First-Principles Study of Structural and Electronic Properties of Monolayer PtX and Janus PtXY (X, Y = S, Se, and Te) via Strain Engineering.

In this work, the structural parameters and electronic properties of PtX and Janus PtXY (X, Y = S, Se, and Te) are studied based on the density functional theory. The phonon spectra and the Born criteria of the elastic constant of these six monolayers confirm their stability. All PtX and Janus PtXY monolayers show an outstanding stretchability with Young's modulus ranging from 61.023 to 82.124 N/m, about one-fifth that of graphene and half that of MoS, suggesting highly flexible materials. Our first-principles calculations reveal that the pristine PtX and their Janus counterparts are indirect semiconductors with their band gap ranging from 0.760 to 1.810 eV at the Perdew-Burke-Ernzerhof level (1.128-2.580 eV at the Heyd-Scuseria-Ernzerhof level). By applying biaxial compressive and tensile strain, the electronic properties of all PtX and Janus PtXY monolayers are widely tunable. Under small compressive strain, PtX and Janus PtXY structures remain indirect semiconductors. PtTe, PtSeTe, and PtSTe monolayers undergo a semiconducting to metallic transition when the strain reaches -6, -8, and -10%, respectively. Interestingly, there is a transition from the indirect semiconductor to a quasi-direct one for all PtX and Janus PtXY monolayers when the tensile strain is applied.