# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### Doping and Compositing
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**Dopant**} & \multirow{2}{*}{**Morphology**} & \multicolumn{2}{c}{**Output Voltage**} & \multicolumn{2}{c}{**Output Current**} & \multirow{2}{*}{**Ref.**} \\ \cline{5-6}  & & & **(Undoped)** & **(Doped)** & & & \\  & & & **(V)** & **(V)** & & & \\ \hline ZnO & S & Nanowire & 0.659 & 0.689 & 49.7 nA/cm\({}^{2}\) & 57.3 nA/cm\({}^{2}\) & [5] \\ ZnO & Cl & Nanowire & 2.4 & 3.8 & 240 nA/cm\({}^{2}\) & 600 nA/cm\({}^{2}\) & [76] \\ ZnO & Br & Nanowire & 2.4 & 4.2 & 240 nA/cm\({}^{2}\) & 1000 nA/cm\({}^{2}\) & [76] \\ ZnO & Y & Nanorods & - & 20 & - & - & [86] \\ ZnO & Sn & Nanorods & 2.15 & 4.15 & 17 nA & 36 nA & [87] \\ ZnO & Tb & Nanotaper & 6.90 & 9 & 940 nA & - & [88] \\ ZnO & Ag & nanowire & 0.5 & 2 & 200 nA/cm\({}^{2}\) & 500 nA/cm\({}^{2}\) & [89] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Examples of piezoelectric materials with different dopants. The advantages of composite thin-film-based nanogenerators are that they provide low leakage current, high flexibility, easy large-scale fabrication, eco-friendliness, and robustness. Numerous polymers, such as P(VDF-HFP), PVDF, P(VDF-TrFE), PVC, and PDMS, [102; 103; 104; 105; 106] have been used to fabricate composite thin-film nanogenerators. A study on piezoelectric thin-film composites with BTO and organic polymers as active materials revealed that the piezoelectric performance was higher with organic polymers alone. This indicated that the PENG made of thin-film composite materials was able to fulfill the high-performance requirements and flexibility. Comparing the composite based on PDMS/BTO [106] and PVDF/BTO [99], the BTO nanoparticle/PDMS composite thin films delivered excellent performance. The nonpiezoelectric (PDMS)/BTO thin film, with uniformly distributed BTO in the polymer, formed a nondirectional polymer structure that improved the performance to a certain level. Lead-free materials are environmentally friendly and they provide an excellent piezoelectric response. BTO is the most widely used lead-free perovskite material in piezoelectric applications because of its ease of fabrication and the comprehensive investigations that have been conducted of its use in piezoelectric applications. Among the other lead-free perovskite materials with good piezoelectric properties, KNN is a promising candidate for PENG applications. [PERSON] et al. [107] demonstrated a novel approach towards fabricating a KNN-based PENG with enhanced performance; in their work, they incorporated a KNN/PDMS/C (C--acetylene black) composite into the device. They doped 12 wt% C, which resulted in a higher relative permittivity (173.56) and large remnant polarization (Pr, 1.84 \(\mu\)C/cm\({}^{2}\)). A peak output voltage of 10.55 V was produced, which was 3.3 times higher than that of undoped KNN/PDMS. [PERSON] et al. [108] reported a PNG device with Au as the top electrode and PDMS/Ni foam as the bottom electrode, where ZnO nanoflakes/graphene was incorporated into the PDMS as an active element (Figure 7a). This device powered 68 commercial LEDs, producing an output voltage of 122 V, with a current density of 51 \(\mu\)A/cm\({}^{2}\) (Figure 7b,d). This excellent performance was due to three reasons: the flakes of ZnO provided a high active surface area, the 3D porous network of graphene provided a large surface area with an increased number of active sites for the growth of ZnO nanoflakes, and the PDMS layer functioned as a negative triboelectric material that reduced the screening effect. [PERSON] et al. [109] developed a BFO-based piezoelectric nanogenerator. In their study, they fabricated a BFO copolymer P(VDF-TrFE)-based device, with BFO synthesized via a hydrothermal process. They fabricated two different films of BFO/P(VDF-TrFE) and BFO PVDF-HFP and compared the performance. They observed that the BFO/P(VDF-TrFE) film provided superior performance, and the 6 wt% BFO film improved the \(\beta\)-phase of the polymer.