Energetic materials: flexible approach pays off.

Despite rapid advances in the design and fabrication of miniaturized sensors and devices, their practical applications have been impeded by the lack of suitably miniscule sources of electrical power. Indeed, the generation and storage of electrical energy for mobile devices and systems is one of the most urgent challenges in science and engineering today. Microand nanoscale devices — such as ultrasensitive chemical and biomolecular sensors, nanorobotics, microelectromechanical systems (MEMS), environmental sensors and other personal electronic devices — have energy requirements that are not fully met by available technologies such as batteries. Although the energy requirements of these microand nanoscale devices are rather small, they still require a power source that is compact, fully mobile, robust and sustainable over extended periods of time. Converting these devices into completely autonomous and self-powered units will therefore remain an elusive goal until we are able to develop scalable power generators that can scavenge energy from ambient sources such as mechanical vibrations, acoustic energy, thermal gradients and electromagnetic waves (including light). One possible solution would be a miniaturized power source that literally harvests energy directly from low-frequency environmental vibrations1–3. Writing in Nature, Yong Qin, Xudong Wang and Zhong Lin Wang of the Georgia Institute of Technology demonstrate such a device, which offers the prospect of being able to scavenge enough energy from ambient motion to power nanoscale systems without the need for batteries4. Last year the Georgia Tech team demonstrated a d.c. nanogenerator that was driven by ultrasonic waves5. This nanogenerator consisted of an array of vertically aligned zinc oxide nanowires that was similar to a bed of nails, with a specially designed metal electrode floating on the top. The ZnO nanowires are piezoelectric materials, and they become electrically polarized when they are bent by an external force. The team used ultrasonic waves to move the electrode up and down, bending the nanowires in the process. The resulting electrical polarization of ZnO was converted into electricity through a combination of piezotronic effects6 and the piezoelectric–semiconducting coupling process7. Although this work demonstrated the potential of piezoelectric nanowires, a useful device obviously needs to work in the absence of the ultrasound source (which required it own power source). The group’s latest generator builds on the d.c. nanogenerator through an ingenious design that increases the number of individual nanowires and the number of electrical contacts and also, crucially, exploits soft materials to capture energy directly from low-frequency mechanical vibrations. The essential building block of the new generator is a Kevlar microfibre onto which the ZnO nanowires have been grown radially, creating a microscale ‘bottle brush’ that contains billions of nanoscale ZnO bristles. One microfibre–nanowire structure is coated with gold (to serve as the electrode) and is then entangled with an uncoated brush to ensure intimate contact between the two (Fig. 1). As low-frequency ambient vibrations move the brushes back and forth relative to each other, the resulting bending of the nanowires is converted into electrical energy. The approach offers a novel, adaptable, mobile and cost-effective technical platform for harvesting energy Researchers have managed to extract electrical energy from environmental noise by exploiting the piezoelectric properties of zinc oxide nanowires with a device that could herald a new generation of local power sources. eNeRgeTiC MATeRiAls