Nonthermal Plasma Synthesis of Composition-Tunable Silicon Nitride Nanoparticle Films for Passive Radiative Cooling

Passive radiative cooling (PRC) technologies have seen growing attention due to the increasing need for scalable, low-cost, and low-maintenance cooling devices. PRC devices work by minimizing the absorption of light in the visible spectrum (300-700 nm) while optimizing for high emissivity in the infrared atmospheric transmission window (8-14 μm). However, identifying and synthesizing a material or material structure with these precise properties have been found to be challenging. Recently, simulations of silicon nitride (SiN_x) nanoparticle films showed potential significant cooling power improvements over current PRC structures. In this work, we show a scalable, single step, and tunable synthesis technique to produce such homogeneous SiN_x nanoparticle films. By using SiH₄, Ar, and N₂ injected into nonthermal plasma, the nanoparticle composition can be tuned with plasma power. Characterizing the optical properties of the films, we observe high infrared absorption and visible transparency, as required for PRC. The film composition was found to be tunable between stoichiometric Si₃N₄ and nitrogen-poor SiN_x, depending only on the plasma power. Finally, high plasma powers lead to silicon nanocrystal precipitation, suggesting an optimal plasma power for PRC film formation.