Fabrication of the ZnO thin film piezoelectric device and the PVA/DL-alanine polycrystal‒ZnO thin film heterostructured piezoelectric devices

Prior to depositing the MgO and ZnO thin films, the ITO-coated PET substrates (Hanalintech, South Korea) were cut into a rectangular shape with a size of 2 × 3 cm². Then, the cut ITO-coated PET substrates were cleaned with acetone (OCI company Ltd., South Korea) and ethanol (OCI company Ltd., South Korea) consecutively in an ultrasonic cleaner (WUC, DAIHAN) for 5 min, respectively, at room temperature, and after this, they were washed with deionized water and dried under a halogen lamp for enough time to remove the humidity that could remain on the substrates.

The deposition of the ZnO thin film was performed by using a RF magnetron sputtering system (HVS-340, Hanvac, South Korea). Before depositing the ZnO thin film, pre-sputtering to remove the contaminants that could exist on the ZnO sputtering target (iTASCO, South Korea) was conducted for 10 min at in Ar gas ambient at 10 mtorr, and after the pre-sputtering was done, the deposition of the ZnO thin film was performed immediately by opening the closed shutter which is used to prevent the sputtering while closed. The RF power used to deposit the ZnO thin film was maintained to be 160 W, and the vacuum was fixed to be 10 mtorr during the deposition. The sputtering gas was Ar. O₂ gas was introduced together with Ar, which is to compensate the oxygen deficiency in the ZnO thin film. The flow rate ratio of Ar to O₂ was set to 4 to 1, and the deposition was performed for 30 min at room temperature without heating. See also Figure S5.

For the PVA/DL-alanine polycrystal-ZnO thin film heterostructured piezoelectric devices, MgO thin film was deposited directly over the ZnO thin film. The deposition of the MgO thin film was carried out by employing an electron beam evaporation technique at room temperature, and like the ZnO thin film deposition, the MgO source (iTASCO, South Korea) was pre-evaporated to remove contaminants present on the source material in advance for about 5 min prior to starting the deposition of the MgO thin film. The voltage and current to evaporate the MgO source were set to 7 kV and about 10 mA, and the thickness of the MgO thin film was measured in real-time through a thin-film thickness monitor built in the electron beam evaporator. The evaporation of the MgO was conducted at room temperature without additional heating, and the vacuum was maintained at about 10⁻⁶ torr during the deposition. For MgO thin film, it was deposited in a circular shape with a radius of 0.35 cm by using a shadow mask composed of polylactic acid.

The PVA/DL-alanine polycrystal layer for the heterostructured devices was formed by dropping the PVA-DL-alanine-mixed solution of 0.05 mL on the MgO thin film using a pipette. Owing to the hydrophilicity of the MgO, the solution naturally spread evenly along the circular pattern of the MgO thin film, and thus, the PVA/DL-alanine polycrystal layer formed the identical circular pattern. The top electrode of the piezoelectric energy harvesters was formed by conductive Ag paste (P-100, CANS). The Ag electrode was made in a ring-shaped pattern of with a size of 0.16 cm² by using a shadow mask, and this is because the edge of the PVA/DL-alanine polycrystal layer was formed in a convex shape upward, while the middle part of the layer was relatively flat, and thus, it was expected that the piezoelectric characteristics of the polycrystals occur the most at the edge when the piezoelectric devices were stimulated under the d₃₃ mode.