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Mechanism of fluorescent silicon nanoparticles

Silicon (Si) is known to have an indirect bandgap transition, which means it has poor fluorescence properties. However, when engineered into sub-nm sized particles, Si nanoparticles become emissive due to quantum confinement. However, in unmodified Si particles, this effect is limited to generating red or near-infrared emission with low quantum yield. To resolve these limitations, surface-modification methods have successfully generated Si particles that emit in the blue, cyan, and green with quantum yields up to ~90%.1,2 These modifications have also made the Si nanoparticles watersoluble, making them promising in biological applications. To date, the mechanism of emission in these species is still unclear although it has been speculated that charge transfer of Si-O-N could be responsible. To investigate whether emission by these Si nanoparticles proceeds via a charge transfer mechanism, Stark spectroscopy is used. In this method, an external electric field is applied to the Si nanoparticles. Changes in the absorption and/or emission spectra due to the applied field can be taken as strong evidence for a charge transfer mechanism. From the results of Stark spectroscopy, Si nanoparticles are revealed to have ligand to metal charge transfer mechanism along with electric-field quenching, which is useful information for utilization into applications. Addition to the information found, a method of how to tune the emission maxima based on selection of ligands is prosed.