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Developing silicon nanoparticle (SiNP) synthesis techniques that allow for straightforward control of nanoparticle size and associated optical properties is critical to potential applications of these materials. In addition, it is, in general, hard to probe the absorption threshold in these materials due to silicon's low absorption coefficient. In this study, size is controlled through direct introduction of sulfur hexafluoride (SF6) into the dilute silane precursor of plasma synthesized SiNPs. Size reduction by nearly a factor of two with high crystallinity independent of size is demonstrated. The optical absorption spectra of the SiNPs in the vicinity of the bandgap are measured using photothermal deflection spectroscopy. Bandgap as a function of size is extracted taking into account the polydispersity of the samples. A systematic blue shift in absorption edge due to quantum confinement in the SiNPs is observed with increasing flow of SF6. Photoluminescence (PL) spectra show a similar blue shift with size. However, a ∼300 meV difference in energy between emission and absorption for all sizes suggests that PL emission involves a defect related process. This shows that, while PL may allow size-induced shifts in the bandgap of SiNPs to be monitored, it cannot be relied on to give an accurate value for the bandgap as a function of size.

A dual plasma reactor has been developed for depositing nanocrystalline silicon with quantum confined silicon nanoparticles, either by sequential or concurrent deposition of amorphous silicon and silicon nanoparticles. Sequential deposition allows for complete decoupling of the amorphous and nanoparticle deposition and well-defined layers of silicon nanoparticles between amorphous silicon. The concurrent deposition is similar to conventional deposition of nanocrystalline silicon, which allows for complete mixing of the silicon nanoparticles and amorphous silicon. With the introduction of the silicon nanoparticles into the amorphous silicon, we observe a quenching of the photoluminescence, which has been reported in the literature to be from the transfer of charge from the amorphous silicon to silicon crystallites.

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