Toshio Takiya

Plume dynamics during film and nanoparticles deposition by pulsed laser ablation

The gas dynamics of pulsed laser ablation of silicon target in the helium gas ambient is investigated via direct simulation Monte Carlo method with a real physical scale of target-substrate configuration. A shock driven process is clearly observed. It is shown that the interaction of the shock front with the target surface and the vapor front induce significant backward flux of ablated particles and oscillating behavior of vapor front. A confined layer mixed with high density Si and He atoms is formed around the contact front. Its behavior is important to the nanoparticle formation and deposition.

Thermodynamics of Nanoparticle Formation in Laser Ablation

Nanometer-sized particles, or nanoparticles, are smaller than conventional solid-state materials and possess great potential for new, useful properties due to peculiar quantum effects (Roco, M. C., 1998). Highly functional devices synthesized from nanoparticles have been studied for use in various fields, such as semiconductors (Liqiang, J., 2003; Lu, M., 2006), photocatalysis (Liqiang, J., 2004), secondary batteries (Ito, S., 2005; Kim, K., 2009, 2010), superconductors (Strickland, N. M., 2008), and bonding substances (Ide, E., 2005). In the present chapter, we discuss the thermodynamics related to nanoparticle formation. Cooling processes of expanding vapor evaporated from a solid surface, such as gas evaporation, arc discharge, sputtering, pulsed microplasma and pulsed laser ablation (PLA), have been applied as a method of nanoparticle formation in the gaseous phase (Wegner, K., 2006). The PLA method, under reduced atmospheric pressure, has been found to be especially promising since it provides the following capabilities (Chrisey, D. B., 1994): (i) ablation of target material regardless of melting point due to the high intensity and focused laser beam pulse, (ii) flexibility in choice of atmospheric gaseous species and pressure, (iii) ease of production of the non-equilibrium state of the highpressure field due to the formation of shock waves, (iv) ability to obtain many different structured materials, from thin films to micrometer-sized particles, by controlling vapor association and condensation, and (v) ease of synthesis of nano-compounds of nonstoichiometric composition by preparing target materials with desired compositional ratios. The PLA method has been widely used for nanoparticle formation because the formed nanoparticles have diameters smaller than 10 nm with low size dispersion and can be formed as basic materials for highly functional devices via effective utilization of these capabilities (Li, S., 1998; Li, Q., 1999; Patrone, L., 1999, 2000; Wu, H. P., 2000; Suzuki, N., 2001; Inada, M., 2003; Seto, T., 2006). To understand the process of nanoparticle formation by the PLA method, two perspectives are necessary: (i) the thermodynamics of the microscopic processes associated with the nucleation and growth of nanoparticles, and (ii) the thermodynamics of the macroscopic processes associated with the laser irradiated surface of the target supplying the raw