Nanogenerators for mechanical energy harvesting and self-powered sensor networks
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Energy crisis and internet of things have attracted long term interests due to rapid development of modern society. In this regard, the invention of nanogenerators provide us new insights on scavenging wasted mechanical energy from the ambient environment, and convert kinetic agitations into electricity for powering electronic devices. Piezoelectric nanogenerators are based on the piezoelectric property of semiconductor nanowires. Vertical integration, proper selection of materials, and surface modifications have been applied to enhance its output performance. Triboelectric nanogenerators (TENG), on the other hand, work on the basis of contact electrification and electrostatic induction. Four fundamental working modes have been developed to accommodate different types of mechanical motions. A high output power density of 35.6 W/m2, an excellent energy conversion efficiency of up to 55%, and terrific output stability of over 300,000 cycles have been accomplished with the state-of-art TENG devices. The niche applications of the TENGs have been demonstrated for powering portable electronics toward the goal of fully-integrated self-powered system. Since the nanogenerators are enabled to convert mechanical input into electrical output signals, the information of mechanical stimuli (amplitude and frequency) can be retrieved through analyzing the output performance of the nanogenerators. In this way, nanogenerators were employed as self-powered/active sensors without an external power supply, and multiple functions could be achieved including pressure detection and motion sensing. Both static and dynamic pressure sensing was realized using the open-circuit voltage and short-circuit current from the TENG, respectively. A high sensitivity of 0.31 kPa-1 and a low detection limit of 2.1 Pa were also fulfilled. The integration of pressure sensor array for tactile imaging was further demonstrated for its potential application in electronic skin, human-machine interface, and security monitoring.