Preparation And Luminescent Properties Of Cr³⁺-Based Phosphors With Near Infrared Emissions

The phosphor-converted near infrared LED with unique advantages is expected to replace the traditional near infrared light source in Near-Infrared spectrum analysis,security monitoring,biological identification and other fields.Phosphor plays a key role in determining the spectral structure,output power and lifetime of pc-LED devices.At present,there are many researches on near-infrared phosphors,but inevitably there are some shortcomings,such as narrow emission spectrum,low luminous efficiency and poor thermal stability.Therefore,it is of great significance to develop near-infrared phosphors with broadband emission,high luminous efficiency,good thermal stability and practical value in pc-LED devices.In this paper,Cr³⁺ions are used as activator ions.Firstly,LiScSi₂O₆ is used as the matrix material to obtain a new type of near-infrared phosphor,and then a new broadband near-infrared LiInSi₂O₆:Cr³⁺is designed by element replacement to reduce the preparation cost.The powder crystal structure,absorption spectrum,excitation and emission spectrum,quantum efficiency,fluorescence lifetime,thermal stability,and pc-LED packaging performance are systematically studied.In addition,a series of Cr³⁺doped ABC₂O₆（A=Li、Na,B=Sc、In,C=Si、Ge）phosphors were synthesized.The crystal structure analysis,spectrum analysis and thermal stability test were carried out to study the luminescent behavior and luminescent law of these phosphors.The main conclusions are as follows:LiScSi₂O₆:Cr³⁺phosphor can be effectively excited by blue and red light.Under excitation at 450 nm,the phosphor shows broadband NIR emission peaking at 839 nm with a full width at half maximum（FWHM）about 145 nm.The near-infrared emission is originating from the spin-allowed ⁴T₂→⁴A₂ transition of Cr³⁺ions.From the refined results,it shows that Cr³⁺ions occupy the site of Sc³⁺ions in the lattice.The optimum Cr³⁺concentration in LiScSi₂O₆ host was found to be 5 mol%,and the concentration quenching mechanism was analyzed.Temperature dependent measurements were performed,the phosphor exhibits a good thermal stability,retaining 84%of the initial intensity at 150°C.By combining the LiScSi₂O₆:0.05 Cr³⁺phosphor with a 445 nm LED chip,the NIR output power of 42.2 m W and photoelectric efficiency of 14.4%can be obtained at the drive current of 100 m A.LiInSi₂O₆:Cr³⁺phosphors have strong broad absorption bands in the blue and red bands.Under the excitation of 450 nm,the phosphor shows broadband near infrared emission peaking at 838 nm and the half peak width is 143 nm.When the doping concentration of Cr³⁺ions is 6mol%,the luminescence intensity reaches the highest.With further increasing the concentration of Cr³⁺ions,the reason for the decrease of the intensity is the radiative energy transfer process between the ions.LiInSi₂O₆:Cr³⁺phosphor has a quantum yield of 75%and can retain 76%of the initial intensity at 150°C.The high efficiency and thermal stability of the phosphor is caused by the weak crystal field and relatively rigid[In O₆]octahedron introduced by Cr³⁺.The output power of the near infrared pc-LED is 51.6 mw and the photoelectric efficiency is 17.8%under the driving current of 100 m A.A series of ABC₂O₆:0.06Cr³⁺phosphors can be effectively excited by blue light and emit broadband near-infrared light from 700 nm to 1100 nm.As the ions at the A,B or C positions are replaced by ions of larger radius,the lattice expansion will be caused.The crystallographic constants of the main lattice such as lattice parameters a,b,c,unit cell volume V,and the effective radius of cations and coordination anions R.The increase of those parameters cause the crystal field environment of Cr³⁺ions are weakened,which shows that the absorption,excitation and emission spectra of phosphors shift to long wavelength in different degrees.At the same time,in this series of phosphors,the smaller the lattice constant,unit cell volume and effective radius means the stronger the structural rigidity and the smaller the Stokes shift,the better the phosphor’s resistance to thermal quenching.The above researches are significant for the design and research of new Cr³⁺-doped near infrared phosphors.