Device Made from 2D Material Could Power Wearables, Large-Area Electronics, and Medical Devices

Picture a world where a range of electronics, including laptops, smartphones, and wearables, run without batteries.

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.” (Image credit: Christine Daniloff)

Taking a step in that direction, scientists from MIT and elsewhere have developed the world’s first fully flexible device that is capable of converting the energy released from Wi-Fi signals into electricity, which, in turn, could be used to power electronics.

“Rectennas” are devices that transform AC electromagnetic waves into DC electricity. The researchers have reported a novel type of rectenna that utilizes a flexible radio-frequency (RF) antenna. The RF antenna captures electromagnetic waves, including those that carry Wi-Fi, as AC waveforms. The team has described this latest breakthrough in a study published in Nature.

The RF antenna is subsequently linked to a new device fabricated from a two-dimensional (2D) semiconductor that has a thickness of just a few atoms. When the AC signal passes inside the semiconductor, it is converted into a DC voltage by the semiconductor; this DC voltage can be used for powering recharge batteries or electronic circuits.

In this fashion, the ever-present Wi-Fi signals are passively captured and converted into useful DC power by the battery-free device. In addition, the device is also flexible and can be produced in a roll-to-roll process to conceal extremely large areas.

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.

Tomás Palacios, Study Co-Author and Professor, Department of Electrical Engineering and Computer Science, MIT.

Palacios is also the director of the MIT/MTL Center for Graphene Devices and 2D Systems in the Microsystems Technology Laboratories.

The proposed rectenna could be used for powering wearable and flexible electronics, sensors, and medical devices for the “internet of things.” For example, flexible smartphones are a new, trending market for top technology companies. In their experiments, the researchers observed that their device has the potential to generate approximately 40 µW of power upon exposure to the normal power levels of Wi-Fi signals (up to 150 µW). That power is more than sufficient to light up drive silicon chips or an LED.

Fueling the data communications of implantable medical devices is another potential application of the device, stated Jesús Grajal, co-author of the study and a researcher at the Technical University of Madrid. For instance, researchers are starting to create pills that can be ingested by patients and can transfer health data back to a computer for diagnostic purposes.

Ideally you don’t want to use batteries to power these systems, because if they leak lithium, the patient could die,”. “It is much better to harvest energy from the environment to power up these small labs inside the body and communicate data to external computers.

Jesús Grajal, Study Co-Author and Researcher, Technical University of Madrid.

“Rectifier”—a component relied upon by all rectennas—transforms the AC input signal into DC power. Conventional rectennas either utilize gallium arsenide or silicon arsenide for the rectifier. Although such materials can cover the Wi-Fi band, they tend to be rather stiff. In addition, while these materials can be used to develop tiny devices in a relatively cheaper manner, using them to conceal huge areas, like the surfaces of walls and buildings, would be extremely costly. As such, investigators have long been trying to solve these issues, but the few flexible rectennas that have been reported thus far can function only at low frequencies and are not capable of capturing and changing signals in gigahertz frequencies, where the majority of the pertinent Wi-Fi and cellphone signals exist.

In order to develop their rectifier, the team applied a new kind of 2D material known as molybdenum disulfide (MoS2) that is only three atoms thick, making it one of the world’s thinnest semiconductors. In doing so, the researchers exploited an extraordinary behavior of MoS2—upon exposure to specific chemicals, the atoms of the material reorganize in a way that behaves similar to a switch, causing a phase transition from a semiconductor to a metallic material. The structure, thus obtained, is called a Schottky diode—the junction of a semiconductor with a metal.

By engineering MoS2 into a 2D semiconducting-metallic phase junction, we built an atomically thin, ultrafast Schottky diode that simultaneously minimizes the series resistance and parasitic capacitance.

Xu Zhang, Study First Author and Postdoc, Department of Electrical Engineering and Computer Science, MIT.

Zhang will soon join as an assistant professor at Carnegie Mellon University.

An inevitable situation in electronics is parasitic capacitance, where some materials store a small amount of electrical charge, slowing down the circuit. Hence, lower capacitance means higher operating frequencies and increased rectifier speeds. The parasitic capacitance of the Schottky diode developed by the researchers is an order of magnitude smaller than the present-generation of sophisticated flexible rectifiers, and hence it is extremely faster at signal conversion and enables the diode to capture and transform around 10 GHz of wireless signals.

Such a design has allowed a fully flexible device that is fast enough to cover most of the radio-frequency bands used by our daily electronics, including Wi-Fi, Bluetooth, cellular LTE, and many others,” stated Zhang.

The latest study offers blueprints for other flexible Wi-Fi-to-electricity devices that have considerable efficiency and output. For the current device, the maximum output efficiency is 40%, based on the input power of the Wi-Fi input. At the normal Wi-Fi power level, the MoS2 rectifier’s power efficiency is approximately 30%. For reference, present-day rectennas, which are made from stiff and more costly gallium or silicon arsenide, can achieve a power efficiency of around 50% to 60%.

Fifteen other paper co-authors are available from MIT, the Army Research Laboratory, Technical University of Madrid, Boston University, Charles III University of Madrid, and the University of Southern California.

Currently, the researchers are intending to develop more intricate systems and enhance efficiency. The study was partly made possible by an association with the Technical University of Madrid via the MIT International Science and Technology Initiatives (MISTI). It was also supported in part by the Institute for Soldier Nanotechnologies, the National Science Foundation’s Center for Integrated Quantum Materials, the Army Research Laboratory, and the Air Force Office of Scientific Research.

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