Doctor of Philosophy (PhD)
Over the past decades, the development of light-emitting diodes (LEDs) to produce a wide range of wavelengths has revolutionized the solid-state lighting industry due to their higher energy efficiency and operational lifetimes. These LEDs employ rare earth (RE) doped phosphors due to their stable emission wavelengths which can be amplified when sensitized by other RE dopants (Yb, Ce) or shell layer passivation. However, there has been a push to replace the RE elements in LEDs due to increased socioeconomic issues. One proposed alternative, transition metal (TM) dopants, is typically avoided due to their susceptibility to the local crystal environment resulting in a wide range of colors based on the host materials. Therefore, this work aims to control the optical and electronic properties of TM doped hosts in a reversible, but stable manner for their deployment in steady luminescent and magnetic applications. Further, the adaptive optical absorption of TM ions is utilized to sensitize the RE emission in a core-shell architecture to obtain dynamic luminescence for solid-state lighting applications. Owing to their multifunctionality, chemical stability, and strong optical responses, TiO2:Ni2+ thin films were chosen as the prototypical system for TM doped hosts. The surface of these films was functionalized with surface dipoles to engineer the interfacial energetics. Since these dipoles primarily act at the surface, surface-sensitive X-ray and UV spectroscopic techniques were employed to probe the changes in the electronic and geometric structure of the inorganic film. Additionally, these results were corroborated with first-principles simulations to elucidate the dipole-dopant interactions and deduce a quantitative relationship between the dipole moment and the change in the optoelectronic properties of TiO2:Ni2+. As a proof-of-concept, the photoluminescence spectra of the RE doped core (NaYF4:Er3+) with TM doped shell layer (TiO2:Ni2+) showed an enhancement in the emission intensity of the Er3+, suggesting energy transfer between the Er3+-Ni2+ ions. Further, surface functionalization of these core-shell nanoparticles demonstrated adaptive absorption of Ni2+, indicating potentiality for dynamic luminescence. Ultimately, these tunable luminescent nanophosphors will align with the interests of the Department of Energy in reducing the RE dependence in clean energy technologies, especially LED lighting.
Darapaneni, Pragathi, "Tunable Luminescence of Rare Earth Doped Nanophosphors via Adaptive Optical Properties of Transition Metals" (2020). LSU Doctoral Dissertations. 5199.