David Sanchez from the Universidad de les Illes Balears in Spain and Heiner Linke from Lund University in Sweden, the guest editors of "Focus on Thermoelectric Effects in Nanostructures" recently published by New Journal of Physics, describe some of the advantages offered by nanostructures. In their editorial they point out that nanostructures have more to offer than improvements to the thermoelectric efficiency of a material quantified by the ZT factor and emphasize the benefits of "novel thermoelectric functionalities due to their lower dimensionality and the large variety of well characterized nanosystems at our disposal".

Tackling thermoelectric efficiency

That said, the improvements to ZT are significant. In 1993 when Lyndon Hicks and Mildred Dresselhaus published their pioneering research on the thermoelectric figure of merit for quantum-well superlattice structures, progress in enhancing the ZT factor of thermoelectric materials had been slow since the 1960s.

It was the interdependence of the various contributing factors that stalled efforts to improve the thermoelectric function of materials. A high figure of merit requires a high electrical conductivity, a high Seebeck coefficient (which quantifies the thermoelectric power) and low thermal conductivity, and improving one factor tended to adversely affect another. The genius of the work by Hicks and Dresselhaus was to recognize that the confinement of electrons in low-dimensional nanostructures and the increased phonon scattering at interfaces in a superlattice would provide a route around this impasse.

Quantum wells versus quantum dots

The work catalysed activity in thermoelectric studies of nanostructures that persists to this day. In fact a recent study reported in the New Journal of Physics focus collection shows that despite the advantages of quantum dots due to their sharp spectral features, quantum-well structures still compare favourably for energy harvesting. Rafael Sánchez at the Instituto de Ciencia de Materiales de Madrid, Andrew N Jordan from the University of Rochester, and Björn Sothmann and Markus Büttiker from the Université de Genéve demonstrate how their quantum-well device generates 0.18 W cm–2 for a temperature difference of 1 K, double the energy extracted from a similar heat engine based on quantum dots.

All the same, there are good reasons to expect that devices based on quantum dots will remain firmly rooted in the research interests of thermoelectric scientists for some time yet. In the same special issue an international collaboration of researchers led by Sofia Fahlvik Svensson at Lund University in Sweden and Eric Hoffmann at the Technische Universität München in Germany explore the nonlinear responses of quantum-dot systems and show how controlling these effects "expands the range in which quantum thermoelectric effects may be used for efficient energy conversion". Sánchez, Sothmann, Jordan and Büttiker also probe the performance of a quantum-dot-based thermoelectric engine by investigating the noise properties of electric charge, heat and currents and analysing the correlations between them.

The reports in the special issue are among the last pieces of research by Markus Büttiker who sadly died in November 2013. His active career brought several significant contributions to the understanding of conductivity, scattering and quantum-transport behaviour, and the contributions of his life’s work will continue to inform the studies of researchers in these fields for many years to come.

Real-world devices

Thermoelectric devices may not yet be central to the world’s energy resources, but early devices highlight their potential. Reporting in Nanotechnology Fang Fang Song, Liming Wu and Shengde Liang at Renmin University in China used their thermoelectric material to power an LED. The material was made from MnO2 powder and demonstrated a Seebeck coefficient 100 times greater than the "state-of-the-art of Bi2Te3".

Other examples exist such as a wearable glass fabric generator that can use naturally produced body heat to power small devices, reported by researchers at the Korean Advanced Institute of Science and Technology (KAIST) in Korea. Another tempting heat source for thermoelectric devices is the waste heat generated in computers, which would reduce the need for cooling systems while powering the device as well. With the improving thermoelectric performance of nanomaterials, the ready application of thermoelectric power for small devices and the current proliferation of energy-hungry mobile devices, it is possible that the contribution of thermoelectric devices may little by little provide a significant portion of the world’s energy needs.