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[Display omitted] • We achieved ultrathin epitaxial ZnO thin-film actuators on flexible Hastelloy tapes. • We discovered higher piezoelectric constants in thinner epitaxial ZnO thin films. • We demonstrated tunable linear piezoelectric properties in strained ZnO thin films. Linear piezoelectric materials and devices potentially serve as high-accuracy and fast-response sensors and actuators. However, linear piezoelectrics have been scarcely studied, while tuning the linear piezoelectric response of undoped ZnO remains a challenge. Here, linear piezoelectric zinc oxide (ZnO) thin film actuators are fabricated with a sandwich structure of AZO/ZnO/AZO epitaxially on CeO 2 /Hastelloy substrates, with AZO denoting the aluminum-doped ZnO electrodes and CeO 2 denoting the cerium oxide buffer. Due to the lattice mismatch between ZnO and CeO 2 , compressive in-plane lattice strains are detected. Through varying the ZnO layer thickness, a strong correlation is found between the in-plane epitaxial strain and the piezoelectric constant which proves an effective method to improve the linear piezoelectric response of ZnO. Further, the better linear piezoelectric response of the thinner flexible ZnO layer resolves the controversy between the performance and the deformation-durability of flexible piezoelectric ZnO devices. The vertical piezoelectric constant of the ultrathin (∼32 nm) ZnO film is ∼ 8.14 pm/V, which is highly suitable for positioners working at the picometer scale. Future works harnessing the epitaxial strains to further improve the piezoelectric constant of ZnO materials potentially lead to the prosperity of linear piezoelectrics. [ABSTRACT FROM AUTHOR]

[Display omitted] • We achieved epitaxial linear-piezoelectric ZnO films on flexible substrates. • We enhanced the linear-piezoelectric constant of ZnO with epitaxial strains. • We proved the trivial impacts of textures on the piezoelectric constant of ZnO. Ferroelectrics are popular for sensors, actuators and transducers. However, the energy barrier for reversing the polarization leads to extra energy losses, while the hysteresis causes phase-lags and non-linearities between input and output signals. The high piezoelectric constants of ferroelectrics dictate the figure-of-merit of piezoelectric devices. Enhancing the piezoelectric constant of linear-piezoelectrics potentially leads to next-generation piezoelectric devices free from issues associated with ferroelectrics. By epitaxial growth of undoped ZnO films on flexible Hastelloy substrates, we demonstrated an ~85% improvement in the piezoelectric constant, and more importantly maintained the linearity of the piezoelectric response. Strong out-of-plane and in-plane crystallographic alignments are detected in the epitaxial sample, yet modeling of the effective thin-film piezoelectric constant shows only a limited contribution of the optimized textures. The improvement in the piezoelectric response is ascribed mostly to the in-plane epitaxial strain resulted from the lattice mismatch of ZnO to the substrate. The epitaxial ZnO film on Hastelloy promisingly offer high-precision positioners with nano-/pico-meter resolutions. Enhancing the piezoelectric constant by strain engineering potentially removes the barrier for applications of linear-piezoelectrics and enriches the piezoelectric research community. [ABSTRACT FROM AUTHOR]