Two-dimensional tetragonal and hexagonal lattices of transition metal carbides MC (M = Ti, Zr, Hf): Observation of two nodal loops and strong light-harvesting ability

Two-dimensional (2D) transition metal carbides (TMCs) have captured immense interest for their fascinating physical and chemical properties. Among them, bare carbide MXenes exhibit a metallic feature, greatly limiting the potential applications in optoelectronic devices. Herein, based on the particle swarm optimization and first-principles calculations, we predicted the existence of two single layer (SL) TMCs, namely, tetragonal beta-MC and hexagonal gamma-MC (M = Ti, Zr, Hf). The two structures are more stable than previously reported alpha-MC and possess semimetal and semiconductor properties, respectively. We further reveal that the beta-MC monolayers have two nodal loops around the Fermi level. SL gamma-TiC, ZrC, and HfC have a bandgap of 1.59, 1.24, and 1.57 eV, respectively, and display strong light-harvesting ability in the visible light range. Our research expands the 2D TMCs family beyond the extensively existing MXenes and greatly enriches the carbon chemistry. Published under an exclusive license by AIP Publishing.