A mirror asymmetric Janus structure induces Rashba spin splitting (RSS) and a piezoelectric response. Inspired by the recently synthesized layered material CrSSe [Yang, Shi, Wang, Yue, Zheng, Zhang, Gu, Yang, Shadike, Li, and Fu, J. Mater. Chem. A 8, 25739 (2020)], we use first-principles calculations to systematically study the Rashba effect and piezoelectricity of Janus chromium dichalcogenide monolayers $\mathrm{Cr}XY$ ($X\ne Y=\text{S,}\text{Se,}\text{Te}$), as well as their regulation with biaxial strain. Our results reveal that spin-orbit coupling (SOC) plays an important role in the electronic properties (such as the semiconductor type, RSS, and valley polarization) of a $\mathrm{Cr}XY$ monolayer. Due to the mirror symmetry break and strong SOC, the strain-free $\mathrm{Cr}XY$ exhibits large Rashba parameters. Specifically, the Rashba parameter of CrSeTe is as high as 1.23 eV Å. Due to the ${\mathit{k}}^3$ term in the valence-band edge, the CrSeTe exhibits a strong hexagonal warping effect along with a nonzero out-of-plane spin polarization ${\mathit{S}}_z$, which can also be found in the CrSSe and CrSTe monolayers in the lower energy valence bands. Moreover, the Janus $\mathrm{Cr}XY$ monolayer exhibits superior intrinsic piezoelectric responses (${\mathit{d}}_{31}$ = $0.4–0.83$ pm/V), which are orders of magnitude larger than those of the $\mathrm{Mo}XY$ monolayer. Furthermore, we reveal in detail the modulation of the band structure, RSS, and piezoelectric properties with biaxial strain. Tensile strain suppresses the band gap, whereas compressive strain increases the band gap. Thus, strain engineering can effectively tune the band structures resulting in semiconductor-metal and indirect-direct transitions. In addition, the strain has opposite effects on the RSS and the piezoelectricity; that is, unlike compressive strain-enhanced RSS, the tensile strain can significantly elevate the piezoelectric coefficients. Our results indicate that a Janus $\mathrm{Cr}XY$ monolayer has coexisting large intrinsic RSS and piezoelectricity, which can be efficiently regulated by strain engineering, opening opportunities for applications in spintronic and piezoelectric devices.

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.

The newly modeled ScSeCl and ScSeBr monolayers are promising photocatalysts for water splitting with high carrier mobilities.

With the synthesis of single-layer lead iodide (PbI₂) in the laboratory in recent years, it has been widely used due to its stable structure and suitable bandgap (2.5 eV). We carried out first-principles calculations and a theoretical analysis to investigate the mechanical properties, electronic structure, and thermal transport properties of monolayer PbI₂. We computed the elastic constant C₁₁ = 14.69 N m⁻¹, layer modulus ${\gamma }_0$ = 9.35 N m⁻¹, and Young’s modulus E^2D = 13.63 N m⁻¹. Our results indicate that monolayer PbI₂ is more flexible than other two-dimensional materials, and it has good ductility. The Poisson’s ratio v of monolayer PbI₂ is 0.27, which means that ionic forces play a major role in the interatomic bonding for PbI₂ layered by van der Waals forces. We also did first-principles calculations for the lattice thermal transport properties of monolayer PbI₂ through the ShengBTE code. The calculated thermal conductivity $k_l$ of monolayer PbI₂ at 300 K is 0.66 W/m K, which agrees well with the experimental data of 0.68 W/m K. Furthermore, its thermal conductivity is much lower than that of other two-dimensional materials. We concluded that the acoustic modes of monolayer PbI₂ make a major contribution to its thermal conductivity.