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A two-dimensional (2D) Ga 2 O 3 monolayer with an asymmetric quintuple-layer configuration was reported as a novel 2D material with excellent stability and strain tunability. This unusual asymmetrical structure opens up new possibilities for improving the selectivity and sensitivity of gas sensors by using selected surface orientations. In this study, the surface adsorptions of nine molecular gases, namely, O 2 , CO 2 , CO, SO 2 , NO 2 , H 2 S, NO, NH 3 , and H 2 O, on the 2D Ga 2 O 3 monolayer are systematically investigated through first-principles calculations. The intrinsic dipole of the system leads to different adsorption energies and changes in the electronic structures between the top- and bottom-surface adsorptions. Analyses of electronic structures and charge transport calculations indicate a potential application of the 2D Ga 2 O 3 monolayer as a room-temperature NO gas-sensing device with high sensitivity and tunable adsorption energy using plenary strain-induced lattice distortion.

A novel two-dimensional (2D) Ga 2 O 3 monolayer was constructed and systematically investigated by first-principles calculations. The 2D Ga 2 O 3 has an asymmetric configuration with a quintuple-layer atomic structure, the same as the well-studied α-In 2 Se 3 , and is expected to be experimentally synthesized. The dynamic and thermodynamic calculations show excellent stability properties of this monolayer material. The relaxed Ga 2 O 3 monolayer has an indirect band gap of 3.16 eV, smaller than that of β-Ga 2 O 3 bulk, and shows tunable electronic and optoelectronic properties with biaxial strain engineering. An attractive feature is that the asymmetric configuration spontaneously introduces an intrinsic dipole and thus the electrostatic potential difference between the top and bottom surfaces of the Ga 2 O 3 monolayer, which helps to separate photon-generated electrons and holes within the quintuple-layer structure. By applying compressive strain, the Ga 2 O 3 monolayer can be converted to a direct band gap semiconductor with a wider gap reaching 3.5 eV. Also, enhancement of hybridization between orbitals leads to an increase of electron mobility, from the initial 5000 to 7000 cm 2 V -1 s -1 . Excellent optical absorption ability is confirmed, which can be effectively tuned by strain engineering. With superior stability, as well as strain-tunable electronic properties, carrier mobility, and optical absorption, the studied novel Ga 2 O 3 monolayer sheds light on low-dimensional electronic and optoelectronic device applications.