Nanosensors – sensor devices built at the nanoscale (1/100,000 the width of a human hair) – need energy to run. Nanobatteries are one approach under development. Another technology is nanogenerators, nanoscale devices that create electricity from the mechanical energy provided by the environment such as wave action, wind motion, and body movement. Researchers at the Georgia Institute of Technology (USA) have produced the first nanosensor – nanogenerator combination. These become fully independent sensors, invisible, and with electrical generation that has no moving parts and nothing to run down or wear out.
The electricity is generated by the flexing of zinc oxide nanowires in what is called the piezoelectric effect. Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering and his team have been working on piezoelectric nanoscale devices for many years.
The technology for nanowires of zinc oxide is also not new, but Wang and team have focused on the manufacturing aspects – working to make the construction of the devices scalable (the ability to produce large numbers) and to coax more efficiency from the design. The latest advance comes from embedding the nanowire structure in a polymer substrate (essentially a plastic wafer). The wires then generate a current as they are compressed. Since they are completely enclosed in the polymer, they can be used in many different environments. The nanogenerators are produced in a multistep process that includes the important step of connecting electrodes to the nanowires. Two common connections, Ohmic (linear and symmetric current) and Shottky (non-linear and asymmetric current) are available. The wire arrays can be built both vertically and horizontally providing flexibility for manufacturing formats. 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. These are obviously producing minute amounts of electricity, but hooked to nanoscale sensors – the energy requirements are also minimal.
This is the sort of detail that can make or break the transition of nanotechnology to commercial production.
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.
“Building devices that are small isn’t sufficient,” Wang noted. “We must also be able to power them in a sustainable way that allows them to be mobile. Using our new nanogenerator, we can put these devices into the environment where they can work independently and sustainably without requiring a battery.”