Stability of the M2 phase of vanadium dioxide induced by coherent epitaxial strain

Tensile strain along the $c_R$ axis in epitaxial ${\mathrm{VO}}_2$ films raises the temperature of the metal insulator transition and is expected to stabilize the intermediate monoclinic M2 phase. We employ surface-sensitive x-ray spectroscopy to distinguish from the ${\mathrm{TiO}}_2$ substrate and identify the phases of ${\mathrm{VO}}_2$ as a function of temperature in epitaxial ${\mathrm{VO}}_2/{\mathrm{TiO}}_2$ thin films with well-defined biaxial strain. Although qualitatively similar to our Landau-Ginzburg theory predicted phase diagrams, the M2 phase is stabilized by nearly an order of magnitude more strain than expected for the measured temperature window. Our results reveal that the elongation of the $c_R$ axis is insufficient for describing the transition pathway of ${\mathrm{VO}}_2$ epitaxial films and that a strain induced increase of electron correlation effects must be considered.

Figure 1

Schematic representations of the crystal structure of epitaxial ${\mathrm{VO}}_2$ film grown on (a) ${\mathrm{TiO}}_2\left(001\right)$ and (b) ${\mathrm{TiO}}_2\left(100\right)$, showing the orientation of the V-V dimers. (c) ADF STEM image of a 7.5 nm ${\mathrm{VO}}_2/{\mathrm{TiO}}_2\left(100\right)$ epitaxial film viewed along the [001] axis.

Figure 2

(Left) Temperature dependent O $K$-edge XAS measurements of ${\mathrm{VO}}_2/{\mathrm{TiO}}_2\left(001\right)$ and ${\mathrm{VO}}_2/{\mathrm{TiO}}_2\left(100\right)$ with (right) an expanded view of the $d_{\parallel }$ feature and its peak intensity as a function of temperature.

Figure 3

The $t_{2g}\left(\pi \right)$ projected DOS comparing the M1 and M2 phase calculated by hybrid DFT. The highlighted region indicates the $d_{\parallel }$ band in each structure arising from dimerized vanadium ions.

Figure 4

Transformation temperature as a function of mismatch strain for (001) ${\mathrm{VO}}_2$ (top) and (100) ${\mathrm{VO}}_2$ (bottom) thin films. In the (001) case, the epitaxial strain is equal in the $a_R$ and $b_R$ directions, while in the (100) case, the $x$ axis represents the epitaxial strain along the $b_R$ direction (${\epsilon }_{S1}$), with the strain along $c_R$ direction (${\epsilon }_{S2}$) fixed to $4.35{\epsilon }_{S1}$ in order to reflect the anisotropic epitaxial strain. The white boxes (along with dotted lines to guide the eye) represent experimentally determined transformation temperature values.

Figure 5

Valence band HAXPES of ${\mathrm{VO}}_2/{\mathrm{TiO}}_2\left(100\right)$ and ${\mathrm{VO}}_2/{\mathrm{TiO}}_2\left(001\right)$ collected above and below $T_{\text{MIT}}$. (Bottom) HSE06 hybrid DFT calculations showing the total DOS for the M1 and R phase. Solid bars indicate the coherent (red) and incoherent (black) contributions to the V $3d$ feature.