An SF6-based plasma has been employed to perform in-flight etching of silicon nanocrystals (Si-NCs) after they were synthesized in an SiH4-based plasma. The photoluminescence of the Si-NCs blue-shifts after etching, indicating an etching-induced size reduction of the Si-NCs. It is shown that both the SF6 plasma power and the flow rate can be utilized to control the etch rate (and thus the size reduction) of the Si-NCs. The SF6 etched Si-NCs show only low concentrations of residual impurities other than fluorine. Quantum yields as high as 50% have been observed from these SF6 etched Si-NCs despite oxidation.