Mechanisms of Nanoparticle Formation by Laser Ablation

Abstract

During the last decade, numerous experiments have been performed, clearly demonstrating that an interaction of a short (several nanoseconds or shorter) laser pulse with a solid target leads to the formation of nanoparticles. This technique, known as pulsed laser ablation (PLA), has become a promising method of the synthesis of nanoclusters for photonics, electronics and medicine (Movtchan et al., 1995; Yamada et al., 1996; Makimura et al., 1996; Geohegan et al., 1998; Albert et al., 2003). The PLA method has several advantages compared to more traditional techniques. In particular, it was shown that this method provides a possibility for chemically clean synthesis, which is difficult to achieve under more conventional nanoparticle production conditions. In addition, several experimental studies indicated that the cluster size distribution could be controlled in PLA by carefully choosing the laser irradiation parameters and properties of the background gas. Furthermore, laser ablation allows for an easy production of colloidal metal nanoparticles for biological and medical applications. Despite the large number of the experimental results, the theoretical understanding of the physical and chemical mechanisms leading to the formation of nanoparticles during the PLA is still lacking. The number of theoretical studies of these mechanisms remains limited because both the continuum hydrodynamic models and the classical nucleation theory become inapplicable under the typical PLA conditions. Under these conditions, laser plume expansion and all collisional processes inside the plume occur so rapidly that equilibrium conditions are not attained. In addition, fast laser energy deposition may induce an explosive volume ejection rather than an equilibrium surface evaporation. Nevertheless, the laser ablation process is often described by a thermal desorption model, which considers the ablation as a rather slow layer-by-layer evaporation of monomers from the target surface (Anisimov, 19968). The presence of nanoparticles in the laser plume is then explained by using a Zel’dovich-Raizer condensation model (Zeldovich, 1966; Luk’yanchuk et al., 1998; Gusarov et al., 2000). This approach is appropriate for interpretation of the experimental findings obtained in laser ablation with hundred of nanoseconds and longer laser pulses, and in the presence of a background gas (Ohkubo et al., 2003; Boldarev et al., 2001). In the PLA with shorter laser pulses, however, clusters can be ejected directly from the target as a result of the target disintegration by laser-induced explosion-like process (Bulgakov, 2004; Amoruso et al., 2004; Zhigilei, 2003). In this case, the common thermal desorption and