Polymorph separation induced by angle distortion and electron delocalization effect via orbital modification in VO2 epitaxial thin films

Since Morin discovered that vanadium dioxide (VO2) undergoes a reversible and dramatic structural phase transition coupled with an abrupt metal-insulator transition, extensive attention has been paid to VO2 due to its importance in fundamental condensed state physics and its potential technological applications. Here, we observed that the precipitated phases of VO2 (insulating and metallic polymorphs) could be controlled by relaxing the dimerization of the vanadium-vanadium (V-V) atomic chain. In particular, the monoclinic metallic phase can be stabilized even at room temperature with the assistance of the angle-distortion-induced (beta = 120 degrees.) metallization through symmetry matching between the VO2 epitaxial thin films and the (0001)-oriented sapphire substrates. Concomitantly, the insulating phase (M1, beta = 122.6.) that separates from the metallic matrix may supply another driving force for stabilizing the metallic phase, as indicated by scattering-type scanning near-field optical infrared microscopy and further confirmed by synchrotron radiation high-resolution x-ray diffraction characterizations. Soft x-ray absorption spectroscopy results showed that the orbital features of the monoclinic metallic phase are analogous to those of the high-temperature metallic rutile VO2 (R) phase. First-principles calculations further demonstrate the angle-distortion-induced reduction of the V-V atomic dimerization, which enhances the electron delocalization and thus the conductivity. Therefore, the angle distortion results in the metallic monoclinic phase and stabilizes it with the assistance of the nanoscale insulating VO2 (M1) domains at room temperature. These results are of great importance for understanding the contributions of various polymorphs to the metal-insulator transition and for the design of novel artificially heterointerfacial devices based on VO2 nanoscale polymorphs.