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The production of nanostructured bulk materials from silicon powders has been well documented as being one way of bringing down the thermal conductivity of silicon while still maintaining its high power factor. This reduction of thermal conductivity is predicted to lead to significant increases in its figure-of-merit, ZT. The size of the starting particles has a major effect on the nanostructuring and grain size of the final silicon-based materials. Using particles of differing size and distribution, pellets were produced using spark plasma sintering. The results show a significant lowering in the thermal diffusivity as the particle size in the powders is decreased. As the starting particle size deceases from 1 lm to 60 nm, we see a tenfold decrease in the thermal diffusivity at 300 K, from 20 mm 2 S À1 to 2 mm 2 S À1 . Both these show a significant decrease from the thermal diffusivity of 88 mm 2 S À1 observed from bulk silicon. A further decrease to 1 mm 2 S À1 is observed when the particle size of the starting material is decreased from 60 nm to sub-10 nm. The results also highlight the potential of using particles from solution approaches as a potential starting point for the prediction of nanostructured bulk materials.

Silicon nanoparticles (SiNPs) functionalized with conjugated molecules are a promising potential pathway for generating an alternative category of thermoelectric materials. While the thermoelectric performance of materials based on phenylacetylene-capped SiNPs has been proven, their low conductivity is still a problem for their general application. A muon study of phenylacetylenecapped SiNPs was recently carried out using the HIFI spectrometer at the Rutherford Appleton Laboratory, measuring the avoided level-crossing spectra as a function of temperature. The results show a reduction in the measured line width of the resonance above room temperature, suggesting an activated behaviour for this system. This study shows that the muon study could be a powerful method for investigating microscopic conductivity of hybrid thermoelectric materials.

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Phenylacetylene capped silicon nanoparticles (Phenyl-SiNPs) have attracted interest as a novel thermoelectric material. Here we report a combined muon spectroscopic (µSR) and computational study of this material in solution to investigate microscopic electronic structure in this system. For comparison, the model molecular compound tetrakis (2-phenylethynyl) silane has also been investigated. µSR measurements have shown that the muon isotropic hyperfine coupling constant, Aµ, which depends on spin density at the muon, is greatly reduced for the Phenyl-SiNPs system when compared to the model compound. Results have also demonstrated that the temperature dependence of Aµ for the Phenyl-SiNPs is of opposite sign and proportionally larger when compared to the model compound. Ab initio DFT methods have allowed us to determine the muon addition site in the model compound, while a wider computational study using both DFTB+ and CASTEP offers a qualitative explanation for the reduced coupling seen in the Phenyl-SiNPs system and also the contrasting temperature dependence of Aµ for the two materials. Calculations suggest an increase in the density of electronic states at the energy level of the highest occupied molecular state for the Phenyl-SiNPs, even in the presence of an organic cap, suggesting a mechanism for enhanced electron transport in this system when compared to the tetrakis model compound.