First-principles study on physical properties of a single ZnO monolayer with graphene-like structure

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The elastic, piezoelectric, electronic, and optical properties of a single ZnO monolayer (SZOML) with graphene-like structure are investigated from the first-principles calculations. The phonon dispersion curves contain three acoustic and three optical branches. At Γ point, the out-of-plane acoustic mode has an asymptotic behavior ω(q)=Bq² with B=1.385 × 10⁻⁷ m²/s, while two in-plane acoustic modes have sound velocities 2.801 km/s and 8.095 km/s; the other three optical modes have frequencies 250 cm⁻¹, 566 cm⁻¹, and 631 cm⁻¹. The elastic and piezoelectric constants are obtained from the relaxed ion model. It is found that the SZOML is much softer than graphene, while it is a piezoelectric material. The electronic band gap is 3.576 eV, which implies that the SZOML is a wide band gap semiconductor. Many peaks exist in the linear optical spectra, where the first peak at 3.58 eV corresponds to the band gap of SZOML.