Modification Of Structure And Performance In Cr³⁺-doped Borate/gallate Near-infrared Phosphors

Near infrared light technology has unique advantages such as non-invasive,real-time monitoring,and rapid analysis,and is widely used in food detection,biological imaging,night vision security,agriculture,and other fields.Near infrared fluorescence conversion light-emitting diodes（NIR pc-LED）,as a new generation of solid-state light sources,are highly favored by researchers due to their advantages such as fast response,low energy consumption,small size,and long lifespan.In NIR pc-LED,fluorescent powder is an important factor determining its performance.Traditional near-infrared phosphors still face problems such as narrow emission bands,low absorption coefficients,low quantum efficiency,and poor thermal stability.Cr³⁺ions have a unique 3d³ electron configuration and exhibit broadband absorption and tunable emission spectra,making them an ideal near-infrared luminescent activator.In this paper,the problems of relatively narrow emission spectrum,low quantum efficiency,poor thermal stability and strong electron phonon coupling of Cr³⁺-activated near-infrared phosphors were studied.The strategies of two-sites occupation,energy transfer cation substitution,cosolvent,chemical unit co substitution and other strategies were used to improve the luminescence performance of Cr3+activated near-infrared phosphors.（1）Expand the emission spectrum by utilizing two-sites occupation and energy transfer strategies.ZnNb₂O₆:Cr³⁺and Mg Nb₂O₆:Cr³⁺,Yb³⁺near-infrared phosphors were synthesized by high-temperature solid-state method.Under excitation at 510 nm,the emission center of ZnNb₂O₆:Cr³⁺phosphor is located at 940 nm and the FWHM is 210 nm.Based on the analysis of crystal structure,steady-state spectra,time-resolved spectra,and theoretical calculations,it was determined that Cr³⁺ions simultaneously replace the lattice sites of Zn²⁺and Nb₅₊in the matrix ZnNb₂O₆,revealing that double lattice occupation is the fundamental reason for the unfolding of emission spectra.In addition,a Cr³⁺-Yb³⁺energy transfer chain was constructed to achieve broadening of the emission spectra of Mg Nb₂O₆:Cr³⁺,Yb³⁺near-infrared phosphors.Under 510 nm excitation,Mg Nb₂O₆:Cr³⁺,Yb³⁺phosphors exhibit Cr³⁺characteristic broadband emission and Yb³⁺characteristic narrowband emission,with emission centers located at 930 nm and 1003 nm,respectively.The steady-state and time-resolved spectra indicate energy transfer between Cr³⁺and Yb³⁺.The construction of the Cr³⁺-Yb³⁺energy transfer chain broadened the FWHM of the fluorescent powder to 230 nm,improved quantum efficiency by 17.7%,and improved thermal stability by 18.6%.（2）Improving the quantum efficiency and thermal stability of ZnGa₂O₄:Cr³⁺using a cationic substitution strategy.ZnGa₂O₄:Cr³⁺near-infrared fluorescent powder was synthesized by high-temperature solid-state method.Under excitation at 410 nm,ZnGa₂O₄:Cr³⁺exhibits multiple narrowband emission peaks at 710 nm,with a FWHM of 68 nm.Optimize the local structure of the sample by utilizing the lattice of Zn²⁺in Mg²⁺substituted ZnGa₂O₄:Cr³⁺.The optimized thermal stability reached 63.6%(I_{298 K}/I_{473 K}),an increase of 11.3%.According to the calculation of Debye temperature,cation substitution improved the rigidity of the sample structure.The optimized sample quantum efficiency reached 87.9%,an increase of 14.4%.（3）Improve the thermal stability and quantum efficiency of SrGa₁₂O₁₉:Cr³⁺through flux strategy.SrGa₁₂O₁₉:0.1Cr³⁺near-infrared phosphor was synthesized using a high-temperature solid-state method.Under excitation at430 nm,the emission spectrum center of SrGa₁₂O₁₉:0.1Cr³⁺was located at 780nm,with a FWHM of 115 nm.The SrGa₁₂O₁₉:0.1Cr³⁺phosphor was synthesized using Li₂CO₃ as a flux,and its luminescence intensity was 1.38times that of the untreated flux.The internal quantum efficiency（IQE）and external quantum efficiency（EQE）of the optimized sample were 81.9%and36.1%,respectively.The optimized thermal stability reached 88.1%,an increase of 5.9%compared to before optimization.Under the driving current of 20 m A,the power and Solar-cell efficiency of the pc-LED synthesized with the optimized sample are 0.052 W and 8.3%respectively.（4）By utilizing chemical unit co substitution to regulate the properties of the characteristic emission peaks of Gd₃Ga₅O₁₂:Cr³⁺,the quantum efficiency and thermal stability of Gd₃Ga₅O₁₂:Cr³⁺are improved through condition optimization strategies.Chemical unit co substitution of Gd₃Ga₅O₁₂:Cr₃₊using Al³⁺and In³⁺,i.e.,Al³⁺-In³⁺replacing Ga³⁺-Ga³⁺.The addition of Al³⁺causes a blue shift in the emission peak,while the addition of In³⁺causes a red shift in the emission peak.The co substitution of Al³⁺-In³⁺chemical units increased the emission wavelength of Gd₃Ga₅O₁₂:Cr³⁺from 731 nm to 782 nm.Design new synthesis conditions and use Li₂CO₃ as a flux to synthesize Gd₃In_1.5Al_0.5Ga_4.5O₁₂:Cr³⁺in a CO reducing atmosphere.The optimized sample has a thermal stability of 90.3%,which is 9.7%higher than before optimization.The quantum efficiency is 86.7%,an increase of 20%.