Intense photoluminescence in CaTiO3:Sm3+ phosphors, effect of co-doping singly, doubly and triply ionized elements and their applications in LEDs

Abstract

In this work, Sm³⁺-doped and Sm³⁺/Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺ co-doped CaTiO₃ phosphors were synthesized by a solid-state reaction method at 1473 K. The phase of phosphors was identified to be orthorhombic with space group Pnma (62) by XRD measurements. The morphological properties of the prepared samples were studied by SEM measurements. The average crystallite and particle sizes were found to increase in the presence of modifiers and they follow the trend Li⁺ > Mg²⁺ > Gd³⁺ > K⁺ > Bi³⁺ > Ba²⁺. EDX measurements were used to verify the presence of Ca, Ti, O, Sm, K, Mg, Ba, Gd and Bi atoms in the prepared phosphor samples. The Sm³⁺ ion shows emission peaks at 564, 599 and 646 nm due to ⁴G_5/2 → ⁶H_5/2, ⁶H_7/2 and ⁶H_9/2 transitions upon 407 nm excitation, among which the peak situated at 599 nm has maximum emission intensity. Concentration quenching was observed above 2 mol% of Sm³⁺ ions in this host. However, the emission intensity of Sm³⁺ peaks can be enhanced using different modifier (Li⁺/K⁺/Mg²⁺/Ba²⁺/Gd³⁺/Bi³⁺) ions. It was found that the size (ionic radii) and charge compensation of the ion together play a dominant role. The enhancement is more after co-doping with smaller radius ions (Li⁺, Mg²⁺ and Gd³⁺), among which Li⁺ shows the largest enhancement. This is because ions of smaller size will be able to go closer to the activator ion and the charge imbalance causes a larger field. The CIE color coordinates, correlated color temperature (CCT) and color purity of the phosphors were calculated and show orange-red color emissions with a maximum color purity of ∼93% in the case of CaTiO₃:2Sm³⁺/1.0Li⁺ phosphor. The lifetime value is increased in the presence of these ions. It is again maximum for the Li⁺ co-doped CaTiO₃:2Sm³⁺ phosphor sample. Thus, the prepared phosphor samples are suitable sources for orange-red light.