Strain effects on insulator-to-metal transition and electronic structure in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>VO</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:math>

Abstract

Tunability of near-room temperature insulator-to-metal transition (IMT) of ${\mathrm{VO}}_{2}$ is a prerequisite for its applications in switching and sensing devices. IMT in ${\mathrm{VO}}_{2}$ is accompanied with structural transition, monoclinic (insulating) to rutile (metallic), where the V-V dimer of the monoclinic phase becomes equidistant in the rutile phase. Tuning of the V-V dimer distances can result in dramatic changes in IMT characteristics. However, understanding of such processes has been limited due to intrigued relations between structure, electronic structure, and IMT of ${\mathrm{VO}}_{2}$. By utilizing the substrate and Cr doping-induced strain, we have grown ${\mathrm{VO}}_{2}$ thin films with distinct V-V dimers, which has enabled us to study the effect of these dimers on the structure, IMT, and electronic structures of ${\mathrm{VO}}_{2}$. In addition to the usual M1 phase of ${\mathrm{VO}}_{2}$, strain-mediated $T$ and M2 phases have been stabilized with the help of both Cr doping and tensile strain along the ${c}_{R}$ axis. We have observed that a small compressive strain ($\ensuremath{\approx}0.19%$) along the ${c}_{R}$ axis ($\ensuremath{\approx}\mathrm{monoclinic}\phantom{\rule{4pt}{0ex}}{a}_{M}$ axis) lowers the transition temperature significantly (by $\ensuremath{\approx}10{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) compared to bulk. Temperature-dependent Raman spectroscopy measurement is used to track the exact structural transformation route followed by these insulating phases of ${\mathrm{VO}}_{2}$. Dependent on the nature of strain along the ${c}_{R}$ axis, IMT temperature is found to vary---increases (for tensile) or decreases (for compressive)---while Cr doping-induced strain has a less significant impact on the IMT temperature compared to the nature of the strain. X-ray absorption near-edge spectroscopy (XANES) has been utilized to examine the electronic structure of the grown ${\mathrm{VO}}_{2}$ thin films. Temperature variation of pre-edge features (vanadium $3d$ orbitals) in XANES directly scales with the insulator-to-metal transition, which suggests that the electronic structure of ${\mathrm{VO}}_{2}$ is strongly influenced by the nature (compressive/tensile) of strain, whereas minimal changes in electronic structure have been observed due to different insulating phases (M1, $T$, and M2). Our study underscores the important role of the nature of strain in tailoring the IMT in ${\mathrm{VO}}_{2}$.

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