A study on microstructure and luminescent properties of oxyfluoride silicate glass-ceramics with (Ho3+,Yb3+):NaYF4 crystallites

Glass-ceramics containing NaYF4 nanocrystals were prepared by heat-treatment from oxyfluoride silicate-based glass doped with Ho3+ and Yb3+ ions. The formation of crystalline fluoride phase was confirmed by X-ray diffraction and transmission electron microscopy. Absorption and emission spectra revealed that a fraction of Ho3+ and Yb3+ ions is incorporated into the NaYF4 ordered lattice influencing spectroscopic features of glass-ceramics in comparison with those of precursor glass. Green up-conversion emission (545 nm) originating in the S-5(2) level in glass-ceramics and up-converted red emission (650 nm) originating in the F-5(5) level in as-melted glass were observed under optical pumping into Yb3+ absorption band and analyzed. Although both emissions in both materials are achieved by two-photon excitations, the relation between green and red emission intensity in glass-ceramics and glass implies that processes relevant to up-conversion phenomena are different. Based on a careful analysis of relaxation dynamics of Ho3+ and Yb3+ excited states, the mechanisms involved in conversion of the infrared radiation into the visible emission in these materials are proposed and discussed. (C) 2011 Elsevier B.V. All rights reserved.