<title>Synthesis of nanoparticles by a laser-vaporization-controlled condensation technique</title>

A method which combines laser vaporization of metal targets with controlled condensation from the vapor phase is used to synthesize nanoscale metal oxide, carbide and nitride particles (10 - 20 nm) of well-defined composition. The metal vapor is generated by pulsed laser vaporization using the second harmonic (532 nm) of a Nd-YAG laser. Following the laser pulse, the ejected atoms react with the reactive gas within the ambient atmosphere and condense to form nanoparticles. The role of convection in the experiments is to remove the small particles away from the nucleation zone (once condensed out of the vapor phase) before they can grow into larger particles. Surface-oxidized silicon nanocrystals, produced by this method, aggregate into a novel weblike microstructure. These aggregates are very porous and have a large surface area. The nanoparticles show a short-lived (less than 20 ns) blue emission at 450 nm characteristic of the SiO₂ coating and a biexponential longer-lived red emission characteristic of the Si core. Other examples of nanoparticles discussed include ZnO and magnetic FeO. Examples of the effects of solvents and temperature on the morphology of the nanoparticles are presented. The advantages of microgravity in the synthesis of multilayer nanoparticles of engineered compositions and morphology are outlined.