Multiple temperature effects on up-conversion fluorescences of Er3+-Y b3+-Mo6+ codoped TiO2 and high thermal sensitivity

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.

Multiple Temperature-Sensing Behavior of Green and Red Upconversion Emissions from Stark Sublevels of Er3+

“…The upconversion mechanism of Er 3+ after the addition of Mo 6+ was reported in a previous study on the sensitization of the Yb 3+ -MoO 4 2− dimer to Er 3+ [20,21,22]. Through a cooperative sensitization process in the Yb 3+ -MoO 4 2− dimer, two excited Yb 3+ ions nonradiatively transfer their energy to MoO 4 2− .…”

“…Conversely, the intensity of green emissions increased with increasing temperature, which does not satisfy the classical theory of thermal quenching, likely due to the increased Yb 3+ absorption cross-section at elevated temperatures [22,24]. A general theoretical description of the green upconversion emission can be given by [22]: $I_{green}=B{\left[1-\exp \left(-\frac{h\nu }{kT}\right)\right]}^{-2}$ where B is a constant, and hν is the phonon energy participating in the multiphonon-assisted excitation.…”

“…A general theoretical description of the green upconversion emission can be given by [22]: $I_{green}=B{\left[1-\exp \left(-\frac{h\nu }{kT}\right)\right]}^{-2}$ where B is a constant, and hν is the phonon energy participating in the multiphonon-assisted excitation. The dependence of green upconversion emissions on temperature fits well to Equation (2).…”