Highly bright and sensitive thermometric LiYF 4 :Yb, Er upconversion nanocrystals through Mg 2+ 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 (2H11/2/4S3/2). 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.