Uwe Kortshagen1
University of Minnesota1
Chemically reactive nonthermal plasmas at low pressure are an interesting medium for the growth of nanocrystals. Molecular precursors are dissociated by electron impact reactions and the resulting molecular fragments and radicals, many of them charged, nucleate to form clusters and nanocrystals. Energetic surface reactions can heat these initial clusters to temperatures that exceed the gas temperature by hundreds of Kelvin. This enables plasmas to form crystalline nanoparticles even of materials with very high melting points. The unipolar negative charging of nanoparticles in the plasma virtually eliminates particle aggregation, leading to very monodisperse size distributions. Furthermore, plasma synthesis is an all-gas-phase method that does not require solvents or ligands and is thus a relatively sustainable synthesis method. In this presentation, we will discuss the plasma synthesis of silicon quantum dots with diameters on the order of ~3 nm for luminescent applications as well as the synthesis of larger silicon nanocrystals, 60-200 nm in diameter, that show interesting scattering behaviors.<br/><br/>Plasma-synthesized silicon quantum dots have shown good luminescence properties with photoluminescence quantum yields of up to 60%. To attain such high quantum yields, passivation of surface defects is required that has traditionally been achieved by hydrosilylation reactions. However, hydrosilylation requires organic solvents which partly offsets the sustainability advantage of plasma synthesis. Here, we report on recent progress in performing the surface functionalization in the gas phase right after the synthesis step. We discuss various methods to attach organic ligands to the surfaces of silicon quantum dots while in flight in the afterglow of the synthesis plasma.<br/><br/>One application of luminescent silicon quantum dots is in luminescent solar concentrators (LSCs). These are semitransparent waveguides that are doped with silicon quantum dots. The silicon dots absorb solar radiation primarily in the blue range of the spectrum and reemit it in the near-infrared. Waveguiding concentrates this radiation on small solar cells that can be edge or surface mounted. We discuss results of a numerical model that investigates the application of silicon LSCs to greenhouses and demonstrate that LSC roof panels have the ability to enable net-zero-energy greenhouses in certain climates.<br/><br/>In the final part of the presentation, we discuss recent progress in the plasma synthesis of larger, highly monodisperse silicon nanocrystals with diameters of 60-200 nm. These nanocrystals exhibit intriguing optical scattering through overlapping electric and magnetic dipole modes. We show that ensembles of silicon nanocrystals produced by plasmas show scattering behavior that is essentially consistent with single particle scattering models due to their very narrow size distribution.<br/><br/>This work was supported by the Army Research Office MURI grant W911NF-18-1-0240.