MaNEP e-Newsletter MaNEP Nr. 28
Scientific Highlight – p1

Energy harvesting: Converting waste heat into electricity at the nanoscale
by Markus Büttiker and Björn Sothmann
After an article published in Physical Review B

The efficient use of available energy sources is of increasing importance. Of interest are energy harvesters which can convert waste energy from the environment into useful work.  A particular class of such harvesters consists of thermoelectric devices that convert waste heat, e.g., in a computer chip, back into electricity. Therefore, a key challenge in material science is to find thermoelectric materials with large conversion efficiency. Despite decades of research in this area, there has only been very slow progress. Mesoscopic solid-state physics might help to overcome the limitations of current thermoelectric devices. Quantum dots - nanometer-sized, artificially made structures - in the Coulomb-blockade regime are known to be highly efficient heat-to-current converters. Unfortunately, they only deliver very little power.

In a recent publication in Physical Review B, Björn Sothmann and Markus Büttiker from the University of Geneva together with their collaborators Andrew Jordan from the University of Rochester and Rafael Sánchez from the Instituto de Ciencia de Materiales de Madrid propose a new type of nanoscale energy harvester that is both highly efficient and powerful. The device is based on two adjacent quantum dots that connect a central hot region to two cold reservoirs. Due to a quantum mechanical effect called resonant tunneling, the dots act as energy filters, i.e., they only allow electrons of particular energy to pass through. When the dot properties are tuned appropriately, electrons can leave the system at an energy that is higher than the energy at which they entered. The energy needed to move up the (potential) hill is taken from the central hot region.

While a single such heat engine still delivers a rather tiny power, combining many of them in parallel can give enough energy to power a light bulb. A simple way to achieve this scaling consists of using self-assembled quantum dots in what the authors term a Swiss Cheese Sandwich configuration (see image). One square inch of such a structure can deliver one Watt of power for each degree of temperature difference between the hot and the cold part of the system. Importantly, due to the large level-spacing in self-assembled quantum dots, the device can even operate at room temperature. Furthermore, the authors demonstrate that its performance is robust with respect to fluctuations inherent to any fabrication process.

Schematics of nanoscale energy harvester

Swiss cheese sandwich

[page 2]

- 4 -