Threshold phenomena in photoluminescence of upconversion micro- and nanophosphors containing Er3+ and Yb3+ ions

Abstract

During electron beam evaporation of green-emitting (λex = 980 nm) silicate upconversion (UC-PL) bulk phosphors Sr2Y6.8YbEr0.2Si6O26, amorphous nanoparticles of size 2.7 nm are formed. The nanoparticles are globules formed during condensation of chains consisting of SiO4 tetrahedra. When the globules are excited with radiation of λex = 980 nm, the luminescence color becomes red and its intensity increases 80 times compared with bulk phosphors. It has been shown that luminescence is influenced by nonradiative processes between different transitions of Er3+ ions. There is a threshold population of the Er3+ and Yb3+ levels (level 2F5/2) in the region of relatively low pumping powers. Above this threshold, a sharp increase in the intensity of upconversion photoluminescence occurs. We show that the produced nanophosphors have a core consisting of a combination of SiO tetrahedra (n = 0, 1, 2), while Er, Yb, Sr, and Y ions are on the surface of the globules. Owing to this structure of the globules, the interaction of Er and Yb ions with nucleus defects, which usually suppress the luminescence, decreases. Probably, for this reason the luminescence intensity increases. Our findings reveal that new effective non-linear materials can be designed for converting IR radiation into visible radiation. The produced effective phosphors can hold promise as fluorescent probes in bioresearch, for fundamental therapy, as well as for new display technologies.

Keywords

• Upconversion photoluminescence
• Er3+ ions
• Yb3+ ions
• Raman spectra