Nanoparticle-based Photonic Materials from Non-thermal Plasma Synthesis

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Nanoparticle-based Photonic Materials from Non-thermal Plasma Synthesis

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2022-12

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Light-matter interactions dictate innumerable modern technologies and the desire for further control over purpose-built optical interfaces will continue to grow. Controlling sub-wavelength geometry has shown to be extremely successful in advancing this goal. In this work, silicon-nitride and crystalline silicon nanoparticles were produced and optically characterized to further understand how nanoscale composition or diameter changes can affect macroscopic optical properties with potential applications ranging from sustainable passive cooling films to color-tuning in crystalline silicon nanoparticle-based inks.Passive radiative cooling (PRC) technologies have seen growing attention due to the increasing need for scalable, low-cost, low-maintenance cooling devices. PRC devices work by minimizing 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 has been found to be challenging. Recently, a simulation of silicon nitride (SiNx) 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 homogenous (SiNx) nanoparticle films. By using SiH4, Ar, and N2 injected into a non-thermal plasma, the nanoparticle’s composition can be tuned with plasma power. Characterizing the optical properties of the films with UV-Vis and Fourier Transform Infrared (FTIR) spectroscopy, we observe high IR absorption and visible transparency, as required for PRC. Using X-ray Photoelectron Spectroscopy (XPS), the film composition was found to be tunable between stoichiometric Si3N4 and lower amounts of nitrogen, depending only on discharge power. Finally, Transmission Electron Microscopy (TEM) revealed silicon nanocrystal precipitation at high discharge powers suggesting an optimal power for PRC films. Optically Mie-resonant crystalline silicon nanoparticles are synthesized using a bottom-up nonthermal plasma process. Highly controllable particle sizes between 60 to 214 nm with standard deviations smaller than 5.4% are achieved via temporary trapping the nanoparticles inside a continuous-flow plasma reactor. The particle size is simply tuned by adjusting the precursor gas residence time. By dispersing the nanoparticles in deionized water, optical extinction measurements show stable colloidal solutions of a metafluid, supporting both strong magnetic and electric dipole resonances in the visible. The spectral overlap of these resonances is related to anomalous directional Kerker scattering. The extinction measurements show excellent agreement with Mie theory, indicating that the fabrication process is precise in maintaining a narrow deviation in size, shape, and material constraints for most samples. These particle characteristics are also independently verified via TEM analysis. This single-step gas-phase process is capable of synthesizing Mie-resonant nanoparticles of different dielectric materials and directly depositing them on desired substrates.

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University of Minnesota Ph.D. dissertation. December v2022. Major: Material Science and Engineering. Advisors: Uwe Kortshagen, Ognjen Ilic. 1 computer file (PDF); x, 77 pages.

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Nelson, Gunnar. (2022). Nanoparticle-based Photonic Materials from Non-thermal Plasma Synthesis. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/252523.

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