  Most recently the formation of boron monoxide (BO) in the two-dimensional
(2D) form has been confirmed experimentally (J. Am. Chem. Soc. 2023, 145,
14660). Motivated by the aforementioned finding, herein we theoretically
explore the key physical properties of the single-layer and suspended BO.
Density functional theory (DFT) results reveal that BO monolayer yields a large
indirect band gap of 3.78 (2.18) eV on the basis of HSE06(PBE) functional.
Ab-initio molecular dynamics results reveal the remarkable thermal stability of
the BO monolayer at 1000 K. The thermal and mechanical properties at room
temperature are furthermore investigated using a machine learning interatomic
potential (MLIP). The developed MLIP-based model close to the ground state
could very precisely reproduce the DFT predictions for the mechanical
properties of the BO monolayer. The elastic modulus, tensile strength and
lattice thermal conductivity of the BO monolayer at room temperature are
predicted to be 107 GPa, 25 GPa and 5.6 W/mK, respectively. At the room
temperature the BO monolayer is noticeably predicted to yield an ultrahigh
negative thermal expansion coefficient, by almost 17 folds larger than that of
the single-layer graphene. The presented results reveal the large indirect
electronic band gap, decent thermal and dynamical stability, anomalously low
elastic modulus to tensile strength ratio, ultrahigh negative thermal expansion
coefficients and low lattice thermal conductivity of the BO monolayer.
