Synthesis And Properties Of Cr³⁺ Doped Broadband Near Infrared Luminescence Materials

Near-infrared light has been widely used in night vision lighting,monitoring system,iris recognition,infrared spectrum analysis and biological imaging due to its strong penetration,low thermal effect and no damage.Recent years,near-infrared phosphor converted light-emitting diode(NIR pc-LED)has attracted wide attention due to its small size,low price and high luminous efficiency compared with traditional light sources.As an important part of NIR pc-LED,phosphor plays a decisive role in the whole device.At present,the most NIR luminescence ion is the Cr3+ ion,whose unfilled 3d3 electron shell is highly susceptible to crystal field,showing spectral tunable near-infrared luminescence.When in octahedral crystal field,Cr3+ will show the emission,so it has the possibility of luminescence in garnet,olivine,perovskite and clinopyroxene structure with octahedral position.This paper systematically describes the research status of Cr3+doped near-infrared luminescent materials.In view of the problems existing in Cr3+ activated near-infrared light emitting materials(low luminous intensity,poor thermal stability and narrow spectral range),several Cr3+ doped wideband near-infrared luminescent materials are synthesized and their luminescence performance are improved by synthesis technology,cation site substitution,energy transfer and other strategies.Besides,this paper studies the luminescence mechanism and performance improvement mechanism of Cr3+ ions in different crystal field environments and explores the application prospect of the prepared near-infrared luminescent materials in NIR pc-LED light source.The details are as follows:1)A series of Cr3+doped AScSi2O6(A=Na/Li)luminescent materials were synthesized by high temperature solid phase method.X-ray diffraction and Rietveld refinement confirm that the Cr3+entered the AScSi2O6 structure.Under blue light excitation,AScSi206:Cr3+phosphors exhibit broadband NIR emission from 700 to 1100 nm with a full width at half-maximum(FWHM)of～150 nm resulting from the 4T2→4A2 electron transition of Cr3+.An efficient LiScSi2O6:Cr3+ phosphor with an external quantum efficiency(EQE)of 33.4%was obtained by adjusting the flux and sintering conditions.In addition,the optimized LiScSi2O6:Cr3+exhibits good thermal stability(75%@150℃)with a high activation energy(0.33 eV).The optimized LiScSi2O6:Cr3+was combined with blue LED chip,demonstrating bright NIR light and a high luminous efficacy.The output power reached 203 mW at 300 mA,while the luminous efficiency decreased from 8.01 to 5.72 lm/W with driving current from 50 mA to 300 mA.The device was used as a NIR light source to realize the important applications in night vision lighting,showing its application potential in NIR pc-LED.2)Cr34+ activated LiInGeO4 phosphor was successfully synthesized by high temperature solid phase method,X-ray diffraction and Rietveld refinement determines the pure phase structure.Under blue excitation,LiInGeO4:Cr3+exhibits ultra-wideband long-wavelength near-infrared emission in the range of 800-1600 nm,with a peak value at 1165 nm and a FWHM of 305 nm.Electron paramagnetic resonance(EPR),X-ray absorption near-side structures(XANES)and steady-state/transient photoluminescence spectra have proved that long-wavelength near-infrared emission orginates from Cr3+ rather than Cr4+.The emission spectra have achieved a blue shift of nearly 200 nm by designing LiIn1-zSczGeO4/(LiIn)1-yMg2yGeO4 continuous solid solution as the host,which is derived from the enhancement of crystal field intensity due to the lattice contraction caused by small-radius ion substitution.The finite solid solution(LiIn)1-yZn2yGeO4:Cr3+ was designed to tune the emission spectra in the range of 1165 nm to 875 nm,and its FWHM was expanded from 305 nm to 508 nm.The formation of finite solid solution causes the local structure change of Cr3+,and finally produces the ultra broadband NIR luminescence.Finally,the above near-infrared luminescent materials and blue LED are fabricated into devices,which have good current stability and show good application potential in spectral detection and non-destructive detection of food.3)A series of Ca2TbHf2Al3O12:Ce3+,Cr3+(CTHAO:Ce3+,Cr3+)near-infrared luminescent materials were prepared by high temperature solid state method.The structure of CTHAO:Ce3+,Cr3+was determined by X-ray diffraction and Rietveld refinement.In this sample,Tb3+ is composed as the substrate,together with the dopant ion Ce3+ acts as the sensitizer of Cr3+.There are various energy transfer effects among the three ions of Ce3+→Tb3+,Ce3+→Cr3+,Tb3+→Cr3+,which confirmed by the spectra and fluorescence lifetime characterization.The energy transfer mechanism from Tb3+to Cr3+ is dipole-quadrupole interaction,and the energy transfer efficiency can reach 79.5%.CTHAO:Ce3+,0.02Cr3+ has good thermal stability(43%@100℃),and the calculated activation energy is 0.30 eV.Combining this sample with a 410 nm chip to make a device,the output power can reach 40 mw and the photoelectric conversion efficiency is 6.1%when the driving current is 200 mA.As a near-infrared light source,this device shows the application value in night vision lighting.