Self-assembled monolayers of perfluoroterphenyl-substituted alkanethiols: specific characteristics and odd–even effectsw Frederick Chesneau, a Bjo¨rn Schu¨pbach, b Katarzyna Szela ˛ gowska-Kunstman, c Nirmalya Ballav,z a Piotr Cyganik,* c Andreas Terfort* b and Michael Zharnikov* a Received 22nd April 2010, Accepted 2nd July 2010 DOI: 10.1039/c0cp00317d Self-assembled monolayers (SAMs) formed by perfluoroterphenyl-substituted alkanethiols (C 6 F 5 –C 6 F 4 –C 6 F 4 –(CH 2 ) n –SH, FTPn) with variable length of the aliphatic linker (n = 2 and 3) were prepared on (111) Au and Ag and characterized by a combination of several complementary spectroscopic and microscopic techniques. A specific feature of these systems is the helical conformation of the FTP moieties, which, along with the high electronegativity of fluorine, distinguishes them from the analogous non-fluorinated systems and makes them attractive for different applications. The SAMs were found to be well-defined, highly ordered, and densely packed, which suggests a perfect correlation between the orientations and, in particular, twists of the FTP helices in the adjacent molecules. Significantly, the SAM exhibited pronounced odd–even effects, i.e. a dependence of the molecular orientation and packing density on the length of the aliphatic linker in the target molecules, with parity of n being the decisive parameter and the direction of the effects on Au opposite to that on Ag. The presence of the odd–even effects in the FTPn system brings new aspects into the discussion about the origin and mechanism of these phenomena. Specifically, the helical conformation of the FTP moieties in the dense phase excludes a variation of the intramolecular torsion and molecular twist as the mechanism behind the odd–even effects. 1. Introduction The current progress in microelectronics is driven by the increasing density of the individual building blocks. This demand, along with the growing costs of standard top-down fabrication procedures upon further miniaturization, requires new strategies and materials. Promising systems in this regard are functional molecules, which can self-assemble into highly ordered aggregates (bottom-up behavior), revealing a variety of useful properties. In particular, self-assembled monolayers (SAMs), 1–6 the properties of which can be ‘pre-programmed’ by the design of their molecular components, have been used for a number of applications in the area of microelectronics, such as insulators, 7,8 charge-carrier injection layers, 9–14 or for the control of organic semiconductor growth. 12,15 In addition, SAM-like assemblies of the individual ‘‘device’’ molecules serve as prototypes of future molecular electronics. 16,17 To achieve efficient charge transport through the mono- layer, the use of aromatic molecules is essential. 18–20 However, direct attachment of such moieties to metal substrates (electrodes) usually does not result in the formation of well- ordered monolayers, 21–24 hampering application of aromatic SAMs in prototype and real devices. A reason for this behavior is presumably a strong interaction between the aromatic moieties in the film, which are frequently prone to form an arrangement that is not completely commensurate with the substrate. 25 The arising stress results in the formation of small domains and appearance of dislocations and other defects. 23,24,26 For the most popular thiolate–gold systems, one of the remedies is the substitution of the sulfur atoms by selenium ones, since these experience a lower corrugation of the headgroup–substrate binding potential that permits the adoption of less preferred adsorption sites, decoupling the optimal molecular arrangement and the structural template provided by the substrate. 27–30 Another, presumably more general approach is the introduction of a short alkyl linker between the oligophenyl moiety and the headgroup. 31–42 In the respective monolayers, such a linker decouples the structure adopted by the aromatic part from that of the substrate and transfers the predominant bonding geometry given by the substrate-specific bending potential to the aromatic moiety. 31–33,35 This results in either smaller or larger inclination of the latter moiety depending on the bond angle at the binding atom (which itself is a function of the substrate metal) and the parity of the number of methylene groups in the linker. 31,32,35,38,41,43 a Angewandte Physikalische Chemie, Universita ¨t Heidelberg, 69120 Heidelberg, Germany. E-mail: Michael.Zharnikov@urz.uni-heidelberg.de; Fax: +49 6221-54-6199; Tel: +49 6221-54-4921 b Institut fu ¨r Anorganische und Analytische Chemie, Universita ¨t Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany. E-mail: aterfort@chemie.uni-frankfurt.de; Fax: +49 69-798-29188; Tel: +49 69-798-29180 c Research Centre for Nanometer-Scale Science and Advanced Materials (NANOSAM), Faculty of Physics, Astronomy, and Applied Computer Science, Jagiellonian University, Reymonta 4, 30-059 Krako ´w, Poland. E-mail: piotr.cyganik@.uj.edu.pl; Fax: +48 12 6324888; Tel: +48 12 6635838 w Electronic supplementary information (ESI) available: Experimental set-up, conditions, and additional spectroscopic data. See DOI: 10.1039/c0cp00317d z Present address: Laboratory for Micro and Nanotechnology, Paul Scherrer Institut, 5232 Villigen, Switzerland. This journal is  c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 12123–12137 | 12123 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Downloaded by UB Heidelberg on 27 September 2010 Published on 09 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00317D View Online