(388d) Reversible Tuning of the Optoelectronic Properties of Transition Metal Doped Semiconductor Materials for Adaptive Luminescence

Inorganic semiconductor-based flexible electronics are receiving widespread attention in multiple fields such as flat-panel displays, electronic textiles, sensors, batteries, etc. owing to their superior electrical properties and excellent stability. However, the main challenge in these materials is their ability to switch their response in a reversible fashion. Currently, external stimuli (e.g. pressure, temperature) are applied on these inorganic semiconductor materials to modify their optoelectronic response, ultimately reverting to their initial state upon relaxation. The current work demonstrates the reversible tuning of transition metal-based nanophosphors using weak external fields (chemical dipoles). As a proof-of-concept, the adaptive optical properties of these phosphors are coupled with steady emissions of rare earth (RE) dopants for dynamic luminescent applications.

Ni doped TiO₂ thin films, synthesized via sol-gel chemistry, were characterized to identify effect of surface dipoles (para-substituted benzoic acid ligands) on their optoelectronic and structural properties. The local structural and electronic changes around Ni²⁺ with the ligand were investigated via soft x-ray absorption spectroscopy (XAS) and in situ ultraviolet photoelectron spectroscopy (UPS), respectively. Upon functionalization with an electron withdrawing ligand, a distinct shift in the t_2g:e_g filling and energy levels were observed in the Ni L_II edge XAS spectra and was attributed to a local geometry distortion with a change in the electron density based on DFT simulations. Additionally, the direction of the valence band bending and valence band electronic structure of the TiO₂:Ni²⁺ thin films was characterized with UPS. The bending was observed to be a function of the surface dipole and coverage, and the original state could be recovered due to the weak chemisorption of the benzoic-acid ligand molecules on the surface of the TiO₂:Ni²⁺. Finally, a core-shell structure was synthesized to facilitate energy transfer between the Ni and RE dopants for enhanced upconversion luminescence. Er³⁺ doped NaYF₄ nanoparticles were synthesized as the core NPs using colloidal chemistry and Ni²⁺ doped shell layer was coated to have ligand dependent absorption. The ligand-induced shifts in the optical absorption (~ 50 nm) and the emission spectra of these surface-modified core-shell phosphors were determined by UV-Vis and photoluminescence (PL) measurements. Additionally, the excited state energy transfer kinetics between the Er-Ni ion couple were extracted from the lifetime decay measurements. Ultimately, these adaptive luminescent phosphors have the potential to reduce the RE dependence in light-emitting diodes, anti-counterfeit technologies, bio-detection, etc.