Degradation of functionalized alkanethiolate monolayers by 0–18 eV electrons

Abstract

Electron stimulated desorption of neutral molecular fragments is used to study degradation of ordered organic thin films under low-energy (0–18 eV) electron impact, and total electron doses ranging between 180–550 μC/cm2. Different saturated linear thiols HS(CH2)nX (n=2 or 15, and X=CH3 or COOH) are adsorbed from solution onto a gold surface to produce a self-assembled monolayer (SAM). Here, we present yield function measurements for electron stimulated desorption of moities such as H2, CH3, CH3CH2, CH3CH2CH2, CO, and CO2 from such thin chemisorbed films. For CH3-terminated SAMs, neutral fragment desorption thresholds lie between 5–7 eV, whereas for COOH-terminated SAMs, desorption thresholds as low as 0.2 and 3–5 eV are observed. The results suggest that the incident electrons interact with functional groups localized at the film–vacuum interface, which then leads to predominantly methyl group C–H, and C–COOH bond cleavage. In addition to nonresonant degradation mechanisms, which vary monotonically from threshold with increasing incident electron energy, structures in the neutral fragment desorption yield functions are related to resonant electron attachment. Particularly for Au–S(CH2)15COOH monolayers, this mechanism leads to a desorption peak of CO fragments at incident electron energies near 1.0 eV.

Electronically induced modification of thin layers on surfaces

“…Here we focus on the last point, using them for investigations of basic temperature effects on beam induced modifications. Many studies of irradiation effects in SAMs by electrons and to a lesser extent also by photons are reported in the literature, from which we cite three articles in which the reader can find most of the relevant references [8][9][10]. The majority of these previous reports are room temperature studies, thus neglecting any low-temperature effects.…”

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

“…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.…”

“…4,12,18,19 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. 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.…”

“…High-energy electrons (e.g., E > 1 keV), as they impact the substrate, produce large quantities of secondary LEEs which possess energies within a distribution lying mostly below 20 eV. 4 The interactions of these secondary electrons, in addition to the multiple electron scattering effects, limit the spatial resolution obtainable with high energy electrons by broadening the effective area of irradiation. 4,13 Such problems can be minimized when using low-energy beams by irradiation through a mask 14,15 or by tunneling LEEs from the tip of a scanning tunnel microscope (STM) into the SAM, to induce specific reactions at different positions on the surface.…”

“…4 The interactions of these secondary electrons, in addition to the multiple electron scattering effects, limit the spatial resolution obtainable with high energy electrons by broadening the effective area of irradiation. 4,13 Such problems can be minimized when using low-energy beams by irradiation through a mask 14,15 or by tunneling LEEs from the tip of a scanning tunnel microscope (STM) into the SAM, to induce specific reactions at different positions on the surface. 16,17 Effectively, another potential advantage of SAM modification by LEE irradiation is the selectivity of damage by the precise control of the incident electron energy (E i ).…”