The structure, metal-insulator transition (MIT), and related Terahertz (THz) transmission characteristics of VO2 thin films obtained by sputtering deposition on c-, r-, and m-plane sapphire substrates were investigated by different techniques. On c-sapphire, monoclinic VO2 films were characterized to be epitaxial films with triple domain structure caused by β-angle mismatch. Monoclinic VO2 β angle of 122.2° and the two angles of V4+–V4+ chain deviating from the am axis of 4.4° and 4.3° are determined. On r-sapphire, tetragonal VO2 was determined to be epitaxially deposited with VO2 (011)T perpendicular to the growth direction, while the structural phase transformation into lower symmetric monoclinic phase results in (2¯11) and (200) orientations forming a twinned structure. VO2 on m-sapphire has several growth orientations, related with the uneven substrate surface and possible inter-diffusion between film and substrate. Measurements of the electrical properties show that the sample on r-sapphire has MIT property superior to the other two samples, with a resistivity change as large as 9 × 104 times and a transition window as narrow as 3.9 K, and it has the highest resistivity with the lowest free carrier density in the insulating phase. THz transmission measurements on VO2 films grown on r-plane sapphire substrates revealed intensity modulation depth as large as 98% over a broadband THz region, suggesting that VO2 films are ideal material candidates for THz modulation applications.

We report the epitaxial relationship of VO2 thin-films on c-plane sapphire and their terahertz transmission modulation with temperature. The films exhibit a triple-domain structure caused by the lattice mismatch between monoclinic VO2 and sapphire hexagon. The epitaxial relationship is determined to be VO2[010]∥Al2O3[0001] and VO2(2¯02)∥Al2O3{112¯0}, with the in-plane lattice mismatch of 2.66% (tensile) along [2¯02] and the out-of-plane lattice mismatch of −2.19% (compressive). Terahertz measurements revealed that VO2 films have over fourfold change in transmission during the metal-insulator transition, indicating a strong potential for terahertz wave switching and modulation applications.

The synthesis of pure δ-MoN with desired superconducting properties usually requires extreme conditions, such as high temperature and high pressure, which hinders its fundamental studies and applications. Herein, by using a chemical solution method, epitaxial δ-MoN thin films have been grown on c-cut Al(2)O(3) substrates at a temperature lower than 900 °C and an ambient pressure. The films are phase pure and show a T(c) of 13.0 K with a sharp transition. In addition, the films show a high critical field and excellent current carrying capabilities, which further prove the superior quality of these chemically prepared epitaxial thin films.

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