*3.1.2. Semiconductor and oxide nanoparticles*

**Figure 10** shows a series of TEM images illustrating the change in morphology for ZnO nanoparticles formed by PLA in water at pressures of 0.1, 15, 22, and 31 MPa [27]. At a pressure of 0.1 MPa, the size distribution is quite large, with maximum particle sizes reaching values of 50 nm up to 100 nm. By increasing the water pressure, the authors found that the particle size

**Figure 10.** TEM images of ZnO nanoparticles obtained by PLA in water as a function of pressure. **(a)** Atmospheric pressure (0.1 MPa). **(b)** 15 MPa. **(c)** 22 MPa. **(d)** 31 MPa. The magnification of all images is the same, and the length of the scale bars is 100 nm. Reprinted with permission from Ref. [27]. Copyright (2013) American Institute of Physics.

Pulsed Laser Ablation in High-Pressure Gases, Pressurized Liquids and Supercritical Fluids: Generation... http://dx.doi.org/10.5772/65455 233

**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].

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

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

**Figure 10** shows a series of TEM images illustrating the change in morphology for ZnO nanoparticles formed by PLA in water at pressures of 0.1, 15, 22, and 31 MPa [27]. At a pressure of 0.1 MPa, the size distribution is quite large, with maximum particle sizes reaching values of 50 nm up to 100 nm. By increasing the water pressure, the authors found that the particle size

**Figure 10.** TEM images of ZnO nanoparticles obtained by PLA in water as a function of pressure. **(a)** Atmospheric pressure (0.1 MPa). **(b)** 15 MPa. **(c)** 22 MPa. **(d)** 31 MPa. The magnification of all images is the same, and the length of the scale bars is 100 nm. Reprinted with permission from Ref. [27]. Copyright (2013) American Institute of Physics.

Copyright (2008) American Chemical Society.

20 MPa was realized on gold and silver targets [26].

*3.1.2. Semiconductor and oxide nanoparticles*

**Figure 11.** Photoluminescence spectra and luminescence images of silicon nanocrystals. **(a)** Red light-emitting silicon nanocrystals generated at *p* = 4.56 MPa in supercritical CO2. **(b)** Green light-emitting silicon nanocrystals generated at *p* = 14.8 MPa in supercritical CO2. **(c)** Blue light-emitting silicon nanocrystals generated at *p* = 11 MPa in supercritical CO2. **(d)** Near-IR light-emitting bulk silicon measured by excitation at 632.8 nm of a He-Ne laser. **(e)** Photoluminescence images measured by a fluorescence microscope at an excitation wavelength of 375 nm. Reprinted with permission from Ref. [29]. Copyright (2009) American Chemical Society.

decreased and the difference between particle sizes also was reduced. Independent XRD measurements indicated particle sizes of from about 35 nm to ∼ 15 − 20 nm.

Using ZnO nanoparticles with well-defined defects, mainly interstitial zinc atoms (Zni ) and charged oxygen vacancies (VO + ) showed promising properties as highly sensitive oxygen sensors [28].

PLA of silicon (Si) targets in scCO2 permitted to control the size of Si-nanocrystals (Si-nc) [29].

The size distribution of Si-nc can be estimated from different characteristic cooling times *τ*, which can be expressed by

$$\tau = \frac{R\_{\rm si}^2 \rho\_{\rm si}^2 C\_{\rm si}^2}{9 \,\rho\_{\rm co\_2} C\_{\rm co\_2} \lambda\_{\rm co\_2}} \tag{4}$$

with RSi the radius of the Si-nc, *ρ*Si its density, *C*Si its specific heat capacity, and *ρC02* and *C*C02 the corresponding values of the supercritical CO2, λC02 being the thermal conductivity. By adjusting the pressure of the supercritical fluid, different cooling rates could be realized, and the obtained nanoparticles showed varying photoluminescence spectra. The different colors could be attributed to different types of defects caused by changes in the cooling rates (τ−1). (**Figure 11**).

Using a similar approach of PLA in supercritical CO2, white light-emitting Si nanoparticles could be obtained [30].

### *3.1.3. Carbon nanomaterials*

In addition to metallic and semiconductor nanoparticles, PLA has also been used for the synthesis of carbon nanomaterials. One group used single and double pulse laser ablation of graphite targets placed in water that was pressurized at values ranging from = 1 − 146 atm [31]. At = 146 atm, the cooling rate was highest, resulting in the formation of carbon nanotubes.

**Figure 12** shows the Raman spectra of the collected particles for water at 1 and 146 atm. At atmospheric pressure (**Figure 12 (a)**), the Raman spectrum of the ablated particles exhibit features that are characteristic of diamond-like carbon (DLC), that is, a mixture of carbon with different degrees of amorphousness [32]. As shown in **Figure 12(b)**, at a pressure of 146 atm, the Raman spectrum contains features that are characteristic of carbon nanotubes [33].

**Figure 12.** Raman spectra of particles produced by PLA of graphite target in H2O. **(a)** Single pulse PLA at 1 atm liquid pressure. **(b)** Single pulse PLA at 146 atm. Data adapted with permission from Ref. [31].

PLA in supercritical xenon [7] and CO2 [34] has also been employed for the synthesis of molecular diamond, so-called "diamondoids" [35]. Diamondoids are carbon nanomaterials, consisting of a C(*sp*<sup>3</sup> ) – *C*(*sp*<sup>3</sup> hybridized carbon cage structure in the form of adamantane units, which can be superimposed on a diamond lattice, and a H-terminated surface. Except for their isolation from crude oil and gas sources [36], diamondiods consisting of more than 4 units ( 4) are very difficult or even impossible to synthesize. It was shown that by PLA in supercritical fluids diamondoids up to 10 units could be synthesized. **Figure 13** shows examples of mass spectra of diamondoids synthesized by PLA in supercritical xenon: diamantane ( = 2, molecular ion peak at mass-to-charge ratio / 188, **Figure 13(a)**); pentamantane ( = 5, molecular ion peak at / 330, **Figure 13(b)**), and octamantane ( = 8, molecular ion peak at / 472, **Figure 13(c)**).

nanoparticles showed varying photoluminescence spectra. The different colors could be attributed to different types of defects caused by changes in the cooling rates (τ−1). (**Figure 11**).

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

Using a similar approach of PLA in supercritical CO2, white light-emitting Si nanoparticles

In addition to metallic and semiconductor nanoparticles, PLA has also been used for the synthesis of carbon nanomaterials. One group used single and double pulse laser ablation of graphite targets placed in water that was pressurized at values ranging from = 1 − 146 atm [31]. At = 146 atm, the cooling rate was highest, resulting in the formation of carbon nano-

**Figure 12** shows the Raman spectra of the collected particles for water at 1 and 146 atm. At atmospheric pressure (**Figure 12 (a)**), the Raman spectrum of the ablated particles exhibit features that are characteristic of diamond-like carbon (DLC), that is, a mixture of carbon with different degrees of amorphousness [32]. As shown in **Figure 12(b)**, at a pressure of 146 atm, the Raman spectrum contains features that are characteristic of carbon nanotubes [33].

**Figure 12.** Raman spectra of particles produced by PLA of graphite target in H2O. **(a)** Single pulse PLA at 1 atm liquid

pressure. **(b)** Single pulse PLA at 146 atm. Data adapted with permission from Ref. [31].

could be obtained [30].

tubes.

*3.1.3. Carbon nanomaterials*

From samples fabricated by PLA in supercritical CO2, traces that could be attributed to "superadamantane," a highly symmetric diamondoid (point group *Td*, C35H36) consisting of = 10 units, were found [34].

**Figure 13.** Mass spectra of products obtained by PLA in supercritical xenon. **(a)** Mass spectrum mad molecular structure of diamantane (molecular weight 188). **(b)** Mass spectrum with molecular ion peak (M+•) at / 330 that may be attributed to pentamantane (C25H30). **(c)** Mass spectrum of species with molecular weight of 472 that could be assigned to octamantane C36H40). Data adapted with permission from Ref. [7].

**Figure 14.** Variation of the number of diamondoid cages obtained by CO2 with and without cyclohexane as a function of the relative retention time in the gas chromatography-mass spectrometry measurements. The retention time of diamantane is the reference retention time, and the color map indicates the molecular weight (MW) of the detected diamondoids. Diamondoids with higher MWs need increasingly longer elution times for being detected by mass spectrometry, the increase being almost linear. Data adapted with permission from Ref. [34].

**Figure 14** shows the possible types of diamondoids (indicated by the number of cages ) that were obtained by PLA in CO2, as a function of the relative retention time.

These examples illustrate that PLA in high-density media allows the synthesis of nanomaterials far from thermodynamic equilibrium that would be difficult or impossible to be achieved by other methods, the main reasons being the high pressures and temperatures that can be achieved by the PLA plasma, and the cooling rates that can be modified by changing the density of the fluid. At present, the main drawback of nanomaterials fabrication by PLA in high-density media is that the quantities of the formed nanomaterials cannot be increased easily to industrially relevant quantities.

### **3.2. Characterization of nanoparticle formation**

**Figure 15** illustrates a schematic of the experimental setup for carrying out in situ Small Angle X-ray Scattering (SAXS) measurements of nanoparticles during PLA. For continuous refreshing of the target surface, the target is in the form of a moving metallic ribbon (In this case silver (Ag), speed 10 cm s−1). To avoid convolution of the SAXS measurements and overlapping of ablated material from previous laser shots, the fluid (H2O) is also continuous (in this specific case, the authors used a flow rate of 25 l h−1).

These time-resolved SAXS measurements of pulsed laser ablation in liquid water at atmospheric pressure revealed that after laser absorption by the target, a vapor-filled cavitation bubble is formed at the target surface which undergoes oscillation including a rebound and final collapses after 220 ms. Inside the cavitation bubble two types of particles can be identified, namely, compact primary particles of 8 − 10 nm size and bigger agglomerates of 40 − 60 nm  size. While it cannot be ruled out, presently, SAXS experimental detection limits cannot prove or disprove the presence of very small particles or particle clusters with sizes < 2 nm.

**Figure 15.** Schematic of PLA and synchrotron measurement during formation of nanoparticles. **(a)** Experimental setup of an X-ray scattering experiment, liquid flow conditions and moving target to provide reproducible experimental conditions for every laser pulse during time-gated data accumulation. **(b)** Schematic of the stroboscopic data acquisition with the detector being gated active for a fixed interval with delay with respect to the laser impact. The oscilloscope traces of the transmission change are recorded at the same time. Figure adapted with permission from Ref. [37].

**Figure 14.** Variation of the number of diamondoid cages obtained by CO2 with and without cyclohexane as a function of the relative retention time in the gas chromatography-mass spectrometry measurements. The retention time of diamantane is the reference retention time, and the color map indicates the molecular weight (MW) of the detected diamondoids. Diamondoids with higher MWs need increasingly longer elution times for being detected by mass

**Figure 14** shows the possible types of diamondoids (indicated by the number of cages ) that

These examples illustrate that PLA in high-density media allows the synthesis of nanomaterials far from thermodynamic equilibrium that would be difficult or impossible to be achieved by other methods, the main reasons being the high pressures and temperatures that can be achieved by the PLA plasma, and the cooling rates that can be modified by changing the density of the fluid. At present, the main drawback of nanomaterials fabrication by PLA in high-density media is that the quantities of the formed nanomaterials cannot be increased easily to indus-

**Figure 15** illustrates a schematic of the experimental setup for carrying out in situ Small Angle X-ray Scattering (SAXS) measurements of nanoparticles during PLA. For continuous refreshing of the target surface, the target is in the form of a moving metallic ribbon (In this case silver (Ag), speed 10 cm s−1). To avoid convolution of the SAXS measurements and overlapping of ablated material from previous laser shots, the fluid (H2O) is also continuous (in this specific

These time-resolved SAXS measurements of pulsed laser ablation in liquid water at atmospheric pressure revealed that after laser absorption by the target, a vapor-filled cavitation bubble is formed at the target surface which undergoes oscillation including a rebound and final collapses after 220 ms. Inside the cavitation bubble two types of particles can be identified, namely, compact primary particles of 8 − 10 nm size and bigger agglomerates of 40 − 60 nm

spectrometry, the increase being almost linear. Data adapted with permission from Ref. [34].

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

were obtained by PLA in CO2, as a function of the relative retention time.

trially relevant quantities.

**3.2. Characterization of nanoparticle formation**

case, the authors used a flow rate of 25 l h−1).

Similar trends were observed using different water-ethanol mixtures from 0 to 100 % and pressures up to 13 MPa of the fluid, which enabled to control various types of defects in ZnO nanoparticles [38]. The higher pressures were associated with shorter lifetimes of the cavitation bubble, leading to higher quenching rates of the nanoparticles produced. The faster cooling rates also resulted in less agglomeration of the particles and higher surface-to-volume ratios. As a consequence, for higher pressure, the number of interstitial oxygen (Oi ) defects was found to increase and at the same time, the contribution of near-band-edge emission, that is, emission in the UV (at 400 nm), was found to decrease.

**Figure 16.** Schematic illustration of particle formation inside a laser-induced cavitation bubble and distribution at the time of its largest size for PLA inside atmospheric pressure water. **(a)** Time-line of laser irradiation and evolution of bubble height. **(b)** Evolution of cavitation bubble and evolution of primary and secondary particles. Figure adapted with permission from Ref. [40].

**Figure 16** illustrates the particle formation inside a cavitation bubble generated by laserirradiation of a target surface placed inside a liquid [39].

The time shown is at the instant of the largest extent of the bubble. After irradiation of the target by the laser, primary particles are formed. Over time, these primary particles coagulate to form larger, secondary particles. While the secondary particles are trapped inside the cavitation bubble, some of the primary particles can escape from the inside of the cavitation bubble to the surrounding fluid. In other words, the interface of the cavitation bubble is not an impenetrable wall or membrane, but instead can be crossed by particles.

Agglomeration occurs for the confined particles in the second cavitation bubble, which forms after the first bubble has collapsed. Additionally, upon the collapse of the second bubble, a jet of confined material is ejected perpendicularly to the target surface.

From this one can see that the lifetime of the cavitation bubble, the pressures and temperatures reached inside the cavitation bubble, and the conditions of the surrounding fluid all influence the nucleation and growth of nanoparticles.

In another study, to monitor the formation of nanoparticles in situ, a multipurpose timeresolved spectrometer was developed, that allows following the formation of nanoparticles over several timescales [41]. The spectrometer consists of three different parts, one that can be used for following the nanoparticle formation by time-resolved absorption spectroscopy in the wavelength range of 350 − 850 nm and on timescales of nanoseconds to milliseconds in the time following the laser pulse. The second part consists of an absorption spectrometer that allows following the nanoparticle formation on timescales of seconds to hours between 220 and 900 nm, and the third component, which dynamic light scattering for tracking nanoparticles with sizes ranging from 10 nm to 10, over timescales of seconds to hours.
