New Two-Dimensional Wide Band Gap Hydrocarbon Insulator by Hydrogenation of a Biphenylene Sheet

Based on first-principles calculations, the ground state configuration (<i>Cmma</i>-CH) of a hydrogenated biphenylene sheet (Science 2021, 372, 852) is carefully identified from hundreds of possible candidates generated by RG² code (Phys. Rev. B. 2018, 97, 014104). <i>Cmma</i>-CH contains four inequivalent benzene molecules in its crystalline cell due to its <i>Cmma</i> symmetry. Hydrogen atoms bond to carbon atoms in each benzene with a boat-like (DDUDDU) up/down sequence and reversed boat-1 (UUDUUD) sequence in adjacent benzene rings. <i>Cmma</i>-CH is energetically less stable than the proposed allotropes of hydrogenated graphene, but the formation energy for hydrogenating a biphenylene sheet is remarkably lower than that for hydrogenating graphene to graphane. Our results of mechanical and dynamical stability also confirm that <i>Cmma</i>-CH is a stable 2D hydrocarbon, which is expected to be realized experimentally. Especially, biphenylene undergoes a transition from normal metal to a wide band gap insulator (4.645 eV) by hydrogenation to <i>Cmma</i>-CH, which has potential applications in nanodevices at elevated temperatures and high voltages.