Nanoparticle formation by laser ablation

The properties of nanoparticle aerosols of size ranging from 4.9 nm to 13 nm, generated by laser ablation of solid surfaces are described. The experimental system consisted of a pulsed excimer laser, which irradiated a rotating target mounted in a cylindrical chamber 4 cm in diameter and 18-cm long. Aerosols of oxides of aluminum, titanium, iron, niobium, tungsten and silicon were generated in an oxygen carrier gas as a result of a reactive laser ablation process. Gold and carbon aerosols were generated in nitrogen by non-reactive laser ablation. The aerosols were produced in the form of aggregates of primary particles in the nanometer size range. The aggregates were characterized using a differential mobility analyzer and electron microscopy. Aggregate mass and number concentration, electrical mobility size distribution, primary particle size distribution and fractal dimension were measured. System operating parameters including laser power (100 mJ/pulse) and frequency (2 Hz), and carrier gas flowrate (1 l/min) were held constant. A striking result was the similarity in the properties of the aerosols. Primary particle size ranged between 4.9 and 13 nm for the eight substances studied. The previous studies with flame reactors produced a wider spread in primary particle size, but the order of increasing primary particle size follows the same trend. While the solid-state diffusion coefficient probably influences the size of the aerosol in flame reactors, its effect is reduced for aerosols generated by laser ablation. It is hypothesized that the reduced effect can be explained by the collision-coalescence mechanism and the very fast quenching of the laser generated aerosol.