(LaREYb)OS Nanophosphors Converted from Layered Hydroxyl Sulfate and Investigation of Upconversion Photoluminescence (RE=Ho, Er).

Phase-pure (LaREYb)OS upconversion (UC) nanophosphors (average crystallite size ~ 45 nm; RE=Ho, Er) were annealed from their hydrothermally crystallized layered hydroxyl sulfate precursors in flowing hydrogen at 1200 °C for 1 h, with water vapor as the only exhaust. Under 978-nm laser excitation (up to 2.0 W), the Ho-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at ~ 546 (F → I), 658 (F → I), and 763 nm (F → I), respectively, and has the stable chromaticity coordinates of about (0.30, 0.66) in the visible-light region (400-700 nm). The Er-doped UC phosphor, on the other hand, showed weak green (~ 527/549 nm, H,S → I), weak red (668/672 nm, F → I), and strong near-infrared ( 807/58 nm, I → I) luminescence, whose emission color in the visible region drifted from yellowish-green [(0.36, 0.61)] to green [(0.32, 0.64)] with increasing excitation power. Analysis of the power-dependent UC luminescence found three- and two-photon processes for RE=Ho and Er, respectively, and the possible UC mechanisms were proposed.