No abstract

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 T e 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.

In situ extinction spectroscopy and transient absorption spectroscopy of the femtosecond laser-induced fragmentation of 60 nm diameter aqueous gold nanoparticles were performed. The threshold laser fluences of fragmentation determined by in situ spectroscopy and transmission electron microscopy, (7.3 ± 1.5) mJ·cm−2 for excitation at 400 nm and (3.6 ± 0.5) mJ·cm−2 at 532 nm, agreed well with the values of 6.0−7.4 and 3.4−4.1 mJ·cm−2 calculated by our simulation based on the twotemperature and liquid drop models. The transient absorption study revealed that real-time observation of fragmentation is possible at picosecond time scales. When monitored at 490 nm, at which the effect of fast relaxation dynamics is minimal, excitation at 400 nm afforded a reduced extinction signal of the localized surface plasmon resonance (LSPR) band of gold nanoparticles at laser fluences greater than or equal to (6.1 ± 1) mJ·cm−2. The reduction can be ascribed to nanoparticle fragmentation because the intensity (I) of the LSPR band depends on particle radius (R), I ∝ R 3. The signal reduction occurred not instantaneously but gradually within 100 ps, suggesting separation of initial densely packed small clusters during the observation period. The onset of the size reduction was laser-fluence-dependent, and it occurred earlier at higher fluences. This fluence dependence was explained well within the framework of our model: fragmentation occurs for liquid rather than solid gold, and the onset suggests the initiation of particle melting. The present result demonstrated that femtosecond laser-induced fragmentation is dominated by the Coulomb explosion mechanism, discussed many times without experimental verification. We believe we can provide information long needed in the field.

No abstract

An in situ spectroscopic study of the nanosecond laser-induced melting and size reduction of pseudospherical gold nanoparticles with 54 +/- 7 nm diameter allowed the observation of a heating efficiency that was very dependent on the excitation wavelength. A remarkably greater efficiency was observed for the photothermal effect of interband excitation than that of intraband excitation. This noteworthy observation is ascribed to an altered electron heat capacity, c(e), during photoexcitation depending on the excitation energy, which is a phenomenon that has not been realized previously. As a result, a 60% reduction of the specific heat capacity, c(p), compared to that of bulk gold was obtained for interband excitation at 266 nm whereas the c(p) value for the excitation of the intraband transition at 532 nm was unaltered. A semiquantitative explanation was given for this striking phenomenon induced by interband excitation in which excitation-relaxation cycles of electrons upon excitation of 5d electrons to the 6sp band lead to a reduced number of electrons contributing to the electron temperature rise in the vicinity of the Fermi level during the nanosecond laser pulse duration. By contrast, electronic excitation within the 6sp band results in no net reduction in the number of electrons near the Fermi level, giving rise to a value of c(p) similar to that of bulk gold. Our finding that the heat capacity of gold nanoparticles can be changed upon UV laser excitation is important for understanding the fundamental nature of noble metal nanoparticles. Furthermore, this finding might be useful for preparing new metal alloy particles as well as for manipulating the thermodynamic properties of the nanoparticles.