Modification and Stability of Aromatic Self-Assembled Monolayers upon Irradiation with Energetic Particles

Abstract

We have studied ion and electron irradiation of self-assembled monolayers (SAMs) of 2-(4'-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3-C6H4C6H4CH2CH2-SH), phenyl mercaptan (PEM, C6H5CH2CH2-SH), and 4'-methyl-biphenyl-4-thiol (BP0, CH3-C6H4C6H4-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of molecules. This feature is the result of a desorption process caused by a chemical reaction initiated by an ion impact. Both for ions and electrons, experiments indicate that the possibility for scission of the Au-S bond strongly depends on the chemical nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of molecules. The analysis of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact molecule or specific fragment. Analysis of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymerization process induced by electron irradiation.

Ion‐Beam‐Induced Desorption as a Method for Probing the Stability of the Molecule‐Substrate Interface in Self‐Assembled Monolayers

“…Herein we propose using ion‐induced desorption in combination with neutral fragment mass spectrometry as a method to analyze the chemical stability of the molecule–substrate interface in complicated and technologically relevant SAMs. This work builds on previous studies analyzing the behavior of SAMs on a Au(111) substrate upon bombardment with beams of Ar + ions in the keV range 5–8. It was shown that the bombardment of aromatic thiol SAMs on a Au(111) substrate results in the ejection of neutral molecular fragments via two distinct desorption mechanisms.…”

“…Finally, we note that in the presented method, chemical reactions of reactive molecular fragments produced by the primary ion impact probe the chemical stability of the molecule–substrate bonding in SAMs. Since a low dose of primary ions (∼10 10 ions/cm 2 , see the Experimental Section) is used for this analysis (static conditions), and the produced reactive species quickly move out of the primary ion impact zone,8 the proposed method analyzes, fast and locally, the unperturbed structure of the molecule–substrate interface in SAMs. This is in contrast to relatively slow thermal or electrochemical desorption experiments where, due to the global temperature19–21, 29, 35 or potential36 ramping, respectively, the molecule–substrate interface in SAMs can reconstruct before the desorption process takes place.…”

Odd‐Even Effects in Ion‐Beam‐Induced Desorption of Biphenyl‐Substituted Alkanethiol Self‐Assembled Monolayers

“…[24,25] The bombardment of aromatic thiol SAMs on a Au(111) substrate with beams of Ar + ions in the keV range results in the ejection of neutral molecular fragments via two distinct desorption mechanisms. [24,[26][27][28][29] A small percentage of the SAM fragments is desorbed with high kinetic energies (~eV) as a result of the direct momentum transfer from the collision cascade that develops in the substrate upon primary ion impact. The majority of desorbing particles, however, leaves the surface with low kinetic energies (~10 À2 eV) as a result of gentle (i.e.…”

“…Performing photon-fluence dependence measurements on gas-phase molecules demonstrated that the fragment m/z 168, the desulphurized molecule (BPnS -S), and the parent molecule (BPnS) are direct products of the ion-induced desorption process, while the other molecular fragments, including m/z 181, at least partly stem from photofragmentation of larger molecular entities. [24,25,29] A normalization of the mass spectra is necessary in order to correct for small experimental variations between the different sets of measurements. For normalization we use the total detected amount of desorbed molecular material with low kinetic energy.…”

Fabrication of carbon nanomembranes by helium ion beam lithography

“…Ar + , Ga + , Si + , etc. ), which leads to the desorption and the fragmentation of molecules [14–15]. High energy helium ions passing through polymer films modify the macroscopic properties of these films, too.…”