Crystalline Submicrometer Spherical Particles with High Refractive Index for Optical Applications

Nanoparticles are generally aggregated in liquid to reduce surface energy, and therefore surface modification of the nanoparticles is important to obtain well dispersed nanoparticles in liquid. Thus the aggregation of nanoparticles should be avoided for practical use of nanoparticles as much as possible.

Recently we found that B₄C submicrometer spherical particles by irradiating low-fluence laser onto the B nanoparticles dispersed in organic solvent through the high-temperature carbonization reaction. Furthermore, we also found that similar submicrometer spherical particles were produced for various kinds of materials, such as metals (Au, Ag, etc.), oxides (ZnO, TiO₂, etc.) and semiconductors (Si, GaP, etc.) using nanoparticles as raw materials and by unfocused laser beam irradiation.

In these cases, raw nanoparticles are presumably not vaporized nor ablated but melted by nanosecond laser irradiation to form liquid droplets, resulting in spherical particle formation by quenching. Thus, this process “laser melting in liquid” is evidently different from the conventional technique, “laser ablation in liquid” for nanoparticle fabrication using high-fluence laser irradiation in liquid.

More importantly, the particle size increased from nano-size to sub-micrometer size by laser melting in liquid. This is because aggregated nanoparticles melt together by optical absorption and subsequent temperature increase over melting temperature to form larger spherical particles. Thus the aggregation of nanoparticles plays an important role in submicrometer spherical particle formation.

Here we present our recent results on the laser melting in liquid, such as the effect of processing parameters on the products. Especially we discuss the effect of raw particle size by dynamic light scattering and zeta potential on the product morphology and generated particle size. Physicochemical calculation of temperature estimation based on the particle heating-melting-quenching model by laser irradiation is compared with the experimental results. Optical and medical applications of submicrometer spheres is also demonstrated.