## 3 Structures
### Thin Films  
Thin films are another significant structure of PENG [80]. Newly, an increased flexible nanogenerator relying on a composite thin film made of poly (vinylidene fluoride-co-hexafluoropropene) P(VDF-HFP) and hemispherical clustered BaTiO\({}_{3}\) nanoparticles (NP) has been developed [81]. By evaporating solvents containing tetragonal BTO-NP, P(VDF-HFP), dimethylformamide (DMF), and acetone, the composite thin film was created. A schematic representation of the device manufacturing process is shown in Figure 16. A silicon substrate is shown in Figure 16a. Following the solvent evaporation of the spin-coated solution, a BTO-P(VDFP-HFP) composite thin film was cured in an incubator at 80 \({}^{\circ}\)C for 1 h under ambient circumstances. During the curing process, clusters develop. Following a high-field poling procedure, the subsequent composite thin film is peeled off from the Si substrate containing BTO clusters, as shown in Figure 16c. Finally, the top and bottom of the thin composite layer were enclosed by the coated PDMS before the spin-coated PDMS cured, coated metal electrodes AI on the polyimide (PI) film, as illustrated in Figure 16d,e. Figure 16f illustrates the flexibility of the device by providing an optical view of the manufactured nanogenerator in its bent condition. Piezoelectric potentials were  
Figure 15: **(a)** Schematic diagram of the PCNG device **(i)** morphology of the manufactured nanoparticle **(ii)** thickness of the composite film **(iii)** digital image of the PCNG device; **(b)** PCNG device working mechanism; **(c)** output voltage of the PCNG device; **(d)** output current of the PCNG device; **(e)** output comparison of the PCNG devices reproduced with permission from [79], Elsevier, 2019.  
produced for two distinct BTO NP topologies using the simulation tool COMSOL. One is the hemispherically packed BTO clusters (Figure 17a), and the other is the planar distribution of the lower region depicted in Figure 17b. In both simulations, the same number of BTO-NPs were employed. At a force applied of 0.23 Mpa, the flexible nanogenerator displays an open-circuit voltage of 75 V and a short-circuit current of 15 A, highlighting the significant significance of the hemispherical BTO clusters and producing enough power to turn on light-emitting diodes (LEDs). A PDMS layer increases the output power. To verify the reliability of the output signal, a switching polarity test was run. In this test, the nanogenerator was measured in two different modes. Figure 17c shows the nanogenerator measured in forward link mode, and Figure 17d shows the nanogenerator measured in reverse link mode. The output signal in reverse link mode is the same as in forward link mode; only the polarity of the signal is reversed. Furthermore, the durability and reproducibility of the nanogenerator were tested, which produced an output of \(\sim\)5 V and \(\sim\)750 nA, by repeated bending tests during the bending stage shown in Figure 17e. Similarly, another BaTiO\({}_{3}\) piezoelectric thin film was deposited on a plastic substrate to show a direct piezo effect [82]. The manufacturing of flexible BaTiO\({}_{3}\) nanogenerators on plastic substrates is depicted in Figure 18a. On Pt/Ti/SiO\({}_{2}\)/(111)Si substrates, perovskite ceramic BaTiO\({}_{3}\) thin films were formed by RF magnetron sputtering, crystallized at 700 \({}^{\circ}\)C, and poled to enhance piezoelectric characteristics. The BaTiO\({}_{3}\) thin film was then printed using soft lithography and nanofabrication techniques onto a substrate material and linked with electrodes. Figure 18b depicts the SEM image of the Au/BaTiO\({}_{3}\)/Pt layers on the SiO\({}_{2}\)/Si substrate that make up the metal-insulator-metal (MIM) structure. The spectrum of an annealed BaTiO\({}_{3}\) thin film, which includes Ba, Ti, O\({}_{2}\) and tiny C peaks, is shown in the inset.