Research On Nano-Effect Mechanisms Of Eu³⁺-Doped Phosphors Under Charge Transfer Excitation

Among the lanthanide luminescent materials, Eu³⁺-doped phosphors have been most extensively applied due to their excellent luminescent properties and unique spectrum properties. For the light emission of Eu³⁺-doped phosphors, the predominant energy input approach is charge transfer (CT) excitation. With the development of nanotechnology, the superiorities of nanophosphors gradually emerge, nano-sized phosphors exhibit a series of unique properties. In view of these properties, the applications of lanthanide luminescent materials have been expanded, meanwhile, serious problems have risen. In this paper, taking Y₂O₃:Eu³⁺and La₂O₃:Eu³⁺ nanophosphors as the researching objects, the quenching mechanisms of the optical centers in Eu³⁺-doped nanophosphors have been typically investigated.Based on the quenching mechanisms, the improvement of luminescence efficiency has been achieved for La₂O₃:Eu³⁺ nanophosphor. On the other hand, the red shift of CT excitation spectra for Eu³⁺-doped nanophosphors has been specifically investigated, and the intrinsic linkages between the decrease of luminescence efficiency and the red shift of CT excitation spectra have been revealed. In addition, the preparation processes of the experimental samples have also been investigated. With the size decrease of Eu³⁺-doped phosphors in nanoscale, the environment rigidity of the optical centers is decreased and the CT state (CTS) coordinate offset is enlarged. The volume deformation in Eu³⁺-doped nanophosphors leads to the decrease in zero-phonon CT energy of the optical centers. The decrease in zero-phonon CT energy and the enlargement of CTS coordinate offset mean the displacement of CTS in the configurational coordinate diagram (CCD), and the CTS displacement in CCD results in the relaxation of the excited optical centers by sending phonons to the host lattice. Based on Jodd-Ofelt theory and spectral experiments, the decrease of CTS feeding probability to the 5 D states of Eu³⁺ ions and the emission efficiency of Eu³⁺-ions have been quantitatively investigated. The quenching mechanisms of optical centers in Eu³⁺-doped nanophosphors have been revealed and the mechanisms leading to the decline of luminescence efficiency have been progressively clarified.In view of the decrease in the rigidity of lattice environment and the presentation of quenching centers in the surface, proper surface coating is adopted to improve the luminescence efficiency for La₂O₃:Eu³⁺ nanophosphor under CT excitation. La₂O₃:Eu³⁺ nanophosphor was coated with SiO₂ layer of proper thickness, a series of characterizing techniques (e.g., TEM, XRD, FT-IR) have been utilized before and after the surface coating and the annealing processes. In order to determine the effects of the surface coating and annealing processes on the luminescence improvement of La₂O₃:Eu³⁺ nanophosphor, proper spectral experiments have been designed; the environment rigidity of the optical centers was improved, the quenching centers in the surface were efficiently eliminated, and the luminescence efficiency was improved. With experimental determination and theoretical calculations, the increasing magnitudes of CTS feeding probability to the 5D states of Eu³⁺ ions and the emission efficiency of Eu³⁺ ions have been quantitatively given after the surface coating and the annealing processes. As a significant surface effect, the declining magnitude in luminescent efficiency for a certain nanophosphor should be specifically determined. In this paper, the concept of equivalent quenching layer has been presented, as a proper parameter, the thickness of equivalent quenching layer could be used to measure the surface effect in luminescent efficiency. Based on the spectral data, the calculating method of the thickness of equivalent quenching layer was presented. The corresponding thickness of La₂O₃:Eu³⁺phosphor has been determined, and the size limit for effective luminescent emission has been given. By surface coating, the thickness of equivalent quenching layer is lowered. The thickness of equivalent quenching layer of La₂O₃:Eu³⁺phosphor is decreased in the surface coating of SiO₂ layer, and further decreased after a subsequent annealing process.Based on Kronig-Penney model, the linear relation between the bandgap (or a particular level) and the volume deformation in crystalline materials has been derived. On the basis of this changing tendency, the zero-phonon charge transfer (CT) energy is deduced to be decreased when the size of Y₂O₃:Eu³⁺ phosphor decreases in nanoscale. In addition, the rigidity decrease of lattice environment in Y₂O₃:Eu³⁺nanophosphor leads to the enlargement of CT state coordinate offset, this means an optical center would reach a higher vibration level in CT excitation. The increasing magnitude of vibration energy is smaller than the decreasing magnitude of zero-phonon CT energy while the size of the Y₂O₃:Eu³⁺ phosphor decreases in nanoscale. As a result, the CT energy is decreased, and the CT excitation spectrum shifts to lower energy.As two different surface effects, the redshift of CT excitation spectra and the decrease of luminescence efficiency for Eu³⁺-doped nanophosphors are originated from the same microscopic mechanisms, i.e., the decrease of zero-phonon CT energy and the enlargement of CTS coordinate offset for Eu³⁺-doped nanophosphors.In addition, the optimized preparation procedure of the experimental samples, especially the glycine-nitrate solution combustion synthesis and the surface coating of La₂O₃:Eu³⁺nanophosphor, were experimentally determined, and proper experiment has been designed to examine the effect of surface coating. The preparation of Y₂O₃:Eu³⁺ micronphosphor was experimentally investigated.