First-Principles Study on Physical Properties of a Single ZnO Monolayer with Graphene-Like Structure

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 Gamma point, the out-of-plane acoustic mode has an asymptotic behavior omega(q) = Bq(2) with B = 1.385 x 10(-7) m(2)/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.