Heterogeneous Oxysulfide@Fluoride Core/Shell Nanocrystals for Upconversion-Based Nanothermometry.

Lanthanide (Ln 3+ )-doped upconversion nanoparticles (UCNPs) often suffer from weak luminescence, especially when their sizes are ultrasmall (less than 10 nm). Enhancing the upconversion luminescence (UCL) efficiency of ultrasmall UCNPs has remained a challenge that must be undertaken if any practical applications are to be envisaged. Herein, we present a Ln 3+ -doped oxysulfide@fluoride core/shell heterostructure which shows efficient UCL properties under 980 nm excitation and good stability in solution. Through epitaxial heterogeneous growth, a ∼4 nm optically inert β-NaYF 4 shell was coated onto ∼5 nm ultrasmall Gd 2 O 2 S:20%Yb,1%Tm. These Gd 2 O 2 S:20%Yb,1%Tm@NaYF 4 core/shell UCNPs exhibit a more than 800-fold increase in UCL intensity compared to the unprotected core, a 180-fold increase in luminescence decay time of the 3 H 4 → 3 H 6 Tm 3+ transition from 5 to 900 μs, and an upconversion quantum yield (UCQY) of 0.76% at an excitation power density of 155 W/cm 2 . Likewise, Gd 2 O 2 S:20%Yb,2%Er@NaYF 4 core/shell UCNPs show a nearly 5000-fold increase of their UCL intensity compared to the Gd 2 O 2 S:20%Yb,2%Er core and a maximum UCQY of 0.61%. In the Yb/Er core-shell UCNP system, the observed variation of luminescence intensity ratio seems to originate from a change in lattice strain as the temperature is elevated. For nanothermometry applications, the thermal sensitivities based on thermally coupled levels are estimated for both Yb/Tm and Yb/Er doped Gd 2 O 2 S@NaYF 4 core/shell UCNPs.