Temperature-Dependent Structure Formation in the Wetting Layer of the Ionic Liquid [C₂C₁Im][OTf] on Au(111)

Growth and structure formation in ultrathin ionic liquid (IL) films have recently received considerable attention. We report on in situ studies of the structure of ultrathin films of the IL 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([C(2)C(1)Im][OTf]) on Au(111). The IL thin films were prepared by physical vapor deposition and investigated by scanning tunneling microscopy (STM). We carried out measurements at sample temperatures between 100 and 350 K on submonolayers and monolayers of [C(2)C(1)Im][OTf]. In this temperature range, we identified three different surface structures. At submonolayer coverage, [C(2)C(1)Im][OTf] tends to exhibit a 2D glass structure growing close to gold terraces and the elbows of the herringbone structure of Au(111). Oblique and hexagonal structures of the IL are visible at higher coverage. The molecular arrangement within these structures is analyzed by STM in combination with simulations using density functional theory and molecular dynamics. We investigated the phase transitions and melting behavior of these wetting layers by temperature-dependent STM measurements. Small glass-like islands of IL melt at 190-200 K, resulting in a mobile 2D liquid/gas phase. At low coverage, a mobile phase coexists with islands with highly ordered domains. At higher coverage, ordered domains are visible much above the melting temperature, i.e., at temperatures up to 350 K.