Modification Of Structure And Performance In Cr³⁺-activated Borate/Gallate Near-Infrared Phosphors

As a new-generation near-infrared(NIR)light source,NIR phosphor-converted LED(pc-LED)in the package of NIR-emitting phosphors integrated with blue LED chips,has currently attracted increasing attention owing to its simple fabrication process,low cost,and wavelength-tunable behavior,et al.As one of the key materials of pc-LED,phosphor can directly determine the luminous efficiency and spectral continuity of NIR LED devices However,studies on NIR phosphor have just started with very limited available materials.In addition,the NIR phosphors suffer from poor luminescence efficiency and low thermal stability owing to large Stokes shift,which need to be improved.Cr3+-doped phosphor possesses wide and strong absorption ranging from blue to visible light,as well as wavelength-tunable behavior.Additionally,borates/gallates,which could provide a more stable crystallographic site for luminescent center Cr3+,have the advantages of abundant crystallographic sites and diversified structures.In this dissertation,based on Cr3+-activated borate/gallate,we intend to modify the luminescence performance and elucidate the relationship of luminescence performance with lattice structure and crystal-field environment to develop novel NIR phosphors with high quality by structural design on borate/gallate via chemical unit substitution and non-equivalent co-substitution.Meanwhile,the luminescence intensity of NIR phosphors can be enhanced via energy transfer,and the mechanism of enhancement is investigated in detail.This dissertation work includes the following four aspects.A series of NIR phosphors LaSc3-2xCaxSixB4O12:Cr3+(0≤x≤0.9)have been designed and prepared via non-equivalent co-substitution of Ca2+-Si4+→Sc3+-Sc3+.We investigate the evolution of structure with non-equivalent co-substitution.The broad emission band exhibits a shift toward longer wavelength from 871 nm to 880 nm along with the variation of local structure via Ca2+-Si4+→Sc3+-Sc3+co-substitution.Moreover,the as-prepared phosphor shows good thermal stability in application for pc-LED,indicating its potential for the application in blue-pumped NIR pc-LED towards IP camera for car plate,medical and food testing.Targeted phosphors LaSc2.93-yGayB4O12:0.07Cr3+(0≤y≤1.5)have been designed via the chemical unit substitution of Sc3+by Ga3+.The substitution of Sc3+by smaller Ga3+in LaSc2.93-yGayB4O12:0.07Cr3+ leads to crystal shrinkage and decreasing structural polyhedron distortion of[Sc1O6]and[Sc2O6]octahedrons,consequently resulting in crystal structure evolution and luminescence performance modification.Blue shift(871-824 nm)of broadband emission and enhanced sharp emission peak(689 nm,2E→4A2 transition)are observed.Interestingly,the NIR phosphor LaSc2.93-yGayB4O12:0.07Cr3+(y=0.6)shows broadband NIR emission centered at 850 nm under blue light excitation.The as-prepared phosphor meets the needs of blue-pumped LED,which could be applied in security-monitoring,medical and food testing.In addition,the improved luminescence intensity and enhanced thermal stability of LaSc2.93-yGayB4O12:0.07Cr3+(y=0.6)are interpreted by decreasing structural polyhedron distortion and band gap variation.Targeted NIR phosphors LaSc3-xYxB4O12:Cr3+ have been designed from the initial structure of LaSc2.93B4O12:0.07Cr3+via the chemical unit substitution of Sc3+by Y3+.The crystal structure evolution and luminescence performance variation have been investigated.The octahedron where Cr3+accommodated is directly expanded through substituting the Sc3+by Y3+in LaSc2.93B4O12 host,resulting in weakened crystal field.Consequently,the broadband emission shifts toward longer wavelength(871-883 nm).In addition,the increasing band gap of LaSc3-xYxB4O12 explains the luminescence intensity enhancement of NIR phosphor.According to the configurational coordinate model and thermal ionization model,the thermal stability is enhanced with Y3+incorporating.The above results indicate that a proper chemical composition modification is beneficial for optimizing the luminescence performance of phosphors and developing novel phosphors toward diversified applications.The enhanced NIR luminescence performance can be realized in La3Ga5GeO14:Cr3+due to the energy transfer from Pr3+ to Cr3+.For co-doped sample,Cr3+ luminescence intensity,centered at 980 nm,enhancement by a factor of 3 is achieved by doping Pr3+compared to that of La3Ga5GeO14:Cr3+.In addition,the as-prepared phosphor has good thermal stability and reliability,which holds promise in NIR pc-LED toward food testing and standard light source.Cr is identified to preferentially substitute for octahedral Gal and tetrahedral Ga3 crystallographic sites simultaneously according to valence state,ionic radius,EPR spectrum,Rietveld refinement,and the first-principles DFT calculations,which fully explains the super broadband emission(FWHM=330 nm).