Semi-metallic Be(5)C(2) monolayer global minimum with quasi-planar pentacoordinate carbons and negative Poisson's ratio

Designing new materials with novel topological properties and reduced dimensionality is always desirable for material innovation. Here we report the design of a two-dimensional material, namely Be(5)C(2) monolayer on the basis of density functional theory computations. In Be(5)C(2) monolayer, each carbon atom binds with five beryllium atoms in almost the same plane, forming a quasi-planar pentacoordinate carbon moiety. Be(5)C(2) monolayer appears to have good stability as revealed by its moderate cohesive energy, positive phonon modes and high melting point. It is the lowest-energy structure with the Be(5)C(2) stoichiometry in two-dimensional space and therefore holds some promise to be realized experimentally. Be(5)C(2) monolayer is a gapless semiconductor with a Dirac-like point in the band structure and also has an unusual negative Poisson's ratio. If synthesized, Be(5)C(2) monolayer may find applications in electronics and mechanics.