Design,Preparation And Properties Of Gallate-based Color-tunable Up-conversion Luminescence And Persistent Phosphors

Up-conversion luminescence?UCL?is a photoluminescence process,which emit a short wavelength photon after absorbing two or more long-wavelength photons,also called as anti-Stokes fluorescence.Persistent luminescence?PL?refers to self-sustained luminescence that continue emitting luminescence after the stoppage of excitation.Combining the above two special luminescence,a new luminescence process called as up-conversion persistent luminescence?UCPL?can be formed.That is,UCPL materials can emit high-energy persistent luminescence after excited by low-energy irradiation.Therefore,it is not only challenging to realize the above three special luminescence phenomena in a same matrix material,but also having far-reaching significance and broad application prospects.In order to better satisfy production and application requirements,it usually requires various colors of luminescence to enrich the diversity of fluorescent paints and luminescent marks.If luminescent materials can emit regular and controllable polychromatic fluorescence,the application range can certainly be expanded.Besides,Er³⁺ ion is one of the most frequently used activator ions in up-conversionluminescence.If we can adjust the green up-conversion emission intensity and the red up-conversion emission intensity respectively,then the difficulty of different energy level transitions occurred in Er³⁺ ions can be evaluated indirectly,which has certain research significance.What's more,due to the great potential application in decoration,military,biological imaging and other fields,developing and producing more kinds of up-conversion and persistent phosphors are still the research focus with plenty of research space in the field of luminescence.In this thesis,we chose gallate materials as the matrix materials.Adjusting the doping matrix cations(including divalent cation Zn²⁺/Mg²⁺,trivalent cation Ga³⁺/Al³⁺and tetravalent cation Sn⁴⁺/Ge⁴⁺),a series of phosphors doped with rare earth ions(Yb³⁺,Er³⁺,Tm³⁺)and transition metal ions(Cr³⁺),were designed and fabricated.Meanwhile,properties of up-conversion luminescence,persistent luminescence and up-conversion persistent luminescence were all studied.Main study content has beensummarized as follows:?1?Yb³⁺-Er³⁺ doubly-doped gallate based up-conversion phosphors were synthesized by high temperature solid state reaction and sol-gel method,while the GRR?green-to-red ratio?could be adjusted by the different doping ratio of rare earth ions and doping different valent cations into the matrix.Under the excitation of a980 nm near infrared laser,Yb³⁺-Er³⁺ doubly-doped gallate based phosphors have obvious green up-conversion emissions at 524 nm,549nm and red up-conversion emission at 659 nm respectively generated by ²H_11/2?⁴I_15/2?⁴S_3/2?4^I_15/2 and ⁴F_9/2?⁴I_15/2 transition of Er³⁺ ions.Up-conversion luminescence intensity and GRR of phosphors decreased with the increasing molar ratio of Yb³⁺-Er³⁺ and the doping of divalent cation(Mg²⁺),making the red up-conversion intensity higher than the green one?GRR<1.0?.It means Er³⁺ ions on ⁴S_3/2 state are more likely to relax nonradiatively to ⁴F_9/2 state increasing ⁴F_9/2?⁴I_15/2 transition.However,after doping trivalent cation(Al³⁺)or tetravalent cation(Sn⁴⁺-Ge⁴⁺)in matrix,the up-conversion intensity and GRR of phosphors can be greatly improved?GRR>1.0?,which means the green up-conversion intensity is stronger.That is,the non-radiative relaxation from⁴S_3/2 state to ⁴F_9/2 state is decreased while ⁴S_3/2?⁴I_15/2 transition of Er³⁺ ion is increased.The particle size of powder prepared by sol-gel method can reduce to nanometer level,but its up-conversion luminescent intensity is much lower than powder prepared by high temperature solid state reaction.?2? Yb³⁺-Er³⁺-Tm³⁺ triply-doped gallate based up-conversion phosphors were prepared by high temperature solid state method,while the up-conversion emission color could be adjusted by the different doping ratio of rare earth ions and doping different valent cations into the matrix.Under the excitation of a 980 nm near infrared laser,besides the green and red up-conversion emissions of Er³⁺ ions, phosphors generate blue up-conversion emission at 477 nm and red up-conversion emission at694 nm respectively attributed to ¹G₄?³H₆ and ³F₃?³H₆ transition of Tm³⁺ ions.In other words,blue,green and red emission appeared simultaneously in the same phosphor.According to the different molar ratio of Tm³⁺-Er³⁺,ZnGa₂O₄:3.5%Yb³⁺,0.33%Er³⁺.0.1 65%Tm³⁺ emit red-purple visible light,ZnGa₂O₄:3.5%Yb³⁺.0.25%Er³⁺,0.25%Tm³⁺ emit purple-pink visible light,ZnGa₂O₄:3.5%Yb³⁺,0.165%Er³⁺,0.33%Tm³⁺ emit purple visible light.In addition,the up-conversion luminescent colorof phosphors can be adjusted by controlling the doping amount of trivalent cation in matrix.When the molar ratio of Al³⁺-Ga³⁺ is smaller than 1:1,the increasing doping of Al³⁺ ions can make the up-conversion luminescence gradually move to the blue region.However,when the ratio of Al³⁺-Ga³⁺ is alrger than 1:1,increasing the doping amount of Al³⁺ ions can return the up-conversion luminescence to pink region.Also,doping different tetravalent cations in the matrix can lead to different emission colors.Specifically,Zn₃Ga₂SnO₈:Yb³⁺,Er³⁺,Tm³⁺ show blue-green luminescence, Zn₃Ga₂GeO₈:Yb³⁺,Er³⁺,Tm³⁺ show blue-purple luminescence,Zn₃Ga₂Ge_0.5Sn_0.5O₈: Yb³⁺,Er³⁺,Tm³⁺ show blue luminescence.However,co-doping divalent cation(Mg²⁺)in the matrix can not change the luminescent color of phosphors.?3?Cr³⁺ doped gallate based long persistent luminescent materials were prepared by high temperature solid state method and sol-gel method.The afterglow of powder phosphors prepared by high temperature solid state method,which peaked at 689 nm,can last more than 60 min after excited at 266 nm for 15 min with a xenon lamp.When the molar ratio of Cr3³⁺ ions is 0.5%,the afterglow decay speed of ZnGa₂O₄:0.5%Cr³⁺phosphors is the slowest.Only when trivalent Al³⁺ ions are co-doped in the matrix and the doping molar ratio of Al³⁺-Ga³⁺ is 1:9,the afterglow decay speed can be decreased and the afterglow time can be increased.However,doping bivalent Mgig ions and tetravalent Ge⁴⁺-Sn⁴⁺ ions in the matrix would accelerate the afterglow decay speed and decrease the afterglow time.Besides,compared with phosphors prepared by high temperature solid state method,the particle size of powder prepared by sol-gel method can reduce to nanometer level,but the afterglow decays faster.?4?Yb³⁺-Er³⁺-Cr³⁺ gallate based up-conversion persistent phosphors were prepared by high temperature solid state method.After excited by a xenon lamp at266 nm or a 980 nm near infrared laser for 15 min,phosphors can produce afterglow at about 700 nm.After doping Yb³⁺ ions and Er³⁺ ions,the afterglow decay speed of phosphors would be accelerated.Furthermore,the afterglow time of phosphors becomes shorter and shorter with the increased doping amount of Yb³⁺ ions and Er³⁺ions.After doping tetravalent Ge⁴⁺-Sn⁴⁺ ions in the matrix,the luminescence intensity,afterglow time and up-conversion afterglow time of phosphors can be improved by controlling the molar ratio of Ge⁴⁺-Sn⁴⁺ ions.Specifically,Zn₃Ga₂Ge_0.3Sn_0.7O₈:Yb³⁺,Er³⁺,Cr³⁺ and Zn₃Ga₂Ge_0.7Sn_0.3O₈:Yb³⁺,Er³⁺,Cr³⁺ phosphors have slower afterglow decay speed than ZGO:Cr phosphors.Also,their up-conversion afterglow time is longer than other phosphors.In conclusion,we designed and prepared double-doped up-conversion phosphors ZGO:Yb,Er,triple-doped up-conversion phosphors ZGO:Yb,Er,Tm,long afterglow phosphors ZGO:Cr and up-conversion afterglow phosphors ZGO:Yb,Er,Cr.Moreover,doping cations with different valence in the matrix of above phosphors(including divalent cation Zn²⁺/Mg²⁺,trivalent cation Ga³⁺/Al³⁺ and tetravalent cation Sn⁴⁺/Ge⁴⁺),the different energy level transitions occurred in rare earth ions can be adjusted and color-controllable emission can be obtained in up-conversion phosphors,while afterglow decay speed can be slowed down in long persistent phosphors.