Eu²⁺/Eu³⁺ activated phosphate glasses synthesized via melting with multi-wall carbon nanotubes

Glasses of BaO:P₂O₅ composition were melted with fixed Eu₂O₃ content and varying amounts of multi-wall carbon nanotubes (MWCNTs) for a spectroscopic and calorimetric study focusing on luminescent properties of interest to light-emitting devices. The optical transmission data indicated favorable Eu³⁺ → Eu²⁺ reduction with MWCNTs up to 1.0 wt% as judged by the 4f⁷ → 4f⁶5d transitions of Eu²⁺ ions around 320 nm. A small amount of MWCNTs at 0.3 wt% lead to an improved Eu³⁺ luminescence under 320 nm relative to the Eu³⁺-doped reference likely due to a Eu²⁺ → Eu³⁺ energy transfer. On the other hand, the most intense emission around 450 nm corresponding to 4f⁶5d → 4f⁷ radiative relaxation in Eu²⁺ was obtained for a glass melted with 0.7 wt% MWCNTs. Concentration quenching and optical band gap reduction are suggested to cause the decreased 450 nm band emission in the glass melted with 1.0 wt% MWCNTs. The glasses presented concomitant luminescence due to remaining Eu³⁺ ions, and the resulting emitted light under excitation at 320 nm was characterized by CIE diagram. Further, Raman spectroscopy was employed for a structural characterization, which was ultimately correlated with the thermal properties measured by differential scanning calorimetry.