Condensed-Matter Physics & Materials Science Seminar

Independent control over electrical and thermal properties is one of the central goals in the search for efficient thermoelectric materials for waste heat energy conversion [1]. Nature, however, does not seem to favor this dissociation, and the best synthesized thermoelectrics in use today remain at about the same efficiency they had 50 years ago. Nanostructuring offers the opportunity to engineer materials at length scales where phonon heat conduction is affected much more than charge transport [2,3]. Certain nanostructured thermoelectrics more than double the efficiency of commercial devices, mostly as a result of increased boundary scattering, and therefore large thermal conductivity reduction, in high surface-to-volume ratio geometries [4]. Though this is a great leap for thermoelectrics, the efficiency still falls short of what is needed for a viable renewable energy technology. Further size reductions are not expected to give better results because the capacity of the material to convert heat becomes adversely affected. In fact, the attempts to lower thermal conductivity by increasing incoherent phonon scattering mechanisms may have already given us their best.
In this talk I will show that nanostructures can also enable coherent control of heat transport [5]. In our silicon nanomeshes, which are thin films patterned into a regular array of holes approximately 20 nm in diameter and 20 nm apart, we measured thermal conductivity a factor of two lower than expected from all size and material considerations. I will argue that even though thermal-phonon wavelengths are an order of magnitude shorter, the nanomesh is tailored at the right mesoscopic scale to behave as a phononic metamaterial. Phonons are forced to incorporate the periodicity of the superstructure into an altered phonon dispersion with markedly lower group velocities [5, 6]. The effect is significant even at room temperature. Furthermore, the nanomesh passes the requirement of preserving electrical p