In this work, Yb3+, Tm3+ co-doped NaYF4 powders with several morphologies (spheres, micro-needles, and
microrods) were successfully synthesized by varying the NaOH concentration from 0 to 2 M. The synthesis was
carried out with pressure-assisted hydrothermal reactions. XRD diffraction patterns confirmed the presence of a
pure crystalline hexagonal phase in all the NaYF4 samples. Structural data, crystallite sizes, lattice parameters, and cell volumes were obtained with the Rietveld method.
The up-conversion spectra of the NaYF4:Yb3+(20%),Tm3+(1%) powders synthesized with different concentration of NaOH were obtained under infrared excitation at λ=980 nm. These spectra showed the characteristic visible emissions of Tm3+ at 455 (1D2→3F4), 478 nm (1G4→3H6) and 805 nm (3H4→3H6) for NaOH concentrations below 1 M. However, the visible emission was completely quenched for NaOH concentrations from 1M and only a pure IR emission was observed at 805 nm. Hence, the NaYF4 powders studied in this work could be candidates as active media of solid state laser at 805 nm, which are currently important for biomedical applications and photothermal therapy.