Synthesis and luminescence spectroscopy of inorganic materials doped with rare earth ions

Inorganic hosts doped with rare earth ions have always been in recent times a research hotspot because of their broad applications, such as: luminescent transparent ceramics, catalyst, optical temperature sensor and fluorescent anti-counterfeit materials, as well as white light emission diodes and so on. The energy transfer involving trivalent rare earth ions in these inorganic solids has been and still is a fundamental field in modern luminescence research, which plays an essential role in improving luminescence performance. The area of energy transfer has been intensively studied since the early 1960’s, and the contribution provided by Professor George Blasse has been crucial for the advancement of the understanding of energy transfer and migration processes involving luminescent ions. Particularly, Prof. Blasse studied in detail the migration of excitation energy in concentrated materials, especially based on Gd3+, Eu3+ and Tb3+. But it still attracts a lot of attention due to unanswered questions regarding the mechanisms at play, and the technological applications of this type of process. For this reason, several studies have been performed here. (a) Energy transfer processes of the types Tb3+ → Nd3+ and Tb3+ → Sm3+ have been studied at room temperature in eulytite double phosphate materials with stoichiometry Sr3Tb0.99Nd0.01(PO4)3 and Sr3Tb0.99Sm0.01(PO4)3. As already found for (Ba,Sr)3Tb1-xEux(PO4)3, the transfer of excitation from the Tb3+ to the dopant is assisted by very fast energy migration in the 5D4 subset of levels, and effectively occurs between nearest neighbor ions. It is for this reason that the transfer efficiency results to be compared to what observed in other hosts. (b) The stoichiometry Sr3Y1−x−yTbxTmy(PO4)3 have been studied to explore the energy transfer processes involving the Tb3+ and Tm3+ ions. It revealed that the Tb3+ → Tm3+ transfer of excitation could quench strongly the 5D4 level of Tb3+, and internal Tm3+ → Tm3+ cross relaxation processes would take place upon excitation in the emissive 5D4 (Tb3+) level. (c) A detailed study of doubly doped LaInO3: Bi3+/Tb3+ and LaInO3: Tb3+/Eu3+ and triply doped LaInO3: Bi3+/Tb3+/Eu3+ samples showed that the Bi3+ → Tb3+ energy transfer is dominated by electric dipole- electric dipole (EDD) mechanisms, and the largest energy transfer efficiency can be achieved through increasing the concentration of lanthanide ions (Tb3+ and Eu3+). The co-presence of blue, green, and red color light in the right ratio to obtain white light, is ensured by tuning the Bi3+ → Tb3+ and Tb3+ → Eu3+ energy transfer efficiencies. In turn, these efficiencies are strongly related to the relative amount of the three dopant ions and therefore to their interaction distances. (d) Fluoride materials have been broadly exploited as convenient hosts for rare earth dopant ions due to their high refractive index, low phonon energy (<350 cm-1) as well as excellent thermal stability. The energy transfer mechanisms Tb3+ → Eu3+ ions have been explored in samples NaBi1-x-yTbxEuyF4, and it has been found the energy transfer efficiency is easily influenced by the quenching groups on the surface of nanoparticles. Also, the superior temperature sensing performance for samples has been proposed. Therefore, the materials could be a promising candidate for determination of water content in organic solvents as well as optical thermometry. (e) Taking advantage of the outstanding merits belonging to CaAl2O4: Eu, Nd (CAO) persistent phosphors, such as long-lasting afterglow as well as unique luminescence spectra, it was adopted as excitation source to irritate Y3Al5O12: Ce phosphors. Based on this fact, one desired yellow emission persistent phosphor has been achieved by depositing these two kinds of phosphors as a polymer layer with a certain thickness. The process of radiative energy transfer from CAO to YAG has been indicated. The complicated energy transfer mechanisms system allows an endless exploration in the future. Even with the results obtained so far, an enormous contribution towards the improvement of luminescence intensity, the regulation of emission color, as well as the achievement of white light have been made. Anyway, energy transfer as a fascinating phenomenon in luminescent materials, is beneficial for various aspects in our life.