28-02-2019 | | By Rob Coppinger
Devices could be powered from WIFI signals using a two-dimensional semiconductor, a few atoms thick, which can turn the transmission’s alternating current (AC) into direct current (DC) voltages.
The AC converting semiconductor, called a rectifier, has been made of silicon or gallium arsenide to date. They are rigid, limiting their applications and where they can be located for WIFI energy capture. There are flexible rectifiers, but they have had the limitation of not being able to capture the gigahertz signal frequencies used by WIFI. The two-dimensional semiconducting-metallic phase junction rectifier, which is a few atoms thick, is made of molybdenum disulphide. It could be manufactured for large arrays and still be flexible for whatever surface it must cover.
“The next step is to scale up. It will still be molybdenum disulphide. We will use chemical vapour deposition to make this molybdenum disulphide in large scale, to make large scale rectennas, to build arrays of rectennas,” says Xu Zhang, an assistant professor who is now at Carnegie Mellon University. “The key is to make the antenna flexible so it can be integrated with anything. This work shows it is possible to demonstrate with high enough frequency to cover the WIFI band and show pretty good efficiency,” he adds. While using the WIFI signal for power, any device can still receive the data being transmitted.
Researchers from MIT and elsewhere have designed the first fully flexible, battery-free “rectenna” — a device that converts energy from Wi-Fi signals into electricity — that could be used to power flexible and wearable electronics, medical devices, and sensors for the “internet of things.” Credit: Christine Daniloff
The molybdenum disulphide semiconducting-metallic phase junction is only three atoms thick making it eligible to be called a two-dimensional object. This rectifier is known as a Schottky diode, which is able to switch very fast with a smaller drop in voltage than other diode designs. The maximum output efficiency for the 2D Schottky diode stands at 40%, depending on the power of the WIFI input. The silicon or gallium arsenide rectifiers have an output efficiency of up to 60% but are rigid.
A challenge for the rectenna is parasitic capacitance, which is where a structure, material retains a small amount of the energy, diminishing the efficiency. The 2D design of the Schottky diode minimises series resistance and parasitic capacitance. Lower capacitance means the rectifier will be faster and more efficient for the rectenna. Zhang explains that when scaling up the rectenna technology, the parasitic capacitance will still need to be minimised. The scaled-up rectifier array’s design and the use of the chemical vapour deposition process is expected to aid that minimisation.
The research, which started in 2016, was partially supported by the United States’ Institute for Soldier Nanotechnologies, the Army Research Laboratory, the US government’s National Science Foundation’s Center for Integrated Quantum Materials, and the US Air Force Office of Scientific Research. The research was carried out at the Massachusetts Institute of Technology (MIT), Boston University, and the University of Southern California. Through the MIT International Science and Technology Initiatives, the Technical University of Madrid was also involved.