METAL NANOPARTICLES GENERATED BY LASER-ABLATION

We study a new method for producing ultrafine metal particles (nanopar ticles) that employs Laser Ablation of Microparticles (LAM). Pulsed ex cimer laser radiation at 248 nm wavelength was used to ablate similar to 2 mu m feedstock of silver, gold, and permalloy(Ni-81%:Fe-19%) unde r both normal atmospheric conditions and in other gases and pressures. A model for nanoparticle formation by LAM is proposed that includes p lasma breakdown anal shock-wave propagation through the initial microp article. Behind the shock a large fraction of the original micropartic le mass is converted to nanoparticles that diffuse to silicon substrat es and TEM grids for collection and analysis. Nanoparticle morphologie s are spherical except for gold nanoparticles >100 nm that are general ly cubes. Electron micrographs of the samples were analyzed by compute r-aided image processing to determine the effect of irradiation condit ions on the nanoparticle size distribution. The results showed that me an particle diameters were normally in the range from 10 to 100 nm and that the particle size distributions were generally log-normal, with dispersion (diameter/standard deviation) ranging from 0.2 to 0.5. For metallic microparticle feedstock, the mean size of the produced nanopa rticles generally increased with increasing laser fluence and were sma llest for fluences not too far above the breakdown threshold. (C) 1998 Acta Metallurgica Inc.