Metal–insulator transition of vanadium dioxide and the role of grain boundaries

Abstract

The metal–insulator transition (MIT) in polycrystalline vanadium dioxide (VO2) is less sharp than in single-crystal VO2 due to the presence of grain boundaries (GBs). The MIT causes a resistivity change (Ron/Roff) across the MIT to decline from the single-crystal value of ∼104 and also causes a larger hysteresis across the transition. The properties of GBs in VO2 are studied as a function of GB tilt angle. We find that V–V site pairing, which causes a monoclinic phase VO2 to be semiconducting in bulk, also controls bandgap opening at GBs, thus influencing the overall metal–insulator behavior. The almost linear relationship between the ratio of adhesion energy divided by two times the surface energy and the GB energy indicates that the GB energy of twin GBs is primarily determined by re-binding across the GB rather than by the corresponding surface energy. Moreover, a steeper linear fit, characterized by a slope of 0.62, observed in the monoclinic phase VO2 suggests that the GB energy and stability have a higher tolerance to interfacial distortion at the boundary, compared to the rutile phase VO2 and TiO2.