*2.2.2.3. Nanostructuring in Fe(99.9%)*

with 200 mJ/cm2

444 Radiation Effects in Materials

appreciated.

cavities at different magnifications.

c) Micrographies at larger magnifications to show the porosity.

. The SEM micrographies are shown in **Figure 9a**. It shows a sequence of

contiguous cavities each drilled with 25 laser pulses each. **Figure 9b**,c shows the details of the

**Figure 9.** SEM micrographies (a) contiguous cavities machined on the copper coating of a PCB with 25 pulses each. (b,

The appearance of a porous coating of copper and copper oxide is observed as the number of laser pulses impinging on the same sample area is increased. The lower density of porous

**Figure 10** shows the superficial modifications originated by laser irradiation of sub-micro‐ metric structures of copper oxide. **Figures 10a,b** are SEM micrographies of the surface before

**Figure 10.** SEM micrographies of a copper oxide surface: (a, b) before irradiation; (c, d) after irradiation with one square laser shot. Cu nanoparticles of lighter color than the rest of the surface due to their larger conductivity can be

copper makes it suitable to be used as a lighter material for electrodes and catalysis.

irradiation; **Figure 10c,d** after irradiation with one square laser shot.

The nanostructuring found in Ta and Cu encouraged the irradiation of Fe(99.9%) samples. Only sub-micrometric structures in Fe obtained with sub-picosecond laser pulses have been reported in the literature. The HAZ effect produced on the surface of steel plates is used as anticorrosive. In reference [41], Yang et al. report that the zones in the surroundings of a folding axis are more liable to corrosion due to the larger mechanical strains to which they are subjected. On the other hand, the authors found that steel plates which had been irradiated with laser pulses before being folded became more resistant to oxidation. **Figure 11** shows the surface of a Fe(99.99%) plate after irradiation with laser pulses of 200 mJ/cm2 in which details of the induced surface nanostructuring can be observed.

**Figure 11.** Fe (99.99%) plate surface after irradiation laser pulses of 200 mJ/cm2 . The magnification of the electronic mi‐ croscope (SEM) increases from left to right disclosing details of the surface nanostructuring.

#### *2.2.3. Fe(99.9%), Au(99.9%), Ag(99.9%) nanoparticles in different media*

The manufacture method of NPs of different metals and the diverse analysis performed will be described in the following. The first NPs were generated in air in an ad hoc chamber suitable for obtaining metallic NPs in gaseous media. The substrate was fixed to the container which was then fixed to the machining system. The laser was focused on the surface of the substrate, and the movement of the positioning system of the substrate was controlled by a PC. To ease the subsequent analysis with an electronic microscope, the NPs formed were stuck on carbon tapes.

**Figure 12** shows the Fe NPs generated in air at ambient temperature and pressure with two different irradiation processes. Those shown in **Figure 12a** were generated by drawing lines with a speed of 10 μm/s with pulses of 70 μm spot size. The irradiated zones and the number of pulses are detailed in **Figure 12b**. The NPs shown in **Figure 12c**,d were generated by drilling holes by percussion with 1000 pulses. The difference between both methods is the generation of clusters of NPs. The size of the NPs generated by overlapping pulses on the substrate's surface (**Figure 12a**) is ~200 nm, and, as can be seen, they are grouped in clusters. On the other hand, the size of the NPs generated by drilling is ~50 nm and they are also grouped in clusters. In addition to NPs clusters, the formation of dense spherical microparticles is observed (**Figure 12c**). This effect could be produced by the change of the superficial crystalline structure introduced by the ns 532 nm laser pulse on the metal surface, Fe(99.9%) in this case [43, 44].

**Figure 12.** (a) Fe NPs manufactured by drawing lines with up to seven contiguous pulses, (b) Fe NPs generated by percussion (1000 pulses in the same sample area). In both processes, the laser fluence was 220 mJ/cm2 . All microgra‐ phies were obtained by SEM.

The generation of NPs in liquid media was performed at ambient temperature and pressure. The substrate was immersed in the liquid contained in a beaker. Laser pulses were focused on the substrate's surface, while the position of the substrate was continuously being changed so that each pulse would impact in a non-irradiated surface area. Metallic NPs dispersions in different liquid media such as ultrapure H2O, isopropanol, and a solution of sodium dodecyl sulfate (SDS) were obtained following this method.

**Figure 13.** NPs generated in air and in different liquid media. (a) Tyndall effect in dispersions and solutions: (from left to right) Ag NPs in ultrapure H2O, ultrapure H2O, Fe NPs in isopropanol, Fe NPs in SDS, and Fe NPs in ultrapure H2O. (b) Characterization of the Fe NPs in a SDS solution by microscopy (HR-TEM).

Different methods were used for the NPs characterization:


The advantage of this last method is that it allows preserving the NPs' properties *in situ*, avoiding alterations due to drying effects. Clustering or oxidation effects usually appear when the NPs are separated from the medium in which they were originated (**Figure 14**).

Finally, the size distribution of the NPs in their original medium was estimated from the absorbance spectra of the dispersions and their comparison with those calculated with the MiePlot algorithm (Appendix E, reference [30]).

**Figure 14.** (a) Absorbance spectrum of a dispersion of Au NPs in ultrapure H2O compared to that of the same disper‐ sion diluted 10 times, (b) comparison of the absorbance spectrum of Ag NPs in H2O experimentally obtained with that calculated for Ag NPs of 20 nm of radius in H2O with a log-normal distribution with different standard deviations.

#### *2.2.3.1. Core–shell nanoparticles (preliminary results)*

**Figure 12.** (a) Fe NPs manufactured by drawing lines with up to seven contiguous pulses, (b) Fe NPs generated by

The generation of NPs in liquid media was performed at ambient temperature and pressure. The substrate was immersed in the liquid contained in a beaker. Laser pulses were focused on the substrate's surface, while the position of the substrate was continuously being changed so that each pulse would impact in a non-irradiated surface area. Metallic NPs dispersions in different liquid media such as ultrapure H2O, isopropanol, and a solution of sodium dodecyl

**Figure 13.** NPs generated in air and in different liquid media. (a) Tyndall effect in dispersions and solutions: (from left to right) Ag NPs in ultrapure H2O, ultrapure H2O, Fe NPs in isopropanol, Fe NPs in SDS, and Fe NPs in ultrapure

H2O. (b) Characterization of the Fe NPs in a SDS solution by microscopy (HR-TEM).

Different methods were used for the NPs characterization:

. All microgra‐

percussion (1000 pulses in the same sample area). In both processes, the laser fluence was 220 mJ/cm2

phies were obtained by SEM.

446 Radiation Effects in Materials

sulfate (SDS) were obtained following this method.

Nanoparticles of different nucleus' materials and *core–shell* were produced as prototypes for *drug delivery*. The particles were required to have a ferromagnetic nucleus to be able to be delivered with magnets to the desired zones and a *core–shell* of a material assimilable by the organism so as not to be rejected by it.

The following method was used for the synthesis of *core–shell* NPs. First, a suspension of Fe NPs in ultrapure H2O was obtained by laser ablation of a Fe(99.9%) plate. An Au(99.9%) plate was subsequently irradiated in the Fe NPs suspension. The Fe NPs acted as nucleation centers for Au crystallization according to the nucleation principles described in Section 1.1.1.2.

The absorbance spectrum of the final suspension obtained, shown in **Figure 15**, resulted in very good agreement with those reported by other authors in references [45, 46].

**Figure 15.** Absorbance spectrum of dispersions of Fe NPs, Au NPs, and Fe@Au NPs in ultrapure H2O.
