Enhancing Near Infrared Persistent Luminescence From Cr³⁺-activated Zinc Gallogermanate With Spinel Structure

Near infrared?NIR?long-persistent luminescence materials hold strong commercial promise for applications ranging from night-vision surveillance to in vivo bioimaging,as they are non-toxic,stable in harsh environment,and can emit NIR luminescence for a long time?from hours to days?after the stoppage of the excitation light.Despite these advantages,the luminescence brightness and afterglow period remain insufficient for the reported NIR persistent phosphors with both the activator content optimized and the host material defined.This thesis aims to propose an innovative route to enhance the luminescence profile of the NIR persistent phosphors of Cr³⁺-activated spinel zinc gallogermanate(emission at 650 nm-850 nm from the 2E ? 4A2 of Cr³⁺)through incorporation of non-luminescent ions(Ca²⁺,Li⁺,Mg²⁺,and Ca²⁺/Li⁺)into the host lattice.A set of Zn₃Ga_1.99Ge₂O₁₀: Cr³⁺,M(M = Ca²⁺,Li⁺,Mg²⁺,and Ca²⁺/Li⁺)samples with an optimized Cr³⁺ ion concentration of 0.5 %,and defined amount of non-luminescent M concentration were synthesized by the high temperature solid-state reaction method.The doping concentration of non-luminescent M was determined by the appropriate solid precursors with designated stoicchiometic ratios.We systematically investigated the concentration effect of non-luminescent ions(Ca²⁺,Li⁺,Mg²⁺)on the performance of afterglow,photoluminescence,excitation spectra of Zn₃Ga_1.99Ge₂O₁₀: 0.5 % Cr³⁺ long-persistent luminescence powders.Moreover,scanning electron microscopic?SEM?and x-ray diffraction?XRD?were used to show the doping effect on the morphology and structure of the as-prepared samples.Our experimental results showed that the optimum doping concentrations of Ca²⁺,Li⁺,and Mg²⁺ were 3 %,3 %,and 10 %,respectively.Under the optimum doping concentration,the sample doped with Ca²⁺ 3 % exhibited about 15-fold higher persistent luminescence intensity than the one with null Ca²⁺,the Li⁺ 3% sample exhibited 2-fold higher persistent luminescence than Li⁺ 0 % sample,while the Mg²⁺ 10 % sample exhibited 2.6-fold higher persistent luminescence than the Mg²⁺ 0% sample,at 300 s post the stoppage of the light excitation.Besides,the sample codoped with both Ca²⁺ 3 % and Li⁺ 1% exhibited about 21-fold higher persistent luminescence than the sample without doping,demonstrating the synergistic enhancement effect of Ca²⁺ and Li⁺.Mechanistic analyses indicated that doping non-luminescent ion with appropriate concentration could result in the size increase and the formation of single crystal phase of the resulting particle size,favoring the reduction of quenching centers in the crystal.Moreover,non-luminescent ions in the samples can also increase the number of antisite defects,i.e.,the increase of the quantity of Cr³⁺ located in an disordered environment,thus enhancing the long-persistent luminescence intensity.However,when doping concentration was too high,the luminescence quenching centers in the crystals would be increased,thereby increasing the deactivation rates of excited state energy and thus correspondingly decreasing the afterglow time.The thermoluminescence curves demonstrated that doping with Ca²⁺ could increase the number of electronic traps that capture more excited state electrons.Finally,the energy transfer mechanism of Zn₃Ga₂Ge₂O₁₀: Cr³⁺,M(M = Ca²⁺,Mg²⁺,Li⁺,Ca²⁺/Li⁺)long-persistent luminescence powders was proposed to explain the observed phenomena,constituting a solid step forward to improve the performance of long-persistent luminescence.