Investigation On The Mechanism Of Eu²⁺â†'Mn²⁺ Energy Transfer In Chlorophosphate Phosphors

White light-emitting diodes (w-LEDs) have been considered as the new-generation lighting source, and they can be assembled in different ways, including the combination of "blue LED chip and yellow-emitting phosphor" as well as "two or even more phosphors and near-ultraviolet LED (nUV-LED) chips". However, the two approaches both have some disadvantages. While assembly of a single-phase full-color phosphor with nUV-LED chip is expected to be an effective way to solve these disadvantages, but it is lack of this kind of high-efficiency phosphors that can be applied directly. Therefore, it is imperative to investigate it deeply. Single-phase full-color phosphors can be produced by co-doping a sensitizer and an activator into a host, in which the co-doping pairs may be Eu2+-Mn2+ or Ce3+-Mn2+ etc. At present, the research on the Eu2+-Mn2+ co-doping system is the most attractive, but many problems are expected to be solved:the phosphors that can be efficiently excited by nUV-LED is quite few; Eu2+cannot efficiently transfer its absorbed energy to Mn2+, so that the luminescent efficiency of Mn2+ is low; in some cases, apparently the transfer efficiency of Eu2+ to Mn2+ is quite high, but the quantum efficiency of the Mn2+ emission is still relatively low. Therefore, further investigations may be necessary to Eu2+â†'Mn2+ energy transfer mechanism.In this thesis, we choose the Eu2+-Mn2+ co-doped halophosphates as the model materials to study the correlations between the composition and PL properties as well as the energy transfer mechanism of Eu2+â†'Mn2+. Powder samples of Eu2+ or Mn2+ singly doped and Eu2+-Mn2+co-doped M5(PO4)3Cl(M=Ca, Sr, Ba) were synthesized at 850-1200℃ in a CO reductive atmosphere by conventional solid state reactions. The phase characterization was carried out using powder X-ray diffractometer (XRD), the photoluminescence (PL) properties were characterized by the excitation and emission spectra as well as PL decay behaviors were characterized by the transient decay curves, respectively. Energy dispersive spectrometer (EDS) was used for the qualitative analysis of Eu2+ and Mn2+ in the hosts. The phonon energy of the host materials was measured by Raman spectra. The main results are as follows:(1) PL properties of Eu2+ or Mn2+ singly doped and Eu2+-Mn2+ co-doped M5(PO4)3Cl (M=Ca, Sr, Ba) phosphors were measured and compared, and the factors that influence the formation of the solid solutions and their PL properties were discussed, in which the size matching between the doped cations and the host cations and the phonon energy of the hosts were mainly considered. The results show that a better size match between the doped cation and the host cation allows a wider solid solution range (e.g. Mn2+/Ca2+) and a narrower emission band (e.g. Eu2+/Sr2+ and Mn2+/Ca2+). For the strong coupling system between the activator and the coordination field, which is similar to the weak coupling system, a lower phonon energy of the host (e.g. the Sr phase) reduces the non-radiation probability and enhances the PL efficiency. The PL intensity of the M=Ba system is exceptionally low, possibly because in this phase Eu2+ tends to disperse into the two cation sites and Mn2+ cannot be doped into the lattice due to large size difference between Mn2+ and Ba2+, which is supported by the EDS results. But whether these laws are universal in other systems need to be further verified.(2) The steady energy transfer efficiency (ηst) and the transient efficiency (ηtr) from Eu2+ to Mn2+ as well as the quantum efficiency (Q) of the Mn2+ emission in M5(PO4)3C1: Eu2+,Mn2+(M=Ca, Sr) phosphors were determined by analyzing their PL spectra and decay curves, and the least square fittings were conducted on the Eu2+ decay curves. Based on the above analyses, the energy transfer mechanism of Eu2+â†'Mn2+ was analyzed. The data indicate that the energy transfer from Eu2+ to Mn2+ takes the exchange mechanism. It is also found that the energy transfer efficiency is quite high, superficially, but the Q is not as high as expected, which implies some energy loss processes. These processes may include the "inverse bottleneck process" that is related to the lifetime mismatch between Eu2+ and Mn2+, and the "charge transfer process" in the Eu2+-Mn2+ clusters. In the M=Sr system, the energy loss is mainly related the "inverse bottleneck process", while in the M= Ca system, it may be the combined effect of the "inverse bottleneck process" and the "charge transfer process".(3) To further understand the energy loss mechanism of the "charge transfer process", the influences of synthesis temperature and heating time on the PL behaviors of M5(PO4)3Cl:Eu2+,Mn2+(M=Ca, Sr) phosphors were investigated by analyzing their PL spectra and decay curves. The results indicate that for the Sr phase the loss is mainly caused by the "inverse bottleneck process", which is not affected much by synthesis temperature. However for the M=Ca system, besides the "inverse bottleneck process", the "charge transfer process" in the Eu2+-Mn2+ clusters plays an important role in energy loss, and with an increase of the synthesis temperature, the loss becomes more serious and the PL intensity reduces since more Eu2+ and Mn2+ ions are involved in the clusters. However, it is seemly not in agreement with the thermodynamic expectation. According to the Gibbs free energy function:AG=â–³H-Tâ–³S, a lower synthesis temperature is more beneficial for the formation of Eu2+-Mn2+clusters, leading to poorer PL intensity. To solve this contradiction, we propose that the formation of Eu2+-Mn2+ clusters is kinetically blocked at lower synthesis temperatures. With an increase in heating time for the phosphors synthesized at lower temperature, the PL intensity decreases and the decay curves are more bended, which supports the above proposal.The results in this thesis give a comprehensive understanding of the correlations between the composition and PL properties, and the energy transfer mechanism from Eu2+ to Mn2+in M5(PO4)3Cl and may provide some clues for designing and screening novel single-phase full-color phosphors in our further work. For choosing sensitizer and activator, not only energy match but also lifetime match between them should be considered. For designing host, it is better to select the hosts with low phonon energy and two cation sites. These two sites should be suitable on both the valences and radii of Eu2+ and Mn2+ respectively, and it is also hoped that the two cation sites are located separately in a suitable distance in the lattice, so that the possibility of "charge transfer" between Eu2+and Mn2+ would be reduced. In this case, energy transfer efficiency(η) may be reduced in some extent, but the transferred energy would be utilized well by Mn2+ and a high efficiency of the Mn2+ emission Q would be achieved.