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

Abstract

[a] During recent years self-assembled monolayers (SAMs) have been recognized as model structures for different areas in nanotechnology, ranging from nanolithography and molecular electronics to sensors and biocompatible materials.[1] A growing range of possible new SAM applications demands better understanding of their detailed molecular structure, which is a result of balancing molecule-molecule and molecule-substrate interactions in the monolayer. Despite the numerous structural studies of SAMs available nowadays, the structure and stability of the SAM-substrate interface is still poorly understood. Even for a most simple SAM system of methanethiol on Au(111), identification of the adsorption geometry (the AuÀS interface) remains controversial. [2,3] As a consequence, the experimental and theoretical analysis of the bonding geometry and the stability of the molecule-substrate interface for technologically relevant, and therefore more complicated SAMs, is extremely difficult. Importantly, this missing information remains fundamental for most of SAMs potential applications with molecular electronics in particular.[4]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][6][7][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. A minor percentage of the SAM fragments leave the surface with high kinetic energy (~eV) as a result of the direct momentum transfer from the collision cascade developed in the substrate upon primary ion impact. The overwhelming majority of desorbing particles leaves the surface with low kinetic energies (~10 À2 eV) as a result of "gentle"cleavage of chemical bonds (with no significant momentum transfer) within the organic layer by chemical reactions initiated by reactive fragments (e.g. radicals) created in the organic film as a result of the primary ion impact. Following these findings, ion-induced desorption experiments were performed for aromatic SAMs that form different structural phases on the Au(111) surface. [9] These experiments unambiguously demonstrated that the ion-induced cleavage of chemical bonds in SAMs is extremely sensitive to details of their geometric and electronic configuration, due to the chemical reaction mechanisms involved. To further investigate this effect, a homologue series of 4,4'-biphenyl-substituted alkanethiols [CH 3 ÀC 6 H 4 ÀC 6 H 4 À(CH 2 ) n ÀSH; BPnS; n = 1-6] deposited on a Au(111) substrate was analyzed.[10] Former microscopic [11] and spectroscopic [12,13] experiments demonstrated that, depending on the parity of the number of CH 2 units (the parameter n is odd or even), two different structure...

Oscillations in the Stability of Consecutive Chemical Bonds Revealed by Ion‐Induced Desorption

“…Two distinct desorption mechanisms leading to the emission of neutral molecular fragments were identified by previous studies on collision-cascade and chemical-reaction based processes. [11][12][13][14] Whereas the chemical reaction mechanism quite naturally explains the high sensitivity of emission towards the stability of the respective cleaved bond, such sensitivity for the energetic collision-cascade based emission is not obvious. To address this issue we have performed simulations of BP3S/Au(111) sputtering induced by 8 keV Bi atoms using MD simulations.…”

“…In short, two issues must be considered: the desorption and the ionization mechanism. Two distinct desorption mechanisms leading to the emission of neutral molecular fragments were identified by previous studies on collision‐cascade and chemical‐reaction based processes 11–14. Whereas the chemical reaction mechanism quite naturally explains the high sensitivity of emission towards the stability of the respective cleaved bond, such sensitivity for the energetic collision‐cascade based emission is not obvious.…”

“…As detailed in the Supporting Information, this presumably is not the case. Generally, experiments analyzing emission of neutral fragments13, 14 and the DFT calculations for these SAMs15 with the possible ionization process by electron transfer16 or by excited precursor fragmentation16 show that the ionization process does not influence significantly the correlation between desorption process and stability of a cleaved chemical bond.…”

Nachweis der Oszillation in der Stabilität konsekutiver chemischer Bindungen durch Ionen‐induzierte Desorption

“…Es stellt sich aber auch die Frage, ob ein ähnlicher Effekt innerhalb der beiden homologen Serien separat existiert und wie sich die jeweiligen Veränderungen zueinander verhalten. Wie in Abbildung 2 a-c dargestellt, existiert eine signifikante gerade/ [11][12][13][14] Während sich die hohe Sensitivität der Emission bei chemischen Reaktionsmechanismen intuitiv durch die Stabilität der zu spaltenden Bindung erklären lässt, ist ein solcher Zusammenhang der Sensitivität bei Emission aufgrund energetischer Kollisionskaskaden nicht offensichtlich. Um diese Fragestellung zu klären, haben wir MD-Simulationen für den Sputter-Prozess von BP3S/Au-(111)-SAMs mit Bi + bei 8 keV durchgeführt.…”

“…Wie in den Hintergrundinformationen detailliert dargestellt wird, ist dies vermutlich nicht der Fall. Allgemein lässt sich festhalten, dass die experimentelle Analyse der Neutralfragmentemission [13,14] sowie DFT-Rechnungen für diese SAMs [15] mit den mçglichen Ionisierungsmechanismen via Elektronentransfer [16] oder via Fragmentierung angeregter Präkursoren [16] zeigen, dass der Ionisierungsprozess keinen signifikanten Einfluss auf die Korrelation zwischen Desorptionsprozess und der Stabilität der gespaltenen chemischen Bindung hat.…”