Low-Temperature Fluorination Route to Lanthanide-Doped Monoclinic ScOF Host Material for Tunable and Nearly Single Band Up-Conversion Luminescence

Abstract

Lanthanide upconversion (UC) materials that convert near-infrared excitations into visible emissions are of extensive current interest owing to their potential applications in biosensing, 3D displays, and solar cells. A wise choice of the host lattice is crucial for high-quality UC luminescence with desired emission wavelengths. From the viewpoint of structural chemistry, here we propose monoclinic scandium oxyfluoride (M-ScOF) as a promising UC host material for the following reasons: (1) the shortest Sc3+–Sc3+ distance (3.234 Å, versus 3.584 Å of Y3+–Y3+ in hexagonal NaYF4); (2) the unique crystallographic site of Sc in the structure; (3) specific coordination environment of Sc with 4O + 3F in C1 symmetry. Lanthanide doping in an individual host with such structural features is highly expected to achieve single band emission and fast energy migration for high-efficiency UC process. Experimentally, we employ a low temperature fluorination method to synthesize pure and lanthanides doped M-ScOF samples successfully by using polytetrafluoroethylene as the fluridizer. The Yb3+/Ho3+-codoped M-ScOF nanoparticles exhibit tunable UC emissions with various red/green ratios under excitation of λex = 980 nm. Nearly single-band red (∼660 nm) and near-infrared (∼805 nm) UC luminescence have been achieved in Yb3+/Er3+- and Yb3+/Tm3+-incorporated samples, respectively. We believe that more attention to M-ScOF and the search for other advanced host materials based on structural chemistry perspective will greatly boost the development of high-efficiency UC phosphors in various applications such as bioprobes and chromatic displays.