# The Effect of Laser Pulse Widths on Laser--Ag Nanoparticle Interaction: Femto- to Nanosecond Lasers
## 4 Results and Discussion
### Nanoparticle Size Distribution  
Nanoparticle shape was visualized after laser fragmentation for \(\geq\)1 h. Solutions of nanoparticles produced using femtosecond and picosecond laser pulse widths were yellow in color, and no precipitate was present. However, solutions of nanoparticles produced using nanosecond laser pulse widths were gray in color, and clusters had precipitated. The Ag nanoparticles thus generated can change physically or chemically as time passes. Ag nanoparticles can generate colloids and can be converted into oxides such as AgO or Ag\({}_{2}\)O as they bind to oxygen. These issues are observed not only in nanoparticles generated by laser fragmentation, but also in other nanoparticle production processes. Spherical and cylindrical Ag nanoparticles, some in the form of linked clusters, were detected by TEM (Figure 5). In addition, the nanoparticle size distribution was evaluated (Figure 6). Nanoparticles produced using femtosecond laser pulse widths were small, having a diameter of around 25 nm (Figures 5a and 6a). In the nanosecond region, as the pulse width increased, the nanoparticle size distribution became broader. Therefore, the size distribution of nanoparticles is determined by the laser pulse width. To analyze nanoparticle generation using femtosecond laser pulse widths, the mechanisms of nanoparticle generation according to pulse width must be evaluated. These mechanisms are near-field ablation, Coulomb explosion, photothermal ablation, and mechanical breakdown due to shockwaves. Among these, near-field enhancement and Coulomb explosion are likely involved in nanoparticle generation by femtosecond laser pulse widths. In addition, the laser beam-induced shockwave affects the generation of nanoparticles and the formation of clusters. Using femtosecond laser pulse widths, spherical nanoparticles were created by Coulomb explosion, and nonspherical nanoparticles by near-field enhancement. However, distinguishing between these two mechanisms is problematic because of the nonuniform nanoparticle shape, which is a result of both the interaction of the laser beam with the target and laser fragmentation. The shapes of nanoparticles generated using picosecond and 164 fs laser pulses were similar (Figure 5b). In addition, their size distribution was similar to that of nanoparticles produced using femtosecond pulse widths (Figure 6b). Thermal diffusion within the particle resulting from electron-phonon scattering occurs at laser pulse widths of around a few picoseconds [41]. However, thermal diffusion at the boundary takes several hundred picoseconds [28; 41]. Therefore, heat transfer at the particle surface does not occur during laser irradiation for 5 ps. Therefore, the mechanism of laser fragmentation with a pulse width of 5 ps is similar to that of lasers with femtosecond pulse widths. The energy accumulation by particles subjected to successive laser beam pulses must also be considered. Based on its repetition rate, the laser pulse has a delay time of 2 to 5 \(\upmu\)s. Thus, on the basis of the difference between delay time and laser pulse width, the particle energy accumulation is not significant and can be disregarded. The irradiation with a nanosecond pulse-width laser beam produced particles of diameters from 25 nm to more than 200 nm, which were in a linked-cluster form (Figure 5c-f). For lasers with nanosecond pulse widths, the shorter the pulse width, the smaller the maximum particle diameter. The broad particle size distribution is due to photothermal ablation. Our simulation showed that the thermal effect predominated at nanosecond laser pulse widths and induced peeling off of the outermost layers. By using this exfoliation process, it is possible to produce nanoparticles of various sizes. Moreover, laser-induced plasma generation and particle fragmentation due to the shockwave are also involved in particle formation.