(La0.97RE0.01Yb0.02)2O2S Nanophosphors Converted from Layered Hydroxyl Sulfate and Investigation of Upconversion Photoluminescence (RE=Ho, Er)

Phase-pure (La0.97RE0.01Yb0.02)(2)O2S upconversion (UC) nanophosphors (average crystallite size similar to 45 nm; RE=Ho, Er) were annealed from their hydrothermally crystallized layered hydroxyl sulfate precursors in flowing hydrogen at 1200 degrees C for 1 h, with water vapor as the only exhaust. Under 978-nm laser excitation (up to 2.0 W), the Ho3+-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at similar to 546 (F-5(4) -> I-5(8)), 658 (F-5(7) -> I-5(8)), and 763 nm (F-5(4) -> I-5(7)), respectively, and has the stable chromaticity coordinates of about (0.30, 0.66) in the visible-light region (400-700 nm). The Er3+-doped UC phosphor, on the other hand, showed weak green (similar to 527/549 nm, H-2(11/2),S-4(3/2) -> I-4(15/2)), weak red (similar to 668/672 nm, F-4(9/2) -> I-4(15/2)), and strong near-infrared (similar to 807/58 nm, I-4(9/2) -> I-4(15/2)) 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.