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[Display omitted] Linear piezoelectric materials offer new opportunities for next-generation multifunctional micro/nano systems on chip aside from ferroelectrics. However, the advantage of quantitative pressure sensing of linear piezoelectric materials over ferroelectrics has not been demonstrated, while the thermal stability of linear piezoelectric zinc oxide (ZnO) is yet to be evaluated. In this work, pressure sensors based on linear piezoelectric undoped ZnO and ferroelectric vanadium doped ZnO (V-ZnO) films are studied side by side. Both films possess similar fiber textures oriented along the polar ZnO [0 0 2] direction, yet each of the V-ZnO grains consists of strip-like domains observed by SEM. Applying triangular-wave pressures, the linear piezoelectric ZnO sensor generates triangular-wave voltage signals that are one-to-one linearly correlated to the applied pressure, showing decisive advantages for quantitative pressure sensing compared to the 'sail-like' voltage signals of the ferroelectric V-ZnO sensor that is many-to-one correlated to the applied pressure. Superior thermal stability is demonstrated in the linear piezoelectric ZnO sensor showing stable pressure sensing properties up to 450 °C compared to the degraded sensing properties of the ferroelectric V-ZnO sensor at 150 °C. The thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric ZnO films serve as candidates for high-performance multifunctional micro/nano systems. [ABSTRACT FROM AUTHOR]

[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 ultrathin flexible AZO electrodes with low resistivities (less than100 μΩ•cm). • We demonstrated the mechanical robustness of ultrathin flexible AZO electrodes. • We discovered abnormally high carrier concentrations in strained epitaxial AZO. Aluminum-doped zinc oxide (AZO) thin films are popular transparent electrodes for optoelectronics and photovoltaics. However, next generation flexible devices demand not only high conductivities but also low thicknesses in AZO electrodes to stay robust against deformations. The two requirements contradict each other due to the scattering of carriers at grain boundaries in polycrystalline AZO electrodes. Herein, this work develops ultrathin, highly conductive, single-crystalline, and mechanically robust AZO electrodes on flexible Hastelloy substrates. The epitaxial growth of AZO on polycrystalline Hastelloy is empowered by an advanced buffer architecture topped with a biaxially-textured MgO layer. Thereby, a c -axis epitaxial strain is generated in the AZO electrodes and then regulated by varying the thickness of the electrodes. Abnormally high carrier concentrations are detected in the AZO electrodes likely caused by the piezopotential generated by the epitaxial strain. While the highest resistivity value of the epitaxial AZO electrodes is within the mainstream values of AZO electrodes on various substrates, a resistivity as low as ~22 μΩ•cm is demonstrated in the ultrathin electrode with a thickness of ~28 nm. The mechanical durability of the ultrathin AZO electrode is validated by bending tests of 1000 cycles. [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]

• Niobium-doped indium oxide (InNbO) was prepared by co-sputtering technique. • InNbO is compact and uniform without obvious pores and cracks. • InNbO is cubic bixbyite polycrystalline structure. • The resistivity of InNbO is as low as 8.29 × 10–3 Ω· cm when the Nb content is 1.8 at%. • InNbO exhibits high optical transmittance from visible to near-infrared wavelength regions. Indium oxide (In 2 O 3) based thin films have attracted much attention due to their high transparency and low resistivity. In order to obtain good transmittance and high conductivity, niobium (Nb) element was chosen as dopant in In 2 O 3 in this study. Nb-doped In 2 O 3 thin films (InNbO) were deposited on substrate by co-sputtering technique. The influence of Nb content on the structural, optical and electrical properties of InNbO thin films was systematically investigated. These thin films are compact and uniform without obvious pores and cracks. And InNbO is cubic bixbyite structure similar to In 2 O 3. When Nb content is 1.8 at%, the resistivity of InNbO is as low as 8.29 × 10−3 Ω·cm. The prepared thin films exhibit high optical transmittance from visible to near-infrared wavelength regions. The average transmittance of InNbO thin films can reach 80.74 % in visible light range (380 ∼ 750 nm), and 75.71 % in near-infrared region (750 ∼ 2400 nm). [ABSTRACT FROM AUTHOR]

This work presents a materially and structurally designed barrier thin film with a self-wrinkling morphology, which consists of composites of amorphous alumina (a-Al 2 O 3) and polycrystalline zinc oxide (ZnO). The pure ZnO, Al 2 O 3 films, and Al 2 O 3 -ZnO composited film are deposited on polyethylene terephthalate (PET) by magnetron co-sputtering at room temperature. The water vapor transmission rate (WVTR) for the composited film with about 30 % ratio of Al 2 O 3 to ZnO is down to 0.026 g·m−2·day−1 from 1.184 g·m−2·day−1 of pure ZnO film at 38 °C/90 % RH, indicating that the nanocomposite structure of amorphous Al 2 O 3 and crystalline ZnO can significantly improve the water-resistance performance. Theoretical calculations demonstrate that the amorphous Al 2 O 3 can dramatically suppress the defect fraction within ZnO from 4.65 % to 0.08 %. Furthermore, a self-wrinkling morphology with Al 2 O 3 -ZnO composited film deposited on PET/acrylic is designed to have the WVTR value as low as 1.30 × 10−3 g·m−2·day−1. Due to the stress release when forming micro-scale wrinkles, the more smooth and dense Al 2 O 3 -ZnO composited film can markedly sharpen up the barrier property. The low WVTR barriers via amorphous and crystalline hybrid composites with self-wrinkling morphology by magnetron sputtering potentially serve as a low-cost and large-scale production path for electronics encapsulation as compared to atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD). [Display omitted] • Al 2 O 3 -ZnO film with an appropriate Al 2 O 3 ratio reduces water vapor transmission rate. • Amorphous Al 2 O 3 -crystalline ZnO hybrid film is deposited by magnetron co-sputtering. • A defect-mediated model for a single film on PET is proposed and applied to the system. • Al 2 O 3 -ZnO film with self-wrinkling morphology markedly sharpens up barrier property. [ABSTRACT FROM AUTHOR]

This work presents a materially and structurally designed barrier thin film with a self-wrinkling morphology, which consists of composites of amorphous alumina (a-Al 2 O 3) and polycrystalline zinc oxide (ZnO). The pure ZnO, Al 2 O 3 films, and Al 2 O 3 -ZnO composited film are deposited on polyethylene terephthalate (PET) by magnetron co-sputtering at room temperature. The water vapor transmission rate (WVTR) for the composited film with about 30 % ratio of Al 2 O 3 to ZnO is down to 0.026 g·m−2·day−1 from 1.184 g·m−2·day−1 of pure ZnO film at 38 °C/90 % RH, indicating that the nanocomposite structure of amorphous Al 2 O 3 and crystalline ZnO can significantly improve the water-resistance performance. Theoretical calculations demonstrate that the amorphous Al 2 O 3 can dramatically suppress the defect fraction within ZnO from 4.65 % to 0.08 %. Furthermore, a self-wrinkling morphology with Al 2 O 3 -ZnO composited film deposited on PET/acrylic is designed to have the WVTR value as low as 1.30 × 10−3 g·m−2·day−1. Due to the stress release when forming micro-scale wrinkles, the more smooth and dense Al 2 O 3 -ZnO composited film can markedly sharpen up the barrier property. The low WVTR barriers via amorphous and crystalline hybrid composites with self-wrinkling morphology by magnetron sputtering potentially serve as a low-cost and large-scale production path for electronics encapsulation as compared to atomic layer deposition (ALD) and plasma-enhanced chemical vapor deposition (PECVD). [Display omitted] • Al 2 O 3 -ZnO film with an appropriate Al 2 O 3 ratio reduces water vapor transmission rate. • Amorphous Al 2 O 3 -crystalline ZnO hybrid film is deposited by magnetron co-sputtering. • A defect-mediated model for a single film on PET is proposed and applied to the system. • Al 2 O 3 -ZnO film with self-wrinkling morphology markedly sharpens up barrier property. [ABSTRACT FROM AUTHOR]