Charge transfer state-mediated red and near-infrared upconversion emission in NaLaS₂:Yb/Tm phosphor.

The red and near-infrared (600-1100 nm) regions, known as biological "optical window" due to minimized tissue absorption and scattering, are vital for high-contrast deep-tissue imaging. Here, we report intense red (≈696 nm) and near-infrared (≈797 nm) emissions from NaLaS2:Yb3+,Tm3+ under 980 nm excitation, both falling within this window. In addition to its enhanced chemical stability compared with conventional NaYF4 host, the NaLaS2 lattice effectively suppresses the typical blue emissions of Tm3+ (1D₂ → 3F₄/1G₄ → 3H₆, 450-500 nm) due to the overlap between the 1D2/1G₄ states and the low-lying S2- → Yb3+ charge transfer state (CTS) with an absorption edge extending to 560 nm. As a result, the red emission at 696 nm (3F2,3 → 3H6) and the near-infrared emission at 797 nm (3H4 → 3H6) dominate the spectrum. Furthermore, the thermal sensing performance of the NaLaS₂:Yb3+, Tm3+ sample was evaluated over a wide temperature (303-573 K) using fluorescence intensity ratio of the 696 nm and 797 nm emissions. It exhibits a maximum absolute sensitivity (SA) of 0.122 K-1 and a high relative sensitivity (SR) of 0.896 % K-1, which is significantly higher than that of other Yb3+, Ln3+ co-doped samples. These results highlight NaLaS₂:Yb3+, Tm3+ as a promising multifunctional luminescent material for deep-tissue bioimaging and high-performance optical thermometry.