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We demonstrate that surface stresses in epitaxially grown VO₂ nanowires (NWs) have a strong effect on the appearance and stability of intermediate insulating M₂ phases, as well as the spatial distribution of insulating and metallic domains during structural phase transitions. During the transition from an insulating M1 phase to a metallic R phase, the coexistence of insulating M₁ and M₂ phases with the absence of a metallic R phase was observed at atmospheric pressure. In addition, we show that, for a VO₂ NW without the presence of an epitaxial interface, surface stresses dominantly lead to spatially inhomogeneous phase transitions between insulating and metallic phases. In contrast, for a VO₂ NW with the presence of an epitaxial interface, the strong epitaxial interface interaction leads to additional stresses resulting in uniformly alternating insulating and metallic domains along the NW length.

PACS 71.30.+h -Metal-insulator transitions and other electronic transitions PACS 64.70.Nd -Structural transitions in nanoscale materials PACS 78.20.-e -Optical properties of bulk materials and thin films Abstract -We report on the epitaxial growth of vanadium dioxide (VO2) thin films on (0001) GaN substrates using a radio frequency magnetron sputtering method and discuss their unusual M2-mediated metal-insulator transition (MIT) properties. We found that large lattice misfits between the VO2 film and the GaN substrate could favor the stabilization of the intermediate insulating M2 phase, which is known to be observed only in either doped or uniaxially strained samples. We demonstrated that the MIT in VO2 films on GaN substrates could be mediated via a monoclinic M2 phase during the transition from a monoclinic M1 to a rutile R phase. This was confirmed by temperature-dependent Raman studies that exhibited both an evident upshift of a high-frequency phonon mode (ωV-O) from 618 cm −1 (M1) to 645 cm −1 (M2) and a distinct peak splitting of a low-frequency phonon mode (ωV-V) at 221 cm −1 (M2) for increasing temperatures. Moreover, a resistance change of four orders of magnitude was observed for VO2 thin films on GaN substrates, being indicative of the high quality of VO2 thin films. This study may offer great opportunities not only to improve the understanding of M2-mediated MIT behavior in VO2 thin films, but also to realize novel electronic and optoelectronic devices.

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Graphene has been gradually studied as a high‐frequency transmission line material owing to high carrier mobility with frequency independence up to a few THz. However, the graphene‐based transmission lines have poor conductivity due to their low carrier concentration. Here, it is observed that the radio frequency (RF) transmission performance could be severely hampered by the defect‐induced scattering, even though the carrier concentration is increased. As a possible solution, the deposition of the amorphous carbon on the graphene is studied in the high‐frequency region up to 110 GHz. The DC resistance is reduced by as much as 60%, and the RF transmission property is also enhanced by 3 dB. Also, the amorphous carbon covered graphene shows stable performance under a harsh environment. These results prove that the carrier concentration control is an effective and a facile method to improve the transmission performance of graphene. It opens up the possibilities of using graphene as interconnects in the ultrahigh‐frequency region.