Speaker
ye li
(uwo)
Description
Strontium aluminate co-doped with europium and dysprosium [SrAl2O4(Eu2+,Dy3+)] is a popular long-lasting (~10 hrs) phosphor for security signs, medical diagnostics, and other applications. Although it has been 20 years since the discovery of persistent luminescence (PL) in SrAl2O4(Eu2+,Dy3+), the associated physical mechanism is still unclear. Electron spin resonance (ESR) is a powerful technique to investigate the excitation of paramagnetic centers in solids, and the associated energy transfer processes. It was previously observed [1] that the intensity of the ESR signal of SrAl2O4(Eu2+,Dy3+) decreases under illumination and is restored upon remission of light during PL, a phenomenon that has been taken as evidence of the transformation of ESR-active Eu2+ centers into diamagnetic Eu3+ under illumination, and their recovery upon PL remission. Here, we present the ESR spectra of SrAl2O4(Eu2+,Dy3+) in the X microwave band, in the dark and under illumination by violet (405 nm) light. At high enough microwave field or sufficiently long and intense illumination, a "negative" ESR signal is observed, which, clearly, cannot be associated to negative concentrations of Eu2+ centers. Our findings put in question the validity of the current interpretation of the ESR spectra and PL mechanism in SrAl2O4 (Eu2+,Dy3+). We propose that the transformation of the ESR spectra at high microwave power and under illumination is due to dephasing of the ESR signal as a consequence of extremely long spin-lattice relaxation times in (Eu2+,Dy3+)SrAl2O4, a phenomenon that may also be responsible to persistent luminescence.
[1] Clabau, Frédéric, et al. Chemistry of materials 17.15 (2005): 3904-3912.
Author
ye li
(uwo)
Co-authors
Dr
Arash Akbari-Sharbaf
(western university)
Giovanni Fanchini
(The University of Western Ontario)
Mr
Patrick Murphy
(western university)
Prof.
yan zhao
(beihang university)
