**2. Research methods**

The nanostructures described in this chapter were found when machining micro-devices with a micromachining station based on laser ablation [30]. In particular, the monocrystalline silicon nanostructure was obtained in wafers coated with a sacrificial layer. The nanoparticles were found while analyzing the material ejected during the micro-devices' machining. Later, adequate containers for their generation were developed.

The micromachining station was composed of a laser and a substrates' positioning system. The laser was a frequency doubled Nd:YAG laser (532 nm) emitting pulses of 8 ns FWHM at a repetition rate of 10 Hz. The maximum achievable fluence, for this wavelength, was 250 mJ/ cm2 . The substrates' positioning system consisted of eight motors: six stepper motors with micrometric resolution and two piezoelectric motors with nanometric resolution. A diagram of the micromachining station is shown in **Figure 1**.

**Figure 1.** Micromachining system diagram.

**4.** Mechanical exfoliation or chipping.

to 105

to generate the nanoparticles are produced [21].

The high temperature and density of the ejected material near the target's surface develops a pressure exceeding in several orders of magnitude the atmospheric pressure, leading to an expansion of the vapor. During the adiabatic expansion (occurring at low pressure or vacuum) that follows, the thermal energy is converted into kinetic energy, causing a very fast cooling

to a supersaturation condition in which nucleation becomes energetically favorable [19].

From nucleation theory [20], it is known that the nucleation barrier to form a spherical cluster depends on the cohesive forces between the atoms in the liquid phase, the energetic barrier due to the surface tension, and the plasma ionization (in dielectric materials the electric field leads to polarization). If the radius of the particles with packed atoms is very small, the particle will continue growing, while if it is large, the particle will stop growing and may even break up. The laser beam polarizes the atoms and this effect tends to pack them, so that the critical radius will be even smaller. So, the nucleation energy decreases and a higher amount of nuclei

Nucleation is said to be homogeneous if clusters are produced from the vaporized material, and the NPs are composed of a few tens of atoms. On the other hand, if clusters are already present during the vapor plume condensation, it is considered a heterogeneous condensation. The already existing clusters are considered nucleation centers and play a predominant role

The number of particles is decreased both by collisions and coalescence producing an increase of the NPs' average size. Coalescence in a vapor–liquid medium spontaneously occurs as a reduction of the total surface area during this process, corresponding to a reduction of Gibbs free energy. It takes place up to a few ms after the laser pulse. Then, the NPs cluster due to Van der Waals and electrostatic forces. Clustering is a characteristic feature of NPs' synthesis by laser ablation in gases or liquids that do not contain added stabilizing agents [21–24].

NPs suspended in transparent liquid media can be irradiated, too. Experiments show that when NPs' suspensions are irradiated with pulsed lasers, new structures [25] such as disks,

The nanostructures described in this chapter were found when machining micro-devices with a micromachining station based on laser ablation [30]. In particular, the monocrystalline silicon nanostructure was obtained in wafers coated with a sacrificial layer. The nanoparticles were found while analyzing the material ejected during the micro-devices' machining. Later,

The micromachining station was composed of a laser and a substrates' positioning system. The laser was a frequency doubled Nd:YAG laser (532 nm) emitting pulses of 8 ns FWHM at

segments, cubes, and pyramids of nanoscale dimensions [26–29] are generated.

adequate containers for their generation were developed.

K in 1 μs for a ns laser). The extreme cooling rate leads the plasma

**5.** Hydrodynamic spraying.

of the plasma (from 104

436 Radiation Effects in Materials

in the condensation stage.

**2. Research methods**

Different samples of 2 cm × 2 cm of monocrystalline silicon were cut from c-Si wafers of 4" diameter and 525 μm thickness. These samples were later coated with different films: Si2O, Si3N4, positive photoresins, resins with dyes, or in some cases, a combination of these films.

The plasma-enhanced chemical vapor deposition (PECVD) technique [30–32] was used to perform Si2O and Si3N4 films coatings. The effect of laser ablation on films of different thicknesses such as 110, 300, 430, 510, 1220, and 2400 nm was studied. The c-Si samples coated with Si2O were named c-Si + Si2O(*x*), where *x* indicates the film thickness in nm. The thicknesses of the Si3N4 films were 180, 420, 500, 570, and 960 nm, and the naming: Si + Si3N4(*x*), *x* indicating the film thickness in nm. Commercial grade c-Si wafers were also used. These wafers have a 50 nm thick Si3N4 film coating on either one or both sides.

Metals such as Fe (99.9%), Ag (99.9%), Au (99.9%), Ta (99.9%), and Cu were other materials processed. Results obtained on Tantalum coated with a 2.5 μm thick AZ1518 resin with the spin-coating technique [33] will be also presented.
