Hexagonal M2C3 (M = As, Sb, and Bi) monolayers: new functional materials with desirable band gaps and ultrahigh carrier mobility

Abstract

Based on first-principles calculations, we propose a new type of two-dimensional (2D) material M₂C₃ (M = As, Sb, and Bi) showing an infinite hexagonal lattice, in which C atoms adopt sp² hybridization and M atoms prefer three-fold coordination with lone pair electrons. Such monolayers are calculated to be stable verified by their moderate cohesive energies, the absence of imaginary modes in their phonon spectra, and the high melting points predicted via molecular dynamics simulations. Sb₂C₃ and Bi₂C₃ monolayers possess intrinsic band gaps of 1.58 and 1.23 eV (based on HSE06 calculations), values suitable for photovoltaic applications. The intrinsic acoustic-phonon-limited carrier mobility of the As₂C₃ sheet can reach up to 4.45 × 10⁵ cm² V⁻¹ s⁻¹ for electrons at room temperature, higher than that of (60–200 cm² V⁻¹ s⁻¹) MoS₂ and (∼10³ cm² V⁻¹ s⁻¹) few-layer phosphorene, approaching the figure of merit in graphene (3 × 10⁵ cm² V⁻¹ s⁻¹). The well-located band edge and visible light absorption make stretched Sb₂C₃ a potentially promising optoelectronic material for photocatalytic water splitting. Besides, Sb₂C₃/As₂C₃ excitonic solar cells have been proposed, and their power conversion efficiencies are estimated to exceed 23%. First-principles calculations have demonstrated that Sb₂C₃/Bi₂C₃ heterojunctions are indeed 2D node-line semimetals in the absence of spin–orbit coupling.