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

Abstract

Using a multi-sector ion-surface scattering mass spectrometer, reagent ions of the general form SiR(3) (+) were mass and energy selected and then made to collide with a hydroxy-terminated self-assembled monolayer (HO-SAM) surface at energies of approximately 15 eV. These ion-surface interactions result in covalent transformation of the terminal hydroxy groups at the surface into the corresponding silyl ethers due to Si--O bond formation. The modified surface was characterized in situ by chemical sputtering, a low-energy ion-surface scattering experiment. These data indicate that the ion-surface reactions have high yields (i.e. surface reactants converted to products). Surface reactions with Si(OCH(3))(3) (+), followed by chemical sputtering using CF(3) (+), yielded the reagent ion, Si(OCH(3))(3) (+), and several of its fragments. Other sputtered ions, namely SiH(OCH(3))(2)OH(2) (+) and SiH(2)(OCH(3))OH(2) (+), contain the newly formed Si--O bond and provide direct evidence for the covalent modification reaction. Chemical sputtering of modified surfaces, performed using CF(3) (+), was evaluated over a range of collision energies. The results showed that the energy transferred to the sputtered ions, as measured by their extent of fragmentation in the scattered ion mass spectra, was essentially independent of the collision energy of the projectile, thus pointing to the occurrence of reactive sputtering.A set of silyl cations, including SiBr(3) (+), Si(C(2)H(3))(3) (+) and Si(CH(3))(2)F(+), were similarly used to modify the HO-SAM surface at low collision energies. A reaction mechanism consisting of direct electrophilic attack by the cationic projectiles is supported by evidence of increased reactivity for these reagent ions with increases in the calculated positive charge at the electron-deficient silicon atom of each of these cations. In a sequential set of reactions, 12 eV deuterated trimethylsilyl cations, Si(CD(3))(3) (+), were used first as the reagent ions to modify covalently a HO-SAM surface. Subsequently, 70 eV SiCl(3) (+) ions were used to modify the surface further. In addition to yielding sputtered ions of the modified surface, SiCl(3) (+) reacted with both modified and unmodified groups on the surface, giving rise not only to such scattered product ions as SiCl(2)OH(+) and SiCl(2)H(+), but also to SiCl(2)CD(3) (+) and SiCl(2)D(+). This result demonstrates that selective, multi-step reactions can be performed at a surface through low-energy ionic collisions. Such processes are potentially useful for the construction of novel surfaces from a monolayer substrate and for chemical patterning of surfaces with functional groups.

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, ).…”

“…At the same time, smaller fragile ions fragment more readily at the time of collision while larger ions may withstand further vibrational excitation without fragmentation due to their more numerous internal degrees of freedom (Shen et al, ; Ouyang et al, ; Alvarez et al, ). The properties of the projectile ion and the surface also determine the efficiency of reactive landing, which results in formation of covalent bonds between projectile ions or their fragments and the surface (Shen et al, ; Evans et al, ; Wade et al, ; Volny et al, ; Wang et al, ; Wang & Laskin, ). Reactive landing is particularly important for stable immobilization of molecules on surfaces.…”

“…For example, the ability to focus the ion beam prior to deposition was identified as an important factor in the controlled preparation of protein arrays and subsequent characterization of their biological activity (Blake et al, ). In addition, surface patterning has been achieved by placing a grid in front of the substrate (Evans et al, ; Wade et al, ; Tang et al, ). The kinetic energy of the ion is readily adjusted for soft deposition of labile molecules at low kinetic energies or for reactive landing of molecules and pinning of clusters and nanoparticles to surfaces at high kinetic energies.…”

“…Reactive landing, which constitutes a special type of ion deposition where chemical bonds are formed with the surface, has been applied extensively for modification of surfaces using beams of polyatomic ions (Shen et al, ; Hanley & Sinnott, 2002; Jacobs, ; Wang & Laskin, ). Examples of reactive deposition, including the covalent modification of SAMs (Pradeep et al, ; Evans et al, ; Wade et al, ; Hu et al, ), modification of polymer films using hydrocarbon and fluorocarbon ions (Ada et al, ; Wijesundara et al, ,; Hanley & Sinnott, 2002), formation of metal oxide coatings on MALDI plates for enrichment of phosphopeptides through deposition of metal alkoxide ions (Blacken et al, , ; Krásný et al, ), immobilization of biomolecules (Volny et al, ,; Wang et al, ), dendrimers (Hu & Laskin, ), metal and carbon clusters (Bottcher et al, , ; Loffler et al, ; Ulas et al, ,), and organometallic complexes (Johnson & Laskin, ; Johnson et al, ; Nagaoka et al, ; Pepi et al, ) on surfaces are discussed in detail in section 4c.…”

Differentiation of isomeric oxygenated adsorbates using low-energy ion/surface collisions

“…Aromatic ions, such as the (M − H) + ion of chlorobenzene [6] covalently binds to a carboxylic acid terminated self-assembled monolayer surface through an ion/surface decarboxylation reaction, which is reminiscent of the condensed phase Kolbe reaction. Furthermore, hydroxy-terminated SAMs have been transformed into terminal esters [32] and silyl ethers [33] through reactions with such cations as the benzoyl cation, C 6 H 5 CO + , and the trimethylsilyl cation, Si(CH 3 ) 3 + . The extent of such surface modifications produced by ion/surface reactions has been measured using X-ray photoelectron spectroscopy and Fourier transform infrared external reflectance spectroscopy (FTIR-ERS) [34].…”