Nanoscale structural evolution of electrically driven insulator to metal transition in vanadium dioxide (original) (raw)
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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
The metal-insulator transition of M1 vanadium dioxide
arXiv: Materials Science, 2016
Materials that undergo reversible metal-insulator transitions are obvious candidates for new generations of devices. For such potential to be realised, the underlying microscopic mechanisms of such transitions must be fully determined. In this work we probe the correlation between the energy landscape and electronic structure of the metal-insulator transition of vanadium dioxide and the atomic motions occurring using first principles calculations and high resolution X-ray diffraction. Calculations find an energy barrier between the high and low temperature phases corresponding to contraction followed by expansion of the distances between vanadium atoms on neighbouring sublattices. X-ray diffraction reveals anisotropic strain broadening in the low temperature structure's crystal planes, however only for those with spacings affected by this compression/expansion. GW calculations reveal that traversing this barrier destabilises the bonding/anti-bonding splitting of the low temperature phase. This precise atomic description of the origin of the energy barrier separating the two structures will facilitate more precise control over the transition characteristics for new applications and devices.
Nano Letters, 2010
The ability to synthesize VO 2 in the form of single-crystalline nanobeams and nano-and microcrystals uncovered a number of previously unknown aspects of the metal-insulator transition (MIT) in this oxide. In particular, several reports demonstrated that the MIT can proceed through competition between two monoclinic (insulating) phases M1 and M2 and the tetragonal (metallic) R phase under influence of strain. The nature of such phase behavior has been not identified. Here we show that the competition between M1 and M2 phases is purely lattice-symmetry-driven. Within the framework of the Ginzburg-Landau formalism, both M phases correspond to different directions of the same four-component structural order parameter, and as a consequence, the M2 phase can appear under a small perturbation of the M1 structure such as doping or stress. We analyze the strain-controlled phase diagram of VO 2 in the vicinity of the R-M2-M1 triple point using the Ginzburg-Landau formalism and identify and experimentally verify the pathways for strain-control of the transition. These insights open the door toward more systematic approaches to synthesis of VO 2 nanostructures in desired phase states and to use of external fields in the control of the VO 2 phase states. Additionally, we report observation of the triclinic T phase at the heterophase domain boundaries in strained quasi-two-dimensional VO 2 nanoplatelets, and theoretically predict phases that have not been previously observed.
Non-congruence of thermally driven structural and electronic transitions in VO2
Journal of Applied Physics, 2012
Coupled structural and electronic phase transitions underlie the multifunctional properties of strongly-correlated materials. For example, colossal magnetoresistance 1,2 in manganites involves phase transition from paramagnetic insulator to ferromagnetic metal linked to a structural Jahn-Teller distortion 3. Vanadium dioxide (VO 2) likewise exhibits an insulator-to-metal transition (IMT) at ~67 o C with abrupt changes in transport and optical properties and coupled to a structural phase transition (SPT) from monoclinic to tetragonal 4. The IMT and SPT hystereses are signatures of first-order phase transition tracking the nucleation to stabilization of a new phase. Here we have for the first time measured independently the IMT and SPT hystereses in epitaxial VO 2 films, and shown that the hystereses are not congruent. From the measured volume fractions of the two phases in the region of strong correlation, we have computed the evolving dielectric function under an effective-medium approximation. But the computed dielectric functions could not reproduce the measured IMT, implying that there is a strongly correlated metallic phase that is not in the stable rutile structure, consistent with Qazilbash et al 5. Search for a corresponding macroscopic structural intermediate also yielded negative result. The complex physics of VO 2 phase transition has long been debated 6-10. Unlike other strongly correlated materials 11 exhibiting IMT, such as V 2 O 3 12,13 , where phase transitions are satisfactorily explained by the Mott mechanism alone, the IMT in VO 2 is complicated by accompanying spin-Peierls instability 14 that leads via strong electron-phonon coupling
Electronic and structural transformations near the insulator-to-metal transition in vanadium dioxide
2010
dioxide (VO 2) undergoes an insulator-to-metal transition (IMT) at T ≈ 340 K accompanied by a change in the lattice structure. Numerous studies of this phase transition in VO 2 have focused either on the electronic change or on the structural change. The interplay between the electronic and lattice degrees of freedom has been relatively unexplored. In previous work using scanning near-field infrared microscopy (SNIM), we showed that the electronic IMT in VO 2 films proceeds via nucleation and percolation of nanoscale metallic domains [1,2,3]. Here we present nanoscale X-ray diffraction measurements that image the structural changes in a VO 2 film with 40 nm spatial resolution. In addition, local resistivity and SNIM measurements of the electronic IMT in the VO 2 film allow us to present a coherent picture of this complex phase transition. 1.
The metal–insulator phase change in vanadium dioxide and its applications
Journal of Applied Physics
Vanadium dioxide is an unusual material that undergoes a first-order Metal-Insulator Transition (MIT) at 340 K, attracting considerable interest for its intrinsic properties and its potential applications. However, the nature of MIT has not been fully determined. Variants of density functional theory (DFT) have been widely used to study the MIT in pure and doped VO 2. A full description of MIT is complicated by several related factors such as V-V dimerization, magnetic properties, and spin correlations. Each of these requires careful attention. In this Perspective, we explain why DFT fails, introduce a spin-pairing model of MIT, and propose a new way to estimate the transition temperature. We then use the method to study the doping and alloying process. Finally, we give an overview of some applications of MIT. This work aims to provide insight into and stimulate more research studies in this promising field.
Scientific reports, 2016
Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the e...
physica status solidi (RRL) - Rapid Research Letters, 2011
Vanadium dioxide undergoes a first order metalinsulator transition (MIT) from the high temperature metallic rutile (R) phase to an insulating monoclinic (M1) phase at a commercially accessible temperature. Two separate mechanisms have been proposed to explain the nature of the MIT: the Mott and Peierls mechanisms . Electron-electron interactions drive the MIT according to the Mott mechanism [3, 5] while the Peierls mechanism explains the transition in terms of electron -lattice interactions [1]. Between these two competing models, understanding the intermediate insulating M2 phase has been the most critical issue. The M2 phase of VO 2 is an electronic insulator despite band structure calculations suggesting that undimerized V atoms in the M2 phase should lead to conducting states . For this reason, the structural properties of the M2 phase prepared by applying stress or through Cr doping have been thoroughly investigated .
Nature of the Metal Insulator Transition in Ultrathin Epitaxial Vanadium Dioxide
Nano Letters, 2013
We have combined hard X-ray photoelectron spectroscopy with angular dependent O K-edge and V Ledge X-ray absorption spectroscopy to study the electronic structure of metallic and insulating end point phases in 4.1 nm thick (14 units cells along the c-axis of VO 2) films on TiO 2 (001) substrates, each displaying an abrupt MIT centered at ∼300 K with width <20 K and a resistance change of ΔR/R > 10 3. The dimensions, quality of the films, and stoichiometry were confirmed by a combination of scanning transmission electron microscopy with electron energy loss spectroscopy, X-ray spectroscopy, and resistivity measurements. The measured end point phases agree with their bulk counterparts. This clearly shows that, apart from the strain induced change in transition temperature, the underlying mechanism of the MIT for technologically relevant dimensions must be the same as the bulk for this orientation.
Control of the metal–insulator transition in vanadium dioxide by modifying orbital occupancy
Nature Physics, 2013
External control of the conductivity of correlated oxides is one of the most promising schemes for realizing energy-efficient electronic devices. Vanadium dioxide (VO 2 ), an archetypal correlated oxide compound, undergoes a temperature-driven metal-insulator transition near room temperature with a concomitant change in crystal symmetry. Here, we show that the metal-insulator transition temperature of thin VO 2 (001) films can be changed continuously from ∼285 to ∼345 K by varying the thickness of the RuO 2 buffer layer (resulting in different epitaxial strains). Using strain-, polarizationand temperature-dependent X-ray absorption spectroscopy, in combination with X-ray diffraction and electronic transport measurements, we demonstrate that the transition temperature and the structural distortion across the transition depend on the orbital occupancy in the metallic state. Our findings open up the possibility of controlling the conductivity in atomically thin VO 2 layers by manipulating the orbital occupancy by, for example, heterostructural engineering.