Abstracts

Over the past few years, research towards novel and efficient near-infrared (NIR) emitting persistent luminescent materials makes use of surface sensitizers to incorporate organic dyes and tune light emission within the first and second biological windows, using activators such as transition metals and rare-earth ions (e.g. Cr³⁺, Nd³⁺) [1,2]. In this work, Zn_xMg_ySnO₄:Cr³⁺ NIR-emitting phosphors were prepared via ceramic method and microwave-assisted solid-state synthesis, which were later functionalized with the [Eu(tta)₃(H₂O)₂] complex by microwave-assisted surface silanization. These materials showed high crystallinity in the results of both powder X-ray diffraction and electron microscopy experiments, where SnO₂ by-product formation with increasing Mg content was observed. Luminescence spectra registered intense NIR emission of Cr³⁺ ions, assigned to the allowed ⁴T₂(t²e)→⁴A₂ transition under a weak crystal-field environment. Interestingly, in functionalized materials, a similar response was registered under direct Eu³⁺ excitation, revealing intrinsic europium-to-chromium energy-transfer processes in single-shell phosphors. Said mechanism was further studied in materials functionalized with a double-shell, where Eu³⁺ red emission was observed in the visible range alongside the Cr³⁺ NIR emission. Additionally, NIR persistent luminescence under band-gap excitation was studied by vacuum-ultraviolet spectroscopy, where Cr³⁺ emission was recorded up to 10 min after excitation. Finally, X-ray fluorescence, X-ray absorption near edge structure, and X-ray excited optical luminescence spectroscopy experiments were carried out at the Carnaúba (Coherent X-ray Nanoprobe) beamline of the Brazilian synchrotron (Sirius). Site-selective luminescence was investigated and correlated with local chemical composition, where the results showed intense NIR emission under X-ray excitation related to the Cr K-edge and the Eu L-edge in functionalized materials. Hence, the results obtained in this work outline an important understanding of energy-transfer mechanisms and their role in promoting NIR emission utilizing β-diketonate complexes as sensitizers in Cr³⁺-doped persistent phosphors which are widely studied for optical imaging applications.References:[1] F. Zhao et al., Laser Photonics Rev. 16 (2022) 2200380.[2] X. Zhu et al., ACS Materials Lett. 4 (2022) 1815.