*1.1.1. Applications: Nanostructuring and manufacturing of nanoparticles*

Nanomanufacturing was developed in the last decade driven by the nanotechnology progress. Sub-micrometric structures in the irradiation zones or resulting from the ablation plume were observed since the beginning of pulsed lasers applications to materials' processing. However, at that time, these were a problem and many attempts have been made to mitigate them. It was only in the last decade that they were considered useful [9–12].

The large scope of applications of the sub-micrometric structures' with unique properties that have been lately found [13–15] contributed to this change of perspective. Although the manufacture of nanometric structures and particles with pulsed lasers is quite simple and can be applied to a large variety of materials, the phenomena involved are difficult to analyze and simulate, resulting in a theoretical and experimental research field in continuous growth. In particular, in this chapter, we will present a brief overview of the heuristic functions which describe the laser fluence dependence of the removed material in laser ablation processes of different substrates as well as the nanostructures and nanoparticles found in the analysis with an electronic microscope of the quality of micromachined devices and of the material ejected in the ablation plume, respectively.

#### *1.1.1.1. Surface modification – nanostructuring*

In this chapter, we will present a review of the nanostructures found by our research group during the quality control of the surfaces of micromachined devices. On the other hand, a method for generating metallic nanoparticles which was developed on the basis of the analysis of the material ejected during the micromachining ablation process will be reported. Nano‐ particles were then generated in different media such as air, deionized water, isopropyl alcohol and sodium dodecyl sulfate (SDS) solution. Preliminary results of the generation of Fe@Au

A brief summary of the fundamentals of laser ablation of solid substrates and of the theory of the resulting ablation plume will be given in the next section. A large variety of studies in this field including basic research and applications have been carried out worldwide by many researchers. However, we will focus on the area of micromachining by laser ablation [1–4].

The laser ablation mechanism is one of the most complex phenomena observed when laser radiation interacts with a solid material [1]. Ablation may be produced either with pulsed or with intense continuous wave (cw) lasers [1, 3]. For a given type of material, the onset of ablation takes place around a threshold fluence (∅ *th*), which depends on absorption mecha‐ nisms and particular properties of the material, such as its microstructure, its morphology, and the presence of defects as well as on laser parameters (intensity, wavelength, and pulse duration) [1, 2, 5, 6]. When irradiation is performed with several consecutive pulses impinging on the same sample's area, ∅ *th* can vary due to the accumulation of defects in the previously irradiated zone. At a given wavelength, the amount of material removed per pulse usually shows a logarithmic increase with fluence in accordance with the Lambert–Beer law. On the other hand, Villagran-Muniz et al. [7] reported heuristic equations to describe the fluence dependence of the amount of removed material per pulse of a given substrate with parameters

In particular, for laser pulse durations larger than 10 ps, the laser beam will also interact with the ablation plume as in the case of 10 ns Nd:YAG lasers. In these conditions, the plume will also absorb and scatter radiation. In consequence, the amount of energy reaching the substrate will be less since part of it will be absorbed by the plasma, generating a hot plasma. This excited plasma will then expand and create shock waves in the molten substrate. Frozen shock waves can be observed in the material when the irradiated zone cools [8]. Another effect produced by the excited plume is the explosion of the remnant molten material, which produces splashes. The expelled liquid will inevitably solidify around the irradiated area together with plume's material condensation. In addition, the rapid generation of large thermal gradients may induce excessive thermal stress and thermoelastic excitation of acoustic waves. These stresses may add hardening work, buckling, or cracking [1] to the response of the material, effects which

However, this is not a general feature of all materials. The dynamics of the laser–matter interaction starts with the electronic excitation and relaxation. For weak electric fields, ionization of an atom occurs when the energy of the incident photon exceeds the binding energy of the valence electron. Thus, unlike metals, in a large variety of materials with valence

nanoparticles (core–shell nanoparticles) will be also discussed.

that can be related to physical properties of the substrate.

must be taken into account in each particular application.

**1.1. Laser ablation**

432 Radiation Effects in Materials

Nanostructuring by interaction with pulsed lasers is known since the beginning of lasers' applications to machining. However, in recent years, it is an issue that is gaining large importance due to its potential applications in various fields. Different types of nanostructures (NS), named laser-induced periodic surface structures (LIPSS), have been reported. The low spatial frequency LIPSS (LSFL) are characterized by a spatial period Λ of about the laser wavelength (*λ*), that is, Λ ~ *λ*. Structures called high spatial frequency LIPSS (HSFL) have a spatial period Λ shorter than the laser wavelength, Λ ≪ *λ*. So far, it is accepted that Λ depends on the laser wavelength and on the radiation incidence angle. These LIPPS are generally described by their period Λ and their orientation with respect to the polarization of the laser [9]. However, the origin of the HSFL is still under debate and some theories have been proposed.

Zhang et al. [10] report NS formed with ultrashort lasers pulses (Ti: Sapphire, 1 kHz, 800 nm, 120 fs). They describe the formation of both types of ripples: LSFL and HSFL. They explain that LSFL arise from optical interference effects due to the coherent interaction between the incident radiation and the electromagnetic wave scattered by the surface. Two main mecha‐ nisms are proposed for HSFL formation. One is related to the interaction of the laser pulse with the surface plasma produced by the incident laser. The other mechanism is associated to a combination of interference effects and second harmonic generation. For example, they have experimentally found that the period of the ripples decreases with the increase of the laser fluence on the surface. They report ripples generated perpendicular to the laser polarization in the single-crystal superalloy CMSX-4. According to the experiments of these authors, HSFL (Λ ≪ *λ*) are generated from LSFL (Λ ~ λ) with a period about half of the LSFL spatial period. That is, while the period of the crests of LSFL is about the laser wavelength (*λ<sup>L</sup>* = 800 nm, Λ*LSFl* ~ 760 nm), that of the HSFL is Λ*HSFl* ~ 360 nm, which is about half the laser wavelength.

NS in silicon were observed in areas up to 100 mm2 using a versatile laser machining station. Experiences show that the morphology and frequency of the ripples of these structures do not depend either on the focusing geometry, energy, scan rate, polarization, or incident angle of the radiation [11]. It can be thus inferred that laser-based synthesis of nanomaterials is an area that is in its beginnings.

### *1.1.1.2. Manufacturing of nanoparticles*

The manufacture of nanoparticles by pulsed laser ablation is framed within what is known as laser synthesis of nanomaterials (NM). The nanomaterials have extensive applications in electronics, medicine, and energy generation and storage. The physical and chemical proper‐ ties of these materials strongly depend on their dimensions and shape. The most investigated nanostructures are spheres (nanoparticles (NPs) and "quantum-dots") and cylinders (nano‐ tubes, nanowires, nanorods). In particular, the nanostructures are made by pulsed laser ablation (PLA). In the PLA of a solid, the NPs produced have the same composition of the solid from which they come. If it takes place in air or vacuum, then thin coatings of NPs formed from the ejected clusters can be generated on the surface. When PLA takes place in a liquid, then a colloidal solution is obtained. On the other hand, there is also evidence that the interaction of the laser radiation with nano- and micro-materials in solutions and/or in suspensions in liquids/gases can produce new structures: nanostructures or alloys, as reported in Chapter 7 of reference [1].

Pulsed laser ablation of materials was introduced with the advent of the ruby laser around 1960 [16]. Since then laser for machining research has been directed to obtain new emission wavelengths, increase the repetition rate and output energy and, of course, decrease the pulse duration, nowadays up to attoseconds (10−18 s). This research was mainly encouraged by the fact that the laser processing is a powerful tool that does not wear during machining and, in addition, does not produce pollution. On the other hand, in PLA, materials are subjected to high temperatures and pressures, giving rise to a very particular chemistry with the production of various compounds of oxides, carbides, and nitrides. Besides, since cooling rate is very high due to the rapid expansion of the plasma plume, metastable nanomaterials which are difficult to obtain by other techniques can be produced.

PLA is conceptually simple. However, the material removal mechanisms extend over very long intervals, from fs to ms, at least nine orders of magnitude. When the laser is focused on the surface of a dielectric solid, wavelength dependent non-thermal effects take place. When the solids are metals and semiconductors, thermal processes are also present. The mechanisms and the characteristics of the ejected species strongly depend on the parameters of the laser radiation (wavelength, pulse duration, and fluence).

With a nanosecond laser, PLA is basically a photothermal phenomenon. The incident energy excites the electronic and vibrational levels of the substrate and, in consequence, the material is heated, melted, and evaporated in the first 10–100 ps following the pulse arrival. For relatively low fluence radiation (≤0.3–1 J/cm<sup>2</sup> ), the ejected material, by desorption and evapo‐ ration, is mostly atomic size vapor. Then, the resultant vapor plume expands vertically and is ionized by the photons that keep coming. For fluences near the ablation threshold, the amount of ionized material can be calculated by the Saha equation [17]. If the fluence is larger than the ionization threshold, then optical breakdown occurs and the degree of ionization can be estimated from the Saha-Boltzmann equation.

The plasma role during the laser–matter interaction is not yet fully understood and has been discussed over the past two decades. The plasma is composed of a set of atoms and electrons that remain confined in a high energy field. The laser can produce plasma because during a very short time lapse the photons that interact with the atoms of the solid target can strip off about 15 electrons from each atom. Therefore, the photons that arrive after the solid has been vaporized, contribute to increase the degree of ionization [1]. Ionization strongly influences the dynamics of the plume's condensation.

As the laser fluence increases (Φ ≥ 10 J/cm<sup>2</sup> ), there is a remarkable increase in the amount of ablated material suggesting that a different material ejection mechanism is taking place. Experimentally, it is observed that the removed material consists of a mixture of vapor and micron size droplets (≥10 μm). Different mechanisms have been proposed to explain this behavior. Some suggest that temperature gets close to the critical temperature of the material and then a phase explosion follows [1].

In general, it can be said that in the nanomaterials' synthesis, clusters' ejection and gas to particles condensation, take place. The literature reveals that their final size varies within four orders of magnitude [1]. However, Hubental et al. propose a *tailor*ing method which can be used to homogenize NPs' size [18].

Regardless of the confinement in which the NPs are generated, certain steps taking place in their formation may be outlined:


Zhang et al. [10] report NS formed with ultrashort lasers pulses (Ti: Sapphire, 1 kHz, 800 nm, 120 fs). They describe the formation of both types of ripples: LSFL and HSFL. They explain that LSFL arise from optical interference effects due to the coherent interaction between the incident radiation and the electromagnetic wave scattered by the surface. Two main mecha‐ nisms are proposed for HSFL formation. One is related to the interaction of the laser pulse with the surface plasma produced by the incident laser. The other mechanism is associated to a combination of interference effects and second harmonic generation. For example, they have experimentally found that the period of the ripples decreases with the increase of the laser fluence on the surface. They report ripples generated perpendicular to the laser polarization in the single-crystal superalloy CMSX-4. According to the experiments of these authors, HSFL (Λ ≪ *λ*) are generated from LSFL (Λ ~ λ) with a period about half of the LSFL spatial period. That is, while the period of the crests of LSFL is about the laser wavelength (*λ<sup>L</sup>* = 800 nm, Λ*LSFl* ~ 760 nm), that of the HSFL is Λ*HSFl* ~ 360 nm, which is about half the laser wavelength.

Experiences show that the morphology and frequency of the ripples of these structures do not depend either on the focusing geometry, energy, scan rate, polarization, or incident angle of the radiation [11]. It can be thus inferred that laser-based synthesis of nanomaterials is an area

The manufacture of nanoparticles by pulsed laser ablation is framed within what is known as laser synthesis of nanomaterials (NM). The nanomaterials have extensive applications in electronics, medicine, and energy generation and storage. The physical and chemical proper‐ ties of these materials strongly depend on their dimensions and shape. The most investigated nanostructures are spheres (nanoparticles (NPs) and "quantum-dots") and cylinders (nano‐ tubes, nanowires, nanorods). In particular, the nanostructures are made by pulsed laser ablation (PLA). In the PLA of a solid, the NPs produced have the same composition of the solid from which they come. If it takes place in air or vacuum, then thin coatings of NPs formed from the ejected clusters can be generated on the surface. When PLA takes place in a liquid, then a colloidal solution is obtained. On the other hand, there is also evidence that the interaction of the laser radiation with nano- and micro-materials in solutions and/or in suspensions in liquids/gases can produce new structures: nanostructures or alloys, as reported

Pulsed laser ablation of materials was introduced with the advent of the ruby laser around 1960 [16]. Since then laser for machining research has been directed to obtain new emission wavelengths, increase the repetition rate and output energy and, of course, decrease the pulse duration, nowadays up to attoseconds (10−18 s). This research was mainly encouraged by the fact that the laser processing is a powerful tool that does not wear during machining and, in addition, does not produce pollution. On the other hand, in PLA, materials are subjected to high temperatures and pressures, giving rise to a very particular chemistry with the production of various compounds of oxides, carbides, and nitrides. Besides, since cooling rate is very high

using a versatile laser machining station.

NS in silicon were observed in areas up to 100 mm2

that is in its beginnings.

434 Radiation Effects in Materials

in Chapter 7 of reference [1].

*1.1.1.2. Manufacturing of nanoparticles*

**3.** Fragmentation.


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 of the plasma (from 104 to 105 K in 1 μs for a ns laser). The extreme cooling rate leads the plasma 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 to generate the nanoparticles are produced [21].

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 in the condensation stage.

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