Growth Of Ultralong ZnO Microwire And Its Application In Piezoelectric Strain Sensor

Among semiconducting nanomaterials, ZnO is a wide band gap semiconductor and one of the most functional materials. On account of its many exciting properties, such as superior emission, chemical and thermal stability, transparency, biocompatibility, and wide electrical conductivity range, ZnO has a variety of applications in an emerging area of nanotechnology. By exploiting coupled semiconducting and piezoelectric properties of ZnO, nanogenerators, piezoelectric field effect transistors, and piezoelectric diodes were invented. As the fundamental component of piezotronics of ZnO, piezoelectric strain sensor has attracted extensive interest because of its stable structure and high sensitivity. Currently, there are two major research subjects of the ZnO piezoelectric strain sensor. One is how to obtain controllable high-quality ZnO micro-nano structures. The other is how to get rid of restriction from the substrate and realize self-supporting of strain sensors. This paper focused on the two major issues.A novel chemical vapor deposition (CVD) system with an upstream growth pattern was exploited to prepare the ultralong ZnO microwires. Because the special backflow effect and temperature distribution appeared in a small quartz reacting tube (SQT), the ZnO island-like thin film and nanotetrapods were synthesized in the upstream and downstream region of the SQT, respectively. The ZnO microwires were growth on the ZnO island-like thin film in the second growth, which acted as the catalyst-free layer. It is worth pointing out that the ultralong ZnO microwires exhibited large size and perfect hexagonal morphology. Here, we achieved the controllable growing of ultralong ZnO microwires, which provided the basis for the fabrication of piezoelectric devices.Contact characteristics between ZnO and metal is an important component of the research of piezoelectric devices, which is directly related to the device performances and applications. In this study, we respectively discussed the contact characteristics between ZnO microwires and three materials and found out:In can form ohmic contact to ZnO microwire and the contact exhibited better stability and repeatability; Au can also form ohmic contact to ZnO microwire, but the stability was poor; There were three distinct contact properties between silver paste and ZnO microwires, including rectifying, symmetric, and linear. By utilizing annealing and isolation processes, theâ… -â…¤properties of sliver paste/ZnO microwires became better and stable. With consideration of technics process and contact characteristics, silver paste was finally selected to act as the electrode material of ZnO piezoelectric strain sensor.Piezoelectric strain sensors based on as-grown individual ultralong ZnO microwires have been fabricated. Epoxy polymeric resin (EPR) and a polytetrafluoroethylene (PTFE) substrate were carefully chosen for device isolation and support. In this study, we only focused on the rectifying I-V characteristic, which can be explained by the thermionic emission-diffusion theory quite well. When a stretched strain was applied to the stain sensor, it exhibited upward diode-like or downward diode-like I-V behavior, which was due to the piezoelectric potential and c-axis orientation of ZnO microwire. Under periodically stretching, the ZnO piezoelectric strain sensor exhibited excellent stability and fine reversibility, and a remarkably high gauge factor up to 1010 had been achieved. By using a simple technology, the strain sensor fully packaged by EPR can be easily isolated from the PEFT substrate and became a quasi-free-standing sensor. The isolatable stain sensor can also act as photo-sensitive device or an electromechanical switch with a large ON/OFF ratio of 12. The ZnO piezoelectric strain sensor fabricated here will provide a fine strain measurement method in cell biology, biological fluid mechanics, and micro-electromechanical system (MEMS).