Si Bridges Research Articles

First Principles Investigation of HCl, H2, and Chlorosilane Adsorption on Cu3Si Surfaces with Applications for Polysilicon Production

The hydrochlorination reaction used in polysilicon production is often catalyzed through copper-based materials which have been found to form the Cu3Si intermetallic under reaction conditions. The structure and composition of the η″ phase, present in the reaction temperature range, has been only recently derived. We conduct the first study of Cu3Si surfaces and their interaction with molecules. Two surfaces of the η″ phase were examined through density functional theory, a silicon-rich Si termination in hexagonal geometry and an equal-parts copper and silicon CuSi termination in honeycomb geometry. The adsorption of H2, HCl, SiCl2, dichlorosilane (SiH2Cl2 or DCS), trichlorosilane (SiHCl3 or TCS), and silicon tetrachloride (SiCl4 or STC) was tested on multiple sites. The SiCu termination favorably adsorbed only the SiCl2 molecule, where a bridge position between a Cu and Si atom produced the strongest adsorption. The Si-terminated surface dissociatively adsorbed HCl at all sites with almost equal adsorption energies. The SiCl2 adsorbed at all sites but greatly preferred a Cu–Si bridge site, where it produced the strongest adsorption of this study. The H2 molecule moved away from the surface for all sites. The STC molecule was unfavorable at sites with closely spaced Cu and Si atoms but dissociatively adsorbed at the other sites with more isolated Si atoms. The TCS molecule underwent dissociative chemisorption on all sites while the DCS molecule moved away from the surface for all site types. Therefore, the Si-terminated surface is preferred for molecular adsorption and appears to provide promising starting sites for surface hydrochlorination reactions.

A Si–O–Si bridge assembled from 3-mercaptopropyltrimethoxysilane and silicon carbide for effective charge transfer in photocatalysis

This work demonstrates the functionalization of SiC with the organosilane 3-mercaptopropyltrimethoxysilane via a wet chemical process. The attachment of the covalently bound organosilane onto SiC was indicated by scanning electron microscopy, water contact angle measurements, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. A Si–O–Si bridge was successfully established on the heterogeneous interface, which acts as a fluent carrier transfer channel and can effectively contribute to the quick transfer of photogenerated electrons from the SiC interior to the surface, thus increasing the number of surface active sites and prolonging the charge-carrier lifetime. The photocatalytic H2 production rate under visible light irradiation was improved to 27.5 μmol h−1 g−1, while unmodified SiC resulted in no H2 production under the same conditions. The study definitively shows that the Si–O–Si bridge modelled by organosilanes can directly lead to the observed enhanced rate and pave the way for the heterojunction construction of inorganic and organic materials.

Giant ( 12×12 ) and ( 4×8 ) reconstructions of the 6H -SiC(0001) surface obtained by progressive enrichment in Si atoms

Silicon carbide (SiC) is nowadays a major material for applications in high power electronics, quantum optics, or nitride semiconductors growth. Mastering the surface of SiC substrate is crucial to obtain reproducible results. Previous studies on the 6H-SiC(0001) surface have determined several reconstructions, including the (√ 3× √ 3)-R30 • and the (3×3). Here, we introduce a process of progressive Si enrichment that leads to the formation of two reconstructions, the giant (12×12) and the (4×8). From electron diffraction and tunneling microscopy completed by molecular dynamics simulations, we build models introducing a type of Si adatom bridging two Si surface atoms. Using these Si bridges, we also propose a structure for two other reconstructions, the (2 √ 3×2 √ 3)-R30 • and the (2 √ 3×2 √ 13). We show that five reconstructions follow each other with Si coverage ranging from 1 and 1.444 monolayer. This result opens the way to greatly improve the control of 6H-SiC(0001) at the atomic scale.

Molecular dynamics study of correlations between IR peak position and bond parameters of silica and silicate glasses: Effects of temperature and stress

AbstractIn the IR spectra of the silica and silicate glasses, the shifts of the maximum intensity position of the νSi–O–Si,as band upon heating or applying mechanical stress could be attributed to changes in the distribution of bond parameters such as bond length and bond angle. Upon heating, isotropic expansion occurs and the density changes; upon applying mechanical stress, anisotropic strain is induced and a significant change in the Si–O–Si bond angle is observed. From molecular dynamics simulations of a silica glass, we show that the peak position shift correlates better with the asymmetric change in the Si–O bond length distribution, rather than the Si–O–Si bridge angle, the O–Si–O tetrahedral angle, or the density change. This new finding provides an insight into how and why the νSi–O–Si,as IR peak of soda lime silica (SLS) glass shifts upon chemical strengthening via ion exchange and thermal tempering.

Ab Initio energetics of SiO bond cleavage.

A multilevel approach that combines high-level ab initio quantum chemical methods applied to a molecular model of a single, strain-free SiOSi bridge has been used to derive accurate energetics for SiO bond cleavage. The calculated SiO bond dissociation energy and the activation energy for water-assisted SiO bond cleavage of 624 and 163 kJ mol-1 , respectively, are in excellent agreement with values derived recently from experimental data. In addition, the activation energy for H2 O-assisted SiO bond cleavage is found virtually independent of the amount of water molecules in the vicinity of the reaction site. The estimated reaction energy for this process including zero-point vibrational contribution is in the range of -5 to 19 kJ mol-1 . © 2017 Wiley Periodicals, Inc.

Rare Earth and Main Group Metal Poly(hydrosilyl) Compounds

The potassium poly(hydrosilyl) compound KSi(SiHMe2)3, which contains three β-SiH groups, is synthesized by the reaction of Si(SiHMe2)4 and KOtBu. A single-crystal X-ray diffraction study reveals chains composed of head-to-tail KSi(SiHMe2)3 monomers. This potassium poly(hydrosilyl) anion reacts with divalent metal halide salts in THF to form the bis(silyl) compounds Mg{Si(SiHMe2)3}2THF2 (1·THF2), Ca{Si(SiHMe2)3}2THF3 (2·THF3), and Yb{Si(SiHMe2)3}2THF3 (3·THF3). A trivalent yttrium bis(silyl), as part of a KCl-containing polymeric chain, is supported by K↼H–Si bridging interactions. The N donors pyridine (py) and dimethylaminopyridine (DMAP) readily substitute THF, giving tetrahedral magnesium and octahedral calcium silyl compounds. The one-bond silicon–hydrogen coupling constants (1JSiH), infrared stretching frequencies (νSiH), and solid-state structures of 2·py4 and 3·THF3 indicate classical two center-two electron bonding between the metal center and the silyl ligand, as well as terminal (nonbridging) Si...

Ab Initio Simulation of the Acid Sites at the External Surface of Zeolite Beta

AbstractThe acidity at the external surface of protonic zeolites, because of the finite size of crystallites, has been questioned strongly for decades. We used density functional theory (DFT) calculations to propose atomistic models for the external surface of zeolite Beta, which show that bridging Si−(OH)−Al groups still exist at the pore mouth in what we call open micropores (pores that emerge at the external surface). However, at the outermost surface (no emerging micropores), water molecules adsorbed on Al atoms [Al−(H2O)] prevail. The local structure of those surface Al atoms depends on the temperature and water partial pressure. A detailed vibrational study of adsorbed CO helps in the assignment of the different sites and reveals a generalized vibrational Stark effect. Proton‐transfer ability was quantified by the adsorption of isobutene. Carbenium ions appear to be stabilized on the bridging Si−(OH)−Al groups located on the open micropores of the external surface in a similar way as in the bulk of zeolite Beta. By contrast, the outermost surface is not able to stabilize carbenium ions and promotes the existence of alkoxides. This work brings new atomic‐scale insights into the concept of pore‐mouth catalysis and provides the molecular architecture of potential active sites located in open micropores.

Triangular Triplatinum Complex with Four Bridging Si Ligands: Dynamic Behavior of the Molecule and Catalysis

A triangular triplatinum(0) complex with bridging diphenylsilylene ligands, [{(Pt(PMe3)}3(μ-SiPh2)3] (1a), reacts with H2SiPh2 to produce the 1:1 adduct, [{Pt(PMe3)}3(H)2(μ-SiPh2)4] (2a), which was isolated and characterized by X-ray crystallography. Two Pt–Pt bonds of the triangular Pt3 core are bridged by a diphenylsilylene ligand, while the remaining Pt–Pt bond has two unsymmetrical bridging Si ligands. Dissolution of 1a and H2SiPh2 at a 1:3 molar ratio forms a mixture of complex 2a and unreacted 1a. NMR measurement of the solution at −90 °C revealed the structure of 2a as having two hydride ligands and four bridging silylene ligands. Two 31P{1H} NMR signals of 2a at −90 °C coalesce on warming to −50 °C owing to facile exchange of the four Si ligands. Reversible addition of H2SiPh2 to 1a yielded 2a with ΔG° = −8.0 kJ mol–1, ΔH° = −51.7 kJ mol–1, and ΔS° = −146 J mol–1 K–1. Addition of bis(4-fluorophenyl)silane and bis(4-methylphenyl)silane to the complexes, which have three bis(4-fluorophenyl)silylene ...

Crystalline and amorphous aluminosilicates with different pore structures for the synthesis of pyridines

The heterogeneous catalytic synthesis of pyridine and methylpyridines (picolines) was carried out by the reaction of ethanol with formaldehyde and ammonia over aluminosilicates with different acidic properties and textural characteristics: zeolite framework-type FAU in H-form with microporous (H-Y) and micro-meso-macroporous structure (H-Y-mmm) and mesoporous aluminosilicates, obtained by sol-gel synthesis in an alkaline medium (ASM-1) and under variable pH (ASM-2). It is shown that the highest catalytic activity is characteristic of ASM-2 (Si/Al = 40) with a narrow pore size distribution within the range 2 to 5 nm and containing strongly acidic bridging Si–OH–Al groups on the surface. The products of the reaction of ethanol with formaldehyde and ammonia on the catalysts studied are dominated by picolines; the maximum amount of picolines (60%) is formed on the H-Y-mmm and aluminosilicate ASM-2 samples.

Bimodal distribution of Si–O–Si angles in sodo-silicate glasses

Polarized Raman scattering is performed in a series of sodo-silicate glasses. The Si–O–Si inter-tetrahedral angle and its distribution are extracted using a model developed previously for densified silica, and the result are compared to ab initio atomistic calculations. The two techniques reveal a reduction of the most probable angle of about 0.30°/mol% of Na2O, but the shape of the angular distributions are different. The results suggest that Raman scattering enhances a specific angular distribution of Si–O–Si bridges, likely those close to sodium atoms, highlighting local angular heterogeneities.

Comparative Structural, Optical, and Photoluminescence Studies of YF3:Pr, YF3:Pr@LaF3, and YF3:Pr@LaF3@SiO2 Core–Shell Nanocrystals

Praseodymium (Pr3+)‐doped YF3 (core) and LaF3 ‐covered YF3 :Pr (core–shell) nanocrystals (NCs) were prepared successfully by an ecofriendly, polyol‐based, co‐precipitation process, which were then coated with a silica shell by using a sol–gel‐based Stober method. X‐ray diffraction (XRD), transmission electron microscopy (TEM), thermal analysis, Fourier transform infrared (FTIR), UV/vis, energy bandgap, and photoluminescence studies were used to analyze the crystal structure, morphology, and optical properties of the nanomaterial. XRD and TEM results show that the grain size increases after sequential growth of crystalline LaF3 and the silica shell. The silica surface modification enhances the solubility and colloidal stability of the core–shell‐SiO2 NCs. The results indicate that the surface coating affects the optical properties because of the alteration in crystalline size of the materials. The emission intensity of silica‐modified NCs was significantly enhanced compared to that of core and core–shell NCs. These results are attributed to the formation of chemical bonds between core–shell and noncrystalline SiO2 shell via La–O–Si bridges, which activate the “dormant” Pr3+ ions on the surfaces of the nanoparticles. The luminescence efficiency of the as‐prepared core, core–shell, and core–shell‐SiO2 NCs are comparatively analyzed, and the observed differences are justified on the basis of the surface modification surrounding the luminescent seed core NCs.

Glass structure, phase transformation and microwave dielectric properties of CaO–B2O3–SiO2 glass–ceramics with addition of La2O3

The CaO–B2O3–SiO2 (CBS) glasses with addition of xLa2O3 (0 ≤ x ≤ 20 wt%) were prepared by conventional quenching method, the glass structure, phase transformation and microware dielectric properties were studied. The glass structure analysis by magic angle spinning nuclear magnetic resonance (MAS-NMR) and infrared (IR) spectroscopy indicated that [BO4] transformed into [BO3] and the proportion of Si–O–Si bridge oxygen bond in [SiO4] tetrahedrons decreased obviously with the addition of La2O3. The differential thermal analysis (DTA) revealed that the CBS glass system kept better glass-forming ability and the transition temperature (Tg) decreased from 743 to 724 °C after addition of La2O3. As the content of La2O3 increases, the crystallization tendency of CaB2O4 is suppressed, while that for Ca2SiO4 is promoted. With appropriate content of La2O3 (5 wt%), the sample possess excellent dielectric properties (er ≈ 4.1, tanδ ≈ 1.7 × 10−3 at 15 GHz). The variations of dielectric properties are believed to be appreciably associated with the relative content of the phases present.

Quantitative X-ray pair distribution function analysis of nanocrystalline calcium silicate hydrates: a contribution to the understanding of cement chemistry.

The structural evolution of nanocrystalline calcium silicate hydrate (C-S-H) as a function of its calcium to silicon (Ca/Si) ratio has been probed using qualitative and quantitative X-ray atomic pair distribution function analysis of synchrotron X-ray scattering data. Whatever the Ca/Si ratio, the C-S-H structure is similar to that of tobermorite. When the Ca/Si ratio increases from ∼0.6 to ∼1.2, Si wollastonite-like chains progressively depolymerize through preferential omission of Si bridging tetrahedra. When the Ca/Si ratio approaches ∼1.5, nanosheets of portlandite are detected in samples aged for 1 d, while microcrystalline portlandite is detected in samples aged for 1 year. High-resolution transmission electron microscopy imaging shows that the tobermorite-like structure is maintained to Ca/Si > 3.

Glass structure of the CaO–B2O3–SiO2–Al2O3–ZnO glasses system with different Si content

The aim of the present work is to investigate the effects of SiO2 content on the structure of CaO–B2O3–SiO2–Al2O3–ZnO glass using the MAS-NMR, the fourier transform infrared spectrometer and differential scanning calorimetry (DSC). The results showed that the majority of Al existed in fourfold coordination and Zn acted as glass modifier in glass structure. With the increasing of SiO2 content, the relative amount of BIII units decreased while the BIV units increased and the proportion of Si–O–Si bridge oxygen bond in [SiO4] tetrahedrons increased obviously. The DSC curves revealed that the glass transition temperature increased from 732 to 773 °C with the increasing of SiO2 content, indicating that SiO2 generating more bridge oxygen can increase the degree of polymerization of glass structure.

Adsorption of vinyl trimethoxysilane and formation of vinyl siloxane nanolayers on zinc surface from aqueous solution

The method of quartz crystal microbalance is used to study adsorption of vinyl trimethoxysilane (VS) on the surface of zinc from an aqueous solution. Adsorption isotherms are obtained. Approaches corresponding to the known adsorption isotherms are used for interpretation of adsorption data: Langmuir, BET, Flory–Huggins, Langmuir multicenter, Temkin, and Langmuir–Freundlich. It is shown that silanes are adsorbed on the surface of thermally deposited zinc from aqueous solutions and displace adsorbed water from the surface by occupying more than six adsorption sites on the surface. It is found that monolayer coverage of the zinc surface is reached at a concentration of the VS solution of 1 × 10–4 M. The neighboring adsorbate molecules can interact, forming siloxane dimers and trimers bound to the metal surface by either covalent or hydrogen bonds. Adsorption heats are calculated using different adsorption models. It is shown that VS is chemosorbed on the surface of zinc. An increase in the concentration of the VS solution up to 0.1 M results in formation of polycondensed siloxane oligomers on the surface with polycondensation degree n = 8–12. Oligomer surface fragments are connected with each other by hydrogen bonds and are connected with the surface by Zn–O–Si bridge bonds. The overall thickness of such a layer is 10–12 nm or ten molecular layers.

Systematic study of glass transition in low-molecular phthalonitriles: Insight from computer simulations.

Phthalonitrile compounds with Si bridges were recently suggested for producing thermosetting polymer composites with reduced Tg and thus expanded processing range. The detailed experimental investigation of this class of phthalonitriles is still difficult due to development time and costs limitations and the need to take into account the structural changes during the crosslinking. In this paper, we try to overcome these limitations using computer simulations. We performed full-atomistic molecular dynamics simulations of various phthalonitrile compounds to understand the influence of molecular structure on the bulk glass temperature Tg. Two molecular properties affect Tg of the resulting bulk compound: the size of the residue and the length of the Si bridge. The larger residues lead to higher Tgs, while compounds with longer Si bridges have lower Tgs. We have also studied relaxation mechanisms involved in the classification of the samples. Two different factors influence the relaxation mechanisms: energetic, which is provided by the rigidity of molecules, and entropic, connected with the available volume of the conformational space of the monomer.

Atomic insight into tribochemical wear mechanism of silicon at the Si/SiO2 interface in aqueous environment: Molecular dynamics simulations using ReaxFF reactive force field

In this work, the atomic mechanism of tribochemical wear of silicon at the Si/SiO2 interface in aqueous environment was investigated using ReaxFF molecular dynamics (MD) simulations. Two types of Si atom removal pathways were detected in the wear process. The first is caused by the destruction of stretched Si–O–Si bonds on the Si substrate surface and is assisted by the attachment of H atoms on the bridging oxygen atoms of the bonds. The other is caused by the rupture of Si–Si bonds in the stretched Si–Si–O–Si bond chains at the interface. Both pathways effectively remove Si atoms from the silicon surface via interfacial Si–O–Si bridge bonds. Our simulations also demonstrate that higher pressures applied to the silica phase can cause more Si atoms to be removed due to the formation of increased numbers of interfacial Si–O–Si bridge bonds. Besides, water plays a dual role in the wear mechanism, by oxidizing the Si substrate surface as well as by preventing the close contact of the surfaces. This work shows that the removal of Si atoms from the substrate is a result of both chemical reaction and mechanical effects and contributes to the understanding of tribochemical wear behavior in the microelectromechanical systems (MEMS) and Si chemical mechanical polishing (CMP) process.

Rapid and alternative fabrication method for microfluidic paper based analytical devices

A major application of microfluidic paper-based analytical devices (µPADs) includes the field of point-of-care (POC) diagnostics. It is important for POC diagnostics to possess properties such as ease-of-use and low cost. However, µPADs need multiple instruments and fabrication steps. In this study, two different chemicals (Hexamethyldisilazane and Tetra-ethylorthosilicate) were used, and three different methods (heating, plasma treatment, and microwave irradiation) were compared to develop µPADs. Additionally, an inkjet-printing technique was used for generating a hydrophilic channel and printing certain chemical agents on different regions of a modified filter paper. A rapid and effective fabrication method to develop µPADs within 10min was introduced using an inkjet-printing technique in conjunction with a microwave irradiation method. Environmental scanning electron microscope (ESEM) and x-ray photoelectron spectroscopy (XPS) were used for morphology characterization and determining the surface chemical compositions of the modified filter paper, respectively. Contact angle measurements were used to fulfill the hydrophobicity of the treated filter paper. The highest contact angle value (141°±1) was obtained using the microwave irradiation method over a period of 7min, when the filter paper was modified by TEOS. Furthermore, by using this method, the XPS results of TEOS-modified filter paper revealed Si2p (23%) and Si-O bounds (81.55%) indicating the presence of Si–O–Si bridges and Si(OEt) groups, respectively. The ESEM results revealed changes in the porous structures of the papers and decreases in the pore sizes. Washburn assay measurements tested the efficiency of the generated hydrophilic channels in which similar water penetration rates were observed in the TEOS-modified filter paper and unmodified (plain) filter paper. The validation of the developed µPADs was performed by utilizing the rapid urease test as a model test system. The detection limit of the developed µPADs was measured as 1unitml−1 urease enzyme in detection zones within a period of 3min. The study findings suggested that a combination of microwave irradiation with inkjet-printing technique could improve the fabrication method of µPADs, enabling faster production of µPADs that are easy to use and cost-effective with long shelf lives.

Freestanding Ultrathin Nanoparticle Membranes Assembled at Transient Liquid–Liquid Interfaces

A synthetic route is presented for the realization of ultrathin freestanding nanoparticle membranes that are built of gold nanoparticles protected with trimethoxysilane‐bearing ligands. The mechanism relies on interfacial assembly in an oil–water mixture. Upon shaking, nanoparticles are transported to the liquid–liquid interface of the oil droplets and form a network through the formation of Si–O–Si bridges. Reticulation of the nanoparticles during the dynamic process of droplet coalescence allows the formation of ultrathin membranes of only a few nanoparticle layers in thickness and square centimeters in dimension. The membranes can be manipulated, such as locally perforated, without causing their collapse. Furthermore they can be transferred onto solid or holey substrates. The synthetic route is compatible with a coassembly of dopants. As an example, membranes were doped with single walled carbon nanotubes, which resulted in a sizable increase of their electric conductivity.

Cationic Si-H-Si Bridges in Polysilanes: Their Detection and Targeted Formation in Stable Ion Studies.

The ionization of 1,1‐dihydridocyclopentasilane 7 has been found to yield the cyclic polysilanylsilyl cation 8 instead of the expected hydrogen‐substituted silylium ion 6. The silyl cation 8 is stabilized by the formation of an intramolecular Si−H−Si bridge, which also provides the thermodynamic driving force for its formation. In general, the preference for the formation of Si−H−Si bridges can be used to scavenge and identify transient intermediates in the Lewis acid induced rearrangement of polysilanes. The validity of this concept has been demonstrated for one central step in this chemistry, the ring‐contraction reaction of cyclohexasilanes to form silylcyclopentasilanes.