Synthesis and Spectroscopy of Transition-Metal-Based Fluoride and Oxide Nanostructures

Abstract

Bottom-up synthetic methods facilitate control over the atomic positioning, size, and architecture of crystals. This work leverages the parameter space of two bottom-up routes, non-polar solution-phase and molten-salt, to prepare nanoscale ternary and quaternary metal-fluorides and -oxides with precise morphologies and ion doping concentrations. Chapter 2 describes the versatile hydrofluoric acid-free solution-phase synthesis of A2BF6 (A = Na, K, NH4, Cs, N(CH3)4; B = Zr, Ti, Si) nanocrystal colloids, with an emphasis on Zr4+-based compositions. Various morphologies of (NH4)2ZrF6 nanocrystals, ranging from isotropic hexagons to large-aspect-ratio rods, are accessed by altering the reaction temperature, time, and solvent polarity. Conversion of (NH4)2ZrF6 nanocrystals to isomorphic Cs2ZrF6 via an in-situ A-site cation exchange requires a structural reorganization of the labile anion sublattice. The fluoride and oxide lattices studied in this dissertation are large-bandgap insulators, allowing for unimpaired observation of paramagnetic transition-metal (d-d) and lanthanide (f-f) ligand-field transitions within the gap. Utilizing the solution-phase methodology described in Chapter 2, Chapter 3 presents colloidal near-infrared-emitting Cr3+-based fluoride nanocrystals, e.g., Cs2NaCrF6 and Na3CrF6. Yb3+ ions are substitutionally doped into the octahedral Cr3+-site in the nanocrystals. Inefficient Cr3+ → Yb3+ multipolar energy transfer between the disconnected [MF6]3- (M = Cr3+, Yb3+) octahedra causes simultaneous Cr3+ d-d (650 – 950 nm) and Yb3+ f-f (900 – 1100 nm) emission upon exciting into the Cr3+ d-d band at room temperature. In Chapter 4, Yb3+ ions are doped into YCrO3 microcrystals, a canted antiferromagnet with a Néel temperature of 140 K. While Cr3+ 2Eg → 4A2g luminescence is non-radiatively quenched above 40 K, Yb3+ f-f luminescence is observed up to room temperature and acts as an optical probe of the spontaneous magnetic ordering of the Cr3+ sublattice below 140 K. Above 80 K, Yb3+ luminescence predominantly originates from simultaneous-pair excitations of coupled Cr3+-Yb3+ dimers, instead of sensitization from ligand-field Cr3+ d-d excitation. Altogether, this dissertation highlights the advantages of synthesizing multinary fluoride and oxide lattices on the nanoscale, enabling access to complex morphologies and optical properties previously unexplored.