Fabrication and characterization of piezoelectric nanogenerator using the ferroelectric P(VDF-TrFE)

Abstract

There have been many recent studies about piezoelectric nanogenerators. Most piezoelectric materi-als for nanogenerator use are focused on ceramics (ZnO, $Pb(Zr,Ti)O_3$, $BaTiO_3$, GaN), while only a few studies of ferroelectric PVDF-based polymer have been carried out. It is essential to formation the aligned nanostructure to fabricate the nanogenerator but it might be difficult to form a polymer nanostructure with high aspect ratio because of the low stiffness and relative softness of the polymers. Although many studies regarding fabrication of a one-dimensional polymer nanostructure, such as nanotube and nanowire, have been conducted over the last decade, there are no studies that describe the well-aligned one-dimensional structure of PVDF-based polymer piezoelectrics. Among the ferroelectric polymer materials, poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] has a good property in that it naturally has a stable ferroelectric $\beta$ phase at room temperature without additional pro-cesses such as mechanical stretching or electrical poling. P(VDF-TrFE) has also relatively large piezoelectric voltage constant than ceramic materials. We tried to make the piezoelectric nanogenerators with well-aligned P(VDF-TrFE) nanostructure in order to maximize the piezoelectric effect and enhance the effective surface area.

Firstly, we have studied the fabrication and characterization of polymer piezoelectric nanogenerator with vertically well-aligned P(VDF-TrFE) nanorod arrays using $SiO_2$ template. We have conducted many trials to obtain vertically aligned P(VDF-TrFE) nanorod structures using conven-tional methods. $SiO_2$ templates fabricated by a semiconductor process were used in the formation of the P(VDF-TrFE) nanorod arrays. Compared to the AAO templates, the $SiO_2$ templates offer several advantages such as scalability, compatibility with the semiconductor process, and capability of nanoscale device fabrication. We were aware of the fact that it was difficult to fabricate a one-dimensional nanostructure by traditional template assisted methods through many trials. By combining the merits of traditional template-assisted methods for polymer nanostructure fabrication and previous results, we demonstrate the immersion crystallization(IC) process, which has features of polymer crystallization and template removal simultaneously. The ferroelectric P(VDF-TrFE) nanorod arrays were fabricated for the first time via the IC method, a simple and convenient method for fabricating polymer nanorods. We were able to obtain a highly aligned one-dimensional nanostructure with a high aspect ratio (~10:1) and these results show that the IC method can be suitably applied to the fabrication of one-dimensional nanostructures using polymers as starting materials. However, we did not obtain the piezoelectric outputs from polymer nanogenerator composed of P(VDF-TrFE) nanorod arrays.

Secondly, to solve the above problem, we proposed a new concept of one-dimensionally aligned poly(vinylidene fluoride-trifluoroethylene)[P(VDF-TrFE)]/titanium nitride (TiN) hybrid nanogenerators using the robust TiN nanorod arrays. TiN have several advantages of low cost, good stability against corrosion, high conductivity and mechanical hardness and widely is used in semiconductor process. By converging the ad-vantages of TiN and P(VDF-TrFE), TiN cored nanorod nanogenerators are able to maximize the electrode sur-face area and the stiffness of the nanostructures. We demonstrated a new type of P(VDF-TrFE)/TiN hybrid nanogenerator that has advantages over generators based on poled PVDF nanofibers. We obtained the output current value of ~20 nA/spot from a piezoelectric nanogenerator by deflecting the nanorods using a conductive AFM tip. This piezoelectric current value is larger than other reported values.

Particularly, the P(VDF-TrFE)/TiN nanogenerator showed the good electricity output in P(VDF-TrFE) ferroe-lectrics without the poling process which is comparable to those of other PVDF-based nanogenerators. Notably, these simple fabrication and assembly routes would allow for the facile mass production and minia-turization of this type of nanogenerator. In addition, this study will offer more chances for diverse applications such as highly sensitive sensors and high efficient energy harvesting.