Silicon quantum dots: surface matter, what next?

Silicon quantum dots (SiQDs) are of great interest because they are believed to be harmless to living organisms, mainly due to their low toxicity. They have great potential for various applications such as in optoelectronics, photonics, and photodynamic therapy. As the Si core is susceptible to oxidation, efforts are needed to protect the core or to passivate it. This passivation/functionalization is aimed at both the protection of the core and the enhancement of the light emission. The importance of surface modification of SiQDs concerning the effect of quantum confinement and ligand type on their photoluminescence quantum yield is discussed in this review. Different functionalization reactions are described along with the precursors used. Among these are hydrosilylation reactions between the silicon core and the carbon of alkyl ligands and chromophoric ligands. Alkyl ligands protect the silicon core and provide chemical and light stability to the silicon. Ligands containing chromophoric groups also play a role in increasing the absorption of light, which in turn can increase the emission of PL light. These chromophoric ligands can also be used for optoelectronic, photonic, energy conversion, and biological purposes to enhance photophysical properties through energy transfer mechanisms. The energy transfer between the SiQDs and the ligands allows the triplet-triplet annihilation to enhance the emission and the generation of singlet oxygen. Finally, a brief description is given of the opportunities that SiQDs can offer and the progress made in recent years in the potential applications of SiQDs. They can be used as materials for light-harvesting antennas, bioimaging, luminescent solar concentrators, LEDs, and photosensors by exploiting triplet-triplet annihilation and singlet oxygen.