and found that they exhibit high absorption (13.11%) in the visible light range and possess a diverse range of hyperbolic regions. Hence, it is anticipated that penta-MSeTe materials hold great promise for applications in photocatalytic water splitting and optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this paper, the thermoelectric (TE) properties of Janus MXY monolayers (M = Pd, Pt; X, Y = S, Se, Te) are systematically studied using first principles and the Boltzmann transport theory. The thermal conductivity (k), Seebeck coefficient (S), power factor (PF), and TE figure of merit (ZT) are calculated accurately for various carrier concentrations. The lattice thermal conductivities of these six materials sequentially decrease in the order PtSSe, PtSTe, PtSeTe, PdSSe, PdSTe, and PdSeTe. PdSeTe and PtSeTe monolayers have a high ZT close to one at 300 K. In addition, we predicted the TE properties at high temperatures and found that the maximum ZT (2.54) is achieved for a monolayer of PtSeTe at 900 K. The structural and electronic properties of these six Janus transition-metal dichalcogenide (TMD) monolayers were systematically studied from first principles. Our results show that all six materials are semiconductors with bandgaps between 0.77 eV and 2.26 eV at the Heyd-Scuseria-Ernzerhof (HSE06) level. The present work indicates that the Janus MXY TMD monolayers (M = Pd, Pt; X, Y = S, Se, Te) are potentially TE materials. [ABSTRACT FROM AUTHOR]

In this paper, the electronic band structure, Rashba effect, hexagonal warping, and piezoelectricity of Janus group-VIA binary monolayers STe2, SeTe2, and Se2Te are investigated based on density functional theory (DFT). Due to the inversion asymmetry and spin–orbit coupling (SOC), the STe2, SeTe2 and Se2Te monolayers exhibit large intrinsic Rashba spin splitting (RSS) at the Γ point with the Rashba parameters 0.19 eV Å, 0.39 eV Å, and 0.34 eV Å, respectively. Interestingly, based on the k·p model via symmetry analysis, the hexagonal warping effect and a nonzero spin projection component Sz arise at a larger constant energy surface due to nonlinear k3 terms. Then, the warping strength λ was obtained by fitting the calculated energy band data. Additionally, in-plane biaxial strain can significantly modulate the band structure and RSS. Furthermore, all these systems exhibit large in-plane and out-of-plane piezoelectricity due to inversion and mirror asymmetry. The calculated piezoelectric coefficients d11 and d31 are about 15–40 pm V−1 and 0.2–0.4 pm V−1, respectively, which are superior to those of most reported Janus monolayers. Because of the large RSS and piezoelectricity, the studied materials have great potential for spintronic and piezoelectric applications. [ABSTRACT FROM AUTHOR]

The structural, electronic, and magnetic properties of vanadium disulfide VS2 monolayers were investigated using first-principles calculations and Monte Carlo (MC) simulations. The results of molecular dynamics simulations and phonon dispersion showed that the VS2 monolayer has good dynamic and thermodynamic stabilities. Based on the results of the band structure, we also explore the effect of carrier concentrations on the spin gap, spin polarization and the direction of the easy magnetic axis. Our results demonstrated that doping an appropriate amount of holes can cause the reversal of the easy magnetic axis and maintain nearly 100% spin polarization, which greatly improves the application possibility of the VS2 monolayer as a spintronic device. The contribution of different orbits to the spin–orbit coupling (SOC) effect is given in magnetocrystalline anisotropy energy, which provides a theoretical basis for explaining the origin of magnetic crystal anisotropy. Based on the MC simulations, we also showed the influences of different parameters (carrier concentrations, magnetic field and crystal field) on the magnetothermal properties of the VS2 monolayer. It is found that the increase of hole doping concentrations can promote the increase of the Curie temperature, while the increase of electron doping concentrations will greatly weaken the Curie temperature. Furthermore, according to the influences of different parameters on the Curie temperature and spin polarization, we conclude that a suitably enhanced magnetic field and appropriate hole concentrations will not only make the system maintain high spin polarization, but also make the system exhibit ferromagnetic properties above room temperature. [ABSTRACT FROM AUTHOR]

Bismuth telluride (Bi2Te3) and its derivatives are often focused on as thermoelectric materials around room temperature. In this work, we theoretically predicted two new types of Bi2Te3-based two-dimensional materials Bi2SSe2 and Bi2S2Se using density functional theory (DFT) combined with Boltzmann transport theory. The thermal, dynamic, and mechanical stabilities of Bi2SSe2 and Bi2S2Se monolayers are confirmed using ab initio molecular dynamics (AIMD) simulations, phonon dispersion, and elastic constant calculations. The phonon transport properties, including lattice thermal conductivity, group velocity, Grüneisen parameter, converged scattering rate, and phonon lifetimes contributed by different branches, are systematically investigated. The electronic and thermoelectric properties, including carrier mobility (μ), Seebeck coefficient (S), electrical conductivity (σ), power factors, and figure of merit (zT) along the zigzag and armchair directions as a function of carrier concentration at different temperatures, are also investigated. It is found that the Bi2SSe2 and Bi2S2Se monolayers have moderate indirect band gaps (0.92 eV and 1.08 eV at the PBE level, respectively) and low lattice thermal conductivities (4.35 and 5.37 W m−1 K−1 at 300 K, respectively). The largest zT values of Bi2SSe2 and Bi2S2Se monolayers are 0.50 and 0.28 at 300 K and 1.39 and 0.93 at 700 K for p-doping types, respectively. The Bi2SSe2 and Bi2S2Se monolayers are predicted to show high optical absorption peaks at 8 × 105 cm−1 in the visible and near-UV light region, respectively. Our results indicate that both Bi2SSe2 and Bi2S2Se could be promising candidates in energy conversion, solar cells, and optoelectronic devices. [ABSTRACT FROM AUTHOR]

We presented a comprehensive study of electronic properties and phonon thermal transport properties in layered WX2 (X = Se, Te) using first-principles calculations combined with density functional perturbation theory (DFPT). Our results indicated that the spin–orbit coupling (SOC) and biaxial strains have significant effects on the band gaps and band alignments of the bilayer WX2 (X = Se, Te), and the d-orbital of the W atom played a dominant role at the valence and conduction band edge positions. We found that the band gaps decreased with the increase of strain and WTe2 have the transition from indirect band gap to direct band gap at 2% strain. We also calculated their lattice thermal conductivities k l , group velocities, cumulative thermal conductivities k c along the x and y directions, and scattering rates at room temperature. The calculated k l along x and y directions of WSe2 are 70.81 and 78.38 W/m K, respectively, whereas those of WTe2 were relatively low (55.05 W/m K along x-direction and 59.97 W/m K along y-direction), which mainly originated from their relatively smaller phonon group velocities and atomic weights. Two materials exhibited that k l , k c and group velocities in the y-direction were higher than those in the x-direction. We also investigated the size dependence of the k c and contributions of different acoustic and optical branches to the total k l . It was found that the nanostructure may be efficient to reduce k l . [ABSTRACT FROM AUTHOR]