Researchers in Belgium have drawn up plans for an electronic “nanorefrigerator” device that is driven by high-energy photons, and could, therefore, potentially be directly powered by the sun. The device consists of two electrodes, one of which is cooled by replacing hot electrons with cool ones via photon absorption. While this is not the first system that applies the “cooling by heating” concept, it is the first that can be applied for a nanosized device, with no moving parts or electrical input, allowing a lower temperature to be achieved at the nanoscale.
Bart Cleuren and colleagues at Hasselt University, Belgium have proposed a solid-state device that would potentially use solar energy directly to cool. The electrode to be cooled is electrically connected to another electrode that is much warmer. Between the two electrodes are two quantum dots that act as a highway for electrons to move from one electrode to another. Quantum dots are tiny semiconductor structures where electrons are confined in all three dimensions. They have electronic and optical properties that can be controlled by adjusting the shape and size of the structures. In this work, the adjoining quantum dots each have a lower and higher energy level, such that each dot could hold an electron at any given time in one of the levels.
The researchers then suggest using high-temperature photons – solar radiation is generally at a temperature of about 5800 K – to make cold electrons flow from the warm electrode to the cool one, and vice versa for the warm electrons in the opposite direction.
To create this flow, a cold electron from the warm electrode would hop into the lower energy level of the first dot, where it must absorb a photon for it to jump to the lower energy level of the second dot. From there, the cold electron can make its bid for freedom to the cold electrode, where it would further decrease the temperature of that electrode. Hot electrons would make a similar trip in the opposite direction, hopping across the higher energy level of the dots instead. This net circulation of electrons would allow one of the electrodes to be efficiently cooled.
The researchers say that one can imagine that the electrons are “evaporating” out of the cold object and “condensing” in the warm object, somewhat like an absorption refrigerator.