Mn2+-doped magnesium germanate (MGO:Mn2+) nanoparticles exhibit persistent luminescence in the near-infrared (NIR) region. MGO has two stable stoichiometric forms: MgGeO3 and Mg2GeO4. The octahedra surrounding the Mg2+ ions in these two structures have different extents of distortion. In this work, MGO:Mn2+ with mixed MgGeO3 and Mg2GeO4 is produced by thermal annealing at different temperatures. The photoluminescence intensity and persistent luminescence lifetime are found highly dependent on the annealing temperature and the crystallinities of the MGO host. The site where Mn2+ occupies in MGO with mixed stoichiometries is evaluated using X-ray absorption spectroscopy combined with structure simulation. The distribution of the Mn2+ at different Mg2+ sites can be quantitatively identified. This is further related to the observed optical properties, and the mechanism of luminescence enhancement of high-temperature-treated MGO:Mn2+ is rationalized.

Shine bright: Persistent luminescent Cr-doped ZnGa2O4 (CZGO) is incorporated into an amorphous calcium phosphate (ACP) matrix. Annealing CZGO prior to ACP integration greatly improves its luminescence intensity and duration. Zn2+ redistribution from CZGO to ACP is identified, leading to the formation of Zn3(PO4)2 when immersed in water. The nanocomposite exhibits high stability under prolonged X-ray exposure.

Cr-doped zinc gallate (ZnGa2O4:Cr3+, CZGO) is a long-lasting persistent luminescent material which emits light in the near-infrared region. In the present work, CZGO nanoparticles are synthesized at different hydrothermal synthesis temperatures, including the lowest and highest temperatures reported. We found that although all the synthesized CZGO emits light, the luminescent intensities vary and the synthesized CZGO nanoparticles respond differently to the change of excitation energy. A detailed electronic structure analysis using synchrotron X-ray absorption fine structure is performed, and we propose a structure-luminescence correlation.

PersL nanoparticles with dual emission bands based on MgGeO3 was investigated. Introducing Yb3+ as a second dopant enables a deep-red-to-NIR energy transfer, producing dual-emission at both deep-red and NIR. The NIR emission can be further enhanced by the addition of Li+. A detailed spectroscopy study is performed to investigate the local structure around the light activators. We found that adding the Yb3+ and Li+ changes the preferred site of occupancy for Mn2+, and when Mn2+ is shifted to a site that emits deep-red less efficiently, it acts as an electron trap to extend the PersL of the NIR-II emission.

The formation of a CZGO-ACP nanocomposite, which was found to emit bright NIR luminescence and long-lasting persistent luminescence, could equip a particle with the target-tracking-drug release function. By analyzing the X-ray absorption fine structure (XAFS) and X-ray excited optical luminescence (XEOL) of the composite in comparison with pure CZGO, we found that Zn2+ ions from the CZGO could dissociate into the ACP matrix, substituting Ca2+ ions and causing the luminescent intensity and lifetime to decrease.