Crystalline silicon nanoparticles at the nanometer scale have been attracting great interest in many different optoelectronic applications such as photovoltaic and light-emitting-diode devices. Formation, crystallization, and size control of silicon nanoparticles in nonharsh and nontoxic environments are highly required to achieve outstanding optoelectronic characteristics. The existing methods require high temperature, use of HF solution, and an additional process for the uniform redistribution of nanoparticles on the substrate and there are difficulties in controlling the size. Herein, we report a new self-assembly method that applies the controlled extremely low plasma ion energy near the sputtering threshold energy in rare gas environments as nonharsh and nontoxic environments. This method produces silicon nanoparticles by crystallization nucleation directly at the surface of the amorphous film via plasma surface interactions. It is evidently observed that the nucleation and growth rates of the crystalline silicon nanoparticles are promoted by the enhanced plasma ion energy. The crystalline silicon nanoparticle size is tailored to the nanometer scale by the plasma ion energy control.