Recently, Habib et al. [Nanoscale 11, 20123 (2019)] successfully synthesized two-dimensional (2D) CrS₂ monolayer using the chemical vapor deposition method for the first time, opening a new avenue for the exploration of Cr-based layered materials with astonishing properties. In the present work, we use a first-principles method based on the density-functional theory to investigate the electronic structures, mechanical anisotropy, and piezoelectricity of transition-metal dichalcogenide monolayer CrS₂. It is found that the bandgap is tunable between 1.175 eV and 1.862 eV at the Heyd–Scuseria–Ernzerhof (HSE06) level with applied strain, and a direct-to-indirect bandgap transition occurs at tensile strains larger than 2%. Calculated phonon dispersions suggest that CrS₂ is thermodynamically stable under a given strain and optical phonon splitting is discussed. A new elastic anisotropy measurement method is performed, and the results confirm that the application of strain raises the mechanical anisotropy because of the symmetrical structure being destroyed, which may exploit astonishing properties of 2D layered materials. In addition, tensile strain is more beneficial to improving the piezoelectric strain coefficient d₁₁ due to tensile strain results in a more flexible structure, which reached up to 9.74 pm/V (relaxed-ions) and 7.33 pm/V (clamped-ions) when applying 6% tensile strain. Our investigation suggested that strain engineering is an effective approach with which to modify the electronic, mechanical anisotropy, and piezoelectric properties of 2D CrS₂, raising the possibility of future optoelectronic, mechanical, and piezoelectric applications.

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.

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 Se ₂ Te and SeTe ₂ with 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 SeTe ₂ , and induces a transformation from indirect to direct band gap. The electronic and phononic transport properties of the Se ₂ Te and SeTe ₂ monolayer are calculated and discussed. High carrier mobility (up to 3744.321 and 2295.413 cm ² V ⁻¹ S ⁻¹ for electron and hole, respectively) is exhibited, suggesting great applications in nanoelectronic devices. Furthermore, the maximum thermoelectric figure of merit zT of SeTe ₂ for 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 conductivity k _l (0.25 and 1.89 W m ⁻¹ K ⁻¹ for SeTe ₂ and Se ₂ Te at 300 K), and the origin of ultralow lattice thermal conductivity is revealed. The present results suggest that 1T-phase Se ₂ Te and SeTe ₂ monolayer are promising candidates for thermoelectric applications.

A new group-VI Janus ternary compounds α-TeSSe monolayer exhibits outstanding piezoelectric and thermoelectric performances, which has broad application prospects in the fields of piezoelectrics and thermoelectrics.