Synthesis And Performance Study Of Long Afterglow Luminescent Materials

Persistent luminescence,also known as afterglow,is the phenomenon that a luminescence materials keeps emitting light after removal of the irradiation source.Persistent luminescence materials have found numerous applications in architectural decoration,information display,and emergency signage.The emission colors of the persistent luminescence materials developed by far are mainly in the visible regions and only blue and green long persistent phosphors meet the requirements for high initial intensity and a sufficiently long duration time of afterglow.Rare earth ions are widely used as luminescence centers and could be confined to Eu²⁺ions in the visible long persistent phosphors.Near infrared?NIR?long persistent phosphors have gained considerable attention inrecent years owing to the potential applications in medical imaging,information storage,and in energy engineering.NIR radiation is non noninvasive as well as non-ionizing and has more effective blood and tissue penetration than UV and visible light.However,the progress on NIR long persistent phosphors is too slow to fulfill the increasing demands on medical imaging,labeling and clinical medicine.Up until now,NIR long persistent phosphor with good performance remains rather limited.Therefore,it is an important aspect to investigate and develop the novel long afterglow phosphors to extend this family.The work presented in this thesis aims to develop and synthesize novel long persistent phosphors in the NIR region.For this purpose,suitable luminescence centers whose emission in the NIR region and proper hosts that can enhance the NIR emission are necessary.The transition metal ions is the potential candiates for the luminescent center.Among the transition metal ions activated phosphors,chromium ion in the trivalent states,Cr³⁺,is an ideal luminescent center because of its NIR emission at around 700 nm.A novel series of Zn₃Al₂Ge₃O₁₂:Cr³⁺NIR persistent phosphors were successfully synthesized by high temperature solid-state reaction in air atmosphere,which emit broad bands from 650 nm to 750 nm peaking at 688 nm and 697 nm.This due to the²E?⁴A₂ transitions of Cr³⁺ions occupying two different Al³⁺octahedral sites.The luminescent properties and the afterglow behaviors of the phosphors were systematically investigated.The persistent luminescence mechanisms of Zn₃Al₂Ge₃O₁₂:Cr³⁺were studied based on the results of afterglow decay curves and thermoluminescence measurements and related models were proposed.The thermoluninescence results indicate Cr³⁺doping favors the foreign trap creation and thus influence the trap depth and concentration.Cr³⁺ions not only act as emitters,but also introduce foreign trap centers contributing to the long persistent luminescence process.It appears that the linear dependence of I^-1versus t is a sign that the persistent luminescence in Zn₃Al₂Ge₃O₁₂:Cr³⁺phosphor probably occurs through a tunneling-related process.The NIR persistent luminescence can persist more than 12 hours.Several novel long persistent phosphors were developed,such as Li₅Zn₈Ga₅Ge₉O₃₆:Cr³⁺,Li₅Zn₈Ga₅Ge₉O₃₆:Mn²⁺,Li₅Zn₈Al₅Ge₉O₃₆:Cr³⁺and Li₅Zn₈Al₅Ge₉O₃₆:Mn²⁺.The options for the host lattices and activators are thus broadened.After exposed to 254 nm UV light for five minutes,Li₅Zn₈Ga₅Ge₉O₃₆:Cr³⁺and Li₅Zn₈Al₅Ge₉O₃₆:Cr³⁺exhibit persistent luminescence in the 650-850 nm wavelength range with an afterglow time of more than 10 h.Li₅Zn₈Ga₅Ge₉O₃₆:Mn²⁺and Li₅Zn₈Al₅Ge₉O₃₆:Mn²⁺exhibit persistent luminescence in the 500-600 nm wavelength range dominated at 520 nm originates from the characteristic ⁴T₁?⁴G??⁶A₁?⁶S?transition of Mn²⁺.The green persistent luminescence can persist more than 4hours.The investigation of the new long persistent phosphor enriches the options for the activator and hosts.We try to improve the persistent luminescence performance of LiGa₅O₈:Cr³⁺NIR persistent phosphors.A series of novel LiGa₅O₈:Cr³⁺,M(M=Sn⁴⁺,Si⁴⁺,Ge⁴⁺Ca²⁺,Sr²⁺)phosphors were prepared by a traditional solid-state reaction method in air atmosphere.The influence of codoping M ions on photoluminescence,long persistent luminescence and thermoluminescence properties were systematically investigated.The results showed the persistent luminescence performance of phosphors was significantly enhanced after the incorporation of M ions.Thermoluminescence spectra indicated that the incorporation of Sn⁴⁺or Si⁴⁺contributes to the formation of defects and increase the traps density,thus improving the persistent luminescence properties of LiGa₅O₈:Cr³⁺phosphors.In addition,Cr³⁺doped LiGa₅O₈ phosphors with deficient,stoichiometric or excess amounts of lithium were synthesized by a solid-state reaction method.We found that the addition of excess Li⁺into LiGa₅O₈:Cr³⁺improve its photoluminescence and afterglow properties where Li⁺deficiencies did not favor photoluminescence and afterglow.The underlying reason of photoluminescence and afterglow enhancement is discussed.