Morphology Manipulation and Upconversion Luminescence Enhancement of β-NaLuF₄: Er³⁺ Hexagonal Microtubes via Sr²⁺ Doping

Figure 6

(a) The upconversion emission spectra; (b) the integrated intensity variation of green, red, infrared, and all emissions of 6%Er³⁺, % Sr²⁺ samples with , 5, 10, 18, 25, and 30 under 980 nm excitation (color figure online).