Fig. 1. (a) Construction scheme of the preparation process of GVA@SiO₂@NaYF₄:Yb³⁺/Er³⁺; (b) the typical SEM images of the large-scale and corresponding small-scale GVA; (c) the SEM images of the GVA@SiO₂ (8 nm), and the cross-sectional view of the spin-coated NaYF₄:Yb³⁺/Er³⁺ nanocrystals; (d) the ultraviolet-visible absorption spectrum of GNRs; TEM images of (e) NaYF₄:20%Yb³⁺/2%Er³⁺ and (f) NaYF₄: 40%Yb³⁺/2%Er³⁺ nanocrystals; (g) XRD patterns of NaYF₄:20%Yb³⁺/2%Er³⁺ and NaYF₄: 40%Yb³⁺/2%Er³⁺ nanocrystals.

Fig. 2. Schematic explanation of confocal microscopy setup.

Fig. 3. (a) and (b) Upconversion emission spectra and enhancement factor of different systems under 980 nm excitation; (c) and (d) the peak area of the green and red emission intensity and corresponding R/G ratio of the NaYF₄:20%Yb³⁺/2%Er³⁺ with wafer and different thicknesses of isolation layer about from 0 to 8 nm (RE1: NaYF₄: 20%Yb³⁺/2%Er³⁺; M-RE1, M-RE14 and M-RE18: GVA@SiO₂ (0, 4, 8 nm)@NaYF₄: 20%Yb³⁺/2%Er³⁺).

Fig. 4. (a) and (b) Upconversion emission spectra and enhancement factor of different systems under 980 nm excitation; (c) and (d) the peak area of the green and red emission intensity and corresponding R/G ratio of the NaYF₄:40%Yb³⁺/2%Er³⁺ with wafer and different thicknesses of isolation layer about from 0 to 8 nm (RE2: NaYF₄:40%Yb³⁺/2%Er³⁺; M-RE2, M-RE24, and M-RE28: GVA@SiO₂ (0, 4, 8 nm)@NaYF₄:40%Yb³⁺/2%Er³⁺).

Fig. 5. Energy level transition diagram and enhancement mechanism of LSPR.

Fig. 6. (a) Upconversion emission spectra of GVA@SiO₂ (8 nm)@NaYF₄:40%Yb³⁺/2%Er³⁺ nanocrystals; (b) and (c) are graphs corresponding to the emission intensity and the red-green ratio as a function of the 980 nm laser excitation power from 40 mW to 160 mW.

Fig. 7. Life decay of green (⁴S_3/2 level) upconversion emission of NaYF₄:20%Yb³⁺/2%Er³⁺ nanocrystal with the thickness of the isolation layer under 980 nm excitation.

Fig. 8. Local electromagnetic field distribution of the smallest unit of the GVA@SiO₂ system is simulated under 980 nm excitation: (a) The local electromagnetic field distribution map of the x-y plane; (b) the GVA@SiO₂ (2 nm) of x-z cross-section local electromagnetic field distribution diagram was illustrated; (c) the intensity of the localized electromagnetic field hot spot produced by the LSPR with the SiO₂ thickness changed.