Highly efficient upconversion luminescence in narrow-bandgap YMoO.

Lanthanide-doped upconversion (UC) materials have been extensively investigated for their unique capability to convert low-energy excitation into high-energy emission. Contrary to previous reports suggesting that efficient UC luminescence (UCL) is exclusively observed in materials with a wide bandgap, we have discovered in this study that YMoO:Yb/Tm microcrystals, a narrowband material, exhibit highly efficient UC emission. Remarkably, these microcrystals do not display any four- or five-photon UC emission bands. This particular optical phenomenon is independent of the variation in doping ion concentration, temperature, phonon energy, and excitation power density. Combining theoretical calculations and experimental results, we attribute the vanishing emission bands to the strong interaction between the bandgap of the YMoO host matrix (3.37 eV) and the high-energy levels (I and D) of Tm ions. This interaction can effectively catalyze the UC emission process of Tm ions, which leads to YMoO:Yb/Tm microcrystals possessing very strong UCL intensity. The brightness of these microcrystals outshines commercial UC NaYF:Yb,Er green phosphors by a factor of 10 and is 1.4 times greater than that of UC NaYF:Yb,Tm blue phosphors. Ultimately, YMoO:Yb/Tm microcrystals, with their distinctive optical characteristics, are being tailored for sophisticated anti-counterfeiting and information encryption applications.