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 S^2- → Yb³⁺ charge transfer state (CTS) with an absorption edge extending to 560 nm. As a result, the red emission at 696 nm (³F_2,3 → 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 (S_A) of 0.122 K^-1 and a high relative sensitivity (S_R) 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.