Formation dynamics of SiO2 nanoparticles produced by laser ablation in ambient gases

Pulsed laser deposition (PLD) is a commonly used technique for fabricating thin films. Additionally, it can produce oxide material nanocrystals and nanoporous films by controlling gas pressure during the deposition. Previously, we fabricated nanoporous films with low dielectric constants by depositing SiO2 nanoparticles. In such deposition techniques that use laser-generated nanoparticles as raw materials, parameters, such as ambient gas pressure, gas species, and the distance between the target and substrates, have a considerable influence on the size and structure of the nanoparticles. In this study, to clarify the behavior of nanoparticles during pressure-controlled PLD, such as their formation process and spatial distribution, SiO2 nanoparticles generated by laser ablation are visualized using two-dimensional laser scattering imaging. The spatial distribution of the nanoparticles tends to decrease with increasing ambient gas pressure, and the distribution exhibits various shapes, including spherical and mushroom clouds, depending on the gas species. Nanoparticles generated by laser ablation in the gas phase and collected at different positions from the target surface on a collection substrate placed parallel to the target are observed. Nanoparticles are deposited without aggregation at a collection position below the maximum expansion distance of the laser ablation plasma plume l(p). In the collected position around and above l(p), web-like aggregates consisting of nanoparticles with a particle size of a few nanometers are observed. As the pressure increases, the size of the web-like aggregates and number of deposits increase. Information regarding these spatial distributions and deposited aggregates is useful in optimizing conditions, such as ambient gas species, gas pressure, and deposition distance, for nanoporous film fabrication.

Automated summary

Pulsed laser deposition (PLD) is a commonly used technique for fabricating thin films. This study visualizes SiO2 nanoparticles generated by laser ablation using two-dimensional laser scattering imaging and investigates their behavior during pressure-controlled PLD. The spatial distribution of the nanoparticles decreases with increasing ambient gas pressure, and the distribution exhibits various shapes depending on the gas species. The size and number of web-like aggregates of nanoparticles increase with pressure. The information obtained from this study is useful for optimizing conditions in nanoporous film fabrication.