Controllable synthesis of lanthanide Yb³⁺ and Er³⁺ co-doped AWO₄ (A = Ca, Sr, Ba) micro-structured materials: Phase, morphology and up-conversion luminescence enhancement

Lanthanide ion (Yb³⁺, Er³⁺) co-doped AWO₄ (A = Ca, Sr, Ba) up-conversion (UC) luminescent materials have been synthesized using a hydrothermal method and characterized by various microstructural and optical techniques. The results indicate that AWO₄:Yb³⁺,Er³⁺ samples have an identical body-centered tetragonal scheelite structure with different morphologies, including CaWO₄:Yb³⁺,Er³⁺ microspheres, dumbbell-like SrWO₄:Yb³⁺,Er³⁺ and bipyramid-like BaWO₄:Yb³⁺,Er³⁺. These samples exhibit visible emissions via an UC process under near-infrared (NIR) light (980 nm) excitation. Interestingly, the UC luminescence properties of AWO₄:Yb³⁺,Er³⁺ can be prominently increased after combination with fluorescent carbon dots (CDs) to form CDs@AWO₄:Yb³⁺,Er³⁺ composites. Compared to the corresponding samples without combination with CDs, the UC emission intensities of CDs@CaWO₄:Yb³⁺,Er³⁺, CDs@SrWO₄:Yb³⁺,Er³⁺ and CDs@BaWO₄:Yb³⁺,Er³⁺ composites increase about three, six and seven fold in the green emission area, and two, three and four fold in the red emission area, respectively. The mechanism of UC luminescence enhancement is probably that the loss of non-radiative transitions from the higher energy levels to the lower excited levels could be effectively reduced through the energy capture by the CD energy levels. The fluorescence enhancement for Yb³⁺ and Er³⁺ co-doped AWO₄ through combination with CDs provides a simple strategy for the tungstate system and other UC luminescent host systems.