Zinc oxide (ZnO) simultaneously exhibits semiconducting and piezoelectric properties. ZnO in the form of nanorods has been studied intensively for application in self-powering devices. The power generation in piezoelectric nanogenerators based on ZnO nanorods can be improved via several approaches, including an oxygen plasma treatment. When ZnO nanorods are exposed to oxygen plasma, the charge carrier concentration decreases and the piezoelectric output voltage consequently increases. However, the effects of oxygen plasma on the mechanical properties of ZnO nanorods has not been systematically studied using a precise measurement technique. Given the size of ZnO nanorods, atomic force microscopy (AFM) is a suitable method for manipulating individual ZnO nanorods and measuring their elastic properties. In the present work, we observed the effects of oxygen plasma on the elemental composition and microstructure of ZnO nanorods. First of all, the surface roughness of the ZnO nanorods was analyzed using AFM, revealing that it increased due to the etching effect of the oxygen plasma. From X-ray photoelectron spectroscopy (XPS) measurements, three distinct peaks corresponding to lattice oxygen, oxygen vacancies, and absorbed oxygen on the surface were identified. The XPS analysis results showed that oxygen vacancy defects on the ZnO nanorods were decreased by oxygen plasma treatment. Next, the effects of oxygen plasma on the elastic properties of ZnO nanorods were studied using lateral force microscopy. It was confirmed that the elastic modulus of ZnO nanorods increased due to the reduced number of defects originating from oxygen vacancies.

Flexible polymers, ferroelectric ceramic nanoparticles, and conductive nanomaterials have been intensively studied with the aim of exploiting their unique properties synergistically and producing a ternary composite displaying excellent piezoelectric performance. Therefore, it is important to understand the role of conductive nanomaterials in ternary nanocomposites for piezoelectric power generation. In this study, the effect of Cu nanowire (CuNW) addition on the dielectric, ferroelectric, and piezoelectric properties of poly(vinylidene fluoride) (PVDF)/BaTiO3 nanoparticle (BTNP)/CuNW composite films was systematically investigated. The experimental results reveal that ternary composites with 0.04 vol. % CuNWs generated the highest total charge and power density among samples of varying CuNW content. When 0.04 vol. % CuNWs were incorporated into the PVDF/BTNP binary composite, the remanent polarization (Pr) increased from 0.51 to 1.63μC/cm2 due to an enhanced effective electric field. However, when the CuNW content exceeded 0.04 vol. %, Pr started to decrease owing to an increase in the leakage current and the enhancement in the pinning effect of the PVDF dipoles. When an excessive amount of CuNWs was added to the composite, the piezoelectric performance showed only a moderate decrease owing to the enhanced stress transfer. Conductive nanowires are often incorporated into piezoelectric ternary composites to facilitate the dispersion of piezoelectric nanoparticles and for stress transfer. However, composites with a more than 0.04 vol. % CuNWs have a lower net polarization and piezoelectric power density. When the CuNW content is optimized (0.04 vol. %), the maximum power density of the ternary composite film can be enhanced by up to 520%.

A Comparative Study of BaTiO3/PDMS Composite Film and a PVDF Nanofiber Mat for Application to Flexible Pressure SensorsIntensive research has been conducted to develop flexible piezoelectric pressure sensors, since selfpowering devices are advantageous for wearable electronic applications. Recently, two types of piezoelectric devices, ceramic-PDMS composite film and PVDF nanofiber mats, have drawn attention in the research community. Piezoelectric ceramics such as BaTiO3 (BTO) and PZT exhibit outstanding piezoelectric coefficients, while PDMS provides flexibility. In contrast, a PVDF nanofiber mat simultaneously exhibits piezoelectricity and flexibility. In the present study, a comparative analysis of BTO-PDMS composite film and a PVDF nanofiber mat for application to flexible pressure sensors was carried out. First, step-wise electric poling was conducted on these two types of pressure sensors, after which the open-circuit voltage (Voc) was measured under compressive force. The 1.8 V peak-to-peak Voc was measured in a BTO-PDMS composite with a 30 wt.% BTO content that was poled by 10 kV/mm electric field for 15 min. This peak-to-peak Voc of the BTO-PDMS composite increased further to ~ 4 V when it was poled for 24 hr. Unlike the BTO-PDMS composite films, the maximum Voc (1.1 V) was measured in a PVDF nanofiber mat that did not undergo subsequent electric poling. A BTO-PDMS composite film and a PVDF nanofiber mat were fabricated, and the compressive force and strain-rate dependencies of Voc and the short-circuit current (Isc) were investigated. Overall, the Voc and Isc of the BTO-PDMS composite film exceeded those of the PVDF nanofiber mat in a force range of 1 − 25 N and frequency range of 0.5 − 2.0 Hz. However, the Voc and Isc signals from the PVDF nanofiber mat were more stable than those from the BTO-PDMS composite film due to the longer lifetime of the signals.

Nanofiber networks comprising polymer-metal core–shell structures exhibit several advantages, such as high uniformities and considerable flexibilities. Additionally, the flexibility of the nanofiber network may be further enhanced by engineering the network topology. Therefore, in this study, the topologies of polyvinylidene fluoride (PVDF)-Pt core–shell nanofiber (CS NF) networks were engineered, and their performances as flexible transparent electrodes were comprehensively evaluated. Three distinct topologies of nanofiber networks were induced using circular, square, and rectangular electrode collectors. A highly uniform nanofiber network was obtained using the square electrode collector, which generated a high density of nanofiber junctions (nodes). Consequently, this nanofiber network exhibited the smallest sheet resistance $$\left({R}_{\mathrm{s}}\right)$$ R s and lowest optical transmittance $$\left(T\right)$$ T among the three CS NF networks. In contrast, nanofiber bundles were frequently formed in the randomly aligned CS NF network prepared using the circular electrode collector, reducing the node density. As a result, it simultaneously exhibited a very small $${R}_{\mathrm{s}}$$ R s and high $$T$$ T , generating the largest percolation figure of merit $$\left(\Pi =330.5\right)$$ Π = 330.5 . Under certain strain directions, the CS NF network with the engineered topology exhibited a significantly enhanced mechanical durability. Finally, a flexible piezoelectric pressure sensor with CS NF network electrodes was fabricated and its sensing performance was excellent.

Nanofiber networks comprising polymer-metal core-shell structures exhibit several advantages, such as high uniformities and considerable flexibilities. Additionally, the flexibility of the nanofiber network may be further enhanced by engineering the network topology. Therefore, in this study, the topologies of polyvinylidene fluoride (PVDF)-Pt core-shell nanofiber (CS NF) networks were engineered, and their performances as flexible transparent electrodes were comprehensively evaluated. Three distinct topologies of nanofiber networks were induced using circular, square, and rectangular electrode collectors. A highly uniform nanofiber network was obtained using the square electrode collector, which generated a high density of nanofiber junctions (nodes). Consequently, this nanofiber network exhibited the smallest sheet resistance (Rs) and lowest optical transmittance (T) among the three CS NF networks. In contrast, nanofiber bundles were frequently formed in the randomly aligned CS NF network prepared using the circular electrode collector, reducing the node density. As a result, it simultaneously exhibited a very small Rs and high T, generating the largest percolation figure of merit (Π = 330.5). Under certain strain directions, the CS NF network with the engineered topology exhibited a significantly enhanced mechanical durability. Finally, a flexible piezoelectric pressure sensor with CS NF network electrodes was fabricated and its sensing performance was excellent.