Nanostructures that boost thermoelectrics

Thermoelectric generators directly convert heat into electrical energy. Unlike heat engines, their electrical components for energy-conversion have no moving parts and are not producing any greenhouse gas emissions. However, the relatively high cost and low efficiencies make thermoelectric converters suitable only for niche markets. With low thermal conductivity and high carrier mobility, semiconductor nanowires and their heterostructures in particular are very attractive candidates for energy conversion. Significantly improving thermoelectric properties should open up a number of new applications.

In the EU-funded 'Nanowire structures for energy conversion' (WISE) project, scientists developed novel semiconductor nanostructures with enhanced phonon scattering. WISE investigated the thermoelectric efficiency of homogeneous III-V semiconductor nanowires and heterostructures grown directly on silicon through various techniques.

An important achievement was a new technique of selective nanowire growth within nanotube templates that is catalyst free. The template was selectively filled by homo- and hetero-epitaxial nanowires, with their morphology entirely defined by its geometry. Compared to the metal- or self-catalysed nanowire growth, no intermixing of (hetero-)interfaces and non-intentional core-shell formation was observed.

Scientists fully characterised the electrical and thermal transport properties of indium arsenide (InAs) nanowires. Thermal conductivity was found to be about 20 to 30 times smaller compared to bulk InAs.

WISE successfully demonstrated that one-dimensional structures – nanowires – are efficient thermoelectric materials. They also developed techniques that could serve as a general monolithic integration path for III-V semiconductor-based electronic and optoelectronic devices on silicon.