Atomic Resolution Study of Local Strains in Doped VO2 Nanowires (original) (raw)
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Nanostructure-Dependent Metal−Insulator Transitions in Vanadium-Oxide Nanowires
The Journal of Physical Chemistry C, 2008
Single-crystal VO 2 nanowires were synthesized using atmospheric-pressure and physical vapor deposition and outfitted with electrodes for current-voltage measurements. The Mott insulator-to-metal transition temperatures of several nanowires with varying lateral dimensions were determined by measuring the voltage values at which the sharp current step, signaling that the occurrence of the insulator-to-metal or the reverse transitions, had taken place. The observed Mott transition temperatures, which ranged between 62 and 70°C for the nanowires measured, trended downward with decreasing nanowire width. We ascribe this to strong interactions between the nanowire and the underlying silica substrate. However, the scatter in the Motttemperature versus nanowire width exceeded the experimental uncertainty in the values of the Mott temperature, indicating that other parameters also contribute to the precise value of the Mott transition temperature of nanostructured VO 2 .
Axially Engineered Metal–Insulator Phase Transition by Graded Doping VO 2 Nanowires
Journal of the American Chemical Society, 2013
The abrupt first-order metal−insulator phase transition in single-crystal vanadium dioxide nanowires (NWs) is engineered to be a gradual transition by axially grading the doping level of tungsten. We also demonstrate the potential of these NWs for thermal sensing and actuation applications. At room temperature, the graded-doped NWs show metal phase on the tips and insulator phase near the center of the NW, and the metal phase grows progressively toward the center when the temperature rises. As such, each individual NW acts as a microthermometer that can be simply read out with an optical microscope. The NW resistance decreases gradually with the temperature rise, eventually reaching 2 orders of magnitude drop, in stark contrast to the abrupt resistance change in undoped VO 2 wires. This novel phase transition yields an extremely high temperature coefficient of resistivity ∼10%/K, simultaneously with a very low resistivity down to 0.001 Ω·cm, making these NWs promising infrared sensing materials for uncooled microbolometers. Lastly, they form bimorph thermal actuators that bend with an unusually high curvature, ∼900 m −1 ·K −1 over a wide temperature range (35−80°C ), significantly broadening the response temperature range of previous VO 2 bimorph actuators. Given that the phase transition responds to a diverse range of stimuliheat, electric current, strain, focused light, and electric fieldthe graded-doped NWs may find wide applications in thermo-opto-electro-mechanical sensing and energy conversion.
Synthesis of VO2 Nanowire and Observation of the Metal-Insulator Transition
2007
We have fabricated crystalline nanowires of VO2 using a new synthetic method. A nanowire synthesized at 650 • C shows the semiconducting behavior and a nanowire at 670 • C exhibits the first-order metal-insulator transition which is not the one-dimensional property. The temperature coefficient of resistance in the semiconducting nanowire is 7.06 %/K at 300 K, which is higher than that of commercial bolometer.
Insulator-to-metal transition in ultrathin rutile VO2/TiO2(001)
npj Quantum Materials
An insulator-to-metal transition (IMT) is an emergent characteristic of quantum materials. When the IMT occurs in materials with interacting electronic and lattice degrees of freedom, it is often difficult to determine if the energy gap in the insulating state is formed by Mott electron–electron correlation or by Peierls charge-density wave (CDW) ordering. To solve this problem, we investigate a representative material, vanadium dioxide (VO2), which exhibits both strong electron–electron interaction and CDW ordering. For this research, VO2 films of different thicknesses on rutile (001) TiO2 substrates have been fabricated. X-ray diffraction (XRD) data show that ultrathin VO2 films with thickness below 7.5 nm undergo the IMT between rutile insulator below Tc and rutile metal above Tc, while an ultrathin VO2 film with a thickness of 8 nm experiences the structural phase transition from the monoclinic structure below Tc to the rutile structure above Tc. Infrared and optical measurement...
Fabrication, structural and electrical characterization of VO2 nanowires
Materials Research Bulletin, 2008
The structural and electrical properties of VO 2 nanowires synthesized on Si 3 N 4 /Si substrates or molybdenum grids by a catalystfree vapour transport method were investigated. The grown VO 2 nanowires are single crystalline and rectangular-shaped with a preferential axial growth direction of [1 0 0], as examined with various structural analyses such as transmission electron microscopy, electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. In particular, it was found that growing VO 2 nanowires directly on Si 3 N 4 deposited molybdenum transmission electron microscopy grids is advantageous for direct transmission electron microscopy and electron diffraction characterizations, because it does not involve a nanowire-detachment step from the substrates that may cause chemical residue contamination. In addition to structural analyses, VO 2 nanowires were also fabricated into field effect transistor devices to characterize their electrical properties. The transistor characteristics and metalinsulator transition effects of VO 2 nanowires were investigated.
Scientific reports, 2015
Single-crystalline vanadium dioxide (VO2) nanostructures have recently attracted great attention because of their single domain metal-insulator transition (MIT) nature that differs from a bulk sample. The VO2 nanostructures can also provide new opportunities to explore, understand, and ultimately engineer MIT properties for applications of novel functional devices. Importantly, the MIT properties of the VO2 nanostructures are significantly affected by stoichiometry, doping, size effect, defects, and in particular, strain. Here, we report the effect of substrate-mediated strain on the correlative role of thermal heating and electric field on the MIT in the VO2 nanobeams by altering the strength of the substrate attachment. Our study may provide helpful information on controlling the properties of VO2 nanobeam for the device applications by changing temperature and voltage with a properly engineered strain.
Electrical Transition in Isostructural VO2 Thin-Film Heterostructures
Scientific Reports
Control over the concurrent occurrence of structural (monoclinic to tetragonal) and electrical (insulator to the conductor) transitions presents a formidable challenge for Vo 2-based thin film devices. Speed, lifetime, and reliability of these devices can be significantly improved by utilizing solely electrical transition while eliminating structural transition. We design a novel strain-stabilized isostructural VO 2 epitaxial thin-film system where the electrical transition occurs without any observable structural transition. The thin-film heterostructures with a completely relaxed NiO buffer layer have been synthesized allowing complete control over strains in VO 2 films. The strain trapping in VO 2 thin films occurs below a critical thickness by arresting the formation of misfit dislocations. We discover the structural pinning of the monoclinic phase in (10 ± 1 nm) epitaxial VO 2 films due to bandgap changes throughout the whole temperature regime as the insulator-to-metal transition occurs. Using density functional theory, we calculate that the strain in monoclinic structure reduces the difference between long and short V-V bond-lengths (Δ V−V) in monoclinic structures which leads to a systematic decrease in the electronic bandgap of Vo 2. This decrease in bandgap is additionally attributed to ferromagnetic ordering in the monoclinic phase to facilitate a Mott insulator without going through the structural transition. The metal-insulator transition in strongly correlated materials such as vanadium dioxide (VO 2) is usually coupled with the symmetry-lowering structural transition, which is tetragonal rutile P 4 2 /mnm to monoclinic P 2 1 /c. The fundamental understanding and control over electrical and structural transitions in VO 2 , which occur often simultaneously, are of immense scientific importance with profound impact on technological applications ranging from smart switching to infrared sensing devices. Over the years, numerous efforts have been made in this direction, primarily focusing on the manipulation of these transitions via defect and interface engineering 1-5. However, the switching speed and endurance of such devices are often limited by the complexities that emerge from the kinetically slower occurrence of the structural transition (10 picoseconds) as compared to the electrical transition (0.1 picoseconds) 6-8. This leads to the decoupling between these coexisting transitions in the presence of strain, dopants, and defects in the thermal spectrum and deleteriously affects the performance of such systems 2,4,8,9. The coexistence of electrical and structural transitions presents practical challenges in fabricating electronically-correlated VO 2 based solid-state devices 3. In this respect, the development of materials displaying an isolated electrical transition without an accompanying structural transition provides an ideal solution. This can be achieved by strain management in VO 2 thin films 10-13. It has been shown that the primary mechanisms of metal-insulator transitions are based on electron-electron interactions (Mott transition) and electron-lattice interactions (Peierls transition). The ratio of these can be effectively steered through strain-induced tuning of c/a lattice ratio in VO 2 thin films 1,14-18. This is a result of an interplay between these competing mechanisms of electron-electron interaction and electron-phonon interaction, leading to a tunable electrical transition 1,12,15,19. Previously, several researchers including our group have shown that it is possible to separate structural and electrical transitions 9,10,20-23. However, this raises the question of whether it is possible to totally prevent the occurrence of the structural transition, which had been predicted previously by density functional theory (DFT) calculation suggesting a thermally stable monoclinic metallic phase of VO 2 24. The insulating state in monoclinic VO 2 results from electron-electron correlations and electron-phonon interactions. These correlations can be manipulated by charge, spin, orbital, and lattice degrees of freedom. This means that the ratio of the Mott (electron-electron correlations) and Peierls (electron-phonon interactions) transitions can change depending on these factors. Thus, if