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

Nanoelectronic devices that are self-poweredthroughthe conversionof mechanical energy into electronic energy are of great interestfor the fields of wearables and medical implants. The ability to findnanomaterials that have piezoelectric properties suitable for nanogeneratorscan be a challenge. Zinc oxide (ZnO) is a well-known material thathas one of the highest piezoelectric tensors among tetrahedrally bondedsemiconductors, due to its buckled noncentrosymmetric structure onthe (0001) and (0001(-))] surfaces. However, this piezoelectricresponse is diminished in few-layered two-dimensional (2D) ZnO, asit loses it buckled structure and becomes graphitic. As a consequence,ultrathin 2D ZnO sheets are not suitable for nanoscale devices thatrely on a piezoelectric response. In this work, we show that defects,specifically, zinc and oxygen vacancies, and adsorbed oxygen, canrestore the wurtzite structure and piezoelectric response of freestanding 2D ZnO sheets at specific thicknesses. Using density functionaltheory calculations, we show that both a 1 monolayer (ML) surfacecoverage of oxygen and a 25% concentration of zinc vacancies can restorethe buckling in few-layer 2D ZnO nanosheets. For a 1.48 nm thick sheetwith a 25% Zn vacancy concentration, an e (33) value of 1.26 C/m(2) can be achieved, which is 120% enhancementcompared to the bulk value. These findings provide a methodology forscreening piezoelectric properties of other materials and demonstratethat defects can be used to restore the wurtzite structure and dielectricproperties of ultrathin 2D ZnO sheets that would allow them to beused in nanoscale piezoelectric devices.