Figure 1.

Schematics showing the fabrication and emission of NaYF₄ NPs serving as the nanothermometer³¹. (a) Schematic of the preparation of NaYF₄: Yb³⁺, Ho³⁺, Er³⁺ NPs. The oleic acid-capped NaYF₄:Yb³⁺, Ho³⁺, Er³⁺ NPs were synthesized by a thermal decomposition method and the NPs were finally dispersed in cyclohexane. (b) Schematic illustration of the NaYF₄ NPs emitting at 1150 nm (NIR-II) and 1550 nm (NIR-III) under 980-nm (NIR-I) laser excitation. (c) Simplified energy scheme of the Yb³⁺, Ho³⁺, and Er³⁺ emitting centers representing the excitation, resonance energy transfer, and phonon-assisted energy transfer. Under 980-nm excitation (blue line), they present the emission bands ascribed to Ho³⁺ (1150 nm, green line) and Er³⁺ (1550 nm, red line).