A Study Of Luminescence And Co-doping Effect In Some Designed Eu Activated Novel Oxides As Red Phosphors

As a new solid state lighting technique, white LED ?Light emitting diode? has attracted great attention, due to the advantages of their high efficiency, low power consumption, long lifetime and no pollution. White LED devices are usually assembled by combining a LED chip and a phosphor, for LED itself can only emit monochromatic light. Currently the most simple and popular method to achieve white light is to combine a blue light emitting InGaN chip and a yellow phosphor YAG:Ce.Though this white light LED has high efficiency, YAG:Ce phosphor is deficient in red component in the emission spectrum, leading to low color rendering index. One way to solve this problem is to add a red phosphor which can be excited by the blue LED chip. Other methods are also proposed to solve this problem, such as combining a blue LED chip with green and red phosphors to obtain white light, using an ultraviolet LED chip with tricolor ?red, green, and blue? phosphors to obtain white light. Red phosphors are in urgent need in these white LED methods. Thus, design and developing efficient, stable and low cost red phosphors as well as the corresponding mechanisms are the hot topics in material physics field.Europium ?Eu? is a good luminescent activator for red phosphors. At present,the red phosphors of concern mainly contains Eu³⁺ activated molybdates and tungstates,Eu²⁺ activated alkaline earth sulfides, Eu²⁺ activated silicon based nitrides, and so on.Eu³⁺ activated molybdates and tungstates have high color purity but the efficiencies are not high enough. The chemical stability is poor for Eu²⁺ activated alkaline earth sulfides.And Eu²⁺ activated silicon based nitrides are difficult to synthesize, which leads to high cost. In this paper, we attend to design and develop new stable, efficient, and low-cost red phosphors according to the luminescence principles, which are based on Eu activated materials. We focus on the co-doping effects on the structure and luminescent properties of Eu in order to improve the emission intensity as well as chromaticity property. The main conclusions are as follows:We find a new host material which is very favorable to the red emission of Eu³⁺ based on the analysis of the structure, and an efficient red phosphor has been developed by doping the luminescent center Eu³⁺ to this host material. Intense red emission is observed in Eu³⁺ doped Sr3WO6 which is of double perovskites A2BB'O6 structure. The red emission intensity is further enhanced by co-doping charge compensators Li, Na, and K, among which K is the best one. We also find that K plays a different role in improving the emission intensity of Sr3WO6:Eu³⁺ at different doping concentration. The red emission intensity of Eu³⁺ is more increased at high doping concentration compared with that at low doping concentration. The mechanisms for the different enhancement by doping K are discussed and the results show that at low concentration K only plays the role of charge compensation, decreasing the segregation of Eu³⁺. However for high concentration, K can also increase the transition probability of Eu³⁺ in addition to the charge compensation effect, which further enhances the red emission. Sr3WO6:Eu³⁺ phosphor shows intense red emission under the excitation of ultraviolet or blue LED. Sr3WO6:Eu³⁺ can be as an efficient and promising candidate red phosphor for white LEDs.Based on the investigation and the effects on the structure symmetry of Eu³⁺activated red phosphors by co-doping other ions, K co-doped Ca₃WO₆:Eu³⁺efficient red phosphor has been developed. By choosing Ca₃WO₆:Eu³⁺ as the investigation object, We investigate the effects of co-doping different alkali metals as the charge compensators on the relationship between the structure of Eu³⁺ substitution site and luminescence of Eu³⁺. The results show that the change to the symmetry at Eu³⁺sites are different by co-doping different alkali metals,which leads to the difference of the enhancement of Eu³⁺ emission intensity. The order is K>Li>Na. By comparing the structure and the luminescence of Sr3WO6:Eu³⁺ and Ca₃WO₆:Eu³⁺,vestigate the effect of K co-doping on the luminescence property of Eu³⁺.The results show that K can greatly change the site symmetry of Ca₃WO₆ host, and increases the the red emission transition probability of Eu³⁺, which implies that we must pay more attention to the change of the site symmetry by co-doping in addition to the symmetry of the host.Additionally, Raman and DFT calculations are performed to analyze the substitution sites in Ca₃WO₆:K+,Eu³⁺. K+ and Eu³⁺ are A-site substitutions. The emission intensity of K+ co-doped Ca₃WO₆:Eu³⁺ is 3.5 times as that of commercial red phosphor Y₂O₂S·Eu³⁺ under 395 nm excitation, which can be as an efficient and promising candidate red phosphor for white LEDs.An efficient Eu²⁺ activated oxide red phosphor has been developed by doping S into Eu²⁺ activated oxide phosphor to tune its chromaticity property. Orange emission oxide phosphor HTP-Ca_2.95SiO₄Cl₂:0.05Eu²⁺ is successfully synthesized by improved one step method, which has a broad emission spectrum centered at 600 nm.Based on this experiment, S doping is realized by reducing CaSO4 into CaS for sUlfUration under H2 atmosphere at high temperature, and S2- co-doped HTP-Ca_2.95SiO₄Cl₂:0.05Eu²⁺ is obtained. Maximum red shift of 40 nm is observed for the emission spectra by S2- doping, and additionally the excitation spectra show first red then blue shift with increasing the S2- doping contents. The mechanism for the shift of the luminescence spectra of S2- co-doped HTP-Ca_2.95SiO₄Cl₂:0.05Eu²⁺ is investigated.The results show that S2- doping can increase the crystal field splitting effect and the nephelauxetic effect, which are responsible for the shift of the spectra. S2- co-doped HTP-Ca_2.95SiO₄Cl₂:0.05Eu²⁺ has good chromaticity property which indicates that it can be used as a candidate for red phosphor for white LEDs based on blue or ultraviolet LED.The red emission intensity of Ca₄P₂O₉:Eu²⁺ has been greatly enhanced by doping Ce³⁺, and the mechanism for the enhancement has also been investigated.Luminescence property of CaaP209:Ce³⁺ is firstly reported, which emission spectrum well overlaps the excitation spectrum of Ca₄P₂O₉:Eu²⁺.Based on this phenomenon, we investigate the energy transfer process and efficiency in Ce³⁺ and Eu²⁺ co-doped Ca₄P₂O₉. The result shows high energy transfer efficiency from Ce³⁺ to Eu²⁺. The red emission intensity of Ca₄P₂O₉:Eu²⁺ is greatly enhanced by co-doping Ce³⁺.Themechalisms for the enhancement of the red emission of Eu²⁺ are analyzed and we find that, in addition to energy transfer from Ce³⁺ to Eu²⁺, Ce³⁺ co-doping can also make more Eu³⁺ be reduced to Eu²⁺, which increase the number of the luminescence centers.And Ce³⁺ co-doping can also decrease the Eu³⁺ content, leading to the decrease of the luminescence quenching of Eu²⁺ by Eu³⁺, which further increases the emission intensity of Eu²⁺.