Effect of confined geometry on the size distribution of nanoparticles produced by laser ablation in liquid medium

Abstract

An effective method has been proposed for controlling the size distributions of metallic nanoparticles produced through laser ablation in liquid by employing physical boundaries in the vicinity of the ablation site. An Nd:YAG laser (1064 nm, FWHM 6 ns) has been used to ablate the copper target immersed in two different liquid medium, water and isopropyl alcohol. Water and isopropyl alcohol have been chosen because of the significant differences in their densities while the optical properties are nearly comparable. To demonstrate the efficacy of the method, experiments have been conducted in both the configurations, that is, with and without confining boundaries. To ascertain the observed effect, select experiments have also been carried out with gold as the target material to ensure that the size distribution is not influenced because of oxide-layer formation. The size of the particles were estimated from the SEM images and analysing these images using an indigenously developed code. It has been observed that there are significant differences in the size distributions in the cases of nanoparticles produced with and without confining boundaries. For any given medium, a consistent increase in the mean size of the nanoparticles produced with the targets fitted with physical boundary has been observed as compared to those produced with flat targets. The observed trend has been attributed to the plausible role of the shock wave reflection from the physical boundaries, which alters the plasma parameters. Reflected shock wave–plasma interactions prolonging the thermalisation time of the plasma plume in confined geometry facilitate the particle growth, resulting in the formation of bigger particles. The proposed method, which can be applied to any metallic target, is one of the greener methods for producing nanoparticles and is also relatively simple and cost effective.