Highly bright and sensitive thermometric LiYF₄:Yb, Er upconversion nanocrystals through Mg²⁺ tridoping

A new strategy is reported to enhance the upconversion (UC) luminescence emission of LiYF₄:Yb, Er nanocrystals (NCs) using magnesium as a dopant. We carried out systematic experimental studies on the crystal structure, grain size, and UC emitting property of the tetragonal LiYF₄:Yb, Er with varied concentrations of Mg²⁺. The UC luminescence properties were examined under 980 nm laser illumination with various excitation power densities. At a proper doping concentration, co-doping of Mg²⁺ ions into LiYF₄:Yb, Er is found to result in efficient reinforcement in both the green and red upconverted emissions. Remarkably, the maximum green and red luminescence intensities were reinforced by sevenfold and fivefold, respectively, when 7 mol% Mg²⁺ was co-doped into tetragonal LiYF₄. The possible origin and mechanism for boosting UC emission were explained according to the alteration of the cell volume and the local crystal field surrounding the Er³⁺ ions by co-doping of Mg²⁺. Moreover, the emission-optimized LiYF₄ UCNCs were further investigated to understand thermal-sensing behaviors employing the fluorescence intensity ratio (FIR) approach from the two neighboring thermal coupled states (²H_11/2/⁴S_3/2). The optimization of Mg²⁺ co-doping in LiYF₄:Yb, Er allowed the resultant UCNCs to be an excellent luminescent thermometer over a wide range of temperature. Applying the optimized UCNCs as an optical thermometer, a maximum thermal sensitivity (S) of 5.43 × 10^–2 K⁻¹ was achieved at room temperature and a low-power excited upconversion (1 W cm⁻²). The achieved S value is more advanced than most of the Er-based nanophosphors reported heretofore. This paper provides a perspective scheme to design and grow high-quality upconversion nanomaterials for achieving the preconditions of the pragmatic application in temperature sensing, optically heating, and color display devices.