Preparation of spherical upconversion nanoparticles NaYF4:Yb,Er by laser ablation in liquid and optical properties

Spherical upconversion nanoparticles, NaYF4:Er,Yb, were successfully prepared by laser ablation in liquid. The number of photons related to luminescence from the nanoparticles was 2. Micrometer-sized raw materials were irradiated with a nanosecond-pulsed laser (Nd:YAG, SHG) to prepare nanoparticles. The particle size of prepared spherical nanoparticles was a few hundred nanometers. The composition of nanoparticles was the same as that of the raw material even by using the high-temperature process of laser ablation in liquid. Fine nanoparticles prepared at high laser fluence by a focused laser beam would be melted at low laser fluence and cooled to prepare the spherical nanoparticles. The amount of spherical nanoparticles was increased by using the unfocused laser beam. This phenomenon supported the formation mechanism of spherical nanoparticles.