DOI: 10.1002/cphc.201100413 Imaging an Ionic Liquid Adlayer by Scanning Tunneling Microscopy at the Solid j Vacuum Interface Thomas Waldmann, [a] Hsin-Hui Huang, [a] Harry E. Hoster, [a] Oliver Hçfft, [b] Frank Endres, [b] and R. Jürgen Behm* [a] Ionic liquids (ILs), which are salts usually consisting of sterically hindered organic ions with melting points below 100 8C, are of high interest because of a number of distinct physical proper- ties such as ionic conductivity, electrochemical stability in a wide potential window, a very low vapor pressure or low flam- mability. [1–4] These properties make ILs interesting for various applications, for example, as electrolyte (additives) in lithium ion and lithium air batteries. [5] The physical and chemical prop- erties of ILs can be tuned in their synthesis and the variety of different substances is large. [2] So far, however, there is little in- formation on the structure and processes at the phase boun- dary between solid interfaces (electrodes) and ILs on a submo- lecular scale. It has been shown recently that solvation layers of ILs form on electrodes, and that these have an influence on electrochemical reactions. [6] Such interfacial layering was ob- served by atomic force microscopy and high-energy X-ray re- flectivity measurements. [7–9] The surface structure of ILs on electrode surfaces has mainly been studied with vibrational spectroscopy (sum frequency generation; SFG) and electro- chemical impedance spectroscopy (EIS). [10] Pan and Freyland showed that PF 6 À ions can be imaged by electrochemical (in situ) STM. [11] Atkin et al. demonstrated by in situ STM that the interaction with the IL induces a restructuring of the Au(111) surface at potentials between À0.4 and À1.0 V. [12] In addition to these measurements in an electrochemical en- vironment, the interaction of ILs with solid surfaces was also probed under high-vacuum conditions, allowing to apply typi- cal surface science methods such as electron spectroscopy [13, 14] or temperature-programmed desorption (TPD). [15, 16] This offers new opportunities like measurements without a supernatant solvent and at low temperatures, which allows immobilization of the adsorbates. Monolayers of ILs on different substrates, adsorbed under UHV conditions, were investigated, for exam- ple, by electron spectroscopy, time-of-flight secondary ion mass spectrometry (TOF-SIMS) and infrared reflection absorp- tion spectroscopy (IRAS). [17–19] Recently, Cremer et al. studied the growth of 1,3-dimethyl-imidazolium bis(trifluoromethylsul- fonyl)imide ([MMIm]Tf 2 N) and 1-methyl-3-octylimidazolium bis- (trifluoromethylsulfonyl)imide ([OMIm]Tf 2 N) on Au(111) by angle-resolved X-ray photoelectron spectroscopy (ARXPS), and concluded that both anions and cations are in direct contact with the surface at a coverage of 0.5 monolayers, most likely in an alternating arrangement. [20] Detailed structural information on IL adlayers under UHV conditions, however, is scarce. In analogy to the detailed knowledge on structure formation and self-assembly of large organic molecules, [21–23] scanning tunnel- ing microscopy (STM) measurements with molecular resolution would be highly interesting to learn more about various as- pects of structure formation at the IL j solid interface. STM measurements at variable temperatures could provide detailed information on the ordering behavior of the adlayer, which in analogy to the bulk behavior can range from a two-dimension- al (2D) solid to a 2D liquid. Despite the obvious potential of STM measurements on IL adlayers under UHV conditions, such measurements have not been reported so far. Herein, we report first results of a molecular-scale STM study on the ad- sorption of 1-butyl-1-methylpyrrolidinium tris(pentafluoro- ethyl)trifluorophosphate ([Py 1,4 ] + [FAP] À , [24] see Figure 1) mono- layers on Au(111) under UHV conditions. It has been shown previously that [Py 1,4 ] + [FAP] À can be evaporated without de- composition. [16] Figure 2 a shows an STM image of [Py 1,4 ] + [FAP] À on Au(111) recorded at 298 K. The herringbone reconstruction [25] is visible through the monolayer. This observation resembles findings in previous in situ STM measurements of [Py 1,4 ] + [FAP] À on Au(111) at negative electrode potentials. [24] We did not find any indication of a vertical restructuring of the Au(111) surface in the nm regime, as it was reported for the interaction of bulk 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide [Py 1,4 ]Tf 2 N with Au(111) at low negative potentials, [12] in our Figure 1. Energy-minimized structures of a) [FAP] À and b) [Py 1,4 ] + . They have the maximum sizes that these ions can have (C: grey, H: white, F: yellow, N: blue, P: pink). [a] T. Waldmann, H.-H. Huang, Prof. Dr. H. E. Hoster, + Prof. Dr. R. J. Behm Institute of Surface Chemistry and Catalysis Ulm UniversityD-89069 Ulm (Germany) Fax: (+ 49) (0)731/50-25452 E-mail : juergen.behm@uni-ulm.de [b] Dr. O. Hçfft, Prof. Dr. F. Endres Institute of Particle Technology Clausthal University of Technology D-38678 Clausthal-Zellerfeld (Germany) [ + ] Current address: TUM CREATE Centre for Electromobility Singapore 637459 (Singapore) ChemPhysChem 2011, 12, 2565 – 2567  2011 Wiley-VCH Verlag GmbH& Co. 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