Pronounced Odd−Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Based Thiol SAMs:  A Combined STM and LEED Study

Abstract

Self-assembled monolayers (SAMs) of ω-(4‘-methylbiphenyl-4-yl) alkanethiols CH3(C6H4)2(CH2) n SH (BPn, n = 3 and 4) on Au(111) substrates were studied using scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Preparation at elevated temperature results in highly ordered layers with large domains. Whereas the (2√3 × √3) structure of the BP3 SAMs is similar to that reported previously for other aromatic thiols, SAMs made from BP4 exhibit a very different structure. A (5√3 × 3)rect unit cell containing 8 molecules is found which corresponds to a packing density reduced by 25% compared to that of BP3. The odd−even effect observed in the molecularly resolved STM images of BP3 and BP4, therefore, confirms the pronounced influence of the spacer chain on the structure of these biphenyl-based SAMs.

Temperature-induced phase transition; Polymorphism in BP2 SAMs on Au(111)

“…The variation in topographic heights of the adsorbed molecules can be attributed to the adoption of different sulfur adsorption sites on Au(111). The α-phase is well-known from our previous studies of BPn SAMs, and is characteristic of BPn systems with an even number of CH 2 units prepared from solutions at room temperature or a little higher [32,40] …”

“…The structure formed by the odd-numbered SAMs was found to be (2√3×√3)R30° structure. In contrast, the evennumbered SAMs were described by a much larger (5√3×3) structure with an area per molecule larger by about 25% compared to n=odd (32). However, these structures are not the only structures observed for aromatic thiols on Au(111).…”

“…Unlike alkanethiol-based SAMs, whose structure is largely independent of the chain length, the structure and the ordering mechanism of aromatic SAMs is more complicated. A conspicuous example of this complexity is found in SAMs generated from biphenyl derivatives BPn (CH 3 (C 6 H 4 ) 2 (CH 2 ) n SH) [32,40] and terphenyl derivatives TPn ((C 6 H 4 ) 3 (CH 2 ) n SH) [23]. These films were found to exhibit a pronounced alternation in both molecular packing and orientation with changing the parameter n from odd to even.…”

Existence of isostatic, maximally random jammed monodisperse hard-disk packings

“…In three dimensions (3D), hard particle packings have served as simple, yet powerful models for a wide variety of condensed matter systems including liquids, glasses, colloids, particulate composites, and biological systems, to name a few (1-5). In two dimensions (2D), they have been used to model systems such as the molecular structure of monolayer films (6,7), adsorption of molecules on substrates (8,9), and the organization of epithelial cells (10,11). Moreover, particular interest has been devoted toward packings that are jammed (roughly speaking, packings that are mechanically stable) (12)(13)(14)(15)(16)(17)(18).…”

Effect of Halo Substitution on the Geometry of Arenethiol Films on Cu(111)

“…A FT is similar to a LEED pattern taken on a single-domain film, but it avoids film degradation common with conventional LEED systems. 15 The Cu(111) step height is used as an internal calibration standard, which allows direct comparison between the film structure at 81 K and the Cu(111) periodicity at 15 K.Careful analysis of Figure 1d,f allows us to deduce the periodicity inside the island: BTP exhibits a (4 × 3) pattern, which corresponds to a coverage of 1 BTP per 12 Cu atoms, i.e., 0.083 ML. In the (4 × 3) structure, all molecules are aligned in parallel and the Br and S atoms of neighboring molecules face each other.…”