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

During electron beam evaporation of green-emitting (λ_ex = 980 nm) silicate upconversion (UC-PL) bulk phosphors Sr₂Y_6.8YbEr_0.2Si₆O₂₆, 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 λ_ex = 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 ²F_5/2) 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.