| Long persistent luminescent(LPL)materials have gained widespread attention because of their rapid energy storage under excitation,continuous energy release after excitation,and no harmful and no by-products in the process.They have important application prospects in daily lighting,medical and military fields.Currently,the long afterglow materials that demonstrate the better performance,or are already commercially available,are activated by Eu2+/Mn2+.These materials emit light across a broad-spectrum range,spanning from violet to near-infrared wavelengths.However,the development of long afterglow materials has faced several challenges,including a lack of rich substrate options,limited colour diversity,unclear carrier and trap types in mechanism and the role of co-doping ions in the afterglow process in Eu2+/Mn2+activated afterglow materials is not yet defined.To address this problem,we first start by analyzing the appearance and explore the relationship between the afterglow phenomenon and trap changes,then gradually go deeper and discuss the types of defects and traps as well as the intrinsic connection between them and the possible carrier types by combining the first principle calculation,so as to refine and perfect the afterglow mechanism from the theoretical point of view;Finally,through a series of means such as X-ray absorption near edge structure(XANES),the detection of valence changes caused by electron exchange between the luminescence center and the trap center during the afterglow process,it has been demonstrated experimentally time that trivalent lanthanide co-doping ions exist as electron traps in the afterglow material,solving the key problem that the role of co-doping ions in afterglow materials has been unclear for a long time.This will provide theoretical support to guide the design exploration of new long afterglow luminescent materials and is of great significance to the development of materials in this field.The main contents and results are as follows.:1.Firstly,the blue long persistent luminescent material(Ba,Li)(Si,Ge/P)2O5:Eu2+,Pr3+ was designed and synthesized by a doping,single solid/double solid solution strategy using a high-temperature solid-phase method,and its crystal structure,luminescence properties,trap states and afterglow properties were determined by X-ray diffraction(XRD)structure refinement,photoluminescence or afterglow spectroscopy,thermoluminescence spectroscopy and afterglow decay curves.The material can produce bright blue light with a 478 nm emission peak under near-UV excitation,and the afterglow emission peak is also located near 478 nm.The afterglow time of the material increases from 38 h for BaSi2O5:Eu2+,Pr3+ to 47 h for Ba(Si,Ge)2O5:Eu2+,Pr3+ and 56 h for(Ba,Li)(Si,Ge/P)2O5:Eu2+,Pr3+ with the doping,single solid solution-double solid solution modulation.The changes in the trap depth and concentration of the material during the process reveal that in addition to the conduction band transport,the re-trapping process between the deep and shallow traps and the tunneling effect with the energy level of the luminescence center occurs in the afterglow process,the existence of the tunneling effect was also demonstrated by theoretical derivation at the same time,which provides a preliminary understanding of the influence of electrons and traps on the afterglow process,2.Secondly,in order to further investigate the relationship between defects and traps,as well as to clarify the carrier types in the afterglow process,we introduced theoretical simulations to assist in the study of the afterglow mechanism.The possible lattice occupation and corresponding emission bands of Mn2+in SrMgGe2O6:Mn2+,Sm3+ are calculated and predicted by the first principle,and the reasons for their different luminescence and afterglow emission peaks are analyzed;the energy band structures of SrMgGe2O6:Mn2+ and SrMgGe206:Sm3+ are constructed,and the positions of Mn2+ and Sm3+ in the band gap were discussed to provide the theoretical basis for Mn2+ as the luminescence center and Sm3+ as the trap center,at the same time,the types of defects that may be caused by Mn2+ and Sm3+ doping were listed and their respective formation energies were calculated to determine the types of defects that may exist in the material and to qualitatively correspond to the traps.Guided by the results of this simulation,SrMgGe2O6:Mn2+,Sm3+ afterglow materials were designed and synthesized,which exhibit 658 nm orange-red photoluminescence and afterglow emission under excitation at 254 nm.The fitting results of both spectra also indicate that the luminescence centers involved in the luminescence process are Mnsr,MnMg,and MnGe,while the luminescence centers involved in the afterglow process only the Mn2+ occupying the Sr2+ and Ba2+ site.The thermoluminescence spectra also confirm that Sm3+ is involved in the afterglow process as a trap center,and these experimental results are in good agreement with the predictions of theoretical calculations.;3.Using the defect-trap-afterglow properties as an entry point,the effect of Ca2+solid solution and VGe’’’’ on the afterglow properties of the material was continued to be investigated in(Sr/Ba)1-xCaxGe4-yO9:Mn2+.It was found that when either Ca2+ solid solution or VGe’’’’ was introduced to act singly on the material,no afterglow phenomenon was produced;while when both occurred simultaneously,the material produced a bright orange-red afterglow emission.The corresponding phenomena were simulated by the first principle,and the results showed that when only Ca2+ solid solution was present,a slight downward shift of the conduction band bottom of the material was produced,but the large energy difference between the conduction band bottom and the excited state energy level makes the electrons insufficient to enter the conduction band through thermal perturbation and thus be trapped,then producing an afterglow.When a hole energy level was produced above the valence band when VGe’’’’was introduced.However,the hole energy level is at a large distance from basic state energy level of Mn2+ and cannot produce afterglow.And when the two acts simultaneously,the downward shift of the material conduction band increases,and the electrons have enough energy to enter the conduction band to participate in the afterglow process and make the material produce afterglow.In addition,the simulation results of defects and the images of AC-TEM also suggest that the defects involved in the afterglow process are(VGe’’’’+VO**)and(VGe’’’’+VO**),which further clarifies the defect type attribution in the afterglow mechanism.4.Last,the existence of electron traps in the afterglow process is corroborated by experimental means.The resulting cyan-emitting persistent phosphor BaZrSi3O9:Eu2+,Sm3+(Em=525 nm)was prepared.The role of the trivalent co-doped ion Sm3+in the afterglow process and the types of traps in the afterglow material were elucidated.It was found that BaZrSi3O9:Eu2+,Sm3+ produces Sm2+ during the excitation process by tools such as XANES and low-temperature spectroscopy,thus demonstrating the existence of the co-doping ion Sm3+as an electron trap in the afterglow process.We successfully observed for the first time the formation of Ln2+after excitation,transferring its electrons to the 5d band of Eu2+and returning to Ln3+ by itself during afterglow,where the decrease of Sm2+coincides with the increase of Sm3+and the two decay times τ1 and τ2 of the PL of Sm2+(5D0→7F0)coincide with the two evolution times τ1 and τ2 of the PL of Eu2+(5d→4f).We were able to detect the nonsimple behavior of electron transfer from Sm2+to Eu2+,which is the key to afterglow.Our discovery of "electrons trapped in the afterglow for Ln2+" gives us a clearer understanding of the afterglow with respect to the trapping depth,as the energy map of each Ln2+ is our knowledge.Based on this,a detailed afterglow mechanism is proposed in combination with a systematic analysis of thermoluminescence and defect reactions,which is important for the in-depth study of long afterglow materials and further refinement of the mechanism. |
PDF Full Text Request