**8. Conclusions**

air, the microparticles were notably distinct both near and far from the crater on the brass target. However, in water, the narrow distribution of nanoparticles away from the crater was also sound. It was concluded that water is a more suitable environment in which to produce uniform and large quantities of nanoparticles by laser ablation [11]. Hermann et al. [74] determined that the ablation depth of copper and gold in a vacuum chamber increased

190 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

The laser-ablation process is also used to clean the surfaces of materials. The effectiveness of laser-ablation cleaning methods on silver artefacts in different media (air, water and a vacuum) was characterised. Before cleaning the samples, they were treated with HCl (37%) solutions for several hours to simulate the formation of a chloride patina (AgCl) on marine archeological silver artefacts. It was shown that the patina was removed from a few to 300 μm in air and in water and removed completely under vacuum conditions. However, the formation of a white patina in air and water and an increasing amount of oxygen on the surface of the samples were observed during cleaning process. This situation shows that 'in ambient reach in oxygen laser ablation favourite the oxygen absorbance on surface and the formation of a thin layer of AgO which dulls the surface' [75]. This hypothesis is proven by unobserving surface tarnishing in a vacuum during laser ablation. It can be concluded that the optimal conditions in which to

**7.4. Differences and advantages of different environments to laser ablation and generation**

Many important differences have been observed between laser ablation in a vacuum, air, gas and liquid environments. These differences can be summarised as follows: first, in water, at the liquid-solid interface, a plasma plume is created. In a liquid solution, the ablated materials and plasma plume expansion are confined, which leads to the production of a high temperature and pressure caused by the mechanical effects. Second, chemical reactions are produced at the interfaces between the laser-induced plasma and the liquid and in the laser-induced plasma at high temperature and pressure. Third, in water, the quenching time of the plasma plume is

The advantages of using laser ablation in liquid environments to produce nanomaterials over other methods such as chemicals are as follows: (i) simple and clean production because the process does not need a catalyst and no byproducts are formed; (ii) the process does not require extreme temperature and pressure; (iii) the method leads to the production of a new phase of nanocrystals which may occur in both liquids and solids, which presents more options to choose and combine interesting and desired solid targets and liquids to produce nanocrystals and nanostructures of new compounds or bimodal [57]. Another advantage of using liquid environments in the production of nanoparticles is the ability to use the liquid as a medium in which to collect the nanoparticles. In addition, it has also been shown that the application advantages of nanoparticles produced in liquid environments in comparison with conventionally generated nanoparticles are the following: (i) the number of bimolecules which can be conjugated on the surface of laser-produced nanoparticles in water is three to five times higher

, but that this figure increased considerably at

gradually at low laser fluence up to 0.5 J/cm2

clean silver artefacts is under vacuum conditions [75].

much shorter than in gas and a vacuum [57].

high laser fluence.

**of nanoparticles**

Laser ablation in laser-material processing has been carried out in different environments such as a vacuum, ambient air, different liquid environments and different background gases. Each medium presents different responses to laser ablation; in other words, different ablation mechanisms are observed in each environment. Although, generation of nanoparticles via pulsed-laser ablation of a solid target material in a vacuum chamber, air and gas have been widely developed, but the advancement of techniques performed in liquid environments has been of the greatest interest to researchers. The optimal medium for generation of nanoparticles is deionised water.
