Self-assembled monolayers of a series of ω-(4‘-methyl-biphenyl-4-yl)-alkanethiols (CH3−C6H4−C6H4−(CH2) m −SH, m = 1−6) formed on polycrystalline gold and silver surfaces were characterized in detail by contact angle measurements, optical ellipsometry, X-ray photoelectron spectroscopy (XPS), reflection absorption infrared spectroscopy (IRRAS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The orientation of the biphenyl moiety, determined by combining the results from IRRAS and NEXAFS, exhibits a pronounced dependence on the number of methylene groups. Similar to n-alkanethiols an odd−even effect is observed which on silver is opposite to that on gold. For m = odd on gold and m = even on silver the arrangement of the aromatic moieties agrees well with the bulk structure of biphenyl, and the bonding of the thiols to the substrate is in agreement with an sp3 hybridization of the sulfur on gold and sp on silver, respectively. In the opposite case of m = even on gold and m = odd on silver, the biphenyl moieties adopt a significantly more canted orientation which, as a consequence, results in a lower coverage. The odd−even behavior of the coverage is in sharp contrast to that seen for n-alkanethiols. The experiments provide evidence that a significant driving force exists to pertain the sp3 and sp hybridization of sulfur on gold and silver, respectively. In the case of gold substrates the experimental results are in conflict with available bending potentials derived from ab initio calculations.

Synchrotron-based high-resolution X-ray photoelectron spectroscopy was applied to characterize self-assembled monolayers (SAM) of biphenyl-substituted alkanethiols CH 3 (C 6 H 4 ) 2 (CH 2 ) n SH (BPn, n ) 1-4) on Au and Ag substrates. Beyond previously identified odd-even changes in the packing density and the tilt angle of the biphenyl moieties, the high-resolution spectra reveal a number of additional odd-even effects upon variation of the number of methylene groups in the aliphatic part in the BPn molecule. Their occurrence and mutual correlation suggests that a BPn SAM represents a strongly correlated, highly ordered molecular assembly. In particular, periodical changes of a shake up feature in the C 1s region are observed, which are related to the differences in the arrangement of the aromatic matrix. The width and binding energy position of the S 2p signals also exhibit odd-even changes. The width changes are associated with the occupation of either equivalent or nonequivalent adsorption sites on the polycrystalline (111) Au and Ag substrates. The comparison of the width values with those for conventional alkanethiols implies that the substrate bonding of alkanethiols on gold cannot be described by a single adsorption site.

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Self-assembled monolayers formed by adsorption of 4-methyl-4‘-mercaptobiphenyl, CH3-(C6H4)2−SH (BPT), on Au(111) have been studied using scanning tunneling microscopy. The results show that with increasing coverage (resulting from longer immersion times) BPT forms a series of different structural phases with different molecular arrangements, closely resembling the behavior reported previously for n-alkanethiolate adlayers. For short immersion times, striped structures are observed (α and β), where the molecules are orientated with their axes parallel to the surface. Longer immersion times yield additional phases, namely, an ordered χ-phase, where the molecules are proposed to be oriented with their molecular axis tilted away from the surface, a disordered δ phase, and, finally, a densely packed (2√3 × √3) ε phase where the molecular axes are orientated almost upright. Unexpectedly, for BPT after adsorption small islands are seen on the Au substrate instead of the etch pits commonly observed after formation of organothiolate adlayers.

The low-energy electron-induced damage in self-assembled monolayers (SAMs) formed from ω-(4‘-methylbiphenyl-4-yl)alkanethiols CH3(C6H4)2(CH2) n SH (BPn, n = 0, 1, 4, 5, and 12) on gold substrates was studied. The pristine and heavily (8000 μC/cm2) irradiated films were characterized in detail by X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, infrared reflection absorption spectroscopy, and advancing contact angle measurements. In contrast to SAMs of conventional alkanethiols but similar to pure aromatic thiol-derived systems, only minor damage is observed for the aliphatic−aromatic BPn films. In particular, the orientational order and anchoring to the substrate are retained upon the irradiation. At the same time, C−H bond scissions in the aromatic part occur, leading to a cross-linking between the neighboring biphenyl moieties. Whereas the general behavior of the BPn SAMs with respect to electron irradiation is qualitatively similar, the extent of the irradiation-induced changes depends on the packing of these systems. The densely packed BP1 and BP5 SAMs are much more stable with respect to electron bombardment than the less densely packed BP4 films. The relation between the packing density and the extent of the irradiation-induced changes seems to be a general phenomenon in monomolecular films, which provides a tool to tailor the reaction of these systems toward ionizing radiation for lithographic applications.

Self-assembled monolayers (SAMs) formed from [1,1‘-biphenyl]-4,4‘-dimethanethiol (BPDMT) and [1,1‘;4‘,1‘ ‘-terphenyl]-4,4‘ ‘-dimethanethiol (TPDMT) on Au were characterized by X-ray photoelectron spectroscopy (XPS), high-resolution XPS, infrared reflection absorption spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and water contact angle measurements. The results of all experimental techniques suggest the formation of densely packed and highly oriented SAMs for both BPDMT and TPDMT, with a slightly higher packing density and a smaller molecular inclination in TPDMT/Au. All molecules were found to be bound to the substrate via the thiolate link, i.e., by one of the thiol groups, whereas the second thiol group is located at the SAM−ambient interface. This suggests that aromatic dithiols are well-suited for the fabrication of thiol-terminated SAMs. Such films are of particular importance for molecular electronics, since the thiol group has a high affinity to metals and can be used as a chemical link between a metal nanowire and the molecule.

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Self-assembled monolayers of ω-(4′-methyl-biphenyl-4-yl)-dodecyl thiol [CH3–C6H4-C6H4–(CH2)12–SH,BP12] on gold were patterned via exposure to 300 eV electrons. Subsequent copper deposition in an electrochemical cell revealed behavior opposite to that of electron beam patterned monolayers of alkanethiols. Whereas alkanethiols act as a positive resist and lead to copper deposition only on irradiated parts, the biphenyl based thiol acts as a negative resist. At the irradiated areas the layer exhibits blocking behavior and copper deposition is observed only on the nonirradiated parts.