Equibiaxial strain regulates the electronic structure and mechanical, piezoelectric, and thermal transport properties of the 2H-phase monolayers CrX2 (X = S, Se, Te).

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.

Correlation-driven topological phase transition in 2D valleytronic materials: a mini-review.

Electronic correlation combined with spin-orbit coupling (SOC) may have a significant impact on the physical properties of two-dimensional (2D) transition metal magnetic compounds. Moreover, magnetic anisotropy (MA) is very important in determining magnetic, ferrovalley (FV) and topological properties of these 2D systems. Based on a density-functional theory (DFT) + U approach, it is found that the electronic correlation can induce topological phase transition in some special 2D valleytronic materials (for example FeCl2 and VSi2P4) with out-of-plane MA, and a novel valley-polarized quantum anomalous Hall insulator (VQAHI) and half-valley-metal (HVM) can be produced. These topological phase transitions are connected with a sign-reversible Berry curvature and band inversion between dxy/dx2-y2 and dz2 orbitals. However, for in-plane MA, the FV and nontrivial topological properties will be suppressed. For a given material, the correlation strength is fixed, but these novel electronic states and topological phase transitions can still be exhibited by strain in practice. The mini-review sheds light on the possible role of correlation effects in some special 2D valleytronic materials.

Electric-field induced half-metallic properties in an experimentally synthesized CrSBr monolayer.

Two-dimensional (2D) half-metallic materials are highly desirable for nanoscale spintronic applications. Here, we propose a new mechanism that can achieve half-metallicity in 2D ferromagnetic (FM) materials with two-layer magnetic atoms by electric field tuning. We use a concrete example of an experimentally synthesized CrSBr monolayer to illustrate our proposal through first-principles calculations. It is found that half-metallic properties can be achieved in CrSBr within an appropriate electric field range, and the corresponding amplitude of electric field intensity can be realized experimentally. Janus monolayer Cr2S2BrI is constructed, which possesses a built-in electric field due to broken horizontal mirror symmetry. However, Cr2S2BrI without and with an applied external electric field is always a FM semiconductor. A possible memory device is also proposed based on the CrSBr monolayer. Our work will stimulate the application of 2D FM CrSBr in future spintronic nanodevices.

Two-dimensional Janus Si dichalcogenides: a first-principles study.

Strong structural asymmetry is actively explored in two-dimensional (2D) materials, because it can give rise to many interesting physical properties. Motivated by the recent synthesis of monolayer Si2Te2, we explored a family of 2D materials, named Janus Si dichalcogenides (JSD), which parallel the Janus transition metal dichalcogenides and exhibit even stronger inversion asymmetry. Using first-principles calculations, we show that their strong structural asymmetry leads to a pronounced intrinsic polar field, sizable spin splitting, and large piezoelectric response. The spin splitting involves an out-of-plane spin component, which is beyond the linear Rashba model. The piezoelectric tensor has a large value in both in-plane d11 coefficient and out-of-plane d31 coefficient, making monolayer JSDs distinct among existing 2D piezoelectric materials. In addition, we find interesting strain-induced phase transitions in these materials. Particularly, there are multiple valleys that compete for the conduction band minimum, which will lead to notable changes in the optical and transport properties under strain. Our work reveals a new family of Si based 2D materials, which could find promising applications in spintronic and piezoelectric 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.

Piezoelectric ferromagnetism in Janus monolayer YBrI: a first-principles prediction.

Coexistence of intrinsic ferromagnetism and piezoelectricity, namely piezoelectric ferromagnetism (PFM), is crucial to advance multifunctional spintronic technologies. In this work, we demonstrate that Janus monolayer YBrI is a PFM, which is dynamically, mechanically and thermally stable. The electronic correlation effects on the physical properties of YBrI are investigated by using generalized gradient approximation plus U (GGA+U) approach. For out-of-plane magnetic anisotropy, YBrI is a ferrovalley (FV) material, and its valley splitting is larger than 82 meV within the considered U range. The anomalous valley Hall effect (AVHE) can be achieved under an in-plane electric field. However, for in-plane magnetic anisotropy, YBrI is a common ferromagnetic (FM) semiconductor. When considering intrinsic magnetic anisotropy, the easy axis of YBrI is always in-plane, and its magnetic anisotropy energy (MAE) varies from 0.309 meV to 0.237 meV (U = 0.0 eV to 3.0 eV). However, the magnetization can be adjusted from the in-plane to out-of-plane direction by an external magnetic field, and then lead to the occurrence of valley polarization. Moreover, the missing centrosymmetry along with broken mirror symmetry results in both in-plane and out-of-plane piezoelectricity in the YBrI monolayer. At a typical U = 2.0 eV, the piezoelectric strain coefficient d11 is predicted to be -5.61 pm V-1, which is higher than or comparable with the ones of other known two-dimensional (2D) materials. The electronic and piezoelectric properties of YBrI can be effectively tuned by applying biaxial strain. For example, tensile strain can enhance valley splitting and d11 (absolute value). The predicted magnetic transition temperature of YBrI is higher than those of experimentally synthesized 2D FM materials CrI3 and Cr2Ge2Te6. Our findings of these distinctive properties could pave the way for designing multifunctional spintronic devices, and bring forward a new perspective for constructing 2D materials.

Valley polarization transition driven by biaxial strain in Janus GdClF monolayer.

The valley degree of freedom of carriers in crystals is useful to process information and perform logic operations, and it is a key factor for valley application to realize valley polarization. Here, we propose a model that the valley polarization transition at different valley points (-K and K points) is produced by biaxial strain. Using first-principles calculations, we illustrate our idea with a concrete example of a Janus GdClF monolayer. The predicted GdClF monolayer is dynamically, mechanically and thermally stable, and is a ferromagnetic (FM) semiconductor with perpendicular magnetic anisotropy (PMA), valence band maximum (VBM) at valley points and a high Curie temperature (TC). Due to its intrinsic ferromagnetism and spin-orbit coupling (SOC), a spontaneous valley polarization will be induced, but the valley splitting is only -3.1 meV, which provides an opportunity to achieve valley polarization transition at different valley points by strain. In the considered strain range (a/a0: 0.94-1.06), the strained GdClF monolayer always has an energy bandgap, strong FM coupling and PMA. The compressive strain is in favour of -K valley polarization, while the tensile strain is favorable for K valley polarization. The corresponding valley splittings at 0.96 and 1.04 strains are -44.5 meV and 29.4 meV, respectively, which are higher than the thermal energy at room temperature (25 meV). Due to its special Janus structure, both in-plane and out-of-plane piezoelectric polarizations can be observed. It is found that the direction of in-plane piezoelectric polarization can be overturned by strain, and the d11 values at 0.96 and 1.04 strains are -1.37 pm V-1 and 2.05 pm V-1, respectively. Our work paves the way to design ferrovalley materials for application in multifunctional valleytronic and piezoelectric devices by strain.

A piezoelectric quantum spin Hall insulator VCClBr monolayer with a pure out-of-plane piezoelectric response.

The combination of piezoelectricity with a nontrivial topological insulating phase in two-dimensional (2D) systems, namely piezoelectric quantum spin Hall insulators (PQSHI), is intriguing for exploring novel topological states toward the development of high-speed and dissipationless electronic devices. In this work, we predict a PQSHI Janus monolayer VCClBr constructed from VCCl2, which is dynamically, mechanically and thermally stable. In the absence of spin orbital coupling (SOC), VCClBr is a narrow gap semiconductor with a gap value of 57 meV, which is different from Dirac semimetal VCCl2. The gap of VCClBr is due to a built-in electric field caused by asymmetrical upper and lower atomic layers, which is further confirmed by the external-electric-field induced gap in VCCl2. When including SOC, the gap of VCClBr is increased to 76 meV, which is larger than the thermal energy of room temperature (25 meV). The VCClBr is a 2D topological insulator (TI), which is confirmed by Z2 topological invariant and nontrivial one-dimensional edge states. It is proved that the nontrivial topological properties of VCClBr are robust against strain (biaxial and uniaxial cases) and external electric fields. Due to broken horizontal mirror symmetry, only an out-of-plane piezoelectric response can be observed, when a biaxial or uniaxial in-plane strain is applied. The predicted piezoelectric strain coefficients d31 and d32 are -0.425 pm V-1 and -0.219 pm V-1, respectively, and they are higher than or compared with those of many 2D materials. Finally, Janus monolayer VCFBr and VCFCl (dynamically unstable) are also constructed, and they are still PQSHIs. Moreover, the d31 and d32 of VCFBr and VCFCl are higher than those of VCClBr, and the d31 (absolute value) of VCFBr is larger than one. According to out-of-plane piezoelectric coefficients of VCXY (X ≠ Y = F, Cl and Br), CrX1.5Y1.5 (X = F, Cl and Br; Y = I) and NiXY (X ≠ Y = Cl, Br and I), it is concluded that the size of the out-of-plane piezoelectric coefficient has a positive relation with the electronegativity difference of X and Y atoms. Our studies enrich the diversity of Janus 2D materials, and open a new avenue in the search for PQSHI with a large out-of-plane piezoelectric response, which provides a potential platform in nanoelectronics.

MoSSe/Hf(Zr)S2 heterostructures used for efficient Z-scheme photocatalytic water-splitting.

Direct Z-scheme water-splitting is a promising route to enhancing the photocatalytic performance due to the effective separation of photogenerated carriers while simultaneously preserving the strong oxidation activity of holes and reduction activity of electrons. In this work, the MoSSe/XY2 (X = Hf, Zr; S, Se) heterostructures (HSs) with different contacts are proposed for Z-scheme photocatalytic water-spitting by first principles calculation. The separation of photogenerated carriers for HfSe2/SMoSe and ZrSe2/SMoSe HSs is limited by the type-I band alignment, while the hydrogen production ability of HfSe2/SeMoS and ZrSe2/SeMoS HSs is limited by the lower conduction band edge positions relative to the water reduction potential. The HfS2/SMoSe, HfS2/SeMoS, ZrS2/SMoSe, and ZrS2/SeMoS HSs are direct Z-scheme water-splitting photocatalysts with the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) occurring at the Hf(Zr)S2 layer and MoSSe layer, respectively. More excitingly, the S (or Se) vacancies effectively lower the HER overpotentials. Besides, the solar-to-hydrogen efficiencies are 6.1%, 5.9%, 6.4%, and 6.3% for HfS2/SMoSe, HfS2/SeMoS, ZrS2/SMoSe, and ZrS2/SeMoS HSs, respectively. This work paves the way for designing highly efficient overall water-splitting photocatalysts using 2D materials.

Predicted intrinsic piezoelectric ferromagnetism in Janus monolayer MnSbBiTe4: a first principles study.

Two-dimensional (2D) piezoelectric ferromagnetism (PFM) is essential for the development of the next-generation multifunctional spintronic technologies. Recently, the layered van der Waals (vdW) compound MnBi2Te4 as a platform to realize the quantum anomalous Hall effect (QAHE) has attracted great interest. In this work, the Janus monolayer MnSbBiTe4 with dynamic, mechanical and thermal stabilities is constructed from a synthesized non-piezoelectric MnBi2Te4 monolayer by replacing the top Bi atomic layer with Sb atoms. The calculated results show that monolayer MnSbBiTe4 is an intrinsic ferromagnetic (FM) semiconductor with a gap value of 0.25 eV, whose easy magnetization axis is out-of-plane direction with magnetic anisotropy energy (MAE) of 158 µeV per Mn. The predicted Curie temperature TC is about 20.3 K, which is close to that of monolayer MnBi2Te4. The calculated results show that the in-plane d11 is about 5.56 pm V-1, which is higher than or comparable to those of other 2D known materials. Moreover, it is found that strain engineering can effectively tune the piezoelectric properties of Janus monolayer MnSbBiTe4. The calculated results show that tensile strain can improve the d11, which is improved to be 21.16 pm V-1 at only 1.04 strain. It is proved that the ferromagnetic order, semiconducting properties, out-of-plane easy axis and a large d11 are robust against electronic correlations. Our work provides a possible way to achieve PFM with a large d11 in well-explored vdW compound MnBi2Te4, which makes it possible to use the piezoelectric effect to tune the quantum transport process.

Generalization of piezoelectric quantum anomalous Hall insulator based on monolayer Fe2I2: a first-principles study.

To easily synthesize a piezoelectric quantum anomalous Hall insulator (PQAHI), the Janus monolayer Fe2IBr (FeI0.5Br0.5) as a representative PQAHI, is generalized to monolayer FeI1-xBrx (x = 0.25 and 0.75) with α and ß phases. By first-principles calculations, it is proved that monolayer FeI1-xBrx (x = 0.25 and 0.75) are dynamically, mechanically and thermally stable. They are excellent room-temperature PQAHIs with high Curie temperatures, sizable gaps and high Chern number (C = 2). Because the considered crystal structures of α and ß phases possess Mx and My mirror symmetries, the topological properties of monolayer FeI1-xBrx (x = 0.25 and 0.75) are maintained. Namely, if the constructed structures have Mx and My mirror symmetries, the mixing ratio of Br and I atoms can be generalized for other proportions. It is also found that different crystal phases have important effects on the out-of-plane piezoelectric response, and the piezoelectric strain coefficient, d32, of the ß phase is higher than or comparable with those of other known two-dimensional (2D) materials. To further confirm this idea, the physical and chemical properties of monolayer LiFeSe0.75S0.25 with α and ß phases, as a generalization of PQAHI LiFeSe0.5S0.5, is investigated, as it has a similar electronic structure, magnetic and topological properties as LiFeSe0.5S0.5. Our work provides a practical guide to achieve PQAHIs experimentally, and the combination of piezoelectricity, topological and ferromagnetic (FM) orders makes Fe2I2-based monolayers a potential platform for multi-functional spintronics and piezoelectric electronics.

Coexistence of intrinsic piezoelectricity and ferromagnetism induced by small biaxial strain in septuple-atomic-layer VSi2P4.

The septuple-atomic-layer VSi2P4 with the same structure of experimentally synthesized MoSi2N4 is predicted to be a spin-gapless semiconductor (SGS) with the generalized gradient approximation (GGA). In this work, the biaxial strain is applied to tune the electronic properties of VSi2P4, and it spans a wide range of properties upon increasing the strain from a ferromagnetic metal (FMM) to SGS to a ferromagnetic semiconductor (FMS) to SGS to a ferromagnetic half-metal (FMHM). Due to broken inversion symmetry, the coexistence of ferromagnetism and piezoelectricity can be achieved in FMS VSi2P4 with the strain range of 0% to 4%. The calculated piezoelectric strain coefficients d11 for 1%, 2% and 3% strains are 4.61 pm V-1, 4.94 pm V-1 and 5.27 pm V-1, respectively, which are greater than or close to a typical value of 5 pm V-1 for bulk piezoelectric materials. Finally, similar to VSi2P4, the coexistence of piezoelectricity and ferromagnetism can be realized by strain in the VSi2N4 monolayer. Our works show that VSi2P4 in the FMS phase with intrinsic piezoelectric properties can have potential applications in spin electronic devices.

Predicted Janus SnSSe monolayer: a comprehensive first-principles study.

The Janus structure, by combining properties of different transition metal dichalcogenide (TMD) monolayers in a single polar material, has attracted increasing research interest because of its particular structure and potential application in electronics, optoelectronics and piezoelectronics. In this work, Janus SnSSe monolayer is predicted by means of first-principles calculations, and it exhibits dynamic and mechanical stability. By using the generalized gradient approximation (GGA) and spin-orbit coupling (SOC), the Janus SnSSe monolayer is found to be an indirect band-gap semiconductor, whose gap can easily be tuned by strain. High carrier mobilities are obtained for SnSSe monolayer, and the hole mobility is higher than the electron mobility. For SnSSe monolayer, a uniaxial strain in the basal plane can induce both strong in-plane and much weaker out-of-plane piezoelectric polarizations, which reveals the potential as a piezoelectric two-dimensional (2D) material. High absorption coefficients in the visible light region are observed, suggesting a potential photocatalytic application. Calculated results show that SnSSe monolayer has a very high power factor, making it a promising candidate for thermoelectric applications. Our works reveal that the Janus SnSSe structure can be fabricated with unique electronic, optical, piezoelectric and transport properties, and can motivate related experimental works.

Phonon transport in Janus monolayer MoSSe: a first-principles study.

Transition Metal Dichalcogenide (TMD) monolayers are very widely studied due to their unique physical properties. Recently, Janus TMD monolayer MoSSe, with a sandwiched S-Mo-Se structure, has been synthesized by replacing the top S atomic layer in MoS2 with Se atoms. In this work, we systematically investigate the phonon transport and lattice thermal conductivity (κL) in MoSSe monolayers using first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). The calculated results show that the κL of MoSSe monolayers is much lower than that of MoS2 monolayers, and higher than that of MoSe2 monolayers. The corresponding thermal sheet conductance of MoSSe monolayers is 342.50 W K-1 at room temperature. This can be understood by studying the phonon group velocities and lifetimes. Compared to MoS2 monolayers, the smaller group velocities and shorter phonon lifetimes of MoSSe monolayers give rise to a lower κL. The larger group velocities of MoSSe compared to those of MoSe2 monolayers are the main reason for the higher κL. The elastic properties of MoS2, MoSSe and MoSe2 monolayers are also calculated, and the order of the Young's modulus is identical to that of the κL. The calculated results show that isotope scattering leads to a 5.8% reduction of the κL. The size effects on the κL are also considered, and are usually used in device implementation. When the characteristic length of the MoSSe monolayer is about 110 nm, the κL reduces to half. These results may offer perspectives on thermal management of MoSSe monolayers, for applications in thermoelectrics, thermal circuits and nanoelectronics, and may motivate further theoretical or experimental efforts to investigate thermal transport in Janus TMD monolayers.

Nonmonotonic strain dependence of lattice thermal conductivity in monolayer SiC: a first-principles study.

An increasing number of two-dimensional (2D) materials have already been achieved experimentally or predicted theoretically, which have potential applications in nano- and opto-electronics. Various applications of electronic devices are closely related to their thermal transport properties. In this work, the strain dependence of phonon transport in monolayer SiC with a perfect planar hexagonal honeycomb structure is investigated by solving the linearized phonon Boltzmann equation. It is found that the room-temperature lattice thermal conductivity (κL) of monolayer SiC is two orders of magnitude lower than that of graphene. The low κL is due to small group velocities and short phonon lifetimes, which can also be explained by the polarized covalent bond due to large charge transfer from Si to C atoms. In a considered strain range, it is proved that the SiC monolayer is mechanically and dynamically stable. With increased tensile strain, the κL of the SiC monolayer shows an unusual nonmonotonic up-and-down behavior, which is due to the competition between the change of phonon group velocities and phonon lifetimes of low frequency phonon modes. At low strain values (<8%), the phonon lifetime enhancement induces the increased κL, while at high strain values (>8%) the reduction of group velocities as well as the decrease of the phonon lifetimes are the major mechanisms responsible for decreased κL. Our works further enrich the studies on the phonon transport properties of 2D materials with a perfect planar hexagonal honeycomb structure, and motivate further experimental studies.

Soft phonon modes driven huge difference on lattice thermal conductivity between topological semimetal WC and WN.

Topological semimetals are currently attracting increasing interest due to their potential applications in topological qubits and low-power electronics, which are closely related to their thermal transport properties. Recently, the triply degenerate nodal points near the Fermi level of WC are observed by using angle-resolved photoemission spectroscopy. In this work, by solving the Boltzmann transport equation based on first-principles calculations, we systematically investigate the phonon transport properties of topological semimetals WC and WN. The predicted room-temperature lattice thermal conductivities of WC (WN) along the a and c directions are 1140.64 (7.47) W m-1 K-1 and 1214.69 (5.39) W m-1 K-1. Considering the similar crystal structure of WC and WN, it is quite interesting to find that the thermal conductivity of WC is more than two orders of magnitude higher than that of WN. It is found that, different from WN, the large acoustic-optical (a-o) gap prohibits the acoustic+acoustic → optical (aao) scattering, which gives rise to very long phonon lifetimes, leading to ultrahigh lattice thermal conductivity in WC. For WN, the lack of an a-o gap is due to soft phonon modes in optical branches, which can provide more scattering channels for aao scattering, producing very short phonon lifetimes. Further deep insight can be attained from their different electronic structures. Distinctly different from that in WC, the density of states of WN at the Fermi level becomes very sharp, which leads to destabilization of WN, producing soft phonon modes. It is found that the small shear modulus G and C44 limit the stability of WN, compared with WC. Our studies provide valuable information for phonon transports in WC and WN, and motivate further experimental studies to study their lattice thermal conductivities.

Potential 2D thermoelectric material ATeI (A = Sb and Bi) monolayers from a first-principles study.

Lots of two-dimensional (2D) materials have been predicted theoretically and further confirmed in experiments, and have wide applications in nanoscale electronic, optoelectronic and thermoelectric devices. In this work, the thermoelectric properties of ATeI (A = Sb and Bi) monolayers are systematically investigated according to semiclassical Boltzmann transport theory. It is found that spin-orbit coupling (SOC) has an important effect on the electronic transport coefficients of p-type doping, but a negative influence on n-type doping. The room-temperature sheet thermal conductance is 14.2 [Formula: see text] for SbTeI and 12.6 [Formula: see text] for BiTeI, which is lower than that of most well-known 2D materials, such as the transition-metal dichalcogenide, group IV-VI, group VA and group IV monolayers. The very low sheet thermal conductance of ATeI (A = Sb and Bi) monolayers is mainly due to their small group velocities and short phonon lifetimes. The strongly polarized covalent bonds between A and Te or I atoms induce strong phonon anharmonicity, which gives rise to low lattice thermal conductivity. It is found that the high-frequency optical branches contribute significantly to the total thermal conductivity, which is obviously different from the usual picture, where there is little contribution from the optical branches. According to cumulative lattice thermal conductivity with respect to the phonon mean free path (MFP), it is difficult to further reduce the lattice thermal conductivity using nanostructures. Finally, the possible thermoelectric figure of merit ZT values of the ATeI (A = Sb and Bi) monolayers are calculated. It is found that p-type doping has much better thermoelectric properties than n-type doping. At room temperature, the peak ZT can reach 1.11 for SbTeI and 0.87 for BiTeI, respectively. These results make us believe that ATeI (A = Sb and Bi) monolayers may be potential 2D thermoelectric materials, which could stimulate further experimental work towards the synthesis of these monolayers.

Lower lattice thermal conductivity in SbAs than As or Sb monolayers: a first-principles study.

Phonon transport in group-VA element (As, Sb and Bi) monolayer semiconductors has been widely investigated in theory, and, of them, monolayer Sb (antimonene) has recently been synthesized. In this work, phonon transport in monolayer SbAs is investigated with a combination of first-principles calculations and the linearized phonon Boltzmann equation. It is found that the lattice thermal conductivity of monolayer SbAs is lower than those of both monolayer As and Sb, and the corresponding sheet thermal conductance is 28.8 W K-1 at room temperature. To understand the lower lattice thermal conductivity in monolayer SbAs than those in monolayer As and Sb, the group velocities and phonon lifetimes of monolayer As, SbAs and Sb are calculated. The calculated results show that the group velocities of monolayer SbAs are between those of monolayer As and Sb, but that the phonon lifetimes of SbAs are smaller than those of both monolayer As and Sb. Hence, the low lattice thermal conductivity in monolayer SbAs is attributed to very small phonon lifetimes. Unexpectedly, the ZA branch has very little contribution to the total thermal conductivity, only 2.4%, which is obviously different from those of monolayer As and Sb with very large contributions. This can be explained by very small phonon lifetimes for the ZA branch of monolayer SbAs. The lower lattice thermal conductivity of monolayer SbAs compared to that of monolayer As or Sb can be understood by the alloying of As (Sb) with Sb (As), which should introduce phonon point defect scattering. We also consider the isotope and size effects on the lattice thermal conductivity. It is found that isotope scattering produces a neglectful effect, and the lattice thermal conductivity with a characteristic length smaller than 30 nm can reach a decrease of about 47%. These results may offer perspectives on tuning the lattice thermal conductivity by the mixture of multiple elements for applications of thermal management and thermoelectricity, and motivate further experimental efforts to synthesize monolayer SbAs.

Strain effects on phonon transport in antimonene investigated using a first-principles study.

Strain engineering is a very effective method to continuously tune the electronic, topological, optical and thermoelectric properties of materials. In this work, strain-dependent phonon transport of recently-fabricated antimonene (Sb monolayers) under biaxial strain is investigated using a combination of first-principles calculations and the linearized phonon Boltzmann equation within the single-mode relaxation time approximation (RTA). It is found that the ZA dispersion of antimonene with strain less than -1% gives imaginary frequencies, which suggests that compressive strain can induce structural instability. Experimentally, it is possible to enhance structural stability by tensile strain. The calculated results show that lattice thermal conductivity increases with strain increasing from -1% to 6%, and lattice thermal conductivity at 6% strain is 5.6 times larger than that at -1% strain at room temperature. It is interesting that lattice thermal conductivity is inversely proportional to the buckling parameter h in a considered strain range. Such a strain dependence of lattice thermal conductivity is attributed to enhanced phonon lifetimes caused by increased strain, while group velocities have a decreased effect on lattice thermal conductivity with increasing strain. It is found that acoustic branches dominate the lattice thermal conductivity over the full strain range. The cumulative room-temperature lattice thermal conductivity at -1% strain converges to a maximum with the phonon mean free path (MFP) at 50 nm, while that at 6% strain becomes as large as 44 µm, which suggests that strain can give rise to very strong size effects on lattice thermal conductivity in antimonene. Finally, the increased lattice thermal conductivity caused by increasing strain can be explained by a reduced polarized covalent bond, inducing weak phonon anharmonicity. These results may provide guidance on fabrication techniques of group-VA element (As, Sb, Bi) monolayers, and offer perspectives on tuning lattice thermal conductivity by the size and strain for applications of thermal management and thermoelectricity.

How to produce spin-splitting in antiferromagnetic materials.

Antiferromagnetic (AFM) materials have potential advantages for spintronics due to their robustness, ultrafast dynamics, and magnetotransport effects. However, the missing spontaneous polarization and magnetization hinders the efficient utilization of electronic spin in these AFM materials. Here, we propose a simple way to produce spin-splitting in AFM materials by making the magnetic atoms with opposite spin polarization locating in the different environment (surrounding atomic arrangement), which does not necessarily require the presence of spin-orbital coupling. We confirm our proposal by four different types of two-dimensional AFM materials within the first-principles calculations. Our works provide an intuitional design principle to find or produce spin-splitting in AFM materials.