ИСТИНА

Nanocomplexes doped with lanthanide ions have recently attracted high interest due to their unique luminescent properties associated with the possibility of realization of up-conversion luminescence under infrared excitation. This opens wide opportunities for their use in various applications: for optical imaging in biology and medicine, in optical coding as coding elements, in photoelectric devices. Typically, such nanocomplexes consist of inert matrix doped with two types of lanthanide ions. The first of these ions is a sensitizer ion, which efficiently absorbs exciting IR radiation. The second ion is an activator ion, which has intense luminescence. The most effective mechanism of up-conversion luminescence in lanthanide nanoluminophores is the energy transfer from the sensitizer ion to the activator ion. Luminescent properties depend on many parameters, the most important of which are the ratio of the concentrations of ions of activators and sensitizers, the intensity of exciting radiation, and temperature. The presence of two types of ions in the complex - activators and sensitizers - leads to the fact that the efficiency of up-conversion luminescence is determined by the rates of two competing processes: nonradiative energy transfer from the sensitizer ion to the intermediate metastable level of the activator ion and the rate of linear decay of this intermediate level. Because of this, the intensity of anti-Stokes luminescence depends nonlinearly on the pump intensity. The study of this dependence allows us to draw conclusions about the processes occurring in nanocomplexes based on lanthanides under optical excitation and the mechanisms of the formation of up-conversion luminescence. In this work, we investigated suspensions of new synthesized luminophores based on NaYF4:Yb/Tm in dimethyl sulfoxide, differing from each other in the ratio of the concentrations of ytterbium and thulium ions. Two series of samples were prepared: in one of them, the concentration of the sensitizer was varied, while the concentration of the activator remained unchanged, in the other series, the concentration of the activator was changed at a fixed concentration of the sensitizer. The photoluminescence spectra of all suspensions were obtained upon excitation by continuous laser radiation with a wavelength of 980 nm. The dependences of the luminescence intensity of thulium on the intensity of the exciting radiation were investigated. The optical pumping intensity was varied in the range 0-180 W/cm2. As a result of the analysis of the obtained spectra, the regularities of the influence of the ratio of the concentrations of ytterbium and thulium ions and the laser excitation power on the luminescence intensity of NaYF4: Yb/Tm nanophosphor were established. The obtained results made it possible to determine the optimal ratio of ion concentrations in the nanoluminophore and to refine the models of processes occurring in the lanthanide-based phosphor under laser excitation. The important role of the cross-relaxation process in the formation of the luminescent response of NaYF4: Yb/Tm complexes under optical excitation is confirmed.