Preparation And Performance Study Of MgGa₂O₄ Based Long Afterglow Material

Long persistent phosphors materials are special optical materials that emit radiation for long periods of time even after the excitation source has been switched off.It has gained much interest due to its potential applications in deep tissue bioimaging in vivo,emergency signalling,traffic signs,night vision monitoring,light emitting diodes,security displays etc.As MgGa₂O₄has the stability of a spinel structure（AB₂O₄）with a tightly packed face-centred cubic structure and partially inverted spinel properties,it has excellent physical and chemical properties,such as having a high melting point and high corrosion resistance;we have chosen MgGa₂O₄as the matrix material for the preparation of long afterglow materials in this thesis.In MgGa₂O₄,the Mg²⁺ion occupies the tetrahedral A-site and the Ga³⁺ion occupies the octahedral B-site.In this thesis,we have used A（Mg）site substitution and B（Ga）site substitution for doping or co-doping to prepare long persistent phosphors materials for potential applications in tissue bioimaging and luminescence anti-counterfeiting.The main research of this work is as follows:1.A series of NIR long persistent phosphors MgGa₂O₄:Cr³⁺,Nb⁵⁺were successfully synthesized by high temperature solid phase reaction,and the long afterglow performance was improved by co-doping with Nb⁵⁺ions.The MgGa₂O₄:Cr³⁺,Nb⁵⁺long persistent phosphors were prepared by calculating the critical distance and Dexter’s theory showed that the concentration burst mechanism of the samples was the nearest neighbor ion interaction.The experimental results show that the afterglow intensity is increased by 4 times and the photoluminescence intensity is increased by 2.34 times after co-doping with Nb⁵⁺.The thermoluminescence test shows that the trap density can be enriched and the distribution of traps can be adjusted by co-doping with Nb⁵⁺.A possible afterglow luminescence mechanism is proposed based on the analysis of the experimental results.2.A series of MgGa₂O₄:Mn²⁺long persistent phosphors materials were prepared using high temperature solid phase reactions.The emission spectra of the samples were tested in the range of400～800 nm,and the peak position of the emission band showed a strong peak around 501 nm,which was caused by the ⁴T₁（G）→⁶A₁（S）spin forbidden leap of the Mn²⁺ion.The afterglow properties of the test sample exhibit an intense green long afterglow after 10 min UV irradiation excitation has ceased,with the long afterglow lasting for more than half an hour.The results of the test temperature dependent photoluminescence spectra indicate that the MgGa₂O₄:Mn²⁺phosphor has good thermal stability3.MgGa₂O₄:Mn²⁺,Cr³⁺multimode luminescent long afterglow fluorescent materials for anti-counterfeit message encryption were synthesized based on the analysis of MgGa₂O₄single-doped Mn²⁺and MgGa₂O₄:Cr³⁺co-doping properties.The energy transfer mechanism between Mn²⁺and Cr³⁺ions was analyzed.The energy transfer efficiency between Mn²⁺and Cr³⁺ions was calculated to be as high as 93.14%.The excitation with wavelengths of 254 nm and 306 nm exhibited different emission phenomena,and the green emission was weakened and the NIR emission was enhanced with increasing x value at higher Cr³⁺concentration.The prepared samples have different luminescence colors under different excitations.They continue to emit intense green and near-infrared long afterglow even after the cessation of UV excitation.MgGa₂O₄:Mn²⁺,Cr³⁺long afterglow phosphors exhibit composition-based,excitation-based and time-dependent multimode dynamic luminescence and are a potential information encryption and anti-counterfeiting fluorescent material.4.Based on the analysis of the experimental results,the optimum doping concentration for the photoluminescence of MgGa₂O₄doped with Mn ions has been determined to be 0.001.The MgGa_2-x（Zn/Ge）_xO₄:0.001Mn luminescent material was prepared by co-doping Zn/Ge ions at a doping concentration of 0.001Mn,and the luminescence color and afterglow phenomenon of the samples were controlled by adjusting the doping ratio of Zn²⁺/Ge⁴⁺ions and thus changing the action mechanism of Mn²⁺and Mn⁴⁺in the matrix.With the increase of x value,the luminescence color of the sample gradually changed from green to red,producing two different luminescence colors and afterglow phenomena.The prepared materials have the potential for multi-modal dynamic luminescence because they are very sensitive to the response of detection time,excitation wavelength and composition.Therefore,adjustable multi-colour radiation and different afterglow phenomena can be output by varying the test conditions,thus achieving an anti-counterfeiting effect.