The Luminescence Performance Study Of Zn Coordinated Mn,Eu Co-doped MgGeO₃

The long afterglow material is a kind of green energy storage optical material.Due to the long-time self-illumination ability of the material,it has important practical applications in the fields of detection and diagnosis,night vision instruments,fire emergency,traffic safety instructions and urban landscape and so on.Especially,the long-afterglow materials emitting near-infrared light have become hot in the field of biomedical imaging in recent years.However,it is facing with two major problems.One is that there is a shortage of materials emitting red light,and their luminescence performance is poor.The other is that the luminescence mechanism of many luminescent materials is still unclear.This work mainly focuses on the optical properties of long-afterglow luminescent materials of Mn and Eu co-doped Zn-Mg-Ge-O systems emitting red light.In this paper,the long afterglow luminescent materials of Zn-Mg-Ge-O system doped by Mn and Eu were prepared by high temperature solid phase method.The control variable method was used to prepare a batch of samples with sintering temperature and content of Zn as variables in this experiment.The optimal Zn content and sintering temperature were determined by comparing the crystal structure,surface morphology,composition and optical properties and so on of these samples.The characterization results showed that,MgGeO₃ and Zn₂GeO₄ were the main crystal structures in these samples.The red light emission come from Mn/Eu co-doping MgGeO₃,while the green light emission was caused by Mn/Eu co-doping Zn₂GeO₄.When the sintering temperature was constant,the red light emission of samples was enhanced first(x?0.7)and then decreased with increasing of Zn content.At the same time,the Zn content is 0.7,the red light emission of samples excited by 375 nm is optimal.However,when the content of Zn is higher than 0.7,the afterglow colors of the samples observed orange(x=0.8),yellow(x=0.9)and green(x=1)respectively.When the Zn content up to 1,the emission intensities both of green fluorescence and afterglow increased and then decreased with the increasing of sintering temperature,and is the strongest at the sintering temperature of 1200?.On the basis of the experimental results,the corresponding models of Mn doped MgGeO₃ and Zn₂GeO₄,and Eu doped MgGeO₃ and Zn₂GeO₄ are constructed respectively,and their fine energy band structures are obtained by density functional theory.Shown that,although the emission centers of MgGeO₃:Mn,Eu and Zn₂GeO₄:Mn,Eu both are Mn²⁺,the positions of the electron-hole pair separation and the transition paths of the electron emitting photons are different.The position of electron-hole pair separation of MgGeO₃:Mn,Eu occurs in the ⁶A₁(S)energy level of Mn,while that of Zn₂GeO₄:Mn,Eu occurs in the valence band(VB).The electron's downward transition path is ⁴T₂(D)?⁶A₁(S)in MgGeO₃:Mn,Eu,and ⁴T₂(D)?VB in Zn₂GeO₄:Mn,Eu.Eu³⁺exists as trap centers in MgGeO₃ and Zn₂GeO₄,but the trap depths formed in these two crystals are different.On above basis,we propose a new luminescence mechanism model of Mn,Eu co-doped MgGeO₃ and Zn₂GeO₄ to explain the optical phenomena in the experiment more reasonably.It is hoped that it will be helpful for people who study the luminescent materials doped with Mn.