Researchers have created first self-powered nanometer-scale sensing devices by combining a new generation of piezoelectric nanogenerators with two types of nanowire sensors. The new devices can measure the pH of liquids or detect the presence of ultraviolet light using electrical current produced from mechanical energy in the environment.
Based on arrays containing as many as 20,000 zinc oxide nanowires in each nanogenerator, the devices can produce up to 1.2 volts of output voltage, and are fabricated with a chemical process designed to facilitate low-cost manufacture on flexible substrates. Tests done with nearly one thousand nanogenerators -- which have no mechanical moving parts -- showed that they can be operated over time without loss of generating capacity.
The report said that the nanoscale generators, which use the piezoelectric effects, produces electric charges when wires made from zinc oxides are subjected to strain. The strain can be produced by simply flexing the wires. The total current from many wires can be added up to power small devices. The research effort has recently focused on increasing the amount of current and voltage generated and on making the devices more robust.
The zinc oxide nanowires are embedded at the both ends of them in a polymer substrate. As they are compressed in a flexible nanogenerator enclosure, they can then generate current. That eliminates the need of metallic electrode that was required in earlier devices. Because the generators are completely enclosed, they can be used in a variety of environments. The whole system thus can be grown on folded and flexible substrates with temperatures of less than100 degrees Celsius. That will accord lower cost fabrication and growth on just about any substrate.
The nanogenerators are produced using a multi-step process that includes fabrication of electrodes that provide both ohmic and shottky contacts for the nanowires. The arrays can be grown both vertically and laterally. To maximize current and voltage, the growth and assembly requires alignment of crystalline growth, as well as the synchronization of charging and discharging cycles.
Production of vertical nanogenerators begins with growing zinc oxide nanowires on a gold-coated surface using a wet chemical method. A layer of polymethyl-methacrylate is then spun-coated onto the nanowires, covering them from top to bottom. Oxygen plasma etching is then performed, leaving clean tips on which a piece of silicon wafer coated with platinum is placed. The coated silicon provides a Shottky barrier, which is essential for maintaining electrical current flow.
The alternating current output of the nanogenerators depends on the amount of strain applied. At a strain rate of less than two percent per second, 1.2 volts is the produced output voltage. Lateral nanogenerators integrating 700 rows of zinc oxide nanowires produced a peak voltage of 1.26 volts at a strain of 0.19 percent. In a separate nanogenerator, vertical integration of three layers of zinc oxide nanowire arrays produced a peak power density of 2.7 milliwatts per cubic centimeter. By measuring the amplitude of voltage changes across the device when exposed to different liquids, the pH sensor can measure the acidity of liquids. An ultraviolet nanosensor depends on similar voltage changes to detect when it is struck by ultraviolet light.
The new generator and nanoscale sensors open new possibilities for very small sensing devices that can operate without batteries, powered by mechanical energy harvested from the environment. Energy sources could include the motion of tides, sonic waves, mechanical vibration, the flapping of a flag in the wind, pressure from shoes of a hiker or the movement of clothing.
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