**3. Films fabricated by monodispersed nanoparticle beams**

#### **3.1 Monodispersed nanoparticle by size classification**

In general, the size distribution of nanoparticles *f*x(*x*) growing in vapor phase is known to be described as the log-geometric distribution as follow [29].

$$f\_{X}\left(\mathbf{x}\right) = \frac{1}{\varkappa\sqrt{2\pi}\ln\sigma\_{\boldsymbol{\xi}}}\exp\left[-\frac{1}{2}\left(\frac{\ln\left(\varkappa/m\_{\boldsymbol{\xi}}\right)}{\ln\sigma\_{\boldsymbol{\xi}}}\right)^{2}\right] \quad \mathbf{x} \ge \mathbf{0} \tag{6}$$

**109**

**Figure 3.**

*Nanoparticle Formation and Deposition by Pulsed Laser Ablation*

While sizes of nanoparticles have a certain distribution, the physical property of the nanoparticle may vary dramatically depending on its size. Thus, the sizes of nanoparticles should be in uniform when we need deal with them as macroscopic materials like nanoparticle films. Hence, in the early 2000s, some kinds of device to make the nanoparticle size in uniform has been actively

In those times, the size selection of aerosol particles is generally done in an electric field by deflecting charged particles generated from a source of the outbreaks such as combustion gas in Diesel engine. Afterward, the Differential Mobility Analyzer (DMA), as shown in **Figure 3** [33], which has been used to measure aerosol particles in the atmosphere, has been improved to the desired extent for applying on nanoparticles [34–36]. It is able to measure the particle size distribution by using the natural law in which a flying charged nanoparticle would land on different locations according to the balance between the electric mobility and fluid resistance, being deflected by an electric field [37]. Camata et al. [38] used a DMA system to estimate the geometric standard deviation of silicon nanoparticles with an average particle diameter of 2.8 nm and got a number of the geometric standard deviation between 1.2 ~ 1.3. Suzuki et al. [39] used DMA system on silicon nanoparticles and obtained a result of an average particle diameter of 2.8 nm and a geometric standard deviation of 1.2. A consistent system from a source of nanoparticles to its sampler is shown in **Figure 4** [34]. The nanoparticles generated by laser ablation pass through the gas phase annealing system, where the particle morphologies are controlled, and move to DMA to be classified by its size, and finally reach the

*Illustration of instrument capable of measuring size distributions of aerosol particles on basis of DMA [33].*

*DOI: http://dx.doi.org/10.5772/intechopen.95299*

particle sampler and the measurement systems.

invented and designed.

Here *m*g is the geometric mean and *σ*g is the geometric standard deviation. A model has been proposed to explain the log-geometric distribution by assuming that the growth formula of a nanoparticle follows a certain Equation [30, 31]. It has also been experimentally confirmed that the size distribution of nanoparticles generated by laser ablation is log-geometric [32].

*Nanoparticle Formation and Deposition by Pulsed Laser Ablation DOI: http://dx.doi.org/10.5772/intechopen.95299*

*Practical Applications of Laser Ablation*

the following issues should be considered.

representative time of the system.

temperature.

non-equilibrium.

nanoparticle is kept or not. If we are assuming that the velocity of vapor atoms is represented in the equilibrium Maxwell distribution, we can estimate the approxi-

> *cr kT p m* <sup>=</sup> ρ

In this equation, *ρ*c stands for the internal density of the nanoparticle, while *r* the radius of the nanoparticle, *p* the vapor pressure, and *m* the mass of the vapor atoms. In order to obtain an exact solution, Gillespie [27] analyzed the process of nanoparticle formation as a random walk problem. However, no matter how rigorous the probabilistic analysis is, it is still based on classical nucleation theory. To address the problem related to the formation of nanometer-sized particle in non-equilibrium,

a.The nucleation rate equation based on steady-state theory is valid only when the formation time of a critical nucleus is sufficiently short compared to the

b.The classical theory is an available model at the range of relatively low supersaturation. In the case of high supersaturation, the internal degrees of freedom of the nanoparticle must be taken into an account, because differences are created among translational, rotational and vibrational

c.In spite of small systems, macroscopic physical properties and coefficients such as surface free energy, evaporation enthalpy, and condensation coefficient besides macroscopic concepts such as the Thomson–Gibbs formula are used. We have to correct these values and formula in conformity with the extent of

In recent years, with the development of computers, molecular dynamics or Direct Simulation Monte Carlo analyses which take into consideration the internal degrees of freedom of nanoparticles have been made [28], through which the Gibbs free energy and nanoparticle concentrations are elucidated under more realistic conditions.

In general, the size distribution of nanoparticles *f*x(*x*) growing in vapor phase is

*g g*

=− ≥

*x m*

exp 0

σ

2

(6)

 

**3. Films fabricated by monodispersed nanoparticle beams**

known to be described as the log-geometric distribution as follow [29].

( ) ( *<sup>g</sup>* ) *<sup>X</sup>*

*f x x*

1 1 ln /

2 ln

Here *m*g is the geometric mean and *σ*g is the geometric standard deviation. A model has been proposed to explain the log-geometric distribution by assuming that the growth formula of a nanoparticle follows a certain Equation [30, 31]. It has also been experimentally confirmed that the size distribution of nanoparticles

**3.1 Monodispersed nanoparticle by size classification**

*x*

generated by laser ablation is log-geometric [32].

2 ln π σ

 π2

(5)

mate nanoparticle formation time *τ* with the following equation.

τ

**108**

While sizes of nanoparticles have a certain distribution, the physical property of the nanoparticle may vary dramatically depending on its size. Thus, the sizes of nanoparticles should be in uniform when we need deal with them as macroscopic materials like nanoparticle films. Hence, in the early 2000s, some kinds of device to make the nanoparticle size in uniform has been actively invented and designed.

In those times, the size selection of aerosol particles is generally done in an electric field by deflecting charged particles generated from a source of the outbreaks such as combustion gas in Diesel engine. Afterward, the Differential Mobility Analyzer (DMA), as shown in **Figure 3** [33], which has been used to measure aerosol particles in the atmosphere, has been improved to the desired extent for applying on nanoparticles [34–36]. It is able to measure the particle size distribution by using the natural law in which a flying charged nanoparticle would land on different locations according to the balance between the electric mobility and fluid resistance, being deflected by an electric field [37]. Camata et al. [38] used a DMA system to estimate the geometric standard deviation of silicon nanoparticles with an average particle diameter of 2.8 nm and got a number of the geometric standard deviation between 1.2 ~ 1.3. Suzuki et al. [39] used DMA system on silicon nanoparticles and obtained a result of an average particle diameter of 2.8 nm and a geometric standard deviation of 1.2. A consistent system from a source of nanoparticles to its sampler is shown in **Figure 4** [34]. The nanoparticles generated by laser ablation pass through the gas phase annealing system, where the particle morphologies are controlled, and move to DMA to be classified by its size, and finally reach the particle sampler and the measurement systems.

#### **Figure 4.**

*Schematic of nanoparticle synthesis process using laser ablation, which is composed of (a) particle generator, (b) gas phase annealing, (c) particle classifying, and (d) particle measurement and correction [34].*

Although the above methods require nanoparticles being charged in some way to select the size, the flank attack method proposed by Wu et al. [40] can be used for neutral nanoparticles. In this method, the nanoparticle beam is intersected with the atomic beams of inert gas such as argon or neon, and the nanoparticle can be sorted out in accordance with the size following the law in which the smaller the nanoparticle size, the greater the deflection angle of the beam.

## **3.2 The direct fabrication of monodisperse nanoparticles**

In the previous section, the relationship between the growth rate and the size distribution of nanoparticles has been discussed. By understanding the growth process of nanoparticles, it is possible to control their size distribution. Laser Vaporization with Controlled Condensation (LVCC), as shown in **Figure 5** [41], is a method to control the average size of the nanoparticles by adjusting the conditions about nucleation and growth. By raising the temperature of the lower wall in ablation chamber with a heater and cooling down the upper wall with liquid nitrogen, the temperature gradient would cause the convection of ambient gas. When the vapor created by laser ablation drifts upward by the convection, the vapor becomes supersaturated near the upper cold wall, which results in nucleation

#### **Figure 5.**

*(A) Experimental set-up for the synthesis of nanoparticles using the LVCC method. (B) Experimental setup for the LVCC method coupled with a DMA [41].*

**111**

**Figure 6.**

*Nanoparticle Formation and Deposition by Pulsed Laser Ablation*

and condensation followed by nanoparticle forming. It is found that the higher the supersaturation is, that is, the larger the temperature gradient is, the smaller the average size of the nanoparticles becomes. Therefore, by adjusting the temperature gradient proficiently, it is possible to control the nanoparticle size. Furthermore, by combining LVCC and DMA, to control the average diameter and size distribution of

In order to determine the size distribution by directly adjusting the growth process of the nanoparticles, it is necessary to control not only statically the degree of supersaturation but also dynamically its temporal variation of it. That is, it is necessary to rapidly increase the supersaturation level to complete the nucleation within a short period of time and to inhibit the subsequent nanoparticle growth in

As described in Section 2.3, a shock wave is generated in front of the plume by laser ablation. It is possible to use the shock wave to rapidly change the state quantity in the plume and increase the supersaturation at a fast rate. One example is the laser ablation process done in a closed space such as an ellipsoidal cell, in which the collision between the reflected shock wave and the plume front triggers the formation of nanoparticles instantly in a small area [43, 44]. The equipment fabricating monodispersed nanoparticles by using this phenomenon is called Spatiotemporal Confined Nanoparticle Source (SCCS), which is illustrated in **Figure 6** [45], as the

The possibility of developing new functional materials with nanoparticle films

has been pointed out for a long time [46], and many attempts have been made mainly to create light-emitting devices using the visible light emission from silicon

fabrication process is restrained in the confined space and time.

**3.3 Application of self-ordered nanoparticle films on substrate**

*Schematic view of new laser ablation-type silicon cluster beam system named SCCS [45].*

*DOI: http://dx.doi.org/10.5772/intechopen.95299*

nanoparticles would be possible.

some ways [42].

## *Nanoparticle Formation and Deposition by Pulsed Laser Ablation DOI: http://dx.doi.org/10.5772/intechopen.95299*

*Practical Applications of Laser Ablation*

**Figure 4.**

Although the above methods require nanoparticles being charged in some way to select the size, the flank attack method proposed by Wu et al. [40] can be used for neutral nanoparticles. In this method, the nanoparticle beam is intersected with the atomic beams of inert gas such as argon or neon, and the nanoparticle can be sorted out in accordance with the size following the law in which the smaller the nanopar-

*Schematic of nanoparticle synthesis process using laser ablation, which is composed of (a) particle generator, (b) gas phase annealing, (c) particle classifying, and (d) particle measurement and correction [34].*

In the previous section, the relationship between the growth rate and the size distribution of nanoparticles has been discussed. By understanding the growth process of nanoparticles, it is possible to control their size distribution. Laser Vaporization with Controlled Condensation (LVCC), as shown in **Figure 5** [41], is a method to control the average size of the nanoparticles by adjusting the conditions about nucleation and growth. By raising the temperature of the lower wall in ablation chamber with a heater and cooling down the upper wall with liquid nitrogen, the temperature gradient would cause the convection of ambient gas. When the vapor created by laser ablation drifts upward by the convection, the vapor becomes supersaturated near the upper cold wall, which results in nucleation

*(A) Experimental set-up for the synthesis of nanoparticles using the LVCC method. (B) Experimental setup* 

ticle size, the greater the deflection angle of the beam.

**3.2 The direct fabrication of monodisperse nanoparticles**

**110**

**Figure 5.**

*for the LVCC method coupled with a DMA [41].*

and condensation followed by nanoparticle forming. It is found that the higher the supersaturation is, that is, the larger the temperature gradient is, the smaller the average size of the nanoparticles becomes. Therefore, by adjusting the temperature gradient proficiently, it is possible to control the nanoparticle size. Furthermore, by combining LVCC and DMA, to control the average diameter and size distribution of nanoparticles would be possible.

In order to determine the size distribution by directly adjusting the growth process of the nanoparticles, it is necessary to control not only statically the degree of supersaturation but also dynamically its temporal variation of it. That is, it is necessary to rapidly increase the supersaturation level to complete the nucleation within a short period of time and to inhibit the subsequent nanoparticle growth in some ways [42].

As described in Section 2.3, a shock wave is generated in front of the plume by laser ablation. It is possible to use the shock wave to rapidly change the state quantity in the plume and increase the supersaturation at a fast rate. One example is the laser ablation process done in a closed space such as an ellipsoidal cell, in which the collision between the reflected shock wave and the plume front triggers the formation of nanoparticles instantly in a small area [43, 44]. The equipment fabricating monodispersed nanoparticles by using this phenomenon is called Spatiotemporal Confined Nanoparticle Source (SCCS), which is illustrated in **Figure 6** [45], as the fabrication process is restrained in the confined space and time.
