Bond-selective dissociation of alkanethiol based self-assembled monolayers adsorbed on gold substrates, using low-energy electron beams

Abstract

We have conducted a study of electron-stimulated reactions in butanethiol, octanethiol, dodecanethiol, and hexadecanethiol monolayers adsorbed onto Au/mica substrates, using in situ infrared spectroscopy to quantify the processes; the electron dose dependence of the depletion of various C–H stretching modes has permitted the determination of the first dissociation cross sections for electron stimulated reactions in self-assembled organic monolayers. Electron-induced dehydrogenation of alkanethiol/Au/mica films in the 0–15 eV regime is shown to proceed principally via dissociative electron attachment, thus confirming previous work that directly measured H2 desorption yields during irradiation. The dissociation probabilities exhibit a well-resolved maximum at 10 eV, with a full-width at half-maximum of ∼4 eV. Unlike previous studies, our spectroscopic investigation shows that the dehydrogenation is not uniformly distributed throughout the organic film, but is strongly localized near the methyl terminations of the film. The dissociation cross sections at this interface increase rapidly with increasing chain length. We have shown that these increases are not due to the interaction of the dissociative anionic state with the film via charge-induced dipole forces, nor are they due to interactions with the metal substrate via charge-image charge forces. Our results are consistent with a dipole-image dipole quenching model, whereby the excited state lifetimes are reduced from an estimated ∼26 fs (for a gas-phase electron-alkane collision) to ∼2–10 fs, depending on the chain length. These distance-dependent lifetimes cause the dissociation yields for short-chain systems to be significantly lower than long-chain systems, and it is predicted that the electron-induced dissociation cross sections for alkanethiol monolayers should show much stronger isotopic dependencies than found with the gas-phase alkane species.

Electron induced dissociation of trimethyl (methylcyclopentadienyl) platinum (IV): Total cross section as a function of incident electron energy

“…Quenching occurs when an electron is donated to or from the substrate. 22 As a result of quenching, the probability that an adsorbate molecule dissociates becomes lower and experimentally such an effect would be observed as a smaller tot . However, a more detailed study is necessary to clarify the detailed role of the substrate and the decomposition mechanism.…”

Oxygen Attachment on Alkanethiolate SAMs Induced by Low-Energy Electron Irradiation

“…In the integrated yield functions of 18 OH − (Figure 3b), warming the samples to 150 K results in an increase of the signal, but only for the SAMs formed from long chain alkanes (i.e., DDT and ODT); the maximum integrated yield is observed at 150 K. For SAMs comprised of short chain molecules (i.e., BT and BM), the behavior of the 18 OH − integrated yield as a function of temperature resembles that for 18 O − integrated yield (Figure 3a). This dependence of the 18 OH − integrated yield on the length of the alkane chains is plausibly related to the resistance towards degradation induced by LEE irradiation for short chains compared to long chains (Figure 1), which was previously observed on alkanethiolate SAMs irradiated by LEEs. 4,18,19,41 These results suggest that the binding of 18 O 2 is related to the formation of available sites of attachment in the films during LEE bombardment, thus implying a electron-induced process.…”

“…It has already been established that LEE (1-50 eV) impact induces chemical modification in hydrocarbon SAMs. 4,12,18,19 Cleavage of C-C, Au-S, C-S and C-H bonds occurs, although the latter process seems the most important. 4,12,18 For E i near 10 eV, the cross section for C-H dissociation is at a maximum due to dissociative electron attachment (DEA), and this permits an effective but 'gentle' modification of the chemical structure of the SAM by promoting loss of hydrogen.…”

“…4,12,18 For E i near 10 eV, the cross section for C-H dissociation is at a maximum due to dissociative electron attachment (DEA), and this permits an effective but 'gentle' modification of the chemical structure of the SAM by promoting loss of hydrogen. 18 Dehydrogenation can cause for n-alkanethiolate aliphatic SAMs, a loss of structure in the film by formation of C=C bonds, irregular cross-linking, or reduction of film thickness. 1,12 In the case of aromatic SAMs, cross-linking can happen after dehydrogenation.…”

“…Thus it was shown that the observed enhanced desorption was due to an electron-induced process, and that O 2 can react with LEE induced damage sites in DNA. This paper describing ESD measurements n-alkanethiolate SAMs dosed with 18 O 2 has two objectives: first, to present an outline for a dry method to functionalize a surface with O 2 using LEE irradiation. With this technique, in contrast to plasma treatment, the SAM could remain largely undamaged.…”