Figure 1.

(A) Normalized upconversion luminescence spectra of Yb/Tm (dotted blue line) and Yb/Er-doped (solid green line) multicolor β-NaYF₄ nanocrystals. The UCLNPs were dispersed in cyclohexane (1 mg/mL). (B) Digital photograph of optically transparent colloidal dispersions of corresponding UCLNPs (1 mg/mL) in cyclohexane. The predominantly blue and green emissions of Yb/Tm and Yb/Er-doped UCLNPs upon 980-nm cw laser excitation can easily be seen by the bare eye.