Preparation and Optical Properties of Near Infrared Persistentluminescent CaGdAlO4 : Cr3+

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%similar to 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 (4)A(2)-> T-4(1) (4P), (4)A(2)-> T-4(1) (4F) and (4)A(2) -> T-4(2) (4F) transitions of Cr3+ ions, respectively, and the excitation peaks corresponding to 276 and 313 nm originate from the S-8(2/7)-> I-6(j ) and P-6(2/7) -> S-8(2/7) transitions of Gd(3+ )ionsin the matrix. Under 592 nm excitation, NIR broadband emission peak with a maximum value of 744 nm appears in the range of 650 similar to 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 similar to 1%. After the heat-treatment at 800 degrees C 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 CaGdAlO(4 )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 similar to 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 (T-4(2) ->(4)A(2)) and 744 and 756 rim broadband emissions correspond to the phonon sideband transition T-4(2)-> (4)A(2) . 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.