Multifunctional Flexible Vanadium Dioxide Films

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 degrees 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 degrees 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.