Growth of Multilayers of Ionic Liquids on Au(111) Investigated by Atomic Force Microscopy in Ultrahigh Vacuum

Abstract

Understanding the growth of ultrathin films of ionic liquids (ILs) on metal surfaces is of highest relevance for a variety of applications. We present a detailed study of the growth of the wetting layer and successive multilayers of 1,3-dimethylimidazolium bis- Au(111). By atomic force microscopy (AFM) in ultrahigh vacuum, we follow the temperature-dependent behavior between 110 and 300 K at defined coverages. We initially observe the formation of a wetting layer with a thickness of ∼0.37 nm with anions and cations arranged in a checkerboard structure. Stable AFM imaging up to 280 K allows us to follow the IL growing on top of the wetting layer in bilayers with an average thickness of ∼0.71 nm, that is, double the height of the wetting layer, in a bilayer-by-bilayer fashion. This growth behavior is independently confirmed from the surface morphology, as deduced from AFM and angle-resolved X-ray photoelectron spectroscopy. High-resolution AFM images at 110 K allow for identifying the molecular surface structure of the bilayers as a striped phase, which is one of the phases also seen for the wetting layer (

Time- and Temperature-Dependent Growth Behavior of Ionic Liquids on Au(111) Studied by Atomic Force Microscopy in Ultrahigh Vacuum

“…Ultrapure [C 1 C 1 Im][Tf 2 N] (for synthesis see ref ( 27 )) was deposited following the same routine as described previously. 33 Note that in the literature, [C 1 C 1 Im] + is also referred to as [MMIm] + 42 and [Tf 2 N] − as [NTf 2 ] − 43 or [TFSA] − . 3 The Au(111) crystal was purchased from MaTecK and cleaned by repeated Ar + sputtering and annealing until the terraces were a few hundred nanometres broad and appeared clean in AFM.…”

“…The deposited amount was precisely determined using a calibrated QCM that was positioned besides the crystal during deposition. 33 The deposited amounts are given in WLE, that is, multiples of the dose required to form one complete densely packed wetting layer with a coverage of 1 WL. Please note that in the literature coverages are often given in ML, which for ILs has been defined as the surface fully covered with pairs of cations and anions on top of each other.…”

“… 17 , 27 − 31 Only very recently, the first UHV-based atomic force microscopy (AFM) studies addressing IL growth have been published. 32 , 33 Notably, scanning tunneling microscopy (STM) is not suited to study IL growth behavior, due to too strong tip effects, which occur for layers on top of the first wetting layer. 33 , 34 …”

“… 32 , 33 Notably, scanning tunneling microscopy (STM) is not suited to study IL growth behavior, due to too strong tip effects, which occur for layers on top of the first wetting layer. 33 , 34 …”

“… 17 , 37 , 39 When adsorption is done at low temperature, typically a statistical growth (hit-and-stick) is observed. 33 , 41 Upon heating to RT, for some systems the formation of flat 2D layers is proposed, 33 , 35 while for others dewetting, that is, the formation (ripening) of 3D islands occurs. 18 …”