Penta-CN₂ revisited: Superior stability, synthesis condition exploration, negative Poisson's ratio and quasi-flat bands

Pentagon-based two-dimensional (2D) materials have received tremendous attention owing to their unique atomic configurations, extraordinary physical and chemical properties, and their potential for broad applications. As one member of 2D penta-based materials, penta-CN₂ has been proposed to be a high energy density material. Here, based on an in-depth thermodynamic and ab initio molecular dynamics analysis, we reveal that penta-CN₂ has superior stability with an ultrahigh melting point up to 2500K. Moreover, it remains an amorphous C-N network under the melting state, which makes penta-CN₂ suitable for coating material in aviation area. Encouraged by the prospect of excellent applications, we further propose an experimental condition for synthesizing the penta-CN₂. Specifically, the Nb(100) surface would be a potential substrate, the CH₄ and N₂H₄ could be used as carbon and nitrogen sources under T = 1600K with pressure p(H₂)=p(N₂H₄)=4×10⁻⁴ bar, p(CH₄)=(10∼100)p(H₂). Further calculation of electro-mechanical properties shows that penta-CN₂ has intriguing properties, which includes large-range negative Poisson's ratio and paramagnetic insulator to ferromagnetic half-metal transition induced by hole doping. Our work is expected to give a profound view of penta-CN₂ and guide the future experiment works.