Flexible 2D structure formation of [C1C1Im][Tf2N] on Ag(111).

Abstract

We studied the first layer of the ionic liquid (IL) 1-3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C1C1Im][Tf2N]) on Ag(111) under ultrahigh vacuum conditions using scanning-tunneling and atomic-force microscopy, and compare the observed results to the behavior found on Au(111), Cu(111) and Pt(111). On Ag(111), we observe two stable phases, a stripe phase and a hexagonal phase. The hexagonal phases on Ag(111), Au(111) and Pt(111) exhibit a similar, checkerboard-like structure with surface areas per ion pair ranging from 0.77 to 0.85 nm2. In contrast, the striped phases behave differently: While for Au(111) and Cu(111) we observe similar surface areas per ion pair as for the hexagonal phases, for Ag(111) the area per ion pair varies from 0.50 to 1.14 nm2. This broad range results from a large variation of next neighbor intra- and inter-row distances. We attribute these very substantial differences to the specific interplay of lateral interactions within IL rows and the vertical interaction with the substrate. The pronounced polymorphism observed for [C1C1Im][Tf2N] on Ag(111) suggests that this system represents a special case, with the weakest adsorbate/substrate interaction among the studied systems, due to the absence of covalent interactions as compared to Cu(111), and only moderate van der Waals interactions as compared to Au(111).