As a typical phase transition material, vanadium dioxide (VO₂) has attracted much attention due to its amazing metal–insulator transition (MIT) at the critical temperature of 68 °C, which could be driven by multiple stimuli, including electricity, thermal irradiation, THz waves, strain, etc. In the MIT process, VO₂ exhibits significant changes in its structure from monoclinic structure at low temperature to rutile structure at high temperature, accompanied by significant modulation of physical properties, such as the infrared transmittance from high to low and a resistivity drop over 5 magnitudes. Based on these features, VO₂ has functioned as thermochromic coatings, sensors, switches, electronic devices, actuators, etc. However, the vanadium element possesses multiple valent states and can produce complicated thermodynamics of vanadium oxides. The fabrication of tetravalence VO₂ with desirable phase-transition features usually requires rigorous fabrication conditions with a precisely controlled atmosphere for stoichiometric components and high temperatures for good crystallinity. Under these circumstances, it is difficult to directly fabricate crystalline VO₂ films on flexible polymer substrates because most of them could not stand such high temperatures (usually >400 °C), which will lead to the loss of the substrate functionality. However, the featured phase transition of VO₂ makes it a promising candidate for future flexible devices, while VO₂ owns great potential as flexible optical coatings, flexible sensors, flexible electronics, etc. Hence, research on flexible VO₂ films is necessary and could largely pave the way to the application field of VO₂ material. In this Account, we start with a brief introduction to phase transition properties of VO₂ to reveal the intrinsic advantages as key materials in flexible devices. Next, multifunctional devices based on flexible VO₂ films with different forms and characteristics are presented, including (1) flexible VO₂-film-based thermochromic smart windows for energy-saving function depending on the change of environmental temperatures, (2) flexible VO₂ films optical devices based on the changing emissivity of VO₂, (3) flexible VO₂ films with compatibility as multifunctional sensors, (4) next-generation flexible electronics based on VO₂ films, and (5) flexible VO₂ actuators with significant mechanical motions under external stimuli. Meanwhile, various fabrication technologies of flexible VO₂ films have been introduced and discussed. Finally, we end the Account with an overview of the remaining challenges and new opportunities that have been opened up for vanadium dioxide in new forms of flexible optical and electronic devices. We hope this Account will inspire new innovative designs, fabrication approaches, and more possible functions of flexible VO₂ films in future work.

中文翻译：

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多功能柔性二氧化钒薄膜

作为一种典型的相变材料，二氧化钒（VO ₂）因其在 68 °C 的临界温度下具有惊人的金属-绝缘体转变（MIT）而备受关注，该转变可由电、热辐射等多种刺激驱动。 、太赫兹波、应变等。在MIT过程中，VO ₂的结构从低温的单斜结构到高温的金红石结构发生显着变化，伴随着物理性质的显着调制，如红外透射率从高到低且电阻率下降超过 5 个数量级。基于这些特点，VO ₂已用作热致变色涂层、传感器、开关、电子设备、执行器等。然而，钒元素具有多价态，可以产生复杂的钒氧化物热力学。具有理想相变特征的四价 VO ₂的制造通常需要严格的制造条件，具有精确控制化学计量成分的气氛和高温以获得良好的结晶度。在这种情况下，很难在柔性聚合物基板上直接制备结晶 VO ₂薄膜，因为它们中的大多数不能承受如此高的温度（通常> 400°C），这将导致基板功能的丧失。然而，VO ₂的特征相变使其成为未来柔性器件的有希望的候选者，而 VO ₂在柔性光学涂层、柔性传感器、柔性电子器件等方面具有巨大潜力。因此，对柔性 VO ₂薄膜的研究是必要的，并且可以在很大程度上为该领域的应用铺平道路。 VO ₂材料。在本报告中，我们首先简要介绍 VO _{2 的}相变特性，以揭示其作为柔性器件关键材料的内在优势。接下来，介绍了基于具有不同形式和特性的柔性VO ₂薄膜的多功能器件，包括（1）柔性VO ₂-基于薄膜的热致变色智能窗根据环境温度的变化实现节能功能，（2）基于VO ₂发射率变化的柔性VO ₂薄膜光学器件，（3）与多功能传感器兼容的柔性VO ₂薄膜，（4）基于VO ₂薄膜的下一代柔性电子器件，以及（5）在外部刺激下具有显着机械运动的柔性VO ₂致动器。同时，柔性VO _{2 的}各种制造技术电影已被介绍和讨论。最后，我们将概述在新型柔性光学和电子设备中为二氧化钒带来的剩余挑战和新机遇。我们希望这个帐户将在未来的工作中激发新的创新设计、制造方法和柔性 VO ₂薄膜的更多可能功能。