Since it was realized a few years ago that near-infrared emitting persistent or ‘glow-in-the-dark’ phosphors could be used for medical imaging applications, there is an active research interest in this class of compounds [1]. Persistent phosphors having an emission spectrum in the wavelength range between 650 and 950 nm, in the so-called first tissue transparency window, can be injected and applied as diagnostic and therapeutic agents. To this end, the afterglow should be bright and long enough, and the injected particles should be bio-compatible.

While most of the persistent phosphors for visible light are based on Eu²⁺ doped compounds, most materials for near-IR are based on emission from Cr³⁺ ions [2]. Depending on the host material and the corresponding dopant-lattice interactions, Cr³⁺ shows sharp emission features due to the ²E-⁴A₂ transition (usually accompanied by phonon side-bands), a broad emission band attributed to ⁴T₂-⁴A₂ or a combination of both. In this work, we focus on LiGa₅O₈as a host lattice for the Cr ions, an inverse spinel compound where the Ga-ions occupy both tetrahedrally and octahedrally coordinated lattice sites. Using a combination of x-ray absorption and magnetic resonance measurements, the incorporation of Cr in the lattice was studied. In addition, the effect of co-dopants – increasing the number of trap levels for energy storage – on the afterglow characteristics was investigated.

Next to Cr³⁺, Mn⁴⁺ is a very promising dopant for near-IR persistent luminescence. The ion’s electronic configuration is identical to that of Cr³⁺, therefore the absorption and emission characteristics are also very similar. Mn⁴⁺ is currently a popular dopant for red-emitting LED phosphors, but has hardly been considered as a near-IR dopant [3]. The challenge here is to incorporate the ion in its 4+ valence state in the host, which requires the necessary precautions or charge-compensating co-dopants. In the present work, we selected the perovskite LaAlO₃ as the host for Mn⁴⁺. The afterglow of this material is limited by the limited incorporation of Mn in its 4+ state in the host. The latter is however strongly improved by co-doping with ions with a lower valence state for charge compensation.

[1] Q. le Masne de Chermont et al., Nanoprobes with near-infrared persistent luminescence for in vivo imaging. Proc. Natl. Acad. Sci. 104 (2007) 9266–9271.

[2] P.F. Smet, K. Van den Eeckhout, O.Q. De Clercq, D. Poelman. “Persistent Phosphors.” In Handbook on the Physics and Chemistry of Rare Earths, Including Actinides, ed. Jean-Claude Bünzli and Vitalij K Pecharsky, 48 5 (2015) 1–108. Amsterdam, The Netherlands: Elsevier.

[3] R. Cao et al. , Synthesis and photoluminescence properties of LaAlO₃:Mn⁴⁺, Na⁺ deep red-emitting phosphor, Appl. Phys. A 122 (2016) 299.