Luminescence Enhancement Mechanism Of Long-Wavelength Near-infrared Phosphor Ca₂GeO₄:Cr⁴⁺

In recent years,long-wavelength near-infrared light-emitting diodes(NIR LEDs)light sources have been rapidly developed in the fields of organic detection,non-destructive detection,medical cosmetology and fluorescence imaging.NIR LEDs are mainly realized by NIR chips,but there are technical difficulties in the fabrication of long-wavelength NIR chips.The phosphor-converted LEDs(pc-LEDs)based on blue chip combined with NIR phosphor has the advantages of low cost and strong spectral tunability,and has become an important development trend of NIR light sources.NIR phosphors play a crucial role in NIR LEDs.Currently,there is a shortage of NIR phosphors that can be excited by blue chips,especially broadband long-wavelength NIR phosphors with emission peaks above 1100 nm,and suffer from a general problem of low luminescence efficiency.Therefore,there is an urgent need to design and develop new series of long-wavelength NIR phosphors that can be efficiently excited by blue light.In this paper,we focus on Ca2GeO4:Cr4+phosphors with emission peaks at 1290 nm.To solve the problem of low luminescence intensity,energy transfer and ion substitution are mainly used to improve its luminescence performance.The influence of the sintering temperature,flux addition,and doping concentration of luminescent center on the luminescence intensity of Ca2GeO4:Cr4+phosphors have been investigated.The pure phase of the target product was successfully synthesized by high temperature solid-state method in the air atmosphere,with an optimal sintering temperature of 1300℃.The luminescence intensity was increased by 23.5%and the grain morphology and size were improved with 4%NH4Cl flux addition.The emission spectrum of Ca2GeO4:Cr4+ phosphor shows a broadband emission at 1290 nm,with an optimal doping amount of 0.005 mol.The concentration quenching mechanism is energy transfer between neighboring ions.Based on the energy transfer principle,Eu3+ was selected as the sensitizer to improve the luminescence performance and thermal stability of Cr4+.A series of experiments on Eu3+Cr4+co-doped show that the doping of Eu3+ can effectively enhance the luminescence intensity of Ca2GeO4:xEu3+,yCr4+(0≤x≤0.013,0≤y≤0.013)phosphors.With Eu3+doping x=0.001,Cr4+ doping y=0.005,the luminescence intensity can be doubled and the QY can be increased from 23.16%to 41.32%.In addition,doping with Eu3+ increases the thermal quenching activation energy from 0.3114 eV to 0.3217 eV,which improves the thermal stability of the material.A series of Ca2Ge0.995-xSixO4:Cr4+(0≤x<0.07)phosphors were designed through ion substitution to study the effects of small radius ion Si4+ doping on its crystal structure,luminescence and thermal stability.The experimental results show that Si4+ doping shrinks the unit cell,alleviates the lattice expansion caused by Cr4+ doping,and improves the crystal field environment.When Si4+ is doped with 0.03 mol,the luminescence intensity increases by 50%and the thermal quenching activation energy also increases.The luminescence intensity at 150℃ is 1.4 times that of Ca2GeO4:Cr4+ phosphor sample,showing better thermal stability.