Characterization of self-assembled monolayers by low energy reactive ion scattering spectrometry: Influence of terminal group composition and structures

Low energy (tens of eV) polyatomic cations were used as probes for characterization of monolayers of spontaneously chemisorbed thiols on gold. Characteristics including chemical composition, surface order and orientation of the terminal group of the self-assembled monolayers (SAMs) can be derived by monitoring the products of projectile ion neutralization (electron transfer through SAMs), surface-induced dissociation (SID), and ion-surface reactions.; To study the influence of the terminal group chemical structures and orientations of the SAMs on ion-surface interactions, a series of newly developed semi-fluorinated alkane thiols with difluoromethylenes buried underneath hydrocarbon terminal groups were examined (CH3CF2CH2- and CH3CH2CF2-). Compared to terminally fluorinated SAMs, they showed more projectile ion neutralization and less internal to vibrational energy (T→V) deposition into precursor ions. Projectile ion-hydrocarbon reactions were reduced significantly when difluoromethylenes are one or two bonds away from the terminal methyl or ethyl group. Furthermore, ion-surface reaction results on these surfaces with odd and even chain lengths suggested that they have similar terminal methyl orientations to their hydrocarbon counterparts.; Mixed monolayers of CF3CF2(CH2)14SH (F-SAMs) and CH3(CH2)15SH (H-SAMs) with systematically changing electron transfer, energy deposition and ion-surface reaction characteristics were prepared using mixed thiols solution deposition (homogeneously mixed) and micro-contact printing (mu-CP). The solution mixture system showed linear variations in electron transfer and energy deposition with different F-SAM 30 surface concentrations, while non-linear changes occur for ion-surface reaction suggesting strong lateral interactions between the two components. These interactions are minimized in the mu-CP system which contains domains of each thiol. Energy deposition on the patterned surfaces varies non-linearly with changing F-SAM concentration which differs from the homogenously mixed system.; With the SAMs systems described above, three dimensional arrangements of fluorocarbons and hydrocarbons were achieved. Reactive ion scattering spectrometry (RISS) results provided more in-depth knowledge of low energy ion-surface interactions that will promote usage of RISS as a novel surface characterization technique.; To explore Surface-induced dissociation (SID) with a 90° collision angle, a series of protonated peptide ions were collided with SAMs in an in-line sector Time-Of-Flight (TOF) mass spectrometer. The results were compared to keV collision-induced dissociation (CID) data collected with the same instrument. Fragmentation efficiency for eV SID was higher than keV CID for those peptides. In addition to the excellent control over laboratory collision energies with SID, different amount of translational to vibrational energy conversion can be achieved when varying surface composition, e.g. using mixed F-SAM/H-SAM.