Ligand-Passivated Eu:Y2O3 Nanocrystals as a Phosphor for White Light Emitting Diodes

Abstract

Eu(III)-doped Y₂O₃ nanocrystals are prepared by microwave synthetic methods as spherical 6.4 ± 1.5 nm nanocrystals with a cubic crystal structure. The surface of the nanocrystal is passivated by acetylacetonate (acac) and HDA on the Y exposed facet of the nanocrystal. The presence of acac on the nanocrystal surface gives rise to a strong S₀ → S₁ (π → π*, acac) and acac → Ln³⁺ ligand to metal charge transfer (LMCT) transitions at 270 and 370 nm, respectively, in the Eu:Y₂O₃ nanocrystal. Excitation into the S₀ → S₁ (π → π*) or acac → Ln³⁺ LMCT transition leads to the production of white light emission arising from efficient intramolecular energy transfer to the Y₂O₃ oxygen vacancies and the Eu(III) Judd–Ofelt f–f transitions. The acac passivant is thermally stable below 400 °C, and its presence is evidenced by UV–vis absorption, FT-IR, and NMR measurements. The presence of the low-lying acac levels allows UV LED pumping of the solid phosphor, leading to high quantum efficiency (∼19%) when pumped at 370 nm, high-quality white light color rendering (CIE coordinates 0.33 and 0.35), a high scotopic-to-photopic ratio (S/P = 2.21), and thermal stability. In a LED lighting package luminosities of 100 lm W^–1 were obtained, which are competitive with current commercial lighting technology. The use of the passivant to funnel energy to the lanthanide emitter via a molecular antenna effect represents a new paradigm for designing phosphors for LED-pumped white light.