proposed a Mott-like phase transition in VO 2 thin films induced by a large bi-axial epitaxial strain based on Raman spectroscopy data 26. Furthermore, our recent findings demonstrated an anomalous tetragonal-like to tetragonal structural phase transition but not a conventional monoclinic to tetragonal phase transition in 300-nm-thick VO 2/TiO 2 films (not shown here). A tetragonal phase was identified by transmission electron microscopy in the ultrathin VO 2 layers just adjacent to the TiO 2 substrate in Zou’s work 25 and X-ray diffraction reciprocal space mapping (XRD-RSM) suggested that there is no monoclinic phase in ultrathin VO 2 films at room temperature 25. On the other hand, epitaxial strain due to lattice mismatch between thin film VO 2 and substrates has revealed a different picture of the MIT 22. Concurrently, there are many others who believe that the MIT in VO 2 is a Mott transition 14, 24.
Thus, the mechanism of MIT remains controversial-some researchers 22, 23 believe that the MIT is induced by the broken symmetry of lattice, namely, the Peierls transition. observed that the tetragonal metallic phase did not occur simultaneously with MIT by the ultrafast pump-probe technology-they ascribed the MIT to the Mott mechanism illustrated by photo-assisted hole excitation 21. found evidence for a structure-driven MIT in VO 2 by ultrafast spectroscopy and declared that the mechanism of MIT was not the Mott-Hubbard transition 11.
As early as 1975, Zylbersztejn and Mott showed that the mechanism of MIT in VO 2 was not the simple Mott-Hubbard transition induced only by electron-electron interactions 20. Indeed, there is still a significant debate over whether the mechanism of MIT in VO 2 is a Mott transition 14, 15, Peierls transition 16, 17, or a combination of the two 18, 19. However, this hypothesis does not always hold. Generally, the MIT of bulk VO 2 or nanobeam-like counterparts always accompanies a structural phase transition from a low temperature monoclinic phase to a high temperature tetragonal phase 11, 12, 13.
#IN VICINITY OF WINDOWS#
Due to the optical transmittance changes at the infrared 2 and THz regions 3, 4 and huge resistance jump 5,VO 2 has become a widely-studied material in fundamental studies and for industrial applications such as metamaterials 6, 7, 8, smart windows 9, supercapacitors 10, etc. Vanadium dioxide (VO 2) is an archetypal correlated material discovered by Morin with excellent metal-insulator transition (MIT) characteristics at the critical temperature (~68 ☌ in bulk state) 1. This work offers a better understanding of the mechanism of MIT in the strained VO 2 films. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO 2/TiO 2 thin films. Thus the MIT in the bi-axially strained VO 2 thin films should be only driven by electronic transition without assistance of structural phase transition. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy E F of V 3d orbital, implying that the electronic transition triggers the MIT in the strained 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. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO 2 films in comparison to thicker films. Mechanism of metal-insulator transition (MIT) in strained VO 2 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.
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