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| An array on nano energy harvesters in what the researchers call a "swiss cheese" arrangement. |
A new type of nanoscale engine has been proposed that would use quantum dots to generate electricity from waste heat, potentially making microcircuits more efficient.
"The system is really a simple one, which exploits certain properties of quantum dots to harvest heat," Professor Andrew Jordan of the University of Rochester said. "Despite this simplicity, the power it could generate is still larger than any other nanoengine that has been considered until now." The engines would be microscopic in size, and have no moving parts. Each would only produce a tiny amount of power – a millionth or less of what a light bulb uses. But by combining millions of the engines in a layered structure, Jordan says a device that was a square inch in area could produce about a watt of power for every one degree difference in temperature. Enough of them could make a notable difference in the energy consumption of a computer.
A paper describing the new work is being published in Physical Review B by Jordan, a theoretical physics professor, and his collaborators, Björn Sothmann and Markus Buttiker from the University of Geneva, and Rafael Sánchez from the Material Sciences Institute in Madrid. Jordan explained that each of the proposed nanoengines is based on two adjacent quantum dots, with current flowing through one and then the other. Quantum dots are manufactured systems that due to their small size act as quantum mechanical objects, or artificial atoms.
The path the electrons have to take across both quantum dots can be adjusted to have an uphill slope. To make it up this (electrical) hill, electrons need energy. They take the energy from the middle of the region, which is kept hot, and use this energy to come out the other side, higher up the hill. This removes heat from where it is being generated and converts it into electrical power with a high efficiency.
To do this, the system makes use of a quantum mechanical effect called resonant tunneling, which means the quantum dots act as perfect energy filters. When the system is in the resonant tunneling mode, electrons can only pass through the quantum dots when they have a specific energy that can be adjusted. All other electrons that do not have this energy are blocked.
Quantum dots can be grown in a self-assembling way out of semiconductor materials. This allows for a practical way to produce many of these tiny engines as part of a larger array, and in multiple layers, which the authors refer to as the Swiss Cheese Sandwich configuration.
How much electrical power is produced depends on the temperature difference across the energy harvester – the higher the temperature difference, the higher the power that will be generated. This requires good insulation between the hot and cold regions, Jordan says.
Quantum Dots that Assemble Themselves
Scientists from the U.S. Department of Energy's National Renewable Energy Laboratory and other labs have demonstrated a process whereby quantum dots can self-assemble at optimal locations in nanowires, a breakthrough that could improve solar cells, quantum computing, and lighting devices.
A paper on the new technology, "Self-assembled Quantum Dots in a Nanowire System for Quantum Photonics," appears in the current issue of the scientific journal Nature Materials.
Quantum dots are tiny crystals of semiconductor a few billionths of a meter in diameter. At that size they exhibit beneficial behaviors of quantum physics such as forming electron-hole pairs and harvesting excess energy.
The scientists demonstrated how quantum dots can self-assemble at the apex of thegallium arsenide/aluminum gallium arsenide core/shell nanowire interface. Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire's center. That precision, combined with the materials' ability to providequantum confinement for both the electrons and the holes, makes the approach a potential game-changer.
Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that's the gallium-arsenide core. It's like hitting the bulls-eye rather than the periphery.
The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometers from the surface – a feature that even surprised the scientists.
"Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it," said NREL senior scientist Jun-Wei Luo, one of the co-authors of the study. Luo got to work constructing a quantum-dot-in-nanowire system using NREL's supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium Arsenide core, the thin aluminum-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminum-poor quantum dot. That explains the origin of the highly unusual optical transitions.
Several practical applications are possible. The fact that stable quantum dots can be placed very close to the surface of the nanometers raises a huge potential for their use in detecting local electric and magnetic fields. The quantum dots also could be used to charge converters for better light-harvesting, as in the case of photovoltaic cells.
Quantum Wells as High-Power, Easy-to-make Energy Harvesters
By collecting heat energy from the environment and transforming it into electrical power, thermoelectric energy harvesters have the potential to provide energy for a variety of small electronic devices. Currently, the biggest challenge in developing thermoelectric energy harvesters is to make systems that are both powerful and efficient at the same time.
One material that scientists have experimented with for making thermoelectric energy harvesters is quantum dots, nano-sized crystals with semiconducting properties. Due to their sharp, discrete energy levels, quantum dots are good energy filters, which is an important property for thermoelectric devices.
In a new study published in the New Journal of Physics, a team of researchers from Switzerland, Spain, and the US has investigated a thermoelectric energy harvester design based on quantum wells. Although quantum wells are also made of semiconducting materials, they have different structures and energy-filtering properties than quantum dots.
"We have shown that quantum wells can be used as powerful and efficient energy harvesters," said coauthor Björn Sothmann, a physicist at the University of Geneva in Switzerland. "Compared to our previous proposal based on quantum dots, quantum wells are easier to fabricate and offer the potential to be operated at room temperature."
The energy harvester design that the researchers investigated here consists of a central cavity connected via quantum wells to two electronic reservoirs. The central cavity is kept at a hotter temperature than the two electronic reservoirs, and the quantum wells act as filters that allow electrons of certain energies to pass through. In general, the greater the temperature difference between the central cavity and the reservoirs, the greater the electron flow and output power.
In their analysis, the researchers found that the quantum well energy harvester delivers an output power of about 0.18 W/cm2 for a temperature difference of 1 K, which is nearly double the power of a quantum dot energy harvester. This increased power is due to the ability of quantum wells to deliver larger currents compared to quantum dots as a result of their extra degrees of freedom.
Although the quantum well energy harvester has a good efficiency, the efficiency is slightly lower than that of energy harvesters based on quantum dots. The researchers explain that this difference occurs because of the difference in energy filtering: quantum wells transmit electrons of any energy above a certain level, while quantum dots are more selective and let only electrons of a specific energy pass through. As a result, quantum wells are less efficient energy filters.
Quantum well energy harvesters appear promising for applications. For one thing, they may be easier to fabricate than energy harvesters that use quantum dots, since quantum dots are required to have similar properties in order to achieve good performance, and there is no such requirement for quantum wells. In addition, the fact that they can operate at room temperature may make quantum well energy harvesters suitable for a variety of applications, such as electric circuits.
"The energy harvester can be used to convert waste heat from electric circuits, e.g. in computer chips, back into electricity," Sothmann said. "This way, one can reduce both the consumed power as well as the need for cooling the chip."
The researchers hope that their work encourages experimental groups to build and test the device.
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