Two-dimensional MgX2Se4 (X = Al, Ga) monolayers with tunable electronic properties for optoelectronic and photocatalytic applications

Abstract

Two new two-dimensional (2D) layered materials, namely, MgX₂Se₄ (X = Al, Ga) monolayers, are predicted to possess novel electronic properties. Ab initio electronic structure calculations show that both MgAl₂Se₄ and MgGa₂Se₄ monolayers are direct-gap semiconductors with bandgaps of 3.14 eV and 2.34 eV, respectively. The bandgap of both 2D materials is very sensitive to the in-plane biaxial strain, while the strain induced bandgap changes allow the tuning of optical absorption from the violet to green-light region. Also importantly, the in-plane electron mobility of both 2D materials is predicted to be as high as ∼0.7–1.0 × 10³ cm² V⁻¹ s⁻¹, notably higher than that of the MoS₂ sheet (∼200 cm² V⁻¹ s⁻¹), while it is comparable to that of black phosphorene (∼1000 cm² V⁻¹ s⁻¹), suggesting their potential application in n-type field-effect transistors. Moreover, suitable bandgap and band-edge alignment make the monolayer MgX₂Se₄ a potential photocatalyst for water splitting. Lastly, we show that MgX₂Se₄ possesses a lower monolayer cleavage energy than that of graphite, indicating easy exfoliation of MgX₂Se₄ layers from their bulk.