(La(0.97)RE(0.01)Yb(0.02))(2)O(2)S Nanophosphors Converted from Layered Hydroxyl Sulfate and Investigation of Upconversion Photoluminescence (RE=Ho, Er)

Phase-pure (La(0.97)RE(0.01)Yb(0.02))(2)O(2)S 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(3+)-doped phosphor exhibited green (medium), red (weak), and near-infrared (strong) emissions at ~ 546 ((5)F(4) → (5)I(8)), 658 ((5)F(7) → (5)I(8)), and 763 nm ((5)F(4) → (5)I(7)), respectively, and has the stable chromaticity coordinates of about (0.30, 0.66) in the visible-light region (400–700 nm). The Er(3+)-doped UC phosphor, on the other hand, showed weak green (~ 527/549 nm, (2)H(11/2),(4)S(3/2) → (4)I(15/2)), weak red (~668/672 nm, (4)F(9/2) → (4)I(15/2)), and strong near-infrared (~ 807/58 nm, (4)I(9/2) → (4)I(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. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s11671-017-2277-4) contains supplementary material, which is available to authorized users.