Low-energy electron induced resonant loss of aromaticity: consequences on cross-linking in terphenylthiol SAMs

Abstract

Aromatic self-assembled monolayers (SAMs) can be used as negative tone electron resists in functional surface lithographic fabrication. A dense and resistant molecular network is obtained under electron irradiation through the formation of a cross-linked network. The elementary processes and possible mechanisms involved were investigated through the response of a model aromatic SAM, p-terphenylthiol SAM, to low-energy electron (0-10 eV) irradiation. Energy loss spectra as well as vibrational excitation functions were measured using High Resolution Electron Energy Loss Spectroscopy (HREELS). A resonant electron attachment process was identified around 6 eV through associated enhanced excitation probability of the CH stretching modes ν(CH)(ph) at 378 meV. Electron irradiation at 6 eV was observed to induce a peak around 367 meV in the energy loss spectra, attributed to the formation of sp(3)-hybridized CHx groups within the SAM. This partial loss of aromaticity is interpreted to be the result of resonance formation, which relaxes by reorganization and/or CH bond dissociation mechanisms followed by radical chain reactions. These processes may also account for cross-linking induced by electron irradiation of aromatic SAMs in general.

Classical molecular dynamics investigations of biphenyl-based carbon nanomembranes

“…As panel (C) of Figure (4) shows, the amount of destroyed rings could easily reach 50%. This is in accord with recent experimental investigations of a loss of aromaticity of terphenylthiol SAMs when being irradiated [17]. Nevertheless, the (remaining) intact phenyl rings seem to play an important role as a kind of matrix that stabilizes the resulting structures [28].…”

“…This suggests that the electron exposition (dose, time, energy) as well as the cooling dynamics play an important role for the actual structure. To mention just one aspect: In the real crosslinking process hydrogens are removed one by another in the course of time which could induce correlations for possible crosslinking pathways, namely where two hydrogens are removed at nearby sites [17]. Or is this not important and one can start the simulation – as we did – with hydrogen-free initial configurations?…”

“…In addition it is an interesting question how the theoretical findings can be compared to experiments. One promising observable is certainly the aromaticity, which can be determined spectroscopically [4,17]. Are STM investigations able to discriminate between the scenarios [3,21,23]?…”

“…Our calculations thus assume that the hydrogens are practically removed at once and not stepwise. It would be an interesting question whether a sequential removal of hydrogen would induce strong correlations of binding sites of neighboring biphenyls [17]. For such an investigation classical potentials for hydrocarbons would have to be used [13–14].…”

On-surface synthesis of aligned functional nanoribbons monitored by scanning tunnelling microscopy and vibrational spectroscopy

“…By assigning the vibrational modes, it is possible to evidence the chemical bonding between surface and adsorbate, between several adsorbates, or any chemical modification upon stimulation38394041.…”