First-Principles Study of Structural and Electronic Properties of Monolayer PtX₂ and Janus PtXY (X, Y = S, Se, and Te) via Strain Engineering

In this work, the structural parameters and electronic properties of PtX₂ 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 PtX₂ 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 MoS₂, suggesting highly flexible materials. Our first-principles calculations reveal that the pristine PtX₂ 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 PtX₂ and Janus PtXY monolayers are widely tunable. Under small compressive strain, PtX₂ and Janus PtXY structures remain indirect semiconductors. PtTe₂, 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 PtX₂ and Janus PtXY monolayers when the tensile strain is applied.