Crystal-field Effect On The Luminescent Properties Of Cr³⁺-Doped Garnet-based NIR Phosphor Materials

As an analytical method,near-infrared spectroscopy technology has the advantages of fast,convenient and non-invasive,which has a wide range of applications among agricultural production,food industry,chemical engineering and medical fields.In recent years,NIR detection devices have developed towards miniatrization and integration,thus higher requirements are put forward for the technical characteristics of the NIR light source.The NIR phosphor conversion LED based on the blue InGaN chip has attracted much attention due to its fast response,small size,low cost,and adjustable spectrum.A suitable luminous center is crucial for the design of high-efficiency and broadband emitting NIR luminescent materials.Cr3+ion has a broadband absorption in the visible spectrum range,which matches well with the emission of blue LED chips.The matrix of garnet structure has good stability and abundant ligand environment,which provides the possibility for the regulation of Cr3+ luminescence.Therefore,based on the research of Cr3+-doped garnet-based NIR phosphors,we adopted the regulation strategy of crystal field engineering to explore the influence on the luminescence performance of the materials,and obtained the following results:(1)A series of yttrium aluminum garnet-based NIR phosphors Y3-xCaxAl5xSixO12:yCr3+(x=0-2.0,y=0.1%-1%)were prepared by high-temperature solid-phase sintering,of which Y2CaAl4SiO12:0.6%Cr3+phosphor exhibits broadband emission under the excitation of 440 nm.The emission band can cover the NIR region of 6001100 nm.The emission peak is at 740 nm,the FWHM is 160 nm,and the highest IQE is 75.9%.Characterized by analytical techniques such as luminescence performance and structural refinement,the broadband emission was proved to be originated from the co-emission of 2E and 4T2 energy levels.The octahedral coordination environment is distorted and deformed under the substitution of Ca2+-Si4+ion pair,leading to sub-level splitting,which increases the possibility of energy level mixing and co-emission.The luminescence intensity of Y2CaAl4SiO12:Cr3+phosphor at 200℃ is 78%of that at room temperature,which proves that it has good resistance to thermal quenching.NIR pcLED is prepared by Y2CaAl4SiO12:Cr3+phosphor and blue LED chip,and the resolution of detecting orange peel water content is 30%higher than that of NIR LED.(2)A series of all-aluminum garnet-based NIR phosphors Ln2CaAl4SiO12:0.6%Cr3+(Ln=Gd,Tb,Y,Lu)were prepared by high-temperature solidphase sintering.Under the excitation of 440 nm blue light,the emissiom peak of the above-mentioned phosphors are located between 738-769 nm,with the emission range of 650-1150 nm.As the increase of the Ln3+ionic radius(Ln3+=Gd3+>Tb3+>Y3+>Lu3+),the crystal field intensity Dq/B decreases.Among them,Gd2CaAl4SiO12:Cr3+has the smallest crystal field intensity and FWHM at room temperature of Tb2CaAl4SiO12:Cr3+is 164 nm;Tb2CaAl4SiO12:Cr3+provides a variety of transition ways due to the selfsensitizing characteristics of Tb3+ions,resulting in poor luminescence performance and poor resistance to thermal quenching.Ln2CaAl4SiO12:Cr3+(Ln=Gd,Tb,Y,Lu)phosphors are all in medium crystal field environment with high Racah parameter B,the broadening phenomenon once again verified the assumption of the relationship between the crystal field parameters and the FWHM.