Transparent Phosphor Thin Films Based on Rare-Earth-Doped Garnets: Building Blocks for Versatile Persistent Luminescence Materials

Afterglow properties of persistent phosphors are attracting a great deal of attention in the fields of bioimaging, sensing, labeling, safety, or security. Complex garnet oxides, especially those doped with Ce3+ and Cr3+, are particularly relevant to this end since their persistent luminescence can be tuned through matrix composition and activated by visible light, in contrast to the vast majority of persistent phosphors that require UV excitation. Most extended preparation routes yield micrometer-sized phosphors that display strong light scattering, which limits their versatility and applicability. Herein, nanostructured garnet oxide-based thin films that are transparent and feature persistent luminescence properties are demonstrated. Following a sol–gel route and after high temperature annealing, few hundred nanometre-thick Y3Al2Ga3O12:Ce3+,Cr3+ transparent films showing efficient green emission and afterglow are attained. Gd3Al2Ga3O12:Ce3+,Cr3+ transparent thin films displaying yellow afterglow with distinct persistent kinetics are demonstrated, to prove the generality of the approach herein proposed. Its versatility is further demonstrated by developing layered phosphors with time-dependent chromaticity due to the unique persistent emission color – upon blue light excitation – and kinetics of each layer forming the stack. The results pave an avenue toward nanodevices and multifunctional coatings in which afterglow offers hitherto unexplored properties.This project has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 Research and Innovation Programme (NANOPHOM, grant agreement no. 715832). V.C. thanks Junta de Andalucía for financial support (POSTDOC_21_00694)

Persistent luminescent nanoparticles: Challenges and opportunities for a shimmering future

Persistent phosphors are luminescent sources based on crystalline materials doped with rare-earth or transition metal cations able to produce light after the excitation source vanishes. Although known for centuries, these materials gained renewed interest after the discovery of Eu2+,RE3+ co-doped aluminates and silicates in the late 1990s due to their unprecedented afterglow properties. In contrast, persistent nanophosphors have emerged only recently as a nanoscale alternative to their bulk counterparts, offering exciting opportunities of particular relevance for in vivo imaging, optical data storage, or unconventional light generation. However, taking advantage of the avenues opened by nanoscience demands developing new synthetic strategies that allow precise control of the morphology, surface, and defect chemistry of the nanomaterials, along with a profound understanding of the physical mechanisms occurring in the nanoscale. Besides, advanced physicochemical characterization is required to assess persistent luminescence in a quantitative manner, which allows strict comparison among different persistent nanophosphors, aiming to propel their applicability. Herein, we revisit the main phenomena that determine the emission properties of persistent nanoparticles, discuss the most promising preparation and characterization protocols, highlight recent achievements, and elaborate on the challenges aheadPeer reviewe

Persistent Luminescence of Transparent ZnGa2O4:Cr3+ Thin Films from Colloidal Nanoparticles of Tunable Size

We report on the fabrication of ZnGa2O4:Cr3+ transparent thin films and the evaluation, for the first time in literature, of their persistent red to NIR emission. For this purpose, we have used a simple and economic global strategy based on wet processing methods from colloidal nanospheres with uniform size. A microwave-assisted hydrothermal method was first developed for the synthesis of the precursor particles, which allows size tuning from 300 nm to 30 nm through simple modification of the Zn precursor and the Cr content of the starting solutions. ZnGa2O4:Cr3+ transparent thin films over quartz substrates were then easily fabricated by spin coating, and their structural and optical characteristics were analyzed in detail after annealing at high temperature to elucidate the effect of processing temperature and particle size on films properties. Indeed, our results indicate that high temperature annealing does not compromise the transparency of the films while improves their photoluminescence. In addition, the analysis reveals that persistence luminescence in our films is rather independent of the size of the precursor nanoparticles. Due to their transparency and persistent emission properties, films fabricated from colloidal suspensions of ZnGa2O4:Cr3+ nanoparticles show great potential for applications in the fields of chemical sensing, information storage, labelling, and anti-counterfeiting technology.Peer reviewe