Rectennas convert electromagnetic fields at optical frequencies directly to electrical current. Optical rectennas operate by coupling light’s electromagnetic field to an antenna. The electromagnetic field creates an oscillation in the antenna, producing an alternating flow of electrons. When the electron flow reaches a peak at one end of the antenna, the diode closes, trapping the electrons, then re-opens to capture the next oscillation, creating a current flow. This switching must occur at terahertz frequencies to match the light. The junction between the antenna and diode must provide minimal resistance to electrons flowing through it while open, yet prevent leakage while closed.
Improvements in optical rectennas have been realized by switching to airstable diode materials. The new device design — a combination of a carbon nanotube antenna and diode rectifier — could compete with conventional photovoltaic technologies for producing electricity from sunlight and other sources. The same technology used in the rectennas could also directly convert thermal energy to electricity.
The key is maximizing the number of electrons that get excited in the carbon nanotube and then having a switch that is fast enough to capture them at their peak. The faster the switch, the more electrons that can be caught on one side of the oscillation.
To provide a low work function — ease of electron flow — the researchers initially used calcium as the metal in the oxide insulator-metal diode junction. But calcium breaks down rapidly in air, meaning the device had to be encapsulated during operation and fabricated in a glovebox. That made the optical rectenna both impractical for most applications and difficult to fabricate.
The calcium was replaced with aluminum and a variety of oxide materials were tested on the carbon nanotubes before a bilayer material composed of alumina (Al2O3) and hafnium dioxide (HfO2) was chosen. The combination coating for the carbon nanotube junction, created through an atomic deposition process, provides the quantum mechanical electron tunneling properties required by engineering the oxide electronic properties instead of the metals, which allows air-stable metals with higher work functions than calcium to be used.
The rectennas could be useful for powering Internet of Things (IoT) devices, especially if they can be used to produce electricity from scavenged thermal energy. For converting heat to electricity, the principle is the same as for light — capturing oscillations in a field with the broadband carbon nanotube antenna.