This work compares structural and optical properties of silicon nanocrystals prepared by two fundamentally different methods, namely, electrochemical etching of Si wafers and low-pressure plasma synthesis, completed with a mechano-photo-chemical treatment. This treatment leads to surface passivation of the nanoparticles by methyl groups. Plasma synthesis unlike electrochemical etching allows selecting of the particle sizes. Measured sizes of the nanoparticles by dynamic light scattering show 3 and 20 nm for electrochemically etched and plasma-synthetized samples, respectively. Plasma-synthetized 20-nm particles do not exhibit photoluminescence due to absence of quantum confinement effect, and freshly appeared photoluminescence after surface passivation could indicate presence of organic molecules on the nanoparticle surface, luminescing instead of nanocrystal core. Electrochemically etched sample exhibits dramatic changes in photoluminescence during the mechano-photo-chemical treatment while no photoluminescence is observed for the plasma-synthetized one. We also used the Fourier transform infrared spectroscopy for comparison of the chemical changes happened during the treatment.

Luminescent silicon nanoparticles are promising not only for optoelectronics or photovoltaics, but also for biological and medical research, e.g., as luminescent markers, for drug delivery, or toxicity studies. For intracellular biological research, it is necessary to prepare water-based or isotonic colloidal solutions of nanoparticles which are stable and nontoxic. In this work, we compare structural and optical properties of silicon nanocrystals in colloidal solutions, obtained by two fundamentally different methods: A microplasma-based synthesis (a "bottom-up" technique) and porous silicon prepared by electrochemical etching of a silicon wafer (a "top-down" method). Low-pressure microwave plasma synthesis produces $5-8 nm large silicon nanocrystals while the porous siliconcontains clusters $60-70 nm in diameter, composed of nanometer-sized luminescent nanocrystals. However, both types of nanoparticles are prone to agglomeration, as was confirmed by dynamic light scattering and zeta potential measurements. We have attempted to stabilize the nanoparticles via modification of the their surface by methyl groups; however, the mechano-photo-chemical treatment procedure leading to coverage with methyl groups is less efficient in the case of plasma-synthesized nanoparticles than in porous silicon. The first attempt of steric stabilization of the colloidal solutions of the silicon nanoparticles was succesfully carried out too; good candidates for stabilization are bovine serum albumin and glycine.