Researchers at Georgia Tech have constructed what is believed to be the first self-powered nanometre-scale sensing devices that draw electrical power from mechanical energy.
"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," said the university.
Based on arrays of generators, each containing as many as 20,000 zinc oxide nanowires, the device can produce up to 1.2V.
Tests done with hundreds of nanogenerators - which have no mechanical moving parts - showed that they can be operated over time without loss of generating capacity, claimed the university.
"We have demonstrated a robust way to harvest energy and use it for powering nanometre-scale sensors," said professor Zhong Lin Wang. "We now have a technology roadmap for scaling these nanogenerators up to make truly practical applications."
For the past five years, Wang's research team has been developing generators that use the piezoelectric effect.
Now Wang and collaborators have reported a configuration for nanowires that embeds both ends in a polymer substrate, eliminating direct contact with a metal electrode and allowing the generators to be completely enclosed.
"We can now grow the wires chemically on substrates that are foldable and flexible and the processing can now be done at substrate temperatures of less than 100 degrees Celsius," explained Wang. "That will allow 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 Schottky contacts for the nanowires.
The arrays can be grown both vertically and laterally.
"To maximise current and voltage, the growth and assembly requires alignment of crystalline growth, as well as the synchronisation of charging and discharging cycles," said the university.
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 Schottky barrier, which is said to be 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 2%/s, we can produce output of 1.2V," said Wang. "The power output is matched with the external load."
Lateral nanogenerators integrating 700 rows of zinc oxide nanowires produced a peak voltage of 1.26V at a strain of 0.19%.
In a separate generator, vertical integration of three layers of zinc oxide wire arrays produced a peak power density of 2.7mW/cm³.
Wang's team has so far produced two sensors that are based on zinc oxide nanowires and powered by the nanogenerators.
"By measuring the amplitude of voltage changes across the device when exposed to different liquids, the pH sensor can measure the acidity of liquids," said the university. "An ultraviolet nanosensor depends on similar voltage changes to detect when it is struck by ultraviolet light."
Image below
(a) Fabrication of a vertical-nanowire integrated nanogenerator (VING).
(b) Design of a lateral-nanowire integrated nanogenerator (LING) array.
(c) Scanning electron microscope image of a row of laterally-grown zinc oxide nanowire arrays.
(d) Image of the LING structure.
