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Up-conversion Gd2O3:Er,Yb nanoparticles (NPs) were fabricated by laser ablation in liquid, and the effect of laser irradiation time on their characteristics was investigated. The light source (a focused 532 nm second harmonic laser beam generated from a Nd:YAG laser) was used to irradiate the target pellet in water for 5, 15, or 30 min. Coarse NPs (> 200 nm) and fine NPs (< 100 nm) were observed. It is believed that the angular-shaped NPs resulted from fragmentation, and the spherical NPs were formed by laser melting or laser ablation in liquid (especially the < 80 nm particles). The main primary particle size range (25–75%), for the samples that had been irradiated for 5 min, was approximately 30–94 nm; for 15 min: 11–21 nm; and for 30 min: 11–24 nm. The secondary particle size decreased from 1000 to 200 nm with increasing irradiation time. The photon number was 2.3 for all the irradiation times. In every case, the zeta potential was above 15 mV, indicating a relatively stable dispersion. The transparency of the NP suspensions was almost the same for both the 5 and 15 min laser irradiation times, and higher than that of the 30 min suspension. [ABSTRACT FROM AUTHOR]