Preparation And Luminescent Properties Of Cr³⁺-doped Silicate Garnet Near-infrared Phosphors

Near-infrared phosphor-converted light-emitting diode（NIR pc-LED）has the advantages of small size,long life,low cost and adjustable spectrum.It has important applications in food detection,non-destructive testing,biomedicine,night vision,iris recognition,information anti-counterfeiting and other emerging fields.Cr³⁺has a wide absorption band in the ultraviolet,blue and red regions,which can match the commercial blue light chip,and the luminescence is greatly affected by the crystal field environment.The emission band can be adjusted in the range of 650-1400 nm,which is an ideal broadband near-infrared luminescence center.Cr³⁺activated broadband near-infrared phosphors have attracted wide attention.However,Cr³⁺-activated near-infrared phosphors still have problems such as full width at half-maximum（FWHM）less than160 nm and low thermal stability,which limit their practical applications.Therefore,it is of great significance to develop broadband near-infrared phosphors with excellent performance.Garnet type is a typical material with wide band gap and high rigidity,which is helpful to achieve high thermal stability.The complex cation arrangement and multiple cation substitution of garnet are helpful to adjust the luminescence properties of Cr³⁺and realize the spectral broadening and emission wavelength red shift of Cr³⁺.Therefore,in this paper,Y₂Mg Al₄Si O₁₂was selected as the matrix and Cr³⁺was used as the activator.The matrix composition was changed by the strategy of matrix cation substitution and chemical unit co-substitution,and the local crystal field environment of Cr³⁺was adjusted to adjust the luminescence properties of Cr³⁺.This study provides an effective way to explore new garnet near-infrared phosphors.The specific research contents are as follows:（1）Y₂Mg Al₄Si O₁₂:Cr³⁺near-infrared phosphor was successfully prepared by high temperature solid-state method.A series of Y₂Mg_1+xAl_4-2xSi_1+xO₁₂:0.04Cr³⁺（0≤x≤1）solid solutions were prepared by co-substituting[Al³⁺-Al³⁺]in Y₂Mg Al₄Si O₁₂ with[Mg²⁺-Si⁴⁺].With the increase of[Mg²⁺-Si⁴⁺]co-substitution amount x from 0 to 1,Y₂Mg Al₄Si O₁₂:0.04Cr³⁺is transformed into Y₂Mg₂Al₂Si₂O₁₂:0.04Cr³⁺.The emission peak wavelength is red-shifted from 712 nm to 750 nm,the FWHM is broadened from152 nm to 167 nm,and the emission intensity is enhanced by 1.5 times.The crystal structure,morphology,luminescence properties,fluorescence lifetime,quantum efficiency and thermal quenching behavior of Y₂Mg₂Al₂Si₂O₁₂:Cr³⁺samples were further studied.Under the excitation of 430 nm,the IQE of Y₂Mg₂Al₂Si₂O₁₂:0.04Cr³⁺phosphor is 63%.At 423 K,the emission intensity of the phosphor remains 68%of the initial emission intensity,showing relatively good thermal stability.Finally,a near-infrared pc-LED device was fabricated by combining Y₂Mg₂Al₂Si₂O₁₂:0.04Cr³⁺phosphor with an efficient blue chip.The near-infrared output power and photoelectric conversion efficiency of the device reached 62 m W and 6.2%at 320 m A driving current,respectively.（2）In order to further broaden the FWHM of Y₂Mg₂Al₂Si₂O₁₂:0.04Cr³⁺phosphor and enhance its practical application range.The effect of cation substitution on the luminescence properties of Y₂Mg₂Al₂Si₂O₁₂:0.04Cr³⁺phosphors was studied.A novel of broadband near-infrared Y₂Mg₂Ga₂Si₂O₁₂:Cr³⁺phosphor was successfully prepared by high temperature solid state method by replacing Al³⁺with Ga³⁺.According to EPR spectrum analysis,the valence state of chromium ion in Y₂Mg₂Ga₂Si₂O₁₂ phosphor is+3.Under 440 nm excitation,Y₂Mg₂Ga₂Si₂O₁₂:0.04Cr³⁺phosphor exhibits ultra-wide near-infrared emission with FWHM of 180 nm and emission peak at 790 nm.Compared with the emission spectrum of Y₂Mg₂Ga₂Si₂O₁₂:0.04Cr³⁺,the emission peak is red-shifted by 40 nm and the FWHM is broadened by 13 nm.At 423 K,the emission intensity of Y₂Mg₂Ga₂Si₂O₁₂:0.04Cr³⁺phosphor remains 47%of the initial emission intensity.The fluorescence lifetime is 125.09μs,which is consistent with the ⁴T₂→⁴A₂transition characteristics of Cr³⁺allowed spin.Under 440 nm excitation,the IQE of Y₂Mg₂Ga₂Si₂O₁₂:0.04Cr³⁺is 44.3%.Finally,the near-infrared output power and photoelectric conversion efficiency of the near-infrared pc-LED device prepared by Y₂Mg₂Ga₂Si₂O₁₂:0.04Cr³⁺phosphor at 320 m A driving current are 34 m W and 3.5%,respectively.These results indicate that the broadband near-infrared phosphor with larger FWHM and longer emission wavelength can be obtained by cation substitution,which enhances its application potential in near-infrared pc-LED.