**5. The effect of thermal properties in laser ablation of metal plate**

Normally, the metal nanoparticles are generated using high energy laser pulsed inside an aqueous solution with pico, nano, and femtosecond pulsed laser. Pulsed laser should be melted the target to generate the nanoparticles. If the femtosecond or nanosecond pulses are used to prepare metal nanoparticles, the main problem is the generation of heat in the sample, and the calculation of temperature is difficult. This problem can be significantly simplified based on diffusion length. Indeed, the heat diffusion length is smaller than laser spot size. Hence, the temperature can be controlled during the laser ablation of the metal plate. For example, in some typical experiments, the spot size is about 10 μm, and it is larger than diffusion length. Moreover, the laser beam has a flat-top profile, and the absorbed energy of laser beam causes the increasing of temperature in the target layer. The area of the layer that covers with the laser beam is equal to the spot size of the laser, and it surrounds by liquid that absorbs the heat and decreases the temperature of the layer. In the presence of liquid, the average temperature increases, and from thermal equilibrium, the relationship temperature and other thermodynamic parameter are obtained as follows [63, 64]:

$$T \not\equiv \frac{Aj}{c\rho h} \tag{2}$$

the metal plate, respectively. The temperature has the relationship with the energy density of the laser beam, or it is fluence *j*. The thermal diffusion length *h* depends on the thermal

and the laser pulse duration, respectively, and the increasing of temperature (*T*) depends on

*α*<sup>−</sup><sup>1</sup> ≪ *h*, (4)

Some part of laser power is converted to heat and it is the consequence of the evaporation of

When the laser ablation of the metals plate is considered, the wavelength of laser beam is a significant parameter. Because the optical constants of the material depend on wavelength; hence, the metal nanoparticles and metal targets can absorb the energy of laser beam at the particular wavelength. Metal clusters release in the nano-size from the metal plate and they can provide the condition for absorption of laser energy at each pulse, so the metal target can melt and the generation of nanoparticles becomes faster. Consequently, the higher repetition

Jeon and Yeh were reported about the wavelength dependence of particle size and the formation efficiency in laser ablation [67]. They prepared the silver nanoparticle in inorganic (water) and organic solution (isopropanol) using green laser and infrared laser at nanosecond pulse. They achieved that the particle size using the green laser is larger than the particle size using the infrared laser. Hence, the formation efficiency of nanoparticles using infrared laser was lower than that using the green laser. Moreover, the laser fluence can change the size of the nanoparticle as a function of laser wavelength [65]. Hence, the fragmentation of nanoparticles can improve with enhancement of fluence. The dimension of nanoparticles prepared using infrared photon increases when the laser fluence increases. Consequently, when the wavelength of laser beam decreases, the ablation efficiency increases with the energy of laser beam.

rate of laser pulses can provide the higher rate of generation of nanoparticles [66].

**7. The effect of light absorption with nanoparticles in laser ablation** 

The absorption of laser beam with nanoparticles is the effective factor of the laser ablation process at high laser fluence to prepare the nanoparticles in an aqueous solution. When the prepared nanoparticles have not high mobility in liquid, they are aggregated near the target.

\_\_\_ *a t*

*<sup>p</sup>* (3)

http://dx.doi.org/10.5772/intechopen.80374

are thermal diffusivity

71

*p*

Laser Ablation Technique for Synthesis of Metal Nanoparticle in Liquid

diffusivity of the target materials [65]:

**method**

*<sup>h</sup>* <sup>∝</sup> <sup>√</sup>

where *a* (*k/cρ*, *k* is the thermal conductivity of the metal plate) and *t*

**6. The effect of wavelength in laser ablation of metal plate**

absorption of laser radiation in particular wavelength as follows:

a solution near the laser beam, and this power is very weak.

where *A* (*A* = 1 − *R*, where *R* is the reflectivity coefficient at the laser wavelength) is the absorptivity of the metal plate at the particular wavelength, and *c*, *ρ*, and *h* are the heat capacity of the target material, the density of the metal target, and the thermal diffusion length in the metal plate, respectively. The temperature has the relationship with the energy density of the laser beam, or it is fluence *j*. The thermal diffusion length *h* depends on the thermal diffusivity of the target materials [65]:

$$h \propto \sqrt{a} \,\overline{t\_p} \tag{3}$$

where *a* (*k/cρ*, *k* is the thermal conductivity of the metal plate) and *t p* are thermal diffusivity and the laser pulse duration, respectively, and the increasing of temperature (*T*) depends on absorption of laser radiation in particular wavelength as follows:

$$a^{-1} \ll h,\tag{4}$$

Some part of laser power is converted to heat and it is the consequence of the evaporation of a solution near the laser beam, and this power is very weak.
