Red emitting Eu:ZnO nanorods for highly sensitive fluorescence intensity ratio based optical thermometry

Here, we report the synthesis of Eu:ZnO nanorods by a hydrothermal method and their use as fluorescence intensity ratio (FIR) based optical temperature sensors. The as-prepared nanorods show band to band UV emission along with Eu3+ emissions accompanied by broad intense visible host defect related emissions under UV (355 nm) excitation, whereas only Eu3+ related emissions are observed under visible (532 nm) excitation. The emissions corresponding to D-5(1), D-5(0) -> F-7(1) and D-5(1), D-5(0) -> F-7(2) under 532 nm excitation are considered for FIR based temperature sensing as D-5(1) and D-5(0) bands are very close to each other and can possess a thermal equilibrium between them. The intensity ratio between these transitions follows a Boltzmann type distribution function. The sensitivity obtained by fitting the intensity ratio is very high (3013/T-2 K-1 for D-5(1), D-5(0) -> F-7(2)) as compared to the reported values for Eu3+ based sensors and can be applied in a wide range of temperatures (83-493 K). The sensitivity is highly influenced by the doping concentration of Eu, increases with doping up to a certain extent and then decreases.