A muon spectroscopic and computational study of the microscopic electronic structure in thermoelectric hybrid silicon nanostructures

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 the microscopic electronic structure of 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, A _μ, 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 A _μ 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 A _μ 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.