Abstract:

Luminescent nanoparticles exhibit a broad field of applications like optoelectronic devices, solar cells and fluorescent bio-labelling agents. Since conventional luminescent inorganic nanoparticles usually contain toxic compounds such as Pb and Cd, interest in luminescent silicon nanocrystals arose. In this work we present the generation of luminescent silicon nanocrystals in a microwave plasma reactor on the pilot plant scale. The nanoparticulate powder is synthesized under reduced pressure (70 mbar) using mono silane (SiH4) as precursor. Silicon crystals embedded in a SiO2 matrix can be synthesized by varying the oxygen-to-silane ratio in the range of 0.8 to 5.0. Production rates of 40 g/h were achieved. The specific surface area of the product was determined with nitrogen adsorption (BET). X-ray diffraction (XRD) and photo-luminescence spectroscopy (PL) were used to characterize the morphology and the optical properties of the synthesized material. BET measurements revealed a maximum specific surface area of 315 m2/g that corresponds to a particle size of 7.8 nm. The as-synthesized material did not show any luminescence upon excitation at 450 nm. After temperature-treating at 900°C for 6 h the initial SiO2-x material segregated into SiO2 and Si resulting in luminescence with a maximum at 700 nm and a full width at half maximum of 110 nm. XRD measurements revealed the presence of crystalline silicon. During annealing, the color of the powders changed from light brown to white and the specific surface area decreased with increasing temperature, while the duration of the treatment had a negligible effect on the specific surface area. With increasing oxygen content during the synthesis and increasing annealing temperatures, the luminescence shifts to shorter wavelengths indicating the formation of smaller silicon nanocrystals. The powders could be dispersed in 2-propanol when sonicated for 120 min. Dynamic light scattering (DLS) indicated an agglomerate size of about 230 nm for the as-prepared samples and slightly higher values for the annealed material. Electron microscopy analysis (SEM) of the dried dispersions is in good agreement with the DLS measurements.