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

Abstract

Tm 3+ /Yb 3+ doped CaF 2, SrF 2, and cubic phase NaYF 4 nanoparticles dispersed as colloids in water (H 2 O and D 2 O) 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 (Tm 3+ ions) among the CaF 2, SrF 2, and NaYF 4 hosts indicates that the SrF 2 host leads to the highest emission intensity, 2 orders of magnitude higher than the NaYF 4 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 Tm 3+ /Yb 3+ doped SrF 2 nanoparticles dispersed in a 0.4 g/L concentration solution show detectable upconversion with laser excitation intensities as low as 1 W/cm 2 .