Desirable phosphors for lighting,scintillation and composite films must have good light absorption properties,high concentration quenching,high quantum efficiency,a narrow color emission,and so forth.In this work,we f...Desirable phosphors for lighting,scintillation and composite films must have good light absorption properties,high concentration quenching,high quantum efficiency,a narrow color emission,and so forth.In this work,we first show that undoped yttrium hafnate Y_(2)Hf_(2)O_(7)(YHO)nanoparticles(NPs)display dual blue and red bands after excitation using 330 nm light.Based on density functional theory(DFT)calculations,these two emission bands are correlated with the defect states arising in the band-gap region of YHO owing to the presence of neutral and charged oxygen defects.Once doped with Eu^(3+)ions(YHOE),the YHO NPs show a bright red emission,a long excited state lifetime and stable color coordinates upon near-UV and X-ray excitation.Concentration quenching is active when Eu^(3+)doping reaches 10 mol%with a critical distance of~4.43Å.This phenomenon indicates a high Eu^(3+)solubility within the YHO host and the absence of Eu^(3+)clusters.More importantly,the optical performance of the YHOE NPs has been further improved by lithium co-doping.The origin of the emission,structural stability,and role of Li^(+)-codoping are explored both experimentally and theoretically.DFT calculation results demonstrate that Li^(+)-co-doping increases the covalent character of the Eu^(3+)-O^(2-)bonding in the EuO8 polyhedra.Furthermore,the YHOE NPs have been dispersed into polyvinyl alcohol(PVA)to make transparent nanocomposite films,which show strong red emission under excitation at 270 and 393 nm.Overall,we demonstrate that the YHO NPs with Eu^(3+)and(Eu^(3+)/Li^(+))doping have a high emission intensity and quantum efficiency under UV and X-ray excitation,which makes them suitable for use as phosphors,scintillators and transparent films for lighting,imaging and detection applications.展开更多
Thermal annealing has a profound effect on material characteristics such as defects,crystallinity,surface area,size,morphology,etc.,and is followed by significant influence on their optoelectronic and scintillator pro...Thermal annealing has a profound effect on material characteristics such as defects,crystallinity,surface area,size,morphology,etc.,and is followed by significant influence on their optoelectronic and scintillator properties.As a function of annealing temperature,our as-synthesized La_(2)Hf_(2)O_(7):Eu^(3+)NPs,by a molten-salt method(MSS),showed a systematic increase in quantum efficiency and emission output up to 950℃before a slight reduction at 1050℃.This phenomenon has been attributed to the decrease of surface defects as a function of annealing temperature.In addition,the asymmetry ratio decreases monotonically as a function of annealing temperature and a completely symmetric environment around europium was observed at 1050℃.This can be correlated with the fact that the crystal structure becomes more and more ordered(ideal pyrochlore)as the annealing temperature is increased.The sample annealed at 1050℃displayed site-specific luminescence and can be useful in designing need based orange or red phosphors.The synthesized materials show different emission characteristics in terms of asymmetry and emission output under excitation with highly energetic X-ray beams.The current work highlights the importance of thermal treatment in modulating the light emitting properties of red phosphors and X-ray scintillators.展开更多
基金support from the National Science Foundation under CHE(award#1710160)the IIT startup funds.SKG thanks the United States-India Education Foundation(USIEF)and the Institute of International Education(IIE)for his Fulbright Nehru Postdoctoral Fellowship(award#2268/FNPDR/2017).
文摘Desirable phosphors for lighting,scintillation and composite films must have good light absorption properties,high concentration quenching,high quantum efficiency,a narrow color emission,and so forth.In this work,we first show that undoped yttrium hafnate Y_(2)Hf_(2)O_(7)(YHO)nanoparticles(NPs)display dual blue and red bands after excitation using 330 nm light.Based on density functional theory(DFT)calculations,these two emission bands are correlated with the defect states arising in the band-gap region of YHO owing to the presence of neutral and charged oxygen defects.Once doped with Eu^(3+)ions(YHOE),the YHO NPs show a bright red emission,a long excited state lifetime and stable color coordinates upon near-UV and X-ray excitation.Concentration quenching is active when Eu^(3+)doping reaches 10 mol%with a critical distance of~4.43Å.This phenomenon indicates a high Eu^(3+)solubility within the YHO host and the absence of Eu^(3+)clusters.More importantly,the optical performance of the YHOE NPs has been further improved by lithium co-doping.The origin of the emission,structural stability,and role of Li^(+)-codoping are explored both experimentally and theoretically.DFT calculation results demonstrate that Li^(+)-co-doping increases the covalent character of the Eu^(3+)-O^(2-)bonding in the EuO8 polyhedra.Furthermore,the YHOE NPs have been dispersed into polyvinyl alcohol(PVA)to make transparent nanocomposite films,which show strong red emission under excitation at 270 and 393 nm.Overall,we demonstrate that the YHO NPs with Eu^(3+)and(Eu^(3+)/Li^(+))doping have a high emission intensity and quantum efficiency under UV and X-ray excitation,which makes them suitable for use as phosphors,scintillators and transparent films for lighting,imaging and detection applications.
基金support from the National Science Foundation under the CHE(award#1710160)and DMR(grant#1523577)the USDA National Institute of Food and Agriculture(award#2015-38422-24059)+1 种基金the University of Texas Rio Grande Valley is grateful for the generous support provided by a Departmental Grant from the Robert A.Welch Foundation(Grant No.BX-0048)the United States-India Education Foundation(USIEF)and the Institute of International Education(IIE)for his Fulbright Nehru Postdoctoral Fellowship(award#2268/FNPDR/2017).
文摘Thermal annealing has a profound effect on material characteristics such as defects,crystallinity,surface area,size,morphology,etc.,and is followed by significant influence on their optoelectronic and scintillator properties.As a function of annealing temperature,our as-synthesized La_(2)Hf_(2)O_(7):Eu^(3+)NPs,by a molten-salt method(MSS),showed a systematic increase in quantum efficiency and emission output up to 950℃before a slight reduction at 1050℃.This phenomenon has been attributed to the decrease of surface defects as a function of annealing temperature.In addition,the asymmetry ratio decreases monotonically as a function of annealing temperature and a completely symmetric environment around europium was observed at 1050℃.This can be correlated with the fact that the crystal structure becomes more and more ordered(ideal pyrochlore)as the annealing temperature is increased.The sample annealed at 1050℃displayed site-specific luminescence and can be useful in designing need based orange or red phosphors.The synthesized materials show different emission characteristics in terms of asymmetry and emission output under excitation with highly energetic X-ray beams.The current work highlights the importance of thermal treatment in modulating the light emitting properties of red phosphors and X-ray scintillators.
