Two-dimensional Penta-NiPS Sheets: Two Stable Polymorphs

The discovery of new and stable two-dimensional (2D) materials with exotic properties is essential for technological advancement. Inspired by the recently reported penta-PdPSe, we proposed penta-NiPS as a new member of the penta-2D materials based on first-principles calculations. The penta-NiPS monolayer is stable in two polymorphs including the alpha phase with identical structure as penta-PdPSe, and the newly proposed beta phase with rotated sublayers. Comprehensive analysis indicated that both phases are thermodynamically, dynamically, mechanically, and thermally stable. The penta-NiPS is a soft material with 2D Young's modulus of Ea=208 Nm^-1 and Eb=187 Nm^-1 for the alpha phase and Ea=184 Nm^-1 and Eb=140 Nm^-1 for the beta phase. Interestingly, the alpha-penta-NiPS showed nearly zero Poisson's ratios along the in-plane direction, where its dimensions would be maintained when being extended. For electronic applications, we demonstrated that penta-NiPS is the wide band gap semiconductor with an indirect band gap of 2.35 eV for alpha phase, and 2.20 eV for beta phase.