Dissertation Defense: Orlando Stewart
Candidate: Orlando Stewart
Major: Chemistry
Advisor: Sarah L. Stoll, Ph.D.
Title: Synthetic Strategies to Lanthanide and Actinide Chalcogenide and Oxychalcogenide Nanomaterials
Synthetic routes to lanthanide and actinide chalcogenide and oxychalcogenide nanomaterials were developed using various metal and chalcogen sources and compared with the use of single-source precursors. Nanomaterials of type LnxQy (Ln= La, Sm, Eu, Gd, Q = S and Se) and UOQ (Q= S and Se) were characterized via SQUID magnetometry, SEM, TEM, and PXRD and confirmed the ability of these reactions to yield phase-pure products. Time- and temperature-dependent studies highlighted potential mechanistic pathways to the resulting products. Several comparisons were made in the synthesis of uranium oxychalcogenide nanomaterials such as the choice of metal halide (UCl4 vs UBr4 vs UI4), tertiary phosphine chalcogenide (TOPQ vs TPPQ), and nanoparticle strategy (separate metal-chalcogen sources versus single-source precursors). Magnetic characterization showed antiferromagnetic ordering in UOQ nanomaterials, while electron microscopy studies revealed morphological control through phosphine selection.
A novel class of lanthanide dichalcogenoimidodiphosphinate complexes were developed to act as potential single-source precursors (SSPs) to lanthanide chalcogenide nanomaterials. The luminescent properties of these complexes were explored, along with their ability to form phase-pure nanomaterials. Interestingly, divalent and trivalent europium dichalcogenoimidodiphosphinate complexes were synthesized and exhibited temperature-dependent luminescence, providing rare example of molecular luminescence thermometry in both Eu(II) and Eu(III)-based complexes . The mechanisms behind the observed luminescence thermometry were investigated, and lanthanide luminescence sensitization using soft-chalcogen containing ligands was observed for various trivalent lanthanides spanning NIR to visible emitters.
Lastly, lanthanide tris(trimethylsilyl)amides were used as reactive metal sources in nanoparticle reactions to investigate the effect of chalcogen ligand reactivity on phase formation. When compared to lanthanide-based SSPs bearing these ligands, the use of separate metal-chalcogen sources resulted in the formation of various lanthanide chalcogenide phases that could not be obtained using the SSPs. Additionally, novel morphologies were observed in EuS and EuSe nanomaterials produced from reacting europium tris(trimethylsilyl)amide with the corresponding dichalcogenoimidodiphosphinate (NH(QPPh2)2, Q=S or Se) and diselenophosphinate [Et2NH2][Ph2PSe2] ligands.