Improved Working Model for Interpreting the Excitation Wavelength- and Fluence-Dependent Response in Pulsed Laser-Induced Size Reduction of Aqueous Gold Nanoparticles

We propose a model better describing the pulsed laser-induced size reduction of gold nanoparticles in aqueous solution. A numerical simulation was carried out for energy deposition processes initiated by laser excitation on the basis of the two-temperature model (TTM) of electron temperature T-e, lattice temperature T-l, and the temperature of the medium surrounding the particle. Further improvement was made by rigorous treatments of electron-phonon dynamics, heat losses, and the optical effect of water bubbles surrounding the nanoparticles due to the temperature rise. The most striking effect was brought about through bubble formation by a nanosecond laser pulse irradiation during which a remarkable decrease in the absorption cross section of gold particles takes place, especially in the spectral region of the surface plasmon resonance band. The calculation allowed the clear classification of two mechanisms (the Coulomb explosion and photothermal mechanisms), and a guideline for examining the mechanistic aspect absent previously was provided presently. To initiate the splitting due to the Coulomb explosion, it is necessary to realize T-e high enough to emit electrons thermally while on the other hand the photothermal mechanism is important when T-l exceeds the boiling point of gold nanoparticles. For instance, given that the excitation is carried out by a femtosecond laser that allows T-e and T-l to evolve with time in strong nonequilibrium, fragmentation due to the Coulomb explosion can be observed provided that the laser energy is high enough to raise T-e above 7000 K for liquid gold and above 8000 K for solid gold. In contrast, for a nanosecond laser excitation, the time evolution of T-e and T-l is in quasi-equilibrium during the excitation period. In effect, the photothermal melting evaporation model prevails regardless of the laser intensity because T-l increases steadily to reach the melting and boiling temperatures of gold, leaving Te insufficiently low for the Coulomb explosion to take place. Interestingly, both mechanisms are likely in picosecond laser excitation depending on the laser fluence. The clear classification of the mechanism in terms of T-e and T-l was made for the first time. By using our guideline, we made an assessment on previous mechanistic arguments. At the same time, excitation wavelength-dependent different fragmentation efficiency was also explained more satisfactorily than before.