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.

Self-assembled monolayers (SAM) of ω-(4′-methylbiphenyl-4-yl)alkanethiols CH 3 (C 6 H 4 ) 2 (CH 2 ) n SH (BPn, n ) 1-6) on Au(111) substrates, prepared at room and elevated temperatures, were studied using scanning tunneling microscopy (STM). Molecularly resolved images reveal that all BPn SAMs form well-ordered layers over areas easily exceeding 50 × 50 nm 2 . Only two basic structures are alternatingly adopted with n changing between odd and even. The unit cell of odd-numbered SAMs is described by an oblique (2 3 × 3)R30°s tructure and contains two molecules. In contrast, the even-numbered SAMs are described by a much larger, rectangular (5 3 × 3) structure with eight molecules per unit cell and occupying an area per molecule larger by about 25% compared to n ) odd. With the exception of BP1 and BP6 the preparation at elevated temperatures resulted in a significant improvement in structural quality, yielding very large domains. For BP6 prepared at 343 K a strong domain anisotropy is observed, which is explained by the influence of the alkane spacer chain. For BP1 prepared at 343 K formation of gold islands is concluded.

Selenolate is considered as an alternative to thiolate to serve as a headgroup mediating the formation of self-assembled monolayers (SAMs) on coinage metal substrates. There are, however, ongoing vivid discussions regarding the advantages and disadvantages of these anchor groups, regarding, in particular, the energetics of the headgroup-substrate interface and their efficiency in terms of charge transport/transfer. Here we introduce a well-defined model system of 6-cyanonaphthalene-2-thiolate and -selenolate SAMs on Au(111) to resolve these controversies. The exact structural arrangements in both types of SAMs are somewhat different, suggesting a better SAM-building ability in the case of selenolates. At the same time, both types of SAMs have similar packing densities and molecular orientations. This permitted reliable competitive exchange and ion-beam-induced desorption experiments which provided unequivocal evidence for a stronger bonding of selenolates to the substrate as compared to the thiolates. Regardless of this difference, the dynamic charge transfer properties of the thiolate- and selenolate-based adsorbates were found to be nearly identical, as determined by the core-hole-clock approach, which is explained by a redistribution of electron density along the molecular framework, compensating the difference in the substrate-headgroup bond strength.

Self-assembled monolayers (SAMs) of two omega-(4'-methylbiphenyl-4-yl)alkanethiols (CH(3)(C(6)H(4))(2)(CH(2))(n)SH, BPn, n = 4, 6) on Au(111) substrates, prepared from solution at room temperature and subsequently annealed at temperatures up to 493 K under a nitrogen atmosphere, were studied using scanning tunneling microscopy (STM), high-resolution X-ray photoelectron spectroscopy (HRXPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In striking contrast to BPn SAMs with n = odd, for which only one phase is observed, the even-numbered BPn SAMs exhibit polymorphism. Irreversible phase transitions occur which involve three phases differing substantially in density and stability. Upon annealing, BP4 and BP6 transform into a beta-phase, which is characterized by an exceptionally high structural quality with virtually defect-free domains exceeding 500 nm in diameter. Exchange experiments, monitored by contact angle measurement, reveal that the beta-phase exhibits a dramatically improved stability. The fundamental differences in the phase behavior of even- and odd-numbered BPn SAMs are discussed in terms of two design strategies based on cooperative and competitive effects.

The formation and molecular structure of self-assembled monolayers (SAMs) of anthracene-2-thiol (AnT) on Au(111) have been characterized by reflection adsorption infrared spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption spectroscopy, scanning tunneling microscopy, and low energy electron diffraction. It is demonstrated that highly ordered monolayer films are formed upon immersion, but their quality depends critically on the choice of solvents and rinsing conditions. The saturated monolayer is characterized by a closed packed arrangement of upright standing molecules forming a (2 x 4)rect unit cell. At about 450 K a partial desorption takes place and the remaining molecules form a dilute (4 x 2)-phase with an almost planar adsorption geometry, while further heating above 520 K causes a thermally induced fragmentation. According to their different densities both phases reveal very diverse chemical reactivities. Whereas the saturated monolayer is stable and inert under ambient conditions, the dilute phase does not warrant any protection of the sulfur headgroups which oxidize rapidly in air.

Self-assembled monolayers (SAMs) formed from bis(biphenyl-4-yl) diselenide (BBPDSe) on Au(111) and Ag(111) substrates have been characterized by high-resolution X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared reflection absorption spectroscopy, water contact angle measurements, and scanning tunneling microscopy (STM). BBPDSe was found to form contamination-free, densely packed, and well-ordered biphenyl selenolate (BPSe) SAMs on both Au and Ag. Spectroscopic data suggest very similar packing density, orientational order, and molecular inclination in BPSe/Au and BPSe/Ag. STM data give a similar intermolecular spacing of 5.3 +/- 0.4 A on both Au and Ag but exhibit differences in the exact arrangement of the BPSe molecules on these two substrates, with the (2 square root[3] x square root[3])R30 degrees and (square root[3] x square root[3])R30 degrees unit cells on Au and Ag, respectively. There is strong evidence for adsorbate-mediated substrate restructuring in the case of Au, whereas no clear statement on this issue can be made in the case of Ag. The film quality of the BPSe SAMs is superior to their thiol analogues, which is presumably related to a better ability of the selenolates to adjust the surface lattice of the substrate to the most favorable 2D arrangement of the adsorbate molecules. This suggests that aromatic selenolates represent an attractive alternative to the respective thiols.

Self-assembled monolayers of omega-(4'-methylbiphenyl-4-yl) alkane thiols CH3(C6H4)2(CH2)(n)SH (BPn, n = 2, 3, and 5) on Au(111) substrates, prepared at room and elevated temperatures, were studied using scanning tunneling microscopy. In contrast to the biphenyl thiol analogues with n = 0 or 1, ordered domains of large size are formed which exhibit small, periodic height variations on a length scale of several nanometers. These are attributed to solitons (or domain walls), resulting from structural mismatch between the molecular adlayer and the gold substrate. The implications of these results for the design of aromatic thiols to cope with stress and yield low-defect density self-assembled monolayers are discussed.

Self-assembled monolayers of omega-(4'-methylbiphenyl-4-yl) butanethiol (H3C-C6H4-C6H4-(CH2)n-SH) on Au(111) substrates were investigated with scanning tunneling microscopy and contact angle measurements. A striking polymorphism was observed upon annealing, and structural changes were paralleled by a switch in stability against exchange by other thiols from unstable to stable. The phase formed at temperatures above 413 K was characterized by a very high structural perfection over areas exceeding 105 nm2. The results suggest an additional dimension in the control of structure and properties of thiol monolayers if different factors contributing to the energetics of SAMs enter in a competing rather than a cooperative way.