Nano-rectennas for harnessing infrared energy

Current world energy consumption is ~10 terawatts (TW) per year, and by 2050, it is projected to be about 30 TW. The terawatt challenge is the effort to supply up to 30 TW of carbon-free power by the mid-21st century, to stabilize CO2 in the atmosphere [1]. While all renewable resources (wind, hydroelectric, bio-mass, and geo-thermal) play important role, only solar can meet this level of demand. The sun continuously provides more energy to the planet in an hour than the world consumes in a year. Solar radiation spectrum ranges from a frequency of ~150-1000 THz, corresponding to a wavelength of ~2 - 0.3 m respectively. Over 85% of the radiation energy is contained in the 0.4 -1.6 m range. The energy reaching the earth in both the visible and infra-red (IR) regions and the reradiated IR energy are under-utilized by current technology [2].

Photovoltaic (PV) cells are currently the only mature technology for solar energy harvesting [3]. As a quantum device, the efficiency of PV cell is a function of the bandgap, and ultimately fundamentally limited by the match of the bandgap to the solar spectrum. The average efficiency of a solar PV module is between 6 and 15%. Hence 85-94% of the solar radiation is rejected as waste heat at about 333K. An alternative approach to photovoltaics is the rectenna concept, which is a combination of a receiving antenna and a rectifier.4 Antennas by themselves do not provide a means of converting the collected energy. This is accomplished by associated circuitry, i.e. rectifiers. The virtual large surface area antenna focuses the electromagnetic energy onto the nano-sized energy conversion material fabricated at the antenna feedpoint, which rectifies the alternating current (AC) field across the antenna to direct current (DC) power to an external load. The rectenna concept has been successfully demonstrated for microwave power transmission at 2.45 [4] and 5.8 GHz [5] with high efficiency of ~84%. The capture and conversion of solar energy >200 THz and therefore efficiency of rectennas in the infrared and visible regime, is currently limited by the lack of a diode nanostructure that can work at THz frequencies. High-frequency response of semiconductor diodes, like p-n junctions or Schottky, is limited by charge storage and parasitic capacitance respectively, making them inoperable for rectification at frequencies beyond 5 THz [6]. The practical rectifying mechanism used for high-frequency signals is based on a tunnelling effect of electrons having fast response times of the order of femtoseconds [7]. The device is based on Metal Insulator Metal (MIM) structure, where electrons tunnel from one metal electrode to the other through a thin (< 5 nm) insulator barrier. So far achievements in THz energy harvesting strongly point to non-optimized MIM design structures [8-10] being of less asymmetry and nonlinearity than desired, and with low conversion efficiency defined as THz power in/DC current out. A possible approach to achieve enhanced asymmetry is to change the conduction mechanism for one bias polarity to resonant tunnelling. This scenario can be accomplished by fabricating a multilayer dielectric nanostructure between the metallic electrodes instead of a single dielectric layer.

This project focuses on investigating various metal - double dielectric – metal as well as plasmonic nanostructures [11] (including VO2 insulator-to-metal transition metamaterials [12]) for efficient THz energy conversion. It will entail versatile research work from modelling, design, deposition to characterization and optimization of test nanostructures in superb inter-disciplinary team environment and with state-of-the-art Lab facilities. The project crosses the boundaries of solid state electronics, surface science, ultra-thin high-quality layers deposition technique and aggressive lithography tools for THz engineering, and hence it is aimed at attracting premium PhD students.

To apply for this opportunity, please visit: https://www.liverpool.ac.uk/study/postgraduate-research/how-to-apply/