Influence of Gd 3+ doping concentration on the properties of Na(Y,Gd)F 4 :Yb 3+ , Tm 3+ upconverting nanoparticles and their long-term aging behavior.

We present a systematic study on the properties of Na(Y,Gd)F 4 -based upconverting nanoparticles (UCNP) doped with 18% Yb 3+ , 2% Tm 3+ , and the influence of Gd 3+ (10-50 mol% Gd 3+ ). UCNP were synthesized via the solvothermal method and had a range of diameters within 13 and 50 nm. Structural and photophysical changes were monitored for the UCNP samples after a 24-month incubation period in dry phase and further redispersion. Structural characterization was performed by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) as well as dynamic light scattering (DLS), and the upconversion luminescence (UCL) studies were executed at various temperatures (from 4 to 295 K) using time-resolved and steady-state spectroscopy. An increase in the hexagonal lattice phase with the increase of Gd 3+ content was found, although the cubic phase was prevalent in most samples. The Tm 3+ -luminescence intensity as well as the Tm 3+ -luminescence decay times peaked at the Gd 3+ concentration of 30 mol%. Although the general upconverting luminescence properties of the nanoparticles were preserved, the 24-month incubation period lead to irreversible agglomeration of the UCNP and changes in luminescence band ratios and lifetimes.