Morphology of Imidazolium‐Based Ionic Liquids as Deposited by Vapor Deposition: Micro‐/Nanodroplets and Thin Films

Abstract

The morphology of micro-and nanodroplets and thin films of ionic liquids (ILs) prepared through physical vapor deposition is presented. The morphology of droplets deposited on indium-tin-oxide-coated glass is presented for the extended 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([CnC1im][Ntf2]; n= 1-8) series, and the results show the nano-structuration of ILs. The use of in-vacuum energetic particles enhances/increases the nanodroplets mobility/coalescence mechanisms and can be a pathway to the fabrication of thin IL films. Ionic liquids (ILs) are a family of materials that offer a range of properties that can be adjusted to optimize their performance in a diversity of technological applications. [1-3] ILs share common characteristics, such as ionic conductivity, good thermal stability, and extremely low vapor pressure at room temperature. [4-6] In recent years, extensive studies of the thermo-physical properties of ILs have shown that almost none of these properties are shared across the IL range due to structural/nanostructural modifications. [7-9] ILs are becoming particularly attractive for many applications, such as solvents for electro-chemistry (electrically conducting fluids, electrolytes), sealants, chemical industry, gas handling, pharmaceuticals, cellulose processing, dye-sensitized solar cells, waste recycling, batteries, dispersion of nanomaterials, and electrode-electrolyte interfaces in high-vacuum systems. [10-15] Recently, the liquid/vacuum and liquid/solid interfaces of ILs have been of great relevance for most of the above-mentioned applications, and model surface studies and molecular orientation/ordering investigations of ILs have been performed on surfaces with different chemical properties: Au, Ni, silica, graphene, alumina, glass, and mica. [16-20] In some cases, thin films of ILs have been prepared by using impregnation techniques with solutions of ILs in volatile solvents, followed by solvent removal by evaporation. Thus, nanostructures of ILs can be prepared by spin/dip-coating methods and the films have low thicknesses and exceptional lubrication characteristics. [20] In recent years, the focus has been on physical vapor deposition (PVD) of ILs on different surfaces [16, 18, 19]

Morphology, Structure, and Dynamics of Pentacene Thin Films and Their Nanocomposites with [C2C1im][NTf2] and [C2C1im][OTF] Ionic Liquids

“…The second-order coalescence is a mechanism of coalescence between droplets that had already been coalesced. [25] Attending to the geometry of microdroplets, [C , it is expected that the interfacial tension will be higher since the interaction potential between the IL and the ITO surface is lower. However, at the interface, hydrogen-bonding between the ion pairs is reported to be less relevant than the contribution of the surface entropy.…”

“…Recent experiments have proved that coalescence of all droplets and consequent thin film formation could be induced by inciting energetic particles onto the droplets, for instance by surface treatment using a plasma. [25] The effect of plasma surface treatment in ionic liquids should be analyzed with care since prolonged exposure to a beam can damage the ionic liquid structure. The sputtering process might destroy many IL molecules leaving behind nonvolatile fragments dissolved in the IL droplets.…”

“…[21][22][23] The thermal evaporation of most ILs leads to the formation of droplets instead of compact thin films even by applying large flow rates and/or deposition times. [24][25][26][27][28][29][30][31][32] Recently, the nucleation and growth mechanisms for vapor-deposited micro-and nanodroplets of ILs were found to be based on continuous nucleation and coalescence processes of first and second orders. [24] Typically, the common substrates used in the semiconductor industry like transparent conductive oxides have a low wettability to the ILs and the carbon contamination has a strong influence on their growth mode.…”

“…[17] The formation of thin IL films can be also induced by surface treatment with plasmas. [25] In-vacuum energetic particles have been used in different systems for self-assembly of nanostructures and coalescence processes. [33] In this work, we have used a high-resolution and reproducibility physical vapor deposition (PVD) [34] procedure based on the Knudsen effusion method to produce high-stable, homogeneous, and uniform pentacene films.…”

Chemical Vapor Deposition of Ionic Liquids for the Fabrication of Ionogel Films and Patterns

“…Physical vapor deposition (PVD), through direct IL evaporation and condensation on a surface, has been demonstrated for alkylimidazolium bis(trifluoromethylsulfonyl)imides (Tf 2 N). [32][33][34] However, PVD of ILs requires an ultra-high vacuum (< 10 À8 mbar) due to their low vapor pressure (< 10 À3 mbar at moderate temperatures), resulting in low deposition rates only suited for fundamental studies. [32][33][34][35] In specific cases, the surface-catalyzed cleavage of an ester-functionalized IL with a higher vapor pressure can be exploited to obtain a carboxylate-anchored IL monolayer on the surface.…”