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Harnessing ambient energy for IoT devices and smart dust networks with microscale rectennas
(Nanowerk Spotlight) The rapid proliferation of wireless devices and the Internet of Things (IoT) has been one of the defining technological trends of the 21st century. As billions of smart devices come online, powering them efficiently and sustainably has emerged as a critical challenge. Batteries have long been the go-to solution, but their limited lifetimes, environmental impact, and maintenance requirements pose significant drawbacks for large-scale IoT deployments.
For years, researchers have sought to address this issue by harvesting energy from ambient electromagnetic radiation, such as Wi-Fi, cellular networks, and Bluetooth signals. This approach, known as wireless energy harvesting, promises to provide a virtually unlimited power source for IoT devices while reducing reliance on batteries. However, progress has been hindered by the limitations of existing rectenna technologies, which convert alternating current (AC) from electromagnetic waves into usable direct current (DC).
Conventional rectennas consist of two separate components: an antenna to capture the electromagnetic signal and a rectifier to convert it to DC. Schottky diodes, the most common rectifiers, suffer from limited high-frequency performance and require a minimum input voltage, necessitating a large antenna to amplify the signal. This has made it challenging to miniaturize rectennas for low-power, high-frequency applications.
Recent advances in low-dimensional materials and novel device architectures have opened up new possibilities for wireless energy harvesting. Researchers have explored spin-torque diodes based on magnetic tunnel junctions and non-reciprocal Josephson junctions as potential rectifiers, but these approaches still face challenges in terms of fabrication complexity, temperature requirements, and the need for external magnetic fields.
Against this backdrop, a team of researchers from CIC nanoGUNE BRTA, AGH University of Krakow, and other institutions has made a significant breakthrough by demonstrating wireless energy harvesting at the microscale using chiral tellurium. Their work, published in Advanced Materials ("Microscale Chiral Rectennas for Energy Harvesting"), exploits the intrinsic nonlinear conductivity of tellurium to achieve efficient rectification without the need for a separate antenna.
Tellurium is a fascinating material with a non-centrosymmetric crystal structure that gives rise to unique electronic properties. Its chiral structure, comprising helical chains of covalently bonded atoms, lacks mirror symmetry, resulting in a nonlinear relationship between electric field and current along certain crystallographic axes. The researchers recognized that this inherent nonlinearity could be harnessed for wireless energy harvesting.
To explore this concept, they fabricated microscale devices with electrical contacts aligned to the material's chiral axes. Through a series of elegant experiments, they demonstrated that an incident electromagnetic wave polarized along the appropriate axis generates an asymmetric electric field within the tellurium, resulting in a rectified DC current.
Wireless RF rectification based on the non-linear conductivity in Tellurium
Wireless RF rectification based on the non-linear conductivity in Tellurium. a,b) A current density applied along the a) z-axis (jz) and b) x-axis (jx) generates a linear electric field along the z-axis (Ez) and a non-linear electric field along the x-axis (Ex), respectively. c,d) An incident RF wave aligned with the c) z-axis (Eincz) and d) x-axis (Eincx) generates a symmetric electric field along the z-axis (Ez) and an asymmetric electric field along the x-axis (Ex), respectively. The latter results in a DC current within the material. (Image: Reprinted with permissin by Wiley-VCH Verlag)
Remarkably, this rectification occurs with extremely low input powers, on the order of picowatts, and without the need for an external antenna to amplify the signal. The researchers showcased effective rectification in the gigahertz range, specifically between 5 and 5.3 GHz, which corresponds to the maximum emission frequency of the antenna used in their experimental setup. This is a significant advantage over Schottky diode-based rectennas, which are limited by a minimum input voltage threshold. By eliminating the antenna module, the researchers have paved the way for dramatically miniaturizing wireless energy harvesters, a crucial step towards integrating them into space-constrained IoT devices.
Notably, the chiral tellurium rectennas demonstrated a power conversion efficiency that surpassed previous reports for single-material devices. This highlights the material's potential for more efficient wireless energy harvesting compared to traditional technologies.
The team also demonstrated the ability to modulate the rectified output using an electrostatic gate, a first for a single-material rectifier. This gate-tunability adds an extra layer of control and flexibility to the energy harvesting process. Furthermore, by optimizing the device geometry and material properties, they achieved substantial improvements in rectification efficiency.
The implications of this work are far-reaching. By enabling efficient wireless energy harvesting at the microscale, chiral tellurium rectennas could power a wide range of IoT devices, from wearable sensors to smart dust networks. The elimination of bulky antennas and the ability to operate at low input powers make these devices particularly well-suited for applications where size, weight, and power consumption are critical factors.
Moreover, the demonstration of gate-tunable rectification in a single material opens up new avenues for adaptive and reconfigurable energy harvesting systems. As the IoT continues to evolve and expand, the ability to dynamically tune the performance of energy harvesters could prove invaluable in optimizing power generation across diverse operating conditions and frequency ranges.
Looking ahead, there is still work to be done to further enhance the efficiency and temperature stability of chiral tellurium rectennas. While the rectification signal diminishes as the temperature approaches room temperature, it remains observable, indicating the potential for further optimization. It's important to note that the experiments demonstrating efficient rectification were conducted at low temperatures, specifically at 10 K, which presents a challenge for practical applications. Improving the material's performance at higher temperatures will be crucial for real-world deployment.
As these improvements are realized and the technology matures, chiral tellurium rectennas could become a key enabling component for sustainable, battery-free IoT deployments. By harnessing the power of ambient electromagnetic radiation, they offer a compelling solution to the energy challenges facing the ever-growing universe of connected devices.
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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