## 3 Structures
### Thin Films
The SEM image of the MIM structure (Au/BaTiO\({}_{3}\)/Pt) shown in Figure 18c was created after the underlying Si layer was anisotropic ally etched with 5% tetramethylammonium hydroxide (TMAH) at 80 \({}^{\circ}\)C for 12 min. The inset is the scanning image of the MIM structure partially supported by the Si substrate during etching (corresponding to Figure 18a(ii)). According to Figure 18a(iii), the image in Figure 18d demonstrates that the MIM structure (about 1 cm\({}^{2}\)) was effectively transferred from the bulk Si wafer to the PDMS stamp without fractures. The images of the twisted PDMS stamp and the top view of the MIM structure of the PDMS stamp are shown in the insets at the top and bottom, respectively. A flexible BaTiO\({}_{3}\) nanogenerator with an area of 82 mm\({}^{2}\) and a filling factor of 16.4% is depicted in Figure 18e (equivalent to Figure 18a(iv)) on a plastic substrate. To measure the output voltage and current, silver (Ag) paste was used to affix copper (Cu) wires to metal plates. The top and bottom electrodes of the MIM structure that are connected to the IDE are visible in greater detail in the inset (Au). Figure 19a (i,ii) shows the flexible BaTiO\({}_{3}\) nanogenerators before bending to the original state. Due to polarization in the initial condition of high electric fields without bending, piezoelectric materials have longitudinally oriented dipoles. Figure 19a (iii,iv) shows a flexible BaTiO\({}_{3}\) nanogenerator in a bent state. In the bent state, the longitudinal and compression stresses caused by device deflection generate charges in the MIM structure. The charge then passes into the Au electrodes, generating an output voltage (\(\Delta\)V) across the IDE. Figure 19b,c shows the output voltage (right ii) and output current (left i) of flexible BaTiO\({}_{3}\) nanogenerators on plastic substrates in bent and unbending states. To verify that the output signal was produced by bending the MIM structure, a switching polarity test is done. According to Figure 19b, a positive voltage and current were produced during the forward connection. Negative voltages and currents for reverse connections were noted (according to Figure 19c). When circularly deformed by a bending stage, a BaTiO\({}_{3}\) flexible nanogenerator built on a plastic substrate can provide an output voltage of 1.0 V, an output current signal of 26 nA, an output current density of 0.19 A/cm\({}^{2}\), and a power density of about 7 mW/cm\({}^{3}\). The findings demonstrate that flexible displays can be powered by mechanical motion using nanogenerators for future technologies. Key Highlights of the recent progress on piezoelectric energy harvesters' designs are shown in Table 2.  
Figure 16: (**a–f**) Fabrication process of the BTO NPs schematic diagram; (**g**) SEM of the BTO-P(VDF-HFP) composite film consisting of BTO clusters; (**h**) SEM image of the PDMS-covered BTO-P(VDF-HFP); (**i**) Raman spectrum of the BTO NPs showing BTO NPs have a tetragonal phase, and high piezoelectric coefficient reproduced with permission from [81], American Chemical Society, 2014. Figure 17: Simulated piezoelectric potentials for BTO NP in (**a**) distribution pattern as a hemispherical cluster, (**b**) planner distribution at the bottom; switching polarity test (measure open circuit voltage and short circuit current) in (**c**) forward connection mode and (**d**) reverse connection mode; (**e**) photograph of the nanogenerator in the bending and releasing states reproduced with permission from [81], American Chemical Society, 2014. Figure 18: (**a**) Diagram of a flexible BaTiO\({}_{3}\) nanogenerator fabrication process; (**b**) SEM image of MIM structure (Au/BaTiO\({}_{3}\)/Pt layers); (**c**) SEM image of the MIM structure (Au/BaTiO\({}_{3}\)/Pt) after anisotropic etching of the Si layer. The inset is a magnified cross-sectional view of the MIM structures; (**d**) a magnified image of a PDMS slab inked with MIM structure.