  A simple, yet effective method of controlling the size and size distributions
of nanoparticles produced as a result of laser ablation of target material is
presented. The method employs the presence of physical boundaries on either
sides of the ablation site. In order to demonstrate the potential of the
method, experiments have been conducted with copper and titanium as the target
materials that are placed in two different liquid media (water and isopropyl
alcohol). The ablation of the target material immersed in the liquid medium has
been carried out using an Nd:YAG laser. Significant differences in the size and
size distributions are observed in the cases of nanoparticles produced with and
without confining boundaries. It is seen that for any given liquid medium and
the target material, the mean size of the nanoparticles obtained with the
boundary-fitted target surface is consistently higher than that achieved in the
case of open (flat) targets. The observed trend has been attributed to the
plausible role(s) of the confining boundaries in prolonging the thermalisation
time of the plasma plume. In order to ascertain that the observed differences
in sizes of the nanoparticles produced with and without the presence of the
physical barriers are predominantly because of the prolonged thermalisation of
the plasma plume and not due to the possible formation of oxide layer, select
experiments with gold as the target material in water have also been performed.
The experiments also show that, irrespective of the liquid medium, the increase
in the mean size of the copper-based nanoparticles due to the presence of
physical boundaries is relatively higher than that observed in the case of
titanium target material under similar experimental conditions.
