Ligand Stabilities and Reactivities of Green Photoluminescent Silicon Quantum Dots: Positive Aging in Solution

Bright full-color silicon quantum dots (SiQDs) are promising heavy-metal-free light sources for displays, lighting, and biomedical imaging. However, the relationships linking the stability, ligand reactivity, and SiQD photoluminescence (PL) are not yet fully understood, and obtaining SiQDs that emit true green PL has proved difficult. Herein, we report the synthesis, via SiBr4 reduction followed by ligand exchange with a carbazole derivative, of colloidal SiQDs that emit green PL. The PL and ligand stabilities were tracked over a period of 3 months using two-dimensional (2D) PL, Fourier-transform infrared (FTIR), 2D NMR, and time-resolved spectroscopies. True green PL (lambda(PL) = 530 nm) was emitted by the SiQDs upon excitation using a widely available blue light source (light-emitting diode (LED) flashlight, lambda(ex) = 450 nm), and the width of the emission band was narrow (similar to 60 nm). The PL quantum yield (PLQY) was 60%, and the radiative rate was nearly equal to the value expected for direct transitions of QDs. In acetone, the PLQY was constant, but storing the SiQDs in four different alcohols resulted in PLQY increases in the range of 10-50%. This positive aging effect was attributed to alkoxy ligand formation via the reaction between the Br ligands and alcohols. In addition, a blue PL band appeared as a result of aging, and its intensity was enhanced 7-fold after storage of the SiQDs in O-2-saturated ethanol; evidence of energy transfer between the PL bands was observed in frequency- and time-domain measurements. The findings and characterization of the long-term stability reported herein should prove invaluable for future research and applications of colloidal SiQDs.