Thickness-enhanced piezoelectric performance of [111]-oriented NiO thin films and the underlying atomic-scale mechanism.

Although the piezoelectric effect was discovered 146 years ago, the mechanisms have yet to be fully understood. To uncover the reasons for the thickness-dependent piezoelectric performance and to further validate the proposed piezoelectric mechanism based on polar superstructures, we fabricated [111]-oriented NiO thin films with thicknesses ranging from 130 to 378 nm and investigated their piezoelectric properties. The results show that, with increasing thickness, the piezoelectric current increases from 65.6 nA to 222.0 nA, an enhancement of 2.38 times, and the piezoelectric coefficient, d33, increases from 12.8 to 15.5. Our investigation into how external forces affect the structure and properties of NiO films reveals that the piezoelectric effect originates from a polar superstructure along the NiO [111] direction, possessing a spontaneous electric field (Es). The presence of Es induces electron accumulation at the Ni(111) surface of the NiO films. This leads to a difference in hole carrier concentrations between the upper and lower surfaces of the film. Periodic compression causes NiO to transition from a polar superstructure to a nonpolar structure, thereby generating a periodic current. Increasing the film thickness increases both the number of electrons in the film and the electron density at the Ni(111) surface. Consequently, the difference in hole carrier concentration between the top and bottom surfaces becomes larger, leading to enhanced piezoelectric performance. This finding not only provides a new perspective for understanding the thickness-enhanced piezoelectric mechanism, but also offers further evidence for the establishment of an atomic-scale polar superstructure piezoelectric model.