Aerosol-Phase Synthesis and Processing of Luminescent Silicon Nanocrystals

Abstract

Silicon quantum dots are attractive materials for luminescent devices and bioimaging applications. For these light-emitting applications, higher photoluminescence efficiency is desired in order to achieve better device performance. Nonthermal plasma synthesis successfully allows for the continuous production of silicon nanocrystals, but postprocessing is necessary to improve photoluminescence quantum yields so that nanocrystals can be used for luminescence applications. In this work, we demonstrate an all-aerosol-phase synthesis and processing route that integrates nonthermal plasma synthesis, plasma-assisted surface functionalization with alkene ligands, and in-flight annealing within one flow stream. Here, luminescent silicon nanocrystals are synthesized and postprocessed on a time scale of only 100 ms, which is orders of magnitude faster than previous synthesis and functionalization schemes. The as-produced silicon nanocrystals have photoluminescence quantum yields exceeding 20%, which is a 5-fold increase compared to previous silicon nanocrystals synthesized with all-aerosol-phase approaches. We attribute the enhanced photoluminescence to the reduced “dark” nanocrystal fraction due to reduction of dangling bond density and desorption of surface silyl species induced by the in-flight annealing. We also demonstrate that the ligand coverage plays a minor role for the photoluminescence properties, but that the nature of the silicon hydride surface groups is a major factor.

Poly(methyl methacrylate) Films with High Concentrations of Silicon Quantum Dots for Visibly Transparent Luminescent Solar Concentrators

“…Here, the wt % of the Si QDs refers to the combined weight of Si QD cores and organic ligands. Using the same elemental analysis as previous work, 35 we find that the ligand shell contributes ∼40% of the overall weight of the functionalized Si QDs. Notably, the 1.0 and 3.3 wt % Si QD loadings in these films are significantly higher than in our previous work, where the maximum Si QD loading examined was 0.5 wt %.…”

Water-Soluble Luminescent Silicon Nanocrystals by Plasma-Induced Acrylic Acid Grafting and PEGylation

“…Acrylic acid functionalized Si NCs were synthesized in a nonthermal plasma reactor similar to the ones in previous works. , The reactor consists of a 0.5 cm inner diameter borosilicate tube which expands to 2 cm. Ten to thirty standard cubic centimeters per minute (sccm) of argon and 14 sccm of silane (5% in helium) mixture were flowed through the tube, while the acrylic acid was injected as vapor through a bubbler into the plasma afterglow.…”

“…Plasma-assisted surface functionalization with alkene ligands to produce luminescent Si NCs has been demonstrated earlier, and a similar approach was employed to synthesize acrylic acid grafted Si NCs by adjusting the plasma power and gas flow rates. 26,27 Acrylic acid or poly(acrylic acid) coating of Si NCs has been extensively studied in the solution phase to generate a hydrophilic surface and to reduce nanoparticle agglomeration in water or biological media. [17][18][19]28,29 Acrylic acid ligands with a terminal carboxylic acid group stabilize particles through a combination of steric stabilization and electrostatic repulsion, thus the colloidal stability can be affected by the presence of electrolytes and pH of the medium.…”

“…First, we synthesized acrylic acid grafted Si NCs using an all-gas-phase approach on a millisecond time scale without any postsynthetic processing. Plasma-assisted surface functionalization with alkene ligands to produce luminescent Si NCs has been demonstrated earlier, and a similar approach was employed to synthesize acrylic acid grafted Si NCs by adjusting the plasma power and gas flow rates. , Acrylic acid or poly­(acrylic acid) coating of Si NCs has been extensively studied in the solution phase to generate a hydrophilic surface and to reduce nanoparticle agglomeration in water or biological media. − ,, Acrylic acid ligands with a terminal carboxylic acid group stabilize particles through a combination of steric stabilization and electrostatic repulsion, thus the colloidal stability can be affected by the presence of electrolytes and pH of the medium. ,,− Hence, even though acrylic acid grafting improves the nanocrystal dispersibility in water, the functionalized nanoparticles are typically slightly agglomerated with hydrodynamic diameters around 100 nm …”

Distinct Acoustic and Optical Phonon Dependences on Particle Size, Oxidation, and Temperature in Silicon Nanocrystals

“…Spatially confined silicon systems have drawn an increasing amount of attention since the last century owing to their significantly higher emission efficiencies of photoluminescence (PL) than their bulk counterparts. − Luminescent silicon nanocrystals are widely used in bio and cellular imaging industries as biocompatible, nontoxic, and biodegradable theranostic agents. − They also have tremendous potential in optoelectronic applications. − Despite much work, the mechanism of PL remains a subject of discussion. ,− …”