Strain-Correlated Piezoelectricity in Quasi-Two-Dimensional Zinc Oxide Nanosheets

Two-dimensional (2D) piezoelectric materials have recently drawn intense interest in studying the nanoscale electromechanical coupling phenomenon and device development. A critical knowledge gap exists to correlate the nanoscale piezoelectric property with the static strains often found in 2D materials. Here, we present a study of the out-of-plane piezoelectric property of nanometer-thick 2D ZnO-nanosheets (NS) in correlation to in-plane strains, using in situ via strain-correlated piezoresponse force microscopy (PFM). We show that the strain configuration (either tensile or compressive) can dramatically influence the measured piezoelectric coefficient (d) of 2D ZnO-NS. A comparison of the out-of-plane piezoresponse is made for in-plane tensile and compressive strains approaching 0.50%, where the measured d varies between 2.1 and 20.3 pm V^-1 resulting in an order-of-magnitude change in the piezoelectric property. These results highlight the important role of in-plane strain in the quantification and application of 2D piezoelectric materials.