Lattice Strain in Li+ -Doped NIR-Upconverting Crystals: A Bridge between Photoluminescence Intensity and Local Disorder

Abstract

Explaining the intensification of upconversion luminescence (UCL) from single phase NIR-upconverting (UC) crystal due to non-lanthanide dopant insertion appears challenging in absence of proper mechanistic investigation. This PhD thesis aimed to extend and complement the existing hypothesis of symmetry distortion of UC lattice due to non-lanthanide dopant (Li+) insertion through experimental evidence. Through extensive synchrotron-based X-ray probing, an attempt has been made to understand the variations in the structural attributes of UC crystals generated through non-lanthanide ion (Li+) doping. The context and the significance of this dissertation have been presented in the first chapter as Introduction. First, the Li+ ion was doped in a hexagonal NaYF4 host system containing Yb3+ as a sensitizer with different activator ions (Ho3+, Er3+, Tm3+), and the change in UCL intensity of green and blue emission and other structural parameters were analyzed. While the details are included in the third chapter of this thesis (Crystal Growth and Design, 2020, 20, 468−478 DOI: 10.1021/acs.cgd.9b01426, CrystEnggComm, 2021, 23, 8631–8640 DOI: 10.1039/D1CE01253C), the overall methodology and general characterization techniques have been included in the second chapter. Next, the same study was performed on white light-emitting hexagonal Yb3+/NaYF4 host using a combination of Er3+ and Tm3+ activator ions. The method of thermal decomposition was adopted to synthesize Li+ doped WEN UC crystals. Detailed characterization of these materials was performed using synchrotron X-ray diffraction analysis (SXRD), XAFS, TEM, FESEM, XPS, etc. The details are included in the fourth chapter of this thesis. (The Journal of Physical Chemistry C, 2021, 125, 21211−21222, DOI: 10.1021/acs.jpcc.1c06480). 2 Further, in the fifth chapter, the comparative study was verified on a completely different host system, which is cubic Gd2O3 having Yb3+ as sensitizer and Ho3+ as the activator ion. All the crystalline systems were subjected to synchrotron X-ray diffraction (SXRD)-based in-depth structural analysis. In all three studies, the UCL intensity behavior was found to follow a specific trend. However, no morphological or structural lattice parameters appeared successful to explain the intensity behavior. Next in the sixth chapter, the lattice strain, which often reflects structural intricacy at the local level is conventionally assessed using Williamson-Hall (WH) plot derived from XRD data. The study of crystal lattice strain using the Williamson Hall method on the obtained SXRD data revealed a unique correlation between the variation of UCL intensity and that of lattice strain
Finally, high energy X-ray scattering experiments using Pair Distribution Function Analysis on the selected samples from hexagonal NaYF4 host system and cubic Gd2O3 host system were performed. Concerned analysis showed that the corresponding lattice strain behavior that shared a proportional relationship with the associated UCL intensity was actually linked with the Li+-induced respective local disorders at the atomic scale. The findings reported herein are expected to open up a fresh perspective in further understanding the physics of upconversion host lattices.