Fig. 1. Mechanism of ET in FITC sensitized Er3+-doped CaF2 nanoparticles.

Fig. 2. (a) TEM image and (b) powder XRD pattern of CaF2:Er3+ nanoparticles.

Fig. 3. (a) Absorption spectra; (b) Vis emission spectra (λex=467  nm); and (c) luminescence decay curves (λex=467  nm; λem=538  nm) of FITC (black), CaF2@FITC (blue), CaF2:Er3+@FITC (red). Steady-state spectra are normalized for the absorbed power at excitation wavelength.

Fig. 4. (a) Optimized geometries and (b) MOs calculated by DFT methods at B3LYP/6-311 + G(d,p) level of theory (color codes: Ca, green; C, gray; O, red; N, purple; S, yellow; H, white). The orbitals are reported with a contour value of 0.050.

Fig. 5. (a) NIR emission spectra of CaF2:Er3+ (black, λex=378  nm corresponding to the Er3+4G11/2←4I15/2 transition) and CaF2:Er3+@FITC (red, λex=467  nm corresponding to the maximum of the FITC dye absorption); (b) luminescence decay curves of CaF2:Er3+ (black, λex=378  nm; λem=1530  nm) and CaF2:Er3+@FITC (red, λex=467  nm; λem=1530  nm). Steady-state spectra are normalized for the absorbed power at excitation wavelength.

Fig. 6. Two-dimensional (2D) TA (ΔA) map of (a) FITC; (b) CaF2@FITC; and (c) CaF2:Er3+@FITC in chloroform as a function of wavelength and time, upon photoexcitation at 500 nm; (d) representative selection of TA spectra of CaF2:Er3+@FITC in chloroform at different time delays, the black and gray dashed lines show the inverted PL spectrum and ground-state absorption spectrum, respectively; (e) selection of TA spectra of FITC, CaF2@FITC, and CaF2:Er3+@FITC in chloroform at 7.5 ps time delay; (f) selected kinetics of FITC, CaF2@FITC, and CaF2:Er3+@FITC for the SE signal at 550 nm in the sub-nanosecond time range.

Fig. 7. (a) Spectral overlap of Er3+ absorption cross section (red) and FITC fluorescence spectrum normalized to unity (black curve with shaded area). (b) FITC to Er3+ sensitization efficiency (ηsens) calculated by the Förster’s model as a function of the donor–acceptor distance (R). Inset, DFT-calculated optimized geometry for FITC coordinated to Ca2+ where the light blue arrow represents the TD-DFT calculated S1−S0 transition dipole moment.

Table 1. FITC-Excited Singlet Ultrafast Decay Time Constants and FITC to Er³⁺ Sensitization Efficiency Calculated through Eq. (1)