Upconversion properties of Y2O3:Er,Yb nanoparticles prepared by laser ablation in water

Y2O3:Er,Yb nanoparticles were successfully prepared by laser ablation in liquid, and their upconversion properties were investigated. The particles were prepared by irradiating a Y2O3:Er,Yb target in water with a 532-nm pulsed laser beam. The prepared nanoparticles were determined to be composed of Y2O3, without any by-products, by X-ray diffraction. The average particle size of the nanoparticles measured by electron microscopy was a few tens of nanometers, which depended on the energy density of the irradiating laser beam. Upconversion luminescence of the nanoparticles was observed by the excitation of the laser diode at a wavelength of 980 nm. An increase by 33% in the ratio of red emission (4F9/2→4I15/2) to green emission (2H11/2/4S3/2→4I15/2) was observed with a decrease in the size of the nanoparticles from 21.5 nm to 11.1 nm. This high ratio is useful for biomedical applications because of the high transparency of this wavelength in the human body. The number of photons generated by the upconversion luminescence of the nanoparticles was investigated. The number of photons involved in the process of the nanoparticles was three, which was attributed to nonradiative relaxation due to the large specific surface area of the particles; in contrast, the normal number of photons involved in the process of Y2O3:Er,Yb bulk is two.