Two-dimensional stoichiometric boron carbides with unexpected chemical bonding and promising electronic properties

Exploring new two-dimensional materials with novel properties is becoming a particularly important task due to their potential applications in future nano-mechanics, electronics, and optoelectronics. In the present study, the hitherto unknown stable two-dimensional boron carbides with various stoichiometries are revealed via the structure swarm optimization method combined with first-principles calculations. The predicted new compounds are energetically more favorable compared with the previously proposed counterparts. Counterintuitively, we identify two B-C bonding patterns: pyramidal-geometry tetra-coordinated and hexa-coordinated sp(2) carbon moiety. The intriguing covalent bonding modes create distinct and fascinating physical and chemical properties. For instance, we discover that the predicted B4C3 has an ultrahigh Young's modulus that can even outperform graphene; the B2C sheet is metallic with a relatively high superconducting transition temperature (T-c approximate to 21.20 K). On the other hand, the well-located band edge makes beta-B3C2 a potentially promising metal-free optoelectronic material for visible-light water splitting.