Recent Progress in Vacuum Engineering of Ionic Liquids

Abstract

Since the discovery of ionic liquids (ILs) as a new class of liquid that can survive in a vacuum at room temperature, they have been aimed at being characterized with vacuum analysis techniques and used in vacuum processes for the last two decades. In this review, our state-of-the-art of the vacuum engineering of ILs will be introduced. Beginning with nanoscale vacuum deposition of IL films and their thickness-dependent ionic conductivity, there are presented some new applications of the ellipsometry to in situ monitoring of the thickness of IL films and their glass transitions, and of the surface thermal fluctuation spectroscopy to investigation of the rheological properties of IL films. Furthermore, IL-VLS (vapor-liquid-solid) growth, a vacuum deposition via IL, has been found successful, enhancing the crystallinity of vacuum-deposited crystals and films, and sometimes controlling their surface morphology and polymorphs. Among recent applications of ILs are the use of metal ions-containing IL and thin film nano IL gel. The former is proposed as a low temperature evaporation source of metals, such as Ta, in vacuum deposition, while the latter is demonstrated to work as a gate electrolyte in an electric double layer organic transistor.

Carbon-Induced Changes in the Morphology and Wetting Behavior of Ionic Liquids on the Mesoscale

“…The use of thinner IL films demonstrates the potential for various interfacial applications in electronic devices. The possibility of employing small quantities of carbon as a strategy to spread an IL film holds promise for enhancing the effectiveness of these films as solvents, particularly in vacuum conditions, such as in the crystallization of organic semiconductors. ,, …”

“…The possibility of employing small quantities of carbon as a strategy to spread an IL film holds promise for enhancing the effectiveness of these films as solvents, particularly in vacuum conditions, such as in the crystallization of organic semiconductors. 8,12,60 In recent studies, it was found that long-chain ILs exhibit complete spreading onto Au surfaces, resulting in the growth of the IL film layer by layer. 62,63 This suggests that identical morphologies would likely be obtained on both Au and C/Au surfaces with a homogeneous and compact IL film covering the entire surface.…”

“…These interactions contribute to the 2D growth process. Coalesced films have been experimentally observed, for instance, for [C 8 C 1 im]­[OTf] and [C 8 C 1 im]­[NTf 2 ]. ,, On other surfaces, such as soda-lime glass, glass coated with conductive oxides, silver, graphene, nickel, as well as mica surfaces, the formation of droplets has been consistently observed for imidazolium-based ILs. ,,,,,,− , The formation of larger droplets with enhanced wetting ability is further promoted when longer alkyl chain lengths are attached to the IL cation. ,,, …”

“…The influence of cations, anions, and the length of the alkyl chain on film wetting and morphology has been investigated. Additionally, the effects of deposition rate, substrate temperature, and roughness on nucleation, droplet formation, and spreading during the film deposition process have all been addressed. ,,,,− The nucleation and growth mechanisms of IL films fabricated through thermal evaporation are primarily governed by three key factors: the surface diffusion of ion pairs, the availability of a minimum free area to promote nucleation, and the subsequent coalescence processes . These processes are significantly influenced by the affinity of the ion pairs to the substrate surface as the surface diffusion of ion pairs, driven by their search for favorable binding sites, plays a crucial role in the initial stages of film formation.…”

“…In recent years, there has been extensive research on the properties and applications of ionic liquid (IL) films at the nano- and mesoscale levels. This research interest stems from the remarkable properties exhibited by ILs when used as interfacial materials. − One notable characteristic of most ILs is their low volatility, making them a unique class of compounds exceptionally suitable for a wide range of vacuum applications. − ILs exhibit exceptional thermal and chemical stability, being in the liquid state across a wide temperature range, retaining their structural integrity. − Moreover, ILs demonstrate remarkable solvating properties and can serve as solvents and electrolytes, even under vacuum conditions. − The chemical structure of ILs can be tailored to meet specific requirements related to various properties, such as density, viscosity, surface tension, and adhesion strength. − Moreover, by careful selection of cations and anions in the studied ILs, customizable surface and interfacial properties can be obtained, allowing precise control over wettability, surface energy, and other related characteristics. − These exceptional properties make ILs highly desirable for coating applications. − …”