Researchers from Massachusetts Institute of Technology (MIT) have designed a ‘rectenna’ and conducted the first real application of atomically thin semiconductors for a flexible rectenna for energy harvesting. A device made from inexpensive materials and is fully flexible and battery free. Most importantly, rectena will convert Wi-Fi signals into electricity which could be used to power large area of electronics like, medical devices, and sensors for the internet of things.
What is rectenna?
The device that converts Alternating Current (AC) electromagnetic waves into Direct Current (DC) is known as rectenna. It is combination of antenna and rectifier. The researchers demonstrated a new rectenna that uses a flexible radio-frequency (RF) antenna which catches electromagnetic waves including those carrying Wi-Fi as AC waveforms. Instead of diode to rectify that current to DC, antenna is connected to a device made up of two-dimensional semiconductors which are only a few atoms thick. Thus the AC signal from antenna travels into the semiconductor which converts it into dc voltage. This voltage could be used to power wearables, sensors, various electronics, or medical devices.
Tomás Palacios, a professor in the Department of Electrical Engineering and Computer Science and director of the MIT/MTL Center for Graphene Devices and 2D Systems in the Microsystems Technology Laboratories, also co-author of the paper said, “What if we could develop electronic systems that we wrap around a bridge or cover an entire highway, or the walls of our office and bring electronic intelligence to everything around us? How do you provide energy for those electronics? We have come up with a new way to power the electronics systems of the future—by harvesting Wi-Fi energy in a way that’s easily integrated in large areas—to bring intelligence to every object around us.”
Unique, inexpensive Rectenna, made up of 2D material
In conventional rectennas, silicon or gallium arsenide is used for the rectification. These materials can cover the Wi-Fi band but are rigid and if considered to use it over a large area such as the surfaces of buildings and walls, would be much expensive.
These new rectennas are made up of a 2D material known as molybdenum disulfide (MoS2) which at three atoms thick is one of the thinnest semiconductors. The team carried out certain experiments and found that when exposed to certain chemicals, the material’s atoms rearrange in a way that acts like a switch. These forces a phase transition from semiconductor to metallic material. The junction of a semiconductor with metal is known as Schottky diode.
The team engineered MoS2 into a 2-D semiconductor metallic phase junction and built an atomically thin and ultrafast Schottky diode that could simultaneously minimize the series resistance and parasitic capacitance.
Reduction in the parasitic capacitance is one of the fascinating properties of MoS2. Simply, Reduction is the tendency of materials in electric circuit to act like capacitors and store a certain amount of charge. This slows down the circuit. MoS2 provides lower capacitance hence increase rectifier speed and higher operating frequencies. The parasitic capacitance of this Schottky diode is so small and thus is much faster at signal conversion and can capture and convert signal upto 10 GHz including the range of typical Wi-Fi devices.
This kind of design has allowed a fully flexible device that is fast enough to cover the most of the radio-frequency bands used by daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others.
This way a battery free device inertly captures and transforms omnipresent Wi-Fi signals into DC power.
While performing an experiment the researchers’ device produced about 40 microwatts of power when it was exposed to the typical power levels of Wi-Fi signals around 150 microwatts. This is more than enough power to light up a simple mobile display or light up a LED or silicon chips.
Use in implantable medical devices
Another possible application of the technology is powering data communication of implantable medical devices. For instance, researchers are about to develop pills that can be swallowed by patients and stream health data back to a computer for diagnostics. Moreover, Jesús Grajal, the researcher at the Technical University of Madrid and co-author said that the tradition batteries used to power systems could leak lithium and patient could die. So it is much better to produce energy from the environment to power up the small labs inside the body and communicate data to external computers.
Teamwork makes it happen
This research was made possible by the association with the Technical University of Madrid through the MIT International Science and Technology Initiatives (MISTI). It was moderately supported by the Institute for Soldier Nanotechnologies, the National Science Foundation’s Center for Integrated Quantum Materials, the Air Force Office of Scientific Research, and the Army Research Laboratory.
There are co-authors from MIT, the Army Research Laboratory, Technical University of Madrid, the University of Southern California, Charles III University of Madrid, and Boston University.
At typical Wi-Fi power level, MoS2 rectifier’s power efficiency is about 30 percent, while traditional rectennas like costly silicon and gallium arsenide can achieve around 50 to 60 percent. Depending upon the power of Wi-Fi input the maximum output efficiency of MoS2 rectifiers stands at 40 percent. The team’s next motto is to improve the efficiency and build more complex systems. Current disadvantages will be compensated by the other benefits of the new design, including its flexibility and scalability. Further the inventions promises to power electronics system of the future by producing Wi-Fi energy in much easier ways and that could be integrated in large areas to bring intelligence to every object.
- Xu Zhang, Jesús Grajal, Jose Luis Vazquez-Roy, Ujwal Radhakrishna, Xiaoxue Wang, Winston Chern, Lin Zhou, Yuxuan Lin, Pin-Chun Shen, Xiang Ji, Xi Ling, Ahmad Zubair, Yuhao Zhang, Han Wang, Madan Dubey, Jing Kong, Mildred Dresselhaus and Tomás Palacios. Two-dimensional MoS2-enabled flexible rectenna for Wi-Fi-band wireless energy harvesting. Nature, 2019 DOI: 10.1038/s41586-019-0892-1