Towards efficient upconversion and downconversion of NaYF4:Ho3+,Yb3+ phosphors

NaYF4 microcrystals co-doped with Ho3+ and Yb3+ were prepared by a facile hydrothermal synthesis. The products were characterized by X-ray diffractometer, scanning electron microscopy, and photoluminescence spectroscopy. Upon excitation with a 980 nm laser diode, the sample shows an intense green upconversion emission centered at 540 nm corresponding to the S-5(2)-> I-5(8) transition of Ho3+. The quadratic dependence of the green emission intensity on the excitation power reveals a two-phonon upconversion process. On the contrary, upon excitation with 448 nm, both visible and near-infrared emissions peaked at 483, 540, 644, 749, and 978 nm are simultaneously observed, which could be assigned to the electronic transitions of Ho3+: F-5(3)-> I-5(8), S-5(2)-> I-5(8), F-5(5)-> I-5(8), S-5(2)-> I-5(7), and Yb3+: F-2(5/2)-> F-2(7/2), respectively. The energy transfer processes between Ho3+ and Yb3+ ions and the involved mechanisms have been investigated and discussed. (C) 2011 Elsevier B.V. All rights reserved.