Presentación de los planos

The parameters that are considered chemical, are related to the liquid environment and solid target’s structure. Common sense suggests that target’s structure can be considered the most important element which is going to determine the final char- acteristics of the NPs, nonetheless considering the electromagnetic intensities used in PLAL, the liquid environment in which particles are created can have a strong influence on their chemical composition and physical properties, mainly due to the interaction of chemical active species during or after particle creation.

The level of influence of both elements depends on the energy used for NPs creation but also on the chemical reactivity between them at normal conditions. Both elements represent complex systems and should be analyzed in detail separately.

Liquid or colloidal environment . The main important points of the environ- ment’s influence that should be addressed lies, firstly on the spacial confinement of plasma plume and cavitation bubble, secondly on the interaction between its chem- ical species and the NPs formation process, and finally its further influence on NPs agglomeration process.

As it was mentioned above, the biggest attraction of fabricating nanostructures by ablation in a liquid ambiance is the possibility to have a high control over the final material physical-chemical attributes. When a liquid ambiance is used in ablation process instead of a gas ambiance, aside of a cavitation bubble appearance, the den- sity and viscosity is higher and the relief of energy from the plasma plume becomes more difficult, thus precluding an overgrowth of the plasma plume and therefore pro- moting a more controlled NPs formation [84]. It is also well known that during the ablation process not just target’s material is receiving energy, also chemical species in the liquid ambiance does. The energy absorption from chemical species located at the plasma plume promotes in the most of the cases its dissociation, giving the chance to induce impurities in the NPs, but also react with the energetic species de- tached from the solid target [97]. Normally, regardless purity level of solvents used in the experiments under traditional conditions, the most of them contain atmospheric gasses, being reported CO2, O2, H2O and N2 the most chemically active for the most

of target’s materials [98]. The concentration of each element in the liquid solution may promote different chemical effects resulting in important changes on the created nanomaterials. Among the most important results reported up to know, it has been determined that the biggest consequence lies in the stability, size distribution and final morphology of the NPs [90, 99]. To have a better understanding of this, it’s interest- ing to analyze the special case of metals, due to the extensive research around them.

For metallic solid targets, solvents with a high concentration of oxygen and molecules containing it, the most probable result is the oxidation of NPs surface leading to sta- ble colloids. The oxidation of particles may happen at the beginning of their creation, during this time detached energetic species form non-stable systems and have more chances to loose electrons located at the metallic bonds by interacting with ionized oxygen species [100]. This process is not limited to the plasma plume lifetime, also in cavitation bubble can exist the necessary conditions. Contrary to what happens with oxygen species, when a high concentration of carbon is present a different process happens. It has been observed that carbon has a low reactivity with several metals leading to a capping process instead of a reaction. Apparently, as carbonic species don’t interact with metallic species, when they coexist in the plasma plume, carbonic species tend to interfere in the NPs growth by forming shields around the energetic species contained in the plasma plume, which limits the NPs growth [101]. Neverthe- less, it has been observed that through the time the capping effect of carbonic species also leads to agglomeration of the captured NPs [102]. At this point, it is clear that PLAL is compatible with the use of molecules that act as capping agents, giving the possibility to use dissolution.

The use of dissolution represents endless opportunities, from controlling the par- ticle’s size and stability by the use of surfactants to functionalize particles by the use of more complex molecules [90]. In general it has been reported that the use of dissolution can promote interactions with the detached material at different intensity levels.

The first level of interaction is governed by electrostatic interactions, when molecules in the liquid environment don’t present a strong chemical reaction with the detached species, tend to form a molecular shield which surrounds the particles limiting the growth and possibly avoiding the further agglomeration [103]. When the molecules in the dissolution present a moderated chemical reactivity with the material’s target, chemical bonding with particle’s surface is possible allowing a irreversible capping effect promoting the most efficient way to limit the particle’s growth among oth- ers [89]. When the chemical reactivity between the target’s material and dissolution is so strong, the chemical composition and crystalline phase of particles and dissolu- tion’s molecules can be modified leading to obtaining exotic chemical species [104].

Solid target . Beyond represent the basic material from which NPs are going to be created, solid target also has a strong influence on the ablation mechanism that is going to promote the detachment of material. Perhaps one of the biggest lessons left by the famous Heinrich Hertz’s experiment of photoelectric effect that eventually was

explained by Albert Einstein is that every element has a minimum photon energy to be ionized. In this sense, the choice of solid target has a direct implication on the energy exchange process between laser radiation and the surface’s solid material itself. The values of the photoelectric’s energy threshold depends on the crystalline state of the material, on the presence of electrons at the last electronic layers, on the possible mixtures of materials, and finally the Fermi’s energy in the system [105]. Normally, transition metals like Au or Ag present the highest energy threshold and alkaline metal present the lowest energy threshold [106].

At lower energy thresholds, electron detachment due to the absorption of a single photon increases prioritizing the photo-ionization effect over electron-lattice thermal- ization, preventing in this way the investment of energy in thermal effects during the ablation process [107]. Additionally, the increment of species coming from photo- ionization in the plasma plume can promote a higher density of energetic electrons in the plume zone leading to non-equilibrium conditions in the plasma plume. A strong energetic imbalance in the plasma plume can result in the formation of new materials or materials with non-stable crystalline phases [108].

2.2

Post-irradiation process to control nanomete-