近红外长余辉材料CaGdAlO<sub>4</sub>∶Cr<sup>3+</sup>的制备及其光学特性

doi:10.3964/j.issn.1000-0593(2020)02-0373-06

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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 ⁸S_2/7→⁶I_j and ⁶P_2/7→⁸S_2/7 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.

Key words：Phosphor; Chromium; Photoluminescence; Afterglow

Received: 2018-12-29 Accepted: 2019-04-11

ZTFLH:

TB34

Corresponding Authors: LIU Chun-guang E-mail: liucg@nenu.edu.cn

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https://www.gpxygpfx.com/EN/10.3964/j.issn.1000-0593(2020)02-0373-06 OR https://www.gpxygpfx.com/EN/Y2020/V40/I02/373