Piezoelectricity of ZnO and its nanostructures

The wurtzite ZnO material exhibits excellent piezoelectric property along the [0001]-direction because of the noncentrosymmetric structure. As a typical II-VI wide band gap compound, it has been long studied as a piezoelectric material. Here we review the previous theoretical and experimental researches on the piezoelectric properties of ZnO and its nanostructures. Some practical applications in nanodevices are also exhibited. The present review could serve as a good reference for future investigations in the relative fields, and also indicates potential applications in nanoscale devices. Our contributions on this review topic are focused on the theoretical investigations of piezoelectricity of ZnO nanostructures, including nanowires and nanofilms, by using first-principles calculations. For the nanowires, size-dependent axial piezoelectricity in [0001]-oriented ZnO nanowires with diameters ranging from 0.4 to 3.0 nm is investigated. It is shown that the effective piezoelectric constant e₃₃ of the nanowires increases with increasing diameter, and is approximately one order smaller than the bulk value due to the structural change on the boundary of the nanowires. The absolute value of the axial piezoelectricity induced by the radial strain e₃₁ is around half of the effective e₃₃, which is similar to the bulk case. For the ZnO nanofilms, we find that the effective piezoelectric constant e₃₃ of ZnO nanofilms is also size dependent, and increases with increasing thickness in the nanoscale simulated in our work. When the film thickness is larger than 2.4 nm, the corresponding piezoelectric coefficient becomes higher than that of bulk ZnO. The enhancement over the bulk value reaches 11% when the film thickness is 2.9 nm.