Collisions of Silylium Cations with Hydroxyl-Terminated and Other Self-Assembled Monolayer Surfaces:  Reactions, Dissociation, and Surface Characterization

Abstract

Silylium cations, SiCl3 + and Si(CH3)3 +, undergo dissociative ion/surface reactions in the course of low-energy (20−90 eV) collisions with hydroxyl-terminated (HO−SAM), hydrocarbon (H−SAM), and fluorocarbon (F−SAM) self-assembled monolayer surfaces. Formation of the substitution product, SiCl2F+, upon collision of SiCl3 + with the F−SAM surface is the result of a transhalogenation reaction. In an analogous fashion, one observes substitution of a chlorine in the SiCl3 + projectile ion by either an OH group from the HO−SAM surface or a CH3 group from the H−SAM surface to form the scattered reaction products, SiCl2OH+ and SiCl2CH3 +, respectively. The concomitant transfer of a Cl atom from the projectile ion into the surface is indicated by the sputtered ion, CH2Cl+. The scattered product SiCl(OH)2 + involves disubstitution, and reaction with more than one chain at the surface. These and related reactions involve the activation of C−O, C−F, C−C, C−H, and O−H bonds at the appropriate surface, and they occur after, or in concert with, surface-induced dissociation of the polyatomic projectile. Surface effects on the dissociation of projectile ions are studied using the Si(C2H5)4 •+ ion, and threshold values for translational to internal energy (T ⇒ V) conversion for this ion are measured as 13%, 13%, and 20% for the H−SAM, HO−SAM, and F−SAM surfaces, respectively. At higher collision energies, (>40 eV), the HO−SAM surface demonstrated greater internal energy conversion efficiency than the H−SAM surface. The process of neutralization and the accompanying release of chemically sputtered ions also served to distinguish the three surfaces. Decreased neutralization at the F−SAM surface is associated with increased amounts of dissociatively and reactively scattered product ions. Thermodynamic estimates regarding charge exchange between the surface and the projectile ion are consistent with the relative amounts of chemically sputtered products observed for each of the surfaces.

Soft‐ and reactive landing of ions onto surfaces: Concepts and applications

“…SAM surfaces terminated with amine, hydroxyl, carboxylic acid, phosphate, aldehyde, ester, and halogen groups are susceptible to nucleophilic substitution, esterification, acylation, and nucleophilic addition reactions in solution. As discussed section 4, many of these solution‐phase reactions have also been observed between hyperthermal ions and SAMs (Wade et al, , ,; Wang et al, ; Hu et al, ; Hu & Laskin, ). Furthermore, in certain cases the efficiency of reaction is greatly enhanced for reactively landed ions compared to ions reacted in solution (Wang et al, ).…”

“…The efficiency of reactive landing is determined both by the soft‐landing efficiency described above and by the reaction mechanism with the surface. For example, silylation of HO‐SAMs by reactive deposition of silylium cations involves direct electrophilic attack of the cationic projectile ions on the terminal hydroxyl groups of the surface (Wade et al, ). Due to the fact that the rate of this reaction is determined by the electrophilicity of the reagent ion, the reactive landing efficiency improves with increasing partial charge on the silicon atom of the projectile ion (Wade et al, ).…”

“…Due to the fact that the rate of this reaction is determined by the electrophilicity of the reagent ion, the reactive landing efficiency improves with increasing partial charge on the silicon atom of the projectile ion (Wade et al, ). The reactivity is also affected by steric hindrance and the stability of the projectile ion (Wade et al, ). The effect of the kinetic energy of the ion on the efficiency of silylation reactions with Si(CH 3 ) 3 + cations is presented in Figure a.…”

“…The collision energy that produces the maximum efficiency of reactive landing depends on the reaction mechanism, the size of the projectile ion, and the properties of the surface. For example, while for the majority of small and medium size projectile ions the efficiency of reactive landing reaches its maximum value in the range of 10–50 eV (Shen et al, ; Wade et al, , , ,; Evans et al, ; Volny et al, ; Wang et al, , ; Hu et al, ), the most efficient reactive landing of larger multiply charged trypsin ions on plasma‐treated metal surfaces occurs at higher nominal kinetic energies of 130–200 eV (Volny et al, ).…”

Silylation of an OH‐terminated self‐assembled monolayer surface through low‐energy collisions of ions: a novel route to synthesis and patterning of surfaces

“…In general, the dissociative ion–surface reaction product, SiCl 2 X + , which results from scattering of SiCl 3 + is useful for analysis of surfaces. When fluorocarbon and hydrocarbon monolayer surfaces were examined in previous studies with the SiCl 3 + ion,62 SiCl 2 F + and SiCl 2 CH 3 + were scattered from these respective surfaces. The spectrum recorded as a result of 70 eV collisions of SiCl 3 + with the Si(CD 3 ) 3 + ‐modified HO‐SAM surface (Fig.…”