Influence of the Deposition Rate and Substrate Temperature on the Morphology of Thermally Evaporated Ionic Liquids

Abstract

The wetting behavior of ionic liquids (ILs) on the mesoscopic scale considerably impacts a wide range of scientific fields and technologies. Particularly under vacuum conditions, these materials exhibit unique characteristics. This work explores the effect of the deposition rate and substrate temperature on the nucleation, droplet formation, and droplet spreading of ILs films obtained by thermal evaporation. Four ILs were studied, encompassing an alkylimidazolium cation (CnC1im) and either bis(trifluoromethylsulfonyl)imide (NTf2) or the triflate (OTf) as the anion. Each IL sample was simultaneously deposited on surfaces of indium tin oxide (ITO) and silver (Ag). The mass flow rate was reproducibly controlled using a Knudsen cell as an evaporation source, and the film morphology (micro- and nanodroplets) was evaluated by scanning electron microscopy (SEM). The wettability of the substrates by the ILs was notably affected by changes in mass flow rate and substrate temperature. Specifically, the results indicated that an increase in the deposition rate and/or substrate temperature intensified the droplet coalescence of [C2C1im][NTf2] and [C2C1im][OTf] on ITO surfaces. Conversely, a smaller impact was observed on the Ag surface due to the strong adhesion between the ILs and the metallic film. Furthermore, modifying the deposition parameters resulted in a noticeable differentiation in the droplet morphology obtained for [C8C1im][NTf2] and [C8C1im][OTf]. Nevertheless, droplets from long-chain ILs deposited on ITO surfaces showed intensified coalescence, regardless of the deposition rate or substrate temperature.

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

“…Figure depicts the morphology obtained for the four ILs deposited on ITO (the first column), C/ITO (the second column), Ag (the third column), and C/Ag (the fourth column). The morphology of the droplets of [C 2 C 1 im]­[NTf 2 ] and [C 2 C 1 im]­[OTf] deposited on ITO (Figure a,i) and Ag/ITO surfaces (Figure c,k) is similar, in agreement with recent studies. ,, …”

“…The experimental results are listed in Figures 4 and 6 4a,i) and Ag/ ITO surfaces (Figure 4c,k) is similar, in agreement with recent studies. 62,63,67 There is no significant influence of the anion nature (NTf 2 or OTf) or the substrate (ITO/glass or Ag/ITO/glass) on the morphology of the droplets formed by the thermal evaporation of C 2 C 1 im-based ILs. Larger droplets are formed for…”

“…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.…”

“…8,12,48,53,57,58,[61][62][63]67 The formation of larger droplets with enhanced wetting ability is further promoted when longer alkyl chain lengths are attached to the IL cation. 57,62,63,67 Even in a vacuum environment, every surface inevitably acquires carbon contamination. In fact, in real-world scenarios, it is often challenging to completely eliminate carbon from surfaces.…”