Preparation And Spectroscopic Properties On Bi²⁺ Dopedred Luminescent Materials

Recently,red luminescent materials are essential materials for high efficient white LED and diagnostics and therapeutics.However,few luminescent ions exhibit red emission in the photoluminescent materials.Wherein,when bismuth ions are stable in their divalent state,they exhibit red emission.So far,few studied focused on the enhance of the red emission in Bi²⁺doped luminescent materials and their thermal quenching behaviors.So in this dissertation,we studied the preparation and luminescent properties on Bi²⁺doped red luminescent materials as follows:?1?Phosphor converted white light-emitting diodes?pc-WLEDs?have been recognized as the next generation of lighting sources due to their outstanding features such as energy saving,environment-friendly,small volume and longer lifetime.It can be generalized in domestic and automotive lighting,to imaging,display technologies,agriculture and medical.Red converted phosphors are one of the most important compositions,which have received widespread attention.Red phosphors with blue absorption can be used as an additive applied in WLEDs with blue LED chips+YAG:Ce³⁺yellow phosphors combinations,while red phosphors with NUV absorption can be used as a component applied in WLEDs with NUV chips+RGB primary color phosphors combinations.Usually,Bi²⁺doped red phosphors can absorb blue or NUV,so they have the potential as red phosphors as for application in WLEDs.Bismuth ions have various valence state in different compounds and we use Bi₂O₃ as raw materials in general.So the effective reduction of Bi³⁺to Bi²⁺ions becomes a major subject as preparation of Bi²⁺doped red phosphors.One traditional way is to use the reducing atmosphere.Herein,we presented the luminescent properties of Sr B₆O₁₀:Bi and Sr B₄O₇:Bi after treating with different atmospheres?reducing atmosphere H₂/N₂,protective atmosphere N₂ and ambient atmosphere?.We found out that the red emissions were more intense in samples treating with reducing and protective atmospheres.Amongst,the emission intensity in Sr B₆O₁₀:Bi²⁺increased by 163 times after treating with H₂/N₂ for 60 min.Therefore,it is an appropriate way to prepare the Bi²⁺doped red phosphors to improve their luminescent properties.According to the previous report,the luminescent properties can also be improved by co-doping divalent alkaline earth metal ions in Sr B₄O₇:Bi²⁺,such as Mg²⁺,Ca²⁺,Ba²⁺.But what would happen if we co-dope with other valence state ions in the Bi²⁺doped borate?In this dissertation,we chose Y³⁺ions and alkali metal ions IA⁺?IA=Li,Na,K,Cs?to co-dope into Sr B₆O₁₀:Bi²⁺and Sr B₄O₇:Bi²⁺.The result shows that the red emission in Sr B₆O₁₀:Bi²⁺and Sr B₄O₇:Bi²⁺improve by co-doping with Y³⁺ions in the ambient condition.When the Y³⁺concentration is 10 mol.%,the emission intensity increase by3 times in Sr B₆O₁₀:Bi²⁺and 1.75 times in Sr B₄O₇:Bi²⁺.In contrast,the red emission in the Sr B₄O₇:Bi²⁺decrease gradually by co-doping with IA⁺ions in the following order:Li⁺,K⁺,Na⁺,Cs⁺.And we further studied the Sr B₄O₇:Bi²⁺,Na⁺with different Na⁺concentration.The red emission intensity decrease with increasing the Na⁺concentration.When it is up to 90 mol.%,the sample becomes amorphous.?2?In the practical application of WLEDs,the stability of phosphors are particularly important,especially for their resistance to thermal quenching.But up to now,few and scattered Bi²⁺doped red phosphors have been studied in their thermal quenching behavior.Herein,we studied the thermal quenching behavior in Bi²⁺doped sulfate and pyrophosphate in particular.For Ba SO₄:Bi²⁺,only one barium site in the matrix,the incorporation of Bi²⁺on Sr² would lead to a single emission center.The red emission intensity becomes worse as increasing the temperature due to the classic thermal quenching.And low Bi²⁺concentration coumpound shows the best resistance to thermal quenching.This is in agreement with the previous report on YAG:Ce³⁺phosphors.For M₂P₂O₇:Bi²⁺?M=Ca,Sr,Ba?,we studied the site occupancy preference and thermal quenching of the luminescence in these red phosphors in the low and high temperature.Because all of them have multiple M sites,at which Bi²⁺dopant can be located.We need to find out which sites Bi²⁺would preferentially occupy and the assignment of the photoluminescence properties.Surprisingly,we noticed that the thermal-quenching behavior is better in certain site and high Bi²⁺concentration.This is because the site occupancy preference and energy transfer will lead to the anti-thermal quenching for certain Bi²⁺sites.This work helps us to better understand the complex luminescent behavior of Bi²⁺doped materials,and it will be helpful to design in future the heavily doped phosphor for WLEDs with even better resistance to thermal quenching.?3?In order to further widen the application field for Bi²⁺doped luminescent materials,we reported the luminescence from Bi²⁺-doped pyrophosphate,sulfate and borate under X-ray and found out that they all exhibit red emission,which similar to the photoluminescence exicited by UV-vis.By systematical comparison and analysis,Ba SO₄:Bi²⁺has been chosen as the best material.We also compared this compound with standard Lu_1.8Y_0.2Si O₅:0.11%Ce³⁺(LYSO:Ce³⁺).LYSO:Ce³⁺:15000ph/Me V;Ba SO₄:Bi²⁺:5000ph/Me V.So the emission need to be improved in the future.In addition,we studied the luminescence from Ba SO₄:Bi²⁺under long-time X-ray exposure.The emission intersity decrease 26%after 3h exposure.