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John O | February 2019

Researchers use flexible 2-D materials to build device that converts Wi-Fi signals to electricity

By Josh Perry, Editor


Researchers from the Massachusetts Institute of Technology (MIT) in Cambridge, Mass. created a fully-flexible device that converts the energy from Wi-Fi signals into electricity and, according to a report from the school, could be used to power electronics such as mobile devices and wearables.


Researchers from MIT and elsewhere have designed the first fully flexible, battery-free rectenna. (Christine Daniloff/MIT)


The scientists built a new rectenna, which converts AC electromagnetic waves into DC electricity, with a flexible radio-frequency (RF) antenna to capture the waves and a conversion device made from a 2-D semiconductor material.


“In this way, the battery-free device passively captures and transforms ubiquitous Wi-Fi signals into useful DC power,” the report explained. “Moreover, the device is flexible and can be fabricated in a roll-to-roll process to cover very large areas.”


Experiments demonstrated the device can create 40 microwatts of power through standard Wi-Fi signals (around 150 microwatts). The report noted that this power could be used for an LED or to power silicon chips. Researchers believe this technology could be used in wearables, medical devices, or even flexible smartphones.


“All rectennas rely on a component known as a ‘rectifier,’ which converts the AC input signal into DC power,” the article continued. “Traditional rectennas use either silicon or gallium arsenide for the rectifier. These materials can cover the Wi-Fi band, but they are rigid. And, although using these materials to fabricate small devices is relatively inexpensive, using them to cover vast areas, such as the surfaces of buildings and walls, would be cost-prohibitive.”


MIT researchers turned to molybdenum disulfide (MoS2) to build their rectifier. It is only three atoms thick. The material’s primary benefit is that when exposed to certain chemicals it transitions from semiconductor to metallic material. “The resulting structure is known as a Schottky diode, which is the junction of a semiconductor with a metal,” the article added.


The diode minimizes series resistance and parasitic capacitance, which means it operates at higher speeds and is capable of converting 10 gigahertz of wireless signals. Its efficiency is only about 30 percent, which is nearly half that of standard rectifiers, but establishes proof of concept.


The research was recently published in Nature. The abstract stated:


“The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics. Their atomic thickness and large-scale synthesis capability could enable the development of ‘smart skin’, which could transform ordinary objects into an intelligent distributed sensor network.


“However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics.


“However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting–metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers.


“This flexible MoS2-based rectifier operates up to the X-band (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free).


“Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems.”

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