Restoring Piezoelectric Properties in 2D Zinc Oxide Nanosheets by Surface Modifications: Implications for Piezoelectric Nanogenerators

Abstract

Nanoelectronic devices that are self-powered through the conversion of mechanical energy into electronic energy are of great interest for the fields of wearables and medical implants. The ability to find nanomaterials that have piezoelectric properties suitable for nanogenerators can be a challenge. Zinc oxide (ZnO) is a well-known material that has one of the highest piezoelectric tensors among tetrahedrally bonded semiconductors, due to its buckled noncentrosymmetric structure on the (0001) and (0001̅)] surfaces. However, this piezoelectric response is diminished in few-layered two-dimensional (2D) ZnO, as it loses it buckled structure and becomes graphitic. As a consequence, ultrathin 2D ZnO sheets are not suitable for nanoscale devices that rely on a piezoelectric response. In this work, we show that defects, specifically, zinc and oxygen vacancies, and adsorbed oxygen, can restore the wurtzite structure and piezoelectric response of free standing 2D ZnO sheets at specific thicknesses. Using density functional theory calculations, we show that both a 1 monolayer (ML) surface coverage of oxygen and a 25% concentration of zinc vacancies can restore the buckling in few-layer 2D ZnO nanosheets. For a 1.48 nm thick sheet with a 25% Zn vacancy concentration, an e33 value of 1.26 C/m2 can be achieved, which is 120% enhancement compared to the bulk value. These findings provide a methodology for screening piezoelectric properties of other materials and demonstrate that defects can be used to restore the wurtzite structure and dielectric properties of ultrathin 2D ZnO sheets that would allow them to be used in nanoscale piezoelectric devices.