Though bulk silicon (Si) is an indirect bandgap material and therefore non-emissive, nm-sized Si quantum dots (Si QDs) exhibit direct band-gap characteristics due to quantum confinement. As a result, Si QDs are emissive though their fluorescence is relatively weak and limited to red or near-infrared. Recently, visible and color-tunable emission with up to 90% quantum yield has been achieved through surface-modification of Si QDs with nitrogen-capped ligands. However, the emission mechanism operating in these surface-modified Si QDs is unclear and the factors that determine their emission maxima are still unknown. Here we report that the emission maximum wavelength of these species can be predicted quantitatively from the calculated ground-state dipole moment of the ligand. This is consistent with the origin of the emission being a charge-transfer (CT) transition between the Si surface and the ligand. A detailed study of the photon statistics behavior of isolated Si QDs reveals two types of emission, the dominant one being characteristic of single quantum states and the weaker one being characteristics of a bulk material. Understanding the emission mechanism of these unique systems and how their properties can be tuned synthetically will enable the design of Si QDs with a broader wavelength range and higher quantum yields for applications in light-emitting diodes, bio-imaging and sensing.
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.
Recently, fluorescent Silicon (Si) Quantum Dots (QDs) have attracted much interest due to their high quantum yield, use of non-toxic and environmentally-benign chemicals, and water-solubility. However, more research is necessary to understand the energy level characteristics and single molecule behavior to enable their development for imaging applications. Therefore, single molecule time-resolved fluorescence spectroscopy of fluorescent Si QDs (cyan, green, and yellow) is needed. A rigorous analysis of time-resolved photon correlation spectroscopy and fluorescence lifetime data on single Si QDs at room temperature is presented.