Double-deconvolution method for the separation of thermalised emissions from chromium-doped lanthanum gallate and its potential in luminescence-based thermometry

Abstract

A series of LaGa1–xO3:Cr3+x phosphors (x = 0.002, 0.005, 0.01, 0.02, 0.04) were synthesised by solid-state reaction and structurally characterised by Rietveld refinement from powder X-ray diffraction data. Amongst the five compounds, that with x = 0.01 exhibited the highest photoluminescence quantum yield at room temperature, those with higher doping levels probably suffering from self-quenching of the luminescence. This compound was selected for study of the temperature-dependence of the optical properties. The emission spectra recorded over the range 300–600 K reveal distinct features: a broad band that initially increases with temperature, attributed to fluorescence from the 4T2 excited state, and a series of sharp peaks that monotonically decline with temperature, attributed to phosphorescence from the 2E. The thermometry capabilities of LaGa0.99O3:Cr0.01 were probed by the luminescence intensity ratio (LIR) method using the broad 4T2 band relative to the sharp 2E peaks. To overcome the difficulties associated with the significant overlap of the broad and sharp emissions of Cr3+, a novel method was applied in which the deconvolution of broad peaks was performed in two steps, by fitting the broad bands to the curve sections without the sharp peaks. The ratio of the deconvoluted 4T2 and 2E intensities gave an excellent fit to the Boltzmann distribution, with an energy gap between them of 2172 cm–1. The high relative sensitivity at room temperature of ca. 2.5% K–1 demonstrates very good potential of LaGa0.99O3:Cr0.01 for thermometry in the first biological window of transparency, relevant for in vivo biomedical applications.