Resonance/off-resonance excitations: implications on the thermal evolution of Eu3+ photoluminescence

Abstract

Generally, it is known that the intensity of the emission of trivalent lanthanide ions (Ln³⁺) increases on cooling because of the reduced decay of the excited state population via phonon-mediated nonradiative transitions. In contrast to this, some studies in the recent past have shown that the intensity of Eu³⁺ photoluminescence decreases dramatically on cooling. While this anomalous behaviour has been found to be useful for designing highly sensitive luminescence thermometers, the mechanism underlying this anomalous behaviour remains elusive. Here, we address this issue using a combined experimental and theoretical approach. We identified off-resonance excitation as the exclusive factor which enhances the Eu³⁺ emissions with the increase of temperature. We could tune the temperature dependence of the Eu³⁺ light-emission by varying the bandpass of the excitation source around the off-resonance energy. Using kinetic rate equations, we model the temperature trend of the Eu³⁺ emission intensity in the steady state by considering the independent contribution of the ⁷F₀ and ⁷F₁ ground states in populating the ⁵D₀ excited state. As an illustrative example, we apply this understanding to designing an Er³⁺/Eu³⁺ codoped CaTiO₃ phosphor that shows a considerably large relative thermal sensitivity (S_r = 4.9% K⁻¹) at 83 K. This performance is due to the definition of a thermometric parameter involving the intensity ratio of the ⁵D₀ → ⁷F₂ (Eu³⁺) and ⁴S_3/2 → ⁴I_15/2 (Er³⁺) transitions in which the intensity of the former increases with the increase of the temperature while that of the latter shows the opposite temperature dependence.