Strain-induced electronic structures, mechanical anisotropy, and piezoelectricity of transition-metal dichalcogenide monolayer CrS2

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.