Two-dimensional Janus pentagonal MSeTe (M = Ni, Pd, Pt): promising water-splitting photocatalysts and optoelectronic materials.

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.

Hexagonal warping effect in the Janus group-VIA binary monolayers with large Rashba spin splitting and piezoelectricity.

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.

Ferromagnetic vanadium disulfide VS2 monolayers with high Curie temperature and high spin polarization.

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.

Electronic and transport properties of semimetal ZrBeSi crystal: a first-principles study.

In recent years, semimetals have aroused people's research interest. Here, we systematically study phonon and electronic transport properties of the ZrBeSi with semimetal character by using the first-principles calculations together with the Boltzmann transport theory. Calculated lattice thermal conductivities of the ZrBeSi alongaandcaxes are 31.3 W (m · K)-1and 56.0 W (m · K)-1at room temperature, respectively, which are larger than the most semiconductors and semimetals. By comparing with other semimetals, we find that the larger lattice thermal conductivity of ZrBeSi is due to its smaller Grüneisen parameter, which indicates the weaker phonon scattering. Main contributions to the lattice thermal conductivities alongaandcaxis come from the acoustic branches, and conversely, the contributions of optical branches are very small. In addition, we calculate the Seebeck coefficient and the electron thermal conductivity of ZrBeSi based on the relaxation time approximation. The electronic transport properties of ZrBeSi exhibit strong anisotropy in bothaandbdirections. Calculated electronic thermal conductivities of pristine ZrBeSi alongaandcaxes are 8.8 W (m · K)-1and 9.7 W (m · K)-1at room temperature, respectively. Furthermore, we also obtain the figure of meritZTon the basis of phonon and electron transport. The obtainedZTalongcaxis reaches a maximum of 0.11 at 900 K, demonstrating that ZrBeSi has a generalZT, but it has good heat conduction ability. Our research will help to understand the transport properties of semimetals and expand the application of semimetals to heat conduction devices. At the same time, it also provides some reference for the future experimental work.

An ab initio study of two-dimensional anisotropic monolayers ScXY (X = S and Se; Y = Cl and Br) for photocatalytic water splitting applications with high carrier mobilities.

Recently, metal oxyhalides have been broadly studied due to their hierarchical structures and promising functionalities. Herein, a thorough study of newly modeled monolayers ScXY (X = S and Se; Y = Cl and Br), a class of derivates of ScOBr monolayers, was conducted using first-principles calculations. We theoretically confirm that these ScXY monolayers are mechanically, dynamically, and thermally stable. Young's modulus and Poisson's ratio calculated for all these ScXY monolayers obviously exhibit anisotropic properties. All these monolayers are indirect-gap semiconductors with bandgaps in the range of 2.35-3.18 eV, and their conduction band minimum (CBM) and valence band maximum (VBM) can straddle the reduction and oxidation potential of water very well, respectively. Particularly, ScSeCl and ScSeBr monolayers have the most propitious bandgaps and band alignments to be used as promising photocatalysts, and the predicted carrier mobility is much larger than that of many other two-dimensional materials. Moreover, the predicted anisotropic carrier mobilities and indirect bandgaps will diminish the recombination and facilitate the migration of photo-generated electron and hole pairs. Moreover, biaxial strain (-5% to 5%) effects on the band alignments and bandgaps are discussed. Our findings highlight that ScSeCl and ScSeBr monolayers are envisioned to act as promising photocatalytic and photoelectronic materials with anisotropic ultrahigh carrier mobilities.

Novel thermoelectric performance of 2D 1T- Se2Te and SeTe2with ultralow lattice thermal conductivity but high carrier mobility.

The design and search for efficient thermoelectric materials that can directly convert waste heat into electricity have been of great interest in recent years since they have practical applications in overcoming the challenges of global warming and the energy crisis. In this work, two new two-dimensional 1T-phase group-VI binary compounds Se2Te and SeTe2with outstanding thermoelectric performances are predicted using first-principles calculations combined with Boltzmann transport theory. The dynamic stability is confirmed based on phonon dispersion. It is found that the spin-orbit coupling effect has a significant impact on the band structure of SeTe2, and induces a transformation from indirect to direct band gap. The electronic and phononic transport properties of the Se2Te and SeTe2monolayer are calculated and discussed. High carrier mobility (up to 3744.321 and 2295.413 cm2V-1S-1for electron and hole, respectively) is exhibited, suggesting great applications in nanoelectronic devices. Furthermore, the maximum thermoelectric figure of meritzTof SeTe2for n-type and p-type is 2.88, 1.99 and 5.94, 3.60 at 300 K and 600 K, respectively, which is larger than that of most reported 2D thermoelectric materials. The surprising thermoelectric properties arise from the ultralow lattice thermal conductivitykl(0.25 and 1.89 W m-1K-1for SeTe2and Se2Te at 300 K), and the origin of ultralow lattice thermal conductivity is revealed. The present results suggest that 1T-phase Se2Te and SeTe2monolayer are promising candidates for thermoelectric applications.

Anisotropic Thermoelectric Materials: Pentagonal PtM2 (M = S, Se, Te).

We here report a new pentagonal network structure of the PtM2 (M = S, Se, Te) monolayers with the P21/c (no. 14) space group. The electronic structure and thermoelectric properties of the pentagonal PtM2 monolayers are calculated through the VASP and BoltzTraP codes. We verify their dynamic and thermodynamic stabilities by calculating their phonon spectra and simulating ab initio molecular dynamics. It is found that the new material belongs to the medium-wide indirect band gap semiconductors from the PBE and HSE06 methods. At 300 K, the lattice thermal conductivities (Kl) of the pentagonal PtTe2 in the x and y directions are the smallest among these three materials, being 1.77 and 5.17 W/m K, respectively. The anisotropic zT values (2.60/1.14) in the x/y direction of the pentagonal PtTe2 at 300 K are much greater than those of the pentagonal PtSe2 (1.75/0.82) and the pentagonal PtS2 (0.58/0.16) at 300 K. Importantly, the p-type pentagonal PtTe2 also has excellent thermoelectric properties at 600 K, with a zT value of 5.03 in the x direction, indicating that the p-type pentagonal PtTe2 has a good application potential in the thermoelectric field.

Anisotropic lattice thermal conductivity in topological semimetal ZrGeX(X=S, Se, Te): a first-principles study.

Topological semimetals have attracted significant attentions owing to their potential applications in numerous fields such as low-power electron devices and quantum computation, which are closely related to their thermal transport properties. In this work, the phonon transport properties of topological Dirac nodal-line semimetals ZrGeX(X= S, Se, Te) with the PbClF-type structures are systematically studied using the first-principles calculations combined with the Boltzmann transport theory. The obtained lattice thermal conductivities show an obvious anisotropy, which is caused by the layer structures of ZrGeX(X= S, Se, Te). The room-temperature lattice conductivity of ZrGeTe alongcdirection is found to be as low as 0.24 W m-1 K-1, indicating that it could be of great significance in the fields of thermal coating materials and solar cell absorber. In addition, we extract each phonon branch from group velocities, phonon scattering rates, Grüneisen parameters, and phase space volumes to investigate the mechanism underlying the low thermal conductivity. It is concluded that the difference of thermal conductivities of three materials may be caused by the number of scattering channels and the effect of anharmonic. Furthermore, the phonon mean free path alongadirection is relatively longer. Nanostructures or polycrystalline structures may be effective to reduce the thermal conductivity and improve the thermoelectric properties.

Novel structural phases and the properties of LaX (X = P, As) under high pressure: first-principles study.

The particle swarm optimization algorithm and density functional theory (DFT) are extensively performed to determine the structures, phase transition, mechanical stability, electronic structures, and thermodynamic properties of lanthanide phosphates (LaP and LaAs) in the pressure range of 0 to 100 GPa. Two novel high-pressure structures of LaP and LaAs are first reported here. It is found that LaX (X = P, As) undergo a phase transition from NaCl-type structure (Fm3m) to CsCl-type structure (P4/mmm) at 19.04 GPa and 17.22 GPa, respectively. With the elevation of the pressure, C2/m-LaP and Imma-LaAs are the most stable structures up to 70.08 GPa and 85.53 GPa, respectively. Finally, the analysis of the elastic constants and hardness confirms that the C2/m-LaP possesses hardness values up to 23.24 GPa due to the strong covalent P-P bonding and ionic La-P bonding, indicating that it is a potential hard material.

Anisotropic thermoelectric properties of Weyl semimetal NbX (X = P and As): a potential thermoelectric material.

Weyl semimetal, a newly developed thermoelectric material, has aroused much interest due to its extraordinary transport properties. In this work, the thermoelectric transport properties of NbX (X = P and As), a prototypical Weyl semimetal, are investigated using the first-principles calculations together with Boltzmann transport theory. The calculated room-temperature lattice thermal conductivities along the a and c directions are 2.0 W mK-1 and 0.6 W mK-1 for NbP and 1.4 W mK-1 and 0.4 W mK-1 for NbAs, respectively. The low thermal conductivities may be useful in the thermoelectric applications. It is found that the acoustic branches have obvious contribution to the total lattice thermal conductivity, and the size dependence of the thermal conductivities can provide guidance for designing thermoelectric nanostructures. Our results show that the anisotropic structures of these compounds bring about the anisotropy of transport coefficients along the a and c directions, and the preferred direction is the c direction in thermoelectric applications. Moreover, NbP and NbAs show high ZT values of 0.82 and 0.50 along the c direction for p-type at an optimal carrier concentration, indicating that they are potential thermoelectric materials.

Oxygen-functionalized TlTe buckled honeycomb from first-principles study.

A sizable band gap is crucial for the applications of topological insulators at room temperature. By first-principles calculations, we found that oxygen-functionalized TlTe buckled honeycomb, namely TlTeO, possessed quantum spin Hall (QSH) state with a sizable band gap of 0.17 eV, which owns potential applications at the room temperature. The QSH phase of TlTeO arose from the SOC-induced p-p band gap opening. In addition, the QSH phase was further confirmed by the topological invariant Z2 and gapless edge state in the bulk gap. Significantly, the QSH phase is robustly against the external strain and possesses more than 75% oxygen coverage, making the QSH effect of TlTeO easy to be achieved experimentally. Thus, the oxygen-functionalized TlTeO film is a fine candidate material for the topological device design and fabrication.

Lattice Dynamics Study of Phonon Instability and Thermal Properties of Type-I Clathrate K8Si46 under High Pressure.

For a further understanding of the phase transitions mechanism in type-I silicon clathrates K8Si46, ab initio self-consistent electronic calculations combined with linear-response method have been performed to investigate the vibrational properties of alkali metal K atoms encapsulated type-I silicon-clathrate under pressure within the framework of density functional perturbation theory. Our lattice dynamics simulation results showed that the pressure induced phase transition of K8Si46 was believed to be driven by the phonon instability of the calthrate lattice. Analysis of the evolution of the partial phonon density of state with pressure, a legible dynamic picture for both guest K atoms and host lattice, was given. In addition, based on phonon calculations and combined with quasi-harmonic approximation, the specific heat of K8Si46 was derived, which agreed very well with experimental results. Also, other important thermal properties including the thermal expansion coefficients and Grüneisen parameters of K8Si46 under different temperature and pressure were also predicted.

Study of the thermodynamic properties of CeO2 from ab initio calculations: the effect of phonon-phonon interaction.

The thermodynamic properties of CeO2 have been reevaluated by a simple but accurate scheme. All our calculations are based on the self-consistent ab initio lattice dynamical (SCAILD) method that goes beyond the quasiharmonic approximation. Through this method, the effects of phonon-phonon interactions are included. The obtained thermodynamic properties and phonon dispersion relations are in good agreement with experimental data when considering the correction of phonon-phonon interaction. We find that the correction of phonon-phonon interaction is equally important and should not be neglected. At last, by comparing with quasiharmonic approximation, the present scheme based on SCAILD method is probably more suitable for high temperature systems.

Density functional theory investigation of the phonon instability, thermal equation of state and melting curve of Mo.

The phonon instability and thermal equation of state of Mo are extensively investigated using density functional theory. The calculated phonon dispersion curves agree well with experiments. Under compression, we captured a large softening in the transverse acoustic (TA) branches of body-centred cubic Mo. When the pressure is raised to 716 GPa, the frequencies along Γ-N in the TA branches soften to imaginary frequencies, indicating structural instability. For face-centred cubic Mo, the phonon calculations predicted the stability by promoting the frequencies from imaginary to real. Within quasi-harmonic approximation, we predicted the thermal equation of state and some other properties including the thermal expansion coefficient α, product αK(T), heat capacity C(V), entropy S, Grüneisen parameter γ and Debye temperature Θ(D). The melting curves of Mo were also obtained successfully.

Lattice dynamics and thermodynamics of molybdenum from first-principles calculations.

We calculated the phase transition, elastic constants, full phonon dispersion curves, and thermal properties of molybdenum (Mo) for a wide range of pressures using density functional theory. Mo is stable in the body-centered-cubic (bcc) structure up to 703 +/- 19 GPa and then transforms to the face-centered close-packed (fcc) structure at zero temperature. Under high temperature and pressure, the fcc phase of Mo is more stable than the bcc phase. The calculated phonon dispersion curves accord excellently with experiments. Under pressure, we captured a large softening along H-P in the TA branches. When the volume is compressed to 7.69 A(3), the frequencies along H-P in the TA branches soften to imaginary frequencies, indicating a structural instability. When the pressure increases, the phonon calculations on the fcc Mo predict the stability by promoting the frequencies along Gamma to X and Gamma to L symmetry lines from imaginary to real. The thermal equation of state was also investigated. From the thermal expansion coefficient and the heat capacity, we found that the quasiharmonic approximation was valid only up to about melting point at zero pressure. However, under pressure, the validity can be extended to a much higher temperature.