Abstract：At present, for near infrared (NIR) long afterglow materials doped with Cr3+, the efforts have been focused on the dependence of luminescent centers in the cases of strong and medium crystal fields on the afterglow properties (emission wavelength, afterglow time and storage properties), but the research on the relationship between Cr3+ ions around weak crystal field environments and the afterglow properties is lacking. This studyis very essential for developing novel long afterglow luminescent materials and exploring their applications. In this paper, Cr3+ doped CaGdAlO4 (CaGdAlO4∶x%Cr3+) powders with different Cr3+ doping concentrations were prepared by a self-propagating combustion method. The influences of Cr3+ doping and heat-treatment conditions on the microstructure, morphology, particle size and luminescent properties of the samples were studied by means of X-ray diffraction, scanning electron microscopy, excitation and emission spectra. It was found that, in the concentration range of 0.1%～2.0%, Cr3+ions substitute for Al3+ions in CaGdAlO4 because of their similar radii. From the excitation spectra, it could be found that the excitation peaks at 240, 373 and 592 nm are attributed to 4A2→4T1(4P), 4A2→4T1(4F) and 4A2→4T2(4F) transitions of Cr3+ ions, respectively, and the excitation peaks corresponding to 276 and 313 nm originate from the 8S2/7→6Ij and 6P2/7→8S2/7 transitions of Gd3+ ionsin the matrix. Under 592 nm excitation, NIR broadband emission peak with a maximum value of 744 nm appears in the range of 650～850 nm and several narrowband peaks overlap with it. The NIR emission intensity exhibits an initialrise and a subsequent decrease with the increase of Cr3+ doping concentration and the optimum doping concentration is ~1%. After the heat-treatment at 800 ℃ in vacuum, the average grain size increases from 417 to 843 nm and the luminescent intensity increases by 2 times. It was found that Cr3+ replaces Al3+ site in the weak crystal field environment in CaGdAlO4 host. The origin of NIR emission peaksof the samples was identified by the calculation of crystal field parameters and spectral analysis. It was found that the crystal field strength is ~1.54 and smaller than 2.3, i. e. Dq/B=1.54<2.3, indicating Cr3+ ions around a weak crystal field environment, which is consistent with the experimental result. The 670 nm broadband emission can be attributed to the zero phonon line (4T2→4A2) and 744 and 756 nm broadband emissions correspond to the phonon sideband transition 4T2→4A2. After the heat treatment, the afterglow time of the samples exceeds 60 seconds. In particular, compared with the case of Cr3+ ions around medium and strong crystal field environments (the maximum value of NIR emission peak at 697 nm), the maximum value of NIR emission peak of Cr3+ ions around the weak crystal field environment moves to 744 nm, which is closer to the center of the first biological window, suggesting that Cr3+ doped CaGdAlO4 has potential application in bioimaging.
张 楠，刘春光，张 猛，杨 健，李胜男，祝汉成，严端廷，徐长山，刘玉学. 近红外长余辉材料CaGdAlO4∶Cr3+的制备及其光学特性[J]. 光谱学与光谱分析, 2020, 40(02): 373-378.
ZHANG Nan, LIU Chun-guang, ZHANG Meng, YANG Jian, LI Sheng-nan, ZHU Han-cheng, YAN Duan-ting, XU Chang-shan, LIU Yu-xue. Preparation and Optical Properties of Near Infrared Persistentluminescent CaGdAlO4∶Cr3+. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2020, 40(02): 373-378.
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