Unraveling Yb3+-mediated temperature-dependent upconversion luminescence in Er3+-activated fluoride under 1550 nm excitation.

Abstract

Thermoresponsive upconversion luminescent materials can convert thermal variations into visually distinguishable optical signals, offering potential for noncontact temperature sensing, temperature visualization, information encryption, and anti-counterfeiting. However, it remains challenging for current thermochromic upconversion materials to achieve pronounced temperature-induced color variation under single near-infrared excitation. Herein, NaYbF4:Er3+ nanoparticles with a dense Yb sublattice were employed as a model system, exhibiting a thermochromic evolution from green through yellow and orange to red under 1550 nm excitation. By progressively substituting Yb3+ with optically inert Y3+ to dilute the Yb sublattice and further constructing an inert shell structure to suppress surface-related energy loss, the distinct roles of ErYb back energy transfer, YbYb energy migration, and migration-assisted surface dissipation were differentiated. The distinct thermal responses of the red and green emissions arise from different Yb sublattice-mediated surface energy dissipation mainly affecting the red emission and back energy transfer together with nonradiative processes governing the green emission. Benefiting from the pronounced temperature-induced color evolution, temperature-driven dynamic digital information switching and a flame-shaped temperature-visualization label were demonstrated. These results reveal the nonconventional regulatory role of the Yb sublattice under direct Er3+ excitation and provide a mechanistic basis for designing thermochromic upconversion materials for visual thermal information readout and optical information encoding.

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