Mechanisms of Ultrashort Laser-Induced Fragmentation of Metal Nanoparticles in Liquids: Numerical Insights

Abstract

Femtosecond laser-induced fragmentation of gold nanoparticles in water is examined. Numerical calculations are performed to elucidate the roles of thermal and electrostatic effects due to electron emission in the corresponding decomposition mechanisms. The obtained results demonstrate that particles smaller than a well-defined size R* melt at smaller fluences than the ones required for electrostatic decomposition. The limiting size depends on the absorption coefficient calculated as a function of particle radius, which depends on laser wavelength and on the optical properties of the particle and the background environment. To decompose particles with radii larger than R*, a considerable increase in laser fluence is required. In this case, thermomechanical effects become prevailing. Both the calculated range of particle sizes to be decomposed by the considered laser pulses and the corresponding fluences agree with several experimental measurements. ■ INTRODUCTIONDuring the past decade, nanoparticles (NPs) have found numerous applications in different areas. Thus, metallic NPs are now widely used in photonics, electronics, chemistry, medicine, textile production, and sensors for chemistry and biology. 1−7 A promising recently emerging field of NP application concerns new energy sources. For the development of this application, fusion dynamics in laser−cluster interactions was investigated, with a hope to obtain high neutron yields of about 10 keV−15 MeV. 8−10 Laser−particle interactions are key processes in this case, as well as in the case of cancer treatment 9−16 and sensor and quantum dot development. 17 The unique plasmonic properties of the metallic NPs make them particularly suitable for these applications. In addition, laser-based fragmentation of nanoparticles is found to be a promissing method of control over nanoparticle size distribution.In particular, a constantly growing number of modern applications are based on short (picosecond) and ultrashort (femtosecond) laser interactions with metal NPs. Such laser pulses induce a temperature rise both in the particle and around it, triggering several effects. 18,19 The first one is a trivial thermal expansion of the particle, when acoustic waves are launched. Then, in the presence of an ambient liquid, a microbubble can be formed following the shock wave generation if laser fluence is sufficiently high. 20,21 Particle temperature can reach the melting or boiling point, leading to thermal evaporation and/or decomposition. These processes are typically combined with mechanical and electrostatic effects that can also account for particle fragmentation. 11 Furthermore, intense ultrashort laser interactions with small clusters are known to induce Coulomb explosion in vacuum. 22 In fact, because the electron subsystem is heated first, electrons can be removed, leaving positively charged ions that are subjected to a repulsive force. In this scenario, clusters explode, ejecting energetic ions. For larger particles, furthermore, Mie theory provides a strongly nonhomogen...

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