A facile, rapid, and scalable electrophoretic deposition approach is developed for the fabrication of large-area chemically derived graphene films on conductive substrates based on the electrophoretic deposition of graphene oxide and reduced graphene oxide components. Two distinctive approaches for fabricating conformal graphene films are developed. In the first approach, graphene oxide sheets are electrophoretically deposited from an aqueous solution after the oxidation of graphite to graphite oxide and the subsequent exfoliation of graphite oxide to graphene oxide. Next, the graphene oxide films are reduced via dip-coating in an aqueous solution of hydrazine. In the second approach, graphene oxide is reduced to graphene nanosheets in a strongly alkaline solution and the reduced graphene sheets are directly electrophoretically deposited onto conductive substrates. The film thickness can be modified by the deposition time and the obtained films span several square millimeters in area. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy is used to study the surface chemistry, electronic band structure, and degree of alignment of the electrophoretically deposited films. Polarized NEXAFS measurements verify the presence of epoxide surface functionalities on the graphene basal planes and indicate significant recovery of extended π-bonded networks upon defunctionalization by hydrazine treatment. These measurements further indicate significantly improved alignment of the graphene sheet components of the films parallel to the substrate surface when defunctionalization is performed prior to electrophoretic deposition.

The first-order metal-insulator phase transition in VO(2) is characterized by an ultrafast several-orders-of-magnitude change in electrical conductivity and optical transmittance, which makes this material an attractive candidate for the fabrication of optical limiting elements, thermochromic coatings, and Mott field-effect transistors. Here, we demonstrate that the phase-transition temperature and hysteresis can be tuned by scaling VO(2) to nanoscale dimensions. A simple hydrothermal protocol yields anisotropic free-standing single-crystalline VO(2) nanostructures with a phase-transition temperature depressed to as low as 32 degrees C from 67 degrees C in the bulk. The observations here point to the importance of carefully controlling the stoichiometry and dimensions of VO(2) nanostructures to tune the phase transition in this system.

The peculiarities in the electronic structure of the seemingly simple binary vanadium oxide VO2, as manifested in a pronounced metal−insulator phase transition in proximity to room temperature, have made it the subject of extensive theoretical and experimental investigations over the last several decades. We review some recent advances in theoretical treatments of strongly correlated systems along with ultrafast measurements of VO2 samples that provide unprecedented mechanistic insight into the nature of the phase transition. Scaling VO2 to nanoscale dimensions has recently been possible and has allowed well-defined VO2 nanostructures to serve as model systems for measurements of intrinsic properties without obscuration from grain boundary connectivities and domain dynamics. Geometric confinement, substrate interactions, and varying defect densities of VO2 nanostructures give rise to an electronic and structural phase diagram that is substantially altered from the bulk. We postulate that design principles deduced from fundamental understanding of phase transitions in nanostructures will allow the predictive and rational design of systems with tunable charge and spin ordering.

The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid-solid phase transitions such as those involved in metal-insulator switching, ferroelectricity, and superconductivity. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution W x V 1Àx O 2 . Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal / insulator and insulator / metal phase transformations. Both transitions are depressed to lower critical temperatures upon the incorporation of substitutional tungsten dopants but the impact on the former transition seems far more prominent. In general, the depression in the critical temperatures upon tungsten doping far exceeds corresponding values for bulk W x V 1Àx O 2 of the same composition. Notably, the depression in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W 6+ doping in the W x V 1Àx O 2 nanobelts.

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The substitutional doping of Mo within VO2 substantially alters the electronic and structural phase diagrams of the host lattice, most notably by bringing the technologically relevant metal–insulator phase transition temperature in closer proximity to room temperature. Here, we have used X-ray absorption fine structure (XAFS) spectroscopy at V and Mo K-edges to examine the local electronic and geometric structure of both the dopant atoms and the host lattice. A nominal Mo oxidation state of +5 has been determined, which implies electron doping of the VO2 band structure. In addition, XAFS studies suggest that doping with Mo creates locally symmetric domains that are more akin to the high-temperature rutile phase of VO2, thereby lowering activation energy barriers for structural transformation to the metallic phase. Substantial rectification of octahedral canting is also observed upon Mo doping, which has the effect of decreasing V 3d–O 2p hybridization and likely assists in closing the characteristic band gap of the low-temperature monoclinic phase. A correlated set of cationic interactions is seen to emerge with increasing Mo doping, which can be ascribed to local dimerization along the rutile c axis as has been proposed to be a characteristic structural feature of a correlated metallic phase with intermediate mass.

Results from transport measurements in individual WxV1−xO2 nanowires with varying extents of W doping are presented. An abrupt thermally driven metal-insulator transition (MIT) is observed in these wires and the transition temperature decreases with increasing W content at a pronounced rate of -(48-56) K/at.%W , suggesting a significant alteration of the phase diagram from the bulk. These nanowires can also be driven through a voltage-driven MIT and the temperature dependence of the insulator to metal and metal to insulator switchings are studied. While driving from an insulator to metal, the threshold voltage at which the MIT occurs follows an exponential temperature dependence (V T H↑ ∝ exp( −T /T 0 ))whereas driving from a metal to insulator, the threshold voltage follows V T H↓ ∝ √ Tc − T and the implications of these results are discussed.

Considerable recent research interest has focused on mapping the structural phase diagrams of anisotropic VO(2) nanobeams as model systems for elucidating single-domain behavior within strongly correlated electronic materials, to examine in particular the coupling of lattice and orbital degrees of freedom. Nevertheless, the role of substitutional doping in altering the phase stabilities of competing ground states of VO(2) remains underexplored. In this study, we use individual nanowire Raman microprobe mapping to examine the structural phase progressions underlying the metal-insulator transitions of solution-grown W(x)V(1-x)O(2) nanowires. The structural phase progressions have been monitored for three distinctive modes of inducing the electronic metal-insulator phase transition: as a function of (a) W doping at constant temperature, (b) varying temperature for specific W dopant concentrations, and (c) varying applied voltage for specific W dopant concentrations. Our results suggest the establishment of a coexistence regime within individual nanowires wherein M1 and R phases simultaneously exist before the percolation threshold is reached and the nanowire becomes entirely metallic. Such a coexistence regime has been found to exist during both temperature- and voltage-induced transitions. No evidence of an M2 phase is observed upon inducing the electronic phase transition by any of the three distinctive methods (temperature, doping, and applied voltage), suggesting that substitutional tungsten doping stabilizes the M1 phase over its M2 counterpart and further corroborating that the latter phase is not required to mediate M1→R transformations.