(495b) High-Efficiency Dynamic Lighting with Transition Metal Elements As Sensitizers

The lighting industry has drastically evolved over the past decade in order to provide low-cost, energy efficient lighting with high color quality. In particular, the development of white light light emitting diodes (LEDs), which employ rare earth (RE) based phosphors to produce a wide range of wavelengths due to their unique optical and electronic properties. β-NaYF₄:RE nanoparticles (NPs) are currently employed in personal electronics as stable emission centers because of the steady luminescence of RE elements and low phonon matrix energy of β-NaYF₄ host. Additionally, luminescence can be amplified via co-dopant with a sensitizer agent (Ce³⁺, Yb³⁺, and various transition metals (TM)). These RE doped materials are typically coated with a shell layer to passivate the surface sites and limit the external field effects on the RE. In this work, a new class of TM based phosphors is proposed, specifically a-RE doped core with a TM doped shell layer to maintain LED efficiency while simultaneously controlling the allowed luminescent transitions. The ability to control the emission spectra of these RE elements is due to the dynamic nature of the TM absorption with external stimuli. Despite their susceptible nature, there is motivation to use TMs as a replacement for RE sensitizers based on abundance and absorption cross-section.

For this purpose, β-NaYF₄:Er|TiO₂:Ni core-shell nanoparticles were synthesized by using a combination of thermal decomposition and stöber processes. The surface of the shell layer is functionalized with benzoic acid (BZA) ligands via carboxylic acid chemistry in order to create external chemical dipole. Standard structural and compositional characterizations were performed on these nanoparticles in order to identify the morphology, shape, crystal structure and elemental composition. Next, the optical properties, such as absorption and upconversion luminescence, were investigated via optical spectroscopy (UV-Vis/Photoluminescence), demonstrating the ability to shift the absorption spectra of TM ion and tune the RE-TM energy transfer kinetics for controlled upconversion emission. Furthermore, X-ray Absorption Spectroscopy (XAS) was employed to probe the local site chemical environment surrounding the dopant, indicating the ligand induced dipole, for engineering of the dopant oxidation state within the shell layer without changing the overall charge of the system. Moreover, the system is modeled using time-dependent density functional theory (TD-DFT) simulations based on the concepts of Tanabe-Sugano diagrams and Judd-Ofelt theories, showing good agreement between theory and experiment.