Niobium enhanced europium ion luminescence in hafnia nanocrystals

In this work we demonstrate a method where by adding Nb ions, Ln³⁺ ion luminescence intensity in HfO₂ is increased for up to 15 times (in a sample containing 5 mol%Eu). The effect is described as niobium acting as a charge compensator and neutralizing the charge resulting from Ln³⁺ ion insertion in Hf⁴⁺ site and hence reducing the number of defects present. This is the second system where such an effect was observed, so it is expected that other metal oxides would show the same effect. The optical properties of HfO₂: Eu³⁺ and HfO₂: Eu³⁺, Nb⁵⁺, synthesized using the sol-gel method and annealed at various temperatures are studied. A conclusion that the structure of hafnia does not affect luminescence intensity directly and a larger role is played by factors such as defect presence and the size of the particles is drawn based on XRD and TSL measurements. Time-resolved luminescence measurements were also carried out and significant changes depending on dopant concentration and annealing temperatures were observed. Judd Ofelt theory was used to determine quantum efficiency and the local symmetry of Eu³⁺ ion sites.