The methods currently used for the fabrication of thermoelectric materials either produce really small samples, or such large bulk pieces that they are challenging to tailor at the nanoscale. ALD, on the other hand, can offer nanoscale tailoring of materials that can be deposited over very large areas. Researchers at Aalto University use ALD to create precise nanostructures by depositing the films on track-etched polycarbonate membranes.

The process results in nanotubules that extend through the membrane and the thin film that covers both surfaces of the membrane. As a result the nanostructures show very low out-of-plane thermal conductivity and high in-plane electrical conductivity, as well as sustaining the thermoelectric character of the thin film. Accordingly, the thermal properties of the nanocomposites resemble that of polycarbonate, whereas the electrical behaviour stems from the nanofabricated thin film. In-plane electrical properties of the nanostructures exceed that of films on glass due to the opportunity to utilize both sides of the substrate, suggesting opportunities in applying similar techniques for flexible electronics.

The concept allows fabrication of materials with complex three-dimensional geometries over large-areas and simultaneous tailoring of the nanoscale composition of the material. The work has significance both for basic research as well as a number of applications. The conformal coatings may be utilized to study the properties of thin films, or for studying the various concepts that have been proposed for enhancing thermoelectric performance. For applications, the work paves the way for template-based fabrication for thermoelectric sensors and energy harvesters.

ZnO was chosen as the material to demonstrate the concept as it is a well-studied material in ALD research. However, the concept is directly applicable for higher-performance thermoelectric materials, such as those that are introduced in the full Nanotechnology article.

For more of the latest ALD research visit the Nanotechnology focus collection Atomic layer deposition (ALD) in energy, environment, and sustainability.