Fig. 1. Illustration for the preparation of heterostructure ZIF-67/NaGdF₄: Yb, Er

Fig. 2. The morphological characterization of heterostructure ZIF-67/NaGdF₄: Yb, Er (a) TEM image of NaGdF₄: Yb, Er nanorods, the inset is HRTEM image taken from (a) , (b) TEM image of ZIF-67, (c) TEM image of heterostructure ZIF-67/NaGdF₄: Yb, Er, the inset is 3D model of (c) , (d) The partial enlargement of (c) , the inset is HRTEM image taken from (c) , (e) EDS elemental mapping of heterostructure ZIF-67/NaGdF₄: Yb, Er.

Fig. 3. The XRD and FT-IR spectra of heterostructure ZIF-67/NaGdF₄: Yb, Er

Fig. 4. Characterization and comparison of fluorescence properties of heterostructure ZIF-67/NaGdF₄: Yb, Er (a) UC PL spectra under 980 nm laser excitation, (b) UV-vis spectra, (c) PL emission spectra of heterostructure ZIF-67/NaGdF₄: Yb, Er, the inset is PL emission spectra of NaGdF₄: Yb, Er nanorods and ZIF-67, (d) UC PL spectra of heterostructure ZIF-67/NaGdF₄: Yb, Er with different concentration of NaGdF₄: Yb, Er nanorods, the inset is the variation of PL intensity

Fig. 5. TEM images of heterostructure ZIF-67/NaGdF₄: Yb, Er with different NaGdF₄: Yb, Er nanorods concentration

Fig. 6. Schematic energy level diagram showing the UC process mechanism of heterostructure ZIF-67/NaGdF₄: Yb, Er