Fig. 1. (a) Schematic diagram of resonance energy transfer between donor (ion A) and acceptor (ion B). (b), (c) Schematic diagram of phonon-assisted energy transfer; ΔE is the energy mismatch between donor and acceptor.

Fig. 2. XRD patterns of (a) β-NaLuF4:18%Yb3+, x%Er3+ (x=1, 3, 5, 7) and (b) β-NaLuF4:18%Yb3+, y%Ho3+ (y=0.5, 1, 1.5, 2) hollow microtubes. (c) Raman spectrum of the β-NaLuF4 hollow microtubes at room temperature under 532 nm excitation. (d) SEM images and corresponding elemental mapping images and (e), (f) size distribution histogram of β-NaLuF4:18%Yb3+, 5%Er3+.

Fig. 3. Downshifting emission spectra of (a) β-NaLuF4:18%Yb3+, 5%Er3+ and β-NaLuF4:18%Yb3+, 1.5%Ho3+ and (b) β-NaLuF4:18%Yb3+, 5%Er3+, z%Ho3+ (z=1, 2, 3, 5) under 980 nm excitation. (c) Dependence of NIR-II (Ho3+:1190  nm and Er3+:1530  nm) emission integral intensity on different Ho3+ concentrations. (d) Double logarithmic plots of NIR-II emission intensities versus 980 nm laser power of β-NaLuF4:18%Yb3+, 5%Er3+, 2%Ho3+. (e) Energy level schematic diagram of Er3+, Ho3+, and Yb3+ ions along with the relevant transitions and energy transfers under the excitation of 980 nm.

Fig. 4. Emission spectra of β-NaLuF4:5%Er3+, β-NaLuF4:2%Ho3+, and β-NaLuF4:5%Er3+, m%Ho3+ (m=1, 2, 3, 5) in the range of (a) 500–700 nm and (b) 1100–1700 nm under 980 nm excitation. (c) Intensity of emissions in visible, NIR-II region with the elevated doping concentration of Ho3+. Decay curves of I413/2→I415/2 transitions of (d) β-NaLuF4:5%Er3+ and (e) β-NaLuF4:5%Er3+, m%Ho3+ (m=1, 2), respectively. (f) Calculated Er3+-to-Ho3+ ET efficiency with the elevated doping concentration of Ho3+. (g) Proposed energy transfer scheme of Er3+-Ho3+ co-doped system under 980 nm excitation.

Fig. 5. (a) Emission spectra of β-NaLuF4:18%Yb3+, 5%Er3+, 1%Ho3+ recorded at different temperatures under 980 nm excitation. (b) The plotted emission intensity of Ho3+ and Er3+ dependent on the ambient temperature. (c) The variation trend of integrated emission intensity of Ho3+ and Er3+ at varied Ho3+ doping concentrations with the temperature range from 313 to 523 K. (d) Energy level diagram of the phonon-assisted enhanced ET processes between Er3+ and Ho3+.

Fig. 6. (a) Two-dimensional NIR-II emission topographical mapping with the temperature from 313 to 523 K. (b) The acquired experimental temperature-dependent LIR data fitted with Eq. (4) using the dominant phonon energy at 497  cm−1. (c) The relative temperature sensitivity (Sr) of the investigated β-NaLuF4:18%Yb3+, 5%Er3+, z%Ho3+ (z=1, 2, 5) hollow microtubes with diverse doping contents. (d) Fluctuation of LIR values and the calculated temperature resolution δT at 313 K. (e) Repeatability of LIR over heating and cooling cycles under 980 nm excitation.

Fig. 7. Fourier transform infrared spectrum of the β-NaLuF4.

Fig. 8. (a) Up-conversion emission spectra and (b) NIR-II emission spectra of β-NaLuF4:18%Yb3+, x%Er3+ (x=1, 2, 3, 5, 7) hollow microtubes under 980 nm excitation. (c) Schematic illustration of energy level diagram of different Er3+ concentrations.

Fig. 9. NIR-II emission spectra of (a) β-NaLuF4:18%Yb3+, 0.4%Ho3+, 0.2%Tm3+ and (b) β-NaLuF4:18%Yb3+, 0.2%Er3+, 0.2%Tm3+ hollow microtubes under 980 nm excitation.

Fig. 10. The downshifting emission spectra of (a) β-NaLuF4:18%Yb3+, 5%Er3+, 1%Ho3+, (b) β-NaLuF4:18%Yb3+, 5%Er3+, 3%Ho3+, and (c) β-NaLuF4:18%Yb3+, 5%Er3+, 5%Ho3+ samples excited at different pump powers. (d)–(f) Corresponding to the double-logarithmic plots of emission intensity versus pump power under 980 nm excitation.