Effect of Ring Substitution Position on the Structural Conformation of Mercaptobenzoic Acid Self-Assembled Monolayers on Au(111)

Abstract

Near edge X-ray absorption fine structure (NEX-AFS)spectroscopy, photoemission spectroscopy (PES) and contact angle measurements have been used to examine the structure and bonding of self-assembled monolayers (SAMs) prepared on Au(111) from the positional isomers of mercaptobenzoic acid (MBA). The isomer of MBA and solvent chosen in SAM preparation has considerable bearing upon film morphology. Carbon K -edge NEXAFS measurements indicate that the monomers of 2-, 3-and 4-MBA have well-defined orientations within their respective SAMs. Monomers of 3-and 4-MBA assume an upright orientation on the Au substrates in monolayers prepared using an acetic acid in ethanol solvent. The aryl ring and carboxyl group of these molecules are tilted from the surface normal by a colatitudal angle of ∼ 30• . Preparation of 4-MBA SAMs using pure ethanol solvent, a more traditional means of synthesis, had no appreciable effect upon the monomer orientation. Nonetheless, S(2p) PES measurements illustrate that it results in extensive bilayer formation via carboxyl group hydrogen-bonding between 4-MBA monomers. In 2-MBA monolayers prepared using acetic acid/ethanol solvent, the monomers adopt a more prostrate orientation on the Au substrates, in which the aryl ring and carboxyl group of the molecules are tilted ∼ 50• from the surface normal. This configuration is consistent with an interaction between both the mercaptan sulfur and carboxyl group of 2-MBA with the underlying substrate. S(2p) and C(1s) PES experiments provide supporting evidence for a bidentate interaction between 2-MBA and Au(111).

The Role of a Double Molecular Anchor on the Mobility and Self‐Assembly of Thiols on Au(111): The Case of Mercaptobenzoic Acid

“…For 4-MBAt here are two different states for oxygen, O1 and O2, with binding energies of about 532.7 and 531.4 eV,r espectively.T he first component O1 can be related to the carboxyl of the neutral species, whereas O2 corresponds to the oxygen atoms of the electronically delocalized carboxylate species. [22,27] Therefore, in agreement with previous resultsf rom CK-edge NEXAFS experiments of the MBAi somers on Au(111), [21] XPS data confirmt hat 4-MBA molecules are chemisorbed on the Au(111)s urface through the Sa tom, with the COOH group at the outer part of the SAM, whereas 2-MBA moleculesa re bound to the substrate through both the Sa nd the Oa toms from the carboxylate group. [22,27] Therefore, in agreement with previous resultsf rom CK-edge NEXAFS experiments of the MBAi somers on Au(111), [21] XPS data confirmt hat 4-MBA molecules are chemisorbed on the Au(111)s urface through the Sa tom, with the COOH group at the outer part of the SAM, whereas 2-MBA moleculesa re bound to the substrate through both the Sa nd the Oa toms from the carboxylate group.…”

“…Therefore, based on this result and on the STM images (Figure 1d-e), we can assign the orderedd omains of 2-MBA molecules to the same (4 p 3) lattice but with q = 0.12. [21] This behavior,h owever,h as not been observedf or 2-MBA SAMs,t he surface coverage of which is 0.11, irrespectiveo fb othi mmersion time and solution concentration. The larger q value, the sharper peak, and the shift of E p toward negative potentials [20] (Figure 2, black line) are all consistentw ith ad enser,m ore ordered, and more stable 4-MBA SAM compared with that for 2-MBA.…”

“…The additional anchorh as as tronge ffecto nt he E b value that increases to À3.02 eV,t hus adding 0.53 eV for the two Oa tom-Au interactions (Figure 4b,i nset). [21] Although surface free energy (g)v alues reveal that model Bi st hermodynamically more stable than model A, it is clearly more unstable than the 4-MBA lattice, because the former has half of the surface coverage (0.125 vs. 0.25) (Table 1). [21] Although surface free energy (g)v alues reveal that model Bi st hermodynamically more stable than model A, it is clearly more unstable than the 4-MBA lattice, because the former has half of the surface coverage (0.125 vs. 0.25) (Table 1).…”

XPS and NEXAFS studies of aliphatic and aromatic amine species on functionalized surfaces

“…NEXAFS is well known to be a powerful technique to determine the orientation of molecules adsorbed on surfaces. Intense peaks in the difference spectra are indicative of polarization dependence in the orbital responsible for the NEXAFS resonance and, therefore, of well-defined bond/functional group orientation [36,48]. The mean tilt angle of the carbon chain in SAM 3 relative to the surface normal was estimated to be 48°f ollowing a procedure described in Ref.…”

Surface-Enhanced Raman Spectroscopy on Two-Dimensional Networks of Gold Nanoparticle−Nanocavity Dual Structures Supported on Dielectric Nanosieves

“…The tilting angle of 30°for 4-MBA is adopted from the literature. [61][62][63] The values of for two orientations are 1 and 3.84°, respectively.…”