Luminescence and up-conversion properties in La2Ti2O7:Eu3+,Er3+ oxides under UV and NIR radiations towards a two-color sensor

Abstract

La2-x-yEuxEryTi2O7 oxides have an emission color which depends on the excitation wavelength. Under the UV excitation (310–393 nm), they emit in the red spectral region and, under NIR excitation (785–980 nm), they exhibit a green emission induced by upconversion luminescence from Er3+ ions. The structural and luminescence properties of these oxides synthesized by the solid state reaction method were studied according to the x and y values. The co-substituted oxides are obtained over a composition zone between x ≤ 0.2 and y ≤ 0.05. Beyond these values, a phase mixture consisting of a monoclinic phase (layered-perovskite) and cubic phase (pyrochlore) is observed. Under the UV excitation, the reddish emission color is mainly induced by the Eu3+ ions via an exchange interaction mechanism. The quenching concentration of Eu3+ ions is dependent on the excitation wavelength: x = 0.3 and 0.6, at 310 and 377 nm, respectively. With the measurements of photoluminescence decay curves, the lifetime kinetics was linked to the Eu3+ ions concentration. The CIE 1931 color coordinates were also determined according to the x and y values. This one shows linear evolutions when x is fixed and y is variable and vice versa. The good chemical and thermal stability of these oxides also make it possible to disperse them effectively in a glass of lithium tetraborate. Only a 2% transmittance drop is observed for the insertion of 2 wt. % of the oxide. The upconversion luminescence properties were also measured under NIR excitation and shown an emission in the green spectral region mainly due to Er3+ ions. The study of emission intensity dependence on the laser power led to the conclusion of a two-photon excitation process. Temperature-dependent upconversion emission follows a Mott-Seitz model. The activation energy for 4F9/2 → 4I15/2 quenching was found to be 52.8 meV.