Admittance studies of hydrogen-induced states at the silicon—silicon dioxide interface

The admittance of Pd-thin SiO₂Si MOSCAP devices was studied as a function of the following variables: temperature, measurement frequency, oxide preparation conditions, applied gate voltage and ambient atmospheres of 100 ppm hydrogen in nitrogen and pure oxygen. Transient current, capacitance and annealing studies were also conducted for many of these variables. It is shown that hydrogen atoms produced by the catalytic action of the Pd on hydrogen molecules can be injected into the oxide—semiconductor interface where, depending on the choice of oxidation conditions for growing the oxide, they modify the density and capture cross-sections of the hydrogen-induced interfacial states. It is also demonstrated that below 125 °C, the injected hydrogen can be reversibly removed by changing the ambient gas from the H₂/N₂ mixture to pure oxygen.     

Synthesis of silicon–nitrogen derivatives in quasi-interstellar conditions

At 2–4·10^–5 T, a silicon wafer is the target of a 5–10 keV molecular beam of dinitrogen. The products are extracted by an electric field, and analysed by mass. The ions of this primary spectrum are dissociated in a Kr collision chamber. From the fragments thus obtained, one deduces compositions for the secondary ions, and therefore for the primary products. This is helped by the presence, in silicon, of the isotopes ²⁸Si, ²⁹Si and ³⁰Si. Beside the clusters Si_n (n = 1–7), complex molecular species are thus obtained, such as Si₅N₄⁺, Si₅N₄H⁺, Si₅N₄H₂⁺. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASinterstellar dust / interstellar molecules / atomic impact / silicon / nitrogen derivatives     

1,4-Addition of silicon dienoates to α,β-unsaturated aldehydes catalyzed by in situ-generated silicon Lewis acid

In situ-generated silyl methide species (R(3)Si-CTf(2)R') effectively catalyzed the reaction of β-substituted α,β-unsaturated aldehydes with silicon dienoates such as 3-bromo-2-TESO-furan to give the corresponding γ-adducts with excellent 1,4-selectivity and good anti selectivity.     

Ab initio study of the nuclear spin—spin coupling constants and magnetic shielding constants in silicon derivatives via the equations-of-motion method

Non-empirical equations-of-motion calculations of the nuclear spin—spin coupling constants and magnetic shielding constants in a representative series of molecules featuring siliconsilicon or siliconcarbon single, double and triple bonds are presented. The EOM results, which include the main portion of the electron correlation effects, are in resonable agreement with the available experimental data. On passing from single to double and triple bonding situation the pattern for the ¹J(SiY) parameters resembles that exhibited by ¹J(CY) in the structurally related carbocompounds, whereas an inversion in the relative position of the triply bonded atom is predicted in the case of the ²⁹Si resonance relative to the ¹³C sequence.     

Mechanism of the emergence of the photo-EMF upon silicon liquid crystal–single crystal contact

The effect light has on a silicon liquid crystal–single crystal contact at different temperatures of the surface doping of silicon, and when BaTiO₃ nanoparticles are added to the composition of a liquid crystal, is studied. The mechanism of the emergence of the photo-EMF in the liquid crystal–silicon structure is explained.

     

Switching the photoinduced processes in host-guest complexes of β-cyclodextrin-substituted silicon(IV) phthalocyanines and a tetrasulfonated porphyrin

Porphyrins and phthalocyanines are two attractive classes of functional dyes for the construction of artificial light harvesting and charge separation molecular systems. The assembly of these components by supramolecular approach is of particular interest as this provides a facile route to build multi-chromophoric arrays with various architectures and tuneable photophysical properties. We report herein a series of host-guest complexes formed between a tetrasulfonated porphyrin and several silicon(IV) phthalocyanines substituted axially with two permethylated β-cyclodextrin units via different spacers. As shown by electronic absorption and fluorescence spectroscopic methods, the two components bind spontaneously in a 1:1 manner in water with large binding constants in the range of 1.1 × 10(7) to 3.5 × 10(8) M(-1). The photophysical properties of the resulting supramolecular complexes have also been studied in detail using steady-state and time-resolved optical spectroscopic methods. It has been found that two major photoinduced processes, namely fluorescence resonance energy transfer and charge transfer are involved which are controlled by the spacer between the β-cyclodextrin units and the silicon centre of phthalocyanine. Despite the fact that charge transfer is a thermodynamically favourable process for all the complexes, only the ones with a tetraethylene glycol or oxo linker exhibit an efficient charge transfer from the excited phthalocyanine to the porphyrin entity. The lifetimes of the corresponding charge-separated states have been determined to be 200 and 70 ps by picosecond pump-probe experiments.     

Hydrogenated amorphous silicon nanostructures: novel structure�Creactivity relationships for cyclization and ring opening in the gas phase

The effects of the reactive center connectivity and internal rotations on the reactivity of hydrogenated silicon nanostructures toward cyclization and ring opening pathways have been investigated. Rate coefficients for 25 cyclization and ring opening reactions for hydrides containing up to eight silicon atoms have been calculated using G3//B3LYP. The overall reactions exhibit two elementary steps. Overcoming the first barrier results in the formation of a hydrogen-bridged cyclic intermediate from a substituted silylene. Passing over the second barrier converts this intermediate into a cyclic silicon hydride. The rate-determining step varied according to the ring size formed and the temperature. Assuming a rate-determining step, values for the single-event Arrhenius pre-exponential factor, $ \tilde{A}$ , and the activation energy, E _a, were calculated from G3//B3LYP rate coefficients corrected for internal rotations, and a group additivity scheme was developed to predict $ \tilde{A}$ and E _a. The values predicted by group additivity are more accurate than structure?Creactivity relationships currently used in the literature, which rely on a representative $ \tilde{A}$ value for each reaction class and the Evans-Polanyi correlation to predict E _a. Internal rotation corrections played a prominent role in cyclization pathways, impacting $ \tilde{A}$ values for larger ring formation reactions more strongly than any variations in the connectivity of the reactive center.     

Thermodynamics investigation of the gas-phase reactions in the chemical vapor deposition of silicon borides with BCl3–SiCl4–H2 precursors

The gas-phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing silicon borides with the precursors of BCl₃–SiCl₄–H₂ is investigated with a relatively complete set of 220 species, in which the thermochemistry data are calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels combined with standard statistical thermodynamics. The data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. Based on these data, the distribution of the equilibrium concentration of the 220 species is obtained with the principle of chemical equilibrium. BHCl₂, SiHCl₃, and BH₂Cl are found to be the crucial intermediates. This work provides fundamental data for analyzing the thermochemistry of the CVD process of the BCl₃–SiCl₄–H₂ system, which is instructive to optimize the input precursors and temperatures for controlling the composition of the condensed phase B, SiB₆, and SiB₁₄.     

Determination of the hydrogen concentration in silicon nitride films with the resonant nuclear reaction 1H(15N, αγ)12C

The resonant nuclear reaction ¹H(¹⁵N, αγ)¹²C at resonance energy E(¹⁵N²⁺) = 6.40 MeV has been used to investigate the hydrogen concentration in silicon nitride films.This method is very suitable to determine hydrogen concentration profiles with a good depth resolution (~5 nm) over a large depth (~2 μm) and has a sensitivity of a few tenth of an atomic percent.     

Quantum-chemical modeling of lithiation of a silicon–silicon carbide composite

With the aim of searching for promising anode materials for lithium-ion batteries, quantum-chemical modeling of the introduction of lithium into a silicon layer supported by nitrogen-doped silicon carbide at Li: Si ratios of 1: 1, 2: 1, and 3: 1 has been performed by the density functional theory method with inclusion of gradient correction and periodic boundary conditions. It has been demonstrated that the absorption of lithium by silicon is energetically more favorable than the formation of a metal layer on the silicon surface. As the lithium concentration increases, the energy difference decreases; i.e., the introduction of lithium into silicon becomes increasingly less favorable, the network of silicon atoms is broken down into smaller and smaller pieces, while the layer thickness increases threefold.     

Deposition of silicon dioxide antireflection coatings on glass by sol-gel method in the presence of Pluronic® F127 block-copolymer

Hydrolysis of tetraethoxysilane in the presence of Pluronic® F127 block-copolymer in a water-alcohol mixture was used to obtain sol-formulations from which antireflection coatings were deposited onto silicate glass by the dip-coating method and then calcined at high temperatures (200–500°C). Electron spectroscopy was used to study the dependence of the optical transmission of glass samples with a coating on the concentration of Pluronic® F127 in the starting sol, introduction of organic additives (toluene) into the sol, and calcination temperature of the glasses obtained. The refractive index and hardness of the film coatings were determined and their surface structure was assessed.     

π-Relaxation of the ring strain: design of polycyclic unsaturated silicon molecules

Introduction of a double bond into cyclic silanes lowers the ring strain by the cyclic delocalization of π-electrons through the hyperconjugation with the σ bonds, which is favored by the high π-orbital energy of the SiSi bond and the low σ*-orbital energy of the Si-H bonds. The π-relaxation of strains significantly occurs in the small rings. Unsaturated small silicon ring molecules are less strained than the saturated ones and the unsaturated carbon congeners. We calculated a series of polycyclic silicon molecules to confirm the π-relaxation and suggested that some unknown molecules could be prepared due to the low strain.     

Photoacoustic infrared identification of NHn vibrations in hydrogenated amorphous silicon nitride

Volume and surface component separation in photoacoustic i.r. spectroscopy is used to demonstrate that the peak at 3350 cm⁻¹ in hydrogenated amorphous silicon nitride commonly attributed to NH_n vibrations, is due to a surface contaminant. The magnitude of this surface component obscures the underlying peaks, which become clearly visible on separation.     

Ornstein–Zernike direct correlation function from diffraction experiments in supercooled liquid silicon and in disordered cobalt

Using experimental data for the structure factor S(q) of supercooled liquid silicon, and amorphous and supercooled liquid cobalt, we have calculated and compared the Fourier transform c(q) of the Ornstein–Zernike direct correlation function for these two systems. Differences of c(q) between Si and Co are ascribed to the different atomic coordination and packing, high in Co due to typical metallic bonding, and lower in Si due to more directional bonding. Also, in contrast to previous expectations, differences are found between the direct correlation function of amorphous and supercooled liquid cobalt.     

Quantum-chemical modeling of lithium removal from lithium–silicon–silicon carbide composite

The quantum-chemical modeling of the delithiation-induced reorganization of a Li _m Si _n layer applied to the surface of nitrogen-doped silicon carbide is performed by means of non-empirical molecular dynamics in the frame of the gradient-corrected density functional method with the goal for finding promising anode materials for lithium ion batteries. The ratios Li/Si are considered from 8/3 to 1/4. Partial removal of lithium atoms from the surface of the Li _m Si _n layer and annealing at a moderate temperature (400 K) is found to recover rapidly (as soon as within 10 ps) the uniform metal distribution over the layer when the ratio Li/Si is at least 3/4. At lower values of this ratio, the equalization slows down dramatically.     

Characterization of SiN and other transient species in a silicon tetrachloride–nitrogen discharge

Infrared diode laser absorption spectroscopy has been used to detect the A–X electronic spectrum of diatomic SiN in a SiCl₄/N₂ plasma. The intensity of the SiN transitions with respect to the ratio of SiCl₄:N₂ flow rate was investigated. A correlation between intense signals of SiN and the optimal growth conditions for a-SiN films reported in literature was found. This observation lends support to the importance of diatomic SiN as a film precursor. The electronic emission spectra of silicon-containing transient species were also recorded in the plasma. A brief discussion of the plasma reaction pathways of SiN is presented.     

High electrophilicity at silicon in η3-silane σ-complexes: Lewis base adducts of a silane ligand, featuring octahedral silicon and three Ru-H-Si interactions

New η(3)-silane σ-complexes [PhBP(Ph)(3)]RuH(η(3)-H(2)SiRR') (RR' = PhMe, Ph(2)) were synthesized. Lewis bases [THF, 4-(dimethylamino)pyridine, and PMe(3)] coordinate to the silicon centers of these complexes to form stable adducts. The base adducts, [PhBP(Ph)(3)]Ru(μ-H)(3)SiRR'(base), feature three nonclassical Ru-H-Si interactions and hexacoordinate silicon centers, as determined by multinuclear NMR spectroscopy, X-ray crystallography, and computational investigations.     

X-ray photoelectron spectroscopy for detection of the different Si–O bonding states of silicon

X-ray photoelectron spectroscopy (XPS) was used to detect the bonding between a silica particle surface and attached silanes. In addition to the commonly recorded Si 2p spectrum, the Si 1 s level is also accessible when monochromatic Ag Lalpha X-rays are applied. Furthermore, the spectrum of the Si 1 s level shows a fine structure. After spectrum deconvolution, we assigned the fitted spectral peaks to Si-C bonds of the silanes and to the Si-O bonds of the silica network. The recorded Si 1 s spectra were deconvoluted into peaks originating from Si-C bonds and the Si-O-Si silica network. To check the results of spectrum deconvolution, several differently functionalized silanes containing stoichiometric amounts of heteroatoms were applied for silica surface modification. We conclude that spectra deconvolution of the Si 1 s signal is an appropriate means for quantification of surface attached silane molecules.