Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mengmeng Yang,Chen Gao,Liangxin Wang,Yongqi Dong,Xiaoguang Li,Zhenlin Luo,Kai Hu,Han Xu,Haiping Chen,Huanxin Ju,Jiangtao Zhao,Yueliang Gu,Haoliang Huang,Jun Bao,Bin Hong Bin Hong,Xingyu Gao,Junfa Zhu,Yuanjun Yang,Tieying Yang,Li Song

doi:10.1038/srep23119

Abstract

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 electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. 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)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

Highlights

Vanadium dioxide (VO2) is an archetypal correlated material discovered by Morin with excellent metal-insulator transition (MIT) characteristics at the critical temperature (~68 °C in bulk state)[1]
A tetragonal phase was identified by transmission electron microscopy in the ultrathin VO2 layers just adjacent to the TiO2 substrate in Zou’s work[25], and X-ray diffraction reciprocal space mapping (XRD-RSM) suggested that there is no monoclinic phase in ultrathin VO2 films at room temperature[25]
We investigated the evolutions of crystal structure and electronic states in the ultrathin VO2 films grown on (001)-oriented TiO2 substrates via temperature-dependent XRD-RSM, Raman spectroscopy and Ultraviolet Photoelectron Spectroscopy (UPS)

Summary

Introduction

Vanadium dioxide (VO2) is an archetypal correlated material discovered by Morin with excellent metal-insulator transition (MIT) characteristics at the critical temperature (~68 °C in bulk state)[1]. The MIT of bulk VO2 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]. Jiwei Lu et al proposed a Mott-like phase transition in VO2 thin films induced by a large bi-axial epitaxial strain based on Raman spectroscopy data[26]. Laverock et al observed a crossover from a Mott-Peierls-like transition to a Mott-like transition by large out-of-plane tensile in strained VO2 thin films through various spectroscopic techniques[27]. The low-temperature phase in thin film VO2 under substrate clamping has not yet been fully understood and identified—this is very important for unveiling the MIT mechanism in ultrathin VO2 epitaxial films

Methods

Results

Conclusion