Amine terminated SAMs: Investigating why oxygen is present in these films

Abstract

Self-assembled monolayers (SAMs) on gold prepared from amine-terminated alkanethiols have long been employed as model positively charged surfaces. Yet in previous studies significant amounts of unexpected oxygen containing species are always detected in amine terminated SAMs. Thus, the goal of this investigation was to determine the source of these oxygen species and minimize their presence in the SAM. The surface composition, structure, and order of amine-terminated SAMs on Au were characterized by X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), sum frequency generation (SFG) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. XPS determined compositions of amine-terminated SAMs in the current study exhibited oxygen concentrations of 2.4 ± 0.4 atomic %, a substantially lower amount of oxygen than reported in previously published studies. High-resolution XPS results from the S2p, C1s and N1s regions did not detect any oxidized species. Angle-resolved XPS indicated that the small amount of oxygen detected was located at or near the amine head group. Small amounts of oxidized nitrogen, carbon and sulfur secondary ions, as well as ions attributed to water, were detected in the ToF-SIMS data due to the higher sensitivity of ToF-SIMS. The lack of N-O, S-O, and C-O stretches in the SFG spectra are consistent with the XPS and ToF-SIMS results and together show that oxidation of the amine-terminated thiols alone can only account for, at most, a small fraction of the oxygen detected by XPS. Both the SFG and angle-dependent NEXAFS indicated the presence of gauche defects in the amine SAMs. However, the SFG spectral features near 2865 cm−1, assigned to the stretch of the methylene group next to the terminal amine unit, demonstrate the SAM is reasonably ordered. The SFG results also show another broad feature near 3200 cm−1 related to hydrogen-bonded water. From this multi-technique investigation it is clear that the majority of the oxygen detected within these amine-terminated SAMs arises from the presence of oxygen containing adsorbates such as tightly bound water.

Amine species on self‐assembled monolayers of ω‐aminothiolates on gold as identified by XPS and NEXAFS spectroscopy

“…A minor amide π * resonance occurring at a very similar energy as σ * (N -H) cannot be fully excluded. Another feature is observed at 397.9 eV, which has been recently discussed by Baio et al, [16] and σ * (N -H) which might be π * resonances originating from unsaturated species as N C, N C and N-C C formed in course of beam damage as discussed recently. [3] Figure 2(a) shows representative C K edge NEXAFS of the ATPT SAM with typical features found for polyphenyl systems linked to an aliphatic unit.…”

“…[3,9] With the aliphatic AUDT SAMs we found the protonated amines as the dominating species. In contrast, the major N 1s component for aromatic ABT, APBT and ATPT SAMs originates from NH 2 (cf also Refs [3,16]). In case of the APBT SAM the highest fraction of free amines was obtained.…”

“…In general, two main species were observed which can be assigned to free amines (NH 2 ), found at lower BEs, ∼399.5 eV, and protonated or hydrogen-bonded amino groups (NH 3 + , H· · ·NH 2 ), respectively, at 401-402 eV. [3,16] For simplicity, the latter components will be denoted hereafter as protonated amino species (NH 3 + ) but hydrogen-bonded amino groups (H· · ·NH 2 ) are also included which are indistinguishable by XPS. Both the nitrogen component peaks may interfere with contributions from amide species as well which are located at BEs around 400-401 eV.…”

Smart water channelling through dual wettability by leaves of the bamboo Phyllostachys aurea

“…36,39,40 The latter may also be associated with hydrogen-bonding between the amide nitrogen atom and nearby hydrogen-containing functional groups, such as alcohols. 41,42 The possibilities of N(1s) amine (-C-NH2) or nitrate functionalities were excluded because they typically appear below 400 eV 42,43 and around 407-408 eV, respectively. Nitrogen has been detected in XPS studies of other leaf surface waxes.…”

Oxidative acetylenic coupling reactions as a surface chemistry tool

“…S1a shows a representative survey spectrum that indicates the presence of Si, C, and O only. As suggested by Baio et al 34 and James et al, 30h the oxygen detected is most likely arising from the presence of oxygenated co-adsorbates, such as tightly-bound water. Highresolution narrow scans were collected for the C 1s and Si 2p regions to gain information on bonding configurations and determine the extent of any oxidation of the silicon substrate.…”