4. Nanoscale semiconducting silicides as efficient thermoelectric materials.

Thermoelectric (TE) materials convert heat to electricity and can improve energy efficiency by harvesting waste heat for power generation or improve refrigeration. The performance of TE materials needs to be improved beyond the current state-the-art as benchmarked by the TE figure of merit (ZT) value of 1 for doped Bi2Te3. Dimensional reduction of semiconductors enhances the ZT relative to the bulk values due to surface phonon scattering and/or quantum confinement. Many semiconducting silicides are known as robust, stable, and inexpensive thermoelectric materials, with the most notable being MnSi1.8 and ReSi1.8 that have reported ZT values of 0.7 to 0.8. These silicides are really homologous family of compounds with uniquely complex crystal structures that belong to the “Nowotny chimney ladder” (NCL) phases with one lattice constant as large as tens of nanometers (Fig. 4A). Phonon confinement will occur more readily in nanostructures of these NCL structures with a level of complexity rarely observed in common semiconductors, which together with surface roughness should result in pronounced reduction of the lattice thermal conductivity. If other properties are maintained for single crystal NWs made of NCL silicides with controlled doping, their ZT may be enhanced from the already high bulk values to well above 1 and that of many bulk and nanostructured TE materials.

The general silicide nanowire synthesis we have developed enabled us to make the first nanowire of any Nowotny chimney ladder phase, a higher manganese silicide MnSi1.8 using the SSP Mn(CO)5SiCl3 and structurally characterize it to be Mn19Si33 (Fig. 4B). We have been collaborating with Prof. Li Shi from U. of Texas-Austin, who specializes in TE measurements of 1-D nanomaterials, to measure the TE properties of silicide NWs (Fig. 4C). Our collaboration could open a new direction in TE materials research that combines the complex structure approach and the nanomaterials route and generate a wealth of new fundamental insights and make potential impact on TE energy conversion.

We are also developing unconventional synthetic pathways to bulk quantity nanostructured silicide and silicon materials to enable the practical applications of these nanomaterials in high performance thermoelectrics.

Figure 4. A) Nowotny chimney ladder structures of MnSi1.8;
B) HRTEM of Mn19Si33 NWs and C) TE investigation of the silicide NWs.