2, Louisiana State University, Baton Rouge, Louisiana, United States
3, Center for Advanced Microstructures and Devices, Baton Rouge, Louisiana, United States
Rare earth (RE) elements are frequently employed in many applications, such as catalysis, magnetism, phosphor lighting etc., due to their unique properties stemming from the steady 4f energy levels. Recently, there has been a push to replace these RE elements due to increased economic and national security issues. One proposed family of alternatives; are transition metal (TM) dopants, but are typically avoided because of their field dependent properties. For example, the strong overlap of the TM d orbitals with surrounding ligand is often viewed as an unfavourable phenomenon for luminescent or magnetic applications. In the current project, the field dependent hybridization of TM is utilized to engineer the d energy levels, allowing for controlled optoelectronic properties of TM ion.
In this work, thin films of TiO2:Ni (~ 50 nm) were deposited onto Si substrates by employing sol-gel chemistry and spin-coating techniques. Weak localized external fields were created by surface functionalization of these films with benzoic acid ligands. Initial structural and optical characterization on TiO2:Ni was performed to determine the crystal structure and crystal field splitting. X-ray photoelectron and soft X-ray absorption spectroscopy studies were used to investigate the ability to manipulate Ni core and valence levels without affecting the oxidation states or crystal structures. Additionally, the band structure of surface modified TiO2:Ni, investigated by a combination of ultraviolet photoelectron and X-ray emission spectroscopy techniques, revealed that the interband Ni 3d states suffered a shift towards/away from the valence band depending upon the electron withdrawing/donating nature of the ligand. Furthermore, this phenomenon was theoretically supported by the ligand field multiplet calculations and time-dependent density functional theory simulations on bulk-mimicking TiO2:Ni2+ clusters. These preliminary results on TiO2:Ni demonstrate that the reversible tuning of TM d energy levels can be exploited to as the first step to the substitution of RE elements in phosphors that can have controlled luminescent properties.