Abstract
Phosphors-converted near-infrared LED (pc-NIR LED) possesses applications in various fields including food quality analysis, night vision, biomedical imaging, and biomedicine. The design and development of broadband near-infrared (NIR) phosphors with the required properties are of decisive significance for pc-NIR LED devices. The Cr<sup>3+</sup> doped phosphors are considered to be most promising near-infrared materials for commercialization. Broadband NIR luminescent materials doped with Cr<sup>3+</sup> have attracted more and more attention due to their potential applications in NIR light sources. However, the emission wavelength of Cr<sup>3+</sup> doped phosphor is generally located in the NIR I region of less than 850 nm, and realizing the NIR II region emission is still a challenge. In this work, a series of Cr<sup>3+</sup> doped Na<sub>3</sub>YSi<sub>3</sub>O<sub>9</sub> new silicate phosphors is prepared by solid-state method in N<sub>2</sub> atmosphere at 1150 ℃ for 8 h. We take advantages of the silicate nature and the multi octahedral sites suitable for Cr<sup>3+</sup> in the studied Na<sub>3</sub>YSi<sub>3</sub>O<sub>9</sub> materials to redshift and broaden the spectrum. The phase, crystal structure, microstructure, photoluminescence, main emission peak decay and thermal stability of the samples are systematically studied. The results show that the prepared samples are pure phases, with uneven morphology, slight agglomeration, and the sizes in the micrometer range. The Cr<sup>3+</sup> is located in the weak crystal field environment of Na<sub>3</sub>YSi<sub>3</sub>O<sub>9</sub> lattice, with a <i>Dq</i>/<i>B</i> value of 2.29. Under the excitation of blue light at a wavelength of 485 nm, the strongest emission peaks of Na<sub>3</sub>Y<sub>1–<i>x</i></sub> Si<sub>3</sub>O<sub>9</sub>:<i>x</i> Cr<sup>3+</sup> phosphors are located at 984 nm (NIR II region), which is longer than those of most Cr<sup>3+</sup> activated phosphors. Due to the multi-site occupation of Cr<sup>3+</sup> in the lattice, the full width at half maximum (FWHM) of the emission spectrum is as high as 183 nm. The optimal doping concentration of Na<sub>3</sub>Y<sub>1–<i>x</i></sub> Si<sub>3</sub>O<sub>9</sub>:<i>x</i> Cr<sup>3+</sup> is 3%, and the quenching mechanism is the dipole-dipole interaction between Cr<sup>3+</sup> ions. Fluorescence decay curves show that the luminescence lifetime of Na<sub>3</sub>Y<sub>0.97</sub>Si<sub>3</sub>O<sub>9</sub>:0.03Cr<sup>3+</sup> sample gradually decreases with the increase of doping concentration and temperature. The results of the temperature-dependent spectra show that the emission intensity decreases in a temperature range from 298 K to 423 K, and the activation energy Δ<i>E</i> of Cr<sup>3+</sup> is 0.157 eV.
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