Water (H2O and D2O) Dispersible NIR-to-NIR Upconverting Yb3+/Tm3+ Doped MF2 (M = Ca, Sr) Colloids: Influence of the Host Crystal

Tm3+/Yb3+ doped CaF2, SrF2, and cubic phase NaYF4 nanoparticles dispersed as colloids in water (H2O and D2O) or saline solutions have been directly prepared by a one-step hydrothermal technique, using citrate anions as capping agents, without the need for any postsynthesis reaction. The size monodispersed nanoparticles are directly dispersed in water. Comparison of the upconversion emissions at 800 nm (Tm3+ ions) among the CaF2, SrF2, and NaYF4 hosts indicates that the SrF2 host leads to the highest emission intensity, 2 orders of magnitude higher than the NaYF4 one. Alkali ions (Na+ or K+) counter cations of the citrate salts used as precursors can enter the fluoride host crystals as charge compensators and strongly influence the spectroscopic properties of the lanthanide ions. The Tm3+/Yb3+ doped SrF2 nanoparticles dispersed in a 0.4 g/L concentration solution show detectable upconversion with laser excitation intensities as low as 1 W/cm2.