Atomistic View of Laser Fragmentation of Nanoparticles in a Liquid Environment

Short pulse laser irradiation of a colloidal solution of nanoparticles is an effective method for fragmenting the nanoparticles and producing a population of smaller nanoparticles and atomic clusters with properties desired in various fields of applications, including biology, medicine, and catalysis. To investigate the mechanisms involved in the fragmentation, we develop a computational model capable of realistic treatment of a variety of interrelated processes occurring on different time and length scales, from the electronic excitation by the laser pulse, to the electron-phonon energy transfer and an explosive phase decomposition of the superheated nanoparticle, and to the generation and collapse of a nanobubble in the liquid environment. The application of the model to simulation of laser fragmentation of a Au nanoparticle in water has revealed two distinct channels of the formation of the fragmentation products. The first channel involves the direct injection of compact nanodroplets propelled by the phase explosion of the irradiated nanoparticle deep into the water environment. The second channel of the nanoparticle formation involves a more gradual growth through agglomeration of numerous atomic clusters embedded into a narrow region of water surrounding the laser-induced nanobubble. This channel produces irregularly shaped nanoparticles and leads to a rapid decline of the population of atomic clusters on the timescale of nanoseconds. All the clusters and nanoparticles experience an ultrafast quenching by the water environment and feature a high density of twin boundaries and other crystal defects, which enhance the density of active sites for the catalytic applications of the nanoparticles. The computational predictions of the prompt generation of a high concentration of the fragmentation products in a relatively narrow shell-like region on the outer side of the nanobubble, as well as the rapid solidification of atomic clusters and nanoparticles at the early stage of the nanobubble formation, have important practical implications for the design of new methods aimed at achieving an improved control over the size, shape and defect structures of nanoparticles produced by laser fragmentation in liquids.