C F4 -based plasma has been employed to in-flight etch silicon nanocrystals (Si-NCs) after they are synthesized by Si H4 -based plasma. The authors find that the photoluminescence (PL) of Si-NCs blueshifts when they are etched, indicating the etching-induced size reduction of Si-NCs. It is demonstrated that the power of C F4 -based plasma can be tuned to control the size reduction of Si-NCs. The room-temperature atmospheric oxidation of both etched Si-NCs and unetched ones slows down significantly ∼100 h after production. The PL intensity of etched Si-NCs is smaller than that of unetched ones after oxidation.