Surfactant-free gold nanoparticles were prepared by laser ablation of a gold metal plate in water. The nanoparticles were characterized by absorption spectroscopy, ζ-potential measurements, and XPS spectroscopy. The nanoparticles are negatively charged because the surface atoms are partially oxidized to Au-O -, according to the literature by Sacher and co-workers (J. Phys. Chem. B, 2004, 108, 16864). We further examined electrostatic interactions between nanoparticle and cationic surfactants. It was found that the surfactant cations attach to the particle surface, neutralizing the particle charge. Taking advantage of the electrostatic interactions, we estimated that 3.3-6.6% of the surface gold atoms was oxidized in water.

Gold nanoparticles with an average diameter of ≈11 nm were prepared by laser ablation of a gold metal plate in an SDS aqueous solution. An 80-µm microdroplet of the solution in diameter was ejected in the atmosphere from a microdroplet nozzle. Structural changes of the nanoparticles in the microdroplet, after they are irradiated with a focused single nanosecond laser pulse at the wavelength of 532 nm, were studied by transmission electron microscopy (TEM) and optical absorption spectroscopy. It was revealed that the gold nanoparticles are fragmented into the smaller particles. In order to investigate the dynamics of fragmentation, the nanoparticles were irradiated with delayed double laser pulses, both of which have an identical wavelength and pulse energy (532 nm, 30 mJ). The average diameter of the product fragments was smallest when the two laser pulses simultaneously irradiated the nanoparticles. The diameter increased with an increase in the delay time from the first to the second laser pulse. The delay-time dependence of the particle size indicates that the fragmentation of the gold nanoparticles is caused by the Coulomb explosion of the multiply charged nanoparticles.

Gold nanoparticles with an average diameter of 11 nm (Au(39000)) were prepared in an SDS aqueous solution. A 80-microm liquid droplet (microdroplet) of the solution was ejected into the atmosphere from a microdroplet nozzle. Structural changes of the gold nanoparticles in the microdroplet, after they were irradiated with a focused single-nanosecond laser pulse at the wavelength of 532 nm, were studied by transmission electron microscopy (TEM) and optical absorption spectroscopy. It was revealed that the gold nanoparticles are fragmented into small particles and then the small fragments aggregate with each other. The aggregation was found to be terminated 100 micros after the laser-pulse excitation.

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We developed new methods to form small nanomaterials in a solution, which include a liquid-phase laser ablation and laser-induced structural change of the nanomaterials. Here, we report our recent advances of the methods on gold as the typical example, and on manganese as an application.