Photoemission evidence for crossover from Peierls-like to Mott-like transition in highly strained VO2_2

We present a spectroscopic study that reveals that the metal-insulator transition of strained VO$_2$ thin films may be driven towards a purely electronic transition, which does not rely on the Peierls dimerization, by the application of mechanical strain. Comparison with a moderately strained system, which does involve the lattice, demonstrates the crossover from Peierls- to Mott-like transitions

Strain dependence of bonding and hybridization across the metal-insulator transition of VO2

Soft x-ray spectroscopy is used to investigate the strain dependence of the metal-insulator transition of VO2. Changes in the strength of the V 3d - O 2p hybridization are observed across the transition, and are linked to the structural distortion. Furthermore, although the V-V dimerization is well-described by dynamical mean-field theory, the V-O hybridization is found to have an unexpectedly strong dependence on strain that is not predicted by band theory, emphasizing the relevance of the O ion to the physics of VO2

Effect of inhomogeneities and substrate on the dynamics of the metal-insulator transition in VO2 thin films

We study the thermal relaxation dynamics of VO2 films after the ultrafast photoinduced metal-insulator transition for two VO2 film samples grown on Al2O3 and TiO2 substrates. We find two orders of magnitude difference in the recovery time (a few nanoseconds for the VO2/Al2O3 sample versus hundreds of nanoseconds for the VO2/TiO2 sample). We present a theoretical model to take into account the effect of inhomogeneities in the films on the relaxation dynamics. We obtain quantitative results that show how the microstructure of the VO2 film and the thermal conductivity of the interface between the VO2 film and the substrate affect long time-scale recovery dynamics. We also obtain a simple analytic relationship between the recovery time-scale and the film\u27s parameters

Direct Observation of Decoupled Structural and Electronic Transitions and an Ambient Pressure Monoclinic-Like Metallic Phase of VO2_2

We report the simultaneous measurement of the structural and electronic components of the metal-insulator transition of VO$_2$ using electron and photoelectron spectroscopies and microscopies. We show that these evolve over different temperature scales, and are separated by an unusual monoclinic-like metallic phase. Our results provide conclusive evidence that the new monoclinic-like metallic phase, recently identified in high-pressure and nonequilibrium measurements, is accessible in the thermodynamic transition at ambient pressure, and we discuss the implications of these observations on the nature of the MIT in VO$_2$

Modification of electronic structure in compressively strained vanadium dioxide films

Vanadium dioxide (VO2) undergoes a phase transition between an insulating monoclinic phase and a conducting rutile phase. Like other correlated electron systems, the properties of VO2 can be extremely sensitive to small changes in external parameters such as strain. In this paper, we investigate a compressively strained VO2 film grown on (001) quartz substrate in which the phase transition temperature (T-c) has been depressed to 325 K from the bulk value of 340 K. Infrared and optical spectroscopy reveals that the lattice dynamics of this strained film are similar to unstrained VO2. However, some of the electronic interband transitions of the strained VO2 film are significantly shifted in energy from those in unstrained VO2. The lattice dynamics remain largely unchanged while the T-c and some of the electronic interband transitions differ substantially from the bulk values, which highlights the role of electronic correlations in driving this metal-insulator phase transition

Methanol Adsorption on Vanadium Oxide Surfaces Observed by Ambient Pressure X-ray Photoelectron Spectroscopy

Ambient pressure X-ray photoelectron spectroscopy (APXPS) has been used to study the initial stages of methanol adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and K 2p XPS spectra were collected as a function of relative methanol pressure in a series of isotherm and isobar experiments on two VO2/TiO2 (100) films with different surface vanadium oxidation states. The binding energies and O 1s/C 1s peak area ratios for adsorbates were consistent with a mixture of molecular methanol, methoxide, hydroxide, and water, indicating that both molecular and dissociative methanol adsorption occur. In contrast to water adsorption experiments on similar films, an adsorption onset was observed at a consistent temperature, rather than a consistent relative pressure, indicating that a more complex reaction mechanism is at play. Vanadium oxidation state, C 1s peak position, and the area of carbon and oxygen adsorbate peaks were correlated, suggesting that reduced surface sites play a critical role in enhancing both the dissociative and molecular adsorption of methanol. The two fairly similar VO2/TiO2 (100) films showed quite different behavior, with the more reduced surface showing greater reactivity toward methanol. The difference in reactivity could be linked to different levels of potassium in the two films, which appears to play an important role in determining the vanadium oxidation state and has important practical consequences for the design of catalytic systems

Water adsorption on vanadium oxide thin films in ambient relative humidity

In this work, ambient pressure x-ray photoelectron spectroscopy (APXPS) is used to study the initial stages of water adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and valence band XPS spectra were collected as a function of relative humidity in a series of isotherm and isobar experiments. Experiments were carried out on two VO2 thin films on TiO2 (100) substrates, prepared with different surface cleaning procedures. Hydroxyl and molecular water surface species were identified, with up to 0.5 ML hydroxide present at the minimum relative humidity, and a consistent molecular water adsorption onset occurring around 0.01% relative humidity. The work function was found to increase with increasing relative humidity, suggesting that surface water and hydroxyl species are oriented with the hydrogen atoms directed away from the surface. Changes in the valence band were also observed as a function of relative humidity. The results were similar to those observed in APXPS experiments on other transition metal oxide surfaces, suggesting that H2O–OH and H2O–H2O surface complex formation plays an important role in the oxide wetting process and water dissociation. Compared to polycrystalline vanadium metal, these vanadium oxide films generate less hydroxide and appear to be more favorable for molecular water adsorption

Observation of Weakened V-V Dimers in the Monoclinic Metallic Phase of Strained VO2

Emergent order at mesoscopic length scales in condensed matter can provide fundamental insight into the underlying competing interactions and their relationship with the order parameter. Using spectromicroscopy, we show that mesoscopic stripe order near the metal-insulator transition (MIT) of strained VO2 represents periodic modulations in both crystal symmetry and V - V dimerization. Above the MIT, we unexpectedly find the long-range order of V - V dimer strength and crystal symmetry become dissociated beyond ≈200 nm, whereas the conductivity transition proceeds homogeneously in a narrow temperature range