Silicon nanoparticles are promising new materials for photovoltaic applications that combine materials property tunability on the nanoscale with silicon’s established performance in photovoltaics. We have succeeded in synthesizing crystalline silicon nanoparticles in a continuous flow plasma reactor and established control over particle size. By carefully tuning the plasma parameters, we have synthesized particles with radial precision of a couple atomic layers and characterized them structurally and optically. Such tight size control is the first step toward assembly of nanoparticles into a device layer to produce silicon-based PV devices with tunable bandgaps.