The new electronic 2-D technology can use radio energy to power a range of devices such as hearing aids, sensors and other devices that make up the Internet of Things
A flexible, flat semiconductor material that can extract energy from radio signals in cities could be just the thing for a new generation of electronics.
A team from the Massachusetts Institute of Technology reports in Nature that a film of molybdenum disulphide (MoS2) - a two-dimensional material because it's only three atoms thick - can act as an antenna to transmit radio signals from Wi-Fi, cell phones and radio or radio Radios convert television broadcasts to power for wireless devices. It could power energy-efficient pacemakers, hearing aids, and Internet of Things (IoT) sensors.
The power supply would not be enough to charge cell phones and tablets without dramatic breakthroughs, and even the Fitbit is a bit far away. But a small step towards Wi-Fi power can be at hand. "The future of electronics brings intelligence to every single object, from our clothing to our desks and our infrastructure," says Tomás Palacios, professor of electrical engineering at MIT. "The key missing component is how all these billions of devices can be powered." MoS2 sheets are promising because they are flexible and can be inexpensively produced in roll-to-roll printing.
The group demonstrated a flexible material that can cover radio energy at frequencies up to 10 gigahertz. It covers the widely used 2.4- and 5-gigahertz bands that transmit Wi-Fi signals as well as other radio traffic. Flexibility is important for portable devices and many other sensor applications, but other flexible materials generally absorb little radio power at frequencies above 1.6 gigahertz, limiting their potential for power generation.
According to Palacios, the two-dimensional semiconductor can generate 30 to 50 microwatts of approximately 100 microwatts of WLAN ambient signals, which is sufficient to power pacemakers, hearing aids, strain sensors, communication links, and many low-power IoT objects. Such a system could potentially operate without battery, which lowers weight and prevents leakage of the energy source of a medical implant in the body.
The MIT work "is an important first demonstration of energy production from wireless ambient signals. This is all the more convincing as everything is integrated on the same flexible substrate, "says Deji Akinwande, electrical and computer engineer at the University of Texas at Austin, who was not involved in the work. "The next challenge is to scale the devices to get the higher performance needed for today's mobile applications."
Energy harvesting systems already power other remote devices to prevent frequent battery changes. Most current systems draw power from light, temperature differences, or kinetic movements, says Andreas Schneider, CEO of EnOcean, a German manufacturer of self-powered, batteryless devices that was not involved in the research. He says photovoltaic cells can provide enough light energy to provide indoor lighting with a 100 lux illuminance, which is about the same as corridor illuminance and less than one-third of office normal illuminance.
Pressing a mechanical switch can generate enough energy to send a signal to turn on a lamp in the room or up the stairs. Temperature gradients along hot water lines can send signals to the heating system. However, the company found that the power of the ambient radio signal is insufficient for current equipment unless additional local radio transmitters have been added. Schneider says, "They do not want to sit next to one another", as they fear that electromagnetic fields could reach unhealthy levels.
Looking ahead to the future of 5G wireless networks and the Internet of Things, Palacios says, "You might be able to use solar cells, but you only have sunlight during the day. The other possibility is to gain energy already present in radio frequency signals, "like Wi-Fi, which transmits most of the time".
Both Radio Energy Harvester and Radio Signal Receiver collect energy when radio waves interact with antennas. Electromagnetic forces reciprocate electrons in the conductive material and induce an electric current that changes in direction as the waves alternate in phase. Antennas that collect signals for radio receivers transmit the fluctuating signals to circuits that amplify and convert them into audio or video frequencies. Antennas that receive radio-frequency energy send the fluctuating power to an electronic device, a so-called rectifier, which transmits power in one direction only and converts the incoming alternating current into direct current that powers electronic devices or charges batteries.
Rectifiers are typically semiconductors, while antennas are typically metallic and highly conductive. Molybdenum disulfide is "a really good semiconductor," says Palacios. It can be modified to have high conductivity so it can serve as both an antenna and a rectifier - a device called Rectenna invented in the 1960s and now in RFID and proximity cards ( Radio Frequency Identification) is used.
Today, most rectennas are small, rigid chips of inflexible semiconductors like silicon, which have good frequency response but are limited by their inflexibility. The MIT group is the first to produce large, flexible rectennas that can extract energy from widespread, unlicensed radio frequencies of up to 10 gigahertz without the need for battery voltage to trigger the process. Flexibility and thinness are important for use in portable devices and "smart skins" that can be applied to infrastructure, aircraft, or other objects for continuous monitoring or as part of a distributed network of smart sensors. Layers as thin as three atoms can be grown on a large area at low cost by a process widely used in semiconductor fabrication - chemical vapor deposition - and still operate at very high frequencies.
The technology is still in the lab. The production has to be increased and the films have to be integrated into the devices that supply them with electricity. Another challenge will be to develop devices that can only run at ten microwatts. However, Palacios expects the first commercial applications in five to seven years. "The most important thing you need is scaling, which allows us to produce low-cost sensors over a very large area," he notes. Other applications he expects include lighting small displays by delivering 30 to 50 microwatts of power and linking implantable medical devices to external monitors. At least for a few applications, energy can really be gained from the air.
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