THz spectroscopy of VO ₂ epitaxial films: Controlling the anisotropic properties through strain engineering

We use THz time-domain spectroscopy to investigate the farinfrared properties of vanadium dioxide thin films, strain-engineered through epitaxial growth on (100)R TiO ₂ substrates. The films exhibit a large uniaxial tensile strain along the rutile c-axis. X-ray diffraction measurements reveal a structural transition temperature of 340 K, whereas independent THz conductivity measurements yield a metal-insulator transition temperature of 365K along c _R. Analysis of these results suggests a Mott-Hubbard behavior along the c _R-axis. Along c _R the conductivity is approximately 5500 (Ω cm) ^-1, comparable to bulk single crystals. The tensile strain leads to remarkably uniform cracking oriented along the rutile c-axis, resulting in a large conductivity anisotropy in our single-crystal epitaxial thin films. We discuss our results in the context of previous measurements and calculations of the properties of VO ₂, under different strain conditions. This work demonstrates the potential of strain engineering to tune the properties of complex materials while also serving as a powerful discriminatory tool for probing microscopic responses.