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

Based
on first-principles calculations, the ground state configuration
(Cmma-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). Cmma-CH contains four inequivalent
benzene molecules in its crystalline cell due to its Cmma 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. Cmma-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 Cmma-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 Cmma-CH, which has potential applications in nanodevices
at elevated temperatures and high voltages.