INVESTIGATING THE ORIGINS OF LONG PERSISTENCE IN STRONTIUM ALUMINATE PHOSPHORS BY NANOSCALE RESOLVED IMAGING AND SPECTROSCOPY
Güliz İnan Akmehmet
MAT, PhD Dissertation, 2015
Assoc. Prof Cleva Ow-Yang (Thesis Advisor), Assoc. Prof. Saso Sturm, Prof. Dr. Mehmet Ali Gülgün, Assoc. Prof. Burç Mısırlıoğlu, Prof. Dr. Ender Suvacı
Date & Time: 28.07.2015, 10:30 AM
Place: FENS L068
Keywords : Strontium aluminate phosphors, scanning transmission electron microscopy, long persistence, annular dark/bright field imaging, europium, boron, nanoscale imaging.
The objective of this work is to elucidate the role of B as a dopant in the structural formation of long persistence (SrO)4(Al2O3)7 doped with Eu2+, Dy3+. Boron is known to extend dramatically the afterglow persistence, which was serendipitously discovered when it was used as flux agent during processing of the ceramic phosphor. Preparation of the stoichiometric Sr4Al14O25 compound doped with 1 mol% Eu3+ and 1 mol% Dy3+ by sol-gel processing (a modified Pechini method) produces a mixture of phases as revealed by x-ray diffraction analysis: Sr4Al14O25, SrAl12O19, SrAl4O7, and other non-equilibrium phases. However, the presence of boron (i.e., 0.5, 2, and 4.5 mol% B) results in a predominantly Sr4Al14O25 phase with a limited presence of SrAl12O19. Reducing Eu3+ to Eu2+ is necessary in all compounds, in order for persistence to be observed in the visible spectral range.
We investigated the effect of B incorporation on the crystallization kinetics by thermal analysis, x-ray diffraction analysis, and on microstructural evolution by advanced TEM techniques, such as HAADF, ABF imaging. To image point defects, we take advantage of the sub-angstrom-diameter probe in a spherical aberration-corrected scanning transmission electron microscope (STEM), in order to peer into the atomic arrangements. We performed STEM image simulations in order to interpret the experimental data and determined the conditions to probe Eu-dopant. To monitor the cation stoichiometry throughout the powder processing, we performed inductively coupled plasma-optical emission spectroscopy analysis, in addition to energy dispersive x-ray (EDX) spectroscopy using a probe of sub-nanometer diameter. Finally, we investigated the correlation between structure and optical properties by performing photoluminescence excitation and emission spectroscopy and nanoscale cathodoluminescence (nano-CL).
Atomic resolution STEM imaging reveals a direct confirmation that Eu is incorporated into Sr2+ sub-lattice sites, which has until now been proven only indirectly. Moreover, STEM-EDX and nano-CL reveal that B suppresses the formation of highly anisotropic plate-like grains, which collect most of the Eu and Dy from the surrounding microstructure, in addition to promoting a more uniform distribution of Eu2+ both of the crystallographically non-equivalent Sr2+ sites in S4A7. In fact, this last point is the key result from this study—the uniform distribution of Eu2+ is a necessary condition for the extended persistence in S4A7EDB. With such unprecedented correlation between structure and optical properties at the nanoscale, our results invite revision to existing models of persistence in strontium aluminate phosphors.