Constructing multiform morphologies of YF:Er³⁺/Yb³⁺ up-conversion nano/micro-crystals towards sub-tissue thermometry

Er³⁺/Yb³⁺ co-doped YF₃ samples with uniform rice-like, two-dimension (2D) hexagonal nano-architectures and truncated octahedral micro-crystals were synthesized through a one-step hydrothermal method, and their possible growth mechanisms were also proposed. Upon 980 nm excitation, the up-conversion (UC) emitting color of samples was finely modulated from yellow to pure green with increasing the particle size, which was interpreted in terms of non-radiative process and surface effect. Thermometric capabilities of samples with different morphologies were evaluated based on the fluorescence intensity ratio (FIR) of ²H_11/2 and ⁴S_3/2 levels through temperature-dependent UC spectra, and the optimal absolute sensitivities of samples were all approached to about 0.0026 K^-1 at 490 K. The potentiality of rice-like nano-crystals for temperature detection in sub-tissue was demonstrated by ex vivo experiments. Results display that the sensitivity of YF₃: Er³⁺/Yb³⁺ nano/micro-crystals is independent of morphologies and sizes, which exploit an effective avenue for developing UC-based thermometers in sub-tissue with stable sensitivity.