Ana Maria Pires, Osvaldo Antonio Serra, Stephan Heer and Hans Ulrich Güdel
Low-temperature upconversion spectroscopy of nanosized Y₂O₃:Er,Yb phosphor
J. Appl. Phys. 98, 063529/1-7 (2005)                 

Abstract: This work reports on the structural characterization and on the low-temperature upconversion spectroscopy of the Y₂O₃:2%Er,1%Yb nanophosphor prepared by thermal decomposition of a polymeric resin (Pechini's method [U.S. Patent No. 3,330,697 (July 11 1967)]). The average particle size evaluated from transmission electron microscopy lies in the range of 05–25  nm. The high-resolution upconversion luminescence spectrum at 10  K in the ultraviolet to near infrared (UV-NIR) spectral regions shows narrow lines, characteristic of Er³⁺ transitions occupying both Y³⁺ sites with point symmetries C₂ and C_3i in the oxide cubic system. The excitation spectrum at 10  K in the IR region was used to monitor the green upconversion and IR luminescences and it displayed Er and Yb lines. Power dependence measurements at 298 and 10  K indicate that the main upconversion mechanism is a two-photon excitation process. The temperature dependence of the upconversion luminescence shows a decrease in the emission intensity with increasing temperature, and such decrease is much more evident for the emission in the violet than in the green and the red regions.