Two-dimensional Penta-NiPS Sheets: Two Stable Polymorphs

TL;DR

This paper introduces two stable polymorphs of penta-NiPS, a new 2D material with unique mechanical and electronic properties, expanding the family of stable penta-2D materials for technological applications.

Contribution

The study proposes and characterizes two stable polymorphs of penta-NiPS, including a newly identified beta phase, using first-principles calculations, and analyzes their stability, mechanical, and electronic properties.

Findings

Both phases are thermodynamically, dynamically, mechanically, and thermally stable.

Alpha phase exhibits nearly zero Poisson's ratio, maintaining dimensions under extension.

Both phases are wide band gap semiconductors with indirect band gaps of 2.35 eV and 2.20 eV.

Abstract

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.