Synthesis, characterization, and physical properties of 1D nanostructures
Marley, Peter Mchael
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The roster of materials exhibiting metal--insulator transitions with sharply discontinuous switching of electrical conductivity close to room temperature remains rather sparse despite the fundamental interest in the electronic instabilities manifested in such materials and the plethora of potential technological applications, ranging from frequency-agile metamaterials to electrochromic coatings and Mott field-effect transistors. Vanadium oxide bronzes with the general formula M x V 2 O 5 , provide a wealth of compositions and frameworks where strong electron correlation can be systematically (albeit thus far only empirically) tuned. Charge fluctuations along the quasi-1D frameworks of M x V 2 O 5 bronzes have evinced much recent interest owing to the manifestation of colossal metal--insulator transitions and superconductivity. We start with a general review on the phase transitions, both electronic and structural, of vanadium oxide bronzes in Chapter 1. In Chapter 2, we demonstrate an unprecedented reversible transformation between double-layered (δ) and tunnel (β) quasi-1D geometries for nanowires of a divalent vanadium bronze Ca x V 2 O 5 (x ∼0.23) upon annealing-induced dehydration and hydrothermally-induced hydration. Such a facile hydration/dehydration-induced interconversion between two prominent quasi-1D structures (accompanied by a change in charge ordering motifs) has not been observed in the bulk and is posited to result from the ease of propagation of crystallographic slip processes across the confined nanowire widths for the δ[arrow right]β conversion and the facile diffusion of water molecules within the tunnel geometries for the β[arrow right]δ reversion. We demonstrate in Chapter 3 unprecedented pronounced metal-insulator transitions induced by application of a voltage for nanowires of a vanadium oxide bronze with intercalated divalent cations, β-Pb x V 2 O 5 ( x ∼0.33). The induction of the phase transition through application of an electric field at room temperature makes this system particularly attractive and viable for technological applications. A mechanistic basis for the phase transition is proposed based on charge disproportionation evidenced at room temperature in near-edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements, ab initio density functional theory calculations of the band structure, and electrical transport data suggesting that transformation to the metallic state is induced by melting of specific charge localization and ordering motifs extant in these materials. In Chapter 4, we report the synthesis of single-crystalline δ-Ag 0.88 V 2 O 5 nanowires and unravel pronounced electronic phase transitions induced in response to temperature and applied electric field. Specifically, a pronounced semiconductor--semiconductor transition is evidenced for these materials at ca. 150 K upon heating and a distinctive insulator--conductor transition is observed upon application of an in-plane voltage. An orbital-specific picture of the mechanistic basis of the phase transitions is proposed using a combination of density functional theory (DFT) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Structural refinements above and below the transition temperature, angle-resolved O K-edge NEXAFS spectra, and DFT calculations suggest that the electronic phase transitions in these 2D frameworks are mediated by a change in the overlap of d xy orbitals. The classical orthorhombic layered phase of V 2 O 5 has long been regarded as the thermodynamic sink for binary vanadium oxides and has found great practical utility as a result of its open framework and easily accessible redox states. Concluding with Chapter 5, we exploit a cation-exchange mechanism to synthesize a new stable tunnel-structured polymorph of V 2 O 5 (ζ-V 2 O 5 ) and demonstrate the subsequent ability of this framework to accommodate Li and Mg ions. The facile extraction and insertion of cations and stabilization of the novel tunnel framework is facilitated by the nanometer-sized dimensions of the materials, which leads to accommodation of strain without amorphization. The topotactic approach demonstrated here indicates not just novel intercalation chemistry accessible at nanoscale dimensions but also suggests a facile synthetic route to ternary vanadium oxide bronzes (M x V 2 O 5 ) exhibiting intriguing physical properties that range from electronic phase transitions to charge ordering and superconductivity.