(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)2O2S 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 Ho3+-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at ~ 546 (5F4 → 5I8), 658 (5F7 → 5I8), and 763 nm (5F4 → 5I7), 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 (~ 527/549 nm, 2H11/2,4S3/2 → 4I15/2), weak red (~668/672 nm, 4F9/2 → 4I15/2), and strong near-infrared (~ 807/58 nm, 4I9/2 → 4I15/2) 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.