Moatti Adele, Sachan Ritesh, Gupta Siddharth, Narayan Jagdish
Materials Science and Engineering , North Carolina State University , Raleigh , North Carolina 27606 , United States.
Materials Science Division , Army Research Office , Research Triangle Park , Raleigh , North Carolina 27709 , United States.
ACS Appl Mater Interfaces. 2019 Jan 23;11(3):3547-3554. doi: 10.1021/acsami.8b17879. Epub 2019 Jan 10.
Vanadium dioxide (VO) is a strongly correlated material with 3d electrons, which exhibits temperature-driven insulator-to-metal transition with a concurrent change in the crystal symmetry. Interestingly, even modest changes in stoichiometry-induced orbital occupancy dramatically affect the electrical conductivity of the system. Here, we report a successful transformation of epitaxial monoclinic VO thin films from a conventionally insulating to permanently metallic behavior by manipulating the electron correlations. These ultrathin (∼10 nm) epitaxial VO films were grown on NiO(111)/AlO(0001) pseudomorphically, where the large misfit between NiO and AlO were fully relaxed by domain-matching epitaxy. Complete conversion from an insulator to permanent metallic phase is achieved through injecting oxygen vacancies ( x ∼ 0.20 ± 0.02) into the VO system via annealing under high vacuum (∼5 × 10 Torr) and increased temperature (450 °C). Systematic introduction of oxygen vacancies partially converts V to V and generates unpaired electron charges which result in the emergence of donor states near the Fermi level. Through the detailed study of the vibrational modes by Raman spectroscopy, hardening of the V-V vibrational modes and stabilization of V-V dimers are observed in vacuum-annealed VO films, providing conclusive evidence for stabilization of a monoclinic phase. This ultimately leads to convenient free-electron transport through the oxygen-deficient VO thin films, resulting in metallic characteristics at room temperature. With these results, we propose a defect engineering pathway through the control of oxygen vacancies to tune electrical and optical properties in epitaxial monoclinic VO.
二氧化钒（VO₂）是一种具有3d电子的强关联材料，它表现出温度驱动的绝缘体-金属转变，同时晶体对称性也会发生变化。有趣的是，即使化学计量比的适度变化引起的轨道占据变化也会显著影响系统的电导率。在此，我们报告了通过操纵电子关联成功地将外延单斜VO₂薄膜从传统的绝缘行为转变为永久的金属行为。这些超薄（约10纳米）外延VO₂薄膜在NiO(111)/Al₂O₃(0001)上伪晶生长，其中NiO和Al₂O₃之间的大失配通过畴匹配外延完全弛豫。通过在高真空（约5×10⁻⁶托）和升高温度（450℃）下退火，向VO₂系统中注入氧空位（x≈0.20±0.02），实现了从绝缘体到永久金属相的完全转变。氧空位的系统引入部分地将V⁴⁺转变为V³⁺，并产生未配对的电子电荷，这导致在费米能级附近出现施主态。通过拉曼光谱对振动模式的详细研究，在真空退火的VO₂薄膜中观察到V-V振动模式的硬化和V-V二聚体的稳定，为单斜相的稳定提供了确凿证据。这最终导致通过缺氧的VO₂薄膜实现方便的自由电子传输，从而在室温下呈现金属特性。基于这些结果，我们提出了一条通过控制氧空位来调节外延单斜VO₂电学和光学性质的缺陷工程途径。
