Propionic-Acid-Terminated Silicon Nanoparticles: Synthesis and Optical Characterization

Photoinitiated hydrosilylation was used to attach acrylic acid to the surface of photoluminescent silicon nanoparticles, thereby producing water-dispersible, propionic-acid-terminated particles. From transmission electron microscope (TEM) observations, the average diameters of the synthesized nanocrystals were 1.9-2.4 nm. It is likely that smaller particles (<1.5 nm) were also present but could not be imaged. As the nanocrystal size decreased, both the optical absorption edge and photoluminescence (PL) emission peak blue-shifted, whereas the photoluminescence excitation (PLE) spectrum changed very little and showed a sharp onset indicative of direct interband absorption at Γ. After prolonged ultrasonication in water, the Si nanoparticles showed strong blue PL. This can tentatively be attributed to the formation of radiative centers related to incomplete oxidation of the nanocrystals. The silicon nanocrystals could be transferred into water or methanol by dialysis without inducing this oxidation. The PA-terminated Si nanoparticles were stably dispersed in acrylic acid, water, and methanol and showed essentially the same optical properties in all three solvents. The approach used here provides a general means of producing water-dispersible silicon nanocrystals with size-dependent photoluminescence tunable over a wide range of the visible spectrum.