The effect of nanostructuring on the spectral population of electrons and phonons

A detailed analysis of the spectral population of electrons and phonons in nanostructured materials is presented. It is found that, in addition to the general notion that in nanostructured materials the electrons and phonons mean free paths (MFP) reduce, their populations per mean free path, however, may increase in some ranges of the MFP in some materials. In such cases, the electrical conductivity for a given electron MFP can increase upon nanostructuring; however, the corresponding thermal conductivity does not change significantly. These new understandings can elucidate why nanostructuring does not considerably enhance the thermoelectric figure of merit of some materials. In order to demonstrate these implications, the thermal and electrical properties of three sample materials with different trends were studied and compared. The results are presented in a form that can be validated by the existing thermal conductivity spectroscopy techniques.