*3.1.1. Metallic nanoparticles*

The light source (e.g., a flash lamp or a light-emitting diode) is synchronized with a fast detector, for example, an intensified charge-coupled device (ICCD) or a streak camera. Flash lamps or diodes allow illumination times of typically Δ 10 − 100 μs while Nd:YAG lasers

**Figure 7(a)** shows plasma emission images of PLA realized on a Ti target immersed in water at 0.1 and 30 MPa, respectively [21]. A crosssection of the emission intensity in a direction normal to target, in the middle of the plasma, is presented in **Figure 7(b)**. In both cases, the maximum emission is at a small distance from the target, and as the pressure is increased, the

**Figure 7.** Optical emission intensities for pulsed laser ablation plasma in ambient and pressurized water. **(a)** Optical emission image observed at 0.1 MPa. **(b)** Optical emission image observed at 30 MPa. **(c)** Cross-sections of optical emission images in and along a direction normal to the target surface. Data adapted with permission from Ref. [21].

**Figure 8.** Optical emission spectra measured in CO2 at *p* = 0.1 and 7.4 MPa for PLA on a Ni target. The dashed boxes indicate the domains of the spectrum where Ni lines are dominant. The inset on the left shows a close-up in the wavelength range between 235 and 255 nm, where peaks that can be attributed to atomic and ionized C can be found. The inset on the right shows the detailed spectrum in the region around 777 nm containing lines of atomic O. Data adapted

allow higher fluencies and permit illuminations at pulse durations of Δ 3 − 10 ns.

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

extension of the plasma becomes squeezed along the normal to the target.

*2.2.3. Optical emission characteristics*

with permission from Ref. [5].

Metallic nanoparticles play an increasingly important role in many different fields. These include sensing, catalysis, electronics, and plasmonics, and especially the plasmonic properties of noble metallic nanoparticles has opened new possibilities in biotechnology and medicine, including DNA and protein sensing or new approaches for cancer therapy [22].

PLA in pressurized media is a promising approach for fabricating metallic nanoparticles and tailoring their properties. For example, gold nanoparticles were obtained by ablation in supercritical CO2 at pressures up to 20 MPa [8, 23]. In addition to the particle size, the authors also investigated the influence of the pressure on the ablation depth, which was found to correlate with the constant volume heat capacity (*C*V), that is, the largest particle removal rates were found for conditions 10 MPa.

The density has also been found to affect the morphology and the size distribution of particles. **Figure 9(a)**–**(d)** show the morphologies of nanostructures obtained by PLA of Au targets in supercritical CO2 [24]. For lower pressures ( = 4.29 MPa), the structures obtained resemble chains (**Figure 9(a)** and **(c)**), while at higher pressures ( = 14.5 MPa), spherical Au-nanoparticles are obtained (**Figure 9(b)** and **(d)**).

**Figure 9.** SEM images illustrating the influence of pressure on gold nanoparticle morphologies obtained by PLA in supercritical CO2. **(a)** Nanoparticles generated by laser ablation at *p* = 4.29 MPa. **(b)** Nanoparticles generated by laser ablation at *p* = 14.5 MPa. **(c, d)** Enlarged images of (a) and (b). Reprinted (adapted) with permission from Ref. [24]. Copyright (2008) American Chemical Society.

In one report, the effect of laser fluence and fluidic pressure up to 200 MPa of PLA in water were investigated [25], while in a different study, PLA in pressurized CO2 between 0.1 and 20 MPa was realized on gold and silver targets [26].
