Controlled Synthesis And Upconversion Luminescense Of Lanthanide Doped BaLu₂F₈ Microcrystals

Photon upconversion(UC) in rare earth-doped materials hold promises for applications ranging from volumetric display to biomedcine. In particular, UV emitting upconversion materials are of particular interest for biological detection, light-regulated drug release, and photodynamic therapy. This work intends to use barium lutetium fluoride(Ba Lu2F8) with unique optical properties as the host lattice, which is doped with precisely defined Yb3+/Tm3+ ion concentrations in order to realize intense UV UC luminescence. Manipulation of the synthetic parameters enables us to control the crystallographic phase, size, and morphology of the resulting fluoride microcrystals.We developed a hydrothermal method to prepare phase-controlled barium lutetium fluoride microcrystals, in which the oleic acid is utilized as surfactants. It is shown that when the molar ratio of Ba2+/Lu3+ is below or equal 0.5, pure monoclinic Ba Lu2F8 microcrystals were formed; at the molar ratio of ~ 1, both monoclinic Ba Lu2F8 and cubic Ba Lu F5 microcrystals co-existed; while pure cubic Ba Lu F5 microcrystals could be prepared when the molar ratio reaches above 2. We found that the crystal phase of the resulting product is independent of the reaction time, the volumetric ratio of the C2H5OH/H2 O solvent, as well as the molar ratios of the OA/Lu3+ and F-/Lu3+. As a consequence, it can be concluded that the crystal phase of the synthesized microcrystals is mainly governed by the precursor molar ratio of Ba2+/Lu3+ which impacts the crystal phase composition.In addition, the Yb3+/Tm3+-codoped monoclinic Ba Lu2F8 microcrystals were found to be more efficient than the cubic form. We show that variation of the reaction time, the volumetric ratio of the C2H5OH/H2 O solvent, as well as the molar ratios of OA/Lu3+ and F-/Lu3+ can yield Ba Lu2F8 microcrystals of different shapes such as multiplates, flowers, cubes, and particles. This is because these parameters have important impacts on the nuclei formation as well as the subsequent crystal growth. The dependence of UC luminescence on the shape of the synthesized microcrystals reveals that the multiplate one h as the strongest UV UC output.In comparison to the hexagonal Na YF4: 20%Yb3+/2%Tm3+ nanorods which is considered to be the most efficient UC materials, the microsized Ba Lu2F8: 20%Yb3+/2%Tm3+ multiplates exhibit are 2 times higher UV UC luminescence. Variation of Yb3+ doping concentration can result in 5 times improvement of the UV UC intensity in the microsized Ba Lu2F8:Yb3+/Tm3+ multiplates at an optimized Yb3+ concentration of 90%. We believe that the Ba Lu2F8:Yb3+/Tm3+ multiplates developed in this work have important implications for a broad spectrum of applications such as, security encoding, improvement of photovoltaic efficiency, as well as upconversion photodynamic therapy.