Atomic-level Disorder and Microstrain in Near-Infrared Upconverting Core-Shell Nanocrystals due to Induced Lattice Defects

Abstract

The UC process is dependent on the Laporte parity selection rule, which governs the upconversion luminescence (UCL) intensity through perturbations of crystal symmetry. The non-lanthanide alkali metal ion such as Li+ incorporation in the core upconversion (UC) nanocrystal's host lattice led to UCL intensification. This dissertation is based on providing experimental support to the existing quantum mechanical Laporte forbidden f-f transition hypothesis. The variation of structural attributes in Li+-doped core and core-shell NCs was studied using high-brilliance synchrotron X-ray radiation covering a wide range of energy. The work's significance and background information are described in the introduction of this dissertation. First, three different sets of UC nanocrystals (green emissive, blue emissive, and white emissive) were synthesized by the varying concentration of Li+ through the coprecipitation method, and structural analysis was performed from the high brilliance synchrotron XRD. The single phase was not obtained beyond the 60% of the Li+ incorporation in each set of UCNCs. Therefore, epitaxial shelling was performed on only these core UCNCs to obtain the efficient core-shell UCNCs responsive to 808 nm. Chapters 2 and 3 comprise the synthesis and various characterizations of core and core-shell samples. Rietveld refinement coupled with neutron diffraction (ND) was used to confirm the location of Li+, preferably occupying the octahedral voids along with some lattice positions. The details of structural attributes determination by synchrotron XRD and ND for core samples were described in Chapter 2 of this thesis (Nanoscale, 2025, 17, 2269-2280. DOI: https://doi.org/10.1039/D4NR03951C). All the structural attributes corresponding to core-shell are provided in Chapter 3. The existing hypothesis of symmetry distortion of a UCL intensity enhancement were performed by chemical manipulation through the Introduction of symmetry perturbing agent Li+ in the Host lattice of UCNCs. The effect Li+ was tracked by Williamson-Hall analysis of microstrain calculation using the integral breadth method. The reversal of microstrain behavior was observed from core to core-shell due to epitaxial shell growth over the core UCNCs. The effect of microstrain and misfit strain on Li+ incorporation and epitaxial shell growth was described in chapter4 (Nano Lett. 2024, 24, 21, 6320-6329, DOI: https://doi.org/10.1021/acs.nanolett.4c01077). Further, the electron density maps were described for all the core and core-shell samples. Finally, in Chapter 5, an atomic-level interpretation of such microstrain was performed using atomic pair distribution function analysis (PDF) of the high-energy total scattering and EXAFS study. From the atomic PDF analysis, the various parameters, such as Rw, bond length, and experimental PDF peak width, were measured as the key parameters for local disorder. These parameters, obtained from the PDF, were analyzed for all the core and core-shell samples with the incorporation of Li+. In Chapter 6, the relationship between average structure and local structure variation with UCL intensity were analyzed. Finally, by correlating the UCL intensity with the structural variation (average and local), it was observed that the UCL intensity and symmetry distortion was maximum at a certain degree of Li+ incorporation in UCNCs (core and core-shell). The variation of local disorderedness shared a proportional relationship with the UCL intensity. These results pave the experimental support to the existing hypothesis of quantum mechanical orbital parity selection (Laporte selection) rule for symmetry distortion, which led to UCL intensity enhancement and the atomic level crystal lattice engineering.