Electronic and mechanical properties of stiff rhenium carbide monolayers: A first-principles investigation

Highlights: • Using first-principle calculations, we predicted two stable ReC monolayers. • All stable monolayers display metallic properties. • ReC monolayers with MXene structures were found to be unstable. • ReC monolayers display superior mechanical properties over well-known 2D materials. In this study, we predicted two new stable metallic ReC based monolayer structures with a rectangular (r-ReC₂) and a hexagonal (h-Re₂C) crystal symmetry using first-principle calculations based on density functional theory. Our results obtained from mechanical and phonon calculations and high-temperature molecular dynamic simulations clearly proved the stability of these two-dimensional (2D) crystals. Interestingly, ReC monolayers in common transition metal carbide structures (i.e. MXenes) were found to be unstable, contrary to expectations. We found that the stable structures, i.e. r-ReC₂ and h-Re₂C, display superior mechanical properties over the well-known 2D materials. The Young's modulus for r-ReC₂ and h-Re₂C are extremely high and were calculated as 351 (1310) and 617 (804) N/m (GPa), respectively. Both materials have larger Young's modulus values than the most of the well-known 2D materials. We showed that the combination of the short strong directional p-d bonds, the high coordination number of atoms in the unit-cell and high valence electron density result in strong mechanical properties. Due to its crystal structure, the r-ReC₂ monolayer has anisotropic mechanical properties and the crystallographic direction parallel to the C₂ dimers is stiffer compared to perpendicular direction due to strong covalent bonding within C₂ dimers. h-Re₂C was derived from the corresponding bulk structure for which we determined the critical thickness for the dynamically stable bulk-derived monolayer structures. In addition, we also investigated the electronic of these two stable structures. Both exhibit metallic behavior and Re-5d orbitals dominate the states around the Fermi level. Due to their ultra high mechanical stability and stiffness, these novel ReC monolayers can be exploited in various engineering applications.