Odd-even effects in ion-beam-induced desorption of biphenyl-substituted alkanethiol self-assembled monolayers.

Due to the ease of their preparation and their relatively high stability, self-assembled monolayers (SAMs) are promising candidates to be used in the development of microand nanostructured materials with various functionalities. The formation of SAMs is mainly driven by a combination of molecule– substrate and intermolecular interactions. So far, most of the fundamental studies of SAMs structures have been performed on simple alkanethiols chemisorbed on the Au(111) substrate. 2] The importance of alkane length odd-even effects in SAMs has been recently reviewed. In particular, the odd-even effect was observed in the reaction of low-energy pyrazine and [D6]benzene molecular ions with the terminal group (e.g. CH3 or CF3) of aliphatic SAMs/Au(111). [4–6] Thus, a possibility of using low-energy molecular ion beams for analysis of the molecule–vacuum interface in SAMs was clearly demonstrated. More recently, aromatic thiols have moved into the focus of interest mainly due to their potential use in molecular electronics. However, the stress which originates from the misfit between the structure preferred by the aromatic moieties and the structural template provided by the Au(111) substrate usually results in higher defect concentration in SAMs of aromatic thiols on Au(111). One way to overcome this problem can be realized by introducing an alkane spacer chain between the thiol head group and the aromatic moiety, as demonstrated in previous studies on the model system of biphenyl-substituted alkanethiols BPnS [CH3 (C6H4)2 (CH2)n SH, n=1–6] on a Au(111) substrate. By addition of the alkane spacer the individual thiolates forming these SAMs have additional degrees of freedom through which stress is reduced without breaking up the film structure. However, insertion of the flexible alkane spacer also exerts a crucial influence on the molecular orientation and packing, and has much deeper consequences on the film structure and stability. Earlier microscopic and spectroscopic studies revealed that BPnS/Au(111) SAMs exhibit a pronounced alternation in molecular structure between n= odd and n=even. As a result the odd-numbered BPnS/Au(111) SAMs are characterized by higher density structures, with the BPnS molecules in a more upright orientation, as compared to even-numbered systems (Figure 1). Importantly, this odd-even