Plasma-synthesized silicon nanocrystals with alkene ligands have shown the potential to exhibit high-efficiency photoluminescence, but results reported in the literature have been inconsistent. Here, for the first time, the role of the immediate post-synthesis "afterglow plasma" environment is explored. The significant impact of gas injection into the afterglow plasma on the photoluminescence efficiency of silicon nanocrystals is reprorted. Depending on the afterglow conditions, photoluminescence quantum yields of silicon nanocrystals synthesized under otherwise identical conditions can vary by a factor of almost five. It is demonstrated that achieving a fast quenching of the particle temperature and a high flux of atomic hydrogen to the nanocrystal surface are essential for a high photoluminescence quantum yield of the produced silicon nanocrystals.