Nanointerfacial Layer Effect on Dielectric and Piezoelectric Responses in Chemical Solution-Derived Lead-Free Alkaline Niobate-Based Thin Films.

Since nonpiezoelectric interfacial layers even at the nanoscale significantly affect the performance of lead-free piezoelectric thin films, the quantitative characterization of property changes of thin films due to interfacial layers is of great importance and should be precisely undertaken for piezoelectric microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) devices. In contrast to widely accepted concepts for interfacial layer thickness estimation based on the existing series capacitor model, we find that the interfacial layer thickness at the top and the bottom interfaces is clearly different in chemical solution deposition (CSD)-derived (K0.5,Na0.5)(Mn0.005,Nb0.995)O3 (KNMN) thin films. Interestingly, the thickness of the bottom interface increases linearly with increasing thin-film thickness, while the thickness of the top interface is constant regardless of the thin-film thickness. In this work, nanointerfacial layer effects of CSD-derived KNMN thin films are theoretically and experimentally addressed in a combinatorial way using a modified series capacitor model. The obtained information is used to envisage the origins and the mechanisms of nonpiezoelectric interfacial layers and associated dielectric and ferroelectric properties of KNMN thin films. Our research connects macroscopic properties with microscopic origins and is greatly facilitated by separating intrinsic and extrinsic contributions to phenomenological behaviors, as well as engineering interface-related properties of the films. We believe these studies to be crucial for the further development and applications of KNN-based lead-free piezoelectric devices, which also open the door to future studies on other lead-free piezoelectric material systems for practical MEMS and NEMS applications.