Eu2+ emission from thermally coupled levels – new frontiers for ultrasensitive luminescence thermometry

Abstract

Designing luminescent thermometers operating in a broad T-range, spanning hundreds of degrees, presenting high thermal sensitivity and good resolution is one of the greatest challenges in the field. In this paper, we present SrB₄O₇:Eu²⁺, a novel luminescent thermometer, operating in the T-range of 11–600 K and presenting unprecedentedly high relative thermal sensitivity, i.e., up to ≈10.5%/K and ≈22.6%/K, using either Eu²⁺ luminescence decay time or intensity ratio as thermometric parameters, respectively. Such good performance classifies the SrB₄O₇:Eu²⁺ phosphor as the most sensitive inorganic luminescent thermometer ever reported. The temperature dependences of both luminescence intensity ratios and the emission decay time result from the same physical effect, i.e., thermal coupling of the lowest 5d₁ excited level of Eu²⁺ and the ⁶P_7/2 excited level of the 4f⁷ configuration of the dopant. The latter one is located ≈130 cm⁻¹ below the 5d₁. Such a strong coupling changes the contribution of the emission from the two levels, in response to the temperature change, but without any significant luminescence thermal quenching. This beneficial situation allows measuring spectra with a high signal-to-noise ratio, which lowers the uncertainty of the temperature measurement (improves resolution) in the whole operating T-range. The SrB₄O₇:Eu²⁺ material offers a broad operating T-range, as well as unprecedentedly high relative thermal sensitivity and low uncertainty of the temperature measurement.