Simultaneous Observation of Visible Upconversion and Near-Infrared Downconversion in SrF(2):Nd(3+)/Yb(3+)/Er(3+) Nanocrystals and Their Application for Detecting Metal Ions under Dual-Wavelength Excitation

[Image: see text] In this work, a sequence of Nd(3+), Yb(3+), and Er(3+) tridoped SrF(2) nanocrystals (NCs) is synthesized by a hydrothermal method. Both the efficient near-infrared downconversion luminescence (DCL) and visible upconversion luminescence (UCL) of the Er(3+) and Nd(3+) ions are simultaneously observed and systematically demonstrated under dual-wavelength excitation (808 and 980 nm continuous-wave lasers). Subsequently, the SrF(2):Nd(3+)/Yb(3+)/Er(3+) (15/4/0.2 mol %) NCs with the strongest luminescence were utilized for detecting the metal ion concentrations under 808 nm excitation. The results reveal that both the UCL and DCL gradually decrease as the metal ion concentrations increase, and high sensitivity is obtained for Cu(2+) ions with a detection limit of 0.22 nM (∼650 nm) and 0.63 nM (∼976 nm). In addition, these SrF(2):Nd(3+)/Yb(3+)/Er(3+) NCs are further demonstrated to achieve a solid-state display under 980 nm excitation, exhibiting obvious “red” and “green” patterns by varying the doping rare earth ion concentrations.