Origin of the active luminescence from Sm3+-activated borate phosphors: a correlational study of trap states and decay kinetics

Abstract

The photoluminescent (PL) properties of Sm³⁺-doped strontium hexaborate (SrB₆O₁₀) phosphors are significantly influenced by their crystallinity, phase and dopant concentration. To obtain a high brightness orange-red emitting SrB₆O₁₀:Sm³⁺ phosphor, a controlled and modified soluble-salt reaction method in the presence of a chelating agent (citric acid, C₆H₈O₇) was used at ∼700 °C for about half an hour under an air atmosphere, which is otherwise only possible at very high (>1500 °C) sintering temperatures. The PL spectrum of the SrB₆O₁₀:Sm³⁺ phosphor produced strong and intense PL emission peaks centered at 560 nm (⁴G_5/2 → ⁶H_5/2), 596 nm (⁴G_5/2 → ⁶H_7/2), and 640 nm (⁴G_5/2 → ⁶H_9/2) under excitation of 400 nm (⁶H_5/2→ ⁶H_7/2). This is one of the very few reports that depicts the formation of more shallow traps in the lattice and the concentration quenching phenomenon was attributed to the non-radiative transfer of energy among Sm³⁺ ions via dipole–dipole interactions in the trap states. The optimized phosphor has noticeable reddish-orange Commission International de I’Eclairage color coordinates of (0.459, 0.337). The PL decay kinetics of the synthesized phosphor showed a biexponential decaying characteristic with an overall decay time of milliseconds. The presence of borate ions benefitted the thermal stability of the SrB₆O₁₀:Sm³⁺ phosphor as well. X-ray diffraction studies confirmed the phase purity, while scanning electron microscopy revealed the morphology to be an agglomerated rod-like structure. The PL intensity increased with increasing Sm³⁺ concentration and reached the optimum at 0.1 mol%. The obtained results clearly indicate that the SrB₆O₁₀:Sm³⁺ phosphor is a viable material for providing a red component for white-light emitting diodes (w-LEDs).