Correlation among photoluminescence and the electronic and atomic structures of Sr 2 SiO 4 :xEu 3+ phosphors: X-ray absorption and emission studies

A series of Eu 3+ -activated strontium silicate phosphors, Sr 2 SiO 4 :xEu 3+ (SSO:xEu 3+ , x = 1.0, 2.0 and 5.0%), were synthesized by a sol–gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X-ray powder diffraction analysis revealed the formation of mixed orthorhombic α'-SSO and monoclinic β-SSO phases of the SSO:xEu 3+ phosphors. When SSO:xEu 3+ phosphors are excited under ultraviolet (UV) light (λ = 250 nm, ~ 4.96 eV), they emit yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 and 703 nm) PL bands. These PL emissions typically correspond to 4 f –4 f electronic transitions that involve the multiple excited 5 D 0 → 7 F J levels (J = 1, 2, 3 and 4) of Eu 3+ activators in the host matrix. This mechanism of PL in the SSO:xEu 3+ phosphors is strongly related to the local electronic/atomic structures of the Eu 3+ –O 2− associations and the bandgap of the host lattice, as verified by Sr K -edge and Eu L 3 -edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure, O K -edge XANES and K α X-ray emission spectroscopy. In the synthesis of SSO:xEu 3+ phosphors, interstitial Eu 2 O 3 -like structures are observed in the host matrix that act as donors, providing electrons that are nonradiatively transferred from the Eu 5 d and/or O 2 p –Eu 4 f/ 5 d states (mostly the O 2 p –Eu 5 d states) to the 5 D 0 levels, facilitating the recombination of electrons that have transitioned from the 5 D 0 level to the 7 F J level in the bandgap. This mechanism is primarily responsible for the enhancement of PL emissions in the SSO:xEu 3+ phosphors. This PL-related behavior indicates that SSO:xEu 3+ phosphors are good light-conversion phosphor candidates for use in near-UV chips and can be very effective in UV-based light-emitting diodes.