Methyl-substituted Silanes Research Articles

Gas Separation Silica Membranes Prepared by Chemical Vapor Deposition of Methyl-Substituted Silanes.

The effect on the gas permeance properties and structural morphology of the presence of methyl functional groups in a silica membrane was studied. Membranes were synthesized via chemical vapor deposition (CVD) at 650 °C and atmospheric pressure using three silicon compounds with differing numbers of methyl- and methoxy-functional groups: tetramethyl orthosilicate (TMOS), methyltrimethoxysilane (MTMOS), and dimethyldimethoxysilane (DMDMOS). The residence time of the silica precursors in the CVD process was adjusted for each precursor and optimized in terms of gas permeance and ideal gas selectivity criteria. Final H2 permeances at 600 °C for the TMOS-, MTMOS-, and DMDMOS-derived membranes were respectively 1.7 × 10−7, 2.4 × 10−7, and 4.4 × 10−8 mol∙m−2∙s−1∙Pa−1 and H2/N2 selectivities were 990, 740, and 410. The presence of methyl groups in the membranes fabricated with the MTMOS and DMDMOS precursors was confirmed via Fourier-transform infrared (FTIR) spectroscopy. From FTIR analysis, an increasing methyl signal in the silica structure was correlated with both an improvement in the hydrothermal stability and an increase in the apparent activation energy for hydrogen permeation. In addition, the permeation mechanism for several gas species (He, H2, Ne, CO2, N2, and CH4) was determined by fitting the gas permeance temperature dependence to one of three models: solid state, gas-translational, or surface diffusion.

Role of free-radical chain reactions and silylene chemistry in using methyl-substituted silane molecules in hot-wire chemical vapor deposition

The reaction chemistry of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), has been studied in the hot-wire (catalytic) chemical vapor deposition (HWCVD) process, both on heated tungsten or tantalum filaments and in the gas phase. The four molecules dissociate catalytically on hot W or Ta surfaces to form ·CH3, ·H and H2. In a HWCVD reactor setup where the pressure is relatively high at 8–533Pa, two reaction mechanisms involving silylene and free radical intermediate, respectively, operate with the four precursor gases. For MMS, its reaction chemistry is characterized by the exclusive involvement of methylsilylene intermediate. Reaction products when using DMS as a precursor gas come from silylene chemistry and free-radical chain reactions. Filament temperature and pressure affect strongly the competition between the two mechanisms. Specifically, silylene chemistry dominates at low temperatures of 1200–1300°C and a low pressure of 16Pa, and free-radical chain reactions take over at high temperatures and pressures. With the increasing methyl substitutions in TriMS and TMS, free-radical chain reactions become predominant. A clear transition in the operating reaction mechanism from silylene chemistry to free-radical chain reactions has been shown when the number of SiCH3 bonds increases in the methyl-substituted silane molecules.

Hydrogen elimination from the dissociation of methyl-substituted silanes on tungsten and tantalum surfaces

The elimination of H2 from the dissociation of four methyl-substituted silane molecules, including monomethylsilane (MMS), dimethylsilane (DMS), trimethylsilane (TriMS), and tetramethylsilane (TMS), on a heated tungsten or tantalum filament surface has been studied using laser ionization mass spectrometry. Two complementary ionization methods, i.e., single photon ionization (SPI) using a vacuum ultraviolet wavelength at 118 nm (10.5 eV) and a dual ionization source incorporating both 10.5 eV SPI and laser-induced electron ionization, were employed to detect the production of H2. Examination of the intensity of the H2+ peak from the four molecules has shown that it increases with temperature until reaching a plateau at around 2000−2100 °C on both tungsten and tantalum filaments. These methyl-substituted silanes are dissociatively adsorbed on tungsten and tantalum surfaces by Si−H bond cleavage, and as the temperature is raised, by C−H bond rupture. Experiments with the isotopomers of MMS, DMS, and TriMS have shown that the formation of H2 follows the Langmuir−Hinshelwood mechanism where two adsorbed hydrogen atoms on metal surfaces recombine to produce H2. The determined activation energy (Ea) for H2 formation from MMS, DMS, and TriMS, in the range of 58.2−93.4 kJ mol−1, has been found to increase with the number of methyl substitutions in the precursor molecule. Comparison of these Ea values with the reported values of 51.1−78.8 kJ mol−1 for the methyl radical formation from the same three precursor molecules has led to the conclusion that the initial Si−H bond cleavage in the dissociative adsorption of MMS, DMS, and TriMS is the rate-limiting step for the formation of both H2 molecules and ·CH3 radicals.

Hot wire chemical vapor deposition chemistry in the gas phase and on the catalyst surface with organosilicon compounds.

CONSPECTUS: Hot wire chemical vapor deposition (HWCVD), also referred to as catalytic CVD (Cat-CVD), has been used to produce Si-containing thin films, nanomaterials, and functional polymer coatings that have found wide applications in microelectronic and photovoltaic devices, in automobiles, and in biotechnology. The success of HWCVD is largely due to its various advantages, including high deposition rate, low substrate temperatures, lack of plasma-induced damage, and large-area uniformity. Film growth in HWCVD is induced by reactive species generated from primary decomposition on the metal wire or from secondary reactions in the gas phase. In order to achieve a rational and efficient optimization of the process, it is essential to identify the reactive species and to understand the chemical kinetics that govern the production of these precursor species for film growth. In this Account, we report recent progress in unraveling the complex gas-phase reaction chemistry in the HWCVD growth of silicon carbide thin films using organosilicon compounds as single-source precursors. We have demonstrated that laser ionization mass spectrometry is a powerful diagnostic tool for studying the gas-phase reaction chemistry when combined with the methods of isotope labeling and chemical trapping. The four methyl-substituted silane molecules, belonging to open-chain alkylsilanes, dissociatively adsorb on W and Ta filaments to produce methyl radical and H2 molecule. Under the typical deposition pressures, with increasing number of methyl substitution, the dominant chemistry occurring in the gas phase switches from silylene/silene reactions to free-radical short chain reactions. This change in dominant reaction intermediates from silylene/silene to methyl radicals explains the observation from thin film deposition that silicon carbide films become more C-rich with a decreasing number of Si-H bonds in the four precursor molecules. In the case of cyclic monosilacyclobutanes, we have shown that ring-opening reactions play a vital role in characterizing the reaction chemistry. On the other hand, exocyclic Si-H(CH3) bond cleavages are more important in the less-puckered disilacyclobutane molecules. Metal filaments are essential in HWCVD since they serve as catalysts to decompose precursor gases to reactive species, which initiate gas-phase reaction chemistry and thin film growth. We discuss the structural changes in metal filaments when exposed to various precursor gases. Depending on the nature of the radical intermediates formed from the hot-wire decomposition and subsequent gas-phase reactions, metal silicides and carbides can be formed. Overall, study of the gas-phase reaction chemistry in HWCVD provides important knowledge of the chemical species produced prior to their deposition on a substrate surface. This helps in identifying the major contributor to alloy formation on the filament itself and the film growth, and consequently, in determining the properties of the deposited films. An integrated knowledge of the gas-phase reaction chemistry, filament alloy formation, and thin film deposition is required for an efficient deposition of high-quality thin films and nanomaterials.

Dominance of silylene chemistry in the decomposition of monomethylsilane in the presence of a heated metal filament.

The gas-phase reaction chemistry of the decomposition of monomethylsilane (MMS) has been studied in the presence of a heated metal filament in a hot-wire chemical vapor deposition (HWCVD) reactor. A 10.5 eV vacuum ultraviolet laser single-photon ionization time-of-flight mass spectrometry was employed in combination with isotope labeling and chemical trapping to examine the mechanistic details in the reaction chemistry. We have demonstrated the dominant involvement of the methylsilylene (HSiCH3) intermediate in the gas-phase reaction chemistry. Free radical and silene intermediates do not play a role. Major products are found to be H2, 1,2-dimethyldisilane (DMDS), and 1,3-disilacyclobutane (DSCB). The formation of DMDS proceeds by the insertion reaction of methylsilylene, whereas DSCB originates from the dimerization reaction of methylsilylene. Similar reaction chemistry has been observed when using the different filament materials of tungsten and tantalum in the HWCVD reactor. This indicates that changing the filament material from Ta to W does not affect the gas-phase reaction chemistry when using MMS in the HWCVD process. Finally, comparison of the reaction chemistry of MMS with those of dimethylsilane, trimethylsilane, and tetramethylsilane sheds light on the influence of increasing Si-H bonds. A switch in the dominated chemistry from free-radical short-chain reactions to silylene insertion/dimerization reactions occurs as the number of Si-H bonds increases in the four methyl-substituted silane molecules.

Alkyl-substituted methoxysilanes enhance the activity and stability of d-amino acid oxidase encapsulated in biomimetic silica

Several alkyl-substituted methoxysilanes were evaluated as potential activity and stability enhancing agents for biomimetic silicification of Rhodosporidium toruloides D-amino acid oxidase (RtDAO). When methyl-substituted silanes along with tetramethoxysilane were used as silicic acid precursors for polyallylamine (PAA)--or R5 peptide-catalyzed silicic encapsulation, the RtDAO activity increased with the degree of substitution and the molar ratio up to 15 % of methyl-substituted silanes added. In the presence of 15 mol% trimethylmethoxysilane, the specific activities of encapsulated RtDAO catalyzed by PAA and R5 increased by 1.4- and 4.8-fold, respectively. For PAA-catalyzed encapsulation, a 2.4-fold increase occurred with 30 mol% n-propyltrimethoxysilane; this modification increased the T (m) value by 10 °C and gave a threefold longer half-life in the presence of 10 mM H(2)O(2) as compared to the encapsulation using tetramethoxysilane only.

Preparation of “Si‐Centered” Chiral Silanes by Direct α‐Lithiation of Methylsilanes

The direct alpha-lithiation of methyl-substituted silanes as an efficient method for the preparation and elaboration of Si-chiral compounds is reported. Deprotonation of chiral oligosilanes occurs selectively and with high yields at the methyl group of the stereogenic silicon center, even in the presence of multiple methylsilyl or methylgermyl substituents. Computational studies have confirmed this preference as a consequence of pre-coordination of the lithiating agent by the amino side-arm and repulsion effects in the corresponding transition state. This complexation is also obvious from X-ray structure analyses of the alpha-lithiated silanes, which exhibit intriguing structure formation patterns differing in the type of aggregation and the amount of alkyllithium used. An alternative route to Si-chiral compounds is also presented, which involves desymmetrization of dimethylsilanes mediated by a chiral side-arm. Structure analyses and computational studies have shown that the diastereoselectivity of this alpha-lithiation is influenced by the selectivity of the formation of the stereogenic nitrogen upon complexation of the alkyllithium.

Durable solgel antireflective films with high laser-induced damage thresholds for inertial confinement fusion

We tested the use of two hydrophobic methyl-substituted silane precursors, methyltriethoxysilane and dimethyldiethoxysilane, to synthesize methyl-modified silica sols by a two-step method and a cohydrolysis method to produce durable antireflective films with high laser-induced-damage thresholds (LIDTs). Using small-angle x-ray scattering technology, we obtained details of the microstructure of clusters in sol and found various double fractal structural characteristics in the methyl-modified silica clusters; our findings were confirmed by transmission-electron micrographs. Through a ^29 Si magic-angle spin nuclear magnetic resonance study of the corresponding xerogels, we determined the double-fractal microstructure, which we then related to the LIDTs of AR films. The distribution configuration of methyls in clusters determined the double-fractal microstructure of clusters and then the LIDTs of AR films. The LIDTs of films produced by the cohydrolysis method (the highest was 38 J/cm² for 1-ns, 1064-nm laser action) were much higher than those from the two-step method because of the loose netlike clusters in the former configuration. During the 220-day aging, the transmittance of hydrophobic AR film decreased ∼0.2%. So it is practicable to prepare durable AR films with higher LIDTs than those of normal AR SiO2 films only by introducing hydrophobic methyls into a Si-O-Si matrix of clusters if an appropriate hydrophobic precursor is chosen.

Kinetic studies of the reactions of hydroxyl radicals with the methyl-substituted silanes SiH n(CH 3) 4 − n ( n=0 to 4) at room temperature

The flash-photolysis resonance-fluorescence method has been employed at 293–297 K to measure rate constants for the reactions of OH with SiH 4, Si(CH 3)H 3, Si(CH 3) 2H 2, Si(CH 3) 3H, and Si(CH 3) 4 of (1.2 ± 0.2) × 10 −11, (3.3 ± 0.4) × 10 −11, (4.4 ± 0.5) × 10 −11, (3.1 ± 0.4) × 10 −11, and (1.2 ± 0.2) × 10 −12 molecule −1 cm 3 s −1, respectively. The uncertainties are 95% confidence intervals (see text). The results reveal that the per Si–H bond reactivity towards OH increases with the degree of methylation, and that Si(CH 3) 2H 2 is the most reactive member of the series.

Vibrational overtone spectra of methyl-substituted silanes

Detection photoacoustique intracavite des spectres d'absorption (12800-18200 cm −1 ) de Si(CH 3 ) n H 4−n avec n=1 a 4, pour les harmoniques Δv(CH)=5, 6 et 7 et Δv(SiH)=8 et 9, a l'etat gazeux