The metal–insulator phase change in vanadium dioxide and its applications (original) (raw)
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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 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.
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.
Electronic structure of metallic and insulating phases of vanadium dioxide and its oxide alloys
Physical Review Materials
VO 2 attracts much attention due to its metal-insulator transition. Alloying VO 2 with MgO and GeO 2 allows the band gap and the transition temperature to be varied. We find that the spin order plays a key role in creating the band gap in the low-temperature M 1 phase. For MgO alloying, the alloying fraction n (Mg n V 1−n O 2−n) is varied from 12.5 to 33.3%. The minimum band gap does not change without a structural rearrangement because both band edges of insulating VO 2 consist of only V 3d states on sixfold-coordinated V sites. A crystal search finds that if the Mg fraction in the alloy is large enough (>20%), fivefold-coordinated V sites can have lower energy than the sixfold sites, and the band gaps are doubled. For GeO 2 alloying, the insulating M 1 structure reverts to rutile because GeO 2 has a rutile phase. The result matches the experimental observation and is very important in guiding VO 2 's applications such as smart coating and nonlinear resistor.
Condensed Matter
Among transition metal oxides, VO 2 is a particularly interesting and challenging correlated electron material where an insulator to metal transition (MIT) occurs near room temperature. Here we investigate a 16 nm thick strained vanadium dioxide film, trying to clarify the dynamic behavior of the insulator/metal transition. We measured (resonant) photoemission below and above the MIT transition temperature, focusing on heating and cooling effects at the vanadium L 23-edge using X-ray Absorption Near-Edge Structure (XANES). The vanadium L 23-edges probe the transitions from the 2p core level to final unoccupied states with 3d orbital symmetry above the Fermi level. The dynamics of the 3d unoccupied states both at the L 3-and at the L 2-edge are in agreement with the hysteretic behavior of this thin film. In the first stage of the cooling, the 3d unoccupied states do not change while the transition in the insulating phase appears below 60 • C. Finally, Resonant Photoemission Spectra (ResPES) point out a shift of the Fermi level of~0.75 eV, which can be correlated to the dynamics of the 3d // orbitals, the electron-electron correlation, and the stability of the metallic state.
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...
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.
Infrared spectroscopy and nano-imaging of the insulator-to-metal transition in vanadium dioxide
Physical Review B, 2009
We present a detailed infrared study of the insulator-to-metal transition ͑IMT͒ in vanadium dioxide ͑VO 2 ͒ thin films. Conventional infrared spectroscopy was employed to investigate the IMT in the far field. Scanning near-field infrared microscopy directly revealed the percolative IMT with increasing temperature. We confirmed that the phase transition is also percolative with cooling across the IMT. We present extensive near-field infrared images of phase coexistence in the IMT regime in VO 2. We find that the coexisting insulating and metallic regions at a fixed temperature are static on the time scale of our measurements. A distinctive approach for analyzing the far-field and near-field infrared data within the Bruggeman effective medium theory was employed to extract the optical constants of the incipient metallic puddles at the onset of the IMT. We found divergent effective carrier mass in the metallic puddles that demonstrates the importance of electronic correlations to the IMT in VO 2. We employ the extended dipole model for a quantitative analysis of the observed near-field infrared amplitude contrast and compare the results with those obtained with the basic dipole model.