Strong green upconversion emission from submicron spindle-shaped SrMoO₄:Yb³⁺,Er³

Upconversion luminescence (UCL) is a fluorescence process where two or more lower-energy photons convert into a higher-energy photon. Lanthanide (Ln³⁺)-doped UCL materials often suffer from weak luminescence, especially when directly synthesized by a hydrothermal (HT) process due to the existing hydroxyl group and undesirable arrangement of dopants within host lattices which quench luminescence and limit energy transfer. Therefore, additional heat treatment processes are required to enhance their UCL emission, even though direct hydrothermal synthesis without further heat treatment has the advantages of low energy consumption, fast synthesis, and wide applicability to generate UCL materials. In this study, via a HT process without annealing, we have produced Yb³⁺ and Er³⁺ co-doped SrMoO₄ submicron spindles with a strong green UCL emission which can be seen with the naked eye, which HT produced oxide-based UCL materials often fail to demonstrate. We have investigated different HT synthesis conditions, such as temperature, time, pH and dopant composition, which control the nucleation, growth, lattice structure arrangement, and ultimately their UCL properties through XRD, SEM, EDS and UCL measurements. The bright green UCL from the SrMoO₄:Yb,Er submicron spindles is further enhanced by post-synthesis annealing within a molten NaNO₃/KNO₃ system to prevent particle size growth. The green UCL intensity from the annealed SrMoO₄:Yb,Er submicron spindles surpasses samples produced by the solid-state method and is comparable to that from the commercial NaYF₄:Yb,Er sample. We have further studied the temperature-dependent luminescence of both the HT-prepared and molten-salt annealed SrMoO₄:Yb,Er submicron spindle samples. The strong UCL from our SrMoO₄:Yb,Er submicron spindles could warrant their candidacy for bioimaging and anticounterfeiting applications.