Amine Terminated SAMs: Investigating Why Oxygen is Present in these Films - PubMed (original) (raw)

Amine Terminated SAMs: Investigating Why Oxygen is Present in these Films

J E Baio et al. J Electron Spectros Relat Phenomena. 2009.

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 S(2p), C(1s) and N(1s) 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.

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Figures

Figure 1

Figure 1

High-resolution XPS C1s spectra of the Cl−NH3+(CH2)11SH powder (bottom) and the NH2-SAM on Au (top). In both cases the spectra have peaks at 285eV (CHx) and 286.6eV (C-N/C-O).

Figure 2

Figure 2

High-resolution XPS N1s spectra of the Cl−NH3+(CH2)11SH powder (bottom) and the NH2-SAM on Au (top). The powder spectrum has a single peak at 401.4eV (protonated amine). The NH2-SAM on Au spectrum has peaks at 399.5eV (C-N) and 401.4 eV (protonated amine).

Figure 3

Figure 3

High-resolution XPS S2p spectra of the Cl−NH3+(CH2)11SH powder (bottom) and the NH2-SAM on Au (top). Each spectrum was fit using one S2p doublet with a splitting of 1.2eV.

Figure 4

Figure 4

NEXAFS spectra of the nitrogen _K_-edge for amine SAMs acquired at angles of 70°, 55° and 20° along with the 70 and the 20° difference spectrum.

Figure 5

Figure 5

NEXAFS spectra of the carbon _K_-edge for the NH2- and CH3-terminated SAMs acquired at 55° along with their respective difference spectra (70°-20°).

Figure 6

Figure 6

SFG spectrum of NH2- (upper trace) and CH3-terminated (lower trace) SAMs on gold. Solid lines are best fits based on equation 1.

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