Structure-Composition Relationships and Their Influence on Long Luminescent Lifetimes in Persistent Luminescent Phosphors



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Persistent luminescence (PersL) is an optical phenomenon found in inorganic materials, where a luminescent center, e.g., Eu2+ or Cr3+, is substituted into a host inorganic crystal structure, producing visible light on the order of minutes to hours. The mechanism driving PersL is generally agreed to arise from a relationship between the host structure’s bandgap and any lattice defects stemming from anion vacancies, anti-site defects, and co-dopants, e.g., Dy3+, that form electron traps. Research has primarily focused on establishing methods to improve the long luminescent lifetimes of known persistent luminescent phosphors (PLPs) and the extension of these materials into medical diagnostic applications. However, the PLPs available today have a limited range of emission wavelengths, low efficiency, and lack chemically stability. Developing novel PLPs requires a fundamental understanding of the relationship between the crystal structures and chemical compositions that bring about these distinctive optical properties. Until recently, it was thought that the 5d-orbitals present from a co-dopant were mainly responsible for creating electron traps; yet, thermoluminescence experiments proved a co-dopant was not necessary to observe PersL. This supports the notion that lattice defects are the main electron traps driving PersL. The work presented herein addresses this question using a combination of experimental and computational chemistry tools to identify the intrinsic nature of the electron traps and their relationship to the optical properties. Detailed structural studies were first employed using high resolution synchrotron X-ray diffraction to show compositional control in a solid solution, while X-ray absorption spectroscopy revealed distortions around the luminescent center are linked to anti-site defects. Next, thermoluminescence spectroscopy and computational modeling provided the link between chemical composition and presence of lattice defects to the efficiency of PersL. Finally, a novel synthesis approach to particle size reduction demonstrated an improvement to PersL by inducing surface defects while maintaining chemical composition. The results highlight the importance of investigating the connection between lattice defects, structural properties, e.g., local polyhedral distortion, and optical properties such as PersL. Together, a stronger fundamental understanding of crystal structures and compositions of PLPs will advance the discovery of novel materials.



Lattice defects, Solid solutions


Portions of this document appear in: Finley, Erin, Angelica Cobb, Anna Duke, Andrew Paterson, and Jakoah Brgoch. "Optimizing blue persistent luminescence in (Sr1− δBaδ) 2MgSi2O7: Eu2+, Dy3+ via solid solution for use in point-of-care diagnostics." ACS applied materials & interfaces 8, no. 40 (2016): 26956-26963. And in: Finley, Erin, and Jakoah Brgoch. "Deciphering the loss of persistent red luminescence in ZnGa 2 O 4: Cr 3+ upon Al 3+ substitution." Journal of Materials Chemistry C 7, no. 7 (2019): 2005-2013. And in: Finley, Erin, Aria Mansouri Tehrani, and Jakoah Brgoch. "Intrinsic Defects Drive Persistent Luminescence in Monoclinic SrAl2O4: Eu2+." The Journal of Physical Chemistry C 122, no. 28 (2018): 16309-16314. And in: Finley, Erin, Andrew S. Paterson, Angelica Cobb, Richard C. Willson, and Jakoah Brgoch. "Reducing particle size of persistent luminescent SrAl 2 O 4: Eu 2+, Dy 3+ via microwave-assisted, reverse micelle synthesis." Optical Materials Express 7, no. 7 (2017): 2597-2616.