Enhancing thermal sensitivity in α-NaLuF4:Yb3+, Er3+ upconversion nanocrystals

alpha-NaLuF4:Yb3+, Er3+ nanocrystals were achieved through a hydrothermal method with the assistance of citric acid. Under 980 nm excitation, the temperature dependence of 523 nm and 543 nm upconversion emissions of Er3+ was studied from 150 K to 298 K. The intensity ratio of H-2(11/2) -> I-4(15/2) to S-4(3/2) -> I-4(15/2) increases with elevated temperature for the as-prepared samples. Based on the fluorescence intensity ratio technique, we report the enhanced sensitivity of green-based sensor by simply adding different NaF contents in the initial reaction solution, and the obtained maximum sensitivity is 0.0022K(-1) at 298 K. Moreover, for the annealed sample, the maximum sensitivity can reach 0.0030 K-1 at 298 K. The mechanism of the increasing sensitivity is discussed in details, which is mainly attributed to the enhancement of emission cross section ratio (sigma(H)/sigma(s)). This study not only makes alpha-NaLuF4:Yb3+, Er3+ nanocrystals a promising candidate for the luminescent nanothermometers, but also provides a general and effective approach for enhancing the sensitivity of optical temperature sensors. (C) 2016 Elsevier B.V. All rights reserved.