Two-dimensional graphene-like BeO sheet: a promising deep-ultraviolet nonlinear optical material with strong and highly tunable second harmonic generation
Two-dimensional (2D) materials with ultrawide band gaps and strong, tunable second-harmonic generation (SHG) coefficients are critical for miniaturized deep-ultraviolet (DUV) nonlinear optical (NLO) devices. Despite extensive experimental synthesis of 2D materials, none have satisfied DUV NLO requirements. Here, an experimentally available graphene-like BeO monolayer composed solely of NLO-active [BeO3] units is identified as an excellent 2D DUV NLO material via first-principles calculations. It exhibits an ultrawide band gap of 6.86 eV and a strong SHG coefficient χ22(2)(2D) = 6.81 Å pm/V. Through stacking, strain, and twist engineering, numerous 2D BeO sheets are predicted, and their flexible structural characteristics enable tunable NLO properties. Remarkably, extremely stress-sensitive out-of-plane χ15(2)(2D) and χ33(2)(2D) (with an exceptional 30% change) and robust in-plane χ22(2)(2D) against large strains are achieved together in AC- and ACE-stacked BeO sheets under in-plane biaxial strain, exhibiting emergent phenomena uniquely not observed in other known 2D NLO materials. These results establish 2D BeO systems as a new option for 2D DUV NLO materials.