Flexible 2D Structure Formation of [C₁C₁Im][Tf₂N] on Ag(111)

The first layer of the ionic liquid (IL) 1,3-dimethylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C₁C₁Im][Tf₂N]) on Ag(111) is studied under ultrahigh vacuum using scanning tunneling and atomic force microscopy, and the observed results are compared to the behavior found on Au(111), Cu(111), and Pt(111). On Ag(111), two stable phases are observed, 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 (IP) ranging from 0.77 to 0.85 nm². In contrast, the striped phases behave differently: While for Au(111) and Cu(111), similar surface areas per IP as for the hexagonal phases were observed, the area per ion pair for Ag(111) varies from 0.50 to 1.14 nm². This broad range results from a large variation of next neighbor intra- and interrow distances. These very substantial differences are attributed to the specific interplay of lateral interactions within IL rows and vertical interactions 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).