Nano-harvesting unused signals like Wi-Fi into power for electronic devices

August 7, 2024

(Nanowerk News) We are constantly surrounded by electromagnetic waves such as Wi-Fi and Bluetooth signals. What if we could turn this unused excess into usable energy? Researchers from Tohoku University, the National University of Singapore, and the University of Messina have developed a novel technology to efficiently collect low-power radio frequency (RF) signals from the environment into direct current (DC). This “rectifier” technology can be easily integrated into energy harvesting modules to power electronic devices and sensors, enabling battery-free operation.

The results were published in Nature electronics (“Nanoscale spin rectifiers for harvesting radiofrequency energy from the environment”). Schematic illustration of a wireless network with energy harvesting modules. RF signals that are not used by electronic gadgets and would otherwise be wasted are used to generate usable DC current to power sensors and devices. (Graphics: Shunsuke Fukami and Hyunsoo Yang)

The collection and then conversion of ambient energy sources into usable energy is called “harvesting.” Small devices can harvest energy, which can reduce battery dependence, extend device life, and minimize environmental impact. Instead of physically traveling to devices in remote regions to constantly replace batteries, a device can be powered remotely from ambient energy sources, such as everyday RF wireless signals.

The disadvantage of this method is that the signal source usually has to be in close proximity to the electronic device in question. Existing technologies such as the Schottky diode face challenges in terms of low RF-to-DC conversion efficiency for weak ambient RF signals (typically below -20 dBm).

To address these challenges, the research team developed a compact and sensitive rectifier technology that uses a nanoscale spin-rectifier (SR) to convert ambient RF signals, which are less than -20 dBm, to a DC voltage. The SR consists of a nanoscale magnetic tunnel junction made of CoFeB/MgO, which is used in non-volatile memory technology.

The team optimized the SR devices, paying special attention to the magnetic anisotropy of the material, the device geometry, and the properties of the tunnel barrier. They then tested the RF-to-DC conversion performance of two configurations: 1) a single SR rectifier operating from -62 dBm to -20 dBm and 2) an array of 10 SRs. By integrating the SR device with an energy harvesting module, they were able to power a commercial temperature sensor at -27 dBm. (a) Schematic diagram of a spin rectifier device (magnetic tunnel junction) and its scanning electron microscope image. (b) Demonstration of energy harvesting. The generated output voltage by the spin rectifier array is connected via a capacitor to a DC-DC boost converter. The amplified output voltage by the converter powers a temperature sensor. The multimeter displays the output voltage of the spin rectifier array (24.1 mV), and the temperature sensor displays the room temperature (23.4 °C). (Graphics: Shunsuke Fukami and Hyunsoo Yang)

The researchers are currently investigating on-chip antenna integration to increase efficiency and compactness. The team is also developing series-parallel connections to tune impedance in large SR arrays, using on-chip connections to connect individual SRs. This approach aims to improve the way RF power is harvested. Exploring this technology could lead to the adoption of a self-sufficient, green alternative energy choice that could solve many problems in the future.