# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### Effect of Morphology
An immense output power density of 1.2 mW/cm\({}^{2}\) was achieved for the PENG based on ZnO nanoflakes [63]. Figure 3: (**A**) Fabrication and characterization of the HONG device. (**a**) Schematics of the experimental setup used for transferring the vertically aligned ZnO nanowire onto a flexible substrate by changing their alignment to horizontal. (**b**) SEM image of horizontally aligned ZnO nanowire with an Au electrode printed on it (sputtering). Inset: actual photograph of the fabricated device. (**c**) Open-circuit voltage of HONG. (**d**) Short-circuit current of HONG. Both (**c,d**) were obtained under 0.1% strain, at a 5% s\({}^{-1}\) strain rate, with a deformation frequency of 0.33 Hz. The enlarged view of the red box was shown in the inset. The inset shows an enlarged view of deformation cycles. Reprinted with permission from [14]. Adapted with permission from [PERSON] et al. (2010). Copyright © 2010 American Chemical Society. (**B**) Two-dimensional nanosheets/ionic layer heterojunction-based PENG. (**a**) Schematics of 2D nanosheet-based PENG, with 2D nanosheets and an anionic nanoclay layer heterojunction. (**b,c**) depict the output voltage and output current density, respectively, in the switching-polarity test at 4 KgF [18]. Adapted with permission from [PERSON] et al. (2013). Copyright © 2013 _Nature_. PZT can also be synthesized with various morphologies, such as crystalline nanowires, nanorods, and nanofibers. An energy harvesting device fabricated with a single-crystal PZT nanowire [64] generated an output voltage of 10 V under specific applied pressure, whereas nanorods of PZT delivered an output voltage of 3.3 and 8 V under a compressive force of 10 N and a continuous tapping motion, respectively. At 10 N compressive forces, the device demonstrated a power density of 3.16 \(\upmu\)W/cm\({}^{2}\) and 5.12 \(\upmu\)W/cm\({}^{2}\) under 10 N and a continuous tapping motion [65]. [PERSON] et al. [66] fabricated a PENG based on PZT nanoribbons. They fabricated a flexible PDMS/PZT nanoribbon-based device (shown in Figure 4a) using a stepwise fabrication process. In the first stage, PZT nanoribbons were prepared by means of a standard sputtering process on a MgO substrate. Subsequently, the substrate was undercut and etched, and the PZT nanoribbons were transferred to polydimethylsiloxane (PDMS) gel silica. The fabricated device was examined using a piezoelectric force microscope. The device delivered an open-circuit voltage and a short-circuit current of 0.25 V and 40 nA, respectively. Figure 4d shows the results of measurements in which charge density and pressure oscillated in phase, comparing the d\({}_{31}\) values obtained after and before poling, which were 49 pm/V and 79 pm/V, respectively. In another study, a PENG based on a PZT large-area thin film was prepared using the laser lift-off (LLO) process; the entire area of the PZT thin film was transferred onto the flexible substrate without any mechanical damage using laser radiation. The fabricated device produced an output voltage of 200 V and a current of 150 \(\upmu\)A/cm\({}^{2}\) under periodic bending and releasing motions [67]. [PERSON] et al. [68] fabricated a PENG using PZT nanowires with a unique layered nanostructure, which was synthesized via a hydrothermal method. The nanowires were grafted onto the PDMS polymer to form a composite. The fabricated device generated an output voltage of up to 2.7 V, with a power density of 51.8 \(\upmu\)W/cm\({}^{2}\). These output voltage characteristics were observed at its fundamental resonance (25.2 Hz) under a root mean square acceleration input of 0.1 g. Although PZT-based materials provide excellent performance, the lead content in PZT is a key concern. Lead is a highly toxic material and has numerous adverse effects on the environment. To overcome this drawback, researchers have shifted their focus to implementing lead-free perovskite materials in PENG. Figure 4: Transfer of PZT nanoribbons from an MgO substrate to a flexible rubber substrate.