Looking at Sc(III) and Y(III) Lattices for Upconversion Nanoparticles Complemented by Eu(III) Down-Shifting Luminescence

Upconversion nanoparticles (UCNPs) are valuable materials mostly used in medicine, anticounterfeiting, optical imaging, and solar cells. NaYF4 is mostly used as a host lattice for lanthanides. As an alternative, NaxScF3+ x is worth investigating because it can also provide a host lattice with low phonon energy. The crystal structure and subsequently the phonon energy can be altered when Gd3+ is introduced into the host lattice, which has been frequently shown for NaYF4. Subsequently, the upconversion luminescence properties change. Here, NaYF4:Yb3+:Er3+ and NaxScF3+x:Yb3+:Er3+ upconversion nanoparticles with various amounts of Gd3+ were synthesized using a standard thermal decomposition approach, which has not been optimized for the specific composition of the UCNP. In addition, the UCNPs were doped with Eu3+ as an optical probe to be used in downshifting luminescence measurements. Time-resolved and power-dependent as well as temperature-dependent upconversion luminescence measurements were carried out to collect a standard parameter set for each UCNP. The data showed that Gd3+ addition differently altered the upconversion luminescence properties in both lattices. Distinct changes were especially observed for the samples with the NaxScF3+x lattice. Upconversion luminescence lifetimes of Er3+ increased for NaxScF3+x samples upon adding Gd3+, while they decreased for samples based on NaYF4. Temperature-dependent measurements revealed that the samples of the NaYF4 host lattice showed a slightly larger variation in SR and.E values with a change in the Gd3+ content. On the other hand, for the NaxScF3+ x lattice, Gd3+ addition had only a minor influence on these parameters. The number of IR photons required for the generation of an upconversion luminescence photon was determined as 2 for the red emission (indicating a reduced back energy transfer) and green emission of Er3+ in both lattices. Due to the unique optical properties of Eu3+, complementary information on minute changes occurring in the two host crystal lattices upon Gd3+ addition was obtained. The samples have been doped with 0.5 mol % Eu3+, and a detailed analysis of the Eu3+ luminescence (at room temperature as well as under site-selective excitation conditions at T = similar to 4K) was carried out to track the changes in the crystal structure of the host materials using the method parallel factor analysis (PARAFAC) to process spectral as well as kinetic data. Two Eu3+ species for samples in the NaYF4 host lattice and three Eu3+ species for samples in the NaxScF(3+) x lattice were found based on PARAFAC analysis.