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

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 S-0 -> Si (pi -> pi*, acac) and acac -> Ln(3+) ligand to metal charge transfer (LMCT) transitions at 270 and 370 nm, respectively, in the Eu:Y2O3 nanocrystal. Excitation into the S-0 -> S-1 (pi -> pi*) or acac -> Ln(3+) 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 degrees 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 (similar to 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.