## 3 Structures  
### Nanowires  
Based on the observation of piezoelectricity, ZnO NW-based (Piezoelectric Nanogenerator) PENG was developed. Numerous studies have concentrated on the conceptualization, structure design, operating mechanism, and output optimization of nanogenerators [70]. Many studies show that PENG can harvest energy. Their output voltage signals are up to several hundred volts, enough to power LEDs and LCDs. Recently, piezoelectric-based lead-free NaNbO\({}_{3}\) nanowires with polydimethylsiloxane (PDMS), a polymer-based composite nanogenerator, have been developed [71]. This PENG has been fabricated using lead-free NaNbO\({}_{3}\) NWs. NaNbO\({}_{3}\) NWs are made using a hydrothermal process at low temperatures, and they can be polarized at room temperature using an electric field. The flexible nanogenerator consists of a NaNbO\({}_{3}\) NWs-PDMS composite, and Au/Cr coated films acting as electrodes. Figure 5a,b shows the topographical representation of a NaNbO\({}_{3}\) NW. The piezoelectric nanogenerator's structure is depicted in Figure 5c. It is made up of four layers: two Au/Cr-coated Kapton films functioning as electrodes in the yellow, blue, and light blue layers, a NaNbO\({}_{3}\)-PDMS composite acting as a source of piezo potential, and a solid polyester (PS) substrate film working as the primary driving force of strain. Due to the utilization of all polymer layers in the nanogenerator, the object is flexible and could be bent and released. The power generation process of the nanogenerator is depicted in Figure 5d. Electric dipoles that are neither parallel nor perpendicular to the direction of the NWs are created when a compressive force is exerted. Figure 5d illustrates how the electric dipoles will typically align themselves with the electric field's orientation. Some piezoelectric domains will line up with the electric field's direction, while other zones may tilt away from it. However, the electric dipole element is present in every domain along the green arrow-depicted direction of the electric field. The piezoelectric potential is stronger at the bottom electrode region than at the top electrode area if we apply force parallel to the surface, as shown in Figure 5d. Electric polarization will also align with the dipole direction. The charges should travel from the bottom electrode to the top electrode in the reverse direction if the pressure is relieved, and the piezo potential should dissipate. Thus, both current and voltage are generated as a result of repeatedly applying and relaxing compressive stress. Figure 5e,f shows that the NaNbO\({}_{3}\)-PDMS composite nanogenerator produces an output voltage of 3.2 V and an output current of 72 nA with a current density of 16 nA/cm\({}^{2}\) under a compressive strain of 0.23%. The findings demonstrate the value of NaNbO\({}_{3}\) NWs for applications using lead-free piezoelectric nanogenerators. Similar to the first, a second piezoelectric nanogenerator has been created by converting mechanical energy into electrical energy using cadmium sulfide (CdS) NWs [72]. The CdS NWs arrays, which are grown upwards, are utilized to transform mechanical energy into electrical energy. The hydrothermal approach and the physical vapor deposition technique are the two ways used to manufacture these vertically oriented CdS NWs arrays (PVD). The hexagonal wurtzite (WZ) and cubic zinc blend (ZB) phases with growth directions of WZ <0001> and ZB (111) were alternately present in the NWs produced by the hydrothermal method, while the single-crystalline WZ phase with growth direction <0001> was present in the NWs produced by the PVD process. Figure 6a displays the SEM picture of the hydrothermally generated CdS NWs, and Figure 6b displays the X-ray diffraction pattern (XRD). These NWs have a diameter of 150 nm. 5 nN of normal force and a speed of 150.24 m/s were kept in the AFM contact mode. Figure 6c illustrates how to measure output voltage across a load by moving the tip throughout the sample.