# Strategic Development of Piezoelectric Nanogenerator and Biomedical Applications
## 3 Strategies
### Interfacial and Surface Modification  
Improving the contact between the electrode surface and the piezoelectric nanostructure is another possible approach for improving piezoelectric performance. Interfacial modification is a unique strategy, wherein the output performance of a piezoelectric nano-generator can be improved by enhancing the interfacial contact between the metal electrode and the active material. Designing a p-\(n\) junction or creating a Schottky contact between the metal electrode surface and the piezoelectric nanostructure are considered interfacial modifications. These two mechanisms can drastically improve performance by creating a built-in electrical field near the p-\(n\) junction, thereby reducing the carrier concentration and leakage current with the help of the Schottky barrier. [PERSON] et al. [111] fabricated a PENG based on Ag/(KNN) heterojunctions, which improved the partial voltage applied to the KNN during the poling process and enhanced the performance of the fabricated device. The Ag/KNN heterojunction was created via photoreduction during KNN powder synthesis by means of a solid-state reaction. The KNN/multiwall (MW) CNT/PDMS piezoelectric film was prepared by mixing MW CNTs and KNN powder in the PDMS elastomer. The flexible device generated an open-circuit voltage of 240 V and short-circuit current of 23 \(\upmu\)A. [PERSON] et al. [112] also reported the fabrication of an NiO-coated ZnO heterojunction with an increased DC output voltage of 430 mV, with an output current density of 40 nA/cm\({}^{2}\). [PERSON] et al. [113] also demonstrated an increase in the performance of ZnO-based PENGs using interfacial modification, but they combined this with doping. In their work, they fabricated  
Figure 7: (**a**) Schematic structure of a PENG based on ZnO nanoflakes. (**b**) Image of an actual device. (**c**) Open-circuit voltage of PDMS/ZnO NF/3D Gr/Ni foam-based PENG. (**d**) Short-circuit current density of PDMS/ZnO NF/3D Gr/Ni foam-based PENG [108]. Reprinted with permission from [PERSON] et al. (2018). Copyright © 2018 American Chemical Society.  
a device by doping ZnO with Cl, investigated the effect of different concentrations of Cl on performance, and compared the performance of the device with only interfacial modification and after doping. They observed a notable elevation in performance.  
Nanoscale ZnO exhibits a large specific surface area due to quantum confinement effects. In some cases, piezoelectric performance is hindered by this high surface area. Due to quantum effects, aggregation occurs when the materials are composited with polymers such as PVDF-TrFE; furthermore, poor dispersion of the nanoparticles leads to low performance of the device. In such cases, surface modification is considered to be the best way to improve performance. [PERSON] et al. [114] demonstrated a unique path for the surface modification of ZnO nanoparticles. They utilized an _n_-propylamine (PA) dispersing agent and a silane coupling agent (1H, 2H, 1H, 2H, perfluorooctyltriethylesilane, PFOES). PA is a nonreactive agent that lowers the surface energy of ZnO nanoparticles but does not improve the interfacial binding between the particle surface and the polymer matrix. The output voltage of the fabricated ITO/P-Z-P-P/Au PENG was 24.4 % higher than that of the normal PENG.