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Strain engineering is a very effective method to tune the electronic, optical, topological and thermoelectric properties of materials. In this work, we systematically study the biaxial strain dependence of electronic structures and thermoelectric properties (both electron and phonon parts) of monolayer PtSe2 with generalized gradient approximation (GGA) plus spin–orbit coupling (SOC) for the electron part and GGA for the phonon part. The calculated results show that compressive or tensile strain can induce a change in the position of the conduction band minimum (CBM) or valence band maximum (VBM), which produces important effects on the Seebeck coefficient. It is found that compressive or tensile strain can induce significantly enhanced n- or p-type Seebeck coefficients at the critical strain of change for the position of the CBM or VBM, which can be explained by strain-induced band convergence. Based on GGA+U+SOC, the electron correlation effects on the electron transport coefficients are also considered, the corresponding results agree well with those using GGA+SOC. Another essential strain effect is that tensile strain can produce significantly reduced lattice thermal conductivity, and the room temperature lattice thermal conductivity at the strain of −4.02% can decrease by about 60% compared to the unstrained one, which is very favorable for high ZT. To estimate the efficiency of thermoelectric conversion, the figure of merit ZT can be obtained by the empirical scattering time τ. The calculated ZT values show that strain is indeed a very effective strategy to achieve enhanced thermoelectric properties, especially for p-type doping. Tuning thermoelectric properties with strain can also be applied to other semiconducting transition-metal dichalcogenide monolayers MX2 (M = Zr, Hf, Mo, W and Pt; X = S, Se and Te). [ABSTRACT FROM AUTHOR]

The electronic structures, optical dielectric functions, elastic properties, and lattice dynamics of Ba2ZnWO6 have been investigated by using the generalized gradient approximation. The density of states and distributions of charge density show that O and Ba tend toward ionic bond, but O and W or Zn display the covalent bond character. The calculated energy band structure shows that Ba2ZnWO6 is a wide indirect band gap semiconductor. The static value 2.28 of the refractive index is attained. The analysis of the elastic properties of Ba2ZnWO6 indicates a rather weak elastic anisotropy. The phonon dispersion is calculated, suggesting no structural instability, which is agreement with the recent low temperature neutron diffraction experiments. The mensurability Cv (phonon heat capacity) as the function of the temperature is also calculated to judge our results for future experiment. [ABSTRACT FROM AUTHOR]

The electronic structures, optical dielectric functions, elastic properties, and lattice dynamics of Ba2ZnWO6 have been investigated by using the generalized gradient approximation. The density of states and distributions of charge density show that O and Ba tend toward ionic bond, but O and W or Zn display the covalent bond character. The calculated energy band structure shows that Ba2ZnWO6 is a wide indirect band gap semiconductor. The static value 2.28 of the refractive index is attained. The analysis of the elastic properties of Ba2ZnWO6 indicates a rather weak elastic anisotropy. The phonon dispersion is calculated, suggesting no structural instability, which is agreement with the recent low temperature neutron diffraction experiments. The mensurability Cv (phonon heat capacity) as the function of the temperature is also calculated to judge our results for future experiment. [ABSTRACT FROM AUTHOR]

We use a modified Becke-Johnson exchange plus a local density approximation correlation potential within the density functional theory to investigate the electronic structures of Hg1-xCdxTe and In1-xGaxAs with x being 0, 0.25, 0.5, 0.75, and 1. For both of the two series, our calculated energy gaps and dielectric functions (real part ∊1 and imaginary part ∊2) are in agreement with the corresponding experimental results with x being between 0 and 1. The calculated zero-frequency refractive index varies greatly with x for Hg1-xCdxTe, but changes little with x for In1-xGaxAs, which is consistent with the real parts of their dielectric functions. Therefore, this new approach is satisfactory to describe the electronic structures and the optical properties of the semiconductors. [ABSTRACT FROM AUTHOR]

We investigate the electronic structures of CuFeO 2 by using Tran and Blaha's modified Becke and Johnson exchange potential. The calculated X-ray emission spectra of CuFeO 2 for O–K and Fe–L are quite compatible with experimental data. The calculated energy band gap and optical band gap are 0.15 eV and 1.03 eV, respectively, and the theoretical magnetic moment for Fe atom is 4.11 μ B , which is very close to experimental value 4.2 ± 0.1 μ B . [ABSTRACT FROM AUTHOR]

Graphite-like carbon nitride (g- C 3 N 4) is considered as a promising candidate for energy materials. In this work, the biaxial strain (−4%–4%) effects on piezoelectric properties of g- C 3 N 4 monolayer are studied by density functional theory (DFT). It is found that the increasing strain can reduce the elastic coefficient C 11 - C 12 , and increases piezoelectric stress coefficient e 11 , which lead to the enhanced piezoelectric strain coefficient d 11. Compared to unstrained one, strain of 4% can raise the d 11 by about 330%. From −4% to 4%, strain can induce the improved ionic contribution to e 11 of g- C 3 N 4 , and almost unchanged electronic contribution, which is different from MoS 2 monolayer (the enhanced electronic contribution and reduced ionic contribution). It is found that the biaxial strain indeed is a more effective strategy for g- C 3 N 4 than MoS 2 monolayer with the increase being only 46%. To prohibit current leakage, a piezoelectric material should be a semiconductor, and g- C 3 N 4 monolayer is always a semiconductor in considered strain range. Calculated results show that the gap increases from compressive strain to tensile one. At 4% strain, the first and second valence bands cross, which has important effect on transition dipole moment (TDM). Our works provide a strategy to achieve enhanced piezoelectric effect of g- C 3 N 4 monolayer, which gives a useful guidance for developing efficient energy conversion devices. • Increasing strain can enhance piezoelectric strain coefficient d11 of monolayer g-C3N4. • Compared to unstrained one, strain of 4% can raise the d11 of g-C3N4 by about 330%. • Strain produces important effect on transition dipole moment (TDM). [ABSTRACT FROM AUTHOR]

• The same structural dependence on e 11 and d 11 for MoSi 2 N 4 and WSi 2 N 4. • MA 2 Z 4 containing P have more stronger d11 than one including N. • The d 11 of some MA 2 Z 4 containing P are greater than or close to 5 pm/V. The recently experimentally synthesized monolayer MoSi 2 N 4 and WSi 2 N 4 (Science 369, 670–674 (2020)) lack inversion symmetry, which allows them to become piezoelectric. In this work, based on ab initio calculations, we report structure effect on intrinsic piezoelectricity in septuple-atomic-layer MSi 2 N 4 (M=Mo and W), and six structures ( α i (i =1 to 6)) are considered with the same space group. These structures can connect to each other through translation, mirror and rotation operations of double layer unit Si 2 N 2 . It is found that MSi 2 N 4 (M=Mo and W) with α i (i = 1 to 6) all are indirect band gap semiconductors. Calculated results show that MoSi 2 N 4 and WSi 2 N 4 monolayers have the same structural dependence on piezoelectric strain and stress coefficients ( d 11 and e 11 ), together with the ionic and electronic contributions to e 11 . The α 5 phase has largest d 11 for both MoSi 2 N 4 and WSi 2 N 4 , which are larger than 2.9 pm/V. Finally, we investigate the intrinsic piezoelectricity of monolayer MA 2 Z 4 (M=Cr, Mo and W; A=Si and Ge; Z=N and P) with α 1 and α 2 phases expect CrGe 2 N 4 , because they all are semiconductors and their enthalpies of formation between α 1 and α 2 phases are very close. The most important result is that monolayer MA 2 Z 4 containing P atom have more stronger piezoelectric polarization than one including N atom. The largest d 11 among MA 2 N 4 materials is 1.85 pm/V, which is close to the smallest d 11 of 1.65 pm/V in MA 2 P 4 monolayers. For MA 2 P 4 , the largest d 11 is up to 6.12 pm/V. Among the 22 monolayers, α 1 - CrSi 2 P 4 , α 1 - MoSi 2 P 4 , α 1 - CrGe 2 P 4 , α 1 - MoGe 2 P 4 and α 2 - CrGe 2 P 4 have large d 11 , which are greater than or close to 5 pm/V, a typical value for bulk piezoelectric materials. These materials are recommended for experimental exploration. Our study reveals that the MA 2 Z 4 family have the potential applications in piezoelectric field. [ABSTRACT FROM AUTHOR]

Strain engineering can effectively tune the electronic, topological and piezoelectric properties of materials. In this work, the small strain (−4%–4%) effects on piezoelectric properties of α-AsP monolayer are studied by density functional theory (DFT). The piezoelectric stress tensors e i j and elastic stiffness tensors C i j are reported by using density functional perturbation theory (DFPT) and finite difference method (FDM). It is found that the Young's modulus of α-AsP monolayer shows very strong anisotropy, and the armchair direction is very softer than zigzag direction, which provides possibility for tuning easily piezoelectric coefficients along the armchair direction. In considered strain range, uniaxial compressive (tensile) strain along the armchair (zigzag) direction is found to raise observably both the e 22 and d 22 (absolute value). In fact, both compressive strain along the armchair direction and tensile strain along the zigzag direction essentially reduce the lattice constants along the armchair direction, which can enhance the piezoelectric coefficients. The e i j of β-AsP monolayer as a function of strain is also studied to illustrate the importance of particular puckered structure of α-AsP in enhancing the piezoelectric coefficients. A classic SnSe monolayer with puckered structure is used to further declare that small strain along the armchair direction can effectively improve the piezoelectric coefficients. For example, the d 22 of SnSe monolayer at −3.5% strain is up to 628.8 pm/V from unstrained 175.3 pm/V. For SnSe monolayer, a large peak is observed for e 22 , which is due to a structural phase transition. For e 16 of SnSe monolayer, a large peak is also observed due to the cross of lattice constants a along the zigzag direction and b along the armchair direction. A piezoelectric material should have a band gap for prohibiting current leakage, and they all are semiconductors in considered strain range for all studied materials. Our works imply that small strain can effectually tune piezoelectric properties of materials with puckered structure, and can provide useful guidence for developing efficient nanopiezotronic devices. • Uniaxial strain can raise observably piezoelectric coefficient in α-AsP monolayer. • The d 22 of SnSe monolayer at -3.5% strain is up to 628.8 pm/V from unstrained 175.3 pm/V. • They all are semiconductors in considered strain range for all studied materials. • The piezoelectric properties of a 2D material with puckered structure can be easily tuned by strain. [ABSTRACT FROM AUTHOR]

For two-dimensional (2D) materials, a pure large out-of-plane piezoelectric response, compatible with the nowadays bottom/top gate technologies, is highly desired. In this work, the piezoelectric properties of penta-graphene (CCC) monolayer are studied with pure out-of-plane piezoelectric effect by density functional theory (DFT). However, the d 36 is very small, and only -0.065 pm/V. Two strategies are proposed to enhance piezoelectric properties of CCC monolayer. Firstly, both biaxial and uniaxial strains are applied, but the enhancement is very small, and at -2% biaxial (-4% uniaxial) strain, the d 36 is increased only by 3.1% (13.9%). Secondly, a Janus penta-monolayer (CCB) is constructed by replacing the top C (B) atomic layer in monolayer CCC [pentagonal CB 2 monolayer (CBB)] with B (C) atoms, which shows dynamic and mechanical stability. Fortunately, the pure out-of-plane piezoelectric effect of CCB monolayer still holds, and exhibits a band gap. The calculated d 31 and d 32 are -0.505 pm/V and 0.273 pm/V, respectively, which are very larger than d 36 of CCC monolayer. The out-of-plane piezoelectricity d 31 of CCB monolayer is obviously higher compared with many other 2D known materials. Moreover, its room-temperature electronic mobility along y direction is as high as 8865.23 cm 2 V − 1 s − 1 . Our works provide a new way to achieve pure out-of-plane piezoelectric effect, which is highly desirable for ultrathin piezoelectric devices. • Pure out-of-plane piezoelectric effect is found in penta-graphene. • A Janus penta-monolayer is constructed to improve piezoelectric properties of penta-graphene. • Proposed penta-monolayer with high and pure out-of-plane piezoelectric effect. [ABSTRACT FROM AUTHOR]

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., (© 2024 IOP Publishing Ltd.)

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 CrX 2 (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 CrX 2 , which provides a possibility for the application of flexible electronic devices. Furthermore, the tensile strain can improve the piezoelectric strain coefficient of monolayers CrX 2 . 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.

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 Cr 2 S 2 BrI is constructed, which possesses a built-in electric field due to broken horizontal mirror symmetry. However, Cr 2 S 2 BrI 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.

Coexistence of ferromagnetism, piezoelectricity and valley in two-dimensional (2D) materials is crucial to advance multifunctional electronic technologies. Here, Janus ScXY (X≠Y = Cl, Br and I) monolayers are predicted to be piezoelectric ferromagnetic semiconductors with dynamical, mechanical and thermal stabilities. They all show an in-plane easy axis of magnetization by calculating magnetic anisotropy energy (MAE) including magnetocrystalline anisotropy energy and magnetic shape anisotropy energy. The MAE results show that they intrinsically have no spontaneous valley polarization. The predicted piezoelectric strain coefficients d 11 and d 31 (absolute values) are higher than ones of most 2D materials. Moreover, the d 31 (absolute value) of ScClI reaches up to 1.14 pm V -1 , which is highly desirable for ultrathin piezoelectric device application. To obtain spontaneous valley polarization, charge doping are explored to tune the direction of magnetization of ScXY. By appropriate hole doping, their easy magnetization axis can change from in-plane to out-of-plane, resulting in spontaneous valley polarization. Taking ScBrI with 0.20 holes per f.u. as an example, under the action of an in-plane electric field, the hole carriers of K valley turn towards one edge of the sample, which will produce anomalous valley Hall effect, and the hole carriers of Γ valley move in a straight line. These findings could pave the way for designing piezoelectric and valleytronic devices., (© 2023 IOP Publishing Ltd.)

In this paper, the electronic band structure, Rashba effect, hexagonal warping, and piezoelectricity of Janus group-VIA binary monolayers STe 2 , SeTe 2 , and Se 2 Te are investigated based on density functional theory (DFT). Due to the inversion asymmetry and spin-orbit coupling (SOC), the STe 2 , SeTe 2 and Se 2 Te 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 S z arise at a larger constant energy surface due to nonlinear k 3 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 d 11 and d 31 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.

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 Si 2 Te 2 , 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 d 11 coefficient and out-of-plane d 31 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.

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 d 11 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 d 11 (absolute value). The predicted magnetic transition temperature of YBrI is higher than those of experimentally synthesized 2D FM materials CrI 3 and Cr 2 Ge 2 Te 6 . 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.

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/XY 2 (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 HfSe 2 /SMoSe and ZrSe 2 /SMoSe HSs is limited by the type-I band alignment, while the hydrogen production ability of HfSe 2 /SeMoS and ZrSe 2 /SeMoS HSs is limited by the lower conduction band edge positions relative to the water reduction potential. The HfS 2 /SMoSe, HfS 2 /SeMoS, ZrS 2 /SMoSe, and ZrS 2 /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)S 2 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 HfS 2 /SMoSe, HfS 2 /SeMoS, ZrS 2 /SMoSe, and ZrS 2 /SeMoS HSs, respectively. This work paves the way for designing highly efficient overall water-splitting photocatalysts using 2D materials.

To easily synthesize a piezoelectric quantum anomalous Hall insulator (PQAHI), the Janus monolayer Fe 2 IBr (FeI 0.5 Br 0.5 ) as a representative PQAHI, is generalized to monolayer FeI 1- x Br x ( x = 0.25 and 0.75) with α and β phases. By first-principles calculations, it is proved that monolayer FeI 1- x Br x ( 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 β phases possess M x and M y mirror symmetries, the topological properties of monolayer FeI 1- x Br x ( x = 0.25 and 0.75) are maintained. Namely, if the constructed structures have M x and M y 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, d 32 , of the β 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 LiFeSe 0.75 S 0.25 with α and β phases, as a generalization of PQAHI LiFeSe 0.5 S 0.5 , is investigated, as it has a similar electronic structure, magnetic and topological properties as LiFeSe 0.5 S 0.5 . Our work provides a practical guide to achieve PQAHIs experimentally, and the combination of piezoelectricity, topological and ferromagnetic (FM) orders makes Fe 2 I 2 -based monolayers a potential platform for multi-functional spintronics and piezoelectric electronics.