Vacuum Applications of Ionic Liquids: Fundamentals, Processing, and Emerging Interfaces

Before ionic liquids (ILs) gained popularity, liquids had been rarely used in most vacuum processes because of their nature of volatility and outgassing. ILs can remain in liquid state in high vacuum and have paved a new frontier of vacuum engineering for liquidus interfaces and related nanostructures. This review synthesizes vacuum applications of ILs from the perspective of the vacuum engineering of IL interfaces, linking the interfacial chemistry of structure and reaction to process application and device integration. We shall first summarize thermophysical constraints (vapor pressure, decomposition) and then insights into IL/vacuum and IL/solid interfaces gained in ultrahigh vacuum (UHV) surface science and vacuum-compatible electrochemistry, including electric double layers and interfacial reactions. We then attempt to connect these fundamentals to vacuum-enabled processing such as sputter nanoparticle synthesis, IL-assisted vapor-liquid-solid growth, deposition of ultrathin IL-derived films, and beam-driven patterning. Device-relevant examples include iontropic electronics, tribology and thermal management under vacuum, and IL ion sources for propulsion and focused ion beam technologies. Finally, we outline opportunities to extend the IL interface engineering application from planar models to porous and hybrid solids, including metal-organic frameworks, host-guest architectures, perovskite interfaces, and solid-state IL derivatives.