Optical spectroscopy and upconversion studies of Ho3+-doped bulk and nanocrystalline Y2O3

In this paper we report on the optical spectroscopy and upconversion studies of Ho3+-doped nanocrystalline and bulk Y2O3, as a function of holmium concentration (0.1, 0.5, 1, 2, 5, and 10 mol %). Emission in the blue, green, red, and NIR portions of the spectrum is recorded after 457.9-nm excitation. Red (646 nm) pumping results in blue and green emission through a two-photon excited-state absorption (ESA) upconversion process. NIR (754 nm) pumping results in blue, green, and red emission. Overall luminescence of the nanocrystalline samples under one-photon excitation or 646-nm pumping is severely reduced when compared to that of the bulk sample, or nonexistent in the case of the 754-nm pumping, which is attributed to adsorbed atmospheric carbon dioxide and/or water on the surface of the nanocrystals. A quenching of the green (F-5(4), S-5(2)) --> I-5(8) emission is noted with increasing Ho3+ concentration that is attributed to a cross-relaxation mechanism involving two holmium ions.