Upconversion Luminescence of Silica–Calcia Nanoparticles Co-doped with Tm(3+) and Yb(3+) Ions

Looking for upconverting biocompatible nanoparticles, we have prepared by the sol–gel method, silica–calcia glass nanopowders doped with different concentration of Tm(3+) and Yb(3+) ions (Tm(3+) from 0.15 mol% up to 0.5 mol% and Yb(3+) from 1 mol% up to 4 mol%) and characterized their structure, morphology, and optical properties. X-ray diffraction patterns indicated an amorphous phase of the silica-based glass with partial crystallization of samples with a higher content of lanthanides ions. Transmission electron microscopy images showed that the average size of particles decreased with increasing lanthanides content. The upconversion (UC) emission spectra and fluorescence lifetimes were registered under near infrared excitation (980 nm) at room temperature to study the energy transfer between Yb(3+) and Tm(3+) at various active ions concentrations. Characteristic emission bands of Tm(3+) ions in the range of 350 nm to 850 nm were observed. To understand the mechanism of Yb(3+)–Tm(3+) UC energy transfer in the SiO(2)–CaO powders, the kinetics of luminescence decays were studied.