Long wavelength near-infrared II emitting Na<sub>3</sub>YSi<sub>3</sub>O<sub>9</sub>:<i>x</i> Cr<sup>3+ </sup>silicate and spectral broadening by multi-site occupancy

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 Cr³⁺ doped phosphors are considered to be most promising near-infrared materials for commercialization. Broadband NIR luminescent materials doped with Cr³⁺ have attracted more and more attention due to their potential applications in NIR light sources. However, the emission wavelength of Cr³⁺ 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 Cr³⁺ doped Na₃YSi₃O₉ new silicate phosphors is prepared by solid-state method in N₂ atmosphere at 1150 ℃ for 8 h. We take advantages of the silicate nature and the multi octahedral sites suitable for Cr³⁺ in the studied Na₃YSi₃O₉ 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 Cr³⁺ is located in the weak crystal field environment of Na₃YSi₃O₉ 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 Na₃Y_1–<i>x</i> Si₃O₉:<i>x</i> Cr³⁺ phosphors are located at 984 nm (NIR II region), which is longer than those of most Cr³⁺ activated phosphors. Due to the multi-site occupation of Cr³⁺ 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 Na₃Y_1–<i>x</i> Si₃O₉:<i>x</i> Cr³⁺ is 3%, and the quenching mechanism is the dipole-dipole interaction between Cr³⁺ ions. Fluorescence decay curves show that the luminescence lifetime of Na₃Y_0.97Si₃O₉:0.03Cr³⁺ 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 Cr³⁺ is 0.157 eV.