Preparation And Luminescence Properties Of Novel Rare Earth Eu³⁺/Ce³⁺-doped Oxide Fluorescent Materials

At present,environmental problems(resource exhaustion,water pollution and air pollution,etc.)are becoming more and more prominent and attracting people's attention.Compared with traditional lighting sources such as incandescent lamps and fluorescent lamps,white light emitting diode(LED)have the advantages of low power consumption,high efficiency,long working life and environmental friendliness.Currently,white LEDs are generally prepared by using YAG:Ce³⁺yellow-emitting phosphors with blue In Ga N chips.However,owing to the lack of red light,these kinds of white LEDs give rise to cool white-light emissions with high correlated color temperature(CCT)and low color rendering index(CRI),which prohibits their applications in indoor general lighting.The rapid improvement of people's quality life has increased the demand for high-quality lighting.Therefore,scientific researchers are committed to the developments of efficient white LEDs with low CCT and high CRI.Among them,LED chips coated with tricolor(blue,green and red)phosphors can achieve good results.This lighting scheme has become a research focus.But this combination also has a limitation that there is a so-called cyan gap in emission spectra(480-520 nm),preventing the improvement of the CRI value.Therefore,it is necessary to explore new cyan-emitting phosphors for filling the cyan gap to realize high-CRI white LEDs.In summary,we synthesized the Eu³⁺-doped double-perovskite red phosphors and the Ce³⁺-doped garnet cyan phosphors by the high-temperature solid-phase method.And the synthesis method,phase,structure,morphology,luminescence characteristics and thermal stability of the prepared phosphors were analyzed in detail.The results of the study are as follows:(1)Ca₂Gd Sb O₆:Eu³⁺(CGS:Eu³⁺):under the excitation of 396 nm near-ultraviolet(UV)light,these CGS:Eu³⁺phosphors showed intense red emissions peaking at 612 nm.The strongest luminescence intensity reached when Eu³⁺doping concentration was x=0.5.The CIE color coordinates of the optimal CGS:0.5Eu³⁺phosphors were determined to be(0.6629,0.3367),and the corresponding color purity reached about 94.9%.Importantly,the CGS:0.5Eu³⁺phosphors revealed outstanding internal quantum efficiency of 73%and good thermal stability.The emission intensity of CGS:0.5Eu³⁺phosphors at 423 K still remained about 73%of its initial value at 303 K.Finally,a prototype white LED device was fabricated by coating the phosphor blend of commercial blue-emitting Ba Mg Al₁₀O₁₇:Eu²⁺(BAM:Eu²⁺),green-emitting(Ba,Sr)₂Si O₄:Eu²⁺and our as-prepared red-emitting CGS:0.5Eu³⁺on a 395 nm LED chip.Under 20 m A driven current,the device showed bright warm-white light with CIE color coordinates of(0.3888,0.3943),CCT of 3911 K,and CRI of 88.4.(2)Sr₂La Ta O₆:Eu³⁺(SLT:Eu³⁺):under near-UV excitation at 394 nm,the optimal SLT:0.2Eu³⁺phosphors emitted high-brightness red light around 613 nm with the CIE chromaticity coordinates(0.650,0.349).Notably,the color purity can reach 92%.Impressively,the favorable thermal stability of the SLT:0.2Eu³⁺phosphors were characterized by temperature-dependent emission spectra at different temperatures from 303 to 463 K and the emission intensity at 423 K remained 73%of its value at 303 K.All the results suggested that the as-prepared SLT:0.2Eu³⁺phosphors can be used in near-UV-excitable white light-emitting diodes as red-emitting color converter.(3)Ca₂Lu Hf₂Al₃O₁₂:Ce³⁺(CLHAO:Ce³⁺):these phosphors exhibited a broad excitation band in the 300-450 nm wavelength range with a peak at 400 nm.Upon 400 nm excitation,the optimal CLHAO:0.02Ce³⁺sample showed a broad cyan emission band in the 420-600 nm range peaking at480 nm with a FWHM of 86 nm.Impressively,the IQE and EQE of CLHAO:0.02Ce³⁺cyan phosphors were found to be 84.3%and 60.8%,respectively.Furthermore,the CLHAO:0.02Ce³⁺cyan phosphors showed excellent thermal stability and color stability at high temperatures.The activation energy E_a of CLHAO:0.02Ce³⁺sample was 0.33 e V.By employing the CLHAO:0.02Ce³⁺cyan phosphors as cyan component,the fabricated white LED device exhibited bright warm white light with high CRI value.(4)Ca₂YHf₂Ga₃O₁₂:Ce³⁺(CYHG:Ce³⁺):the obtained samples showed broad excitation bands at the center wavelengths of 346 nm and 398 nm,emitted bright cyan light and the center wavelength were between 479-485 nm.Under the excitation of 398 nm,the CYHG:0.04Ce³⁺sample emittd the strongest light.At this time,the color coordinates were(0.182,0.276),which were located in the cyan region.Under 398 nm excitation,the CIE chromaticity coordinates of CYHG:x Ce³⁺(x=0.01,0.02,0.04,0.07,0.1,and 0.12)phosphors ranged from(0.180,0.267)to(0.209,0.337).All the results implied that CYHG:0.04Ce³⁺cyan phosphors can compensate for the cyan gap of the traditional LED emission spectra.