SrAlxOy:Eu2+, Dy3+ (x = 4) nanostructures: Structure and morphology transformations and long-lasting phosphorescence properties

Abstract

Long-lasting phosphor SrAl_xO_y:Eu²⁺, Dy³⁺ (x = 4) nanostructures were synthesized through a facile but efficient combustion technique followed by a post-annealing reaction at different temperatures. During the synthesis process, triethanolamine (TEA) can not only be used as an organic fuel, but also can control the morphology of nanostructured combustion products. With an increase in post-annealing temperature, their morphology transforms from nanosheets into semi-curled nanobelts due to the crystal growth and the edge curl effect, and moreover, the products with various crystallographic structures can be obtained, whose transition presents the following rule: amorphous → hexagonal β-SrAl₂O₄ (1000 °C) → monoclinic SrAl₄O₇ (1100 °C) → orthorhombic Sr₄Al₁₄O₂₅ (1300 °C). Excess Al facilitates the stabilization of the β-SrAl₂O₄ high-temperature phase at room-temperature. Due to the difference of Eu²⁺ surroundings in various crystallographic structures, their excitation and emission spectra exhibit some discrepancy in peak position and intensity. Surface and boundary defects, originating from small size and poor crystallinity, can both increase trap densities and depth levels; while Sr vacancies (V_Sr), donated by the deficiency of Sr²⁺ ions in β-SrAl₂O₄ and Sr₄Al₁₄O₂₅, mainly increase the concentration of relatively shallow traps. Thus, β-SrAl₂O₄ shows a relatively small afterglow decay constant compared to SrAl₄O₇. Moreover, the afterglow decay constant of Sr₄Al₁₄O₂₅ is initially much smaller than those of β-SrAl₂O₄ and SrAl₄O₇, then increases gradually, and finally exceeds them. Additionally, the maximum value of initial brightness and effective afterglow time emerges in the SrAl₄O₇ and Sr₄Al₁₄O₂₅ composite phosphor annealed at 1200 °C.