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

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.

Selective terminal function modification of SAMs driven by low-energy electrons (0-15 eV).

Low-energy electron induced degradation of a model self-assembled monolayer (SAM) of acid terminated alkanethiol was studied under ultra-high vacuum (UHV) conditions at room and low (~40 K) temperatures. Low-energy electron induced chemical modifications of 11-mercaptoundecanoic acid (MUA, HS-(CH2)10-COOH) SAMs deposited on gold were probed in situ as a function of the irradiation energy (<11 eV) by combining two complementary techniques: High Resolution Electron Energy Loss Spectroscopy (HREELS), a surface sensitive vibrational spectroscopy technique, and Electron Stimulated Desorption (ESD) analysis of neutral fragments. The SAM's terminal functions were observed to be selectively damaged at around 1 eV by a resonant electron attachment mechanism, observed to decay by CO, CO2 and H2O formation and desorption. CO2 and H2O were also directly identified at low temperature by vibrational analysis of the irradiated SAMs. At higher irradiation energy, both terminal functions and spacer alkyl chains are damaged upon electron irradiation, by resonant and non-resonant processes.

Low-energy electron scattering on deuterated nanocrystalline diamond films-a model system for understanding the interplay between density-of-states, excitation mechanisms and surface versus lattice contributions.

Electron energy loss spectrum, elastic reflectivity and selected vibrational excitation functions were measured by High Resolution Electron Energy Loss Spectroscopy (HREELS) for deuterated nanocrystalline dc GD CVD diamond films. The electron elastic reflectivity is strongly enhanced at about 13 eV, as a consequence of the second absolute band gap of diamond preserved up to the surface for D-nano-crystallites. The pure bending modes δ(CD(x)) at 88 meV and 107 meV are dominantly excited through the impact mechanism and their vibration excitation functions mimic the electron elastic reflectivity curve. Pure diamond phonon mode ν(CC) can be probed through the resolved fundamental loss located at 152 meV and through the multiple loss located at 300 meV. In addition to the well-known 8 eV resonance, two supplementary resonances located at 4.5 eV and 11.5 eV were identified and clearly resolved for the first time. A comprehensive set of data is now available on low-energy electron scattering at hydride terminated polycrystalline diamond films grown either by HF (microcrystalline) or dc GD (nanocrystalline) chemical vapour deposition. The careful comparison of the vibrational excitation functions for hydrogen/deuterium termination stretching modes ν(sp(3)-CH(x)) and ν(sp(3)-CD(x)), for hydrogen termination bending modes δ(CH(x)) mixed with diamond lattice modes ν(CC), for deuterium termination bending modes δ(CD(x)), and for multiple loss 2ν(CC) demonstrates the close interplay between three characteristics: (i) the density-of-states of the substrate, (ii) the vibrational excitation mechanisms (dipolar and/or impact scattering including resonant scattering) and (iii) the surface versus lattice character of the excited vibrational modes. This work shows clearly that excitation function measurement provides a powerful and sensitive tool to clarify loss attributions, involved excitation mechanisms, and surface versus lattice characters of the excited vibrational modes.

Chemistry induced by low-energy electrons in condensed multilayers of ammonia and carbon dioxide.

We have investigated by means of HREEL spectroscopy electron induced reactivity in a binary CO2 : NH3 ice mixture. It was shown that the interaction of low energy electrons (9-20 eV) with such mixtures induces the synthesis of neutral carbamic acid NH2COOH and that flashing the sample at 140 K induces the formation of ammonium carbamate. The products have been assigned by FTIR spectroscopy of a CO2 : NH3 mixture heated from 10 K to 240 K. A mechanism involving dissociation of NH3 molecules into NH2* and H* radicals is proposed to explain the product formation.