Structure of Thioaromatic Self-Assembled Monolayers on Gold and Silver

Abstract

Self-assembled monolayers (SAMs) formed from thiophenol, 1,1′-biphenyl-4-thiol, 1,1′;4′,1′′-terphenyl-4-thiol, and anthracene-2-thiol on polycrystalline Au and Ag were characterized by X-ray photoelectron spectroscopy and angle-resolved near-edge X-ray absorption fine structure spectroscopy. With the exception of the poorly defined thiophenol film on Au, all thioaromatic molecules were found to form highly oriented and densely packed SAMs on both substrates. The molecular orientation and orientational order of the adsorbed thioaromatic molecules depends on the number of aromatic rings, the substrate, and the rigidity of the aromatic system. The molecules, which on average are slightly inclined with respect to the surface normal, show a less tilted orientation with increasing length of the aromatic chain, and as observed for aliphatic SAMs, they exhibit smaller tilt angles on Ag than on Au. However, the difference in the tilt angles for aromatic SAMs on Au and Ag is smaller than that observed in the aliphatic films. A comparison of the monolayers formed from p-terphenylthiol and anthracenethiol films suggests that a higher molecular rigidity has only a slight effect on the final molecular orientation within the respective SAMs.

Analysis of the near-edge X-ray-absorption fine-structure of anthracene: A combined theoretical and experimental study

“…In contrast, NEXAFS spectra of oriented films of π-conjugated molecules exhibit a noticeable dichroism of π* resonances which enables a quantitative characterization of molecular orientation [6][7][8][9][10][11]. This approach exploits the fact that the intensity of π* resonances depend on the relative orientation of E according to I(π * ) ∝ |E · T| 2 , where T denotes the transition dipole moment which is oriented normal to the ring plane of aromatic molecules [1].…”

“…Moreover, due to the high brightness of modern synchrotron sources such measurements can be readily carried out for adsorbates even at submonolayer coverage. The recent development in the field of organic electronics has led to a renewed interest in NEXAFS spectroscopy as an important tool to characterize the growth and structure of π-conjugated molecular materials [6][7][8][9][10][11]. Though the large lifetime of excitations into unoccupied π-orbitals in such materials give rise to rather sharp and distinct π*-resonances in the corresponding NEXAFS spectra, a precise assignment and identification of these signatures is required in order to fully exploit the potential of NEXAFS-based analyses.…”

Tunneling across SAMs Containing Oligophenyl Groups

“…22,30 Oligophenylene groups present in a SAM adopt a near-planar conformation and pack in a herringbone structure. 22,31 Characterization of SAMs of C≡CPh n on gold indicates that the acetylene group binds in an upright configuration on gold. [32][33][34] A recent report from Zaba and coworkers demonstrates that it is possible to form highly-ordered SAMs of n-alkyl acetylenes on gold in non-oxidizing environments; 35 Table S7) than saturated n-alkanes where the HOMO-LUMO gap is much larger (~7 eV).…”

Orientation and bonding of biphenyldimethyldithiol

“…Although they are very popular as candidates for use in junction devices, there are clearly problems associated with the use of alkane thiols [6]. Biphenyl and terphenyl functionalized by one or two thiols and one or two methyl groups have been extensively investigated [7][8][9][10][11][12][13][14] by x-ray photoemission spectroscopy (XPS), reflection absorption infrared (RAIR) spectroscopy, near-edge x-ray absorption fine structure (NEXAFS) spectroscopy, scanning tunnelling microscopy (STM), ellipsometry and advancing the water contact angle.…”