What are the possible structural modifications that can be done in order to enhance these properties?
Do you mean zT, the figure of merit? It is determined by the electrical conductivity σ, thermopower S and the thermal conductivity κ, zT=S^2σT/κ. It is a tough work to optimize these parameters because they are inter-correlated.
To enhance it in nanostructures, people use specially engineered surface to reduce thermal conductivity (by selectively screening some phonon spectra). Or you may also notice the energy filtering strategy, which may stem from the surface band bending in nanostructure or grains, that helps to increase the thermopower but has also its tradeoff in electrical conductivity.
That's what I can figure out at present. If I am not right, please point me out.
There were three strategies devised to achieve higher ZT,
(i) creating point defects, vacancies or heavy ion rattlers in a partially filled structures to scatter thermal vibration to bring down thermal conductivity
It is important to note that while engineering the interface structure for thermal conductivity reduction, electron transport properties are affected 'negatively'. But it is possible to scatter more phonons than electrons due their wavelength differences.
(ii) using complex crystals to create a superconductor "like" regions to easily conduct electrons and a glass like regions with dopants and disordered crystals to prevent thermal flow without disturbing electron flow.
Here, increasing the reflectivity perpendicular to the interface by increasing the mismatch between the materials. Density, group velocity, specific heat and the phonon spectrum between adjacent layers are some of the important properties. Interface roughness causes diffuse phonon scattering that increase or decrease the phonon reflectivity at the interface. When the difference in dispersion increases phonon confinement increases more. However, amount not confinement is unknown except in few cases.
(iii) using uniform or non - uniform Nanocomposites.
Superlattices and quantum wells were proved to be good thermoelectric materials, but their production process is difficult and expensive. With SL the transport parameters can be manipulated and with this better understanding of thermoelectric phenomena in nanoscale is possible. Another interesting approach to reduce thermal conductivity is to localize long wavelength phonons with aperiodic superlattices or composite superlattices with different periodicities on addition to defects in low dimensional systems. In superlattices, thermal conductivity can be reduced with band structure changes.
Hope I answered your question...!
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