Odd−Even Effects at the S-Metal Interface and in the Aromatic Matrix of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers

Abstract

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.

Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold

“…The latter value was also found in a recent study of biphenyl-based thiols BPn with even chain length (n = 4, 6) [21]. Heister et al report on a small odd-even effect for BP3 and BP4 with binding energies of 159.94 and 159.90 eV, respectively [20]. The deconvolution of the TF-Az6 compound suggests a small contribution at a binding energy of 161.0 eV (S 2p 3/2 ).…”

“…The sulphur XPS data are dominated by the 2p doublet with an intensity ratio of two (2p 3/2 vs. 2p 1/2 ). The binding energies of 162.0 eV (2p 3/2 ) for Az6, C6, and TF-Az6 are generally assigned to the thiolate species [20]. The latter value was also found in a recent study of biphenyl-based thiols BPn with even chain length (n = 4, 6) [21].…”

“…The latter is also larger than the FWHM of 0.50 eV reported for highly ordered BPn SAMs on gold [20,21]. Moreover, for Az6 and TF-Az6, the Au 4f 7/2 level exhibits an FWHM of 0.62 eV (not shown) as compared to 0.40-0.48 eV for BPn, n = 1-4 [20].…”

“…The deconvolution of the TF-Az6 compound suggests a small contribution at a binding energy of 161.0 eV (S 2p 3/2 ). The latter is attributed either to atomic sulphur or intact molecules bound at distinctly different adsorption sites [20,[22][23][24]. Such a doublet has also been observed for BP4 and BP6, Table 1 XPS data for S 2p 3/2 , C 1s, and N 1s for C6, Az6, and TF-Az6 films on Au(111) on mica, respectively.…”

Near-Edge X-ray Absorption Fine Structure Spectroscopy of Diamondoid Thiol Monolayers on Gold

“…dodecane-1-thiol). In addition, strain in Au-S-C angle reduces the S 2p binding energy [52][53][54] . Structures 1, 3, 6, and 8, with similar local sulfur environments, cannot assume the nominal, highly canted S-C polar angle of 70-76°2 2 observed for the alkane thiols (e.g., the lower pane of Figure 5).…”