Silicon nanocrystals have raised hopes in applications for Si-based devices. Our interest is synthesis of Si/SiO₂ nanoparticles having dimensions < 10 nm, with the final goal "Quantum Dots" for specific application in biology, micro and nanoelectronics. We present the studies of nanometric silicon powders produced by laser ablation. The effects of laser wavelength, fluence, and gas pressure, on the nanoparticle structure and size distribution have been investigated by transmission electron microscopy.
The nanoparticles have been generated by ablation of a pure silicon target with Nd:YAG laser, second and third harmonic, 532 and 355 nm, pulse duration 5-6 ns, fluence 4-8 J/cm². Argon and helium have been used in the pressure range of 250-550 mbar. The nanoparticles have been collected on a filter (pores 100 nm) and analyzed as prepared, without any precaution or special method for deposition on the TEM grids.
The nanoparticles have been in the range of 2-12 nm, 30 percent of them being of 2-4 nm in diameter. The TEM images and SAED patterns reveal spherical silicon nanoparticles with a crystalline silicon core (diamond structure) covered with an amorphous SiO₂ layer (1.2-1.5 nm thick). Some of the clusters are loosely agglomerated, some present chains of welded particles. The size and size distribution of the powders are strong functions of the specific process parameters (laser wavelength, fluence and gas). The experiments showed an optimum at 355 nm, 8 J/cm², in helium at 400 mbar, the particles being smallest, with a much narrower distribution.

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