Anisotropic in-situ stretching-strain engineering of flexible multilayer thin-film nanogenerators with Cu interlayers

Strain engineering has been extensively recognized as an influencing methodology in finely modulating properties of materials. However, there has been no report of proposing the stretching-strain-dependent piezoelectricity in flexible thin-film nanogenerators processed with deliberate lattice strain. Herein, we propose two combined ways of enhancing piezoelectricity and thus electromechanical energy harvesting performance, i.e., imposing a considerable level of internal stress in ZnO thin films by an in-situ deposition method using a substrate-stretching mode and incorporating a metallic interlayer between the ZnO thin films to form a multi-layered structure. The intentional strain results primarily in an elongation of unit cell along the vertical axis and a larger contribution to spontaneous polarization. As a highlight, the highest stretching strain of ~ 4.87% induced a ~ 212% enhancement of output voltage and a ~89% increase of output current in the final optimized thin-film nanogenerators consisting of Cu-interlayered ZnO multilayer thin films.