Anomalous Temperature-Dependent Upconversion Luminescence of α-NaYF4:Yb3+/Er3+ Nanocrystals Synthesized by a Microwave-Assisted Hydrothermal Method

Yb3+/Er3+ co-doped cubic-(alpha-) phase NaYF4 nanocrystals were prepared through a microwave assisted hydrothermal method. Temperature-dependent upconversion luminescence (UCL) and sensing properties were systematically studied. It is interesting that anomalous temperature dependent UCL behavior is observed. With increasing temperature (303-573 K), the UCL intensity of Er3+ does not quench monotonously but reaches a minimum around 483 K and then increases. However, it was found that the UCL spectra change in a different way with decreasing temperature (573-303 K) from the one measured with increasing temperature. The fluorescence intensity ratio of H-2(11/2) -> I-4(15/2) to S-4(3/2) -> I-4(15/2) at any measured temperature point remains almost constant in all measurement processes, indicating the consistency of temperature in each spectrum measurement at all temperature points regardless of the heating or the cooling process in our experiments. The results demonstrate that NaYF4:Yb3+/Er3+ UC nanocrystal has good sensing stability and may have potential application in the nanoscale thermal sensor.