Peter M. Marley

Atomic Origins of Monoclinic-Tetragonal (Rutile) Phase Transition in Doped VO2 Nanowires.

There has been long-standing interest in tuning the metal-insulator phase transition in vanadium dioxide (VO2) via the addition of chemical dopants. However, the underlying mechanisms by which doping elements regulate the phase transition in VO2 are poorly understood. Taking advantage of aberration-corrected scanning transmission electron microscopy, we reveal the atomistic origins by which tungsten (W) dopants influence the phase transition in single crystalline WxV1-xO2 nanowires. Our atomically resolved strain maps clearly show the localized strain normal to the (122̅) lattice planes of the low W-doped monoclinic structure (insulator). These strain maps demonstrate how anisotropic localized stress created by dopants in the monoclinic structure accelerates the phase transition and lead to relaxation of structure in tetragonal form. In contrast, the strain distribution in the high W-doped VO2 structure is relatively uniform as a result of transition to tetragonal (metallic) phase. The directional strain gradients are furthermore corroborated by density functional theory calculations that show the energetic consequences of distortions to the local structure. These findings pave the roadmap for lattice-stress engineering of the MIT behavior in strongly correlated materials for specific applications such as ultrafast electronic switches and electro-optical sensors.

Reversible interconversion of a divalent vanadium bronze between δ and β quasi-1D structures.

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. Depending upon the nature of the intercalating cation (M), distinctive geometries of the V(2)O(5) framework are accessible. Herein, 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 → β conversion and the facile diffusion of water molecules within the tunnel geometries for the β → δ reversion.

Electron lone pair distortion facilitated metal-insulator transition in β-Pb0.33V2O5 nanowires

The electronic structure of β-Pb0.33V2O5 nanowires has been studied with x-ray photoelectron spectroscopy techniques. The recent synthesis of defect-free β-Pb0.33V2O5 nanowires resulted in the discovery of an abrupt voltage-induced metal insulator transition. First principle calculations predicted an additional V-O-Pb hybridized “in-gap” state unique to this vanadium bronze playing a significant role in facilitating the transition. We confirm the existence, energetic position, and orbital character of the “in-gap” state. Moreover, we reveal that this state is a hybridized Pb 6s–O 2p antibonding lone pair state resulting from the asymmetric coordination of the Pb2+ ions.

Proliferation of metallic domains caused by inhomogeneous heating near the electrically driven transition inVO2nanobeams

We discuss the mechanisms behind the electrically driven insulator-metal transition in single crystalline VO

Direct evidence of M2 phase during the monoclinic-tetragonal (rutile) phase transition of W-doped VO2 nanowires

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Structure-induced switching of the band gap, charge order and correlation strength in ternary vanadium oxide bronzes

nanobeams. Our DC and AC transport measurements and the versatile harmonic analysis method employed show that non-uniform Joule heating causes phase inhomogeneities to develop within the nanobeam and is responsible for driving the transition in VO

Atomic Resolution Study of Local Strains in Doped VO 2 Nanowires

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Transformers: the changing phases of low-dimensional vanadium oxide bronzes

. A Poole-Frenkel like purely electric field induced transition is found to be absent and the role of percolation near and away from the electrically driven transition in VO

Emptying and filling a tunnel bronze

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Doped Nanoparticles and Methods of Making and Using Same

is also identified. The results and the harmonic analysis can be generalized to many strongly correlated materials that exhibit electrically driven transitions.