Synthesis of Eu²⁺/Eu³⁺ Co-Doped Gallium oxide nanocrystals as a full colour converter for white light emitting diodes

Eu²⁺ and Eu³⁺ co-doped Ga₂O₃ nanocrystals (Ga₂O₃:Eu NCs) were synthesized in an organic phase at a low reaction temperature of 300 °C. The surface of Ga₂O₃:Eu NCs was passivated by oleylamine (OAm) and acetylacetone (acac). The coexistence of Eu²⁺ and Eu³⁺ as well as passivation by acac and OAm enable Ga₂O₃ to be excited in the broad spectral range of 200–500 nm. The broadened absorption band is attributed to the strong acac → Ln(III) ligand to the metal charge transfer transition at ∼370 nm, Eu(III) f-f allowed ⁷F₀ → ⁵L₆ transition at 395 nm, and ⁷F₀ → ⁵D₂ transition at 465 nm, as well as the efficient electronic transition of Eu(II) 4f → 5d at ∼400 nm. Under near-ultraviolet excitation, white light emission can be achieved by combining orange-red light from f-f electronic transition of Eu(III) with blue-green-yellow light from Ga₂O₃ oxygen defects levels. Furthermore, the resultant Ga₂O₃:Eu NCs with optimized quantum yield of 14.5% were coated onto 395 nm near-ultraviolet chips to fabricate a white light emitting diode. It exhibits a luminous efficiency of 34 lm/W, CIE colour coordinate of (0.2964, 0.2831) and high colour rendering index of 80.