Upconversion Luminescence Properties of NaYF₄:Yb:Er Nanoparticles Codoped with Gd³⁺

The temperature-dependent upconversion luminescence of NaYF₄:Yb:Er nanoparticles (UCNP) containing different contents of Gd³⁺ as additional dopant was characterized. The UCNP were synthesized in a hydrothermal synthesis and stabilized with citrate in order to transfer them to the water phase. Basic characterization was carried out using TEM and DLS to determine the average size of the UCNP. The XRD technique was used to investigate the crystal lattice of the UCNP. It was found that due to the presence of Gd³⁺, an alteration of the lattice phase from α to β was induced which was also reflected in the observed upconversion luminescence properties of the UCNP. A detailed analysis of the upconversion luminescence spectraespecially at ultralow temperaturesrevealed the different effects of phonon coupling between the host lattice and the sensitizer (Yb³⁺) as well as the activator (Er³⁺). Furthermore, the upconversion luminescence intensity reached a maximum between 15 and 250 K depending on Gd³⁺ content. In comparison to the very complex temperature behavior of the upconversion luminescence in the temperature range <273 K, the luminescence intensity ratio of ²H_11/2→⁴I_15/2 to ⁴S_3/2→⁴I_15/2 (<i>R</i> = G1/G2) in a higher temperature range can be described by an Arrhenius-type equation