Enhanced red upconversion luminescence in Er-Tm Codoped NaYF ₄ phosphor

This paper presents a study on the enhanced red upconversion (UC) luminescence via efficient energy transfer (ET) between Er ³⁺ and Tm ³⁺ in Er-Tm codoped NaYF ₄ microtubes. Er doped and Er-Tm codoped NaYF ₄ UC hollow microtubes have been synthesized using a hydrothermal method. Under 1560 nm excitation from a diode laser, the Er doped NaYF4 microtubes emitted dominant green UC luminescence while the Er-Tm codoped NaYF ₄ microtubes emitted dominant red UC luminescence, which implies the energy transfer between Er ³⁺ and Tm ³⁺ plays a key role in the enhanced red UC emissions. The red UC luminescence is significantly enhanced compared with the green UC luminescence with the increase of Tm ³⁺ doping concentration. In addition, our experimental results show that the UC luminescence properties under 980 nm excitation are almost identical with that under 1560 nm excitation. Furthermore, the possible ET mechanism was proposed on the basis of our experimental results.