Abstract
We report multiple temperature effects on green and red up-conversion emissions in Er3+-Y b3+-Mo6+ codoped TiO2 phosphors. With increasing temperature, the decrease of the red emission from 4F9/2→4I15/2, the increase of green emission from 2H11/2→4I15/2 and another unchanged green emission from 4S3/2→4I15/2 were simultaneously observed, which are explained by steady-state rate equations analysis. Due to different evolution with temperature of the two green emissions, higher thermal sensitivity of optical thermal sensor was obtained based on the transitions with the largest fluorescence intensity ratio. Two parameters, maximum theoretical sensitivity (Smax) and optimum operating temperature (Tmax) are given to describe thermal sensing properties of the produced sensors. The intensity ratio and energy difference ΔE of a pair of energy levels are two main factors for the sensitivity and accuracy of sensors, which should be referred to design sensors with optimized sensing properties.
Highlights
Optical temperature sensors based on fluorescence intensity ratio (FIR) technique have attracted much attention due to their wide applications, advantage of reducing dependence on measurement condition and improvement of accuracy and resolution.[1,2,3,4,5,6] FIR technique is applied to a fluorescent system in which two closely spaced energy levels, with separations of the order of thermal energy, are involved following a Boltzmann-type population distribution
In our previous work we reported that codoping with transition metal ions is an efficient method to enhance up-conversion efficiency by the high excited state energy transfer (HESET) mechanism.[5,6]
Due to different evolution of green up-conversion emissions with temperature, an increases in the up-conversion intensity and high FIR value from the green emissions were obtained, which results in a larger sensing sensitivity of optical thermal sensors for Er3+ doped up-conversion materials compared with other reported literature
Summary
Optical temperature sensors based on fluorescence intensity ratio (FIR) technique have attracted much attention due to their wide applications, advantage of reducing dependence on measurement condition and improvement of accuracy and resolution.[1,2,3,4,5,6] FIR technique is applied to a fluorescent system in which two closely spaced energy levels, with separations of the order of thermal energy, are involved following a Boltzmann-type population distribution. Up-conversion emission of rare earth ions doped materials is a popular approach to realize FIR measurement due to large amount of coupled energy levels in several rare earth ions and the accessible up-conversion fluorescence with near-infrared radiation from low-cost commercially available diodes.[1,2,3,4,5] lower up-conversion efficiency and small FIR value can reduce effective sensitivity and accuracy of sensing behavior and limit potential application in optical thermal sensors. Due to different evolution of green up-conversion emissions with temperature, an increases in the up-conversion intensity and high FIR value from the green emissions were obtained, which results in a larger sensing sensitivity of optical thermal sensors for Er3+ doped up-conversion materials compared with other reported literature. FIR value and energy difference of pair energy levels are found to be the two main factors affecting Smax, Tmax and accuracy of sensors, which can be used to design optical thermal sensors with high sensitivity and operating temperature
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