Bi³⁺/Cr³⁺/Fe³⁺-Activated Visible And Near-Infrared Luminescent Materials:Structure Design And Photoluminescence Tuning

Phosphor-converted white light-emitting diodes(LEDs)have been widely applied in lighting display due to their merits of high luminescence efficiency,long lifetime,environmental friendliness,safety,and reliability.Current white LEDs are mainly fabricated by coating yellow-emitting phosphors(YAG:Ce3+)on blue LED chips,the white light from which shows a low color rendering index and a high correlated color temperature owing to the deficiency of red light.In order to obtain high-quality white light illumination,the combinations of near-ultraviolet LED chips with tricolor(red,green,and blue)or single-phased white light-emitting phosphors have attracted much attention.Inspired by white LEDs,a new near-infrared(NIR)light source with the advantages of small size and low cost can be obtained through the technology of "LED chip+NIR-emitting phosphor",which has broad application prospects in night vision,spectroscopy analysis,and biomedicine fields.However,different application scenarios require different luminescence properties for the NIR luminescent materials.The key to the development of the above two light sources is to explore luminescent materials that can meet the requirements of different luminescence performances,which needs precise regulation and directional optimization of luminescence properties.This paper focused on the luminescence properties of Bi3+,Cr3+,and Fe3+ activators,and a variety of visible and NIR luminescent materials with tunable luminescence properties were synthesized by utilizing the strategies of cation substitution,doping concentration adjustment,and energy transfer.The structure-property relationship between crystal structure and luminescence property was studied.It mainly includes the following three aspects:(1)Structural design,spectral regulation,and performance optimization of Bi3+-activated visible luminescent materialsTo address the issue of insufficient red light in commercial white LEDs,yellow/orange-emitting ABZn2Ga2O7:Bi3+(A=Ca2+,Sr2+;B=Ba2+,Sr2+)phosphor materials with tunable luminescence properties were synthesized.The local crystal environment was regulated through Ca/Sr/Ba cation substitution,and the different luminescence properties were attributed to the synergetic effect of centroid shift,crystal field splitting,and Stokes shift.Based on the selective site occupation,emission tuning of CaBaZn2Ga2O7:Bi3+was realized by changing the excitation wavelength and the doping level of Bi3+.The two emission centers of CaBaZn2Ga2O7:Bi3+showed different responses to temperature variation,making it promising in optical thermometer.The broadband orange-emitting Sr2Zn2Ga2O7:Bi3+with better thermal stability was combined with commercial blue phosphor to fabricate a white LED,which contributed to the red light component,resulting in high-quality warm white light.To address the issue of cyan gap in emission spectra of white LEDs,cyan-emitting Ca3Ga4O9:Bi3+,yZn2+ phosphor materials were synthesized.The design of part substitution of Zn2+ for Ca2+/Ga3+ led to the improved crystallinity,charge compensation,and local lattice distortion,which enhanced the intensity of cyan emission(486 nm)by 4.1 times and improved the thermal stability.This broadband cyan-emitting phosphor material was used to close the spectral cyan gap of a white LED,realizng high-quality full-spectrum white light illumination.To address the issue of emission reabsorption of multi-color phosphor materials in white LEDs,single-phased white-emitting Sr2-yBayLaGaO5:Bi3+,zEu3+ phosphor materials were synthesized.Firstly,Sr2yBayLaGaO5:Bi3+solid solution phosphor materials were synthesized by the cation substitution of Ba2+ for Sr2+.Based on the regulation of crystal field by bond length and lattice distortion,tunable emission from 465 to 502 nm was realized.Subsequently,energy transfer from Bi3+to Eu3+was designed to realize further emission tuning from blue-green to orange-red,and single-phased white light emission with adjustable luminescence performances was obtained.(2)Design and photoluminescence tuning of Cr3+-activated ultra-broadband and high-efficiency NIR luminescent materialsAlthough Cr3+-activated phosphors have been widely reported,it is challenging to achieve ultra-broad and tunable NIR emission.In(LiIn)1-yZn2yInSbO6:Cr3+ system,controllable emission tuning from 965 to 892 nm was achieved by the designed chemical unit cosubstitution of[Zn2+-Zn2+]for[Li+-In3+]in LiIn2SbO6 host.Moreover,the emission intensity was increased by 2.24 times,and the full-width at half maximum can reach 235 nm.Based on the analysis of crystallographic site occupation and crystal structure change,this blue-shift behavior was attributed to the increased crystal field around Cr3+.This series of broadband NIR luminescent materials showed potential applications in night vision and NIR spectroscopy analysis.This study provides a way for luminescence tuning of Cr3+.Achievement of high luminescence efficiency and thermal stability is challenging for NIR-emitting phosphor materials.In Ca3Y2-2y(ZnZr)yGe3O12:Cr garnet system,Cr3+ and Cr4+ coexisted in Ca3Y2Ge3O12 host due to the obvious cation size mismatch between Cr3+and Y3+and the cation size match between Cr4+and Ge4+,which reduced the luminescence efficiency of Cr3+.With the designed cosubstitution of[Zn2+-Zr4+]for[Y3+-Y3+],appropriate octahedral sites for Cr3+ ions were reconstructed,promoting the transformation from Cr4+to Cr3+.The introduction of[Zn2+-Zr4+]unit also built a rigid crystal structure.These two aspects significantly improved the luminescence efficiency and thermal stability.This work provides a new perspective for the development of high-performance NIR luminescent materials by chemical unit cosubstitution strategy.(3)Structural design and luminescent properties of Fe3+-activated broadband NIR luminescent materialsExcept for typical Cr3+-activated NIR-emitting phosphors,next-generation Cr3+-free NIR-emitting phosphors with high efficiency and tunable optical properties are highly desired to enrich the types of NIR luminescent materials for different application fields.As an essential element ion of the human body,Fe3+is non-toxic and can be regarded as a friendly dopant.However,its emission commonly occurs in the red and far-red light regions.Here,Fe3+-activated Sr2-yCay(InSb)1-xSn2zO6:Fe3+broadband NIR-emitting phosphor materials were designed and synthesized.The overall emission tuning from 885 to 1005 nm with broadened full-width at half maximum from 108 to 146 nm was achieved through a crystallographic site engineering strategy.The emission redshift and broadening were attributed to the increased crystal field and the reduced lattice symmetry,respectively.In addition,the NIR emission was significantly enhanced after complete Ca2+incorporation.Ca2InSbO6:Fe3+peaking at 935 nm showed an ultra-high internal quantum efficiency of 87%,which demonstrated great potential for NIR spectroscopy detection.This study provides a new insight for Fe3+-activated NIR luminescent materials.