Orientation and order in microcontact-printed, self-assembled monolayers of alkanethiols on gold investigated with near edge x-ray absorption fine structure spectroscopy

Abstract

Time dependent density functional theory study of the near-edge x-ray absorption fine structure of benzene in gas phase and on metal surfaces J. Chem. Phys. 129, 064705 (2008); 10.1063/1.2967190Effect of self-assembled monolayer film order on nanofriction Dipole-induced structure in aromatic-terminated self-assembled monolayers-A study by near edge x-ray absorption fine structure spectroscopyNear edge x-ray absorption fine structure spectroscopy ͑NEXAFS͒ was used to investigate contact-printed ͑CP͒, self-assembled monolayers ͑SAMs͒ of alkanethiols on gold. We studied the influence on order and orientation of the alkyl chains of both the chain length of the thiols and the concentration of the inking solution that was used for stamping. The results are compared to those for monolayers prepared by the conventional technique of immersing the substrate into solution. Our study shows that the CP process, when performed with a sufficiently concentrated solution of the thiol, can result in monolayers indistinguishable from those established by immersion into solution, independent of the thiol chain length. For lower concentrations, however, CP monolayers show a significant deviation from well-ordered films and have to be classified as mainly disordered. As with SAMs prepared from solution, shorter chains ͑dodecanethiol͒ lead to a lower degree of order compared to longer ones ͑hexadecanethiol͒, which can be attributed to their relatively higher percentage of gauche conformations at room temperature. Nevertheless, with lateral force microscopy a slight contrast is still observable between immersed and microcontact-printed ͑CP͒ areas of well-ordered films, as has been previously reported by other groups and attributed to slight differences in the packing density of the chains in the differently prepared regions of the final layer. On the basis of our NEXAFS data the difference of gauche defects due to different average packing densities in the resulting layers can be estimated to be less than 30%.

Assignment of NEXAFS Resonances in Alkanethiols and Their Implication on the Determination of Molecular Orientation of Aliphatic SAMs

“…Applying the same evaluation route to data acquired for ODT SAMs formed on the latter surface, we obtained a remarkably larger tilt of about 41° and a twist β ≈ 47° for the single-chain model and α ≈ 40° in the case of the two-chain model for an orthogonal herringbone arrangement of the CCC-planes. These numbers are comparable to other NEXAFS studies − and reflect the lower level of molecular order in SAMs on polycrystalline substrates due to large density of defects such as steps and grain boundaries, as emphasized in an earlier work . We note that the reduced film quality may not only be attributed to structural defects of the metal substrate but may also be caused by surface contaminations since the polycrystalline gold samples were not subject to additional cleaning (by ion sputtering) prior to the immersion.…”

“…Self-assembling monolayers (SAMs) have become a subject of intense research in surface science due to their useful capability to tailor surface properties, such as, e.g., wettability, adhesion, lubrication, and work function. − A vast majority of related studies have been carried out for organothiol-based SAMs with aliphatic backbones such as alkanethiols on gold surfaces because of their ease of preparation by immersion, hence serving as a prototypical and extensively characterized model system. , Despite intense research, the exact binding geometry and adsorbate coupling of such supposed model systems are, however, still controversially debated. , One important parameter of such films is the orientation of the backbones with respect to the substrate surface. Although densely packed SAMs with saturated hydrocarbon backbones generally reveal an upright molecular orientation on gold, a tilting of the alkyl chains occurs as a consequence of the structural optimization of the intermolecular van der Waals forces with the boundary condition of specific adsorption sites at the metal substrate. , For this tilt angle, α, with respect to the surface normal, very different values are reported in the literature, ranging from 20 to 40°, ,− which on one hand depend on the applied technique, whereas on the other hand might be attributed to the different crystallinity of the used substrates, as will be discussed later.…”

“…Other strategies to determine the average molecular orientation from dichroism measurements involve evaluation of the peak height or area of R * and/or σ*­(C–C), − ,, often using a plane-orbital approximation while assuming a 4-fold symmetric orbital shape . Alternatively, difference spectra are utilized to eliminate angular independent background, however, requiring a well-known reference value. ,,, On the basis of a theoretical analysis for the case of n -octane adsorbed on copper, Weiss et al have shown that the main NEXAFS resonance ( R *) is dominated by two transitions with corresponding TDMs oriented not only perpendicular to the molecular long axis but also perpendicular to each other . To avoid additional electronic substrate interactions due to chemisorption on metal surfaces, similar studies were later also performed for alkanes on graphite or graphene-coated metal substrates. , …”

Highly Oriented, Self-Assembled Alkanephosphate Monolayers on Tantalum(V) Oxide Surfaces

“…33° between the alkyl chain axis and the surface normal). An error of less than 3° was calculated (for details see ref ).…”

“…To compensate for the severe charging of the samples, a low-electron-energy flood gun was used . Data analysis was carried out by means of difference spectra according to the analysis procedure described previously. − NEXAFS spectroscopy probes transitions of core electrons into unoccupied molecular orbitals. Since these excitations are governed by dipole selection rules, the polarization dependence of their signal intensities can be used to extract information on the orientation of molecular adsorbates …”